PALEOCLIMATE VARIATIONS DURING MIDDLE TO LATE EOCENE AT THE WEIHE BASIN,CENTRAL CHINA,REVEALED BY HIGH-RESOLUTION MAGNETIC SUSCEPTIBILITY
The Eocene warm period can provide an analog for understanding current climate warming and its forcing mechanism.The Honghe Formation,an alluvial to lacustrine deposits,is widely distributed in deep part of the Cenozoic deposit of the Weihe Basin,Central China.There are more than 330 m thick in the outcrop and more than 1000 m deep in the depocenter,the Honghe Formation is red sedimentary sequence with detritus and chemical deposits.There are carbonate,mudstone,sandstone and few conglomerate at the margin of basin and mainly argillaceous limestone,mudstone and siltstone in depocenter in the Weihe Basin during the Eocene.We took samples from the 330 m sedimentary stratum of the Honghe Formation at 10 cm intervals from Wangpingcun(WPC)site(34°18′N,109°12′E).The sequence of Honghe Formation is dated between 46～33 Ma by a magnetostratigraphy analysis.Our low-and high-frequency magnetic susceptibility(MS)analyses of 2910 bulk samples show that the MS varied in 0～20×10-8 m3/kg,these are lower MS value compared with the Chinese loess-paleosol deposit sequence at same site,but corresponding the low MS value of Paleogene sediments in North China;meanwhile,the frequency MS is relatively lower with value of 0～11％,showing a strongly weathering thus magnetite and maghematite were oxidized to hematite,along with low MS and/or anti-magnetic sediments input,caused the lower MS value of sediments of the Honghe Formation.Our MS time series show multi-cycle and long-term hydroclimate variations,which are influenced by variations of insolation and earth surface feedback.Our analysis also show that mudstone and siltstone samples have higher MS,but sandstone samples have lower MS.The thermal demagnetization curve reveal that hematite is the main magnetic mineral in the bulk samples.Moreover,grain size variations of Honghe Formation sedimentary sequence were controlled by variations in lake levels.During high lake level,sediment in lakeshore was finer and MS values were higher,and vice versa.Therefore,variations of MS values can indicate fluctuations of lake level and variations of hydroclimate.The multi-cycle variations of lake level and stepwise drying in the Weihe Basin during the Middle to Late Eocene can be well compared with decrease in global temperature,which may modulate water cycle and aridification of interior East Asia during the Cenozoic era.
Middle to Late EoceneWeihe Basinmagnetic susceptibilityfluvial and lacustrine depositpaleoclimate variations
万方数据
维普
