中国南方二叠-三叠纪过渡时期的碳同位素旋回地层与突变事件

Gradual and abrupt shifts in carbon isotope of limestonesduring Permian Triassic transitional period in South China

在中国南方晚二叠世至早三叠世初碳酸盐岩的Ｃ 同位素演化基础上， 考察了长兴阶层型剖面的高分辨率Ｃ 同位素旋回地层，发现长兴阶有三个Ｃ 同位素四级旋回，与吴家坪阶晚期的一个四级旋回共同组成了一个完整的三级旋回。与这种缓慢变化形成强烈对比的是：Ｐ／ Ｔ 界线上在１ ．５ ～２．６ 万ａ 左右的地球年时间中δ１３ Ｃ 突变为高负值， 形成剧烈负异常， 其中生物集群绝灭因素可能占２／３ 以上，变负幅度约５‰～６ ‰。二叠纪末生物集群绝灭的原因可能与大规模的火山爆发引起的气体增加和缺氧事件有关。

During the Permo Triassic transitional period there occurred many notable events,such as mass extinction, sea level change,oceanic anoxia and extensive volcanism in South China,which is a most important region for study of these events. This paper is to provide more detailed information about high resolution carbon isotope cyclostratigraphy in Upper Permian(especially Changhsingian) and Lower Triassic limestones, to reconstruct gradual and abrupt shifts in carbon isotope of limestones during Permian/Triassic transitional period in South China and to further understand the mass extinction events at the end of Permian.The variation of carbon isotope with time can be recognized during Permo Triassic transitional period. There are two 3rd order cycles in Late Permian, and one in the Earliest Triassic. The early 3rd order cycles are included in the lower and middle Wuchiapingian; while the middle one is included in the upper Wuchiapingian and the Changhsingian, which consists of four the 4th order cycles of carbon isotope stratigraphy; Each of three 4th cycles of carbon and oxygen isotope stratigraphy is perfectly conincident with 'subsequence' at section of Changhsingian stratotype. Based on the biostratigraphic age estimates, the time span of the 11～18 m thick sediments in the 4th order isotpe cyclostratigraphy ranges from 1.2 Ma to 1.7 Ma. The author consider that the increases in atmospheric CO2 thatderived from large scale volcanic eruption,a major greenhouse gas, may have been responsible for mass extinction at Permian/Triassic boundary.