Preprints
https://doi.org/10.5194/bg-2020-448
https://doi.org/10.5194/bg-2020-448

  15 Jan 2021

15 Jan 2021

Review status: this preprint is currently under review for the journal BG.

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

Hana Jurikova1, Osamu Abe2, Fuh-Kwo Shiah3, and Mao-Chang Liang4 Hana Jurikova et al.
  • 1School of Earth and Environmental Sciences, University of St Andrews, KY16 9AL St Andrews, United Kingdom
  • 2Graduate School of Environmental Studies, Nagoya University, 464-8601 Nagoya, Japan
  • 3Research Center for Environmental Changes, Academia Sinica, 11529 Taipei, Taiwan
  • 4Institute of Earth Sciences, Academia Sinica, 11529 Taipei, Taiwan

Abstract. South China Sea (SCS), world’s largest marginal sea, plays an important role in the global as well as regional biogeochemical cycling of carbon and oxygen. However, its overall metabolic balance, primary production rates, and their link to East Asian Monsoon forcing still remain poorly constrained. Here, we report seasonal trends 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 SCS. Our results suggest stable mixed-layer GP rates of 1.8 g C m−2 d−1 and NP of −0.02 g C m−2 d−1 during the summer southwest monsoon, indicating the prevalence of net heterotrophy. During winter months characterised by stronger northeast monsoon forcing, the system is more dynamic with variable production rates, which may shift the metabolism from net heterotrophy to net autotrophy (NP up to ~0.15 g C  m−2 d−1). These findings underscore the importance of monsoon intensity on tilting the carbon balance from source to sink in a warm oligotrophic sea, and on driving the regional circulation pattern. Finally, our data from the deeper regions show that 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 conservative tracer for probing mixing processes from a new perspective.

Hana Jurikova et al.

Status: open (until 17 Mar 2021)

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Hana Jurikova et al.

Hana Jurikova et al.

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