Introduction to the Carbon Biogeochemical Cycle

The cycling of carbon through the biosphere, its biogeochemistry, is of critical concern today in light of global warming and its actual and potential multifold feedbacks to society. The enhanced greenhouse is produced by the reradiation of infrared to the Earth's surface by the anthropogenic greenhouse gases, carbon dioxide being the largest trace gas contributor (water vapor actually accounts for most of the greenhouse effect, but its level is dependent on the independent variation of atmospheric carbon dioxide). Central to all the debate and projections is knowing where the carbon diox ide emitted to the atmosphere ends up, and how this pattern might change as global surface temperature increases. Thus, knowledge of the multifold fluxes in and out of the systems and subsystems of the biosphere and their temporal and spatial variation is critical.

A summary of the global carbon cycle is shown in figure 2-1. First, let us take alook at the natural fluxes. The total photosynthetic flux is about 170 Pg C/year (the prefix P stands for Peta, 1015), 50 for marine biota, and 120 for terrestrial. This flux is almost exactly balanced by a respiration and decay flux of carbon back into the atmosphere/ocean pool, mainly as carbon dioxide. Only a small flux of organic carbon and carbonate of about 0.2 Pg C/year is buried, constituting a sink with respect to the atmosphere/ocean pool. This latter flux balances the net source of carbon to the atmosphere, namely the volcanic source (about 0.1 Pg/year) and an equal flux of carbon from the oxidation of organic carbon present in exposed terrestrial rocks (not shown in figure 2-1).

Turning to anthropogenic fluxes to the atmosphere, these sum up the carbon dioxide from fossil fuel burning (about 5 Pg C/year) and deforestation from the decay of organic carbon (1-2 Pg C/year). Note that this sum (6-7 Pg C/year) is some 60 times the natural flux from volcanism and accounts for the well-known rise ofcarbon dioxide in the atmosphere over the past 100 years and the enhanced greenhouse effect. One critical flux to the long-term carbon cycle is subsumed in that of river-borne material, the flux of bicarbonate and calcium/magnesium ions derived from the weathering of CaMg silicates on land. The latter consist mainly of the following minerals: plagioclase (an NaCa feldspar, an aluminosilicate), biotite (sheet silicate containing magnesium), pyroxenes (single-chain silicates), olivine (single-tetrahedra silicate), and amphiboles (double-chained silicates).

Now consider the inventories of carbon in each reservoir. Carbon in the crust occurs mainly in the form of limestone and its metamorphic product marble and is some 500 times the mass of the total carbon in the atmosphere, biosphere, and ocean combined. [Note that other sources give larger reservoirs of carbonate and kerogen carbon (reduced organic carbon in sediments), 77.5 and 14.2 X 106 Pg, respectively (Holser et al. 1988)]. Oceanic carbon, mainly as bicarbonate ions, is some 54 times the mass in the atmosphere, whereas soil carbon is some four times the atmospheric carbon mass. Although the terrestrial biomass is more than 1000 times that of the oceanic

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