Atmospheric radiocarbon during the Younger Dryas: production, ventilation, or both?

摘要

A new reconstruction of past atmospheric Δ~(14)C_(atm) values (e.g. from the Cariaco basin record) for the beginning of the Younger Dryas cold vent (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 through 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 (330) 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)_(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.