The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 344

Recall from Chapter 5 ( Section 5.2.1 ) that the shelf edge is important to carbon
export as it controls the link between the productive shelf seas and the continental
slope sediments and deep ocean where carbon can be sequestered for long time scales.
Even in the absence of any biological activity, carbon can be exported as DIC if cool,
high-latitude water containing high concentrations of DIC becomes sufficiently dense
to sink below the depth of the ocean's winter mixed layer.

10.9.1 Exporting carbon from upwelling systems: coastal filaments and canyons

Upwelled water will contain both nutrients and DIC. As we noted in Chapter 5 , if the
C:N has the Redfield ratio we might expect there to be no net export of carbon
arising from the increased primary production driven by the nitrate. The physics of
an upwelling system can, however, act to convert DIC to organic carbon and then
export the fixed carbon before it can be remineralised. Indeed, overall the current
best estimate is that the eastern boundaries of the world's oceans, including all of the
major upwelling systems, are overall net exporters of carbon and weak sinks for
atmospheric CO 2 (Jahnke, 2010 ). We will now look at some of the physical processes
in upwelling systems that can export carbon.
Off Iberia and off northwest Africa the offshore transport mechanism provided
by upwelling filaments (described in Section 10.3.1 , and shown in Fig. 10.17 )
provides a conduit for dissolved organic material, including DOC, into the sub-
tropical gyre of the northeast Atlantic (Alvarez-Salgado et al., 2007 ) and poten-
tially providing an important source of organic nutrients to the oligotrophic
subtropical gyre (Torres-Valdes et al., 2009 ). Also, the bathymetric features that
play a large part in the development of upwelling front instabilities and filaments
concentrate the deposition of POC. A remarkably high-resolution study of the
Nazare´ Canyon off the Iberian upwelling region (Masson et al., 2010 ) has shown
how the canyon tends to have larger sedimentation rates than the adjacent slope,
and that seafloor sediments in the canyon have higher organic carbon content
than the slope sediments. However, while we may link the observation of organic
carbon deposition with the primary production triggered by the upwelling, the
study also showed that the canyon was not a deposition site for the products of
pelagic carbon fixation. Canyon sediments were found to be derived from terri-
genous material from the shelf, while slope sediments contained more organic
carbon fixed within the overlying water column. So, off the narrow upwelling shelf
of Iberia, it appears that the filaments export carbon fixed in response to the
upwelling, while the canyons hold onto and sequester carbon originating from the
shelf and the rivers.

10.9.2 Seasonal downwelling and a role for the shelf thermocline

Off the California shelf (Hales et al., 2005 ) the high nitrate concentrations of the
upwelled water, which are completely utilised by photosynthesis, lead to the
surface waters absorbing CO 2 from the atmosphere. Here it is the seasonality of