Distributions Of Radium Isotopes Around The Luzon Strait And In The Northern South China Sea Basin:Their Tracing Significance

The South China Sea(SCS)has characteristics of both a marginal sea and an open ocean.Therefore it becomes an ideal area in studying processes and mechanisms of marine biogeochemical cycles.Furthermore,the residence time,origin and composition of the water masses are key parameters to have insight into the marine biogeochemistry in SCS.As a typical Western Boundary Current(WBC),the Kuroshio carrying the northwestern Pacific water intrudes into the SCS through the Luzon Strait,significantly affecting the physical field and the biogeochemistry of SCS.Prior researches have been studying Kuroshio intrusion into SCS,the residence time,origin and composition of water masses mainly by traditional methods of physical oceanography(hydrological observations and/or models),and the results are controversial.Little research has been done in quantitative analysis by using chemical oceanography methods.Radium isotopes are excellent tracers in studying movements and mixing of water masses in oceanography due to their special geochemical behaviors and various half-lives.Primarily using Ra as tracers,we focused on the northern SCS basin and the area around the Luzon Strait,and aimed to quantitatively examine essential features of Kuroshio intrusion.Furthermore,we estimated the fraction of the Kuroshio surface water in the northeastern SCS,and then calculated the residence time of surface water in this area.The main research achievements are as follows:During the cruise,there was almost no signal of 223Ra in most part of the study area except several stations in the Pearl River Plume(PRP)and near islands.It suggests that continental input is the dominating source of 223Ra.Distributions of 228Ra and(228Ra/226Ra)A.R.followed the same pattern.228Ra and(228Ra/226Ra)A.R.in the northern,western and southwestern parts were higher under the influences of PRP,islands and Mekong River Plume(MRP),respectively.However,228Ra and(228Ra/226Ra)A.R.were lower in the northeastern part due to the intrusion of Kuroshio water characterized by depleted Ra activity.Kuroshio intruded into SCS through the northern Luzon Strait,then forms a tongue shape mixing area of Kuroshio water and SCS water in the area bounded by 1150E-1210E,17°N-21°N during the cruise.The sea surface activity of 226Ra showed little difference compared to 228Ra and(228Ra/226Ra)A.R.,and it also revealed the Kuroshio intrusion area matched with 228Ra and(228Ra/226Ra)A.R..Based on the above distribution patterns and geographical locations,the surface seawater in the study area might be roughly divided into four water masses,Kuroshio Surface Water(KSW),SCS Surface Water(SSW),Northern Shelf Surface Water(NSSW),Western Shelf Surface Water(WSSW).Meanwhile,we found a preliminary influence of surface Ra distributions by mesoscale eddies.Based on a two end-member mixing model using 226Ra activity as conservative tracer,we calculated the Kuroshio fractions to be from 1.6%to 73%in the mixing area,and the average was 41.3±25%.Combined with previous results for paths and flow direction of Kuroshio,the Ra distributions suggested that Kuroshio axis flowed into the southern Luzon Strait along the track northeastward from the northeast coast of Luzon Island.But it changed the flowing direction at around(121 °E,20°N),then flowed out Luzon Strait northeastward in the northern part,and keep flowing northward along the east coast of Taiwan island.The branch of Kuroshio intruded northwestward into SCS in the northern Luzon Strait,then the intrusion direction changed toward the southwest at around(119°E,21.8°N).Based on results of the two end-member mixing model and taking 228Ra decay as a timer,the residence time of the northeastern SCS surface water derived from 228Ra decay ranged from 0.16 to 7.44 a,with an average of 2.93± 1.78 a.Ignoring impacts of mesoscale eddies and small-scale perturbations induced by upwelling,we attributed the residence time of the northeastern SCS surface water was in interannual time scale,which was in good agreement with the previous researches.In summary,Ra isotopes are effective tracers of water mass mixing and movement,as well as in the residence time estimation.In order to take full advantage of Ra as tracer,it is important to improve the spatial and temporal resolution of Ra data in the SCS.Then the accuracy of key parameters in interpreting marine biogeochemistry of SCS could be further enhanced.