Geoscience Reference
In-Depth Information
2009
) supports the idea that open shelves in temperate and high latitude regions are
net sinks for carbon, while shelf seas in low latitudes appear to act as sources; the
net global exchange is estimated to be a transfer of carbon from atmosphere to
ocean of
0.35 GT a
1
or
29% of the total CO
2
absorption by the ocean. The
limited sampling of these fluxes, however, means that such estimates are only
provisional and subject to large uncertainty. In view of the crucial role of CO
2
absorption in mitigating global warming, there is clearly a pressing need for a future
research in this area to improve process understanding and our estimates of fluxes
from different shelf regimes.
∼
∼
11.4.2
Cross-slope fluxes at the shelf edge
It might seem obvious that the way to determine the net contribution of the shelf seas
to carbon export to the deep ocean would be to measure fluxes at the shelf edge. This
would also be the place to observe the transport of nutrients back on to the shelf from
deep water sources which are replenished by re-cycling in the deep ocean. As we have
seen in
Chapter 10
, the principal physical processes operating at the shelf edge and
over the continental slope have been identified and their importance in relation to the
biogeochemistry established. However, quantifying the cross-slope exchange which
they bring about at the continental margins of the ocean remains a very challenging
and largely unsolved problem. There are major uncertainties in our knowledge of the
temporal and spatial variability of physical water fluxes across the shelf edge, and the
implications of this exchange on whole-shelf scales. Some of the cross-shelf transport
mechanisms, such as the Ekman drain associated with the slope current, are relatively
consistent, but others, e.g. cascading and wind-forced transport in the surface layers,
are highly intermittent and spatially variable. In the North Sea, for example, the
observations of carbon trapped below the seasonal thermocline are clear, but calcu-
lating the amount of that carbon which is eventually exported off the shelf edge
involves uncertain estimates of the fraction of the lower layer that is exchanged with
the North Atlantic before convective overturning re-establishes contact between the
bottom layer DIC and the atmosphere.
There is some evidence which indicates that significant quantities of organic matter
from the shelf arrive at the shelf break and descend in the bottom layer flow over the
slope (e.g. the SEEP studies off eastern North America (Biscaye et al.,
1994
)).
Figure 11.6
shows a schematic summary of measurements of organic carbon made
at the Hebridean shelf edge during the UK SES study. While downward fluxes from
the surface waters over the slope, determined by sediment traps, showed a settling flux
of
2gCm
2
a
1
, the measured rate of respiration in the surficial bed sediments was
∼
0.2 g C m
2
a
1
.
Direct observations in the boundary layer over the slope confirmed that large
quantities of organic matter were being advected downslope. Much of the material
was in the form of 'fresh' phytodetritus which was observed down to at least
20 g C m
2
a
1
, while long-term burial was estimated at only
∼
∼
500
metres on the slope, indicating rapid transport from the surface. Interestingly, the
remaining organic material contained a significant component of carbon from
∼
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