Glacial–interglacial stability of ocean pH inferred from foraminifer dissolution rates

Abstract

The pH of the ocean is controlled by the chemistry of calcium carbonate. This system in turn plays a large role in regulating the CO2 concentration of the atmosphere on timescales of thousands of years and longer. Reconstructions of ocean pH and carbonate-ion concentration are therefore needed to understand the ocean's role in the global carbon cycle. During the Last Glacial Maximum (LGM), the pH of the whole ocean is thought to have been significantly more basic1, as inferred from the isotopic composition of boron incorporated into calcium carbonate shells, which would partially explain the lower atmospheric CO2 concentration at that time. Here we reconstruct carbonate-ion concentration—and hence pH—of the glacial oceans, using the extent of calcium carbonate dissolution observed in foraminifer faunal assemblages as compiled in the extensive global CLIMAP data set2. We observe decreased carbonate-ion concentrations in the glacial Atlantic Ocean, by roughly 20 µmol kg-1, while little change occurred in the Indian and Pacific oceans relative to today. In the Pacific Ocean, a small (5 µmol kg-1) increase occurred below 3,000 m. This rearrangement of ocean pH may be due to changing ocean circulation from glacial to present times, but overall we see no evidence for a shift in the whole-ocean pH as previously inferred from boron isotopes1.