湖泊沉积物碳酸盐含量的XRD半定量分析

湖泊沉积中的碳酸盐对于气候环境具有很好的指示意义．利用化学分析方法（气量法）和X衍射（XRD）方法，对青藏高原可可西里库赛湖KS-2006孔沉积物中的碳酸盐含量进行了半定量分析．结果表明：虽然KS-2006孔沉积物中的矿物种类达9种之多，且XRD的测量误差（±5％）高于气量法的测量误差（±0．8％）；但XRD法获得的KS-2006孔沉积物碳酸盐总含量的平均值（18．9％）以及波动范围（7％～39％），同气量法所获得的碳酸盐含量的平均值（19．4％）和波动范围（5．0％～30．7％）基本一致，而且二者具有很好的正相关关系（相关系数R2为0．3940），随深度的波动趋势一致，说明XRD方法在半定量分析湖泊沉积物碳酸盐矿物种类及其含量方面，具有很高的可靠性．

On the Unusual Holocene Carbonate Sediment in Lake Nam Co,Central Tibet

In lacustrine sediments,aragonite is a widespread mineral,whereas monohydrocalcite is a rare carbonate mineral. In the cold and high-attitude Xizang (Tibetan) Plateau,where aragonite has been commonly found in lacustrine sediments,there is no aragonite,but low-Mg calcite,monohydrocalcite and trace dolomite. The lake receives solutes primarily from surface runoffs and remains fairly constant water chemistry for a long time. The total CaCO3 percentage in sediments could be controlled by evaporation and inflow of detrital materials. The absence of aragonite is unusual when compared to other lacustrine sediments from the Tibetan Plateau. This could be due to low Ca/Mg ratio,low salinity,low Mg and Ca concentration. Monohydrocalcite might precipitate from the lake water mediated by biological activities. Low-Mg calcite originated from minor ostracoda shell and the precipitation of lake water with biological activities.