The Carbon Biogeochemical Cycle

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The cycling of carbon through the biosphere is of critical concern today because of accelerated global warming and its impacts on global society. The enhanced greenhouse is a result of the additional warming from infrared radiation 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, first of all, atmospheric carbon dioxide). Of critical concern is where the carbon dioxide 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 essential. Anthropogenic methane also contributes to global warming as a trace greenhouse gas, much more potent than carbon dioxide although at a level c. 1% of the latter's concentration in the atmosphere.

A summary of the global carbon cycle is shown in Figure 1. First, the natural fluxes are discussed. The total photosynthetic flux is about 170 PgC yr-1 (the prefix 'P' stands for peta-, 101 ), 50 for marine biota, 120 for terrestrial. This flux is almost exactly balanced by a respiration and decay flux of C back into the atmosphere/ocean pool, mainly as carbon dioxide. Only a small flux of organic C and carbonate of about 0.2 Pg C yr-1 is buried, constituting a 'sink' with respect to the atmosphere/ocean pool. This latter flux balances the net source of C to the atmosphere, namely the volcanic (about 0.1 Pgyr-1) and a roughly equal flux of C from the oxidation of organic C present in exposed terrestrial rocks.

The anthropogenic fluxes to the atmosphere consist of the carbon dioxide from fossil fuel burning (about 5PgCyr-1) and deforestation from the burning and decay of organic C (1-2 PgC yr-1). Note that this sum (6-7PgCyr-1) is roughly 60 times the natural flux from volcanism, and of course accounts for the well-known rise of carbon dioxide in the atmosphere in the last 100 years, and most of the enhanced greenhouse effect. One critical flux to the long-term carbon cycle consists of 'riverborne material', the flux of bicarbonate and calcium/magnesium ions derived from the weathering of CaMg silicates on land, mainly following minerals:

Carbon dioxide 725 Methane 3

Carbon monoxide 0.2

Air-sea exchange 80 80

Atmosphere

Volcanism Erosion

Photosynthesis

Respiration

Marine biota 3

5 Particulate flux

Ocean

Dissolved inorganic carbon 37 900

Dissolved organic carbon 1000 Particulate carbon 30

Riverborne material

Deforestation 1-2

Photosynthesis Respiration

Deforestation 1-2

Photosynthesis Respiration

Volcanism Erosion

Litter 60

Humus 1500 Peat 165

Litter 60

Humus 1500 Peat 165

Fossil fuel burning

Coal, oil

Continental crust

5000-10000

Limestone and marble 20 million Kerogen 5 million

Sedimentation

Figure 1 The global biogeochemical carbon cycle. Reservoir contents are given in PgC (1015g) and fluxes in PgCyr-1. Modified from Holmen K (1992) The global carbon cycle. In: Butcher SS, Charlson RJ, Orians GH, and Wolfe GV (eds.) Global Biogeochemical Cycles, pp. 239-262. London: Academic Press.

plagioclase (an NaCa feldspar, which are aluminosili-cates), biotite (sheet silicate containing Mg), pyroxenes (single-chain silicates), and amphiboles (double-chained silicates).

Carbon in the crust occurs mainly in the form of limestone and its metamorphic equivalent marble and is some 500 times the mass of the total C in the atmosphere, biosphere, and ocean combined. Oceanic C, mainly as bicarbonate ions, is some 50 times the mass in the atmosphere, while soil C is some 2 times the atmospheric C mass. While the terrestrial biomass is over 1000 times that of the oceanic, its rate of assimilation (photosynthesis) of carbon dioxide from the atmosphere is little more than 2 times the oceanic rate; most of the terrestrial biomass is in the form of dead wood in trees.

The photosynthetic flux of oxygen is almost exactly balanced by the respiration and decay flux (the small burial of organic carbon is thus an oxygen source balancing natural sinks such as oxidation of ferrous iron in rocks). The flux of fossil fuel burning is about 50 times that of the natural flux of volcanic/metamorphic release of carbon dioxide to the atmosphere. Hence, the anthropogenic rise of atmospheric carbon dioxide and global warming occur on a timescale of decades to hundreds of years.

The short-term carbon cycle is illustrated in Figure 2. In the soil pool, we see a large potential positive feedback of global warming, the release of soil carbon into the atmosphere (Figure 1). Note that the ratio of soil organic carbon to atmosphere carbon is about 2:1. The global estimate of soil organic matter divided by the carbon

Marine photosynthesis

Marine photosynthesis

Marine respiration

Figure 2 Short-term carbon cycle. Box model diagram. Boxes represent reservoirs and arrows represent mass fluxes between reservoirs. Human effects are not shown; deforestation would be an acceleration of terrestrial respiration, but fossil fuel burning is an acceleration of sedimentary organic matter weathering, a flux from the long-term carbon cycle. From Berner RA (1999) A new look at the long-term carbon cycle. GSA Today 9: 1-6.

Marine respiration

Figure 2 Short-term carbon cycle. Box model diagram. Boxes represent reservoirs and arrows represent mass fluxes between reservoirs. Human effects are not shown; deforestation would be an acceleration of terrestrial respiration, but fossil fuel burning is an acceleration of sedimentary organic matter weathering, a flux from the long-term carbon cycle. From Berner RA (1999) A new look at the long-term carbon cycle. GSA Today 9: 1-6.

deposited as litter gives a mean residence time of about 25 years. However, the residence time is apparently significantly reduced as temperature increases with huge releases of soil carbon to the atmosphere expected from global warming, although the kinetics and feedbacks of this process are still under active investigation.

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