Siberian gas venting and the end-Permian environmental crisis

摘要

The end of the Permian period is marked by global warming and the biggest known mass extinction on Earth. The crisis is commonly attributed to the formation of the Siberian Traps Large Igneous Province although the causal mechanisms remain disputed. We show that heating of Tunguska Basin sediments by the ascending magma played a key role in triggering the crisis. Our conclusions are based on extensive field work in Siberia in 2004 and 2006. Heating of organic-rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion. Basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated > 100,000 Gt CO2. The gases were released to the end-Permian atmosphere partly through spectacular pipe structures with kilometre-sized craters. Dating of a sill intrusion by the U-Pb method shows that the gas release occurred at 252.0 +/- 0.4 million years ago, overlapping in time with the end-Permian global warming and mass extinction. Heating experiments to 275 degrees C on petroleum-bearing rock salt from Siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases. The results indicate that global warming and ozone depletion were the two main drivers for the end-Permian environmental crisis. We demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a Large Igneous Provinces causes an environmental crisis or not. We propose that a similar mechanism could have been responsible for the Triassic-Jurassic (similar to 200 Ma) global warming and mass extinction, based on the presence of thick sill intrusions in the evaporite deposits of the Amazon Basin in Brazil.