晚奥陶世生物灭绝的海洋环境因素

Paleontological fossil records indicate that the first Phanerozoic mass extinction occurred in the Hirnantian Stage, in which the severity of extinction was second only to the Late Permian mass extinction. Accompanying the extinction, the Hirnantian strata also recorded a global positive C- and S-isotope excursion, suggesting dramatic changes in environment and climate during that time. However, the killing mechanism of the extinction, the root cause and paleoenvironmental implications of the C- and S-isotope excursions are still controversial. To further explore the marine environmental factors of the Late Ordovician mass extinction, we propose to constrain the redox evolution of the Late Ordovician ocean in South China by analyzing multiple S-isotopes, organic carbon isotopes, and elemental geochemical proxies of sedimentary rocks from Huanggexi, Nanbazi, and Huanghuachang section located in Yunnan, Guizhou, and Hubei province respectively, which deposited in different water depths and basinal settings. In combination with the preliminary results, we propose to reconstruct the marine chemistry changes in space and time during the Late Ordovician, explore their impacts on the mass extinction, and further improve our understanding of the causes of the Late Ordovician mass extinction as well as the interactions between life and environment in Earth history.

古生物化石记录显示，显生宙最古老的生物大灭绝事件发生于晚奥陶世赫南特阶，其物种灭绝程度仅次于晚二叠世生物大灭绝。与生物灭绝相对应，该时期的沉积地层记录了一次全球性显著的碳、硫同位素正偏移，表明当时环境气候条件发生了显著变化。然而，这次生物灭绝的驱动机制和碳、硫同位素正偏的根本原因及其古环境意义仍然存在争议。为深入探究晚奥陶世生物灭绝的海洋环境因素，本项目拟以多硫同位素、有机碳同位素以及其他元素地球化学为主要研究手段，通过对沉积于不同水深和不同闭塞程度海盆的云南黄葛溪、贵州南坝子以及湖北黄花场剖面进行多化学指标的分析测定，追索华南晚奥陶世海洋化学条件的变化规律；并结合前期成果，重建晚奥陶世海洋氧化还原条件的时空演化特征、探讨其对生物灭绝的影响，深化我们对晚奥陶世生物灭绝机制以及生物与环境相互作用的认识。

为深入探究晚奥陶世环境气候的变化机制及其对生物灭绝的影响，我们以多个稳定同位素体系(即碳、氧、汞和多硫同位素)为主要研究手段，结合沉积岩石学和元素地球化学，对采自美国内华达和我国华南的7条代表性剖面/钻孔样品进行了高分辨率的总硫含量、总有机碳含量、汞浓度、有机碳同位素、无机碳同位素、碳酸盐氧同位素、多硫同位素以及汞同位素的分析测试，取得以下成果：(1)提出平流层火山喷发是造成晚奥陶世生物灭绝的驱动机制这一新观点，首次在23亿年以来的地质记录中发现了硫同位素非质量相关分馏信号。(2)汞化学地层学研究揭示晚奥陶火山活动频繁且具有全球性意义。(3)提出长时间尺度的有机碳埋藏增加导致的气候变冷以及海洋氧化是奥陶纪生物大辐射的驱动因素。通过本项目的顺利实施，我们对奥陶纪海洋碳、硫、汞循环及其古环境指示意义有了更深入的理解，为奥陶纪生物大辐射以及晚奥陶世生物大灭绝的驱动机制提供了新的认识。