琵琶湖湖底堆積物の化学的特徴(タンデトロン加速器質量分析計業績報告2003(平成15)年度)

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  • Chemical characteristics of lake sediments in Lake Biwa(Summaries of Researches Using AMS)

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The chemical composition of major and trace elements in bottom sediments of Lake Biwa was determined by X-ray fluorescence spectrometry (XRF) and instrumental neutron activation analysis (INAA). The sediments were collected in the northern basin, the southern basin and its boundary part. The vertical distribution patterns of some elements in the northern basin, such as Al, Ti and Sc, are similar each other. The concentrations of Al, Ti and Sc in the northern basin increase from the surface to the depth of about 18cm, where they reach maxima. The variations in concentrations are probably due to temporal influx of Allvium which consists of clay, mud and gravel layer. The concentrations of As and Mn are extremely high in the surface layer in the northern and southern basins. Manganese in sediments exists in different forms of oxidation state together with variation of redox condition. Manganese precipitates in oxic layer as Mn(IV)-oxides. Therefore, the concentration of Mn is high in the surface layer. The high concentration of As in the oxidized surface layer is due to Mn acting as carrier, so that the vertical distribution pattern of As is similar to that of Mn. The plots of major elements against TiO_2 show correlations in granites, river sediments and lake sediments. The lower concentrations of Na_2O, CaO and SiO_2 in river and lake sediments compared to granites are caused by weathering of plagioclase. The higher concentrations of Fe_2O_3 and MgO are relative increase in samples due to resolution of some elements sensitive to weathering.

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