The carbon isotope record of the Cenozoic: history of organic carbon burial and of oxygen in the ocean and atmosphere

Summary Carbon isotope measurements in deep-sea sediments may be used to obtain the history of the mean 13 C content of CO 2 dissolved in the ocean, of 13 C gradients within the ocean, and of the globally averaged 13 C content of the carbonate accumulated on the sea floor. The last has some interest in relation to energy reserves, because it provides a means of monitoring the changing rate of accumulation of organic matter on the globe. Data are shown for the 13 C composition of Cenozoic marine carbonates, based on a suite of sites from DSDP (Deep Sea Drilling Project) Leg 74 in the South Atlantic. If the published estimates of global accumulation rates for marine carbonate are recalculated on the basis of recent timescales for the Cenozoic it is found that changes were rather subtle, contrary to the conclusion of Davies and Worsley (1981), so that a model of constant carbon input may be used to estimate changes in organic carbon accumulation from the 13 C data. A cumulation of changes in the global organic carbon reservoir is also obtained and is used to infer changes in the amount of oxygen in the atmosphere, which was 20% higher than today during most of the Cenozoic. However, the solubility of oxygen in the warm early Cenozoic oceans was about 20% lower than today’s value, so that the early Cenozoic ocean had a similar dissolved oxygen content to today’s ocean.