Mechanisms of organic carbon burial marine productivity and sedimentation

We can expect that the sedimentary burial flux of carbon will scale in some way with the strength of productivity in the overlying ocean. All we have to do in order to obtain a stronger geologic sink is to increase primary productivity. One way would be to increase the rate of upwelling of deep waters, thus supplying more nutrients such as phosphate (PO4) to the ocean surface where photosynthesis takes place. However, most ocean circulation models predict that the ocean is likely to become more stratified in the future as the surface warms, reducing rather than enhancing ocean productivity (Sarmiento et al., 1998; Plattner et al., 2001; Schmittner, 2005).

As an alternative to increasing the rate of upwelling, we could increase the total amount of PO4 dissolved in the ocean (i.e. raising the concentration everywhere). Phosphate is supplied to the ocean by the dissolution of phosphate-bearing minerals (e.g. apatites) in exposed rocks and soils on land. An increase in weathering rate, which we already expect might occur as a result of higher CO2 and surface temperatures (Section 6.2.3), will thus act in the 'right' direction. However, PO4 has a relatively long residence time in the ocean - estimated to be 10,000-80,000 years (Benitez-Nelson, 2000). Thus, even if the global weathering rate were to be instantaneously doubled, we would have to wait at least until the year 10,000 for PO4 concentrations (and productivity) to have increased by 10-55% (depending on the residence time chosen). Assuming that burial scaled linearly with productivity, this equates to an increase in the strength of the geologic carbon sink of 0.005-0.027 Pg C/year, which is insignificant compared to a total fossil fuel CO2 release of 4176 Pg C. An increase in weathering rate would also increase the rate of erosion and oxidation of ancient organic matter (kerogens) sequestered in sedimentary rocks and, rather unhelpfully, release additional CO2 to the atmosphere.

Finally, not everywhere in the ocean is all the PO4 that is supplied to the ocean surface fully utilized by the phytoplank-ton. In places such as the Southern Ocean, and Eastern Equatorial and North Pacific, insufficient iron availability (a micronutri-ent essential for parts of the photosynthetic machinery of cells) limits productivity (Jickells et al., 2005). Supply of this iron ultimately comes from dust deposited to the ocean surface. Thus, if dust supply were to increase, presumably so would productivity (e.g. Watson et al., 2000), and with it, an increase in the rate of burial of organic carbon in sediments and sequestration of CO2. Although the factors affecting dust production, transport and deposition are complex (Jickells et al., 2005; Ridgwell and Kohfeld, in press), it seems that on balance, dust supply is likely to decrease rather than increase in response to future climate change, further restricting biological productivity. Overall, therefore, future productivity changes in the open ocean would seem to be of little help in the sequestering of fossil fuel CO2.

Bulk sedimentation rate also appears to be an important control on carbon burial (see Arthur and Sageman, 1994; Hedges and Keil, 1995). This is because a faster accumulation rate reduces the residence time of organic matter in the surface sediments, giving bacteria and benthic animals less chance to consume and metabolize it. However, if productivity were to decline in the future, there is little a priori reason to expect an increase in bulk sedimentation rate, particularly if carbonate production were also to be suppressed (which could give rise to a decrease in the efficiency of transport of organic matter to the sediments if the 'ballast hypothesis' is correct - see Section 6.2.4). In contrast, soil erosion and poor land use management might be expected to contribute to increased sedimentation rates on the continental margins, which could result in increased rates of organic matter burial. For instance, increased Himalayan erosion during the Neogene has been hypothesized to have driven lower atmospheric CO2 by just such a mechanism (France-Lanord and Derry, 1997). However, the damming of rivers for irrigation and power generation could prevent much of the clay mineral supply from reaching the continental margins, thus reducing the potential importance of this effect.

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