Early Pleistocene pCO2 revisited: Boron-based records of carbon dioxide from Atlantic ODP Site 999

Various mechanisms have been proposed for the cooling climate from the warm Pliocene into the Pleistocene ice ages. Such studies often invoke tectonic changes such as uplift and erosion of the Himalayan-Tibetan Plateau and the closure of the Panama seaway as drivers for this shift in global climate. Atmospheric CO2 also plays a central role in climate and declined from the Pliocene into the Pleistocene. Here we examine ocean pH, atmospheric pCO2, and carbonate ion concentration from the latest Pliocene into the early Pleistocene through marine geochemical reconstructions from Atlantic ODP Site 999. Boron isotope ratios (δ11B) from the tests of fossil planktic foraminifera offer an opportunity to reconstruct ocean pH and carbon cycle changes, though independent estimates of other ocean parameters, such as temperature, are also required. Here we present a new record of planktic foraminiferal Mg/Ca (a sea surface temperature proxy) from Site 999 from 4.0 to 2.0 million years ago. Combined with a previously published δ11B record [Bartoli et al., 2011, Paleoceanography] and reasonable assumptions for surface ocean alkalinity, we recalculate pCO2 using a consistent method. Our results suggest the reduction in atmospheric carbon dioxide concentration was gradual over the Plio-Pleistocene transition. Though the pCO2 decline was not dramatic, climate thresholds and feedbacks could have resulted in a relatively rapid buildup of Northern Hemisphere ice sheets.