CARBON CYCLE 337 and calcite-shelled coccoliths Just as photosynthesis always consumes CO2, the reverse process, organic respiration or decay, returns that CO2 after death of a plant or animal; this process is catalysed by the enzymatic action of chiefly anaerobic microbes that can be found in the digestive tracts of animals, in the soil, or beneath the seafloor Because organic decay is essentially an oxidation process, oxygen is consumed instead of being released, which is why oceanic regions of high productivity are commonly underlain by ‘oxygen minimum zones’, i.e., zones of low oxygen concentration caused by the decomposition of organic matter in the water column Organic raindown not only transfers CO2 to the deep ocean but also acts as a biological pump for nutrients that, once stored in the deep ocean ‘conveyor belt’, generally reemerge only in regions of upwelling and high productivity Reoxidation of organic matter that has reached the sediment may take place by a variety of mechanisms (oxic respiration, denitrification, or sulphate reduction) However, in addition, in the deepest, most reducing environments, methane may be produced by the actions of methanogenic bacteria using two main pathways (eqns [2a] and [2b]): CH3 COOH ! CO2 ỵ CH4 ẵ2a 4H2 ỵ CO2 ! CH4 ỵ 2H2 O ẵ2b or Methane produced in this fashion may seep back into seawater to be reoxidized to CO2 or may be stored temporarily for thousands to millions of years as the volatile methane clathrate Under normal circumstances, any particular ecosystem will be experiencing both photosynthesis and respiration, but in different proportions If photosynthesis outweighs respiration, then the ecosystem will act as a sink for CO2, whereas in the reverse case it will act as a source This ‘breathing’ of the biosphere can best be seen in the annual fluctuations of atmospheric CO2 in the northern hemisphere (Figure 3) During the northern hemisphere summer, when terrestrial photosynthesis and leaf growth surpass respiration and decomposition, CO2 levels decrease by as much as 15 ppm in the boreal forest zone (55–65 N), a decrease almost completely balanced by the winter increase in CO2 caused by the inevitable decomposition of fallen leaves In the southern hemisphere, the cycle is reversed but the effect never attains more than ppm This hemisphere inequity confirms that such seasonal CO2 fluctuations are driven by the terrestrial carbon cycle, rather than by the oceanic carbon cycle, there being far more terrestrial biomass in the northern than in the southern hemisphere Because the fluxes involved in the short-term carbon cycle are so large, persistent imbalances would lead to intolerable fluctuations in atmospheric CO2 Therefore, on timescales longer than about a century, this cycle of productivity and decay must be perfectly in balance Nevertheless, not all CO2 is eventually returned to the atmosphere A small proportion of the carbon locked up in soils as organic matter may be washed away to be buried indefinitely in coastal and marine sedimentary basins Similarly, a small proportion of marine organic matter will also escape the processes of decay by being swiftly buried in areas of high sedimentation rate Organic burial constitutes a leak in the short-term carbon cycle, whereby a relatively small proportion of CO2 is continually removed from the surface environment to be stored within Earth’s crust Some of this leaked CO2 may eventually be Figure A three dimensional perspective of terrestrial biosphere breathing Reprinted from Volk T (1998) Gaia’s Body: Toward a Physiology of the Earth New York: Springer Verlag