The burial of organic carbon produced as a result of biological photosynthetic activity represents a net sink for CO2, and can be encapsulated in the reaction CO2 + H2O (+ sunlight) ® CH2O + O2. This reaction explains how burial of organic matter could have driven the changes in atmospheric oxygen concentrations during the Phanerozoic period (see Berner et al., 2000, 2003 and references therein). Considerably more organic carbon is 'fixed' from CO2 by photosynthesis than is ever eventually buried. For instance, global primary productivity by phytoplankton in the ocean is estimated to be ~45 Pg C/year (Houghton et al., 2001). Of this, 11 Pg C/year escapes consumption and respiration (called 'remineralization') by zooplankton grazers and bacteria at the surface, and sinks in particulate form to the ocean interior. In turn, only a tiny fraction of this flux, - 0.05 Pg C/year, is ever buried in marine sediments. The remainder (>99%) is remineralized either in the water column or in the surface sediments. This gives us some clues as to ways in which the strength of the geologic organic carbon sink could be enhanced and thus help sequester fossil fuel CO2 from the atmosphere. We will outline the individual mechanisms that could give rise to a higher burial flux in the following sections, and discuss their possible response (if any) to future global change. The terrestrial geologic organic carbon sink (coal) is discussed separately at the end of the chapter.
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