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 marginal sea, playing an important role in the regional biogeochemical cycling of carbon and oxygen. However, its overall metabolic balance, primary production rates and links to East Asian Monsoon forcing remain poorly constrained. Here, we report seasonal variations in triple oxygen isotope composition (17Δ) of dissolved O2, a tracer for biological O2, gross primary production (GP; inferred from δ17O and δ18O values) and net community production (NP; evaluated from oxygen–argon ratios) from the SouthEast Asian Time-series Study (SEATS) in the 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 summer southwest monsoon season. These values indicate, within uncertainties and variabilities observed, that the metabolism of the system was in net balance. 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 the deeper regions show that the SCS circulation is strongly affected by monsoon wind forcing, with a larger part of the water column down to at least 400 m depth fully exchanged during a winter, suggesting the 17Δ of deep O2 as a valuable novel tracer for probing mixing processes. Altogether, our findings underscore the importance of monsoon intensity on shifting the carbon balance in this warm oligotrophic sea and on driving the regional circulation pattern.