The ocean is the major carbon reservoir in the ocean-atmosphere-biosphere system. Depending upon the demands of the biosphere, the ocean alternately acts as a source or sink of carbon. Because organic carbon has a ^dgrC¹³ composition of approximately -25 ppm (PDB), a 15% variation in the size of our modern biosphere (living and humus) would result in a 0.2 ppm ^dgrC¹³ variation in the CO₂ of the world ocean. A -0.4 ppm ^dgrC¹³ variation in modern surface water has been measured in a 200-year-old coral. Carbon-14 measurements in the same coral confirm that the cause of the ^dgrC¹³ variation

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during the past century is the burning of fossil fuel. The carbon isotopic composition of certain species of planktonic foraminfera accurately reflects the C-13 composition of CO₂ in the ocean surface water, and also records the -0.4 ppm change in surface water. This change was measured by comparing living planktonic foraminifera C-13 composition with the C-13 values measured in fossil planktonic foraminifera from core tops. During deglaciation periods in the Pleistocene, certain species of planktonic foraminifera and benthic foraminifera record synchronous global C-13 fluctuations of approximately +0.8 ppm which indicates a flux of 1.2 × 10¹⁸ g carbon from the ocean to the biosphere. These figures suggest that the biosphere doubled in mass in less than 10,0 0 years, presumably owing to rapid climate change. We can extrapolate these findings in modern and Pleistocene planktonic and benthic foraminifera to measure carbon fluxes in the Tertiary and Late Cretaceous. How well periods of rapid organic carbon production correspond to periods of significant carbon burial and maturation is not yet known.

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