THE PALEOCLIMATIC SIGNIFICANCE INDICATED BY THE EARLY PALEOGENE GRAY-BLACK MUDSTONE LAYERS LIES IN NANXIONG BASIN
During the Early Paleogene,a typical greenhouse climate period in the Cenozoic era,a series of rapid and transient warming events known as"hyperthermals"took place.During these hyperthermals,there was a significant negative carbon isotope excursion(CIE)accompanied by a shoaling of the carbonate compensation depth(CCD),an increase in atmospheric CO2 levels,leading to environmental changes and ecological effects.Currently,most global hyperthermal records are concentrated in marine sediments,while terrestrial records are relatively scarce.The Nanxiong Basin(25°03′～25°16′N,114°08′～114°40′E),located in Southeastern China,has complete preservation of Early Paleogene terrestrial strata and is a hotspot for the study of paleoclimate reconstruction.This terrestrial stratum is dominated by red silty mudstone and muddy siltstone.In the Guchengcun Formation,there are several interbedded gray-black mudstone layers with varying thicknesses.These layers date back to the Late Paleocene and are distinct from red beds,indicating a unique depositional environment.In order to discuss the depositional environment of these gray-black mudstone layers,a-2.6 m stratum was chosen as a representative profile(named Maojiwan profile,MJW:25° 16′22″N,114°30′22″E)in the Maojiwan Member of the Guchengcun Formation in the Nanxiong Basin.This stratum is characterized by significant lithological variations from the bottom to the top and contains gray-black,brown-yellow,and brown-red mudstones.In this study,we conducted multiproxy measurements including stable carbon isotopes,diffuse reflectance spectroscopy(DRS),total organic carbon(TOC),and chroma of the samples.Based on these proxies,we analyzed the paleoenvironment in which the gray-black layer formed and further discussed its potential connection with hyperthermals.The results show that:(1)The gray-black layer exhibits high b*and TOC values,a low FDV570/FDV435 value,and the dominant magnetic mineral is goethite.The brown-red layer shows a high a*and FDV570/FDV435 value.The dominant magnetic mineral is hematite with very low TOC values.The brown-yellow layer exhibits a high TOC value,with goethite being the dominant magnetic mineral.However,this value is lower than that of the gray-black layer but significantly higher than that of the brown-red layer.This indicates that the three mudstone layers experienced different sedimentary environments.From the bottom to the top,a cycle of semi-arid oxidized environment,humid oxidized environment,typical lake environment,humid oxidized environment,and semi-arid oxidized environment was recorded by the MJW profile.(2)The carbon isotope(δ13Ccarb)result of the gray-black layer shows a significant negative deviation of more than 3‰ compared to the brown-yellow and brown-red layers.The significant negative excursion of δ13Ccarb in the gray-black mudstone layer is consistent with the negative carbon isotope excursions recorded globally during hyperthermals.Combined with the published terrestrial paleoenvironmental changes during hyperthermals worldwide,we suggest that the grey-black mudstone layer in this study could correspond to a hyperthermal in the Early Paleogene.
万方数据
维普
