Release of CO2 from the subarctic Pacific Ocean over the last deglaciation

Abstract:

The ~90 ppmv increase in atmospheric CO₂ over the last deglaciation was most likely driven by changes in the way the high latitude oceans regulate the exchange of dissolved inorganic carbon (DIC) between the ocean and atmosphere, however the mechanisms involved in this repartitioning remain unclear. The subarctic Pacific Ocean is currently a significant source of CO₂ to the atmosphere due to mixing of DIC-rich intermediate waters into the surface ocean and incomplete nutrient utilisation. It is hypothesised that during the last glacial period, greater stratification in the high latitudes, including the subarctic Pacific, reduced CO₂ outgassing and increased DIC storage within the deep ocean. During deglaciation, a breakdown in ocean stratification returned the deeply sequestered DIC back to the surface ocean and atmosphere.

We present a new deglacial record of carbonate ion concentration [CO₃^2-] and bottom water temperature from the intermediate to deep subarctic Pacific Ocean based on paired Li/Ca and Mg/Ca ratios in benthic foraminifera Uvigerina senticosa from marine sediment core MD01-2416 (51.268°N, 167.725°E, 2317 m water depth). The data indicate a decrease in [CO₃^2-] within the intermediate-deep waters of ~40 µmol/kg during glacial times, equivalent to a ~150 µmol/kg increase in DIC using modern alkalinity. At ~16 ka, an increase in [CO₃^2-], coeval with warmer bottom waters and an increase in benthic ∆¹⁴C, denotes a transfer of DIC out of the intermediate-deep subarctic Pacific Ocean. Boron isotopes measured on planktonic foraminifera Neogloboquadrina pachyderma (s) in the same core show surface waters in the subarctic Pacific had a higher pH during glacial times, and a reduction in pH over deglaciation. This trend supports the hypothesis that CO₂ outgassing from the subarctic Pacific was reduced during the last glacial and increased during deglaciation.