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Two basic mechanisms control the natural distribution of carbon in the ocean interior: the solubility pump and the biological pump. Because biology is not limited by carbon in the oceans, it is thought that increasing CO2 levels have not significantly impacted ocean biology and that the solubility pump is the primary mechanism for getting anthropogenic CO2 into the ocean interior. The solubility pump is driven by two principle factors. First, more CO2 can dissolve into cold polar waters than in the warm equatorial waters. As major ocean currents (e.g. the Gulf Stream) move waters from the tropics to the poles, they are cooled and can take up more CO2 from the atmosphere. Second, the high latitude zones are places where deep waters are formed. As the waters are cooled, they become denser and sink into the ocean’s interior taking with them the CO2 accumulated at the surface.
 

 
The figure above shows representative sections of anthropogenic CO2 in the Atlantic, Pacific and Indian oceans. Concentrations range from greater than 60 µmol kg-1 in North Atlantic surface waters to zero throughout most of the abyssal waters. Because anthropogenic CO2 invades the ocean through gas exchange across the air-sea interface, highest concentrations of anthropogenic CO2 are found in near surface waters. Variations in surface concentrations are related to the length of time that the waters have been exposed to the atmosphere and to the buffer capacity which determines the total amount of CO2 that can be absorbed from the atmosphere. Approximately 30% of the oceanic inventory of anthropogenic CO2 is found shallower than 200 m and nearly 50% above 400 m depth. The average depth of the 5 µmol kg-1 contour, which represents the detection limit of the technique, is approximately 1000 m. Variations in the depth of penetration of anthropogenic CO2 are related to how the anthropogenic CO2 that has accumulated in the near surface waters is transported into the interior of the ocean. Shallow penetration is generally observed in regions of upwelling, where waters with low anthropogenic CO2 concentrations are brought to the near surface. Deepest penetrations are associated with convergence zones, where waters with relatively high anthropogenic concentrations are moving into the ocean's interior.
 

 
The easiest way to see the spatial distribution of anthropogenic CO2 in the ocean is by looking at a map of the vertically integrated concentrations (shown above). Highest vertically integrated concentrations are found in the North Atlantic where anthropogenic CO2 penetrates to the bottom as a result of the formation and downward spreading of NADW. Roughly 25% of the total inventory of anthropogenic carbon is found in the North Atlantic. The high latitude Southern Ocean generally has very low anthropogenic CO2 inventories and very shallow penetration. The southern hemisphere mode and intermediate waters at around 40-50°S, on the other hand, contain some of the largest inventories of anthropogenic CO2. Approximately 60% of the total anthropogenic CO2 inventory is stored in the Southern Hemisphere.
 
The anthropogenic CO2 inventory estimate for the region shown above is 106±17 Pg C for 1994. The integration covers all latitudes from Antarctica to 65°N, excluding marginal seas. The estimated global inventory of anthropogenic CO2, including the marginal seas and the Arctic Ocean, is 118.0±19 Pg C for 1994. Although the oceans have the potential to absorb 85% of the anthropogenic CO2 released to the atmosphere, today’s oceans are only at about 15% capacity.
 
Through the repeat hydrography program, the PMEL carbon group is working to document carbon cycle changes in the water column over time. In the future, some of the assumptions of steady state biology and circulation may no longer be valid. We are continually striving to improve our estimates of the carbon cycle changes and to attribute the differences observed in the measurements to the appropriate mechanisms.

 
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