中国煤山剖面二叠/三叠系事件界线地层中石膏的负硫同位素异常

Evidence of Extremely Light Gypsum from the Permian-Triassic (P/T) Event Boundary at the Meishan Section of South China

中国煤山剖面二叠系 /三叠系 (P/T)事件界线地层以赋存白粘土和黑粘土著称。本文采用六氟化硫方法经质谱测定发现 ,白粘土中的自形晶石膏、白粘土中的全溶提取石膏和黑粘土中的全溶提取石膏 ,其S同位素均呈显著负值 :δ3 4 SV CDT均值依次为 - 9.2‰、- 10 .7‰和 - 15 .6‰ ;δ3 3 SV CDT均值依次为 - 4 .6‰、- 5 .5‰和 - 8.2‰。P/T事件界线地层中硫酸盐 (石膏 )S同位素如此极端负异常在煤山剖面尚属首次发现 ;在其他P/T剖面乃至在与生物灭绝议题相关的全球其他重要地层界线剖面 ,迄今也未见类似报道。本文还测定出与白粘土密切相关并紧邻的似黄铁矿的铁硫化物 (Fe S) ,其S同位素的δ3 4SV CDT均值为 + 1.3‰和δ3 3 SV CDT均值为 + 0 .7‰ ,且研究中未见任何生物或有机成因黄铁矿痕迹。这在相当大的程度上排除了这里的极轻硫石膏的生物或有机次生来源的可能性。因此 ,本文揭示的显著负硫同位素石膏很可能在自然界中也是罕见的 ,值得进一步积累数据和深入研究。另一方面 ,煤山剖面P/T事件界线层原本富含石膏 ,其极轻硫同位素特征的发现很可能成为一个重要突破口 ,有利于从新的角度认识和探索P/T转折期的灾变与环境变化。

There are famous 'white clay' and 'black clay' existing in the Permian Triassic (P/T) event boundary at the Meishan section that is located in Changxing County, Zhejiang Province of South China.This study focuses on detecting sulfur composition of the gypsum naturally trapped in both of the boundary clays, by isotopic mass spectrometry assistant with standard method of SF 6 preparation. To our surprise, the gypsum proves to have extraordinarily negative delta values, including -9.2‰ (δ 34 S V CDT ) or -4.6‰ (δ 33 S V CDT ) in average for the gypsum in euhedral crystal selected form the white clay, -10.7‰ or -5.5‰ for the whole gypsum water extracted from the same clay, and -15.6‰ or -8.2‰ for those water extracted from the black clay. To our know ledge, such extremely light sulfur dominated gypsum, which now offers the most negative anomaly of sulfur isotope ratio of sulfate towards the P/T event boundary at Meishan section, is originally recognized in this study, not only for the P/T sections but also for other mass extinction related boundary sections in the world. This finding would open a new window for insight into the P/T catastrophic event as well as its subsequent changes of environment in the past of the Earth. Furthermore, native Fe S nuggets close to the white clay are also determined to be usual isotopic composition in average values of +1.3‰ (δ 34 S V CDT ) and +0.7‰ ( δ 33 S V CDT ),and any trace of expecting organic genetic pyrites as well as their analogs fails to prove in our efforts, both of which probably excludes a possibility of organic genesis for the light gypsum we find. Thus, origin of this unexpected gypsum remains to be understood in future.