Regional variation of ⁸⁷Sr/⁸⁶Sr ratio and compositions of the surface sediment in the eastern South China Sea

Isotopic studies have provided significant information about the geological provenance in the eastern South China Sea.Noncarbonate fraction in sediments is composed of weathering products of silica rocks and of young volcanic rock of basaltic composition.There is negative correlation between both components.The volcanic activities are the main source of basaltic composition in the basin.The weathering products of silica rocks of continent are supplied by the Changjiang River,part of them are transported into the study area through the Taiwan Strait.The ⁸⁷Sr/⁸⁶Sr ratios vary systematically in the investigation area:(1) High values were found in sediments collected around continental shelf and continental slope(0.718 994).Lower values(0.713 545) occur in the area lyingon east of the continental slope to the edge of the deep basin and beyond.(2) Somewhat lower values occur around the Dongsha Islands and the Zhongsha Islands(0.709 232),the lowest value(0.704 111) was obtained in sample collected around the Huangyan Island,this value is consistent with that of basaltic component.(3) ⁸⁷Sr/⁸⁶Sr ratios in sediments obtained from the southern Taiwan Strait were high(0.717 940),close to that of average value of the earth crust.The ⁸⁷Sr/⁸⁶Sr ratios decrease markedly in southwest direction toward the deep basin.The weight percentages of the basaltic component and silica component were calculated for each sample in terms of two-component mixing model.This treatment of the data provides estimates of the concentrations of the two components that are at least qualitatively useful.The concentration of volcanogenic detritus in the southern Taiwan Strait is less than 10%.However,this parameter increases to about 50% in the vicinity of the Huangyan Island.Weathering products of silica rock transported throagh the Taiwan Strait gradually deposit in the NNW-NE deep basin.Such a theoretical distribution is quite consistent with the actual occurrance of volcanic detritus and silica detritus.