Inferring Surface Water Equilibrium Calcite δ18O during the Last Deglacial Period from Benthic Foraminiferal δ18O Records: Implications for Ocean Circulation

Abstract:

The ocean mass transport communicates tracers from the ocean mixed layer (ML) to the deep ocean on many time scales. When they are sufficiently well-resolved in time, paleoceanographic tracer proxy time series at multiple locations sample the propagation of tracer signals through the global ocean and can be used to constrain the ocean circulation, including during times of climate transience. Using a Green function approach, we derive a time-dependent solution for ML equilibrium calcite δ¹⁸O (δ¹⁸O_c) during the last deglaciation from a least-squares fit of eight benthic foraminiferal δ¹⁸O records to an estimate of the modern ocean circulation. Within uncertainty, inferred deglacial histories of ML δ¹⁸O_c in the Northern North Atlantic and in the Southern Ocean mixed layers are indistinguishable. A comparison of the derived solution to planktonic foraminiferal δ¹⁸O records is unable to disprove the null hypothesis that the modern ocean circulation persisted during the last deglaciation. In order to disprove the null hypothesis, it is necessary to reduce uncertainties in inferred ML δ¹⁸O_c histories, perhaps by including additional records. The utility of benthic records to reduce uncertainties is quantified, with implications for future paleoceanographic sampling strategies.