| Summary: | A new reconstruction of past atmospheric Δ 14 C (Δ 14 C atm ) based on Polish lake varved sediments has suggested that previous Δ 14 C atm values (e.g. from the Cariaco basin record) for the beginning of the Younger Dryas cold event (YD) are overestimates and that the Δ 14 C atm rise at the YD onset could only be due to changes in atmospheric 14 C production ( P C-14 ). This result would have profound climatic implications, for the YD is a paradigm example of abrupt climate change which is usually thought to have been triggered by a reduction in the northward heat flux by the Atlantic thermohaline circulation. Here we examine results from a large number of simulations (300) based on a zonally averaged ocean circulation model, to constrain the effect on Δ 14 C atm of P C-14 changes during the YD as reconstructed from a Greenland ice core record of 10 Be flux. Our results suggest that the scatter in the lake data set is too large to exclude the probable change in deep ocean ventilation at the onset of the YD. By contrast, the model fit to the higher Δ 14 C atm levels throughout the YD detected in the marine record is generally better when a substantial decrease in deep ocean ventilation is simulated. The early Δ 14 C atm drawdown that initiated during the first half of the YD, however, could entirely be due to production changes. If this was the case, the drawdown would not reflect an increasing formation of North Atlantic Intermediate Water or Southern Ocean water, as previously suggested. The rapid Δ 14 C atm rise at the YD onset documented in the marine record, however, remains unexplained.
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