The Indicating Significance Of Biogenic Silica In Surface Sediments For Primary Productivity From Taiwan Strait And South China Sea

Biogenic silica(BSi)is attributed to amorphous silica,also called opal.Marine biogenic silica is mainly compromised of diatom,radiolarian,siliceous flagellate and siliceous sponge,etc.BSi has been widely studied in spheres of primary productivity indication and silicon cycle,due to its unique protecting mechanism.In this study,we detected and calculated the contents and fluxes of BSi in sediments from the Western Taiwan Strait,Northern South China Sea and station called S0204,data and results are showen below:(1)The horizontal range of surficial BSi contents,in surface sediments from Western Taiwan Strait,is 0.030?1.7%,and the average content is(0.66 ± 0.44)%.The maximum content is about 57 times larger than the minimum.The horizontal range(average value)of surficial sedimentary mass accumulation rates concerning biogenic silica(MARBSi)is 0.765?122(54.1)?mol Si cm-2·yr-1,about 0.28?45(20)times larger than global ocean average value(2.7 ± 2.0 ?mol Si cm-2·yr-1).The distribution features of BSi contents and MARBSi are similar,declining from coastal areas to central strait.The distribution of BSi contents is controlled by three main factors:grain size,dilution of terrigenous particulate matters and modern primary production.Biogenic contents and MARBSi can be used to indicate modern primary production process in upper water column with two significant guarantee conditions,uniform sedimentary environment and the main contribution of diatom to modern primary production parameters.The Zhe-Min coastal mud belt is accord with above-mentioned requirements.Sampling area and data number are of much significance in the indicating application of BSi.(2)Horizontal range of sedimentary BSi contents(average value)is 0.062?5.5(2.3 ± 1.3)%,in surface sediments from the Northern South China Sea.Maximum content is about 88 times larger than the minimum.The distribution ranges(average value)of total sediments and BSi are 0.28?40(6.0)mg·cm-2·yr-1,0.048?8.6(1.7)?mol Si·cm-2·yr-1 separately,and the multiples between extremes are as large as 143 and 179.The BSi contents have positive correlation with water depth as a whole,behaviors of which are mainly controlled by grain size,dilution of terrigenous particulate matters and modem primary productivity.Terrestrial particulate mineral is the main constitute of sedimentary matters,and CaCO3 plays roles mainly in areas close to Zhujiang River estuary.By comparison,BSi has a small proportion in the total sediment constitutes.Sedimentary fluxes of sediments and BSi have similar distribution pattern:declining from coastal areas of the Taiwan-Luzon Islands to deep basin,until the minimum of continental shelf.This pattern reflects the weakening feature of clay fluxes from land runoff,and the sorption of clay mineral of much importance in exporting process of BSi,especially smectite,relative to illite and chlorite.Sedimentary BSi fluxes can't be used to indicate modern primary productivity in upper water column directly,owing to the great difference of total sedimentary fluxes and relative low contribution of diatom in modern primary productivity.In further indicating application,we need to select areas of relative homogeneity,ascertaining the specific contribution ratios of diatoms to modern primary productivity.(3)The vertical range(average value)of sedimentary BSi contents in sediment core,at station S0204,is 0.47?1.5(1.0)%.The occurence of maximum and minimum contents were at 15.4 kyr B.P.and 18.9 kyr B.P.separately,and their value difference is 3.3 times.The declining trend,from Holocene to last glacial period,was controlled by increasing fluxes of sedimentary BSi.The vertical range(average value)of sedimentary BSi fluxes is 0.83?2.5(1.4)?mol Si cm-2·yr-1.The occurence of maximum and minimum fluxes were at 21.1 kyr B.P.and 8.85 kyr B.P.separately,with 3.1 times value difference.The declining tendency of sedimentary BSi fluxes,from last glacial period to Holocene,indicating the weakening of East Asian winter monsoon and primary paleoproductivity.Sedimentary BSi fluxes can also indicate the suppressing effect in primary paleoproductivity of paleoclimate events,as the first Heinrich Event and Younger Dryas Event.These events palyed their roles in paleoproductivity with rapid temperature reduction in large scale,depressing the normal growth and reproduction processes of diatom,radiolarian,etc.The research makes knowen that climate events had strong influence on paleoproductivity of Northern South China Sea,and the significant status of Northern South China Sea in climate evolution,from last glacial period to Holocene.