The SHACKLETON and HALL (1984) record of carbon isotope ratios for bulk CaCO3 from DSDP sites 525 to 528 in the South Atlantic has been widely used as an indicator of how the C-13/C-12 for CaCO3 leaving the ocean has changed over the last 68 million years. Of particular interest is the approximately 2 parts per thousand decrease which occurred during the last 15 million years, for it suggests a change in the O2 content of the atmosphere (SHACKLETON, 1987). As a check on the assumption that this record serves as a global indicator, we have compiled measurements on planktonic foraminifera from a number of tropical cores. We find that this data set suggests a much smaller C-13/C-12 decrease over the same time interval. We can find no satisfactory explanation for this difference and hence can conclude only that more measurements and thought will be required before the time trend in carbon isotope composition can be established.
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