Evidence for increased deep ocean carbon storage during the mid Pleistocene climate transition

The mid Pleistocene transition (MPT) is an intensely studied interval of recent geologic time because it offers an opportunity to better understand internal climate feedbacks and thresholds. Here we show that the evolution towards larger, 100 kyr paced, glacial maxima may have been associated with increased carbon storage in the deep ocean. Our multiproxy approach combines proxies of paleoproductivity (alkenone mass accumulation rates from Lawrence et al. (2013), Uvigerina and Radiolarian counts) with deep water redox conditions (planktic foraminiferal U/Ca), nutrient content (benthic foraminiferal Cd/Ca), and carbonate saturation state (benthic foraminiferal B/Ca, fragmentation index) from Site 607. Lying in the core of North Atlantic Deep Waters, this site has been critical in reconstructing Plio/Pleistocene deep water circulation. Trace metal results indicate a tight coupling between benthic foraminiferal δ13C values, B/Ca, and Cd/Ca ratios across the MPT. Low δ13C is associated with low B/Ca and high Cd/Ca suggesting that the incursion of Antarctic Bottom Water during glacial intervals occurred in conjunction with reduced bottom water saturation state and increased nutrient content. The similarity among U/Ca ratios and alkenone mass accumulation rates and fluctuations in radiolarian abundances suggests that contemporaneous changes in regional productivity also impacted porewater oxygenation. Results demonstrate that while the amplitude of benthic foraminiferal δ13C variations increases during the mid-point of the MPT, the amplitude of the metal/Ca ratios remains the same. To explain this discrepancy we propose that δ13C values must have been affected by source region processes such as reduced air-sea CO2 exchange as potentially associated with the expansion of sea ice. Reduced CO2 outgassing provides a mechanism for lower glacial atmospheric pCO2 thus providing an internal positive feedback mechanism for the growth of increasingly larger glaciations.