Monsoon- and ENSO-driven surface-water pCO2 variation in the tropical West Pacific since the Last Glacial Maximum

The East Asian monsoon (EAM) and El Nino-Southern Oscillation (ENSO) are large-scale oceanic-atmospheric fluctuations that dominate climate variability in the tropical Pacific Ocean. Although the effects of EAM and ENSO on physical and biological processes are increasingly understood, little is known about their influence on seawater carbonate chemistry in the tropical Pacific, especially in the geological past. Here, we present reconstructed variations in surface-water pCO(2) (pCO(2(sw))) and their deviation from atmospheric pCO(2) (Delta pCO(2(sw-atm))) in the western Philippine Sea (WPS) since 27 ka. Our record displays covariation between Delta pCO(2(sw-atm)) and the intensity of the East Asian summer monsoon (EASM) since the Last Glacial Maximum (LGM), suggesting that EASM-driven upwelling controls long-term changes in surface-water carbonate chemistry and air-sea CO2 fluxes. Rapid changes in Delta pCO(2(sw-atm)) were linked to the ENSO-like state and, to a lesser extent, the East Asian winter monsoon (EAWM) during the Last Deglaciation, with low values corresponding to La Nina-like phases and strong EAWM during Heinrich Event 1, the Allerod and the Younger Dryas, and high values corresponding to El Nino-like phases and weak EAWM during the Bolling and Pre-Boreal. This interpretation is supported by the relationship of EAM and ENSO to modern surface-water carbonate chemistry in the WPS. Our new record, combined with previously published data, suggests that the tropical Pacific played a minimal role in sequestering CO2(atm) during the LGM. Tropical Pacific surface waters overall became a pronounced CO2 source to the atmosphere during the Last Deglaciation, possibly making a substantial contribution to the deglacial pCO(2(atm)) rise. We infer that this flux was mainly due to ENSO-related patterns of vertical stratification or lateral advection, perhaps in addition to equatorial upwelling of southern-sourced waters already enriched in dissolved inorganic carbon. Our findings indicate that tropical conditions (i.e., EAM and ENSO-like state) played a crucial role in glacial-interglacial pCO(2(atm)) changes, suggesting that this is an important area for future research into the causes of glacial pCO(2(atm)) cycles. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the influence of the East Asian monsoon (EAM) and El Nino-Southern Oscillation (ENSO) on seawater carbonate chemistry in the tropical Pacific. The results show a correlation between the intensity of the East Asian summer monsoon and variations in surface-water carbonate chemistry. Rapid changes in carbonate chemistry are linked to ENSO-like states and the East Asian winter monsoon. These findings suggest that tropical conditions play a crucial role in glacial-interglacial atmospheric CO2 concentration changes.