地球化学揭示早—中侏罗世陆相地层气候环境变化特征——以滇中川街盆地老文村剖面为例

Geochemical analysis of climatic and environmental changes in Early-Middle Jurassic continental strata--A case study of the Laowencun section in Chuanjie Basin,central Yunnan

侏罗纪温室气候是中生代以来重要的气候变暖事件,全球关于该事件的研究主要集中在海相地层中,关于陆相地层的研究较少,滇中禄丰川街盆地沉积了完整的侏罗纪地层,为研究侏罗纪温室地球气候环境提供了良好的载体。本次工作通过对川街盆地早—中侏罗世陆相红层的岩石地层、微量元素地球化学研究认为,滇中地区早侏罗世早期主要为高温、湿润的气候环境,早侏罗世晚期气候波动剧烈且频繁,湖泊盐度急剧上升,气温缓慢下降,在即将进入中侏罗世时气温骤然上升,水体盐度快速降低,发生“T—OAE事件”,之后则保持较为稳定的高温、干燥的气候环境。通过对滇中侏罗纪恐龙埋藏层位的研究发现,早侏罗世晚期气候环境的突变层位与该层位位置相当,沉积特征及地球化学特征表明恐龙的集群性埋藏与该事件有着密不可分的关系。

Objectives:The Jurassic greenhouse climate is an important climate warming event since Mesozoic.The global research on this event mainly focuses on marine strata,but less on continental strata.The Jurassic strata have been deposited in Chuanjie Basin in Lufeng,central Yunnan,which provides a good carrier for studying the Jurassic greenhouse climate environment.Methods:The research object of this work is another fully exposed section newly discovered of the Early and Middle Jurassic in the Chuanjie Basin,sampling and analysis of petrogeochemistry,trace elements,etc,the sedimentary facies have been redefined,build the chemical stratigraphic framework of the upper section of the Lufeng—Chuanjie Formation through the longitudinal variation of CIA,B/Ga,U/Th,Sr/Cu,etc.of fine clastic rock samples.Results:Through petro-and trace element geochemical signatures we systematically restored the climate change characteristics of the Chuanjie Basin in the Early and Middle Jurassic,which can be divided into three periods:warm humid—hot dry alternation period in the early and middle Early Jurassic,hot dry period with frequent extreme weather(extreme drought,rainstorm)in the late Early Jurassic(similar to the current tropical grassland climate),and climate stability period in the early Middle Jurassic,identified the response of the T—OAE event in the basin in the upper Lufeng Formation,manifested as the increases of proportion of dark purple red mudstone,extensive development of dolostone,argillaceous dolostone,dolomitic mudstone,and paleosol,significantly reduced supply of sandstone and coarse debris clarified its impact on the mass extinction and rapid burial of dinosaurs,find the culprits are extreme drought,water shortage and short-term rainstorm,and filled the gap in previous research.Conclusions:Based on the study of Jurassic dinosaur burial horizon in central Yunnan,it is found that the abrupt change horizon of climate environment in the late Early Jurassic is equivalent to this horizon,and the sedimentary and geochemical characteristics show that the cluster burial of dinosaurs is closely related to this event.