川东北地区中侏罗世新田沟期古气候波动的地球化学响应

四川盆地中侏罗统新田沟组作为侏罗纪大面积红层之下的重要岩石地层单元,见证了盆地由潮湿向干旱转变的整个过程。然而,以往的研究主要集中在古气候类型方面,缺乏系统深入的气候变化研究。本文以川东北地区宣汉七里乡塔坝剖面为对象,系统分析了泥岩的主微量元素地球化学特征,结果表明:化学蚀变指数(CIA)、C值、Sr/Cu、Sr/Ba、Rb/Ti、Cs/Ti、Rb/K等7种古气候替代指标综合显示新田沟期气候存在从炎热潮湿逐渐向干旱转变的趋势,整体以半干旱-半潮湿气候为主。且该时期气候变化过程并不稳定,存在多个短期的湿热与干旱的气候波动。在新田沟组沉积末期,气候条件变得极端干燥。川东北地区中侏罗世的元素地球化学特征记录了区域内重大地质转折期将至时的气候变化,在侏罗纪大面积红层发生之前,气候条件已经开始响应。在中侏罗世早期,四川盆地与欧洲地区并不处于同一个气候带内。

Geochemical response to paleoclimate fluctuation during Xintiangou Period (Middle Jurassic), Northeastern Sichuan

Xintiangou Formation is a key stratigraphic interval beneath the wide- spread Jurassic red strata in the Sichuan basin, and records the climatic change from humidity to drought in the early Middle Jurassic. However, previous researches focused mainly on palaeoclimate patterns, and lack of deep and systematic study of climatic change. This paper investigated geochemical characteristic of mudstone from the Taba section of the Middle Jurassic Xintiangou Formation. The results show that seven climatic proxies (CIA, C- value, Sr/Cu, Sr/Ba, Rb/Ti, Cs/Ti, Rb/K) are indicative of the palaeoclimate change from hot and humid to drought, and dominantly semi- arid- semi- humid climate. However, the process of the climate change was not stable, fluctuating in several hot/humid and dry periods. At the end of the Xintiangou Formation sedimentary period, climate condition became extremely dry. The geochemical characteristics of the Xintiangou Formation mudstones in the northeastern Sichuan Basin records the climate changes at a crucial turning point of geological history, indicating that the climate started to respond before occurrence of large- scale deposition of Jurassic red bed. During the early Middle Jurassic, the Sichuan basin was no in the same climate zone as the Europe.