New constraints on biological production and mixing processes in the South China Sea from triple isotope composition of dissolved oxygen

The South China Sea (SCS) is the world's largest marginalsea, playing an important role in the regional biogeochemical cycling ofcarbon and oxygen. However, its overall metabolic balance, primaryproduction rates and links to East Asian Monsoon forcing remain poorlyconstrained. Here, we report seasonal variations in triple oxygen isotopecomposition (¹⁷Δ) of dissolved O₂, a tracer for biologicalO₂, gross primary production (GP; inferred from δ¹⁷O andδ¹⁸O values) and net community production (NP; evaluated fromoxygen–argon ratios) from the SouthEast Asian Time-series Study (SEATS) inthe SCS. Our results suggest rather stable mixed-layer mean GP rates of∼ 1500 ± 350 mg C m⁻² d⁻¹ and mean NP of∼ −13 ± 20 mg C m⁻² d⁻¹ during the summersouthwest monsoon season. These values indicate, within uncertainties andvariabilities observed, that the metabolism of the system was in netbalance. During months influenced by the stronger northeast monsoon forcing,the system appears to be more dynamic and with variable production rates,which may shift the metabolism to net autotrophy (with NP rates up to∼ 140 mg C m⁻² d⁻¹). Furthermore, our data from thedeeper regions show that the SCS circulation is strongly affected by monsoonwind forcing, with a larger part of the water column down to at least 400 mdepth fully exchanged during a winter, suggesting the ¹⁷Δ ofdeep O₂ as a valuable novel tracer for probing mixing processes.Altogether, our findings underscore the importance of monsoon intensity onshifting the carbon balance in this warm oligotrophic sea and on drivingthe regional circulation pattern.