Geological Processes

The discussion thus far has focused on reservoirs and processes relevant to human timescales. With a residence time of about 300 million years, the huge reserves of carbon stored in the sedimentary rocks are not expected to play a large role in the short-term carbon budget. On timescales greater than 500,000 years, about 80 percent of the CO2 exchange between the solid earth and the atmosphere is controlled by the carbonate-silicate cycle (Kasting et al. 1988). In this cycle, atmospheric CO2 is used to weather calcium-silicate rock minerals on land, which are then transported to the ocean via rivers as calcium and bicarbonate ions. In the oceans, plankton and other organisms incorporate the ions into calcium carbonate shells. A portion of the calcium carbonate is deposited onto the ocean floor, and eventually CO2 is returned to the atmosphere through volcanic and diagenetic processes.

Vast quantities of carbon stored are in ocean sediments as methane hydrates and as calcium carbonate. The methane hydrates are relatively stable but could be released if ocean temperatures increase sufficiently through global warming (Harvey and Huang 1995). The carbonate sediments are likely to be a significant sink for fossil fuel CO2 on millennial timescales (Archer et al. 1999). As the oceans continue to take up anthropogenic CO2, the CO2 will penetrate deeper into the water column, lowering the pH and making the waters more corrosive to calcium carbonate. Dissolution of sedimentary carbonates binds the carbon in a dissolved form that is not easily converted back into atmospheric CO2. Carbonate dissolution is typically thought to occur in the deep ocean, well removed from the anthropogenic CO2 taken up in the surface waters. In portions of the North Atlantic and North Pacific Oceans, however, anthropogenic CO2 may have already penetrated deep enough to influence the dissolution of calcium carbonate in the water column and shallow sediments (Feely et al. 2002).

Although the processes of CO2 uptake through weathering and CO2 release from volcanism and diagenesis appear to have a small net effect on the global carbon cycle on millennial timescales, short-term variability in one of these fluxes can affect the carbon cycle on timescales relevant to humans. For example, explosive volcanic eruptions result in the emission of CO2, dust, ash, and sulfur components. These sulfur particles may rain out and promote acid rain. Atmospheric sulfur particles block sunlight and cool the regional climate. Particles that reach the stratosphere reduce global temperatures for several years. The incidental occurrence of cataclysmic volcanic eruptions or the coincidence of several large eruptions (e.g., the eruptions of 1783 or the 1991

Mount Pinatubo eruption) affect the global carbon cycle directly by their CO2 emissions and indirectly by their impact on marine and terrestrial primary production (Hamblyn 2001; Sarmiento and Gruber 2002).

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