卢氏盆地中始新世沉积序列自生碳酸盐及其古气候意义

AUTHIGENIC CARBONATE IN LACUSTRINE SEDIMENTS OF LUSHI BASIN DURING THE MIDDLE EOCENE AND THE PALEOCLIMATE IMPLICATION

始新世暖期可作为当代全球变暖的历史相似型,研究始新世气候变化及其机制对于理解现代气候变暖的幅度和速率具有重要意义。卢氏盆地位于中国中部秦岭东段,发育了连续的、富含碳酸盐矿物的始新世湖相沉积物,是重建始新世暖期陆地气候的良好材料。本研究在卢氏盆地郑家山-三角沟一带(34°04′N, 111°10′E)采集了张家村组中上部和卢氏组的样品,采样层厚约288 m,共采集样品505个。根据岩石薄片镜下鉴定,卢氏盆地中始新世湖相沉积物物质组成以碳酸盐、粘土和粉砂为主,其中碳酸盐沉积为表生条件下封闭或半封闭淡水湖或微咸水湖化学作用自生成因。使用傅里叶红外光谱(FTIR)漫反射法测定沉积物碳酸盐含量,发现其与磁化率具有一定的负相关关系,分别指示了内源化学沉积和外源碎屑输入的贡献量,受控于流域蒸发量与降水量的共同作用,揭示了中始新世卢氏盆地的干湿变化。当流域更加湿润,即降水量增加(蒸发量减少)时,径流量相应增加,不利于碳酸盐沉淀而有利于外源碎屑输入,表现为沉积物碳酸盐含量降低、磁化率数值升高,反之同理。在已有年代框架的基础上,建立了卢氏盆地湖相沉积物碳酸盐含量时间序列,其记录了在中始新世气候适宜期(MECO)卢氏盆地较为湿润的气候环境,并且具有多旋回变化,与地球运动轨道的周期变化相关。

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).