Most of the organic carbon that is consumed in the water column and surface sediments is preferentially utilized by bacteria and small animals that metabolize aerobically. Oxygen is used by these organisms because it allows the maximum energy to be extracted out of each molecule of organic matter (represented by 'CH2O'): CH2O + O2 ® CO2 + H2O (+ metabolic energy). If oxygen runs out and conditions become 'anoxic', other bacteria that can utilize nitrate (NO-) or even sulphate (SOf") as the electron receptor for the reaction take over. It is less efficient doing it this way and toxic by-products such as H2S can build up. The result is that the fraction of organic matter that escapes degradation should be greater under anoxic conditions (Hedges and Keil, 1995). Another effect of anoxia is that burrowing animals, efficient scavengers and consumers of particulate organic matter from the surface sediments are excluded. This will also tend to increase the preservation and burial of organic carbon. In the geological record, the occurrence of 'organic-rich' (defined as sediments containing >1% carbon by dry weight) sedimentary formations called 'black shales' (of which 'sapropels' are a specific case) have often been interpreted as being caused by local anoxia (Arthur and Sageman, 1994). A contemporary example is the Black Sea, where anoxic conditions prevail deeper than about 150 m in the water column and the organic carbon content of the underly ing sediments is typically 1-3% (Arthur and Dean, 1998).
Any increase in the occurrence of anoxia in the ocean should therefore enhance the geologic organic carbon sink. The solubility of oxygen decreases with increasing water temperatures, so a warmer Earth should have a less oxygenated ocean (all other things being equal). Increased stratification will also reduce the downward transport of oxygenated surface waters to depth. An increase in carbon preservation and burial in marine sediments in response to global warming is therefore starting to look like a distinct possibility.
The story gets more complicated because NO3- is consumed in anoxic regions, and lower ocean oxygen concentrations means less NO3-. Because nitrate, like PO4 and Fe, is an essential nutrient for plant growth, lower NO3- concentrations will restrict productivity, and thus carbon burial. Decreased productivity will result in less oxygen consumed during remineralization, and ocean waters will be driven back away from anoxia. This is a 'negative' feedback loop, helping to stabilize oceanic oxygen concentrations. The net outcome of all these competing processes is not obvious, and this is still not including all the relevant factors (see Berner, 1999). This is where the importance of computer models is felt that can calculate the net impact on oceanic oxygen concentrations due to the interacting effects of surface warming, circulation changes and reduced productivity. The results of long-term climate experiments with such models suggest that no deep anoxia is likely to develop in the future, although in regions such as the eastern equatorial Pacific Ocean, an increase in the volume of hypoxic (i.e. not completely anoxic, but with <10 |mmol/kg oxygen) thermocline waters is possible (Matear and Hirst, 2003). The oceanic distributions of dissolved oxygen predicted in the GENIE-1 model under our fossil fuel carbon dioxide release experiment also remained oxygenated, although hypoxia became more extensive within the thermocline of the northern Indian Ocean in this particular model (not shown).
To put the potential importance of this mechanism into some sort of perspective, even if ocean anoxia doubled the global rate of carbon sequestration (adding another 0.05 Gt
C/year) in the future, it would still take almost 84,000 years for all of the 4176 Pg C of emitted fossil fuel CO2 to be removed by this mechanism operating alone.
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