湖泊沉积物碳酸盐含量3种测定方法的比较

湖泊沉积物中碳酸盐含量是指示气候和环境变化的常用指标之一,其测定方法多样,但缺乏不同测定方法结果之间的比较,尤其是对同一沉积序列样品的比较.选择新疆天山大龙池DLC12孔110 cm岩芯沉积物,按1 cm间隔取样,利用烧失量法、酸碱滴定法和气量法分别进行碳酸盐含量分析,并比较3种分析方法在表达湖泊沉积物碳酸盐含量时的差异性.结果表明:3种方法测定的碳酸盐含量随岩芯深度的变化趋势并无太大差别,其中烧失量法测定的结果较滴定法和气量法测定的结果平均分别偏高5%和3%,气量法结果与滴定法结果较为接近,平均差值不到2%,表明烧失量测定过程中有其他矿物分解导致碳酸盐含量被高估.将3种方法测定的结果与岩芯中20个样品的X衍射(XRD)得到的碳酸盐含量对比,发现气量法的测定结果与XRD测定结果的相关性最高.通过比较各种方法的优缺点,考虑到分析精度、操作技巧和分析费用,认为气量法比较适合湖泊沉积物的碳酸盐含量分析.

Evaluation of three methods used in carbonate content determination for lacustrine sediments

The carbonate content is one of the most frequently used climatic or environmental proxies in lake sediments. There are several conventional ways for determining the carbonate content of lacustrine sediments, but the carbonate content determined by different methods have not been compared, especially for the same samples of a sedimentary sequence. Here we present a comparison of the carbonate content determined by three methods (loss on ignition, acid-base titration and gasometric method) applied to a short core taken from Lake Dalongchi (DLC12, 110 cm, at 1 cm resolution) in Xinjiang, NW China. Our results show that there is generally no significant difference in the down-core variation trend of the carbonate content among the three methods. The carbonate content determined by loss on ignition (LOI) is cal 5% higher than that measured by acid-base titration (AT) and cal 3% higher than that by gasometric method (GM). The GM and AT methods produce similar results and their difference is less than 2% in average. Therefore, the LOI method may have over-estimated the carbonate content due to the decomposition of minerals other than carbonates in the samples. Further comparison of the carbonate content by the three methods with that by X-ray diffraction (XRD) suggest the carbonate content by GM has the highest correlation with that by XRD. Considering the advantages and disadvantages of each method, GM is suggested for conventional carbonate content analysis in lake sediments in terms of accuracy, expense, analytical skills required and time consumed for analysis.