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Biological Pump

Fecal pellets and marine snow aggregates drive the biological carbon pump via sedimentation of organic matter from the surface ocean to the deep ocean where carbon can be sequestered for hundreds to thousands of years. However, the controlling mechanisms for carbon export from the surface ocean are still unclear. It is often observed that fecal pellet fluxes comprise less of the POC flux than would be expected from their production rates (see Turner 2002). This has been explained by high grazing on the fecal pellets by copepods, especially the cyclopoid copepods Oithona sp. (Gonzalez and Smetacek 1994), but direct evidence from the field is still missing. Recent observations have shown that carbonate and organic carbon fluxes have close correlations in the bathypelagic zones of the ocean (e.g., Armstrong et al. 2002). This has lead to the hypothesis that biominerals in phytoplankton, e.g., carbonate and opal, promote carbon preservation in the sinking flux either via increasing the sinking velocity of aggregates due to increased aggregate densities and/or via protection of the organic matter in the cells from degradation (Armstrong et al. 2002; Francois et al. 2002; Klaas and Archer 2002). However, the effect of ballast minerals on sinking velocity and degradation rates in sinking aggregates is still unclear.
Vertical profiles of particulate organic carbon (POC) often show exponential decline with increasing depth (Martin et al. 1987; Schlitzer 2000; Suess 1980), indicating that most of the POC is decomposed in the surface ocean. Therefore, the surface ocean is likely to have the most intensive degradation of marine aggregates. In fact less than half of the primary production are exported out of the euphotic zone and only a few percent reach the deep ocean and sediment (e.g., Martin et al. 1987) (Fig. 2).
Fig. 2. Mean annual global POC flux at different depths in the ocean. The data is compiled from Martin et al. (1987), Suess et al. (1980), Lutz et al. (2002), Andersson et al. (2004) (Figure is modified from Lutz et al. (2002) by C. De La Rocha).