Continental Records Of Early Paleogene Hyperthermals In Nanxiong Basin

Many rapid warming events occurred in geological history,accompanied by the increase of atmospheric CO₂ concentration,which could be regarded as reference for the global warming at present.The Late Cretaceous to Early Paleogene（ca.67～52 Ma）is a typical greenhouse climate period,during which a series of rapid and transient warming events（termed“hyperthermals”）occurred.These hyperthermals were characterized by significantly negative carbon isotope excursion（CIE）,associated with shoaling of the carbonate compensation depth（CCD）,and increasing atmospheric CO₂and temperature.Some hyperthermals also caused environmental changes and ecological effects.Studying these hyperthermals are benefit for understanding the coupling correlation between climate change and the carbon cycle,so that can help us to predict the climate,environmental and biological changes in the future.However,terrestrial hyperthermals are mostly single-point and single-event records,and there is no record of long time series,which limits the in-depth study of the forcing mechanism of terrestrial hyperthermals.Therefore,it is necessary to find and identify more terrestrial hyperthermals to provide abundant data for sea-land comparison.The Nanxiong Basin has a complete preservation of Cretaceous-Early Paleogene red-purple mudstones-sandstones,while the rhyolitic deposition of red muddy siltstone/silty mudstone and greenish-gray mudstone interbedded occurs in Guchengcun Formation,with a special depositional environment.In this study,we test diffuse reflectance spectroscopy（DRS）,elemental geochemistry,total organic carbon（TOC）,calcium carbonate（CaCO₃）,and carbon isotope of the Maojiwan profile of the Guchengcun Formation in the Nanxiong Basin to discuss the depositional environment of the red layers and the greenish-gray layers,identify the layers where hyperthermals occurred,and preliminarily explore the forcing mechanism of hyperthermals in the basin.The main conclusions obtained are as follows:（1）the red layers show a high a*value,a significant hematite characteristic first derivative peak of DRS around 570 nm,and low TOC and CaCO₃ content,indicating an arid or semi-arid oxidized environment.The greenish-gray layers show obvious underwater formation color and horizontal laminae,high b*value,a significant goethite characteristic first derivative peak of DRS around 435 nm,no obvious hematite feature peaks,and high TOC and CaCO₃ content,indicating that precipitation and runoff increase,and the layers are in underwater restoration environment.（2）The carbon isotope result of the typical greenish-gray layer shows a significant negative excursion with a magnitude of more than 5‰,which is consistent with the records of hyperthermals in the marine and other terrestrial,indicating that the greenish-gray layer is the occurrence of hyperthermals.Combining TOC,DRS,CaCO₃ content,carbon isotope and other environmental proxies,ten hyperthermals can be identified in the study profile.（3）The hematite first derivative peak of DRS of the Maojiwan profile is analyzed by periodic analysis software.The spectral analysis shows the existence of a stable 5.5m period.According to the sedimentation rate of 8cm/kyr of Nongshan Formation calculated by the age of the basin’s inner layers and the optimal sedimentation rate of5.6cm/kyr of Maojiwan section calculated by the correlation coefficient method,the occurrence of hyperthermals in the study section is likely to be controlled by the eccentricity of the solar radiation track,which is consistent with the triggering mechanism of hyperthermals revealed by the deep-sea record.