Measurement Of Biogenic Silica (BSi) In Marine Sediment Cores From East China Sea And Their Stratigraphical Application

As a proxy of paleproductivity and paleoclimatic, Biogenic silica (BSi) in marine sediments can record historical events of dissolve silica in sea water, and carry a valuable record of paleoclimatic changes. Water contents, ignition loss, radionuclides and BSi contents in four sediment cores from East China Sea, i.e. S0508-1, S0703-2, S0801-1 and S1004-1, are analyzed in the paper. Sedimentary rate is calculated by ²¹⁰Pb_ex and chronologies of sediment cores are established. Comparisons of water contents, ignition loss with BSi contents,the stratigraphical and environmental implications are studied.²³⁸U,²²⁶Ra,²¹⁰Pb,²²⁸Ra,²²⁸Th,⁴⁰K and ¹³⁷Cs in four cores are detected by HPGe-γspectrometer. Sedimentary rates of S0508-1, S0703-2, S0801-1 and S1004-1, calculated by ²¹⁰Pb_ex, are 0.375, 1.708, 2.320 and 0.677gcm^-2a-1 respectively, their chronological times respectively are 422, 103, 94 and 315a. Horizontal distributions of radionuclides may be affected by the distance off shore or water depth: the content levels of ²³⁸U, ²²⁸Ra,²²⁸Th and 40K from inner shelf (S0801-1and S0703-2) are higher than those from outer shelf (S1004-1 and S0508-1), in good agreement with"a zonal pattern along the coast of element in sediment"of East China Sea, while the contents of ²²⁶Ra in four sediment cores are comparable within the experimental error.Na₂CO₃ solutions of different concentrates(0.5, 1.0 and 2.0 mol/L) are selected to digest the different—depth sediment samples from the same core,or from different cores. By comparison of BSi coutents,the single time point and the sequential alkaline extraction methods, the factors affecting the accuracy of BSi are disused to establish a fast method to determine BSi content in sediments from East China Sea—the single time(5h) point 1.0 mol/L Na₂CO₃ (85℃)extraction method. BSi contents (SiO2%) in S0508-1, S0703-2, S0801-1 and S1004-1 cores are 1.01～1.84,1.04～2.01, 1.12～2.48 and 1.11～1.69 respectively, and the mean values are 1.39, 1.53, 1.81and 1.40;BSi contents in surfaces of four cores are 1.56～1.83,1.67～1.86, 1.62～2.06 and 1.33～1.42 respectively,and the mean values are 1.69, 1.78, 1.79and 1.36。As far as horizontal distribution of BSi of four cores in East China Sea are concerned, BSi content in the whole and surface part are of the same order of magnitude, while they are different quantitatively: high BSi contents are in inner shelf /off shore, which is due to increasing human colonization and activity; BSi contents are in good agreement with distributions of diatoms in surface sediments and the concentration of SiO₃^2- in surface water of East China Sea, due to the input of Yangzi River. Ignition loss level in the whole and surface of four cores is of the same order of magnitude: Ignition loss in total cores is higher off shore; high ignition loss in surface parts of cores is in the north area, low ignition loss is in the south area.The marine environmental change is not triggered by a simple and single factor, but many integrative and complex factors.Profiles of water contents, ignition loss and BSi contents in each core together with its sediment geochronology are compared and analyzed to study the environmental implications. For Core S0508-1, collected from North east of East China Sea, ignition loss and BSi contents of upper part of this core are higher due to human activities since 1980; the varieties of water contents, ignition loss, BSi content and sediment grain size occur in 46～50gcm^-2 part, which maybe be caused that Old Yellow River changed its route in 1855. The sediment part below 75gcm^-2 depth maybe is not contemporary infra-littoral sediment, but"rudimental sediment"in littoral region formed in Late Pleistocene or Upper Holocene.Core S0703-2 is suited in a strong upwelling area where a lot of plankton including diatom flourish due to high content of nutrients. Ignition loss and BSi content of this core are generally increasing with time due to the human activities, development of industry and environmental pollution. The peaks of ignition loss and BSi content are consistent with the second year after El Nino event by and large, indicating that the primary productivity and siliceous plankton maybe be related to climatic change.Core S0801-1 is situated in the southeast coastal area of Dongtou Island in Zhejiang province, where the high content of nutrients—N, P and Si and eutrophication are affected by Jiangzhe longshore current, the runoffs from Oujing River etc. and the salinity front in the offshore area. The encounters of fresh and salt water fronts, the runoffs of industrial and domestic wastewater may accelerate to the red tide occur. The peaks of ignition loss and BSi content are consistent with the year typhoon and storm surge happened, implicating that typhoon and storm surge may make sediment and nutrients replace. Comparison with there cores mentioned above, ignition loss and BSi content in Core S1004-1 vary in a small rang. The primary productivity is lower and the original matter into marine sediment is smaller due to a high salinity warm current from Taiwan. A break of radionuclide, water contents, ignition loss and BSi contents occur at the part of～14gcm^-2 depth because the three strong earthquakes happened in Taiwan in 1986.