Glacial to interglacial changes in ocean chemistry

Abstract

Surface waters of the modern Western Tropical Pacific (WTP) are in equilibrium with atmospheric CO2. However, air-sea exchange of CO2 in this region may have been modulated in the past by oceanic-atmospheric fluctuations in the tropical Pacific such as the East Asian monsoon and the El Niño-Southern Oscillation (ENSO) and extratropical mode waters such as Antarctic Intermediate Water. Thus, understanding controls on the sea-surface carbonate system in the WTP is important for forecasting future carbon-cycle changes in this region. Here, we reconstruct sea-surface pH and pCO2 since Marine Isotope Stage 6 (MIS 6; 155 ka) based on B/Ca ratios of the planktic foraminifer Globigerinoides ruber (white) in sediment Core MD06–3052 from the western Philippine Sea, and we then calculate the difference between oceanic and atmospheric pCO2 (ΔpCO2(sw-atm)) in order to evaluate the history of air-sea CO2 exchange. ΔpCO2(sw-atm) changes were strongly modulated by the ∼20-kyr precession cycle. The results of cross-spectral analysis demonstrate a close connection between the East Asian summer monsoon (EASM) and air-sea CO2 exchange since MIS 6, demonstrating that precession-driven EASM can affect air-sea CO2 exchange through regulation of surface productivity and thermocline depth. In contrast, the East Asian winter monsoon (EAWM) and ENSO-like conditions are not major influences on air-sea CO2 exchange in the study area at precession-band frequencies. In addition, enhanced upwelling of Southern Ocean-sourced deepwater rich in dissolved inorganic carbon (DIC) affected the upper water column during transitions from cold to warm stages (i.e., deglaciations). In conclusion, these findings suggest that orbital precession influences can affect oceanic conditions not only through climate change and biological processes but also through sea-surface carbonate chemistry.