鄂西地区上泥盆统碳质粉砂岩及黄铁矿地球化学特征

Geochemical characteristics of carbonaceous siltstones and pyrite of the Upper Devonian in Western Hubei

晚泥盆世弗拉期—法门期(F-F)之交生物大灭绝事件一直是被学者关注的热点,事件的起因与机制并没有定论。前人对华南地区事件期沉积序列、地层同位素、底栖、微体生物和维管植物的类型及演化进行了研究。但对事件起始期下Kellwasser事件研究较为薄弱,对其古气候、古环境认识尚不足。针对这些问题,本文对鄂西地区上泥盆统黄家磴组碳质粉砂岩与沉积黄铁矿微量元素与硫同位素进行测试分析,探讨了下Kellwasser事件期气候与环境。研究获得下列认识:1)晚泥盆世弗拉期鄂西地区为淡水环境,气候潮湿,水温约为31℃;2)受陆源营养物质供给的影响,海洋中生物生产力较高,大量沉积有机质分解,厌氧带向下扩张,造成水体氧化—还原带分层;3)大气氧含量介于17.23%~21.18%,黄铁矿δ^(34)S介于-12.01‰~-4.67‰,均未见明显波动;4)下Kellwasser事件未表现出明显的火山喷发、海水缺氧与构造作用成因的特征。研究成果为理解F-F生物大灭绝事件起因提供了参考,也丰富了中-上扬子地区晚泥盆世古气候、古环境认识。

Late Devonian Frasnian-Famennian period(F-F)biological extinction event has always been the hot spot of the attention,the cause of the event and the mechanism were not determined.The types and evolution of sedimentary sequences,stratigraphic isotopes,benthos,microorganisms and vascular plants during the event period in South China have been studied previously.However,the study of Kellwasser event under the initial stage of the event is relatively weak,and the paleoclimate and paleo-environment are not well understood.In order to address these problems,this paper analyzes the trace elements and sulfur isotopes of carbonaceous siltstone and sedimentary pyrite in the Upper Devonian Huangjiadeng Formation in western Hubei Province,and discusses the climate and environment during the lower Kellwasser event.The following conclusions are obtained:1)During the Late Devonian Frasnian,the western Hubei Province was a fresh water sedimentary environment with a humid climate and a water temperature of about 31℃.2)Influenced by the supply of terrestrial nutrients,the biological productivity in the ocean is elevated,and a large number of sedimentary organic matters decompose,and the anaerobic zone expands downward,resulting in the stratification of the redox zone in the water.3)The atmospheric oxygen content ranged from 17.23%to 21.18%,and the pyrite δ^(34)S ranged from-12.01‰to-4.67‰,showing no significant fluctuation.4)The lower Kellwasser event did not show the characteristics of volcanic eruption,seawater anoxia and tectonism.Our results provide a reference for understanding the cause of the F-F mass extinction event and enrich the understanding of the paleoclimate and paleo-environment of the Late Devonian in the Middle and Upper Yangtze region.