西宁盆地岩石磁学记录揭示的始新世-中新世古环境演化及驱动机制

Paleoenvironmental evolution and driving mechanism of Eocene to Miocene revealed by rock magnetic records in Xining Basin

亚洲气候环境以东部湿润的季风气候区和西部干旱-半干旱区为典型特征,影响着世界上超过一半人口的生活。河湖相沉积是研究气候变化和环境演变的重要地质记录,而岩石磁学作为一种简便的方法在古气候重建中得到广泛应用。位于青藏高原东北缘的西宁盆地沉积了巨厚的跨越始新世-中新世的连续河湖相地层,是研究亚洲季风湿润气候与内陆干旱气候过渡区新生代环境演变的宝贵档案。本研究对西宁盆地关家山和西宁东复合剖面始新世至中新世厚约900 m的河湖相沉积物进行了系统的岩石磁学研究。首先,对185个泥岩或粉砂岩样品进行传统的磁化率测试,然后在其中挑选出103个样品进行柠檬酸钠-碳酸氢钠-连二亚硫酸钠(CBD)地球化学溶解实验,成功分离出了来自源区的粗颗粒碎屑磁性矿物组分以及沉积后新生成的细颗粒成壤磁性矿物组分的信号。结果显示,新生代以来,成壤作用产生的细颗粒磁性矿物含量变化与全球温度有着较好的对应关系,是影响西宁盆地沉积物磁化率变化的主要原因,碎屑组分的磁化率与全岩磁化率的比值可以半定量估计不同时间段构造或气候对磁化率的影响。始新世期间,西宁盆地气候温暖湿润,成壤作用较强,河湖相沉积物中细颗粒磁性矿物的占比较高;始新世-渐新世气候转型以来,全球变冷导致西宁盆地干旱化加剧,成壤作用减弱;渐新世晚期-中新世期间,碎屑组分磁性矿物对磁化率贡献的占比增加至约60%,碎屑成因磁性矿物的输入对磁化率变化的影响增大;此外,约25 Ma磁化率的增加可能受到全球气候变化和青藏高原隆升的共同控制。

The climate in Asia is characterized by a humid monsoon climate in the east and an arid and semi-arid region in the west,significantly impacting over half of the global population.Fluvial lacustrine deposition serves as a crucial geological record for studying climate change and environmental evolution,and rock magnetism is a widely employed in paleoclimate reconstruction as a convenient method.The Xining Basin,located on the northeastern edge of the Tibetan Plateau,contains extensive,thick,and continuous fluvial lacustrine strata from the Eocene to the Miocene,which is a valuable archive for investigating Cenozoic environmental evolution in the transitional area between the humid Asian monsoon climate and the arid inland climate.This study conducted a systematic rock magnetic analysis of the Eocene to Miocene fluvial lacustrine sediments spanning approximately 900 m of the Guanjiashan-East Xining composite section(Guanjiashan section:from 36°39.48′N,101°52.31′E to 36°40.55′N,101°50.24′E;East Xining section:from 36°34.77′N,101°53.81′E to 36°35.05′N,101°53.56′E)in the Xining Basin.Firstly,185 mudstone or siltstone samples underwent traditional magnetic susceptibility testing,and then 103 samples were chosen for geochemical dissolution experiments of citrate-bicarbonate-dithionite(CBD),and successfully isolated signals of coarse detrital magnetic mineral components from the source area and newly formed fine pedogenic magnetic mineral components.The findings indicate that the variation in the fine magnetic mineral content resulting from pedogenesis since the Cenozoic correlates well with global temperatures,which primarily influences magnetic susceptibility of sediments in the Xining Basin.The ratio of the magnetic susceptibility of the clastic component to that of the bulk samples can serves as a semi-quantitative method to estimate the impact of tectonic activities or climate on magnetic susceptibility during different periods.During the Eocene,the warm and humid climate of Xining Basin led to strong pedogenesis,consequently leading to a relatively high proportion of fine-grained magnetic minerals in the fluvial and lacustrine sediments.Since the Eocene-Oligocene climate transition,global cooling has resulted in the intensification of aridity and the weakening of pedogenesis.During the Late Oligocene to Miocene,the proportion of clastic component magnetic minerals contributing to susceptibility increased to approximately 60%,amplifying the impact of these minerals input on magnetic susceptibility.Additionally,the increase of ca.25 Ma magnetic susceptibility may be controlled by global climate change and the uplift of the Tibetan Plateau.