Study On Water Mass Transportation And Decadal Changes Of Anthropogenic CO₂ In The South China Sea And The Western Pacific Ocean Based On CFC-12 And SF6 Measurements

The South China Sea is one of the largest marginal seas in the western Pacific Ocean.The Luzon Strait is the only deep channel linking the South China Sea(SCS)and the Pacific.It plays an important role in the exchange of water between the two basins.The flow parttern around the Luzon Strait is extremely complex wich could be clearly shown from the physical and chemical properties.As the transient tracers,CFCs and SF6 have well time-dependent source functions and are widely used to understand oceanic processes.However,few transient traccer studies have been carried out in the SCS and the western Pacific.In this study,the mean ages and Cant concentrations in the SCS and the western Pacific were calculated with the TTD method based on the CFC-12 and SF6 measurements.A purge-trap gas chromatography system was established to measure CFC-12 and SF6,simultaneously.The detection limit were 0.03 fmol kg-1 for SF6 and 0.02 pmol kg-1 for CFC-12,the analytical precision of SF6 was ±0.5%and that of CFC-12 was ±1.2%,respectively.The correlation coefficients(R2)of all the calibration curves were greater than 0.9995.In the mixed layer of the SCS and the western Pacific,the SF6 concentration was close to equilibrium with the present atmosphere.The concentration of SF6 decreased with depth monotonically.The horizontal distribution of SF6 in the western South China Sea shown that there are significant horizontal gradients in the central area off Vietnam throughout the entire water column.The result was consistent with the previous studies that upwelling takes place off the Vietnam coast and there was a northeastward coastal jet in summer during the southwest monsoon.The influence of the upwelling takes place off the Vietnam could not reach 500 m.The existence of salinity maximum and minimum in the SCS and the resemblance between the deep waters of the two seas reflected the influence of water intrusion from the west Pacific to the SCS.The salinity increased for the intermediate water in the SCS likely came from the overlying main thermocline since the salinity in the underlying deep water remained almost unchanged.For subsurface and upper intermediate layers,the tendency of CFC-12 and mean age distribution were consistent with an annual mean cyclonic circulation in the SCS.The Luzon Strait acted as a transition zone in the upper hundred meters.The CFC-12 observed in the Luzon Strait supported the notion of intrusion of subsurface and upper intermediate waters from the western Pacific.A mean transit time of 77±15 years for the North Pacific Intermediate Water in the SCS interior can be estimated.Compared the CFC-12 distribution in western Pacific along 130°E transect in the 1996 from the 2016,we found the concentration of CFC-12 above 1 000 m in 1996 was significantly lower than that of 2016,which revealed the migration process of CFC-12 from the surface into the interior of the ocean in the past 20 years.The Cant column inventory along 130°E transect in western Pacific was about 32.23±6.45 mol m'2 in the upper 1 500 m in the 2016,higher compared to the 1996 about 24.47±4.89 mol m-2.The Cant column inventory in the 130°E section increased by approximately 31.7%(7.76 mol m'2)over 20 years.The horizontal distribution of Cant showed that the western Pacific water had an important influence on the Cant distribution of the South China Sea.The CFCs/SF6 tracer combination were used to calculated the Cant concentrations.The mean Cant column inventories in SCS were estimated to 33,29±6.66 mol m-2,which was close to 31.18±6.24 mol m-2 only calculated with CFC-12 data.Comparing the mean Cant column inventories in the SCS with the result estimated by Huang(2016),the Cant storage increased by approximately 16.2%(4.64 mol m-2)during 5 years.The total Cant inventory of the SCS is estimated to 0.77±0.15 Pg.