Recent deep water ventilation in the South China Sea and its paleoceanographic implications

The South China Sea (SCS), being the largest marginal sea with high productivity in the western Pacific, was recently found to be a weak CO2 source based on the modern observation, probably caused by the vertical mixing of deep water within this basin. In order to reveal the distribution of apparent ventilation age of intermediate (water depth of 500-1500 m) and deep (water depth > 1500 m) waters and hence to explore deep water circulation in the SCS, the accelerator mass spectrometry (AMS) radiocarbon ages of co-existing benthic and planktic foraminifers from 13 surface and 4 core-top samples were measured. The results show that the recent deep water ventilation ages (Δ14CB-P ages) are between ~1790 and 2440 years in the northern SCS, comparable with the modern values of the deep western Pacific, implying the external impact of the penetration of Pacific Deep Water (PDW) across the Luzon Strait. Along the flow direction, the average ventilation ages decrease from ~1850 years to ~1330 years southward in the deep layer and from ~1600 years to ~1390 years northward in the intermediate layer. This phenomena may be attributed to the intra-basin well-ventilated process involving strong vertical deep mixing and advection. Our results not only validate the “sandwich” structure of deep water exchange between the SCS and Pacific: the deep water inflows from the western Pacific while the intermediate water flows out of the SCS, but also find the existence of the intermediate water with old carbon and low dissolved oxygen content in the southern SCS for the first time. Overall, the consistence of deep water ventilation ages with the physical-biogeochemical processes in the SCS is supported by the modern δ13C distribution of benthic foraminifers, further confirming that the deep circulation influences the CO2 release from the SCS to the atmosphere.