Pacific Ocean-originated anthropogenic carbon and its long-term variations in the South China Sea

Coastal oceans, traditionally seen as a conduit for transporting atmospheric carbon dioxide (CO₂)-derived anthropogenic carbon (C_ANT) to open oceans, exhibit complex carbon exchanges at their interface. South China Sea (SCS) exemplifies this complexity, where interactions with the Pacific, particularly through Kuroshio intrusion, challenge the understanding of C_ANT source and variability in a coastal ocean. Contrary to prevailing paradigm expectations, our high-resolution, long-term data reveal that C_ANT in the SCS primarily originates from Pacific water injection across the Luzon Strait rather than atmospheric CO₂ invasion. Over the past two decades, the SCS has experienced increasing C_ANT levels, with notable interannual fluctuations driven by El Niño and La Niña events influencing Kuroshio intrusion, generating anomalously high and low C_ANT inventories, respectively. This highlights an overlooked C_ANT transport pathway from open to coastal oceans, responsible for cumulative ocean acidification that has already affected coral reefs enriched in the SCS located west of the Coral Triangle.