藏南定日地区Cenomanian-Turonian期沉积黄铁矿的多硫同位素研究

During the Cretaceous period, there were several periods of global ocean anoxia. The one took place during the Cenomanian-Turonian transition (94 million years ago) was called Oceanic Anoxic Event 2 (OAE2), which is characterized by a positive C isotope excursion of 2-4‰ in both carbonate and organic carbon. This excursion was reported in coeval marine strata globally, reflecting a major perturbation in the global carbon cycle. A series of paleoenvironmental events occurred during the OAE2 period, which may have contributed to stepwise faunal extinction and turnover. The expansion of anoxic waters was thought to have been responsible for this extinction event. Tingri area of southern Tibet is located in southeastern Tethys during OAE2. The Gongzha section of Tingri records this extinction and turnover event. However, recent studies proposed that there was no significant anoxia during OAE2 in the Gongzha section and therefore, there is no link between biotic events and global ocean anoxia. This hypothesis is evidently in contrast to conventional view that oceanic anoxia may have caused the mass extinction. To better understand the link between ocean chemistry change and biotic event, we propose to carry out multiple sulfur isotope analyses of sedimentary pyrite in the Cenomanian-Turonian sedimentary rocks from Gongzha section. By integrating of multiple S-isotopic changes with the detailed biostratigraphy, we aim to reconstruct paleoenvironmental changes during OAE2 in southeastern Tethys as well as to improve our understanding of environmental influences on the faunal extinction and turnover event.

白垩纪发生过数次大洋缺氧事件，其中发生在距今约94 Ma位于Cenomanian-Turonian界线附近的这次缺氧事件被称为OAE2。OAE2最显著的特征之一是碳同位素发生了大约2-4‰正偏，反映了全球碳循环的扰动。在OAE2期间发生了一系列的环境变化事件和海洋生物的灭绝与更替。海水缺氧被认为是这次灭绝的最主要原因。藏南定日地区在OAE2期间位于东南特提斯区域。定日地区的贡扎剖面很好地记录了这次生物灭绝事件。但是最近有研究表明贡扎剖面并没有发生严重的缺氧，因此海洋缺氧与生物灭绝没有成因上的联系。这一结论显然与海水缺氧导致灭绝的观点有分歧。为弄清OAE2期间海水化学条件变化与生物灭绝的关系，本项目将采用多硫同位素方法对贡扎剖面进行进一步深入的研究。通过沉积黄铁矿的多硫同位素组成的测定来重建OAE2期间东南特提斯区域古海洋环境的变化情况，探索这次海洋生物灭绝与更替事件的环境控制因素。

本项目以硫、碳、氮同位素为主要研究手段，探索白垩纪Cenomanian-Turonian（C-T）界线附近发生的OAE2这次大洋缺氧事件在藏南的记录及其与海洋生物灭绝之间的关系。我们对定日地区贡扎剖面C-T界线附近地层进行了高分辨率的多硫同位素、碳酸盐碳、氧同位素、氮同位素、总氮含量和总有机碳含量测试。结果显示东特提斯的南缘在OAE2期间碳、氮、硫循环都发生了剧烈扰动。我们发现在碳同位素正偏的地层中同样记录了氮同位素的负偏。持续较低的氮同位素组成反映东特提斯区域在OAE2期间同样发生了缺氧扩张，进一步证明OAE2是一次全球性的缺氧事件。多硫同位素的异常显示在缺氧层位发生了多次间歇性的硫化海水上涌事件，这为OAE2这次大洋缺氧事件发生的机制提供了新的信息。另外，缺氧和硫化海水上涌的层位和生物灭绝的层位一致，表明海水缺氧是C-T生物灭绝事件发生的重要原因。