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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 CO
2 levels
have not significantly impacted ocean biology and that the solubility pump is the primary
mechanism for getting anthropogenic CO
2 into the ocean interior.
The solubility pump is driven by two principle factors. First, more CO
2
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
CO
2 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 CO
2 accumulated at the surface.
The figure above shows representative sections of anthropogenic CO
2
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 CO
2 invades the ocean through gas exchange
across the air-sea interface, highest concentrations of anthropogenic CO
2
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 CO
2 that can be absorbed
from the atmosphere. Approximately 30% of the oceanic inventory of anthropogenic
CO
2 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 CO
2 are related to how the anthropogenic
CO
2 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 CO
2 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 CO
2
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
CO
2 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
CO
2 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 CO
2. Approximately 60% of the total anthropogenic
CO
2 inventory is stored in the Southern Hemisphere.
The anthropogenic CO
2 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 CO
2,
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 CO
2 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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