Bibliography - Jorge L Sarmiento

1. Gruber, N, and Jorge L Sarmiento, et al., February 2009: Oceanic sources, sinks, and transport of atmospheric CO₂. Global Biogeochemical Cycles, 23, GB1005, doi:10.1029/2008GB003349.
   [ Abstract ]

   We synthesize estimates of the contemporary net air-sea CO₂ flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO₂ (pCO₂) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO₂ for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a⁻¹. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (∼ −0.3 Pg C a⁻¹) contemporary CO₂ sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO₂ and a strong uptake of anthropogenic CO₂. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO₂-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO₂ is estimated to be −1.7 ± 0.4 Pg C a⁻¹ (inversion) and −1.4 ± 0.7 Pg C a⁻¹ (pCO₂-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a⁻¹, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a⁻¹ (inversion) and −1.9 ± 0.7 Pg C a⁻¹ (pCO₂-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a⁻¹ across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO₂ fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

2. Henson, S A., John Dunne, and Jorge L Sarmiento, April 2009: Decadal variability in North Atlantic phytoplankton blooms. Journal of Geophysical Research, 114, C04013, doi:10.1029/2008JC005139.
   [ Abstract ]

   The interannual to decadal variability in the timing and magnitude of the North Atlantic phytoplankton bloom is examined using a combination of satellite data and output from an ocean biogeochemistry general circulation model. The timing of the bloom as estimated from satellite chlorophyll data is used as a novel metric for validating the model's skill. Maps of bloom timing reveal that the subtropical bloom begins in winter and progresses northward starting in May in subpolar regions. A transition zone, which experiences substantial interannual variability in bloom timing, separates the two regions. Time series of the modeled decadal (1959–2004) variability in bloom timing show no long‐term trend toward earlier or delayed blooms in any of the three regions considered here. However, the timing of the subpolar bloom does show distinct decadal‐scale periodicity, which is found to be correlated with the North Atlantic Oscillation (NAO) index. The mechanism underpinning the relationship is identified as anomalous wind‐driven mixing conditions associated with the NAO. In positive NAO phases, stronger westerly winds result in deeper mixed layers, delaying the start of the subpolar spring bloom by 2–3 weeks. The subpolar region also expands during positive phases, pushing the transition zone further south in the central North Atlantic. The magnitude of the bloom is found to be only weakly dependent on bloom timing, but is more strongly correlated with mixed layer depth. The extensive interannual variability in the timing of the bloom, particularly in the transition region, is expected to strongly impact the availability of food to higher trophic levels.

3. Marinov, I, M Follows, Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, 2008: How does ocean biology affect atmospheric pCO₂? Theory and models. Journal of Geophysical Research, 113, C07032, doi:10.1029/2007JC004598.
   [ Abstract ]

   This paper examines the sensitivity of atmospheric pCO₂ to changes in ocean biology that result in drawdown of nutrients at the ocean surface. We show that the global inventory of preformed nutrients is the key determinant of atmospheric pCO₂ and the oceanic carbon storage due to the soft-tissue pump (OCS_soft ). We develop a new theory showing that under conditions of perfect equilibrium between atmosphere and ocean, atmospheric pCO₂ can be written as a sum of exponential functions of OCS _soft . The theory also demonstrates how the sensitivity of atmospheric pCO₂to changes in the soft-tissue pump depends on the preformed nutrient inventory and on surface buffer chemistry. We validate our theory against simulations of nutrient depletion in a suite of realistic general circulation models (GCMs). The decrease in atmospheric pCO₂ following surface nutrient depletion depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric pCO₂ sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO₂ sensitivity to surface nutrient depletion. Surface CO₂ disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric pCO₂ more sensitive to nutrient depletion in high-ventilation models and less sensitive to nutrient depletion in low-ventilation models. Our findings have potentially important implications for both past and future climates.

4. Marinov, I, Anand Gnanadesikan, Jorge L Sarmiento, J Robert Toggweiler, M Follows, and B K Mignone, July 2008: Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO₂. Global Biogeochemical Cycles, 22, GB3007, doi:10.1029/2007GB002958.
   [ Abstract ]

   We use both theory and ocean biogeochemistry models to examine the role of the soft-tissue biological pump in controlling atmospheric CO₂. We demonstrate that atmospheric CO₂ can be simply related to the amount of inorganic carbon stored in the ocean by the soft-tissue pump, which we term (OCS _soft ). OCS _soft is linearly related to the inventory of remineralized nutrient, which in turn is just the total nutrient inventory minus the preformed nutrient inventory. In a system where total nutrient is conserved, atmospheric CO₂ can thus be simply related to the global inventory of preformed nutrient. Previous model simulations have explored how changes in the surface concentration of nutrients in deepwater formation regions change the global preformed nutrient inventory. We show that changes in physical forcing such as winds, vertical mixing, and lateral mixing can shift the balance of deepwater formation between the North Atlantic (where preformed nutrients are low) and the Southern Ocean (where they are high). Such changes in physical forcing can thus drive large changes in atmospheric CO₂, even with minimal changes in surface nutrient concentration. If Southern Ocean deepwater formation strengthens, the preformed nutrient inventory and thus atmospheric CO₂ increase. An important consequence of these new insights is that the relationship between surface nutrient concentrations, biological export production, and atmospheric CO₂ is more complex than previously predicted. Contrary to conventional wisdom, we show that OCS _soft can increase and atmospheric CO₂ decrease, while surface nutrients show minimal change and export production decreases.

5. Matsumoto, K, and Jorge L Sarmiento, April 2008: A corollary to the silicic acid leakage hypothesis. Paleoceanography, 23, PA2203, doi:10.1029/2007PA001515.
   [ Abstract ]

   The silicic acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric CO₂ changes over the last glacial-interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low-latitude surface waters, driven by a “leakage” of high-Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric CO₂ can be lowered with even a reduced absolute flux of silicic acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.

6. Mignone, B K., R H Socolow, Jorge L Sarmiento, and M Oppenheimer, 2008: Atmospheric stabilization and the timing of carbon mitigation. Climatic Change, 88(3-4), doi:10.1007/s10584-007-9391-8.
   [ Abstract ]

   Stabilization of atmospheric CO₂ concentrations below a pre-industrial doubling (~550 ppm) is a commonly cited target in climate policy assessment. When the rate at which future emissions can fall is assumed to be fixed, the peak atmospheric concentration – or the stabilization “frontier” – is an increasing and convex function of the length of postponement. Here we find that a decline in emissions of 1% year⁻¹ beginning today would place the frontier near 475 ppm and that when mitigation is postponed, options disappear (on average) at the rate of ~9 ppm year⁻¹, meaning that delays of more than a decade will likely preclude stabilization below a doubling. When constraints on the future decline rate of emissions are relaxed, a particular atmospheric target can be realized in many ways, with scenarios that allow longer postponement of emissions reductions requiring greater increases in the intensity of future mitigation. However, the marginal rate of substitution between future mitigation and present delay becomes prohibitively large when the balance is shifted too far toward the future, meaning that some amount of postponement cannot be fully offset by simply increasing the intensity of future mitigation. Consequently, these results suggest that a practical transition path to a given stabilization target in the most commonly cited range can allow, at most, one or two decades of delay.

7. Moore, W S., Jorge L Sarmiento, and Robert M Key, May 2008: Submarine groundwater discharge revealed by ²²⁸Ra distribution in the upper Atlantic Ocean. Nature Geoscience, 1, doi:10.1038/ngeo183.
   [ Abstract ]

   Submarine groundwater discharge is defined as any flow of water at continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force1. The flux of submarine groundwater discharge has been hypothesized to be a pathway for enriching coastal waters in nutrients, carbon and metals. Here, we estimate the submarine groundwater flux from the inventory of ²²⁸Ra in the upper Atlantic Ocean, obtained by interpolating measurements at over 150 stations. Only 46% of the loss in ²²⁸Ra from radioactive decay is replenished by input from dust, rivers and coastal sediments. We infer that the remainder must come from submarine groundwater discharge. Using estimates of ²²⁸Ra concentrations in submarine groundwater discharge, we arrive at a total flux from submarine groundwater discharge of 2–4x10¹³ m³ yr^-1, between 80 and 160% of the amount of freshwater entering the Atlantic Ocean from rivers. Submarine groundwater discharge is not a freshwater flux, but a flux of terrestrial and sea water that has penetrated permeable coastal sediments. Our assessment of the volume of submarine groundwater discharge confirms that this flux represents an important vehicle for the delivery of nutrients, carbon and metal to the ocean.

8. Rodgers, K B., Jorge L Sarmiento, O Aumont, C Crevoisier, C de Boyer Montégut, and N Metzl, June 2008: A wintertime uptake window for anthropogenic CO₂ in the North Pacific. Global Biogeochemical Cycles, 22, GB2020, doi:10.1029/2006GB002920.
   [ Abstract ]

   An ocean model has been forced with NCEP reanalysis fluxes over 1948–2003 to evaluate the pathways and timescales associated with the uptake of anthropogenic CO₂ over the North Pacific. The model reveals that there are two principal regions of uptake, the first in the region bounded by 35–45°N and 140–180°E, and the second along a band between 10–20°N and between 120°W and 180°E. For both of these regions, the dominant timescale of variability in uptake is seasonal, with maximum uptake occurring during winter and uptake being close to zero or slightly negative during summer when integrated over the basin. A decadal trend toward increased uptake of anthropogenic CO₂ consists largely of modulations of the uptake maximum in winter. For detection of anthropogenic changes, this implies that in situ measurements will need to resolve the seasonal cycle in order to capture decadal trends in ΔpCO₂. As uptake of anthropogenic CO₂ occurs preferentially during winter, observationally based estimates which do not resolve the full seasonal cycle may result in underestimates of the rate of uptake of anthropogenic CO₂. There is also a sizable circulation-driven decadal trend in the seasonal cycle of sea surface ΔpCO₂ for the North Pacific, with maximum changes found near the boundary separating the subtropical and subpolar gyres in western and central regions of the basin. These changes are due to a trend in the large-scale circulation of the gyres, which itself is driven by a trend in the wind stress over the basin scale. This trend in the three-dimensional circulation is more important than the local trend in mixed layer depth (MLD) in contributing to the decadal trend in ΔpCO₂.

9. Deutsch, Curtis A., Jorge L Sarmiento, D M Sigman, N Gruber, and John Dunne, 2007: Spatial coupling of nitrogen inputs and losses in the ocean. Nature, 445(7124), doi:10.1038/nature05392.
   [ Abstract ]

   Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world's oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.

10. Dunne, John, Jorge L Sarmiento, and Anand Gnanadesikan, 2007: A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor. Global Biogeochemical Cycles, 21, GB4006, doi:10.1029/2006GB002907.
    [ Abstract ]

    We present a new synthesis of the oceanic cycles of organic carbon, silicon, and calcium carbonate. Our calculations are based on a series of algorithms starting with satellite-based primary production and continuing with conversion of primary production to sinking particle flux, penetration of particle flux to the deep sea, and accumulation in sediments. Regional and global budgets from this synthesis highlight the potential importance of shelves and near-shelf regions for carbon burial. While a high degree of uncertainty remains, this analysis suggests that shelves, less than 50 m water depths accounting for 2% of the total ocean area, may account for 48% of the global flux of organic carbon to the seafloor. Our estimates of organic carbon and nitrogen flux are in generally good agreement with previous work while our estimates for CaCO₃ and SiO₂ fluxes are lower than recent work. Interannual variability in particle export fluxes is found to be relatively small compared to intra-annual variability over large domains with the single exception of the dominating role of El Niño-Southern Oscillation variability in the central tropical Pacific. Comparison with available sediment-based syntheses of benthic remineralization and burial support the recent theory of mineral protection of organic carbon flux through the deep ocean, pointing to lithogenic material as an important carrier phase of organic carbon to the deep seafloor. This work suggests that models which exclude the role of lithogenic material would underestimate the penetration of POC to the deep seafloor by approximately 16–51% globally, and by a much larger fraction in areas with low productivity. Interestingly, atmospheric dust can only account for 31% of the total lithogenic flux and 42% of the lithogenically associated POC flux, implying that a majority of this material is riverine or directly erosional in origin.

11. Jacobson, A R., S E Mikaloff Fletcher, N Gruber, Jorge L Sarmiento, and M Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: 1. Methods and global-scale fluxes. Global Biogeochemical Cycles, 21, GB1019, doi:10.1029/2005GB002556.
    [ Abstract ]

    We have constructed an inverse estimate of surface fluxes of carbon dioxide using both atmospheric and oceanic observational constraints. This global estimate is spatially resolved into 11 land regions and 11 ocean regions, and is calculated as a temporal mean for the period 1992–1996. The method interprets in situ observations of carbon dioxide concentration in the ocean and atmosphere with transport estimates from global circulation models. Uncertainty in the modeled circulation is explicitly considered in this inversion by using a suite of 16 atmospheric and 10 oceanic transport simulations. The inversion analysis, coupled with estimates of river carbon delivery, indicates that the open ocean had a net carbon uptake from the atmosphere during the period 1992–96 of 1.7 PgC yr ^-1, consisting of an uptake of 2.1 PgC yr^-1 of anthropogenic carbon and a natural outgassing of about 0.5 PgC yr^-1 of carbon fixed on land and transported through rivers to the open ocean. The formal uncertainty on this oceanic uptake, despite a comprehensive effort to quantify sources of error due to modeling biases, uncertain riverine carbon load, and biogeochemical assumptions, is driven down to 0.2 PgC yr^-1 by the large number and relatively even spatial distribution of oceanic observations used. Other sources of error, for which quantifiable estimates are not currently available, such as unresolved transport and large region inversion bias, may increase this uncertainty.

12. Jacobson, A R., S E Mikaloff Fletcher, N Gruber, Jorge L Sarmiento, and M Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: 2. Regional results. Global Biogeochemical Cycles, 21, GB1020, doi:10.1029/2006GB002703.
    [ Abstract ]

    We report here the results from a coupled ocean-atmosphere inversion, in which atmospheric CO₂ gradients and transport simulations are combined with observations of ocean interior carbon concentrations and ocean transport simulations to provide a jointly constrained estimate of air-sea and air-land carbon fluxes. While atmospheric data have little impact on regional air-sea flux estimates, the inclusion of ocean data drives a substantial change in terrestrial flux estimates. Our results indicate that the tropical and southern land regions together are a large source of carbon, with a 77% probability that their aggregate source size exceeds 1 PgC yr^-1. This value is of similar magnitude to estimates of fluxes in the tropics due to land-use change alone, making the existence of a large tropical CO₂ fertilization sink unlikely. This terrestrial result is strongly driven by oceanic inversion results that differ from flux estimates based on pCO₂ climatologies, including a relatively small Southern Ocean sink (south of 44°S) and a relatively large sink in the southern temperate latitudes (44°S–18°S). These conclusions are based on a formal error analysis of the results, which includes uncertainties due to observational error transport and other modeling errors, and biogeochemical assumptions. A suite of sensitivity tests shows that these results are generally robust, but they remain subject to potential sources of unquantified error stemming from the use of large inversion regions and transport biases common to the suite of available transport models.

13. Mikaloff Fletcher, S E., N Gruber, A R Jacobson, M Gloor, S C Doney, S Dutkiewicz, M Gerber, M Follows, F Joos, K Lindsay, D Menemenlis, A Mouchet, S A Müller, and Jorge L Sarmiento, 2007: Inverse estimates of the oceanic sources and sinks of natural CO₂ and the implied oceanic carbon transport. Global Biogeochemical Cycles, 21(GB1010), doi:10.1029/2006GB0027.
    [ Abstract ]

    We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO₂, i.e., the component of the CO₂ flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate ΔC _gasex , i.e., the component of the observed DIC that is due to the gas exchange of natural CO₂. We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model's simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO₂ characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO₂ flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed ΔC _gasex , and that it is relatively insensitive to the choice of OGCM or potential biases in ΔC _gasex . Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr⁻¹, most of which occurs in the Atlantic. This is considerably smaller than the 1 Pg C yr⁻¹ of Northern Hemisphere uptake that has been inferred from atmospheric CO₂ observations during the 1980s and 1990s, which supports the hypothesis of a Northern Hemisphere terrestrial sink.

14. Najjar, R G., X Jin, F Louanchi, O Aumont, K Caldeira, S C Doney, J-C Dutay, M Follows, N Gruber, K Lindsay, E Maier-Reimer, R Matear, K Matsumoto, Patrick Monfray, A Mouchet, James C Orr, G-K Plattner, Jorge L Sarmiento, R Schlitzer, Richard D Slater, M-F Weirig, Y Yamanaka, and A Yool, 2007: Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Global Biogeochemical Cycles, 21, GB3007, doi:10.1029/2006GB002857.
    [ Abstract ]

    Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1s) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr^-1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

15. Sarmiento, Jorge L., J Simeon, Anand Gnanadesikan, N Gruber, Robert M Key, and R Schlitzer, 2007: Deep ocean biogeochemistry of silicic acid and nitrate. Global Biogeochemical Cycles, 21, GB1S90, doi:10.1029/2006GB002720.
    [ Abstract ]

    Observations of silicic acid and nitrate along the lower branch of the global conveyor belt circulation show that silicic acid accumulation by diatom opal dissolution occurs at 6.4 times the rate of nitrate addition by organic matter remineralization. The export of opal and organic matter from the surface ocean occurs at a Si:N mole ratio that is much smaller than this almost everywhere (cf. Sarmiento et al., 2004). The preferential increase of silicic acid over nitrate as the deep circulation progresses from the North Atlantic to the North Pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter (Broecker, 1991). However, Sarmiento et al. (2004) showed that the primary reason for the low silicic acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface Southern Ocean are depleted in silicic acid relative to nitrate. By implication, the same Southern Ocean processes that deplete the silicic acid in the surface Southern Ocean must also be responsible for the enhanced silicic acid concentration of the deep ocean. We use observations and results from an updated version of the adjoint model of Schlitzer (2000) to confirm that this is indeed the case.

16. Sweeney, C, M Gloor, A R Jacobson, Robert M Key, Galen McKinley, Jorge L Sarmiento, and R Wanninkhof, 2007: Constraining global air-sea gas exchange for CO2 with recent bomb 14C measurements. Global Biogeochemical Cycles, 21, GB2015, doi:10.1029/2006GB002784.
    [ Abstract ]

    The ¹⁴CO₂ released into the stratosphere during bomb testing in the early 1960s provides a global constraint on air-sea gas exchange of soluble atmospheric gases like CO₂. Using the most complete database of dissolved inorganic radiocarbon, DI¹⁴C, available to date and a suite of ocean general circulation models in an inverse mode we recalculate the ocean inventory of bomb-produced DI¹⁴C in the global ocean and confirm that there is a 25% decrease from previous estimates using older DI¹⁴C data sets. Additionally, we find a 33% lower globally averaged gas transfer velocity for CO₂ compared to previous estimates (Wanninkhof, 1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds are 6.9 m s⁻¹. Unlike some earlier ocean radiocarbon studies, the implied gas transfer velocity finally closes the gap between small-scale deliberate tracer studies and global-scale estimates. Additionally, the total inventory of bomb-produced radiocarbon in the ocean is now in agreement with global budgets based on radiocarbon measurements made in the stratosphere and troposphere. Using the implied relationship between wind speed and gas transfer velocity k _s = 0.27 u ₁₀ ² (Sc/660)^−0.5 and standard partial pressure difference climatology of CO₂ we obtain an net air-sea flux estimate of 1.3 ± 0.5 PgCyr⁻¹ for 1995. After accounting for the carbon transferred from rivers to the deep ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr⁻¹) compares well with estimates based on ocean inventories, ocean transport inversions using ocean concentration data, and model simulations.

17. Behrenfeld, M J., R T O'Malley, D A Siegel, C R McClain, Jorge L Sarmiento, G C Feldman, J Milligan, P G Falkowski, R M Letelier, and E S Boss, 2006: Climate-driven trends in contemporary ocean productivity. Nature, 444(7120), doi:10.1038/nature05317.
    [ Abstract ]

    Contributing roughly half of the biosphere's net primary production (NPP)1, 2, photosynthesis by oceanic phytoplankton is a vital link in the cycling of carbon between living and inorganic stocks. Each day, more than a hundred million tons of carbon in the form of CO2 are fixed into organic material by these ubiquitous, microscopic plants of the upper ocean, and each day a similar amount of organic carbon is transferred into marine ecosystems by sinking and grazing. The distribution of phytoplankton biomass and NPP is defined by the availability of light and nutrients (nitrogen, phosphate, iron). These growth-limiting factors are in turn regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, atmospheric dust deposition, and the solar cycle. Satellite measurements of ocean colour provide a means of quantifying ocean productivity on a global scale and linking its variability to environmental factors. Here we describe global ocean NPP changes detected from space over the past decade. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C yr-1), followed by a prolonged decrease averaging 190 Tg C yr-1. These trends are driven by changes occurring in the expansive stratified low-latitude oceans and are tightly coupled to coincident climate variability. This link between the physical environment and ocean biology functions through changes in upper-ocean temperature and stratification, which influence the availability of nutrients for phytoplankton growth. The observed reductions in ocean productivity during the recent post-1999 warming period provide insight on how future climate change can alter marine food webs

18. Crevoisier, C, M Gloor, E Gloaquen, Larry Horowitz, Jorge L Sarmiento, C Sweeney, and P P Tans, 2006: A direct carbon budgeting approach to infer carbon sources and sinks. Design and synthetic application to complement the NACP observation network. Tellus B, 58B(5), doi:10.1111/j.1600-0889.2006.00214.
    [ Abstract ]

    In order to exploit the upcoming regular measurements of vertical carbon dioxide (CO₂ profiles over North America implemented in the framework of the North American Carbon Program (NACP), we design a direct carbon budgeting approach to infer carbon sources and sinks over the continent using model simulations. Direct budgeting puts a control volume on top of North America, balances air mass in- and outflows into the volume and solves for the surface fluxes. The flows are derived from the observations through a geostatistical interpolation technique called Kriging combined with transport fields from weather analysis. The use of CO₂ vertical profiles simulated by the atmospheric transport model MOZART-2 at the planned 19 stations of the NACP network has given an estimation of the error of 0.39 GtC yr^-1 within the model world. Reducing this error may be achieved through a better estimation of mass fluxes associated with convective processes affecting North America. Complementary stations in the north-west and the north-east are also needed to resolve the variability of CO₂ in these regions. For instance, the addition of a single station near 52°N; 110°W is shown to decrease the estimation error to 0.34 GtC yr^-1.

19. Jin, X, N Gruber, John Dunne, Jorge L Sarmiento, and R A Armstrong, 2006: Diagnosing the contribution of phytoplankton functional groups to the production and export of particulate organic carbon, CaCO3, and opal from global nutrient and alkalinity distributions. Global Biogeochemical Cycles, 20, GB2015, doi:10.1029/2005GB002532.
    [ Abstract ]

    We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO3, and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO3 is diagnosed as 1.1 PgC yr−1 (range of sensitivity cases 0.8 to 1.2 PgC yr−1) and that of opal as 180 Tmol Si yr−1 (range 160 to 180 Tmol Si yr−1). CaCO3 export is found to have three maxima at approximately 40¡ÆS, the equator, and around 40¡ÆN. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60¡ÆS. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45¡ÆS to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of ¡­70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.

20. Marinov, I, Anand Gnanadesikan, J Robert Toggweiler, and Jorge L Sarmiento, 2006: The Southern Ocean biogeochemical divide. Nature, 441(7096), doi:10.1038/nature04883.
    [ Abstract ]

    Modelling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air–sea balance of CO2 and global biological production1, 2, 3, 4, 5, 6, 7, 8. Box model studies1, 2, 3, 4 first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure (pCO2). This early research led to two important ideas: high latitude regions are more important in determining atmospheric pCO2 than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric pCO2 are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling pre-industrial atmospheric CO2 because it serves as a lid to a larger volume of the deep ocean5, 6. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide7 and in controlling global biological production8. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air–sea CO2 balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO2 and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.

21. Mignone, B K., Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, 2006: Central role of Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon. Geophysical Research Letters, 33, L01604, doi:10.1029/2005GL024464.
    [ Abstract ]

    Although the world ocean is known to be a major sink of anthropogenic carbon dioxide, the exact processes governing the magnitude and regional distribution of carbon uptake remain poorly understood. Here we show that Southern Hemisphere winds, by altering the Ekman volume transport out of the Southern Ocean, strongly control the regional distribution of anthropogenic uptake in an ocean general circulation model, while winds and isopycnal thickness mixing together, by altering the volume of light, actively-ventilated ocean water, exert strong control over the absolute magnitude of anthropogenic uptake. These results are provocative in suggesting that climate-mediated changes in pycnocline volume may ultimately control changes in future carbon uptake.

22. Mikaloff Fletcher, S E., N Gruber, A R Jacobson, S C Doney, S Dutkiewicz, M Gerber, M Follows, F Joos, K Lindsay, D Menemenlis, A Mouchet, S A Müller, and Jorge L Sarmiento, 2006: Inverse estimates of anthropogenic CO2 uptake, transport, and storage by the ocean. Global Biogeochemical Cycles, 20, GB2002, doi:10.1029/2005GB00253.
    [ Abstract ]

    Regional air-sea fluxes of anthropogenic CO2 are estimated using a Green's function inversion method that combines data-based estimates of anthropogenic CO2 in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO2 estimates. On the basis of the prescribed anthropogenic CO2 storage, we find a global uptake of 2.2 ± 0.25 Pg C yr−1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO2 uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO2 inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.

23. Patra, P K., K R Gurney, Jorge L Sarmiento, and Song-Miao Fan, et al., 2006: Sensitivity of inverse estimation of annual mean CO2 sources and sinks to ocean-only sites versus all-sites observational networks. Geophysical Research Letters, 33(5), doi:10.1029/2005GL025403.
    [ Abstract ]

    Inverse estimation of carbon dioxide (CO2) sources and sinks uses atmospheric CO2 observations, mostly made near the Earth's surface. However, transport models used in such studies lack perfect representation of atmospheric dynamics and thus often fail to produce unbiased forward simulations. The error is generally larger for observations over the land than those over the remote/marine locations. The range of this error is estimated by using multiple transport models (16 are used here). We have estimated the remaining differences in CO2 fluxes due to the use of ocean-only versus all-sites (i.e., over ocean and land) observations of CO2 in a time-independent inverse modeling framework. The fluxes estimated using the ocean-only networks are more robust compared to those obtained using all-sites networks. This makes the global, hemispheric, and regional flux determination less dependent on the selection of transport model and observation network.

24. Dunne, John, R A Armstrong, Anand Gnanadesikan, and Jorge L Sarmiento, 2005: Empirical and mechanistic models for the particle export ratio. Global Biogeochemical Cycles, 19, GB4026, doi:10.1029/2004GB002390.
    [ Abstract ]

    We present new empirical and mechanistic models for predicting the export of organic carbon out of the surface ocean by sinking particles. To calibrate these models, we have compiled a synthesis of field observations related to ecosystem size structure, primary production and particle export from around the globe. The empirical model captures 61% of the observed variance in the ratio of particle export to primary production (the pe ratio) using sea-surface temperature and chlorophyll concentrations (or primary productivity) as predictor variables. To describe the mechanisms responsible for pe-ratio variability, we present size-based formulations of phytoplankton grazing and sinking particle export, combining them into an alternative, mechanistic model. The formulation of grazing dynamics, using simple power laws as closure terms for small and large phytoplankton, reproduces 74% of the observed variability in phytoplankton community composition wherein large phytoplankton augment small ones as production increases. The formulation for sinking particle export partitions a temperature-dependent fraction of small and large phytoplankton grazing into sinking detritus. The mechanistic model also captures 61% of the observed variance in pe ratio, with large phytoplankton in high biomass and relatively cold regions leading to more efficient export. In this model, variability in primary productivity results in a biomass-modulated switch between small and large phytoplankton pathways.

25. Orr, James C., V J Fabry, O Aumont, L Bopp, S C Doney, R A Feely, Anand Gnanadesikan, N Gruber, A Ishida, F Joos, Robert M Key, K Lindsay, E Maier-Reimer, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, K B Rodgers, C L Sabine, Jorge L Sarmiento, R Schlitzer, Richard D Slater, I J Totterdell, M-F Weirig, Y Yamanaka, and A Yool, 2005: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), doi:10.1038/nature04095.
    [ Abstract ]

    Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms such as corals and some plankton will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean-carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

26. Doney, S C., Anand Gnanadesikan, Jorge L Sarmiento, and Richard D Slater, et al., 2004: Evaluating global ocean carbon models: The importance of realistic physics. Global Biogeochemical Cycles, 18, GB3017, doi:10.1029/2003GB002150.
    [ Abstract ]

    A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

27. Gnanadesikan, Anand, John Dunne, Robert M Key, K Matsumoto, Jorge L Sarmiento, Richard D Slater, and P S Swathi, 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(4), GB4010, doi:10.1029/2003GB002097.
    [ Abstract ]

    Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.

28. Matsumoto, K, Jorge L Sarmiento, Robert M Key, O Aumont, J L Bullister, K Caldeira, J-M Campin, S C Doney, H Drange, J-C Dutay, M Follows, Y Gao, Anand Gnanadesikan, N Gruber, A Ishida, F Joos, K Lindsay, E Maier-Reimer, J Marshall, R Matear, Patrick Monfray, A Mouchet, R G Najjar, G-K Plattner, R Schlitzer, Richard D Slater, P S Swathi, I J Totterdell, M-F Weirig, Y Yamanaka, A Yool, and James C Orr, 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31, L07303, doi:10.1029/2003GL018970.
    [ Abstract ]

    New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

29. Mignone, B K., Jorge L Sarmiento, Richard D Slater, and Anand Gnanadesikan, 2004: Sensitivity of sequestration efficiency to mixing processes in the global ocean. Energy, 29(9-10), 1467-1478.
    [ Abstract PDF ]

    A number of large-scale sequestration strategies have been considered to help mitigate rising levels of atmospheric carbon dioxide (CO2). Here, we use an ocean general circulation model (OGCM) to evaluate the efficiency of one such strategy currently receiving much attention, the direct injection of liquid CO2 into selected regions of the abyssal ocean. We find that currents typically transport the injected plumes quite far before they are able to return to the surface and release CO2 through air–sea gas exchange. When injected at sufficient depth (well within or below the main thermocline), most of the injected CO2 outgasses in high latitudes (mainly in the Southern Ocean) where vertical exchange is most favored. Virtually all OGCMs that have performed similar simulations confirm these global patterns, but regional differences are significant, leading efficiency estimates to vary widely among models even when identical protocols are followed. In this paper, we make a first attempt at reconciling some of these differences by performing a sensitivity analysis in one OGCM, the Princeton Modular Ocean Model. Using techniques we have developed to maintain both the modeled density structure and the absolute magnitude of the overturning circulation while varying important mixing parameters, we estimate the sensitivity of sequestration efficiency to the magnitude of vertical exchange within the low-latitude pycnocline. Combining these model results with available tracer data permits us to narrow the range of model behavior, which in turn places important constraints on sequestration efficiency.

30. Sarmiento, Jorge L., N Gruber, M Brzezinski, and John Dunne, 2004: High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature, 427, 56-60.
    [ Abstract PDF ]

    The ocean's biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere and North Atlantic Ocean, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.

31. Sarmiento, Jorge L., Richard D Slater, R T Barber, L Bopp, S C Doney, A C Hirst, J Kieypas, R Matear, U Mikolajewicz, Patrick Monfray, V Soldatov, S A Spall, and Ronald J Stouffer, 2004: Response of ocean ecosystems to climate warming. Global Biogeochemical Cycles, 18, GB3003, doi:10.1029/2003/GB002134.
    [ Abstract ]

    We examine six different coupled climate model simulations to determine the ocean biological response to climate warming between the beginning of the industrial revolution and 2050. We use vertical velocity, maximum winter mixed layer depth, and sea ice cover to define six biomes. Climate warming leads to a contraction of the highly productive marginal sea ice biome by 42% in the Northern Hemisphere and 17% in the Southern Hemisphere, and leads to an expansion of the low productivity permanently stratified subtropical gyre biome by 4.0% in the Northern Hemisphere and 9.4% in the Southern Hemisphere. In between these, the subpolar gyre biome expands by 16% in the Northern Hemisphere and 7% in the Southern Hemisphere, and the seasonally stratified subtropical gyre contracts by 11% in both hemispheres. The low-latitude (mostly coastal) upwelling biome area changes only modestly. Vertical stratification increases, which would be expected to decrease nutrient supply everywhere, but increase the growing season length in high latitudes. We use satellite ocean color and climatological observations to develop an empirical model for predicting chlorophyll from the physical properties of the global warming simulations. Four features stand out in the response to global warming: (1) a drop in chlorophyll in the North Pacific due primarily to retreat of the marginal sea ice biome, (2) a tendency toward an increase in chlorophyll in the North Atlantic due to a complex combination of factors, (3) an increase in chlorophyll in the Southern Ocean due primarily to the retreat of and changes at the northern boundary of the marginal sea ice zone, and (4) a tendency toward a decrease in chlorophyll adjacent to the Antarctic continent due primarily to freshening within the marginal sea ice zone. We use three different primary production algorithms to estimate the response of primary production to climate warming based on our estimated chlorophyll concentrations. The three algorithms give a global increase in primary production of 0.7% at the low end to 8.1% at the high end, with very large regional differences. The main cause of both the response to warming and the variation between algorithms is the temperature sensitivity of the primary production algorithms. We also show results for the period between the industrial revolution and 2050 and 2090.

32. Gao, Y, Song-Miao Fan, and Jorge L Sarmiento, 2003: Aeolian iron input to the ocean through precipitation scavenging: A modeling perspective and its implication for natural iron fertilization in the ocean. Journal of Geophysical Research, 108(D7), 4221, doi:10.1029/2002JD002420.
    [ Abstract ]

    Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for ~40% of the total deposition over the coastal oceans and ~60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 × 10¹² g yr^-1, which is 4-30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6-2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles.

33. Gloor, M, N Gruber, Jorge L Sarmiento, C L Sabine, R A Feely, and A Abe-Ouchi, 2003: A first estimate of present and preindustrial air-sea (CO2 flux patterns based on ocean interior carbon measurements and models. Geophysical Research Letters, 30(1), 1010, doi:10.1029/2002GL015594.
    [ Abstract ]

    The exchange of CO₂ across the air-sea interface is a main determinant of the distribution of atmospheric CO₂ from which major conclusions about the carbon cycle are drawn, yet our knowledge of atmosphere-ocean fluxes still has major gaps. A new analysis based on recent ocean dissolved inorganic carbon data and on models permits us to separately estimate the preindiustrial and present air-sea CO₂ flux distributions without requiring knowledge of the gas exchange coefficient. We find a smaller carbon sink at mid to high latitudes of the sourhtern hemisphere than previous data based estimates and a shift of ocean uptake to lower latitude regions compared to estimates and simulations. The total uptake of anthropogenic CO₂ for 1990 is 1.8 (±0.4) Pg C yr^-1 . Our ocean based results support the interpretation of the latitudinal distribution of atmospheric CO₂ data as evidence for a large northern hemisphere land carbon sink.

34. Gnanadesikan, Anand, Jorge L Sarmiento, and Richard D Slater, 2003: Effects of patchy ocean fertilization on atmospheric carbon dioxide and biological production. Global Biogeochemical Cycles, 17(2), 1050, doi:10.1029/2002GB001940.
    [ Abstract ]

    Increasing oceanic productivity by fertilizing nutrient-rich regions with iron has been proposed as a mechanism to offset anthropogenic emissions of carbon dioxide. Earlier studies examined the impact of large-scale fertilization of vast reaches of the ocean for long periods of time. We use an ocean general circulation model to consider more realistic scenarios involving fertilizing small regions (a few hundred kilometers on a side) for limited periods of time (of order 1 month). A century after such a fertilization event, the reduction of atmospheric carbon dioxide is between 2% and 44% of the initial pulse of organic carbon export to the abyssal ocean. The fraction depends on how rapidly the surface nutrient and carbon fields recover from the fertilization event. The modeled recovery is very sensitive to the representation of biological productivity and remineralization. Direct verification of the uptake would be nearly impossible since changes in the air-sea flux due to fertilization would be much smaller than those resulting from natural spatial variability. Because of the sensitivity of the uptake to the long-term fate of the iron and organic matter, indirect verification by measurement of the organic matter flux would require high vertical resolution and long-term monitoring. Finally, the downward displacement of the nutrient profile resulting from an iron-induced productivity spurt may paradoxically lead to a long-term reduction in biological productivity. In the worst-case scenario, removing 1 ton of carbon from the atmosphere for a century is associated with a 30-ton reduction in biological export of carbon.

35. Gurney, K R., A Lauer, A S Denning, P Rayner, D F Baker, P Bousquet, L Bruhwiler, Y-H Chen, P Ciais, Song-Miao Fan, I Y Fung, M Gloor, M Heimann, K Higuchi, Jasmin John, E Kowalczyk, T Maki, S Maksyutov, P Peylin, M J Prather, B Pak, Jorge L Sarmiento, S Taguchi, T Takahashi, and C-W Yuen, 2003: TransCom 3 CO2 inversion intercomparison: 1. Annual mean control results and sensitivity to transport and prior flux information. Tellus B, 55B(2), 555-579.
    [ Abstract PDF ]

    Spatial and temporal variations of atmospheric CO₂ concentration contain information about surface sources and sinks, which can be quantitatively interpreted through tracer transport inversion. Previous CO₂ inversion calculations obtained differing results due to different data, methods and transport models used. To isolate the sources of uncertainty, we have conducted a set of annual mean inversion experiments in which 17 different transport models or model variants were used to calculate regional carbon sources and sinks from the same data with a standardized method. Simulated transport is a significant source of uncertainty in these calculations, particularly in the response to prescribed "background" fluxes due to fossil fuel combustion, a balanced terrestrial biosphere, and air-sea gas exchange. Individual model-estimated fluxes are often a direct reflection of their response to these background fluxes. Models that generate strong surface maxima near background exchange locations tend to require larger uptake near those locations. Models with weak surface maxima tend to have less uptake in those same regions but may infer small sources downwind. In some cases, individual model flux estimates cannot be analyzed through simple relationships to background flux responses but are likely due to local transport differences or particular responses at individual CO₂ observing locations. The response to the background biosphere exchange generates the greatest variation in the estimated fluxes, particularly over land in the Northern Hemisphere. More observational data in the tropical regions may help in both lowering the uncertain tropical land flux uncertainties and constraining the northern land estimates because of compensation between these two broad regions in the inversion. More optimistically, examination of the model-mean retrieved fluxes indicates a general insensitivity to the prior fluxes and the prior flux uncertainties. Less uptake in the Southern Ocean than implied by oceanographic observations, and an evenly distributed northern land sink, remain in spite of changes in this aspect of the inversion setup.

36. Matsumoto, K, Anand Gnanadesikan, N Gruber, Robert M Key, and Jorge L Sarmiento, 2003: Inconsistent model uptake of anthropogenic tracers in the Southern Ocean. Geochimica et Cosmochimica Acta, 67(18), Suppl 1, A278.
    [ PDF ]
37. McNeil, B I., R Matear, Robert M Key, J L Bullister, and Jorge L Sarmiento, 2003: Anthropogenic CO₂ uptake by the ocean based on the Global Chlorofluorocarbon Data Set. Science, 299(5604), 235-239.
    [ Abstract PDF ]

    We estimated the oceanic inventory of anthropogenic carbon dioxide (CO₂) from 1980 to 1999 using a technique based on the global chorofluorocarbon data set. Our analysis suggests that the ocean stored 14.8 petagrams of anthropogenic carbon from mid-1980 to mid-1989 and 17.9 petagrams of carbon from mid-1990 to mid-1999, indicating an oceanwide net uptake of 1.6 and 2.0 ± 0.4 petagrams of carbon per year, respectively. Our results provide an upper limit on the solubility-driven anthropogenic CO₂ flux into the ocean, and they suggest that most ocean general circulation models are overestimating oceanic anthropogenic CO₂ uptake over the past two decades.

38. Toggweiler, J R., Anand Gnanadesikan, S Carson, R Murnane, and Jorge L Sarmiento, 2003: Representation of the carbon cycle in box models and GCMs: 1. Solubility pump. Global Biogeochemical Cycles, 17(1), 1026, doi:10.1029/2001GB001401.
    [ Abstract ]

    Bacastow [1996], Broecker et al. [1999], and Archer et al.[2000] have called attention recently to the fact that box models and general circulation models (GCMs) represent the thermal partitioning of CO₂ between the warm surface ocean and cold deep ocean in different ways. They attribute these differences to mixing and circulation effects in GCMs that are not resolved in box models. The message that emerges from these studies is that box models have overstated the importance of the ocean's polar regions in the carbon cycle. A reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial - interglacial changes in atmospheric CO₂. In parts 1 and 2 of this paper, a new analysis of the ocean's carbon pumps is carried out to examine these findings. This paper, part 1, shows that unresolved mixing and circulation effects in box models are not the main reason for box model-GCM differences. The main factor is very different kinds of restrictions on gas exchange in polar areas. Polar outcrops in GCMs are much smaller than in box models, and they are assumed to be ice covered in an unrealistic way. This finding does not support a reduced role for the ocean's polar regions in the cycling of organic carbon, the subject taken up in part 2.

39. Toggweiler, J R., R Murnane, S Carson, Anand Gnanadesikan, and Jorge L Sarmiento, 2003: Representation of the carbon cycle in box models and GCMs: 2. Organic pump. Global Biogeochemical Cycles, 17(1), 1027, doi:10.1029/2001GB001841.
    [ Abstract ]

    Box models of the ocean/atmosphere CO₂ system rely on mechanisms at polar outcrops to alter the strength of the ocean's organic carbon pump. GCM-based carbon system models are reportedly less sensitive to the same processes. Here we separate the carbon pumps in a three-box model and the GCM-based Princeton Ocean Biogeochemistry Model to show how the organic pumps operate in the two kinds of models. The organic pumps are found to be quite different in two respects. Deep water in the three-box model is relatively well equilibrated with respect to the pCO₂ of the atmosphere while deep water in the GCM tends to be poorly equilibrated. This makes the organic pump inherently stronger in the GCM than in the three-box model. The second difference has to do with the role of polar nutrient utilization. The organic pump in the GCM is shown to have natural upper and lower limits that are set by the initial PO₄ concentrations in the deep water formed in the North Atlantic and Southern Ocean. The strength of the organic pump can swing between these limits in response to changes in deep-water formation that alter the mix of northern and southern deep water. Thus, unlike the situation in the three-box model, the organic pump in the GCM can become weaker or stronger without changes in polar nutrient utilization.

40. Watson, A J., James C Orr, Anand Gnanadesikan, Robert M Key, Jorge L Sarmiento, and Richard D Slater, 2003: Carbon dioxide fluxes in the global ocean In Ocean Biogeochemistry: A Synthesis of the Joint Global Ocean Flux Study (JGOFS), Berlin, Germany, Springer-Verlag, 123-143.
41. Brzezinski, M, C J Pride, V M Franck, D M Sigman, Jorge L Sarmiento, K Matsumoto, N Gruber, G H Rau, and K H Coale, 2002: A switch from Si(OH)4 to NO3- depletion in the glacial Southern Ocean. Geophysical Research Letters, 29(12), doi:10.1029/2001GL014349.
    [ Abstract ]

    Phytoplankton in the Antarctic deplete silicic acid (Si(OH)4) to a far greater extent than they do nitrate (NO3-). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)4:NO3- uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO3- depletion with excess Si(OH)4 remaining in surface waters. New 30Si and 15N records from Antarctic sediments confirm diminished Si(OH)4 use and enhanced NO3- depletion during the last three glaciations. The present low-Si(OH)4 water is transported northward to at least the subtropics. We postulate that the glacial high-Si(OH)4 water similarly may have been transported to the subtropics and beyond. This input of Si(OH)4 may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO2 by as much as 60 ppm.

42. Doney, S C., A Kleypas, Jorge L Sarmiento, and P G Falkowski, 2002: The US JGOFS Synthesis and modeling project - an introduction. Deep-Sea Research, Part II, 49(1-3), 1-20.
    [ Abstract PDF ]

    The field data collected as part of the international Joint Global Ocean Flux Study (JGOFS) provide an unprecedented view of marine biogeochemistry and the ocean carbon cycle. Following the completion of a series of regional process studies, a global CO2 survey, and a decade of sampling at two open-ocean time-series, US JGOFS initiated in 1997 a final research phase, the Synthesis and Modeling Project (SMP). The objective of the US JGOFS SMP is to "synthesize knowledge gained from the US JGOFS and related studies into a set of models that reflect our current understanding of the oceanic carbon cycle". Here we present an overview of the SMP and highlight the early scientific results from the project.

43. Dutay, J-C, J L Bullister, S C Doney, James C Orr, R G Najjar, Jorge L Sarmiento, and Richard D Slater, et al., 2002: Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models. Ocean Modelling, 4(2), 89-120.
    [ Abstract PDF ]

    We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.

44. Gnanadesikan, Anand, Richard D Slater, N Gruber, and Jorge L Sarmiento, 2002: Oceanic vertical exchange and new production: a comparison between models and observations. Deep-Sea Research, Part II, 49(1-3), 363-401.
    [ Abstract PDF ]

    This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of new production (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively low lateral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m, while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr -1. On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modelled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate water with the right properties. The results demonstrate the potential usefulness of new production for evaluating circulation models.

45. Gurney, K R., R M Law, A S Denning, S Fan, and Jorge L Sarmiento, et al., 2002: Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models. Nature, 415(6872), 626-630.
    [ Abstract PDF ]

    Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models 1-9. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive in transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network with the tropics.

46. Iglesias-Rodriguez, M D., R A Armstrong, R A Feely, R Hood, A Kleypas, J D Milliman, C L Sabine, and Jorge L Sarmiento, 2002: Progress made in study of ocean's calcium carbonate budget. EOS, 83(34), 365, 374-375.
47. Matsumoto, K, Jorge L Sarmiento, and M Brzezinski, 2002: Silicic acid leakage from the Southern Ocean: a possible explanation for glacial atmospheric pCO2. Global Biogeochemical Cycles, 16(3), doi:10.1029/2001GB001442.
    [ Abstract ]

    Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ~3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this "silicic acid leakage" scenario.

48. Peylin, P, D F Baker, Jorge L Sarmiento, P Ciais, and P Bousquet, 2002: Influence of transport uncertainty on annual mean and seasonal inversions of atmospheric CO2 data. Journal of Geophysical Research, 107(D19), 4385, doi:10.1029/2001JD000857.
    [ Abstract ]

    Inversion methods are often used to estimate surface CO2 fluxes from atmospheric CO2 concentration measurements, given an atmospheric transport model to relate the two. The published estimates disagree strongly on the location of the main sources and sinks, however. Are these differences due to the different time spans considered, or are they artifacts of the method and data used? Here we assess the uncertainty in such estimates due to the choice of time discretization of the measurements and fluxes, the spatial resolution of the fluxes, and the transport model. A suite of 27 Bayesian least squares inversions has been run, given by varying the number of flux regions solved for (7, 12, and 17), the time discretization (annual/annual, annual/monthly, and monthly/monthly for the fluxes/data), and the transport model (TM2, TM3, and GCTM), while holding all other inversion details constant. The estimated fluxes from this ensemble of inversions for the land + ocean sum are stable over large zonal bands, but the spread in the results increases when considering the longitudinal flux distribution inside these bands. On average for 1990–1994 the inversions place a large CO2 uptake north of 30°N (3.2 ± 0.3 GtC yr-1), mostly over the land regions, with more in Eurasia than North America. The ocean fluxes are generally smaller than given by Takahashi et al. [1999], especially south of 15°S and in the global total, where they are less than half as large. A small uptake is found for the tropical land regions, suggesting that growth more than compensates for deforestation there. The results for the different transport models are consistent with their known mixing properties; the longitudinal pattern of their land biosphere rectifier, in particular, strongly influences the regional partitioning of the flux in the north. While differences between the transport models contribute significantly to the spread of the results, an equivalent or even larger spread is due to the time discretization method used: Solving for annual mean fluxes with monthly mean measurements tended to give spurious land/ocean flux partition in the north. We suggest then that this time discretization method be avoided. Overall, the uncertainty quoted for the estimated fluxes should include not only the random error calculated by the inversion equations but also all the systematic errors in the problem, such as those addressed in this study.

49. Sarmiento, Jorge L., John Dunne, Anand Gnanadesikan, Robert M Key, K Matsumoto, and Richard D Slater, 2002: A new estimate of the CaCO3 to organic carbon export ratio. Global Biogeochemical Cycles, 16(4), doi:10.1029/2002/GB001919.
    [ Abstract ]

    We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO3 to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO3 production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.

50. Sarmiento, Jorge L., and N Gruber, 2002: Sinks for anthropogenic carbon. Physics Today, 55(8), 30-36.
    [ Abstract PDF ]

    Organic carbon buried in sediments as coal, natural gas, and oil over literally hundreds of millions of years is being consumed as a result of human activities and returned to the atmosphere as carbon dioxide (CO2) on a time scale of a few centuries. The energy harvested from this transformation of fossil fuels supplies us with electricity, heat, transportation, and industrial power. The clearing of forests for agricultural lands and the harvesting of wood, both of which remove carbon-bearing vegetation, have also added CO2 to the atmosphere, in amounts equivalent to more than half of the fossil fuel source. The CO2 added to the atmosphere because of man's activities, and the way it is currently distributed within the land, air, and sea, is depicted in the carbon cycle diagram shown in figure 1.

51. Deutsch, Curtis A., N Gruber, Robert M Key, Jorge L Sarmiento, and A Ganachaud, 2001: Denitrification and N₂ fixation in the Pacific Ocean. Global Biogeochemical Cycles, 15(2), 483-506.
    [ Abstract PDF ]

    We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48 ± 5 Tg N yr^-1 is calculated for the Eastern Tropical Pacific using N* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15 ± 3 Tg N yr^-1 . We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4 ± 12 Tg N yr^-1 . Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59 ± 14 Tg N yr^-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron supply.

52. Gloor, M, N Gruber, T M C Hughes, and Jorge L Sarmiento, 2001: Estimating net air-sea fluxes from ocean bulk data: Methodology and application to the heat cycle. Global Biogeochemical Cycles, 15(4), 767-782.
    [ Abstract PDF ]

    A novel method to estimate annual mean heat, water, and gas exchange fluxes between the ocean and the atmosphere is proposed that is complementary to the traditional approach based on air-sea gradients and bulk exchange parameterization. The new approach exploits the information on surface exchange fluxes contained in the distribution of temperature, salinity, and dissolved gases in the ocean interior. We use an Ocean General Circulation Model to determine how the distribution in the ocean interior is linked to surface fluxes. We then determine with least squares the surface fluxes that are most compatible with the observations. To establish and test the method, we apply it to ocean temperature data to estimate heat fluxes across the air-sea interface for which a number of climatological estimates exists. We also test the sensitivity of the inversion results to data converage, differences in ocean transport, variations in the surface flux pattern and a range of spatial resolutions. We find, on the basis of the World Ocean Circulation Experiment (WOCE) data network augmented with selected high-quality pre-WOCE data, that we are able to constrain heat exchange fluxes for 10 - 15 regions of the ocean, whereby these fluxes nearly balance globally without enforcing a conservation constraint. Our results agree well with heat flux estimates on the basis of bulk exchange parameterization, which generally require constraints to ensure a global net heat flux of zero. We also find that the heat transports implied by our inversely estimated fluxes are in good agreement with a large range of heat transport estimates based on hydrographic data. Increasing the number of regions beyond the 10 - 15 regions considered here is severely limited because of modeling errors. The inverse method is fairly robust to the modeling of the spatial patterns of the surface fluxes; however, it is quite sensitive to the modeling of ocean transport. The most striking difference between our estimates and the heat flux climatologies is a large heat loss of 0.64 PW to the atmosphere from the Southern Ocean and a large heat gain by the subpolar South Atlantic of 0.56 PW. These results are consistent with the large gain of carbon dioxide called for by Takahashi et al. [1999] in his recent analysis of the air-sea flux of carbon dioxide but inconsistent with the large loss of oxygen and carbon dioxide such as those of Stephens et al. [1998].

53. Gruber, N, M Gloor, Song-Miao Fan, and Jorge L Sarmiento, 2001: Air-sea flux of oxygen estimated from bulk data: Implications for the marine and atmospheric oxygen cycles. Global Biogeochemical Cycles, 15(4), 783-803.
    [ Abstract PDF ]

    We estimate the annual net air-sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air-sea gas exchange parameterizations. The inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13°S-13°N) emit ~212 Tmol O₂ yr ^-1 , which is compensated by uptake of 148 Tmol yr^-1 in the Northern Hemisphere (>13°N) and by uptake of 65 Tmol yr^-1 in the Southern Hemisphere (<13°S). These results imply that the dominant feature of oxygen transport in the combined ocean-atmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of O₂ uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where O₂ is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving O₂ uptake in the temperate regions of the Northern Hemisphere versus loss of O₂ in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36°S, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of O₂ in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APO O₂ + CO₂ ). Our results indicate that the north-south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APO. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of O₂ in the ocean, such as proposed by Stephens et al. [1998]. However, we find that uncertainties in the modeled APO distribution stemming from seasonal atmospheric rectification effects and the limited APO data coverage prevent the currently available APO data from providing strong constraints on the magnitude of interhemispheric transport.

54. Orr, James C., E Maier-Reimer, U Mikolajewicz, Patrick Monfray, Jorge L Sarmiento, J Robert Toggweiler, N K Taylor, J Palmer, N Gruber, C L Sabine, C Le Quéré, Robert M Key, and J Boutin, 2001: Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochemical Cycles, 15(1), 43-60.
    [ Abstract PDF ]

    We have compared simulations of anthropogenic CO₂ in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ± 19%, giving a range of 1.85±0.35 Pg C yr ^-1 for the 1980-1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO₂ in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO₂ suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data-based estimates of anthropogenic CO₂ and bomb ¹⁴C. Column inventories of bomb ¹⁴C have become more similar to those for anthropogenic CO₂ with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO₂ would improve if systematic errors associated with the date-based estimates could be provided regionally.

55. Gloor, M, Song-Miao Fan, S W Pacala, and Jorge L Sarmiento, 2000: Optimal sampling of the atmosphere for purpose of inverse modeling: A model study. Global Biogeochemical Cycles, 14(1), 407-428.
    [ Abstract PDF ]

    The 66 stations of the GLOBALVIEW-CO2 sampling network (GLOBALVIEW-CO2: Cooperative Atmospheric Data Integration Project - Carbon Dioxide, (1997)) are located primarily remotely from continents where signals of fossil fuel consumption and biospheric exchange are diluted. It is thus not surprising that inversion studies are able to estimate terrestrial sources and sinks only to a very limited extent. The poor constraint on terrestrial fluxes propagates to the oceans and strongly limits estimates of oceanic fluxes as well, at least if no use is made of other information such as isotopic ratios. We analyze here the resolving power of the GLOBALVIEW-CO2 network, compare the efficiency of different measurement strategies, and determine optimal extensions to the present network. We find the following: (1) GLOBALVIEW-CO2 is well suited to characterize the meridional distribution of sources and sinks but is poorly suited to separate terrestrial from oceanic sinks at the same latitude. The most poorly constrained regions are South America, Africa, and southern hemispheric oceans. (2) To improve the network, observing stations need to be positioned on the continents near to the largest biospheric signals despite the large diurnal and seasonal fluctuations associated with biological activity and the dynamics of the PBL. The mixing in the atmosphere is too strong to allow positioning of stations remote from large fluxes. Our optimization results prove to be fairly insensitive to the details of model transport and the inversion model with the addition of ~ 10 optimally positioned stations. (3) The best measurement strategy among surface observations, N-S airplane transects, and vertical profiles proves to be vertical profiles. (4) Approximately 12 optimally positioned vertical profiles or 30 surface stations in addition to GLOBALVIEW-CO2 would reduce estimate uncertainties caused by insufficient data coverage from ~ 1 Pg C yr -1 per region to ~ 0.2 Pg C yr -1 per region.

56. Murnane, R, and Jorge L Sarmiento, 2000: Roles of biology and gas exchange in determining the 13C distribution in the ocean and the preindustrial gradient in atmospheric 13C. Global Biogeochemical Cycles, 14(1), 389-405.
    [ Abstract PDF ]

    We examine the processes responsible for the distribution of 13C in a global ocean model. The dominant sources of gradients are biological processes and the temperature effect on isotopic fractionation. However, in a model without biology developed to examine the temperature effect of isotropic fractionation in isolation, we find an almost uniform 13C distribution. Extremely slow 13C air-sea equilibrium does not permit the surface ocean to come into equilibrium with the atmosphere and 13C in the ocean thus becomes well mixed. However biological effects, which are interior to the ocean, are strongly expressed and minimally effected by air-sea exchange. Biological fractionation thus dominates the oceanic 13C distribution. An important feature of the model is an extremely large northward transport of isotopic anomaly. The transfer from the ocean to the Northern Hemisphere atmosphere of 120 Pg C 0/00 is equivalent in magnitude to the signal that would be generated by a net terrestrial biospheric uptake of 5 Pg C yr -1 from the Northern Hemisphere atmosphere, or an 1-2 0/00 disequilibrium between terrestrial respiration and photosynthesis. Improved ocean model simulations and observational analysis are required to test for the possible existence of such a large oceanic transport of isotopic anomaly,

57. Sarmiento, Jorge L., 2000: That sinking feeling. Nature, 408(6809), 155-156.
    [ PDF ]
58. Sarmiento, Jorge L., Patrick Monfray, E Maier-Reimer, O Aumont, R Murnane, and James C Orr, 2000: Sea-air CO2 fluxes and carbon transport: A comparison of three ocean general circulation models. Global Biogeochemical Cycles, 14(4), 1267-1281.
    [ Abstract PDF ]

    Many estimates of the atmospheric carbon budget suggest that most of the sink for CO2 produced by fossil fuel burning and cement production must be in the Northern Hemisphere. Keeling et al. [1989] hypothesized that this asymmetry could be explained instead by a northward preindustrial transport of ~1 Pg C y-1 in the atmosphere balanced by an equal and opposite southward transport in the ocean. We explore this hypothesis by examining the processes that determine the magnitude of the preindustrial interhemispheric flux of carbon in three ocean carbon models. This study is part of the first stage of the Ocean Carbon Model Intercomparison Project organized by International Geosphere Biosphere Programme Global Analysis, Interpretation, and Modelling Task Force. We find that the combination of interhemispheric heat transport (with its associated carbon transport), a finite gas exchange, and the biological pump, yield a carbon flux of only -0.12 to +0.04 Pg C y-1 across the equator (positive to the north). An important reason for the low carbon transport is the decoupling of the carbon flux from the interhemispheric heat transport due to the long sea-air equilibration time for surface CO2. A possible additional influence on the interhemispheric exchange is oceanic transport of carbon from rivers.

59. Suntharalingam, P, and Jorge L Sarmiento, 2000: Factors governing the oceanic nitrous oxide distribution: Simulations with an ocean general circulation model. Global Biogeochemical Cycles, 14(1), 429-454.
    [ Abstract PDF ]

    A global model of the oceanic nitrous oxide distribution is developed to evaluate current understanding of the processes governing nitrous oxide formation and distribution in the open ocean. N2O is treated as a nonconserved tracer in a global ocean general circulation model subject to biological sources in the oceanic interior and gas exchange at the ocean surface. A simple scalar parameterization linking N2O production to oxygen consumption (and based on observed correlations between excess N2O and apparent oxygen utilization) is suceessful in reproducing the large-scale features of the observed distribution, namely, high surface supersaturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic subtropical gyres. The majority of the oceanic N2O source is produced in the upper water column (over 75% above 600 m) and effluxes directly to the atmosphere in the latitude band of formation. The observed structure at depth is not as well reproduced by this model, which displays excessive N2O production in the deep ocean. An alternative source of parameterization, which accounts for processes which result in a depth variation in the relationship between N2O production and oxygen consumption, yields an improved representation of the deep distribution. The surface distribution and sea-air flux are, however, determined primarily by the upper ocean source and, therefore, are relatively insensitive to changes in the nature of deep oceanic N2O production.

60. Suntharalingam, P, Jorge L Sarmiento, and J Robert Toggweiler, 2000: Global significance of nitrous-oxide production and transport from oceanic low-oxygen zones: A modeling study. Global Biogeochemical Cycles, 14(4), 1353-1370.
    [ Abstract PDF ]

    Recent studies of marine nitrous oxide have focused attention on the suboxic and low-oxygen zones associated with ocean basin eastern boundaries. It has been suggested that complex N2O cycling mechanisms in these regions may provide a net source to the oceanic interior and a significant portion of the ocean-atmosphere flux. In this study we evaluate the global significance of N2O formation in these regions. N2O is treated as a nonconserved tracer in an ocean general circulation model: a simple source function is developed which models N2O production as a function of organic matter remineralization and local oxygen concentration. Model results are evaluated against both surface and deep observational data sets. The oceanic oxygen minimum zones are predominantly found in the upperwater column of tropical latitudes and overlain by regions of strong upwelling in the surface ocean. Simulations of increased N2O production under low-oxygen conditions indicate that the majority of the N2O thus formed escapes directly to the atmosphere and is not subject to significant meridional transport. Results indicate that while enhanced N2O production in these regions cannot be held accountable for the majority of the sea-air flux and interior distribution, it may, however, have significance for the local distribution and provide as much as 25-50% of the global oceanic source.

61. Fan, Song-Miao, T Blaine, and Jorge L Sarmiento, 1999: Terrestrial carbon sink in the Northern Hemisphere estimated from the atmospheric CO₂ difference between Mauna Loa and the South Pole since 1959. Tellus B, 51B(5), 863-870.
    [ Abstract PDF ]

    The difference between Mauna Loa and South Pole atmospheric CO₂ concentrations from 1959 to the present scales linearly with CO₂ emissions from fossil fuel burning and cement production (together called fossil CO₂). An extrapolation to zero fossil CO₂ emission has been used to suggest that the atmospheric CO₂ concentration at Mauna Loa was 0.8 ppm less than that at the South Pole before the industrial revolution, associated with a northward atmospheric transport of about 1 Gt C yr^-1 (Keeling et al., 1989a). Mass conservation requires an equal southward transport in the ocean. However, our ocean general circulation and biogeochemistry model predicts a much smaller pre-industrial carbon transport. Here, we present a new analysis of the Mauna Loa and South Pole CO₂ data, using a general circulation model and a 2-box model of the atmosphere. It is suggested that the present CO₂ difference between Mauna Loa and the South Pole is caused by, in addition to fossil CO2 sources and sinks, a pre-industrial interhemispheric flux of 0.5-0.7 Gt C yr^-1 , and a terrestrial sink of 0.8-1.2 Gt C yr^-1 in the mid-latitude Northern Hemisphere, balanced by a tropical deforestation source that has been operating continuously in the period from 1959 to the present.

62. Fan, Song-Miao, Jorge L Sarmiento, M Gloor, and S W Pacala, 1999: On the use of regularization techniques in the inverse modeling of atmospheric carbon dioxide.. Journal of Geophysical Research, 104(D17), 21,503-21,512.
    [ Abstract PDF ]

    The global distribution of carbon sources and sinks is estimated from atmospheric CO₂ measurements using an inverse method based on the Geophysical Fluid Dynamics Laboratory SKYHI atmospheric general circulation model. Applying the inverse model without any regularization yields unrealistically large CO₂ fluxes in the tropical regions. We examine the use of three regularization techniques that are commonly used to stabilize inversions: truncated singular value decomposition, imposition of a priori flux estimates, and use of a quadratic inequality constraint. The regularization techniques can all be made to minimize the unrealistic fluxes in the tropical regions. This brings inversion estimated CO₂ fluxes for oceanic regions in the tropics and in the Southern Hemisphere into better agreement with independent estimates of the air-sea exchange. However, one cannot assume that stabilized inversions give accurate estimates, as regularization merely holds the fluxes to a priori estimates or simply reduces them in magnitude in regions that are not resolvable by observations. By contrast, estimates of flux and uncertainty for the temperate North Atlantic, temperate North Pacific, and boreal and temperate North American regions are far less sensitive to the regularization parameters, consistent with the fact that these regions are better constrained by the present observations.

63. Gloor, M, Song-Miao Fan, S W Pacala, Jorge L Sarmiento, and M Ramonet, 1999: A model-based evaluation of inversions of atmospheric transport, using annual mean mixing ratios, as a tool to monitor fluxes of nonreactive trace substances like CO₂ on a continental scale. Journal of Geophysical Research, 104(D12), 14,245-14,260.
    [ Abstract PDF ]

    The inversion of atmospheric transport of CO₂ may potentially be a means for monitoring compliance with emission treaties in the future. There are two types of errors though, which may cause errors in inversions: (1) amplification of high-frequency data variability given the information loss in the atmosphere by mixing and (2) systematic errors in the CO₂flux estimates caused by various approximations used to formulate the inversions. In this study we use simulations with atmospheric transport models and a time independent inverse scheme to estimate these errors as a function of network size and the number of flux regions solved for. Our main results are as follows: (1) When solving for 10-20 source regions, the average uncertainty of flux estimates caused by amplification of high-frequency data variability alone decreases strongly with increasing number of stations for up to ~150 randomly positions stations and then levels off (for 150 stations of the order of ±0.2 Pg C yr^-1). As a rule of thumb, about 10 observing stations are needed per region to be estimated. (2) Of all the sources of systematic errors, modeling error is the largest. Our estimates of SF₆ emissions from five continental regions simulated with 12 different AGCMs differ by up to a factor of 2. The number of observations needed to overcome the information loss due to atmospheric mixing is hence small enough to permit monitoring of fluxes with inversions on a continental scale in principle. Nevertheless errors in transport modeling are still too large for inversions to be a quantitatively reliable option for flux monitoring.

64. Murnane, R, Jorge L Sarmiento, and C Le Quéré, 1999: Spatial distribution of air-sea CO₂ fluxes and the interhemispheric transport of carbon by the oceans. Global Biogeochemical Cycles, 13(2), 287-305.
    [ Abstract PDF ]

    The dominant processes controlling the magnitude and spatial distribution of the preindustrial air-sea flux of CO₂ are atmosphere-ocean heat exchange and the biological pump, coupled with the direct influence of ocean circulation resulting from the slow time-scale of air-sea CO₂ gas exchange equilibration. The influence of the biological pump is greatest in surface outcrops of deep water, where the excess deep ocean carbon resulting from net remineralization can escape to the atmosphere. In a steady state other regions compensate for this loss by taking up CO₂ to give a global net air-sea CO₂ flux of zero. The predominant outcrop region is the Southern Ocean, where the loss to the atmosphere of biological pump CO₂ is large enough to cancel the gain of CO₂ due to cooling. The influence of the biological pump on uptake of anthropogenic CO₂ is small: a model including biology takes up 4.9% less than a model without it. Our model does not predict the large southward interhemispheric transport of CO₂ that has been suggested by atmospheric carbon transport constraints.

65. Sabine, C L., Robert M Key, K M Johnson, F J Millero, A Poisson, Jorge L Sarmiento, D W R Wallace, and C D Winn, 1999: Anthropogenic CO₂ inventory of the Indian Ocean. Global Biogeochemical Cycles, 13(1), 179-198.
    [ Abstract PDF ]

    This study presents basin-wide anthropogenic CO₂ inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified d C* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO₂ and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO₂ column inventories are observed south of 50°S. The total anthropogenic CO₂ inventory north of 35°S was 13.6 ± 2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1 ± 1 Pg C for the same area. Approximately 6.7 ± 1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ± 3 Pg C for 1995. These estimates are compared to anthropogenic CO₂ inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61-0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.

66. Sarmiento, Jorge L., and T M C Hughes, 1999: Anthropogenic CO2 uptake in a warming ocean. Tellus B, 51B(2), 560-561.
    [ PDF ]
67. Fan, Song-Miao, M Gloor, Jerry D Mahlman, S W Pacala, Jorge L Sarmiento, T Takahashi, and P P Tans, 1998: A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science, 282(5388), 442-446.
    [ Abstract PDF ]

    Atmospheric carbon dioxide increased at a rate of 2.8 petagrams of carbon per year (Pg C year-1) during 1988 to 1992 (1 Pg = 1015 grams). Given estimates of fossil carbon dioxide emissions, and net oceanic uptake, this implies a global terrestrial uptake of 1.0 to 2.2 Pg C year-1. The spatial distribution of the terrestrial carbon dioxide uptake is estimated by means of the observed spatial patterns of the greatly incresed atmospheric carbon dioxide data set available from 1988 onward, together with two atmospheric transport models, two estimates of the sea-air flux, and an estimate of the spatial distribution of fossil carbon dioxide emissions. North America is the best constrained continent, with a mean uptake of 1.7 ± 0.5 Pg C year-1, mostly south of 51 degrees north. Eurasia-North Africa is relatively weakly constrained, with a mean uptake of 0.1 ± 0.6 Pg C year-1. The rest of the world's land surface is poorly constrained, with a mean source of 0.2 ± 0.9 Pg C year-1.

68. Sarmiento, Jorge L., T M C Hughes, Ronald J Stouffer, and Syukuro Manabe, 1998: Simulated response of the ocean carbon cycle to anthropogenic climate warming. Nature, 393(6682), 245-249.
    [ Abstract PDF ]

    A 1995 report of the Intergovernmental Panel on Climate Change provides a set of illustrative anthropogenic CO2 emission models leading to stabilization of atmospheric CO2 concentrations ranging from 350 to 1,000 p.p.m. (refs 1 - 4). Ocean carbon-cycle models used in calculating these scenarios assume that oceanic circulation and biology remain unchanged through time. Here we examine the importance of this assumption by using a coupled atmosphere-ocean model of global warming for the period 1765 to 2065. We find a large potential modification to the ocean carbon sink in a vast region of the Southern Ocean where increased rainfall leads to surface freshening and increased stratification. The increased stratification reduces the downward flux of carbon and the loss of heat to the atmosphere, both of which decrease the oceanic uptake of anthropogenic CO2 relative to a constant-climate control scenario. Changes in the formation, transport and cycling of biological material may counteract the reduced uptake, but the response of the biological community to the climate change is difficult to predict on present understanding. Our simulation suggests that such physical and biological changes might already be occurring, and that they could substantially affect the ocean carbon sink over the next few decades.

69. Gruber, N, and Jorge L Sarmiento, 1997: Global patterns of marine nitrogen fixation and denitrification. Global Biogeochemical Cycles, 11(2), 235-266.
    [ Abstract PDF ]

    A new quasi-conservative tracer N*, defined as a linear combination of nitrate and phosphate, is proposed to investigate the distribution of nitrogen fixation and denitrification in the world oceans. Spatial patterns of N* are determined in the difference ocean basins using data from the Geochemical Ocean Sections Study (GEOSECS) cruises (1972-1978) and from eight additional cruises in the Atlantic Ocean. N* is low (< -3 µmol kg^-1) in the Arabian Sea and in the eastern tropical North and South Pacific. This distribution is consistent with direct observations of water column denitrification in these oxygen minimum zones. Low N*concentrations in the Bering Sea and near the continental shelves of the east and west coasts of North America also indicate a sink of N* due to benthic denitrification. High concentrations of N* (>2.0 µmol kg-^-1) indicative of prevailing nitrogen fixation are found in the thermocline of the tropical and subtropical North Atlantic and in the Mediterranean. This suggests that on a global scale these basins are acting as sources of fixed nitrogen, while the Indian Ocean and parts of the Pacific Ocean are acting as sinks. Nitrogen fixation is estimated in the North Atlantic Ocean (10° N - 50° N) using the N* distribution along isopycnal surfaces and information about the water age. We calculate a fixation rate of 28 Tg N yr^-1 which is about 3 times larger than the most recent global estimate. Our result is in line, however, with some recent suggestions that pelagic nitrogen fixation may be seriously underestimated. The implied flux of 0.072 mol N m^-2 yr^-1 is sufficient to meet all the nitrogen requirement of the estimated net community production in the mixed layer during summer at the Bermuda Atlantic Time-series Study (BATS) site in the northwestern Sargasso Sea. Extrapolation of our North Atlantic estimate to the global ocean suggests that the present-day budget of nitrogen in the ocean may be in approximate balance.

70. Gruber, N, Jorge L Sarmiento, and T F Stocker, 1996: An improved method for detecting anthropogenic CO2 in the oceans. Global Biogeochemical Cycles, 10(4), 809-837.
    [ Abstract PDF ]

    An improved method has been developed for the separation of the anthropogenic CO₂ from the large natural background variability of dissolved inorganic carbon (C) in the ocean. This technique employs a new quasi-conservative carbon tracer Delta C*, which reflects the uptake of anthropogenic CO₂ and the air-sea disequilibrium when a water parcel loses contact with the atmosphere. The air-sea disequilibrium component can be discriminated from the anthropogenic signal using either information about the water age or the distribution of Delta C* in regions not affected by the anthropogenic transient. This technique has been applied to data from the North Atlantic sampled during the Transient Tracers in the Ocean North Atlantic (TTO NAS) and Tropical Atlantic study (TTO TAS) cruises in 1981-1983. The highest anthropogenic CO₂ concentrations and specific inventories (inventory per square meter) are found in the subtropical convergence zone. In the North Atlantic, anthropogenic CO₂ has already invaded deeply into the interior of the ocean, north of 50°N it has even reached the bottom. Only waters below 3000 m and south of 30°N are not yet affected. We estimate an anthropogenic CO₂ inventory of 20 plus or minus 4 Gt C in the North Atlantic between 10°N and 80°N. The 2.5-dimensional ocean circulation model of Stocker et al. [1994] and the three-dimensional ocean general circulation biogeochemistry model of Sarmiento et al. [1995] predict anthropogenic CO₂ inventories of 18.7 Gt C and 18.4 Gt C, respectively, in good agreement with the observed inventory. Important differences exist on a more regional scale, associated with known deficiencies of the models.

71. Joos, F, M S Bruno, R Fink, U Siegenthaler, T F Stocker, C Le Quéré, and Jorge L Sarmiento, 1996: An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake. Tellus B, 48B, 397-417.
    [ Abstract PDF ]

    Establishing th link between atmospheric CO2 concentration and anthropogenic carbon emissions requires the development of complex carbon cycle models of the primary sinks, the ocean and terrestrial biosphere. Once such models have been developed, the potential exists to use pulse response functions to characterize their behaviour. However, the application of response functions based on a pulse increase in atmospheric CO₂ to characterize oceanic uptake, the conventional technique, does not yield a very accurate result due to nonlinearities in the aquatic carbon chemistry. Here, we propose the useof an ocean mixed-layer pulse response function that characterizes the surface to deep ocean mixing in combination with a separate equation describing air-sea exchange. The use of a mixed-layer pulse response function avoids the problem arising from the nonlinearities of the carbon chemistry and gives therefore more accurate results. The response function is also valid for tracers other than carbon. We found that tracer uptake of the HILDA and Box-Diffusion model can be represented exactly by the new method. For the Princeton 3-D model, we find that the agreement between the complete model and its pulse substitute is better than 4% for the cumulative uptake of anthropogenic carbon for the period 1765 to 2300 applying the IPCC stabilization scenarios S450 and S750 and better than 2% for the simulated inventory and surface concentration of bomb-produced radiocarbon. By contrast, the use of atmospheric response functions gives deviations up to 73% for the cumulative CO₂ uptake as calculated with the Princeton 3-D model. We introduce the use of a decay response function for calculating the potential carbon storage on land as a substitute for terrestrial biosphere models that describe the overturning of assimilated carbon. This, in combination with an equation describing the net primary productivity permits us to exactly characterize simple biosphere models. As the time scales of biospheric overturning are one key aspect to determine the amount of anthropogenic carbon which might be sequestered by the biosphere, we suggest that decay response functions should be used as a simple and standardized measure to compare different models and to improve understanding of their behaviour. We provide analytical formulations for mixed-layer and terrestrial biosphere decay pulse response functions which permit us to easily build a substitute for the "Bern" carbon cycle model (HILDA). Furthermore, mixed-layer response functions for the Box-Diffusion, a 2-D model, and the Princeton 3-D model are given.

72. Sarmiento, Jorge L., and C Le Quéré, 1996: Oceanic carbon dioxide uptake in a model of century-scale global warming. Science, 274(5291), 1346-1350.
    [ Abstract PDF ]

    In a model of ocean-atmosphere interaction that excluded biological proceses, the oceanic uptake of atmospheric carbon dioxide (CO₂) was substantially reduced in scenarios involving global warming relative to control scenarios. The primary reason for the reduced uptake was the weakening or collapse of the ocean thermohaline circulation. Such a large reduction in this ocean uptake would have a major inpact on the future growth rate of atmospheric CO₂. Model simulations that include a simple representation of biological processes show a potentially large offsetting effect resulting from the downward flux of biogenic carbon. However, the magnitude of the offset is difficult to quantify with present knowledge.

73. Anderson, L A., and Jorge L Sarmiento, 1995: Global ocean phosphate and oxygen simulations. Global Biogeochemical Cycles, 9(4), 621-636.
    [ Abstract PDF ]

    We examine the role of dissolved organic matter (DOM) and the stoichiometric ratios of organic matter remineralization in determining the magnitude and distribution of remineralization of organic matter in the oceanic water column, and the impact of this remineralization on tracer distributions. Our aim is to improve the parameterization of relevant processes in ocean general circulation models by bringing the models into closer agreement with new observational constraints that suggest substantial differences from previous work (lower DOM levels and higher -O₂/P ratios). We used phosphate and apparent oxygen utilization (AOU) to analyze the effect of the remineralization profile on water column tracer distributions. The primary impact of DOM cycling is to modify the distribution of remineralization over that obtained from a model with particle cycling only. Changing the oxygen-to-phosphorous stoichiometric ratio modifies the magnitude of oxygen utilization, but preserves its basic distribution. We find that even a small amount of DOM is sufficient to prevent the problem of nutrient trapping. Improved phosphate and AOU simulations are obtained when the amount of DOM is reduced and the deep ocean -O₂/P ratio is increased in accord with recent observations of these properties.

74. Joos, F, and Jorge L Sarmiento, 1995: Der anstieg des atmospharischen kohlendioxids. Physikalische Blatter, 51(5), 405-411.
75. Sarmiento, Jorge L., C Le Quéré, and S W Pacala, 1995: Limiting future atmospheric carbon dioxide. Global Biogeochemical Cycles, 9(1), 121-137.
    [ Abstract PDF ]

    We estimate anthropogenic carbon emissions required to stabilize future atmospheric CO at various levels ranging from 350 ppm to 750 ppm. Over the next three centuries, uptake by the ocean and terrestrial biosphere would permit emissions to be 3 to 6 times greater than the total atmospheric increase, with each of them contributing approximately equal amounts. Owing to the nonlinear dependence of oceanic and terrestrial biospheric uptake on CO concentration, the uptake by these two sinks decreases substantially at higher atmospheric CO levels. The uptake also decreases with increased atmospheric CO growth rate. All the stabilization scenarios require a substantial future reduction in emissions.

76. Shaffer, G, and Jorge L Sarmiento, 1995: Biogeochemical cycling in the global ocean. 1. A new, analytical model with continuous vertical resolution and high-latitude dynamics. Journal of Geophysical Research, 100(C2), 2659-2672.
    [ Abstract PDF ]

    A new, simple analytical model of ocean chemistry is presented which includes continuous vertical resolution, high-latitude dynamics, air-sea exchange and sea ice cover. In this high-latitude exchange/interior diffusion-advection (HILDA) model, ocean physics are represented by four parameters: k and w, an eddy diffusion coefficient and a deep upwelling velocity in the stratified interior; q, a rate of lateral exchange between the interior and a well-mixed, deep polar ocean; and u, an exchange velocity between surface and deep layers in the polar ocean. First, estimates are made of ice-free and ice-covered areas at high latitudes, surface temperatures, and air-sea exchange velocities from available data. Then values of the physical parameters are estimated from simultaneous, least mean square fits of model solutions for temperature (T) and "abiotic" carbon 14 to interior profiles of T and carbon 14 and surface layer carbon 14 values all derived from available data. Best fit values for k, w, q, and u are 3.2x10^-5 m² s^-1, 2.0x10^-8 m s ^-1, 7.5x10^-11 s^-1 and 1.9x10-6 m s^-1 respectively. These results are interpreted in terms of modes of ocean circulation and mixing and compared with results from other simpler and more complex models. In parts 2 and 3 of this series, these values for k, w, q and u are taken as inputs for studying phosphorus, oxygen, and carbon cycling in the global ocean with the HILDA model.

77. Suntharalingam, P, and Jorge L Sarmiento, 1995: Modeling global air-sea N₂O fluxes - A sensitivity analysis of the gas-exchange formulation In Air-Water Gas Transfer, Selected papers from the Third International Symposium on Air-Water Gas Transfer, July 24-27, 1995,, AEON Verlag & Studio, 843-853.
    [ Abstract ]

    Two models of the global oceanic N₂O flux distribution are presented, and the sensitivity of these models to the gas-exchange formulation is examined. The N₂O models discussed are a multi-variate regression method based on surface delta p N₂O measurements from Weiss et al. [1992], and a model of the oceanic N₂O cycle embedded in the ocean general circulation model (OGCM) of Toggweiler et al. [1989]. The gas-exchange parameterizations considered are from Wanninkhof [1992] and Liss and Merlivat [1986]. Results indicate that the formulation of Wanninkhof [1992] may be better suited for modeling oceanic N₂O fluxes using a data-based model or an OGCM.

78. Anderson, L A., and Jorge L Sarmiento, 1994: Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochemical Cycles, 8(1), 65-80.
    [ Abstract PDF ]

    A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of "preformed" concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time-mean, basin-wide Redfield ratios calculated. It is found that the P/N/C_org-O₂ ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitification) are approximately constant with depth and basin, at a value of 1/16 ± 1/117 ± 14/170 ± 10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast-sinking matter produced by sporadic, high-productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 ± 2 between 1000 and 3000 m. In the Indian and Pacific basins the C_org/C_inorg regeneration ratio decreases from approximately 7 ± 3 at 400 m to 3 ± 1 at 1000 m and to 1 ± 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in the Indian and Pacific Oceans.

79. Murnane, R, J K Cochran, and Jorge L Sarmiento, 1994: Estimates of particle- and thorium-cycling rates in the northwest Atlantic Ocean. Journal of Geophysical Research, 99(C2), 3373-3392.
    [ Abstract PDF ]

    We provide least squares estimates of particle-cycling rate constants and their errors at 13 depths in the Northwest Atlantic Ocean using a compilation of published results and conservation equations for thorium and particle-cycling. The predicted rates of particle aggregation and disaggregation vary through the water column. The means and standard deviations, based on lognormal probability distributions, for the lowest and highest rates of aggregation (B2) and disaggregation (B-2) in the water column are 8 ± 27 y-1 < B2 <18 ± 90 y-1, and 580 ± 2000 y-1< B-2<3 X103 ± 104 y-1. Median values for these rates are 2.1 y-1< B2< 3.2 y-1, and 149 y-1< B-2 < 156 y-1. Predicted rate constants for thorium adsorption (k1 = 5.0 ± 1.0x 104 m3 kg y-1) and desorption (k-1 = 3.1 ± 1.5 y-1) are consistent with previous estimates. Least squares estimates of the sum of the time dependence and transport terms from the particle and thorium conservation equations are on the same order as other terms in the conservation equations. Forcing this sum to equal zero would change the predicted rates. Better estimates of the time dependence of thorium activities and particle concentrations and of the concentration and flux of particulate organic matter would help to constrain estimates of B2 and B-2.

80. Fasham, M J., Jorge L Sarmiento, Richard D Slater, H W Ducklow, and R G Williams, 1993: Ecosystem behavior at Bermuda Station "S" and Ocean Weather Station "India": A general circulation model and observational analysis. Global Biogeochemical Cycles, 7(2), 379-415.
    [ Abstract ]

    A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a seven-compartment nitrogen-based ecosystem model with a three-dimensional seasonal general circulation model. The predicted seasonal cycles of phytoplankton, zooplankton, bacteria, nitrate, ammonium, primary production, and particle flux have been compared to data from Bermuda Station "S" and Ocean Weather Station "India". Bearing in mind the simplicity of the model and the paucity of data, the results are encouraging. However, deficiencies in the physical model lead to winter nitrate values at Bermuda being overestimated, and at both positions the predicted magnitude of the spring phytoplankton bloom was too high. Simulations were carried out with different detrital sinking rates and it was found that a sinking rate of 10 m d^-1 gave the best agreement with observations. The model was used to investigate the factors affecting the population growth of phytoplankton and it was found that the model supported the generally held theory that the spring bloom is initiated by the cessation of physical mixing. After the bloom, phytoplankton are controlled by zooplankton grazing. At Ocean Weather Station "India" the model reproduced the observed high summer nitrate levels and suggested that these high values are caused by a combination of high vertical nitrate transport, ammonium inhibition of nitrate uptake, and zooplankton grazing control. The model demonstrated the critical importance of zooplankton in understanding ecosystem dynamics and highlights the need for more observational data on the seasonal cycles of zooplankton biomass and growth rates.

81. Murphy, E, and Jorge L Sarmiento, et al., 1993: Global extrapolation In Towards a Model of Ocean Biogeochemical Processes, NATO Series I, Vol. 10, Berlin, Germany, Springer-Verlag, 21-46.
82. Sarmiento, Jorge L., 1993: Atmospheric CO₂ stalled. Nature, 365, 697-698.
    [ PDF ]
83. Sarmiento, Jorge L., Richard D Slater, M J Fasham, H W Ducklow, J Robert Toggweiler, and G T Evans, 1993: A seasonal three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone. Global Biogeochemical Cycles, 7(2), 417-450.
    [ Abstract ]

    A seven-component upper ocean ecosystem model of nitrogen cycling calibrated with observations at Bermuda Station "S" has been coupled to a three-dimensional seasonal general circulation model (GCM) of the North Atlantic Ocean. The aim of this project is to improve our understanding of the role of upper ocean biological processes in controlling surface chemical distributions, and to develop approaches for assimilating large data sets relevant to this problem. A comparison of model predicted chlorophyll with satellite coastal zone color scanner observations shows that the ecosystem model is capable of responding realistically to a variety of physical forcing environments. Most of the discrepancies identified are due to problems with the GCM model. The new production predicted by the model is equivalent to 2 to 2.8 mol m^-2 yr^-1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern half of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m-3). Despite this, almost the entire model is nitrate limited in the sense that any addition of nitrate supply would go predominately into photosynthesis. The only exceptions are some coastal upwelling regions and the high latitudes during winter, where nitrate goes as high as ~10 mmol m^-3 .

84. Siegenthaler, U, and Jorge L Sarmiento, 1993: Atmospheric carbon dioxide and the ocean. Nature, 365(6442), 119-125.
    [ Abstract PDF ]

    The ocean is a significant sink for anthropogenic carbon dioxide, taking up about a third of the emissions arising from fossil-fuel use and tropical deforestation. Increases in the atmospheric carbon dioxide concentration account for most of the remaining emissions, but there still appears to be a 'missing sink' which may be located in the terrestrial biosphere. Atmospheric CO₂ measurements made from ice cores recovered ar Siple and Amundsen-Scott Stations were used in the preparation of this review.

85. Slater, Richard D., Jorge L Sarmiento, and M J Fasham, 1993: Some parametric and structural simulations with a three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone In Towards a Model of Ocean Biogeochemical Processes, NATO Series I, Vol. 10, Berlin, Germany, Springer-Verlag, 261-294.
86. Najjar, R G., Jorge L Sarmiento, and J Robert Toggweiler, 1992: Downward transport and fate of organic matter in the ocean: Simulations with a general circulation model. Global Biogeochemical Cycles, 6(1), 45-76.
    [ Abstract ]

    A phosphorous-based model of nutrient cycling has been developed and used in conjunction with a general circulation model to evaluate the roles of the dissolved and sinking particulate phases in the downward transport of organic matter in the ocean. If sinking particles dominate the downward transport and remineralize in accord with observations made primarily with sediment traps, we find in equatorial upwelling regions that particle fluxes and thermocline nutrient concentrations are higher than observed. These enhanced fluxes and concentrations are a result of what we term "nutrient trapping," a positive feedback whereby high upwelling produces high new production that results in remineralization and enhanced nutrient concentrations in the upwelling water, which further increases new production. Nutrient trapping in shallow upwelling zones can be eliminated by increasing the particle flux length scale, which suggests that if sinking particles dominate the downward transport of organic matter then the flux length scale is longer than observed. Even with a longer particle flux length scale, we find that nutrients are trapped in some deep convective regions of the southern ocean, where new production is predicted to be much higher than the observed primary production. In simulations where the downward transport of organic matter takes place primarily in a dissolved phase, nutrient trapping is completely eliminated, in both upwelling and convective regions. The models with dissolved organic matter also agree fairly well with nutrient transports in the north Atlantic Ocean calculated from observed nutrient and hydrographic data (Rintoul and Wunsch, 1991). Our results therefore support the dissolved organic nitrogen and carbon measurements made with the high-temperature combustion technique of Suzuki, et al. (1985) and Sugimura and Suzuki (1988) and suggest that there exists an as-yet undiscovered pool of dissolved organic phosphorous in the ocean. We also use the various models to make an estimate of global new production of 2.9 to 3.6 mol C/m²/yr (12 to 15 Gt C/yr).

87. Sarmiento, Jorge L., 1992: Biogeochemical ocean models In Climate System Modeling, Cambridge University Press, 519-551.
88. Sarmiento, Jorge L., James C Orr, and U Siegenthaler, 1992: A perturbation simulation of CO₂ uptake in an ocean general circulation model. Journal of Geophysical Research, 97(C3), 3621-3645.
    [ Abstract ]

    The uptake of anthropogenic CO₂ by the ocean is simulated using a perturbation approach in a three-dimensional global general circulation model. Atmospheric pCO₂ is prescribed for the period 1750-1990 using the combined Siple ice core and Mauna Loa records. For the period 1980 to 1989, the average flux of CO₂ into the ocean is 1.9 GtC/yr. However, the bomb radiocarbon simulation of Toggweiler, et al. (1989b) shows that the surface to deep ocean exchange in this model is too sluggish. Hence the CO₂ uptake calculated by the model is probably below the actual value. The observed atmospheric increase in 1980 to 1989 is 3.2 GtC/yr., for a combined atmosphere-ocean total of 5.1 GtC/yr. This is comparable to the estimated fossil CO₂ production of 5.4 GtC/yr., implying that other souces and sinks (such as from deforestation, enhanced growth of land biota, and changes in the ocean carbon cycle) must be approximately in balance. The sensitivity of the uptake to the gas exchange rate is small: a 100% increase in gas exchange rate gives only a 9.2% increase in cumulative oceanic uptake. Details of the penetration into different oceanic regions are discussed.

89. Sarmiento, Jorge L., and E T Sundquist, 1992: Revised budget for the oceanic uptake of anthropogenic carbon dioxide. Nature, 356(6370), 589-593.
    [ Abstract PDF ]

    Tracer-calibrated models of the total uptake of anthropogenic CO₂ by the world's oceans give estimates of about 2 gigatones carbon per year, significantly larger than a recent estimate of 0.3-0.8 Gt C yr^-1 for the synoptic air-to-sea CO₂ influx. Although both estimates require that the global CO₂ budget must be balanced by a large unknown terrestrial sink, the latter estimate implies a much larger terrestrial sink, and challenges the ocean model calculations on which previous CO₂ budgets were based. The discrepancy is due in part to the net flux of carbon to the ocean by rivers and rain, which must be added to the synoptic air-to-sea CO₂flux to obtain the total oceanic uptake of anthropogenic CO₂. Here we estimate the magnitude of this correction and of several other recently proposed adjustments to the synoptic air-sea CO₂ exchange. These combined adjustments minimize the apparent inconsistency, and restore estimates of the terrestrial sink to values implied by the modelled oceanic uptake.

90. Joos, F, Jorge L Sarmiento, and U Siegenthaler, 1991: Estimates of the effect of Southern Ocean iron fertilization on atmospheric CO₂ concentrations. Nature, 349(6312), 772-774.
    [ Abstract PDF ]

    It has been suggested that fertilizing the ocean with iron might offset the continuing increase in atmospheric CO₂ by enhancing the biological uptake of carbon, thereby decreasing the surface-ocean partial pressure of CO₂ and drawing down CO₂ from the atmosphere. Using a box model, we present estimates of the maximum possible effect of iron fertilization, assuming that iron is continuously added to the phosphate-rich waters of the Southern Ocean, which corresponds to 16% of the world ocean surface. We find that after 100 years of fertilization, the atmospheric CO₂ concentration would be 59 p.p.m. below what it would have been with no fertilization, assuming no anthropogenic CO₂ emissions, and 90-107 p.p.m. less when anthropogenic emissions are included in the calculation. Such a large uptake of CO₂ is unllikely to be achieved in practice, owing to a variety of constraints that require further study; the effect of iron fertilization on the ecology of the Southern Ocean also remains to be evaluated. Thus, the most effective and reliable strategy for reducing future increases in atmospheric CO₂ continues to be control of anthropogenic emissions.

91. Joos, F, U Siegenthaler, and Jorge L Sarmiento, 1991: Possible effects of iron fertilization in the southern ocean on atmospheric CO₂ concentration. Global Biogeochemical Cycles, 5(2), 135-150.
    [ Abstract PDF ]

    Recently, it was proposed (Baum, 1990 and Martin, et al., 1990a, 1990b) that the southern ocean should be fertilized with iron to stimulate biological productivity, thus enhancing the flux of organic carbon from surface to depth, thereby lowering the concentration of inorganic carbon in surface water and in turn the atmospheric CO₂ concentration. We explore the possible impact of a hypothetical iron fertilization on atmospheric CO₂ levels during the next century using a high-latitude exchange/interior diffusion advection model. Assuming as an upper-limit scenario that it is possible to stimulate the uptake of the abundant nutrients in the southern ocean, the maximum atmospheric CO₂ depletion is 58 ppm after 50 years and 107 ppm after 100 years. This scenario requires completely effective Fe fertilization to be carried out over 16% of the world ocean area. Sensitivity studies and comparison with other models suggest that the errors in these limits due to uncertainties in the transport parameters, which are determined by calibrating the model with radiocarbon and validated with CFC-11 measurements, range from -29% to +17%. If iron-stimulated biological productivity is halted during the six winter months, the additional oceanic CO₂ uptake is reduced by 18%. Possible changes in surface water alkalinity alter the result of iron fertilization by less than +9% to -28%. Burial of the iron-induced particle flux as opposed to remineralization in the deep ocean has virtually no influence on the atmospheric response for the considered time scale of 100 years. If iron fertilization were terminated, CO₂ would escape from the ocean and soon cancel the effect of the fertilization. The factors which determine the atmospheric CO₂ reduction most strongly are the area of fertilization, the extent to which biology utilizes the abundant nutrients, and the magnitude of future CO₂ emissions. The possible effect of fertilizing the ocean with iron is small compared to the expected atmospheric CO₂ increase over the next century, unless the increase is kept small by means of stringent measures to control CO₂ emissions.

92. Nuttle, W K., J S Wroblewski, and Jorge L Sarmiento, 1991: Advances in modeling ocean primary production and its role in the global carbon cycle In Global Change and Relevant Space Observations, Oxford, UK, Pergamon Press, Inc., 67-76.
    [ Abstract ]

    The oceans contain a large fraction of the carbon in the Earth's biosphere. Therefore, understanding the global carbon cycle, particularly the changes in atmospheric CO₂ and their effects on climate, requires an accounting of CO₂ exchanges between the atmosphere and the ocean. Primary production in the ocean, i.e., uptake and assimilation of CO₂ by phytoplankton, plays an important role in this exchange. Ocean production is linked to nutrient cycles, mixing and circulation on a number of scales. Several university research groups are using Coastal Zone Color Scanner imagery to study ocean production and the links between physical and biological oceanographic processes and the carbon cycle. We review their recent accomplishments.

93. Sarmiento, Jorge L., 1991: Oceanic uptake of anthropogenic CO₂: The major uncertainties. Global Biogeochemical Cycles, 5(4), 309-313.
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94. Sarmiento, Jorge L., 1991: Slowing the buildup of fossil CO₂ in the atmosphere by iron fertilization: A comment. Global Biogeochemical Cycles, 5(1), 1-2.
    [ PDF ]
95. Sarmiento, Jorge L., and James C Orr, 1991: Three-dimensional simulations of the impact of the southern ocean nutrients depletion on atmospheric CO₂ and ocean chemistry. Limnology and Oceanography, 36(8), 1928-1950.
    [ Abstract PDF ]

    Surface nutrient concentrations in the Southern Ocean are an important indicator of the atmosphere-ocean chemical balance that played a key role in ice-age reduction of atmospheric pCO₂ and would play a role in any Fe fertilization scenario for increasing oceanic uptake of anthropogenic CO₂. The response of the ocean and atmosphere to a scenario of extreme depletion of Southern Ocean surface nutrients by an increase in the organic matter flux to the deep ocean is examined with a three-dimensional model of ocean circulation coupled to a one-box model of the atmosphere. After 100 years, the increase in the organic matter flux is 6-30 Gt C yr^-1 -about twice the global new production determined by the same model for the present ocean. The removal of nutrients from surface waters of the Southern Ocean reduces the nutrient content of the near-surface and intermediate depth waters of the entire ocean, resulting in a 0.5-1.9 Gt C yr^-1 reduction of low-latitude new production. The deep circumpolar waters, enriched in nutrients by regeneration of organic matter, spread into the deep and bottom waters of the remainder of the ocean, giving an overall downward shift of nutrients from surface and intermediate to circumpolar and deep waters. The oceanic total C distribution is also shifted downward, resulting in uptake of atmospheric CO₂ of 46-85 ppm (98-181 Gt C) in the first 100 yr. The oxygen content shifts upward in the water column, approximately mirroring the downward shift of nutrients. Some of the oxygen shifted to the upper ocean escapes to the atmosphere. As a consequence, the global average oceanic content of oxygen, presently 168 µmol kg^-1, is reduced by 6-20 µmol kg^-1, with anoxia developing in the southwestern Indian Ocean.

96. Murnane, R, Jorge L Sarmiento, and M P Bacon, 1990: Thorium isotopes, particle cycling models, and inverse calculations of model rate constants. Journal of Geophysical Research, 95(C9), 16,195-16,206.
    [ Abstract PDF ]

    Generalized models of thorium and particle cycling, data from Station P, and an inversion technique are used to obtain rate estimates of important biological and chemical transformations occuring in the water column. We first verify the inversion technique using an idealized data set generated by a finite difference model, and then apply the inversion technique to data from Station P. With the Station P data, predicted rate constants for adsorption and release of thorium between the dissolved and small particle phases are consistent with the results from other workers. The predicted rate constants for the interaction between small and large particles are smaller than previous estimates. The predicted concentration of large rapidly sinking particles is greater than the concentration of suspended non-sinking particles is 20 m d-1. This sinking rate is an order of magnitude smaller than the large particle sinking rate inferred from sediment trap mass fluxes at two levels in the water column. The reason we predict a high large particle concentration and slow settling velocity has not been uniquely determined. Possible modifications of the current model that could help to reconcile the differences between observations and model predictions include: 1) two classes of rapidly sinking particles or rate constants that change with depth, 2) direct interactions between the large particle and dissolved phases, and 3) incorporation of a continuous distribution of particle size and settling velocity.

97. Sarmiento, Jorge L., G Thiele, Robert M Key, and W S Moore, 1990: Oxygen and nitrate new production and remineralizaion in the North Atlantic subtropical gyre. Journal of Geophysical Research, 95(C10), 18,303-18,315.
    [ Abstract PDF ]

    New estimates are obtained of oxygen utilization rates on isopycnal surfaces in the North Atlantic subtropical gyre thermocline based on tritium inventories (2.4-3.5 mol m^-2yr^-1) and ²²⁸Ra measurements (8.5 ± 0.8 mol m^-2yr^-1). Arguments are given for why the tritium inventory oxygen utilization rate estimate may be too low. The ²²⁸Ra results are combined with recent estimates of oxygen utlization within the thermocline (Jenkins, 1987) as well as estimates of oxygen production in the mixed layer (Spitzer and Jenkins, 1989; Musgrave et al., 1988), to suggest a tentative overall oxygen balance for the whole water column. The new production of oxygen in the surface ocean (~4.6 ± 1.6 mol m^-2yr^-1) appears to be lower than the estimated utilization within the thermocline (~8.5 ± 0.8 mol m^-2yr^-1), suggesting that there may be a net lateral import of organic matter into the thermocline equivalent to a new production of ~3.9 ± 1.8 mol m^-2yr^-1. The nitrogen balance is consistent with these results. An estimate for the total nitrogen remineralization rate in the thermocline is obtained from the oxygen utilization rate by using an -O₂:N Redfield ratio of 9.1 ± 0.4 for remineralization (Minster and Boulahdid, 1987), giving a nitrogen remineralization rate of ~0.93 ± 0.10 mol m^-2yr^-1. Subtracting off the estimated lateral export of nitrate of ~0.51 ± 0.21 mol m^-2yr^-1, which is presumed to be balanced by a lateral import of dissolved organic nitrogen (Rintoul and Wunsch, 1990), gives a nitrate flux into the surface of ~0.42 ± 0.23 mol m^-2yr^-1, which is comparable to the estimate of 0.6 ± 0.2 mol m^-2yr^-1 obtained by Jenkins (1988) near Bermuda as well as the 100-m particulate nitrogen flux of 0.33 mol m^-2yr^-1 obtained Altabet (1989) near Bermuda.

98. Thiele, G, and Jorge L Sarmiento, 1990: Tracer dating and ocean ventilation. Journal of Geophysical Research, 95(C6), 9377-9391.
    [ Abstract PDF ]

    The interpretation of transient tracer observations depends on difficult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions. Recent studies have attempted to circumvent this problem by making use of a derived quantity, age, based on the simultaneous distribution of two complementary tracers, such as tritium and its daughter, helium 3. The age is defined with reference to the surface such that the boundary condition takes on a constant value of zero. We use a two-dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms. An interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age. We define an idealized "ventilation age tracer" that is conservative with respect to mixing, and we explore how its behavior compares with that of the tracer-based ages over a range of advective and diffusive parameters.

99. Sarmiento, Jorge L., M J Fasham, U Siegenthaler, R G Najjar, and J Robert Toggweiler, 1989: In Models of Chemical Cycling in the Oceans: Progress Report II, Ocean Tracers Laboratory Report #6, Princeton, NJ, Princeton University, 46 pp.
    [ Abstract ]

    In a previous progress report (Toggweiler, et al., 1987) we argued that the most difficult obstacle that needed to be overcome in developing predictive, dynamical, 3-D models of geochemical cycling in the oceans was to develop approaches for simulating the role of biological processes. In this report we update our progress on developing an ecosystem-level description of upper ocean fluxes and on simulating the penetration of anthropogenic CO₂ into the ocean. if the natural carbon cycle and ocean circulation are in steady state, one needs to know only the pre-anthropogenic surface total carbon and alkalinity to predict the uptake of fossil CO₂ by the oceans. As a first simple approximation, we fix the surface alkalinity at a constant value of 2300 ueq kg^-1, and fix the pre-anthropogenic surface total carbon to the value that gives the pre-anthropogenic pCO₂ of 280 ppm everywhere. This neglects details of the natural cycles of CO₂ due to temperature as well as biology that give rise to non-equilibrium pre-anthropogenic pCO₂ levels over much of the ocean. Several fossil CO₂ uptake experiments have been performed with this approach, both with 3-D ocean circulation models and with a new box model that incorporates features not included in previous box models. Another approach we are working on is a determination of the pre-anthropogenic surface total carbon and alkalinity based on the observed surface nutrient distributions and an assumed Redfield stochiometry. This approach gives us the concentration of pre-anthropogenic total carbon and alkalinity that we need for the steady state simulations of fossil CO₂ uptake discussed above. It also provides a simple way of simulating the effects of biology and temperature in the euphotic region of the ocean, allowing us to put major emphasis on processes occurring below the euphotic zone. Our simulations of processes below the euphotic zone suggest an important role for substances not caught in sediment traps, such as dissolved organic matter. We have made considerable progress on the development of ecosystem models of the upper ocean and have performed a first experiment incorporating these models into a 3-D ocean circulation model of the North Atlantic. Such models are necessary for predicting how the atmospheric pCO₂ will be affected should the ocean circulation and biology begin to change in response to a greenhouse climate.

100. Sarmiento, Jorge L., 1988: In A Chemical Tracer Strategy for WOCE: Report of a Workshop Held in Seattle, Washington, U.S. WOCE Planning Report Number 10, 181 pp.
101. Sarmiento, Jorge L., T D Herbert, and J Robert Toggweiler, 1988: Causes of anoxia in the world ocean. Global Biogeochemical Cycles, 2(2), 115-128.
     [ Abstract PDF ]

     We examine the hypothesis that global scale episodes of anoxia such as occurred in the Cretaceous are due to high productivity and/or stagnation of the circulation. Two modes of ocean circulation are considered: a thermohaline overturning cell, essentially vertical, which involves global scale upwelling into the surface followed by sinking in deep water formation regions; and an approximately horizontal cell which connects the abyss directly with deeply convecting waters in deep water formation regions. Modern analogs for these processes are formation of North Atlantic Deep Water and Antarctic Bottom Water, respectively. Over most of the oceans the surface new production is nutrient limited and thus directly proportional to the supply of nutrients by the vertical overturning cell. A reduction in oxygen can only be brought about by increased vertical overturning associated with increased production. In addition, the model shows that as the deep ocean becomes lower in oxygen, the sensitivity of the oxygen levels to the meridional circulation decreases such that it becomes difficult or impossible to achieve.

102. Sarmiento, Jorge L., T D Herbert, and J Robert Toggweiler, 1988: Mediterranean nutrient balance and episodes of anoxia. Global Biogeochemical Cycles, 2(4), 427-444.
     [ Abstract PDF ]

     We examine the causes of anoxia in regions such as the Eastern Mediterranean, which have exchange over sills with adjacent basins. Box models show that the concentration of the limiting nutrient is the major determinant of deep oxygen levels. The most effective way of increasing nutrient concentrations to the point where anoxia occurs is to change the flow pattern across the sills ventilating the basins. With a sill exchange pattern such as that in the present Strait of Sicily, it is difficult to obtain anoxia in the Eastern Mediterranean without also driving the Western Mediterranean to low oxygen and high nutrient levels. Episodes of anoxia in the Eastern Mediterranean are associated with a freshening of surface waters. A reversal in flow directions, presumably resulting from the observed freshening, will inevitably lead to anoxia asociated with increased sediment burial rates of the limiting nutrient and will leave the Western Mediterranean largely unaffected, in keeping with the observational evidence.

103. Sarmiento, Jorge L., J Robert Toggweiler, and R G Najjar, 1988: Ocean carbon-cycle dynamics and atmospheric p_CO2. Philosophical Transactions of the Royal Society of London, A, 325, 3-21.
     [ Abstract ]

     Mechanisms are identified whereby processes internal to the oceans can give rise to rapid changes in atmospheric p_CO2. One such mechanism involves exchange between the atmosphere and deep ocean through the high-latitude outcrop regions of the deep waters. The effectiveness of communication between the atmosphere and deep ocean is determined by the rate of exchange between the surface and deep ocean against the rate of biological uptake of the excess carbon brought up from the abyss by this exchange. Changes in the relative magnitude of these two processes can lead to atmospheric p_CO2 values ranging between 165 p.p.m. (by volume) and 425 p.p.m. compared with a pre-industrial value of 280 p.p.m. Another such mechanism involves the separation between regeneration of alkalinity and total carbon that occurs in the oceans because of the fact that organic carbon is regenerated primarily in the upper ocean whereas CaCO₃ is dissolved primarily in the deep ocean. The extent of separation depends on the rate of CaCO₃ formation at the surface against the rate of upward mixing of deep waters. This mechanism can lead to atmospheric values in excess of 20000 p.p.m., although values greater than 1100 p.p.m. are unlikely because calcareous organisms would have difficulty surviving in the undersaturated surface waters that develop at this point. A three-dimensional model that is being developed to further study these and other problems provides illustrations of them and also suggests the possibility that there is a long-lived form of non-sinking carbon playing a major role in carbon cycling.

104. Sarmiento, Jorge L., 1987: Tracers and modeling. Reviews of Geophysics, 25(6), 1417-1419.
     [ PDF ]
105. Kawase, M, and Jorge L Sarmiento, 1986: Circulation and nutrients in middepth Atlantic waters. Journal of Geophysical Research, 91(C8), 9749-9770.
     [ Abstract PDF ]

     Isopycnal analyses of distributions of salinity, oxygen, apparent oxygen utilization, nitrate, and silica in the depth range 800-3000 m in th North and Tropical Atlantic Ocean are carried out using data from the Transient Tracers in the Oceans, Metero cruise 56 leg 5, Atlantis II cruise 109 legs 1 and 3, and the GEOSECS Atlantic Study. The results are summarized in isopycnal distribution maps and property-property plots. The layer between 800 and 1500 m is oxygen poor and nutrient rich and is poorly ventilated. Below this is a better aerated layer with the deep western boundary current, part of which separates along the equator. Regeneration of nutrients and consumption of oxygen persist into the deep ocean in regions off the west coast of Africa. The Mediterranean Outflow Water apparently is causing significant cross-isopycnal mixing through salt fingering. The resultant cross-isopycnal velocity may be large enough to cause significant vorticity stretching.

106. Moore, W S., Jorge L Sarmiento, and Robert M Key, 1986: Tracing the Amazon component of surface Atlantic water using ²²⁸Ra, salinity and silica. Journal of Geophysical Research, 91(C2), 2574-2580.
     [ Abstract PDF ]

     High ²²⁸Ra/²²⁶Ra activity ratios characteristic of waters in the Amazon estuary provide a sensitive indicator of the presence of these waters in the Atlantic Ocean. A conservative mixing model utilizing the ²²⁸Ra/²²⁶activity ratio (AR) tied to absolute measurements in the estuary allows us to estimate that 20-34% of the surface water east of the Antilles during June, 5-9% from the same area during December, and 15-20% of the eastern Caribbean surface water during December are derived from the Amazon estuary. Differences in ²²⁸Ra input occur in response to variable stratification of water near the river mouth. During high discharge, intense vertical mixing enriches the water in the estuary in ²²⁸Ra. A large fraction of this water moves to the north and east of Antilles, where its relatively high ²²⁸Ra/²²⁶ AR distinguishes it over 1500 km from its source. During low discharge (northern hemisphere fall) a significant fraction of river water passes northwest of the zone of intense mixing into a vertically stratified region where ²²⁸Ra gain is lower. This water is transported by the Guiana Current along the coast of South America and into the Caribbean.

107. Olson, D B., G H Ostlund, and Jorge L Sarmiento, 1986: Western boundary undercurrent off the Bahamas. Journal of Physical Oceanography, 16(2), 233-240.
     [ Abstract PDF ]

     Two tritium sections through the deep western boundary current east of the Bahamas, taken in late 1980 and early 1981, are presented. Tritium from the bomb tests in the late 1950s and early 1960s is used to identify recently formed deep waters in the sections. High concentrations are found in the North Atlantic deep water. Low tritium values occur in the Labrador Sea water found above the core of this deep water. This is consistent with the suggestion by Talley and McCartney that this water mass has not been ventilated at the temperatures observed in these sections since the mid-1950s. Tritium in the sections is correlated with maxima in potential vorticity. This is inconsistent with deep convection as a direct source for the water mass. The potential vorticity maxima may be associated with plume dynamics near the overflow regions or with the dynamics of the deep western boundary current. The sections are south of the section discussed by Jenkins and Rhines, where high tritium concentrations were found along the topography on the Blake-Bahama Outer Ridge between 3.5- and 4.5-km depth in late 1977. In the sections farther south, a similar maximum is found, but it is at a 0.6 degrees C warmer potential temperature and separated from the topography. Tritium is found at the temperature it appears in the Jenkins and Rhines section. In contrast to their concentrated feature, the tritium in the later sections is spread out into a layer that extends into the ocean interior to the limit of the sections in these temperature ranges. This, coupled with dynamic height fields, suggests that the boundary current feeds an offshore flow into the ocean interior east of the Bahamas. The change in the temperature where the tritium maximum is found implies variations in the formation and spread of North Atlantic deep water on fairly short time scales.

108. Sarmiento, Jorge L., 1986: Modeling oceanic transport of dissolved constituents In The Role of Air-Sea Exchange in Geochemical Cycling, Amsterdam, The Netherlands, Reidel Publishing Co, 65-82.
109. Sarmiento, Jorge L., 1986: On the north and tropical Atlantic heat balance. Journal of Geophysical Research, 91(C10), 11,677-11,689.
     [ Abstract ]

     The heat balance deduced from a three-dimensional, seasonally driven primitive equation model of the North Atlantic is described and compared with observations. The greatest response to the seasonal forcing occurs in the region between ~5 degrees N and 10 degrees N, where the northward heat transport goes from a minimum of <0 in summer, when the North Equatorial Countercurrent dominates the surface flow and the Brazil Current is at a minimum, to a maximum of >1.4 x 10¹⁵ W in the winter, when the Brazil Current and northward Ekman transport are at a maximum. Elsewhere in the tropics and subtropics the range in transport is smaller. In the northern hemisphere (~12 degrees to 32 degrees N), there is a signficant semiannual component due to Ekman transport. The large seasonal changes in heat storage in the tropics are caused primarily by transport divergence rather than surface heat flux. In the annual mean, the equatorial region has a large surface heat flux gain associated primarily with the conversion of ~6 x 10⁶ m³ s^-1 of northward geostrophic flow with theta < 20 degrees C in the southern hemisphere, to a surface Ekman flow with theta ~ 27 degrees C in the northern hemisphere. The seasonal variation in heat storage within the subtropical and subpolar gyres is due almost entirely to the surface heat flux. However, seasonal variations in Ekman transport do lead to a relatively large annual cycle in northward heat transport (e.g., 0.5 x 10¹⁵ W to 0.9 x 1015 W at ~35 degrees N), with maximum transport occuring during the summer when the southward Ekman transport is at a minimum. A comparison of the model results with heat storage estimates shows that the model and data agree very well in the tropics but that at higher latitudes the model underestimates the seasonal variations because of inadequate vertical penetration of heating during periods of warming.

110. Sarmiento, Jorge L., 1986: Three-dimensional ocean models for predicting the distribution of three-dimensional ocean models for predicting the distribution of CO₂ between the ocean and atmosphere In Changing Carbon Cycle: A Global Analysis, Springer-Verlag, 279-294.
111. Sarmiento, Jorge L., and P E Biscaye, 1986: Radon 222 in the benthic boundary layer. Journal of Geophysical Research, 91(C1), 833-844.
     [ Abstract ]

     A detailed survey of radon 222 and temperature profiles of the benthic boundary layer in the Hatteras Abyssal Plain shows a strong correlation between the structure of both. The apparent vertical diffusivities estimated from radon 222 are of the order of 50 cm² s^-1 in the mixed layer, and of the order of 1 cm-² s^-1 above it. One profile appears to have been taken in a frontal zone where isotherms dip sharply into the sediments. This is the only profile where there is significant penetration of radon above the mixed layer. Several other profiles suggest that the temperature may take longer than several radon half-lives to adjust to new mixing and advection regimes. In such cases, one often sees considerable structure in the radon profile within the region where the potential temperature is well mixed.

112. Sarmiento, Jorge L., and E Gwinn, 1986: Strontium 90 fallout prediction. Journal of Geophysical Research, 91(C6), 7631-7646.
113. Sarmiento, Jorge L., and J Robert Toggweiler, 1986: A preliminary model of the role of upper ocean chemical dynamics in determining oceanic oxygen and atmospheric carbon dioxide levels In Dynamic Processes in the Chemistry of the Upper Ocean, Plenum Press, 233-240.
     [ Abstract ]

     A first version is presented of equations for a three-dimensional model of nutrient and carbon cycling in the oceans. An analytical solution of these equations has been obtained for a one-and-a-half-dimensional "pipe" model. This solution shows that atmospheric CO₂ can be varied by changing the level of preformed nutrients. It is suggested that this mechanism may explain the lower p_CO values of the last ice age.

114. Brewer, P G., Jorge L Sarmiento, and W M Smethie, Jr, 1985: Transient Tracers in the Ocean (TTO) Program: The North Atlantic Study, 1981; The Tropical Atlantic Study, 1983. Journal of Geophysical Research, 90(C4), 6903-6905.
     [ PDF ]
115. Bryan, Kirk, and Jorge L Sarmiento, 1985: Modeling ocean circulation. Advances in Geophysics, 28A, 433-459.
116. Kawase, M, and Jorge L Sarmiento, 1985: Nutrients in the Atlantic thermocline. Journal of Geophysical Research, 90(C5), 8961-8979.
     [ Abstract PDF ]

     A set of maps are presented of nutrient distribution on isopycnal surfaces in the North and tropical Atlantic Ocean main thermocline. The data used in producing these maps are from the Transient Tracers in the Ocean (TTO) North Atlantic Study and Tropical Atlantic Study, an associated German study (Meteor 56/5), two cross-Atlantic sections from cruise 109 of the Atlantis II, and the GEOSECS program. The nutrient distributions reflect primarily the sources at the northern and southern outcrops of the isopycnal surfaces, the in situ regeneration due to decomposition of sinking organic materials, and the interior physical processes as inferred from thermocline models and the distribution of conservative properties such as salinity. However, silica also exhibits behavior that cannot be explained by in situ regeneration. A simple phenomenological model suggests that cross-isopycnal advection and mixing in the equatorial region may play an important role in the nutrient dynamics. These data should prove of great value in constraining models of physical as well as biogeochemical processes.

117. Key, Robert M., R F Stallard, W S Moore, and Jorge L Sarmiento, 1985: Distribution and flux of ²²⁶Ra and ²²⁸Ra in the Amazon River estuary. Journal of Geophysical Research, 90(C4), 6995-7004.
     [ Abstract PDF ]

     Measurements of ²²⁶Ra and ²²⁸Ra in the Amazon River estuary show that desorption from river-borne suspended particulate matter in the estuary increases the riverine flux of both isotopes to the ocean by a factor of approximately 5 over the flux attributable to radium dissolved in the river water alone. The total Amazon flux supplies approximately 0.20% of the ²²⁶Ra and approximately 2.6% of the ²²⁸Ra standing crops in the near-surface Atlantic (0-200 m). Diffusive flux from estuarine and shelf sediments and desorption from resuspended sediments in the region of the estuary approximately double the estuarine ²²⁶Ra concentration and quadruple the estuarine ²²⁸Ra concentration above that caused by the dissolved and desorbed river components alone.

118. Moore, W S., Robert M Key, and Jorge L Sarmiento, 1985: Techniques for precise mapping of ²²⁶Ra and ²²⁸Ra in the Ocean. Journal of Geophysical Research, 90(C4), 6983-6994.
     [ Abstract ]

     Improvements in the analyses of ²²⁶Ra and ²²⁸Ra in seawater made possible by better extraction and processing techniques reduce significantly the errors associated with these measurements. These improvements and the extensive sampling for Ra isotopes conducted on the TTO North Atlantic Study should enable us to use the distribution of ²²⁸Ra to study mixing processes on a 3-15 year time scale in both the upper and deep North Atlantic. The ²²⁸Ra profiles already analyzed show a closer resemblance to GEOSECS tritium data than to TTO tritium data in the upper ocean. This is because the transient tracer tritium was responding on a 10-year time scale during GEOSECS and a 20-year time scale during TTO. The steady state tracer ²²⁸Ra should always respond on a time scale of 8 years. Thus the ²²⁸Ra data obtained on TTO should provide a means to extend the features of the GEOSECS tritium field to the regions of the TTO study. The ²²⁶Ra data are of high enough quality to identify features associated with different water masses. Changes in the positions of the deep-water masses since the GEOSECS cruise are revealed by the ²²⁶Ra data

119. Toggweiler, J R., and Jorge L Sarmiento, 1985: Glacial to interglacial changes in atmospheric carbon dioxide: The critical role of ocean surface water in high latitudes In The Carbon Cycle and Atmospheric CO₂: Natural Variations Archean to Present, Geophysical Monograph 32, Washington, DC, American Geophysical Union, 163-184.
     [ Abstract PDF ]

     Recent measurements of the CO₂ content of air bubbles trapped in glacial ice have shown that the partial pressure of atmospheric COsub>2 during the last ice age was baout 70 ppm lower than during the interglacial. Isotopic measurements on surface- and bottom-dwelling forams living during the ice age have shown that the ¹³C/¹²C gradient between the ocean's surface and bottom layers was 25% larger during the last ice age than at present. Broecker (1982) proposed that an increase in the phospate content of the deep sea could explain these observations. We follow up here on a proposal by Sarmiento and Toggweiler (1984) that glacial to interglacial changes in P_CO2 are related to changes in the nutrient content of high-latitude surface water. We develop a four-box model of the ocean and atmosphere which includes low- and high-latitude surface boxes, an atmosphere, and a deep ocean. In simplest form the model equations show that the CO₂ content of high-latitude surface water is directly connected to the huge reservoir of CO₂ in deep water through the nutrient content of high-latitude surface water. The relationship between the CO₂ content of low latitude surface water and the deep sea is more indirect and depends to a large extent on transport of CO₂ through the atmosphere from high latitudes. We illustrate how the 14C content of the atmosphere and that of high-latitude surface water constrain model solutions for the present ocean and how ice age ¹³C observations constrain ice age parameters. We propose that the low ice age P_CO2 can be produced by a reduction in local exchange between high-latitude surface water and deep water. The model requires that the current exchange rate of about 50 Sv be reduced to about 10 Sv. We review evidence in the geologic record for widespread changes in deep convection around Antarctica about 14,000 years ago which are synchronous with the change in atmospheric P_CO2.

120. Sarmiento, Jorge L., and J Robert Toggweiler, 1984: New model for the role of the oceans in determining atmospheric P_CO2. Nature, 308(5960), 621-624.
121. Sarmiento, Jorge L., 1983: A simulation of bomb tritium entry into the Atlantic Ocean. Journal of Physical Oceanography, 13(10), 1924-1939.
     [ Abstract PDF ]

     Tritium is used in a model calibration study that is aimed at developing three-dimensional ocean circulation and mixing models for climate and geochemical simulations. The North Atlantic tritium distribution is modeled using a three-dimensional advective field predicted by a primitive equation ocean circulation model. The effect of wintertime convection is parameterized by homogenizing the tracer to the observed March mixed-layer depth. Mixing is parameterized by horizontal and vertical Fickian diffusivities of 5 x 10^-6 cm² s^-1 and 0.5 cm² s^-1, respectively. The spreading of tritium in the model is dominated by advection in the horizontal, and by wintertime convection and advection in the vertical. The horizontal and vertical mixing provided by the model have negligible effect. A comparison of the model tracer fields with observations shows that most of the basic patterns of the tritium field are reproduced. The model's mean vertical penetration of 543 m in 1972 is comparable to the 592 m penetration obtained from the data. The major discrepancy between model and data is an inadequate penetration into deeper portions of the northwestern subtropical gyre main thermocline. Some of the problems that may contribute to this are identified. A tritium simulation with a smoothed input gives a penetration depth of only 395 m. The smoothing puts a high fraction of the tritium into low-latitude, low-penetration regions such as the equator. This suggests that great care needs to be exercised in using simplified models of tritium observations to predict the behavior of tracers with different input functions, like fossil fuel CO₂.

122. Sarmiento, Jorge L., 1983: A tritium box model of the North Atlantic thermocline. Journal of Physical Oceanography, 13(7), 1269-1274.
     [ Abstract PDF ]

     A box model of 1972 tritium observations on isopycnal surfaces in the main thermocline of the North Atlantic subtropical gyre is used to estimate the time scales and volume of exchange of the thermocline with respect to surface waters. The flux of water between the surface and the thermocline implied by this model (~ 40 x 10⁶ m³ s^-1) greatly exceeds the downward Ekman pumping (~8 x 10⁶ m³ s^-1). This suggests that mixing and convective overturning are the dominant mechanisms for exchange between surface waters and the interior geostrophic flow. The flux rate is approximately the same size as conventional estimates of the Sverdrup transport. This suggests that ventilation of the thermocline may occur by recirculation combined with a very efficient exchange across the poleward boundary of the gyre.

123. Sarmiento, Jorge L., and Kirk Bryan, 1982: An ocean transport model for the North Atlantic. Journal of Geophysical Research, 87(C1), 394-408.
     [ Abstract ]

     Three-dimensional solutions are obtained for the circulation of the North Atlantic using a robust diagnostic model. In contrast to previous diagnostic models the robust diagnostic model incorporates the conservation of the large-scale fields of heat and salinity as well as momentum. An approximate fit to observed fields of temperature and salinity is obtained by a closure condition. The method is robust in the sense that it does not have the extreme sensitivity to the density input fields of the classical diagnostic method. Equilibrium solutions are obtained by numerical integration of the time-dependent equations. Error estimates for the velocity field can be obtained indirectly from the numerical solutions. Temperature observations used as input have an effective resolution of 3 degrees x 3 degrees of latitude and longitude and a sampling error of plus or minus 0.15 degrees C. The equivalent vertically integrated velocity error is estimated to be plus or minus 0.5-1.0 cm/s depending on bottom topography. The suitability of the model for geochemical work is judged by comparison with heat and salinity balance estimates. Best results are obtained for the case in which the model has a minimum observational constraint below the surface.

124. Sarmiento, Jorge L., C G H Rooth, and W Broecker, 1982: Radium 228 as a tracer of basin wide processes in the abyssal ocean. Journal of Geophysical Research, 87(C12), 9694-9698.
     [ Abstract ]

     simple model of isopycnal mixing in a circular basin is developed in order to examine the utility of the 5.75-year half-life tracer radium 228 for studying basin wide processes in the deep ocean. The model shows that it is possible to resolve diffusivities of approximately less than 8 x 10⁷ cm² s^-1 in a basin of ~3000-km diameter with profiles measured near the center and edge of the basin. A least squares fit of the model to four abyssal profiles measured during GEOSECS in the North Atlantic Basin gives an isopycnal diffusivity of 6 x 10⁷ cm² s^-1.

125. Sarmiento, Jorge L., C G H Rooth, and W Roether, 1982: The North Atlantic tritium distribution in 1972. Journal of Geophysical Research, 87(C10), 8047-8056.
     [ Abstract ]

     The distribution of tritium in the North Atlantic in 1972 is compared with the distribution of salinity and the first-order potential vorticity as mapped on six constant potential density surfaces in the North Atlantic. The picture presented suggests an advective transport regime which is consistent with currently developing notions of the large-scale gyre structure. Lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre, and vertical (cross-isopycnal) mixing needs to be invoked only in near surface layers.

126. Sarmiento, Jorge L., and C G H Rooth, 1980: A comparison of vertical and isopycnal mixing models in the deep sea based on Radon 222 measurements. Journal of Geophysical Research, 85(C3), 1515-1518.
     [ Abstract ]

     A two-dimensional model is developed for the one-dimensional depth-dependent distribution of a radioactive tracer with a bottom source (e.g., radon 222) in a weakly baroclinic fluid. The tracer transport away from the boundary is separated into two components with cardinal directions along and perpendicular to isopycnal surfaces in the fluid. Contrary to the case of a strictly one-dimensional mixing model, where properties such as heat and buoyancy have the same vertical eddy diffusivity as the tracer so that their vertical fluxes are uniquely related, this model yields a relation between vertical buoyancy and heat fluxes and the tracer flux which depends on the relative magnitude of the isopycnal and cross-isopycnal diffusivities. To close the problem, weassume that the interior vertical buoyancy flux is balanced by a horizontal cross-isopycnal Ekman drift at the bottom. As an example, a bottom radon 222 profile (Geosecs station 31) is analyzed. The buoyancy flux implied by the apparent vertical radon 222 diffusivity of A two-dimensional model is developed for the one-dimensional depth-dependent distribution of a radioactive tracer with a bottom source (e.g., radon 222) in a weakly baroclinic fluid. The tracer transport away from the boundary is separated into two components with cardinal directions along and perpendicular to isopycnal surfaces in the fluid. Contrary to the case of a strictly one-dimensional mixing model, where properties such as heat and buoyancy have the same vertical eddy diffusivity as the tracer so that their vertical fluxes are uniquely related, this model yields a relation betweenvertical buoyancy and heat fluxes and the tracer flux which depends on the relative magnitude of the isopycnal and cross-isopycnal diffusivities. To close the problem, we assume that the interior vertical buoyancy flux is balanced by a horizontal cross-isopycnal Ekman drift at the bottom. As an example, a bottom radon 222 profile (Geosecs station 31) is analyzed. The buoyancy flux implied by the apparent vertical radon 222 diffusivity of 46 cm² s^-1 is 2.1 x 10^-6 cm² s^-3, requiring a bottom friction velocity (u_*) of 0.94 cm s^-1 to balance it in a one-dimensional model. In the two-dimensional isopycnal mixing model the buoyancy flux can take on any value between 0 and 2.1 x 10 ^-6 cm² s -3 with corresponding values for u *, which was not measured, of 0-0.94 cm^-1. For an assumed u_* value of 0.1 cm s^-1 the cross-isopycnal diffusivity is 0.5 cm² s^-1, implying a vertical buoyancy flux of 2.3 x 10^-8 cm² s^-3, and the diffusivity parallel to the isopycnals is 3.8 x 10⁶ cm²s^-1.

127. Sarmiento, Jorge L., W Broecker, and P E Biscaye, 1978: Excess bottom Radon 222 distribution in deep ocean passages. Journal of Geophysical Research, 83(C10), 5068-5076.
     [ Abstract ]

     Radon 222 and STD profiles were obtained as part of the Geosecs program in the Vema Channel in the southwest Atlantic Ocean and in the Samoan, Clarion, and Wake Island passages in the Pacific Ocean. The standing crop of excess radon 222 is higher in the passages than at other nearby locations. The most likely explanation for this is that there is a high flux of radon 222 from the floor of the passages. Since much of the floor is covered with manganese nodules and encrustations, the high flux of radon 222 may be attributable to the high concentrations of radium 226 in the outer few millimeters of such deposits. Laboratory measurements of radon 222 emissivity from manganese encrustations obtained in the Vema Channel support this hypothesis. The excess radon 222 in the Vema Channel and Wake Island Passage is found in substantial quantities at heights above bottom greatly exceeding the heights at which excess radon 222 is found in nonpassage areas. The horizontal diffusion of radon emanating from the walls of the passages is unlikely to be the cause of the observed concentrations because the ratio of wall surface area to water volume is very low. The profiles must therefore be a result of exceptionally high apparent vertical mixing in the passages. Further work is needed to determine the nature of this apparent vertical mixing. The excess radon 222 and STD data in all four passages have been fit with an empirical model in which it is assumed that the buoyancy flux is constant with distance above bottom. The fits are very good and yield apparent buoyancy fluxes that are between 1 and 3 orders of magnitude greater than those obtained at nearby stations outside the passages for three of the four passages.

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