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In an effort to better understand processes that control sources of CO2 in the carbon cycle, the U.S. Global Change Research Program [CEES, 1990] identifies imbonate deposition, and burial of organic matter would deplete the CO2 content of the atmosphere in 10,000 years and the atmosphere-ocean system in 500,000 years [Holland, 1978; Berner et al., 1983]. The CO2 content of the atmosphere-ocean system has varied in the past, but not at the rate expected if CO2 were removed and not replenished. It is assumed, therefore, that CO2 de gassing from the Earth's interior restores the deficit from surficial processes and balances the atmospheric CO2 budget on a time scale of 104–106yr. Earlier atmospheric balancing calculations imply present-day (pre-industrial) CO2 degassing rates of 6–7×1012 mol yr−1 [Holland, 1978; Berner et al., 1983]; recent calculations suggest degassing rates may be as high as 11×1012 mol yr−1 [Berner, 1990].