西藏南部Cenomanian-Turonian缺氧事件:有机地球化学研究

Cenomanian- Turonian anoxic event in southern Tibet: A study of organic geochemistry

基于藏南中白垩统Cenomanian-Turonian缺氧事件沉积的有机地球化学分析，对黑色页岩的有机质输入及其沉积环境进行了系统研究，并探讨了特征生物标志化合物与缺氧事件之间的关系。研究表明，缺氧层内高含量有机碳的黑色页岩与灰绿色页岩、泥灰岩构成二级旋回地层；有机质的母质输入以海洋生物的菌藻类为主。生物标志物与缺氧事件对应研究表明，Pr/Ph在剖面上的相对含量是判别氧化还原的良好指标，胡萝卜烷含量（β-胡萝卜烷和γ-胡萝卜烷）是反映藏南地区沉积环境的氧化还原条件变化的敏感特征标志物。还原条件下的缺氧事件层内，胡萝卜烷含量特别高，而氧化环境下，胡萝卜烷含量相对偏低。

The Cenomanian- Turonian oceanic anoxic event (C/T OAE) is developed in southern Xizang Zizhiqu(Tibet). An organic geochemical study of the Cenomanian- Turonian sediments from Gamba and Tingri areas shows that mid- Cretaceous black shales in southern Tibet are rich in organic carbon content, with values ranging from 0.4％ to 1.68％ . The latest Cenomanian- earliest Turonian in Gamba area is subdivided into five cyclothems according to the organic carbon content and lithology. Each cyclothem consists of total organic carbon (TOC)- enriched black shale interbedded with TOC- low mudstone/marl. This alteration represents the change from anoxic to oxic depositional condition. The distributions of biomarkers indicate that the Cenomanian- Turonian anoxic sediments are dominated by marine organic matter including algae and bacteria. In Gamba area, the organic matter is characterized by the abundance of tricyclic terpanes and pregnane, which are predominant in m/z 191 and m/z 217 mass chromatograms. On the basis of molecular maturity parameters, the organic matter is high mature. The pristane/phytane (Pr/Ph) ratios in the C/T OAE sediments are less than 1, with an average of 0.17, demonstrating the domination of phytane. Our study indicates that the Pr/Ph change in the sedimentary profile may reflect the change of redox condition in the depositional environment. The presence of carotenoid compounds can be regarded as a special mark of oxygen depletion in the C/T OAE sediments in the Tethyan Himalayas. In anoxic sediments, β - carotane and γ - carotane are very abundant. The β - and γ - carotane ratios relative to nC17 in the Cenomanian- Turonian anoxic sediments vary from 32.28 to 42.87 and from 5.10 to 11.01, respectively. Therefore, β - and γ - carotanes can be considered as sensitive indicators of redox conditions in depositional environments in the Tibetan Himalayas.