The Global Carbon Cycle Biotic

Fossil Fuel Emissions

Fossil Fuel Emissions

Net destruction of vegetation

'■-■Z-yo ■J'.v'.-^New Humic ■-■■',,■'.. ' - ■ ':-> ■' ■ Substances

Atmospheric Pool 750 +3.3/yr

92.3 90

Net destruction of vegetation ft® ri^. ^.'ï? a i? 1 • ' ;>? 1

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38,000

Ocean

Fig. 2. Biotic and Anthropogenic Processes Contributing to the Global Carbon Cycle of the Present-Day Earth. Source: Modified from Schlesinger, 1997.

The mean residence time for a molecule of CO2 in Earth's atmosphere -about 5 years2 - is largely determined by the uptake of carbon in photosynthesis. The well-known annual oscillations of CO2 concentration in Earth's atmosphere occur because a large fraction of global photosynthesis occurs in regions with seasonal climate - i.e., where plants grow only during the summer. Annual oscillations of atmospheric O2 are a mirror image to those of CO2, supporting the role of photosynthesis as a major factor affecting the presence of these gases in Earth's atmosphere (Keeling & Shertz, 1992).

Most of the CO2 removed from the atmosphere by photosynthesis is not captured for long, because dead organic matter decomposes rapidly in soils and seawater. The long-term accumulation of carbon in undecomposed materials in soils is about 0.4 Pg C/year (Schlesinger, 1990), while the storage of carbon in marine sediments is only about 0.1 Pg C/year (Berner, 1982).3 The low rate of carbon burial in sediments today is not unlike the rates through most of Earth's history (Garrels & Lerman, 1981); however, over millions of years of geologic time, a huge amount of organic matter has accumulated in the Earth's crust (e 15,600,000 PgC).

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