Ventilation time and anthropogenic CO2 in the South China Sea based on CFC-11 measurements

Abstract

The South China Sea (SCS) is the largest semi-enclosed marginal sea in Southeast Asia, and is bounded by the Asian continent, Philippine Archipelago, and Great Sunda Islands. Due to the wide shelves on its northwestern and southern ends as well as the presence of numerous islets, atolls and reefs, the average depth of the SCS is only 1350m. In this study we used measurements of the transient tracer CFC-11 from the SCS to calculate ventilation time-scales and the concentration of anthropogenic CO2 (Cant) based on the transit time distribution. The CFC-11 concentrations decreased consistently with depth and the deep and bottom water in the SCS had a CFC-11 value close to the detection limit (0.01pmolkg−1 or 0.5ppt). The ventilation times (mean ages) for the deep and bottom water column were ~500–600 years, and based on the mean age profiles the southern part of the intermediate SCS water was older than the northern part. The ventilation time distribution was in agreement with the existence of mean annual cyclonic circulation in the SCS. The mean column inventory of Cant in the northern SCS was 28.9molCm−2 (error range (ER): 22.8–35.6molCm−2), while in the southern SCS it was 28.4molm−2 (ER: 21.9–35.2molCm−2). The total Cant inventory was estimated to be 1 Pg C (ER: 0.8–1.3 Pg C, referenced to the year 2011), suggesting that the SCS stores less Cant then the adjacent seas. The vertical diffusivity was estimated to be 2–4.6×10−4m2s−1 in the SCS based on the “transient steady state”. The upwelling was estimated as 13–34myr−1. The high diffusivity was probably due to the strong internal tide, while the strong upwelling was due to the persistent counterclockwise (cyclonic) circulation.