Environmental Magnetic Characteristics Of Ganzi Loess And Its Environmental Significance Since The Last Glacial

The environmental magnetism parameters have played important role as valuable proxies for reconstructing the past environmental history.The Chuanxi Plateau is located in the eastern Tibetan Plateau?ETP?,which affected by the Indian Monsoon,the westerlies and the Plateau Monsoon.The widely distributed loess is an ideal material to study the rock magnetism and their environmental implications.We analyzed the environmental magnetic characteristics of the loess in the Chuanxi Plateau and the climatic significances from spatial and temporal scales.Detailed rock magenetism and correlation with climate factors analysis were conducted on the modern soils,including magnetic susceptibilities?MS?and antiferromagnetic minerals?hematite and goethite?.Based on the results of the environmental magnetism of the modern soil,we studied the magnetic characteristics of Ganzi loess profile and reconstructed paleoprecipitation since the Last Glacial by different magnetic proxies.The main conclusions of this study are as follows:1)The MS of the topsoils shows significant difference in spatial in the Chuanxi Plateau,with a trend of decreasing from south to north.The temperature and precipitation are the main causes for the MS enhancement of the topsoil samples.The correlations between the MS and climate factors?mean annual precipitation?MAP?and mean annual temperature?MAT??show that the percentage frequency-dependent MS(?_fd%)is correlated with the MAP.Thus,the?_fd%can be used as a proxy for the paleoprecipitation in the region.Measurements of the diffuse reflectance spectroscopy?DRS?show that the contents of the hematite?Hm?and the goethite?Gt?are large spatial different.The correlation coefficient between Hm/?Hm+Gt?and MAP is low.The MAT also has important effects on the magnetic characteristics,but their correlations are not clear in our data,probablely caused by the complex mechanism of MAT and magnetic minerals or other factors,and further work is needed.2)Magnetic parameters show high fluctuation change along the depth of the Ganzi loess-paleosol profile,which can be associated with magnetic minerals,particle size and concentration.The main magnetic minerals are ferromagnetic minerals,such as magnetite and maghematite,and some antiferromagnetic minerals,such as hematite and goethite.Ferromagnetic minerals are main single-domain particles?SD?and pseudo-single-domain?PSD?.The ferromagnetic minerals in loess layers are low and mainly pseudo-single-domain?PSD?.While the paleosol layers have more ferromagnetic minerals with tends to single-domain transition?SD?.The Hm and Gt value is difference in paleosol and loess.The Hm/?Hm+Gt?is higher in the paleosol and lower in the loess,which have good correlations with the?_fd.3)The conversion formula between?_fd%,?_ARM,?_ARM/?_lf,?_ARM/SIRM,?_fd/HIRM and precipitation are employed to reconstruct the precipitations history since the Last Glacial in the Chuanxi Plateau.The results show that the general trends of the reconstructed precipitations were consistent,with large differences in values.The combined average results indicate that the precipitations were low in MIS2 and MIS4period with the precipitation of 515±95mm and 480±130mm,respectively.The precipitations were high in MIS1,MIS3 and MIS5 period characterized by rapid increase.The average precipitation was 605±75mm during MIS1,which was the wettest stage since the Last Glacial.The precipitation was 585±135mm in the early MIS5,and 600±140mm in the late stage.Paleoprecipitation history form the Ganzi loess may reflect the evolution of the Indian Monsoon since the Last Glacial.This study will significantly improve the understanding of the relationship between loess magnetic properties and climate,and the implications for reconstructing palaeoenvironment history in the Chuanxi Plateau since the Last Glacial.