Geoscience Reference
In-Depth Information
This is less true for certain gases with a higher Schmidt number, or
in the presence of very thick layers, for example, during deep
convection events, which, over a limited time, place an important
water reservoir often far from saturation in contact with the
atmosphere. This is even less true for a gas such as DMS, which is
produced during plankton blooms in the ocean and with weak
atmospheric concentrations, because of its rapid chemical alteration in
the atmosphere, and in particular its transfer into the clouds. For other
gases that are stable in the atmosphere, such as O
2
, N
2
O and CO
2
, the
surface concentrations can be far removed from the equilibrium, for
reasons that we will illustrate below in more detail. For example,
concentrations in N
2
O far removed from the equilibrium are found in
narrow regions, linked to the remineralization of organic matter,
which causes a significant air-sea flux of N
2
O. These regions
contribute to approximately 40% of global sources of N
2
O in the
atmosphere.
3.2.4.1.2. Oxygen
For oxygen, the oversaturation of oceanic surface layers increases
during periods of warming and when phytoplankton production
occurs, with typical oversaturation values of 3-10% during these
episodes, lasting over periods of several weeks. However, in a large
majority of oceanic regions, surface concentrations are relatively close
to saturation. Concentrations at saturation being weaker at higher
temperatures, the warming of surface waters over the course of the last
50 years has resulted in a reduction in surface concentration of oxygen
in the sea. This development, perhaps coupled with a reduction in the
ventilation of waters at the subsurface, due to an increase in
stratification and in some cases to a change in the winds, has led to a
reduction in concentration of oxygen beneath the surface in these
regions and in numerous other oceanic regions. This phenomenon is
particularly marked off tropical regions of
upwelling
(zones where
water rises from deep layers) and in eastern boundary currents. In
these regions, the phenomenon is magnified
,
the oxygen being
consumed at the subsurface by bacterial activity, which enables the
oxidation of organic matter. These regions of reduced oxygen,
virtually
anoxic, have tended to increase greatly in area over the
course of he last 40 years, by at least 10-20%, leading to important
