Late Cenozoic East Asian Monsoon Climate Variations Recorded By Loess-red Clay Magnetic Parameters On The Chinese Loess Plateau

During the Late Cenozoic,there was a close relationship between tectonic activities in the northern Tibetan Plateau,aridification in the Asia interior,dust flux in the northern hemisphere,atmospheric carbon dioxide concentration and global ice volume.Understanding the relationship between them is a hot and difficult point in the research of Geoscience.The loess-Red Clay deposit on the Chinese Loess Plateau?CLP?is a key site for connecting the Central Asian dust-source area and the north Pacific dust-deposition area,and the correct analysis of the East Asian monsoon signal encoded in the loess and Red clay sequences is of great significance to understanding the causal relationship between the Tibetan Plateau uplift,the Central Asian aridification and the late Cenozoic climate change.A comparison research between a number of well-established monsoon records and marine sediment records will deepen our comprehension of the mechanism of the East Asian climate change from a unique perspective.The loess and Red Clay environmental magnetism has been developed rapidly in the last several decades.However,most past magnetic paleoclimate studies focused on using bulk magnetic parameters,which represent an integrated response of all the magnetic components in sediments.In addition,the application of magnetic susceptibility as a proxy for the East Asian summer monsoon?EASM?in the Red Clay is controversial.The two key issues are whether magnetic susceptibility has been altered and whether there is a transformation between strong and weak magnetic minerals.In order to resolve the above problems,here we calibrated magnetic parameters with different sensitivities to precipitation and temperature based on surface soil studies across the CLP.Then we generated these parameters'records to infer temperature and precipitation variations.In addition,we unmix the IRM?isothermal remanent magnetization?and hysteresis loop of loess and Red Clay samples.Furthermore,we explore the formation,evolution and driving mechanism of East Asian monsoon based on all magnetism parameters we obtained and their comparison with the marine sediment records.Specific results and conclusions are listed as follows:?1?Joint analysis between magnetic grain size parameters(?_ARM/SIRM,?_ARM and?_ARM/?_lf),EASM indicator and marine sediment records reveals that?_ARM/SIRM and?_ARM are sensitive to precipitation variations while?_ARM/?_lf is more sensitive to temperature changes,especially at low temperatures.This was confirmed by modern surface soil research on the CLP.In addition,based on modern surface soil research,the correlation equation of magnetic grain size parameters with temperature and precipitation was established.?2?Novel IRM and hysteresis loop unmixing results reveal that loess and Red Clay on the CLP contain a similar low-coercivity pedogenic component,while their high-coercivity components differ dramatically.This indicates that loess and Red Clay have the same magnetic enhancement mechanism,and their differences in magnetic properties are concentrated on high coercivity component.In addition,the results of IRM and hysteresis loop unmixing show that the oxidation degree of coarse particles and the changes of pedogenic hematite concentration are affected by the temperature.This finding provides a new idea for the separation of temperature from precipitation in eolian sediments.?3?A comparison between weak magnetic minerals?hematite and goethite?and magnetic susceptibility in the Red Clay sequence demonstrates the validity of using magnetic susceptibility as a reliable indicator for EASM intensity after⁶ Ma,while before⁶ Ma magnetic susceptibility can't record the oscillations of the EASM.?4?We reveal a cooling trend from Late Miocene to Quaternary on the CLP.However,the East Asian precipitation was gradually intensified during 4.5-2.7 Ma.Thus,the climatic trend of the CLP during⁴.5-2.7 Ma was getting cold and humid,significantly different from the other periods.We propose a hitherto unrecognized feedback where persistently intensified EASM during the late Pliocene,triggered by the gradual closure of the Panama Seaway,to explain this unique climate characteristics.This feedback mechanism represents a fundamental reinterpretation of the origin of the Quaternary glaciations and the impact of the monsoon.