AUTHIGENIC CARBONATE IN LACUSTRINE SEDIMENTS OF LUSHI BASIN DURING THE MIDDLE EOCENE AND THE PALEOCLIMATE IMPLICATION
The Eocene climate can serve as an analog to future global warming,making it imperative to scrutinize the climatic variations of the Eocene warm period and understand their underlying forcing mechanisms.The Lushi Basin,situated at Lushi County,Henan Province,China,is a NE-SW trending Cenozoic fault depression basin nestled between the Xiaoshan Mountains and Xiongershan Mountains at 33°57′～34°10′N and 110°35′～111°14′E.With 28 km in length and 10～13 km in width,and a total area of about 300 km2,the Lushi Basin is located in the eastern segment of Qinling Mountains in Central China.This basin has witnessed the continuous deposition of Middle Eocene lacustrine sediments rich in carbonate minerals,providing an exemplary archive for reconstructing the Middle Eocene warm terrestrial climate.A continuously sampling effort yielded 505 samples from both the middle and upper sections of Zhangjiacun Formation and entire Lushi Formation(34° 04′N,111° 10′ E),with a cumulative thickness of 288 m during the Middle Eocene.Microscopic analysis of thin sections reveals that the mineral composition of the Middle Eocene lacustrine sediments in Lushi Basin predominantly comprises carbonates,clays,and silts.The carbonate component is inferred to have formed under surficial conditions in a closed or semi-closed freshwater or slightly saline lake,arising from authigenic chemical processes.Utilizing Fourier transform infrared spectroscopy(FTIR)diffuse reflectance,carbonate content of the sediment was quantified,establishing a negative correlation with magnetic susceptibility of the samples.These parameters reveal the intensity of endogenic chemical sedimentation and exogenic detrital input,respectively,influenced by the evaporation and precipitation.Consequently,they unveil wet and dry variations in Lushi Basin during the Middle Eocene.Increased precipitation or decreased evaporation,indicative of a wet environment,proves unfavorable for carbonate precipitation but conducive to exogenic detrital input,this results in a decrease in carbonate content and an increase in magnetic susceptibility of sediments.Conversely,dry conditions lead to the opposite deposition process.Time-series analysis of the lacustrine sediment carbonate content reveals variations following Earth's orbital cycles,documenting a relatively humid climate during the Middle Eocene Climatic Optimum(MECO).
万方数据
维普
