MASSIVE DISSOCIATION OF GAS HYDRATE DURING OCEANIC ANOXIA AS A CAUSE OF MASS EXTINCTION AT THE END OF PERMIAN
Li Yucheng, Zhou Zhongze (Department of Biological Resource and Environmental Sciences,Anhui University, Hefei 230039 Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008)
Mass extinction at the end of Permian eliminated over 90% of all marine species and had a significant impact on land species as well. Many hypotheses have been proposed to explain this mass extinction, but most of them do not account for observed taxonomic,physiological,ecological, geochemical and biogeographic patterns of extinction. Here, we present a hypothesis for the P/T mass extinction. It is proposed that volcanic eruption might have been the trigger for hydrate dissociation during oceanic anoxia at the end of Permian, such massive dissociation of gas hydrate probably contributed to environmental extreme that led to the mass extinction. This paper provides geological and geochemical evidence from section A in Meishan, Changxing for this hypothesis. As one of the effective methods of distinguishing and interpreting events in carbonate sequences, systematic analyses of carbon and oxygen isotopic composition have manifested stimulating significance in eventostratigraphy. Systematic analyses of 93 pairs of δ 13 C and δ 18 O values across the P/T boundary of the Meishan section within a thickness of approximately 5 m in NW Zhejiang have been conducted by Yan et al. (1991). The following conclusions can be drawn from high resolution carbon and oxygen isotope profile: (1) The anoxic deposition has been characterized by a large drop in δ 13 C with a magnitude of about 6‰, and, a trough in δ 18 O just at the latest Permian within a 66 cm thick unit below the appearance of Hindeodus parvas ; this trough is accompanied by fine lamination,and abundant pyrite in many sections in South China. (2) Based on carbon cyclostratigraphic age estimates, the time span of the anoxia is about 60 100 thousands of years. (3) The oceanic anoxia at the end of Permian might have been produced by increases in oxygen demanding organic materials and atmospheric CO 2. The latter was mostly derived from large scale volcanic eruption leading to rapid transgression,and voluminous and extremely rapid release of CH 4, perhaps, H 2S from gas hydrate contained in deep anoxic oceans. Such events may have been responsible for the mass extinction at the end of Permian.