西藏羌塘盆地早侏罗世Toarcian期缺氧事件及成因机制

During the early Toarcian (early Jurassic), a well-known oceanic anoxic event took place all over the world, which dramatically influenced the paleoceanography, paleoclimate and paleoecology. However, there still are more divergences on its development process and forming mechanism. Located in the northern Qinghai-Tibetan Plateau and eastern tectonic realm of Tethys, Qiangtang basin provides vital links between West Tethys and SE Asian Tethys. A continuous early Jurassic deposition is well preserved along the south basin margin, and various geological factors for the most likely culprit of the anoxic event occur within these black shales. All this can be served as best conditions for analysis Toarcian Oceanic Anoxic Event features and its forming mechanism. This project will focus on these early Jurassic black shales sequence of Quse Formation, and research stratigraphic and sedimentary characteristics of T-OAE in Qiangtang basin, then reconstruct its paleoceanogrphic condition by studying of sedimentology, carbon isotope stratigraphy, organic geochemistry and elemental geochemistry. Additionally, the project will also conduct parallel research on contemporaneous volcanic rocks and authigenic carbonate concretions preserved in these early Jurassic black shales, and try to tease out genetic relationship and coevolution between them. All this effort will be combined together to explore formation and evolution pattern of T-OAE in Qiangtang Basin, and unravel key geological evidence to forming mechanism.

早侏罗世Toarcian期发生过一次全球性大洋缺氧事件，极大的影响了当时古海洋、古气候和古生态条件，目前地学界对该事件的形成过程与成因机制仍存在较大争议。位于青藏高原北部的羌塘盆地地处特提斯域东段，是连接西特提斯及东南亚特提斯的关键区域，盆地南缘广泛发育有连续的早侏罗世沉积，其黑色岩系层段发育有空间上集缺氧事件可能诱因于一体的各种地质素材，为开展缺氧事件及动力机制研究提供了绝佳机会。研究工作拟以羌塘盆地南部早侏罗世曲色组黑色岩系作为主要研究对象，通过系统的沉积学、碳同位素地层学、有机地球化学和元素地球化学分析，明确羌塘盆地T-OAE发生的地层层位，查明它的沉积和地球化学响应特征，恢复其形成的古海洋条件；同时，开展同时代喷发的火山岩和同层段自生碳酸盐岩结核研究，梳理它们与T-OAE成因关联及协同演化关系，探讨T-OAE在羌塘盆地发生、发展过程，为T-OAE成因机制研究寻找关键地质证据。

本项研究重点选择西藏羌塘盆地毕洛错剖面黑色岩系作为研究对象，在孢粉年代学研究基础上，通过系统的沉积学、碳同位素地层学、有机地球化学和元素地球化学分析，明确了羌塘盆地T-OAE事件发生的地层层位，查明了它的沉积和地球化学响应特征，梳理了东特提斯洋T-OAE事件发生的海洋初级生产力、陆源物质注入特征、氧化-还原条件、水体温盐及循环条件、古大气PCO2含量水平等古海洋条件和古气候背景，并通过区域与全球的综合对比，探讨了古海洋、古气候条件与T-OAE成因关联及协同演化关系，明确了T-OAE在羌塘盆地的发生、发展过程。研究表明：羌塘盆地毕洛错剖面油页岩层段孢粉化石主要由陆生球形花粉和少量海生沟鞭藻囊组成，代表性组分Nannoceratopsis gracilis, Mancodinium semitabulatum, Scriniocassis weberi, Scriniocassis priscus, Callialasporites turbatus and C. microvelatus都是欧洲特提斯域Toarcian阶最常见的优势花粉类型，指示毕洛错油页岩层段形成于早侏罗世Toarcian期。这是目前羌塘盆地上侏罗统地层最详尽的孢粉年代学研究，为羌塘盆地早侏罗世地层划分与对比提供了很好约束，也进一步确认了南羌塘盆地早侏罗世Toarcian阶地层的存在。同位素和地球化学分析结果也显示，Toarcian期富油页岩层段TOC含量明显高于上下层段，有机碳同位素和无机碳同位素均存在明显异常，油页岩沉积时海洋环境普遍缺氧和海洋生产力条件偏高，以及大气PCO2含量水平快速升高和风化速率明显增加。因此项目提出：西藏羌塘盆地早侏罗世Toarcian期缺氧事件发生阶段大气PCO2含量水平快速升高，温室气候效应明显增强，同时陆地风化速率增强，海表营养盐组分较高，生物繁盛，联合导致有机碳的大量堆积和碳同位素异常，形成羌塘富有机质页岩沉积。该套黑色岩系富含生烃潜力较好的海相藻类，有机质的成熟度较高，处于生油窗早-中期阶段，干酪根类型为易生烃的II/III型，生烃潜力较好，是羌塘盆地重要的烃源岩层位。