上扬子地区中晚三叠世海洋碳酸盐工厂生产机制转换原因研究

Marine shallow-water carbonate system underwent important changes across the middle to upper Triassic in the Tethys realm. The carbonate factory switched from microbial-dominated to metazoan-dominated both in the western (Alps) and eastern (Sichuan Basin, Upper Yangtze craton) Tethys. The large-scale of these modifications suggests that profound modifications in marine chemistry, possibly induced by global climate changes associated to perturbations of the global carbon cycle, occurred throughout the oceans. Assessing the characteristics of and timing of these events are crucial to understand them and to establish yet unclear cause-effect relationships. To this end, we propose to undertake an integrated stratigraphic study in western Sichuan Basin. Conodont and foraminifer biostratigraphic investigations will be carried out and will contribute to a much-needed refinement of the middle to upper Triassic biostratigraphic framework in South China. Furthermore, new carbonate carbon and oxygen isotopes and organic carbon data encompassing the middle to upper Triassic will be collected. Quantitative petrologic analysis on carbonates will be carried out to verify if a switch from microbial to skeletal carbonate factories can be observed and linked to perturbations in carbon isotope record. Finally, REEs analyses on microbialites will be used to investigate if evidence of changes in seawater chemistry can be found that could help in understanding the causes that brought to the global change in marine carbonate sedimentation occurred across the middle to upper Triassic transition.

中晚三叠世西特提斯地区海洋碳酸盐工厂系统发生重大改变，相似的转变也发生在位于东特提斯洋的四川盆地西缘，表现为中三叠世以来的微生物岩在晚三叠世早期渐变为鲕粒-（后生动物）生屑灰岩。本课题拟利用牙形石和有孔虫微体古生物化石鉴定，结合岩石薄片显微观察、定量岩石学、扫描电镜、碳酸盐岩碳氧同位素、有机碳同位素和微区稀土元素（LA-ICP-MS）等测试手段，厘定川西江油马鞍塘、江油含增等地区中晚三叠世海洋碳酸盐工厂生产机制转换的准确时代，完善川西地区中上三叠统古生物地层格架，定量分析微生物岩颗粒的含量的变化，建立中上三叠统地层碳、氧同位素曲线，借助微区稀土元素的测试数据，分析碳酸盐工厂转变的原因，并提出合理的成因模式，对比分析它们在整个特提斯洋范围的时空联系，探讨晚三叠世地质事件与碳酸盐工厂转换的耦合关系。

以四川盆地西部和西特提斯域中-上三叠统地层为研究对象，以沉积学、古生物学和地球化学等分析方法为研究手段，完善了川西中-晚三叠世年代地层框架，厘定了特提斯域中-晚三叠世碳酸盐工厂转换的时间，明晰了碳酸盐工厂转换的原因，认为特提斯域卡尼期浅海碳酸盐工厂转换与卡尼期洪泛事件（CPE）密切相关。具体研究认识如下：1）认为卡尼期碳酸盐工厂转换时间与CPE启动时间保持一致；2）在CPE发生时期，Wrangellia大火成岩省的喷发改变了全球碳循环，引起海洋碳同位素偏移和大洋酸化，导致海水中碳酸盐饱和度下降，导致碳酸盐工厂类型发生转换。而在事件后期，海洋碳酸盐饱和度升高，促进了化学沉淀，形成了巨厚层的鲕粒灰岩沉积；3）重建了川西中-晚三叠世碳酸盐工厂演化模式，并识别出了全球性的卡尼期Julian亚期大海退；4）恢复了川西中-晚三叠世海水碳同位素记录，在含增剖面识别出3次碳同位素负漂，其中第三次碳同位素负漂可对应于CPE时期全球性的第三次碳同位素偏移事件；5）查明了四川盆地拉丁阶-卡尼阶和卡尼阶Julian-Tuvalian亚阶的古生物地层界线位置；6）认为腕足和同生期方解石胶结物真实记录了同期古海水的稀土元素特征，而部分鲕粒和微生物岩颗粒受到晚卡尼期陆源物质输入污染或者受到了后期成岩作用的改造而导致稀土元素特征呈出钟形的分布曲线；7）对四川盆地（诺利阶-瑞替阶地层）及其周缘地区（云南、鄂尔多斯盆地）中-上三叠统地层进行了探索性研究，进一步完善了四川盆地及周缘地区中-晚三叠世时期的古气候-古生物-古环境演化记录。在国际权威地学期刊《Earth and Planetary Science Letters》、《Global and Planetary Change》以及《Journal of Asian Earth Sciences》等发表SCI学术论文5篇。相关研究成果具有较好的学术价值，为探究中-晚三叠世生物与环境协同演化规律提供研究思路和启示。