Temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon-based age-estimates of biogenic material that do not agree with true calendar age. This problem is particularly severe beyond the limit of the high-resolution radiocarbon calibration based on tree-ring data, which stretches back only to1,2 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 exploited for better understanding the rates and patterns of environmental change during the last deglaciation, extending the well-calibrated radiocarbon timescale back further in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated corals3,4,5 and laminae counts in varved sediments6,7,8,9, show conflicting results. Here 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 BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that North Atlantic Deep Water formation shut down during the Younger Dryas period, but was gradually replaced by an alternative mode of convection, possibly via the formation of North Atlantic Intermediate Water.
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We thank S. Trumbore and M. Kerwin for comments that improved the manuscript; B. Kromer and S. Björck for sharing new data for German pine and oak tree-ring chronologies; T. Goslar and E. Bard for sharing numerical data from Δ14C modelling results; H.-L. Lin for 14C dates from core PL07-39PC; and J. Moore for help making thin sections. This work was supported by the US NSF and the National Oceanic and Atmospheric Administration, as well as the US Department of Energy through Lawrence Livermore National Laboratory, and by NSF funding to the Lamont-Doherty Earth Observatory Deep-Sea Sample Repository.
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