The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 347

the upwelling under a western boundary current, but provides a means to export
carbon rather than supply nutrients.
The LOIS-SES experiment has provided us with a direct observation of a transfer
of organic carbon down the slope (see Fig. 10.10 ). The biogeochemical importance of
such cascades of cool, relatively dense shelf water has been described for the case of
the East China Sea (Tsunogai et al., 1999 ). There the cooling shelf water takes up
CO 2 from the atmosphere, adding to the organic carbon fixed during the preceding
productive months. The net effect is an annual carbon export from the East China
Sea down the adjacent slope equivalent to 35 g C m 2 a 1 , a process named the
continental shelf pump. Incorporating the process into a global circulation model led
to a suggestion that continental shelves could export about 0.6 Gt of carbon every
year (Yool and Fasham, 2001 ), which is about 30% of the annual anthropogenic
supply of carbon to the atmosphere. Dramatic evidence of the channelling of such
seasonal, cooling-driven cascades of dense water containing significant amounts of
carbon is shown in Fig. 10.27 for the Gulf of Lions in the western Mediterranean.
Note the pattern of down-canyon flow ( Fig. 10.27a ) and the Coriolis-driven tendency
for the cascade to be displaced to the right against the canyon wall ( Fig. 10.27b ),
analogous to the modelled cascade shown in Fig. 10.9 . Current speeds within a series
of these observed cascades typically reached 70-80 cm s 1 . The cascade flows have a
significant effect on the morphology of the canyon seabed and contain large amounts
of suspended sediment. At the end of the cascading season chlorophyll fluorescence
was seen down at depths

2000 metres; the overall contribution of the cascades to
carbon export from the shelf area of the Gulf of Lions was estimated to be 50 g
Cm 2 a 1 .

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Summary

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We have seen in this chapter that the edges of the continental shelf are host to a rich
variety of physical processes which mediate important biochemical exchanges
between the shallow, productive shelf seas and the adjacent deep ocean. Two physical
processes in particular exert strong controls on the environment: (1) the steep
bathymetry of the narrow slope region constrains geostrophic current to flow parallel
to the isobaths, thus preventing them from crossing the slope and thereby impeding
exchange between shelf and deep ocean. Some exchange across the slope is still
possible through mechanisms which evade the constraint of bathymetric steering,
notably in the surface and bottom boundary layers where frictional stresses drive
substantial cross-slope flows; (2) the barotropic tidal flow over the slope acts as a
generator of internal wave motions by forcing a stratified water column up and down
the slope.
Surface wind stress and wind-induced upwelling, which brings nutrients in to
the photic zone, plays a key role in driving greatly enhanced primary production
in some shelf edge regions, such as the Benguela system off the SW coast of