Water Mass Processes Between the South China Sea and the Western Pacific Through the Luzon Strait: Insights From Hydrogen and Oxygen Isotopes

Abstract

A comprehensive, depth specific (near‐surface to 3,700 m) water isotope (δD and δ18O) data set of 237 water samples collected from 20 different locations from the Northwest Pacific through the Luzon Strait to the Northern South China Sea (NSCS) are used to clarify the deeper understanding of water mass processes using δ18O‐salinity (S) and δD‐δ18O relations. Our study reveals that the δD‐δ18O correlation has a low slope and the δ18O‐S correlation has a high slope, which are related to the intrusion of the Kuroshio water and the influence of the coastal current above 200 m on the NSCS, respectively. The isotope mixing model showed that 15% is from the Kuroshio water, 40% from the South China Sea (SCS) Tropical Water, and 45% from the coastal current for water above 200 m in the NSCS. In the intermediate layer (300–1,200 m), a significant δ18O‐S correlation was found at depths of 300–700 m on the NSCS side, but no correlation was found on the western Pacific side, suggesting that the mixing of the water masses of the South China Sea Intermediate Water and the North Pacific Intermediate Water mainly occurs at depth in the NSCS. In the deeper water (below 1,200 m), positive isotopic values were found at depths below 1,500 m, which refuted the previous knowledge that positive isotopic values only exist in the Luzon Strait at depths above 1,000 m, and the positive isotope water mass would flow with the Deep Pacific Water into the deeper water of the SCS.

Quantification of the Seasonal Intrusion of Water Masses and Their Impact on Nutrients in the Beibu Gulf Using Dual Water Isotopes

“…To reduce the bioavailability and avoid water isotopic fractionation caused by volatilization, parafilm (PM‐996, country of origin: USA) was tightly wrapped around the gap in the bottle cap to prevent secondary pollution or evaporation and then stored at −20°C before analysis. Similar experimental processes for water isotopes have also been reported previously (Chen, Huang, et al., 2021; Lian et al., 2016; Wu et al., 2021; Yu et al., 2016, 2021; Zhou et al., 2022). After field sampling, the Chl a and nutrient samples were measured within 1 week, and the dual water isotope samples were measured within 2 weeks.…”

Using Stable Isotopes (δ18O and δD) to Study the Dynamics of Upwelling and Other Oceanic Processes in Northwestern South China Sea

“…Some investigators have also studied δ 18 O-S relationships of seawater (e.g., Abrahamsen et al, 2009;Wu et al, 2021). The δ 18 O-S relationships are influenced by physical processes, such as precipitation, evaporation, river influx, upwelling, and advection (Achyuthan et al, 2013;Bigg & Rohling, 2000;Singh et al, 2010).…”

“…However, these methods are difficult to quantify the impact of upwelling. δD and δ 18 O in the water can make up for this defect and quantify the contribution of different water mass processes (Bigg & Rohling, 2000;Wu et al, 2021). The coupling of stable isotopes, salinity, and water temperature have been successfully used to discriminate different water masses (Lansard et al, 2012;Lian et al, 2016;Richardson et al, 2018).…”

“…In general, most studies are based on physical methods to study water processes and upwelling in the South China Sea. Until now, only a few studies have used the δ 18 O values of water to study the hydrographic processes in the northeast area of the South China Sea, and found the δ 18 O values of water in this area were generally ranged from −0.4‰ to 1.1‰ (Chen, Lian, et al, 2020;Ren & Wang, 2000;Wu et al, 2021). Based on water isotopes (δ 18 O and δD) together with salinity, the water mass processes between the South China Sea and the western Pacific through the Luzon Strait were quantified that 15% of the Kuroshio water intrudes into the northern the South China Sea (Wu et al, 2021).…”

Comparison of Nitrate Isotopes Between the South China Sea and Western North Pacific Ocean: Insights Into Biogeochemical Signals and Water Exchange

“…One may argue that the observed seasonal outflow of SCS UIW to the wNPO (Tian et al., 2006) could modify the wNPO UIW characteristics since SCS UIW contains relatively lower $${\delta }^{15}{\mathrm{N}}_{{\text{NO}}_{3}}$$ and $${\delta }^{18}{\mathrm{O}}_{{\text{NO}}_{3}}$$ values (Table 2 and Figure 7). However, according to the distributions of radionuclide 226 Ra and water isotopes, this outflow is mainly blocked by the northward KC and confined to the LS (Chen, 2005; Wu et al., 2021), stretching the plausibility of this argument. In addition, tropical intermediate waters appear to be possible candidates for interpreting our data obtained in the wNPO (e.g., Tropical Pacific Intermediate Water, TPIW (Rafter et al., 2012, 2013) and Mindanao Intermediate Water, MinIW (Lehmann et al., 2018, Table 2 and Figure 7).…”