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Carbonate dynamics in a tropical coastal system in the South China Sea featuring upwelling, river plumes and submarine groundwater discharge

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Abstract

This study examined carbonate dynamics in the northwestern South China Sea (NWSCS), an area jointly influenced by upwelling, river plumes and submarine groundwater discharge. Data were obtained from two cruises conducted in summer 2009 and 2012. In 2009, a high salinity-low temperature water mass occurred nearshore off northeastern Hainan Island, indicative of upwelling, commonly referred to as HNEU. A river plume fueled primarily by local rivers and characterized by low salinity and high temperature was observed in the NWSCS off the mainland roughly along the 30 m isobath. In 2012, coastal upwelling off northeastern Hainan Island was not detectable at the surface, but was observed at a different location off eastern Hainan Island (HEU). River plume waters in 2012 were patchily distributed, with a low salinity zone further westerly than that in 2009 and another on the mid-shelf of the NWSCS sourced from the Pearl River which reached out ∼250 km from the mouth of the Pearl River Estuary. In 2009, elevated dissolved inorganic carbon (DIC) and total alkalinity (TA) occurred in the coastal plume, where submarine groundwater discharge contributed DIC and TA additions of 38.9±20.5 and 42.5±22.3 µmol kg−1, respectively, with a DIC/TA ratio of ∼0.92, which made a minor contribution to the variation of seawater partial pressure of CO2 (pCO2), pH and the aragonite saturation state index (Ωarag). Additionally, high surface phytoplankton production consumed DIC of 10.0±10.4 µmol kg−1 but did not significantly affect TA, which dominated pCO2 drawdown in the coastal plume water and increased the pH and Ωarag at surface. Submarine groundwater discharge was also observed in the region influenced by upwelling, but to a lesser degree than that impacted by coastal plume. Lower pH and Ωarag and higher pCO2 values than in offshore waters were observed downstream of the upwelling system, attributable largely to organic matter remineralization with a DIC addition of 23.8±8.4 µmol kg−1. In 2012, submarine groundwater discharge was not detected but high phytoplankton production dominated carbonate dynamics in the coastal plume water with a net DIC consumption of 104.2 µmol kg−1, which markedly drew down sea surface pCO2 and increased pH and Ωarag. In the Pearl River Plume, the solubility-driven CO2 sink exceeded biological CO2 uptake, resulting in an additional decrease of pH and Ωarag and increase of seawater pCO2. Taken together, this study demonstrated complex spatial and year-to-year variability, and the controls of the carbonate system under the joint modulations of upwelling, river plumes and submarine groundwater discharge. A first order estimate that considered the rise of atmospheric CO2 and seawater temperature further suggested a high risk of ocean acidification in this coastal area by the end of this century, which could be amplified under the stresses of river plumes, submarine groundwater discharge and organic matter remineralization.

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Acknowledgements

We thank Drs. Weidong ZHAI and Pinghe CAI who led the cruises as chief scientists, and Nan ZHENG, Yan YANG, Junqi ZHANG, Qian LI, Xu DONG, Yuancheng SU, Di QI and Bo YANG for their help in data collection. Jia ZHU and Zhenyu SUN assisted with CTD data collection and processing. We are grateful to the crews of the R/V Dongfanghong II for their help during the cruises. The authors declare that the study was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. This study was funded by the National Natural Science Foundation of China (Grant Nos. 42188102, 41206061) and the Hong Kong Research Grants Council (Grant No. T21-602/16-R).

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Correspondence to Minhan Dai.

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Appendix Table A The results of LA-SF-ICP-MS U-Pb dating for detrital zircons from sediments in Gaoping River and northern Manila Trench.

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Yang, W., Guo, X., Cao, Z. et al. Carbonate dynamics in a tropical coastal system in the South China Sea featuring upwelling, river plumes and submarine groundwater discharge. Sci. China Earth Sci. 65, 2267–2284 (2022). https://doi.org/10.1007/s11430-021-9963-8

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