川渝地区早三叠世海相碳酸盐岩的碳同位素研究

The Early Triassic is a critical period in earth history, representing the ecosystem reconstruction and recovery after the end-Permian biotic crisis. As a result of recent discoveries about a series of unusual environmental and evolutionary events, the research of the Early Triassic has become a focus topic in the geological fields. Variations in the carbon isotopic compositions of marine carbonate record the fluctuation of carbon cycle, and give information about ecosystem function and environmental change through the transitional period. However, the relatively scarce occurrence of carbonate depositions throughout the Early Triassic seriously limits the acquisition of paleo-oceanography information. In addition, the Early Triassic segment of the widely cited Phanerozoic curves is poorly constrained for carbon isotopes. Thus, it is in the recent years that the fluctuation of Early Triassic carbon isotopes, with high frequency and large amplitude, has been recognized and emphasized. As a result, the purpose of this research aims to investigate the carbon isotope record of Lower Triassic strata from Sichuan and Chongqing area, in which the paleo-oceanographic information is well-preserved but few attention has been paid to it.On the basis of examinations on diagenetic alternations, the carbon isotopic composition of paleo-sea water would be studied using petrological, paleontological and elementary and isotopic geochemical methods. The establishment of high-resolution carbon isotope curve and its local, regional, and global correlation would reveal the mechanism of carbon cycle in earth's system during the Early Triassic, and also help to further our understanding about the significant geological events and the corresponding ecologic, environmental effects during that catastrophic period.

早三叠世是地球发展史上的特殊时期，代表了P/T之交大灭绝后的生态重建与复苏。随着各种"异常环境事件"信息在近年陆续被揭示，早三叠世地质学已成为地学界新的研究热点。海相碳酸盐碳同位素组成记录了碳循环的变化，其研究对于理解重大突变期的生态环境事件和地球过程至关重要。然而，由于早三叠世全球范围碳酸盐沉积相对缺乏，古海洋信息获取困难，在显生宙碳同位素演化曲线中，该时间段的趋势很不确定，碳同位素组成的高频率、大幅度振荡特征也是近年来才得以认识和重视。本项目以古海水信息保存较好但碳同位素研究水平较低的川渝地区早三叠世碳酸盐岩为研究对象，在样品成岩蚀变检测的基础上，采用岩石学、古生物学、元素及同位素地球化学等手段，开展古海水碳同位素组成研究。建立的高分辨率碳同位素演化曲线及其地区性、区域性和全球性对比可为揭示早三叠世全球碳循环的运行机制，以及地球巨变时期重大地质事件及其生物、环境效应提供重要的基础资料。

早三叠世是地球发展史上的特殊时期，代表了P/T之交大灭绝后的生态复苏。项目在对样品进行系统成岩蚀变检测的基础上，评估了样品碳同位素组成对海水的代表性，获得了川渝地区早三叠世超过1000个碳氧同位素数据，建立了完整的高分辨率碳同位素演化曲线，进行了地区性、区域性和全球性对比，结合岩石学、矿物学、地球化学、生物地层学和沉积相研究，阐明了早三叠世碳同位素曲线的演化样式及控制因素，取得了以下主要成果：1）建立的早三叠世碳同位素曲线具有非常好的全球可对比性，曲线中主要的峰和谷均为全球信号，碳同位素的高频率大幅度变化代表了群体绝灭后生态复苏过程中环境不稳定性对全球碳循环和海水碳同位素组成的影响；2）龙门峡南剖面碳同位素曲线亚阶尺度的年代地层对比表明飞一段、二-四段、嘉一段、嘉二-四段下部可分别与Griesbachian、Dienerian、Smithian和Spathian亚阶对应，嘉四段中上部应归于Aegean亚阶；3）龙门峡南剖面Smithian-Spathian亚阶界线（SSB）附近δ13C从-0.911‰急剧上升至3.679‰，海水碳同位素如此剧烈的变化耗时可能小于36 Ka，但氧同位素在SSB附近先于碳同位素变化，反映海水在碳同位素急剧升高之前，盐度已显著增加，并导致蒸发岩和白云岩的形成；4）开江-梁平海槽东西两侧孤立台地和镶边陆棚早三叠世Induan期具有类似并与全球一致的碳同位素组成与演化样式，但具不同的氧同位素组成，显示全球事件对海水碳同位素组成的控制作用及沉积相对氧同位素组成的影响，孤立台地内海水更加受限，并具更高的温度、盐度和更强的白云石化潜力；5）嘉二段沉积白云石（或准同生交代方解石的白云石）与同沉积方解石（或被交代的先驱方解石）碳同位素组成的对照研究表明，白云石比伴生方解石更富13C，两者δ13C的差值为2-2.6‰，这一数值与前人从实验和理论的研究结果类似，可能说明在碳同位素曲线建立中对白云石样品进行校正的必要；6）从显生宙和全球碳循环角度观察早三叠世海水碳同位素组成表明，早三叠世扮演了一个转折期的角色：海水从古生代δ13C持续增加、通过早三叠世δ13C的高频率大幅度波动转变为中新生代的δ13C值稳定阶段，P/T之交大灭绝可能是古生代后期有机碳长期高速埋藏的必然结果，也可能是地球“大煤期”事件后的必然产物。