华南二叠系－三叠系之交古海水的Ca同位素组成及指示意义

Study of the isotopic variations of calcium is of interest because Ca is important in geochemical and biochemical processes, and is one of few major cations in rocks and minerals with demonstrated isotopic variability. Recently, a transient negative excursion of ～0.3‰ of δ44∕40Ca across the Permian-Triassic boundary from marine limestone in south China were obtained, consistent with the remarkable abrupt shift in style of carbonate sedimentation and a negative excursion in the carbon isotope (δ13C) composition of carbonate minerals. It indicated that CO2-driven ocean acidification can best explains the coincidence of the δ44∕40Ca excursion with negative excursions in the δ13C of carbonates and organic matter and the preferential extinction of heavily calcified marine animals. The biogenic apatice (conodont) are widely used as an archive to reconstruct elemental and isotopic composition of seawater, because of its precise stratigraphic age and relatively resistant for global correlation. Studies, focused on calcium isotope (δ44∕40Ca) composition of marine sediments over the last decades, are increasingly extending to the emerging calcium isotope system. To date, only little attention has been paid to test the reliability of biogenic apatice. To avoid the complexity encountered in the dominant mineralogy in marine carbonate deposits, i.e. the oscillating 'calcite-aragonite seas', we will systematically collect biogenic apatice (conodont) across the Permian-Triassic boundary from marine limestone in south China. The high-resolution δ44∕40Ca will be compared from different depositional setting in the Permian-Triassic transitional sections, where the shallow-water facies from Meishan section and Xiakou section and the deep-water facies from Dongpan section, respectively. Our aim is to obtain the secular variation in the calcium isotope composition to distinguish among these possibilities and thereby constraining the causes of biotic crisis of end-Permian.

二叠系－三叠系（P-Tr）之交生物绝灭事件与海洋酸化是否与有一定关联，一直是人们关注的问题。本研究以代表古海水同位素组成和地层时代限制的磷酸盐壳（牙形石）为载体，建立二叠系－三叠系(P-Tr)之交不同相带连续剖面Ca同位素随时间的演化曲线，比较海相碳酸盐岩Ca同位素分析，在精确的牙形石地层年龄对比的基础上，通过比较P-Tr之交不同相带（浅水相和深水相）钙同位素的变化规律，为P-Tr之交绝灭事件的成因提供新证据。

钙在地球化学和生物化学过程中具有重要的意义，因而，其同位素组成的变化可以作为示踪地质和生物过程的重要工具。本研究通过建立二叠系－三叠系(P–Tr)之交不同相带连续剖面Ca同位素随时间的演化曲线，厘定绝灭事件的性质，取得了以下成果：（1）建立了高精度钙同位素测定方法，在中国地质大学地质过程与矿产资源国家重点实验室（GPMR）和加州大学伯克利分校同位素地球化学中心（CIG）同时对16个美国地质调查局（USGS）的国际地质标样进行了测定，Ca同位素分析的长期重现性均优于0.15‰，达到了国际一流的测试水平；（2）通过建立二叠系－三叠系(P–Tr)之交不同相带连续剖面Ca同位素随时间的演化曲线，同时对比了奥陶系—志留系之交不同沉积相（深水相和浅水台地相）的钙同位素演化规律，发现不同时期的绝灭事件以及不同沉积相海相碳酸盐岩的δ44/40Ca均存在一定程度的负偏，也许与同期的海洋酸化有一定的关联，反映海洋酸化导致碳酸盐岩的再溶解，使得偏轻的同位素进入海洋，值得注意的是，不同沉积相海相碳酸盐岩的δ44/40Ca也存在一定的差异，也许与陆源的风化贡献有关;（3）为了进一步探索陆源碎屑物质贡献在不同沉积相（台地、斜坡和盆地相）δ44/40Ca的影响程度，对长江流域干流以及长江河口海—陆交互相（河水—过渡带—海水）的钙同位素进行了分析，结果表明，河水中作为风化产物的黏土矿物以及盐度的变化均会导致河口海—陆交互相的钙同位素产生明显的分馏，而不能用简单的两端元混合模型进行解释。在项目实施阶段，发表标注论文11篇，其中包括J. Anal. At. Spectrom.，Geostandards and Geoanalytical Research，Chemical Geology，Earth-Science Reviews等国际SCI论文11篇，4篇论文正在整理发表过程中。资助研究生5人次参加American Geophysical Union Meeting 和Goldschmidt Conference国际会议，同时指导博士生1名，硕士研究生3人完成有关钙同位素研究的硕士毕业论文。