Geoscience Reference
In-Depth Information
consequences for the biogeochemical functioning of these systems, as
well as for the food chains and ecology of these regions.
3.2.4.1.3. Carbon dioxide
In the context of gases dissolved in the surface layers which are
close to saturation, the distribution of carbon dioxide gas in these
surface layers is abnormal, with some notable differences saturation
(of 10-30%) with respect to extensive oceanic regions. Equatorial and
tropical latitudes tend to be source regions, whereas the high latitudes
tend to be sink regions, although these sinks have been weakened
greatly over the course of the last 30 years [LE
03], despite CO
2
being
very soluble in seawater. This remarkable property comes from the
fact that carbon dioxide is not an inert gas in seawater, and that it is
associated in acid-base reactions that involve two other forms
dissolved in seawater: carbonates and hydrocarbonates. At the ocean's
typical pH of around 8.1, these two components represent 99.5% of
the stock of inorganic carbon in seawater, whereas CO
2
, in a hydrated
form, (H
2
CO
3
), only represents 0.5% of this stock. As a result, the
majority of inorganic carbon is found in the oceans, and only a small
portion in the atmosphere. It takes a very long time, from a season to a
year, to balance the oceanic surface layer of carbonic gas with the
atmosphere above.
Moreover, CO
2
plays a major role in biological activity, when,
during photosynthesis, it forms organic matter or inorganic skeletons
(calcite and aragonite), as well as during respiration and the
degradation of organic matter through bacterial activity. These
processes are controlled, on the one hand, by the thermal impact on
the partial pressure of surface waters (
pCO
2
), due to seasonal warming
(which tends to increase the
pCO
2
in the surface, and therefore to
cause oversaturation during these warmings) and, on the other hand,
by the impact of biological activity. Phytoplankton blooms contribute
to carbon sinks near the water surface. In addition, the exported matter
leads to carbon dioxide input into the deeper layers through
remineralisation and respiration. Moreover, oceanic circulation,
horizontal as well as vertical (or the vertical mixing of masses of
water), will modify these surface signals which would tend to be
associated with significant vertical gradients.
