CORRECTION OF ACCELERATOR MASS SPECTROMETRY C-14 AGES MEASURED IN PLANKTONIC FORAMINIFERA: PALEOCEANOGRAPHIC IMPLICATIONS

Carbon 14 dates obtained by accelerator mass spectrometry (AMS) on foraminiferal samples from deep-sea sediment cores must be corrected for the difference in C-14 composition between the atmosphere and the sea surface. In the modern ocean, the "apparent age" of carbonate shells formed in surface waters varies between 300 and 1200 years and depends mainly on latitude. The time variation of this parameter during climate oscillations of the last 40,000 years may have been significant: there should have been small changes for most of the ocean between 40 degrees S and 40 degrees N, but an increase of the apparent age by several hundred years should be expected at high latitudes in response to subpolar/subtropical front movements. The North Atlantic is likely to have experienced the most significant changes, due to large variations in the mode and rate of North Atlantic Deep Water production. These hypothetical changes may be measured by coupled AMS C-14 dating of contemporaneous planktonic foraminifera and terrestrial organic matter (pollen, charcoal, wood, etc.) which occur in the same core or are stratigraphically linked by the same volcanic ash layer. The Delta(C-14 atmosphere, C-14 sea surface) can be viewed as a new paleoceanographic tracer which may provide additional information about high latitude surface waters complementary to those obtained with C-13/C-12 and Cd/Ca ratios measured in planktonic foraminifera.