DEGLACIAL CHANGES IN OCEAN CIRCULATION FROM AN EXTENDED RADIOCARBON CALIBRATION

Temporal variations in the atmospheric concentration of radiocarbon so metimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age, This problem is particularl y severe beyond the limit of the high-resolution radiocarbon calibrati on based on tree-ring data, which stretches back only to(1,2) about 11 .8 kyr before present (BP), near the termination of the Younger Dryas cold period If a wide range of palaeoclimate records are to be exploit ed for better understanding the rates and patterns of environmental ch ange during the last deglaciation, extending the well-calibrated radio carbon timescale back further in time is crucial. Several studies atte mpting such an extension, using uranium/thorium-dated corals(3-5) and laminae counts in varved sediments(6-9), show conflicting results, Her e we use radiocarbon data from varved sediments in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate and continuous radiocarbon calibration for the period 9 to 14.5 kyr sp, nearly 3,000 years beyond the tree-ring-based calibration, A simple model compared to the calculated atmospheric radiocarbon concentration and palaeocli mate data from the same sediment core suggests that North Atlantic Dee p Water formation shut down during the Younger Dryas period, but was g radually replaced by an alternative mode of convection, possibly via t he formation of North Atlantic Intermediate Water.