PALEOENVIRONMENTAL EVOLUTION AND DRIVING MECHANISM OF EOCENE TO MIOCENE REVEALED BY ROCK MAGNETIC RECORDS IN XINING BASIN
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.
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维普
