柴达木盆地侏罗纪古气候演变过程:来自化学风化特征的证据

Jurassic Palaeoclimate Evolution of the Qaidam Basin: Evidence from Chemical Weathering Analyses

柴达木盆地侏罗系陆相地层记录了该时期一系列重大的构造气候环境耦合演化事件,然而针对该套地层的古气候、古风化作用等方面研究还很薄弱。为了恢复柴达木盆地侏罗纪时期的古气候和古环境特征,文章研究对柴北缘大煤沟侏罗系标准剖面的沉积岩样品开展了全岩主、微量元素测试,综合采用多种化学风化指数判定柴达木盆地侏罗纪古气候条件及演变过程。元素比值(Rb/Sr、K/Na、Na/Al和Na/Ti)和化学风化指数(CIA、CIW、PIA和ICV)均显示,柴达木盆地侏罗纪化学风化过程和古气候演化过程可划分为4个阶段:早侏罗世早中期和中侏罗世中期,化学风化作用强烈,古气候处于温带亚热带温暖、潮湿气候条件;早侏罗世晚期和中侏罗世晚期晚侏罗世,受到区域降水减少影响,盆地化学风化作用显著减弱,整体处于干旱、半干旱环境。以上结果与前人基于沉积物和植物群面貌研究得出的古气候变化过程相吻合,为盆地及区域陆相侏罗纪古气候研究提供了相对完整而连续的地球化学证据和参考。

A series of distinct tectonic-climatic-environmental coupling evolution events were recorded in terrestrial sequences of the Qaidam Basin during the Jurassic.However,little research has focused on the paleoclimate and paleoweathering variation process of these sequences.To reconstruct the Jurassic paleoclimate and paleoenvironment of the Qaidam Basin,major and trace elemental analyses were carried out on the bulk-rock samples that were collected from the Dameigou section,which is a Jurassic standard section located in the northern basin margin.The reconstruction mainly utilized various chemical weathering indices,including Rb/Sr,K/Na,CIA and CIW.Results indicated that the Jurassic chemical weathering process and paleoclimate evolution of the Qaidam Basin can be divided into four stages.During the early to middle episode of the Early Jurassic and middle episode of the Middle Jurassic,intense chemical weathering occurred in the Qaidam Basin,indicating a temperate-subtropical warm and humid climate.During the late Early Jurassic to Late Jurassic,due to the decrease of the regional precipitation,the chemical weathering of the basin was significantly reduced,and the climate was semi-arid or arid.These geochemical analyses are in agreement with previous results on climate-sensitive sediments and palaeobotany.This investigation will provide complete and continuous geochemical evidence for the change of Jurassic paleoclimate in the Qaidam Basin and adjacent regions.