华南早三叠世缺硅事件研究

The end-Permian saw the largest mass extinction in life's history. Numerous extinction mechanisms have been suggested to explain the causes of the end Permian biotic extinction with ocean anoxia proposed as one of the most important causes.The spread of anoxia coincides with the disappearance of chert deposition in many sections around the world. Thus, collapse of ecosystem during the Permian-Triassic mass extinction led to an Early Triassic 'chert gap'. Recently. in Panthalassa, there are some chert deposition which across the PTB transition have been found in the pelagic sections in Arrow Rocks and Waiheke Island in New Zealand and Japan.The chert deposition of Dalong Formation widespread in South China. The age of Dalong Formation was assigned to Latest Permian.Hence, it is common accepted there was an Early Triassic chert gap in South China. During past few years, based on the conondont study,the research team found some typical Early Triassic conodont species Isarcicella isarcica and Isaricicella staeschei from the upper part of Dalong Formation at the Gaimao section in Guiyang, Guizhou Provice. There maybe also radiolaria-bearing chert deposition extended to Early Triassic at Xinmin section in Anshun, Guizhou Province. The new evidence push us to re-check the age of Dalong Fromation. This project will mainly focus on studying conodont and radiolarian biostratigraphy, microfacies, biotic diversity as well as geochemistry on Dalong Formation in some key areas in South China. Based above study, this project try to establish high-resolution chronological sequence of Dalong Formation, to define the age and palaeoenvironments of "Early Triassic chert". Furthermore, this peoject will also study the palaeogeograpgical background of the Early Triassic chert deposition. This study is of important sigificance for better understanding the Permian-Triassic mass extinction.

二叠纪末发生了生物史最大的一次灭绝事件。因此，人们认为P/T大灭绝引起的生态系的坍塌造成了早三叠世缺硅事件。近年来，在泛大洋地区的新西兰和以及日本等地深水剖面报道了早三叠世硅质岩的存在。广布于华南地区大隆组硅质岩沉积被归于晚二叠世晚期。因此普遍认为华南地区早三叠世缺乏硅质沉积。近年来，本研究组在贵州改毛剖面大隆组上午发现早三叠世早期典型分子Isarcicella isarcica 和 Isaricicella staeschei。因此，新的证据使得我们必须重新检验大隆组上部的时代。本项目拟通过对华南关键地区大隆组顶部地层的牙形石、放射虫以及沉积微相、生物分异度、地球化学等研究，建立精细年代地层序列，厘定"早三叠世的硅质岩"的确切时代及沉积环境，在此基础上研究"早三叠世缺硅事件"的古地理响应，可进一步帮助我们深入了解二叠纪-三叠纪之交这一史上最大生物大绝灭事件。

以验证二叠纪末大灭绝后华南地区是否存在早三叠世的缺硅事件为目标，本项目组在华南及秦岭地区的贵州、广西、湖南、湖北、重庆、陕西和甘肃等省系统测制了15条二叠-三叠系剖面。通过牙形石、双壳类及菊石等化石门类的研究，证实了以硅质岩为主的大隆组的时代存在穿时现象（晚二叠世吴家坪期晚期至长兴期或更高层位），但是华南广大地区的大隆组时代局限在晚二叠世。本研究唯一的特例是贵州遵义新民剖面，根据牙形石生物地层研究，新民剖面大隆组硅质岩延至三叠系底部，其层位火山活动十分发育。因此，本项目组认为因为二叠纪末生物大灭绝造成生态系的坍塌确实造成了华南广大地区早三叠世的缺硅事件。本项目组在华南多条重要剖面上建立了高精度的牙形石生物地层，其中在湖南慈利高化剖面和贵州边阳打讲剖面上，根据牙形石Hindeodus parvus的首次出现厘定了这两条剖面的二叠-三叠系界线以及微生物岩的时代，认为这两条剖面上的微生物出现在早三叠世Hindeodus parvus带内，而不是以前所认为的二叠系顶部。近年来，早三叠世Smithian期-Spathian期之交的生物危机被认为是早三叠世主要的灭绝事件并得到地质学家们的重视。通过对贵州甲戎剖面上牙形石大小的详细测量，本项目组发现牙形石在S-S界线处出现了明显的小型化现象，这一现象与该时期的极热高温相吻合，预示全球造成了牙形石的小型化。通过采用古生物学、古生态学、沉积学、草莓状黄铁矿分析以及地球化学等多种手段，本项目组详细研究了贵州甲戎剖面和明塘剖面上S-S界线附近的氧化还原条件的变化，揭示了在S-S界线附近出现了缺氧事件，进一步证明极热高温及缺氧事件造成了S-S界线处的生物危机。以上工作对于更好地了解华南地区二叠-三叠纪之交深水相和浅水相的生物演化和环境变化之间的因果关系具有重要意义。此外，本项目组在贵州和云南测制了6条晚三叠世海相地层剖面，获得了丰富的牙形石标本，这对该地区上三叠统的划分和对比具有重要意义。通过本项目的工作，证实了晚三叠世卡尼洪水事件在华南地区的存在。