Tracing Study On The Water Mixing Processes On The Northern Slope Of The South China Sea By Radium Isotopes

The South China Sea?SCS?,a semi-enclosed marginal sea in the tropical northwestern Pacific Ocean,has abundant fishery resources and mineral resources.Driven by dynamic factors such as the monsoon and the Kuroshio intrusion,there are large seasonal differences in the water masses structure on the northern SCS,which would strongly affect the ecological structure and materials exchange between the northern SCS and its surrounding waters,and further impact on the ecological environment of the SCS.The study on the water masses mixing processes would improve the understanding of the biogeochemical cycles on the northern SCS.With the application and rapid development of isotope tracer technique in the oceanographic researches,the radium isotopes now have become ideal tracers for the studies on the movement and mixing processes of seawater masses.In the present dissertation,the radium isotopes were used as tracers to study the water mixing processes on the northern slope of the SCS,and conclusions were showed as follows:?1?Significant seasonal variations of the long-lived radium isotopes ²²⁶Ra and²²⁸Ra activities in the surface water of the northern SCS slope were observed.In spring,²²⁶Ra and ²²⁸Ra activities ranged from 64 to 107 dpm m^-3 and from 46 to 260 dpm m^-3,respectively.While in summer,²²⁶Ra and ²²⁸Ra activities ranged from 76 to 120 dpm m^-3 and from 108 to 304 dpm m^-3,respectively.The activities of the two radionuclides in spring were obviously lower than those in summer;?2?By analyzing the spatial distribution of ²²⁶Ra,²²⁸Ra,temperature and salinity in the surface water,and combining with the two-end-member and three-end-member mixed models,the water mass structure of surface water on the northern slope of the SCS in spring and summer were studied.It was found that the water masses structures in spring and summer showed significant differences.In spring,the surface water was composed by 48.9%of SCS water and 51.1%of Kuroshio water,while in summer was composed by 16.8%of shelf water,62.0%of SCS water and 21.2%of Kuroshio water.The Kuroshio intrusion could be observed in both two seasons,but the intrusion intensity in spring was stronger than that in summer;?3?A relatively low-salinity belt?salinity<33‰?was clearly observed in the summer of 2015.The source and proportion of terrigenous water at the low salinity zone of the northern slope of the SCS in summer was analyzed by using radium isotopes.Combined with?¹⁸O,it was concluded that there were other sources in low salinity zone besides the shelf water from the fresh water of Pearl River plume;?4?The vertical distributions of ²²⁶Ra and ²²⁸Ra on the northern slope of the SCS showed that their activities deceased from bottom water to sub-surface layers, indicating that the distinct input of ²²⁶Ra and ²²⁸Ra was from the water-sediment xchange processes.The activities of these two radionuclides ranged from 59 to 297dpm m^-3 and from 26 to 65 dpm m^-3,respectively.By using the two-box model with²²⁶Ra and ²²⁸Ra,the residence time of seawater below 1500 m on the northern slope of the SCS was evaluated as approximately 15⁶5 years.This longer residence time ndicated that the environmental impacts on the exploitations of seabed resources on the northern slope of the SCS might last for decades.