Loess Deposits In North Tianshan Mountains And Paleoclimatic Significance

Loess-paleosol sequences provide key aeolian archives for the terrestrial environment.Loess sediments are also extensively distributed in westerly-dominated Tianshan Mountains area.Studies of loess deposits from this area have the merit of enabling reconstructions of the past climate in inland Asia.Climate conditions under which loess sediments are entrained,transported and deposited,can be recorded by the relevant physical and chemical parameters of the loess-paleosol sequences.As the common paleoclimatic proxies used in loess research,grain size(GS)and magnetic susceptibility(MS),and others.We selected a loess section from Boleregion(BL)and a loess well from Sangongxiang region(SGX),north and south piedmont of Tianshan Mountains,as the object of present study.Bole loess section has a thickness of 12.7 m,SGX well is 6 m deep.High resolution chronology,magnetic susceptibility,particle size,carbonate,color and geochemical elements were carried out on samples we collected,and in efforts to investigate their paleoclimatic implications.Furthermore,we compared these data with the oxygen isotope of the Greenland ice cores and the solar insolation.The following conclusions were obtained.1.The loess-paleosol of BL profile and SGX well was respectively deposited since the last glacial period and the Holocene period,the age of the profile dates 57 ka in the bottom,and the well is 11.5 ka.And the test of AMS 14C could have limitations in the northern Tianshan loess.2.The magnetic susceptibility index analysis showed that Bole loess and SGX loess pedogenesis is weak.MS variabilities are controlled by wind strength,and wind vigor model can apply to the MS enhancement of the BL and SGX loess.3.The BL and SGX loess is a typical aeolian sediment.The BL loess GS is dominated by silt-sized particles,and SGX loess is mainly composed of sand components,and the grain size of SGX loess is coarse in BL loess.The grain size-standard deviation method here has been employed to extract the environmental sensitive components of the BL section and SGX well.Then two components,<31.7 ?m and 31.7?282.5?m,can be distinguished for BL section,and<35.6 ?m?35.6?251.8?m can be distinguished for SGX well.They represent suspension after dust storm and proximal deposit component mainly transported by dust storms,respectively,which can sensitively reflect the climate change.4.Carbonate analysis showed that the BL section carbonate content of high value for the loess,low value corresponding to the paleosol,the characteristics of changes is similar with the Loess Plateau carbonate content,but the performance is not obvious.The change of carbonate content in SGX well may be related to regional temperature change.The changes of carbonate content indicate that the loess deposition in the region is formed in a climate environment with less precipitation,dry,weak weathering and weak biological function.The carbonate content of palaeoclimate is not clear and can only be used as reference for climate proxy.5.The color change characteristics analysis shows that the brightness of loess sediment in north Tianshan area is influenced by the change of red degree and yellow degree.While red and yellow degrees mainly involve hematite and goethite content in the sediment,both values increase with the increase of the hematite and goethite content,can reflect the characteristics of the regional climate change.The characteristics of geochemical elements show that the main constants in the sample of the section and the well are SiO2>Al2O3>CaO>K2O>Fe2O3.The change trend of most trace elements is not obvious.The characteristics of loess element migration indicate that the sedimentary climate environment is arid,the effective humidity is low,the leaching transfer is not significant,and the chemical weathering is weak.The common geochemical weathering indexes have no obvious indication in the area,the process of chemical weathering is restricted by the pattern of regional hydrothermal combination.6.The loess sediments in SGX well show that,the Holocene climate characteristics in North Tianshan area are:early Holocene low humidity,dry climate,and continuous moisr climate in the middle-late Holocene,Holocene optimum period appeared in the late.The evolution of the holocene climate characteristic is mainly controlled by the winter solar insolation.And the BL section respond to the millennial scale climate events during MIS3 better,with the North Atlantic Heinrich and D-O events,and the mid-latitude westerly circulation plays an important role in transmitting the signal from the North Atlantic to the East Asia.