Glacial carbonate compensation in the Pacific Ocean constrained from paired oxygen and carbonate system reconstructions

The response of the deep ocean carbonate system and CaCO_3 dissolution to changes in the carbon cycle (‘carbonate compensation’) is a first order control on atmospheric CO_2 on timescales of ~10^3 to 10^5 years. Although carbonate compensation could account for up to ~half of the glacial drawdown of CO_2, quantitative estimates of changes in ocean alkalinity are lacking. As such, the role of carbonate compensation in driving glacial-interglacial CO_2 variations remains poorly understood.

Here, we combine reconstructions of dissolved oxygen from the infaunal-epifaunal benthic foraminiferal δ^13C proxy (Δδ^13C) and carbonate system reconstructions from boron proxies (B/Ca, δ^11B) in benthic foraminifera to quantify changes in both respired CO_2 storage within the deep Pacific during the last glacial, and the response of the carbonate system to this addition/removal of respired CO_2. Our results provide the first quantitative estimates of the amount and timing of alkalinity changes in the deep Pacific over the last deglaciation. Our results indicate an increase in deep ocean alkalinity during the Last Glacial Maximum, and indicate the buffering of the deep ocean occurs substantially faster than the canonical timescale of ~5 kyr (Broecker and Peng, 1987). We also present results from a series of sensitivity experiments and long-term simulations using the recently coupled iLOVECLIM-MEDUSA climate/carbon-cycle/sediment model, with implications for our understanding of carbonate compensation in both glacial times, and the long-term future.