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

Abstract. The South China Sea (SCS) is the world's largest marginal sea, playing an important role in regional biogeochemical cycling of carbon and oxygen. However, its overall metabolic balance, primary production rates links to East Asian Monsoon forcing remain poorly constrained. Here, we report seasonal variations triple oxygen isotope composition (17Δ) dissolved O2, a tracer for biological gross (GP; inferred from δ17O δ18O values) net community (NP; evaluated oxygen–argon ratios) SouthEast Time-series Study (SEATS) SCS. Our results suggest rather stable mixed-layer mean GP ∼ 1500 ± 350 mg C m−2 d−1 NP −13 20 during summer southwest monsoon season. These values indicate, within uncertainties variabilities observed, that metabolism system was balance. During months influenced by stronger northeast forcing, appears be more dynamic with variable rates, which may shift autotrophy (with up 140 d−1). Furthermore, our data deeper regions show SCS circulation strongly affected wind larger part water column down at least 400 m depth fully exchanged winter, suggesting 17Δ deep O2 as valuable novel probing mixing processes. Altogether, findings underscore importance intensity on shifting balance this warm oligotrophic sea driving pattern.