An investigation of the oceanic redistribution of carbon during the last deglaciation.

摘要

Records from ice and marine sediment cores have revealed significant changes in the ocean and atmospheric carbon reservoirs at the end of the last glacial period: atmospheric CO2 concentrations increased by ∼50%, the atmospheric radiocarbon activity (Delta14C) declined by 190‰, the Delta14C of intermediate depth waters in the eastern North Pacific Ocean became extremely low relative to the contemporaneous atmosphere, and planktic foraminifera in the tropical and southern high latitude oceans experienced a negative excursion in delta13C. A promising theory for explaining these changes involves increased storage of carbon in the deep ocean during the last glacial period due to reduced upwelling and air-sea gas exchange in the Southern Ocean. Poor ventilation would have led this deep water mass to become extremely depleted in 14C and 13C. During deglaciation, as upwelling in the Southern Ocean increased, this isolated deep water would have been mixed back in to ocean surface, spreading low Delta14C and delta13C carbon into the upper ocean and atmosphere.;In this dissertation, I investigate this hypothesis by reconstructing intermediate water Delta14C during the last deglaciation using two marine sediment cores from the northern Arabian Sea and one core from the margin of southern Chile. The Arabian Sea Delta14C records demonstrate that intermediate waters become extremely old during the deglaciation. These results along with previous results from the North Pacific suggest that 14C-depleted waters upwelled to the surface in the Southern Ocean and were transported northward as intermediate waters. Intermediate water Delta14C gradients along the Chile margin, however, were similar to modern, indicating no evidence for large 14C depletions. These seemingly contradictory results can be explained by regional variability in upwelling and intermediate water formation in the deglacial Southern Ocean. I also examine the relationship between the deglacial Delta 14C and delta13C minima using a compilation of published Delta 14C and delta13C records from the deep ocean during the last glacial maximum and new measurements of d13C in planktic foraminifera from the same Baja California and Arabian Sea cores that record the Delta 14C minima. These data demonstrate that the distribution of Delta 14C and delta13C in the glacial deep ocean was significantly different than in the modern ocean and indicate that the deep Southern Ocean could have supplied 14C- and 13C-depleted carbon to the upper ocean and atmosphere during the deglaciation. Declines in planktic delta 13C are observed at the start of the deglaciation coincident with the decline in intermediate water Delta14C. However, the delta 13C records are complicated by changes in local upwelling and productivity at these sites. In summary, the data presented here provide evidence for the redistribution of 14C- and 13C-depleted carbon from the deep ocean to the upper ocean and atmosphere and are consistent with mechanisms involving changes in Southern Ocean stratification and upwelling during the last deglaciation.