Water Mass Exchange In The Nearshore,Offshore And Deep Sea Of The Northern South China Sea: Insights From Hydrogen And Oxygen Isotopes

Hydrogen and oxygen are the constituent elements of water,and the stable isotopic compositions of hydrogen and oxygen(?D and?¹⁸O)and salinity(S)in seawater are effective tracers for studying the traces of modern oceanic physical processes such as runoff,sea ice melting/formation,and brine release due to large-scale ocean mixing.The northern South China Sea has a variety of physical processes such as Kuroshio,Yuexi coastal current,and upwelling,and the movement and mixing among these physical processes have important effects on the transport of materials and human activities in the northern South China Sea.In this study,we took advantage of stable water isotopes(?D and?¹⁸O)to trace the movement of water masses and water exchange,and sampled the Luzon Strait,offshore of western Guangdong and Zhanjiang Bay to reveal the water exchange processes in the nearshore,offshore and deep sea of the northern South China Sea.The main findings are as follows:(1)By sampling and analyzing a total of 20 stations around the Luzon Strait,the water mass exchange processes between the South China Sea water and the West Pacific water at different depths were summarized from the perspective of hydrogen and oxygen isotopes.Based on the correlation between potential temperature and salinity(?-S),three water masses were identified,including surface and subsurface water(above 200 m),representing North Pacific Tropical Water and South China Sea Tropical Water;intermediate layer(300-1200 m),representing North Pacific Intermediate Water and South China Sea Intermediate Water;and deep water(below1200 m),representing Pacific Deep Water.The results show that Kuroshio water accounts for 15%,South China Sea Tropical Water accounts for 40%,and nearshore freshwater accounts for 45%.Combining the correlations of?D-?¹⁸O and?¹⁸O-S,it is found that the mixing of South China Sea Intermediate Water and North Pacific Intermediate Water water masses in the middle water layer(300-1200 m)mainly occurs in the north South China Sea.For deep water(below 1200 m),the PDW with positive isotopic values crosses the Hengchun Ridge into the South China Sea.Here,for the first time in the Luzon Strait,this study suggests that the vertical profiles of water column?D and?¹⁸O can be used to identify water masses and related physical processes in the Luzon Strait.(2)The distribution and exchange processes of water masses between land runoff,nearshore and offshore in western Guangdong are summarized by the distribution characteristics of seawater hydrogen and oxygen isotopes,combined with the distribution characteristics of T and S.In spring,the western Guangdong coastal waters are still under the influence of northeast monsoon,upwelling is not active,and the coastal current of western Guangdong still flows westward along the nearshore.The upwelling water masses in western Guangdong with high S,low T and high?¹⁸O enrichment are concentrated in the bottom layer of western Guangdong offshore waters,while the coastal currents in western Guangdong with high T,low S and high?¹⁸O enrichment occupy the upper layer of western Guangdong offshore waters,and the water exchange is low at this time.The correlation analysis of?¹⁸O-S shows that the oceanic water line in the offshore waters of western Guangdong in spring is closer to the global oceanic water line,while the southwest monsoon in summer strengthens the movement of the western Guangdong upwelling,which climbs toward the nearshore land slope,and deviates from the global oceanic water line with the western Guangdong coastal current influenced by the land runoff.The westward flowing western Guangdong coastal current is strengthened by the influence of the extremely depleted?D and?¹⁸O Jianjiang River alluvial freshwater and the low-saline,?¹⁸O depleted Pearl River alluvial freshwater,while making the?D and?¹⁸O enriched in spring depleted,resulting in the depleted hydrogen and oxygen isotopes in the summer samples.(3)In this study,a total of 189 seawater samples were collected from 26 stations in Zhanjiang Bay in summer and winter of 2018,respectively.These parameters of T,S,?D and?¹⁸O in Zhanjiang Bay seawater were analyzed to understand the mixing of freshwater input from Suixi River and South China Sea water in Zhanjiang Bay waters.The water composition of the semi-enclosed Zhanjiang Bay is mainly controlled by the runoff input represented by the Suixi River and the mixing of South China Sea water.The results show that the South China Sea water is influenced by coastal and upwelling flows in western Guangdong in summer and has a strong water exchange with Zhanjiang Bay water bodies.The?D and?¹⁸O distributions show the mixing of depleted Suixi River water with enriched South China Sea water within Zhanjiang Bay,while there is no significant water exchange in winter.Meanwhile,the analysis combined with the hydrogen and oxygen isotope data from western Guangdong shows that the?D(-19.97‰)and?¹⁸O(-2.50‰)at station L1,which represents the Nandu River(900 million m³ of runoff)within Leizhou Bay,are only slightly lower than the?D(-17.59‰)and?¹⁸O(-2.51‰)at station Z25,which represents the Suixi River(1.4billion m³ of runoff)within Zhanjiang Bay.This indicates that the freshwater flow of Suixi River into Zhanjiang Bay may be decreasing,which will lead to an increase in the share of South China Sea water within Zhanjiang Bay,and the backflow of seawater makes the S rise and even makes the water exchange disappear,which will have a serious impact on the Zhanjiang Bay ecosystem as well as human production activities in the future.Therefore,measures should be taken to control the withdrawal of Suixi River and to ensure the runoff of Suixi River water into Zhanjiang Bay.