The concentration of atmospheric CO₂ increased rapidly during the last deglaciation due to CO₂ outgassing from the oceans. However, records of deglacial surface seawater pCO_2-sw are sparse, hindering our understanding of the process and mechanism of sea-air CO₂ exchange and its influence on glacial-deglacial climate change. Here we reconstructed surface seawater pCO_2-sw for the last deglacial period using carbon isotope composition (δ¹³C) of giant diatom (Ethmodiscus rex) shells from deep-sea sedimentary column collected in the Philippine Sea, West Pacific. The results showed that the Philippine Sea was a source of CO₂ to the atmosphere during intervals of Heinrich Stadial 1 and the Bølling-Allerød. During the Younger Dryas period, surface seawater pCO_2-sw decreased rapidly and the Philippine Sea became a CO₂ sink. The state of source or sink was dominated by thermocline depth, monsoon and biological productivity. During the Heinrich Stadial 1 interval, upwelling in the West Pacific strengthened as the thermocline shallowed and biological productivity decreased, making the Philippine Sea as a source of atmospheric CO₂. During the Bølling-Allerød warm period, the East Asian summer monsoon enhanced and dominated the air-sea exchange over the Philippine Sea, making it a weak source of CO₂. In the Younger Dryas period, with the rapid weakening of East Asian summer monsoon, the difference between seawater and atmospheric pCO_2-atm (△pCO_2(sw-atm)) decreased rapidly, making the Philippine Sea change from source to sink. Analysis of modern observations also supports the role of monsoon in influencing the process of air-sea CO₂ exchange in the Philippine Sea. Our findings highlight the tropical ocean’s role in deglacial CO₂ outgassing and provide insights into the monsoonal drivers of the process.

Ethmodiscus rex, Carbon isotopes, CO2 outgassing, Western Pacific, Last deglaciation