The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 345

CO 2 CO 2

Front

DIC

Autotrophs

DIC

Autotrophs

Thermocline

Heterotrophs

Heterotrophs

POC

Respiration

? N. Atlantic

Southern North Sea
mixed

exchange

DIC

Central and northern North Sea
seasonally stratified

Figure 10.26 Schematic illustration of the role of the thermocline in exporting carbon from
the central and northern North Sea (adapted from (Thomas, Bozec, et al., 2004 )). The question
mark by the N. Atlantic exchange signifies uncertainty in how much of the deep water in
the northern North Sea can be exported before winter convection allows CO 2 out-gassing
to the atmosphere.

the upwelling that plays an important role in carbon export, as the DIC generated
by respiration in the bottom waters of the shelf is exported off-shelf when the
upwelling relaxes and downwelling winds occur in winter. This behaviour of the
Californian system can be understood by considering the example we have already
shown in Fig. 10.21 . Remember that the export of the POC below the thermo-
cline, followed by bacterial degradation, led to significant reductions in the
bottom water dissolved oxygen concentration. These low oxygen conditions are
maintained by the overlying thermocline preventing mixing with the surface layer
and replenishment via contact with the atmosphere. Bacterial breakdown of
organic carbon in the lower layer will generate inorganic carbon, which will also
be kept away from contact with the atmosphere by the thermocline. Thus organic
carbon is removed from the surface layer by sinking, and then the thermocline
acts to trap increasing concentrations of DIC ready for export by the later down-
welling conditions.
The Californian shelf is very narrow, but this trapping role of the shelf sea
thermocline has been highlighted as the key to carbon export from the much more
extensive shelf of the North Sea (Thomas et al., 2004 ). The North Sea is broadly
split into two distinct biogeochemical regimes. The southern North Sea is shallow,
which combined with the strength of the tides results in a vertically mixed water
column all year. The central and northern areas of the North Sea are deeper,
which allows the archetypal temperate seasonal cycle of thermal stratification and
mixing (e.g. see Section 6.1 ). The consequences of this partitioning on the storage,
release and export of carbon are illustrated schematically in Fig. 10.26 . Primary
production in the mixed region draws CO 2 from the atmosphere, but as POC is
cycled through the heterotrophs and remineralised to increase DIC, the water
column is able to ventilate excess CO 2 back to the atmosphere. In the stratifying