Tracing of water mass and anthropogenic CO₂ in the northeastern South China Sea traced by CFC-12 and SF₆

Dichlorodifluoromethane (CFC-12) and sulfur hexafluoride (SF₆) serve as important tracers due to their distinct anthropogenic sources and high conservativeness in oceanographic studies, which could indicate ocean water mass transport, dating, and anthropogenic carbon dioxide (CO₂) storage. The surveys were conducted in the northeastern South China Sea (SCS) from August to September, 2023, to investigate the spatial distributions and influencing environmental parameters and to reveal the tracer role of CFC-12 and SF₆ in water mass transport in the study area. Furthermore, anthropogenic CO₂ storage was quantified using the transit time distribution (TTD) method in SCS. Results showed that the average concentrations of CFC-12 and SF₆ are (1.85±0.82) pmol/L and (0.88±0.48) fmol/L in the surface seawater of SCS, respectively. The tracer distributions exhibited distinct spatial gradients, with concentrations decreasing progressively from northern to southern regions and from the inshore to the offshore. This spatial pattern reflected the combined influences of terrestrial input sources and air-sea exchange processes. The maximum concentrations of CFC-12 and SF₆ were observed in the surface layer, and their concentrations decreased with depth monotonically below the surface layer. The vertical profiles of CFC-12 and SF₆ revealed contrasting tracer gradients across the Luzon Strait, with a decreasing trend from the South China Sea to the Northwest Pacific in the intermediate water, while an opposite trend was observed in the deep water. The results confirmed that the “sandwiched vertical structure” of water mass transport in the Luzon Strait. The mean anthropogenic CO₂ column inventories were estimated to be (37.49 ± 7.50) mol/m² in the South China Sea. The total anthropogenic carbon storage in SCS was approximately (0.92 ± 0.18) Pg, with a growth rate of 20 Tg/a, which stored about 0.05% of global anthropogenic CO₂ emissions annually.