The Review Of Loess Sedimentary Patterns And Westerlies-Monsoonal Climatic Changes In Northern China During Late Quaternary

The climate change of northern China,especially precipitation,is influenced by the monsoon and westerlies.The process and driving mechanisms of climate change at different time scales in various regions of northern China are vital to understand the patterns of climate change and predict future trends.The widespread loess in the arid and semi-arid areas of northern China is a good carrier of paleoclimatic records.The significance of precipitation proxies is still controversial in these regions.At the same time,there is a lack of studies on the integration of paleoclimate change recorded by loess of arid and semi-arid regions of northern China,which limits our understanding of the process and mechanism of monsoon-westerlies changes in central and east Asia during late Quaternary.In this study,anhysteretic remanent magnetization susceptibility/saturation isothermal remanent magnetization(χ_ARM/SIRM)and/or total organic carbon content（TOC）were measured for the XY17 and NLT17 loess sections in the Ili Valley and the HS18 and XY18 loess sections in the northeastern of the Qinghai-Tibetan Plateau（NETP）.The applicability of different precipitation proxies in reconstructing precipitation and effective humidity in above areas was analyzed by correlation analysis.Based on the cross-validation results of multiple climate proxies,the precipitation and effective humidity changes during late Quaternary in Tianshan influenced by westerlies and NETP influenced by monsoons were further reconstructed and verified.On this basis,the study systematically sorted out the late Quaternary loess sedimentary records and paleoclimatic changes in the arid and semi-arid regions of northern China.Based on 86 published high-resolution dating loess records,integrated research was conducted on the loess sedimentation patterns and effective humidity changes in 5 typical loess sedimentation areas in northern China since the last interglacial period.The sedimentation patterns and spatial differences of loess at different time scales in the arid and semi-arid regions of northern China during the late Quaternary were analyzed,and the possible mechanisms were explained.Meanwhile,based on the precipitation changes indicated by the integrated loess records in different regions,the late Quaternary effective humidity change patterns and spatial differences influenced by different climate systems were analyzed.Based on the above research,this thesis concludes the following results:（1）The correlation analysis of various environmental magnetic parameters and TOC shows that the use of low-frequency magnetic susceptibility(χ_lf)to indicate effective humidity changes in Tianshan areas needs to be careful,asχ_lfmay reflect more the changes in local lithology and wind.Percent frequency-dependent magnetic susceptibilityχ_fd（%）andχ_ARM/SIRM can both be used to indicate effective humidity changes in Tianshan and NETP areas since the last glacial period.TOC changes are not only controlled by effective humidity changes,but may also be affected by temperature,plant roots,and other factors.（2）The ¹⁴C dating results of the northern Chinese loess are generally few,and the dating results are concentrated within 25 ka.Quartz optically stimulated luminescence（OSL）dating is mainly used for loess samples within 50 ka,and quartz OSL dating may underestimate the age for samples over 50 ka.For long-time scale loess dating,K-feldspar post-infrared infrared stimulated luminescence（p IRIR）,quartz Re-OSL,and TT-OSL techniques are mainly used.Currently,the dating upper limit of northern Chinese loess has been extended to 350 ka.（3）The integrated results of the loess records in the arid-semiarid areas of northern China show that some loess sections in various regions of northern China have discontinuity since the last glacial period,with a time span ranging from thousands of years to tens of thousands of years.Generally,the discontinuity of loess in northern China shows in MIS2.The loess on the north side of Tianshan Mountains had low deposition rate or discontinuity in warm interglacial or interglacial stage,while the loess in Yili Valley had discontinuity in the last glacial period,especially in MIS2.Since MIS2 in the northeastern Tibetan Plateau,there have been frequent aeolian sand activities and widespread sedimentary discontinuity.The loess in the northern Chinese Loess Plateau（CLP）and the Luzhong Mountain area was discontinuous during the last glacial.The loess depositional discontinuity occurred in the last glacial maximum and last interglacial in Shandong coastal area.The loess in northeastern China may have depositional discontinuity during Holocene and last interglacial.（4）At the scale of the last glacial-interglacial cycle,the loess in all regions of northern China showed a pattern of high glacial-low interglacial deposition rate.In the last glacial period,the loess deposition rate in Yili Valley was affected by regional topography and provenance,and the periods of high and low values were not uniform.The deposition rates of loess near the dust source area,such as in the northern part of the Tianshan Mountains,the NETP,the western part of CLP,the western part of the central part of CLP,and the northeastern China were higher in the relatively cold MIS2and MIS4.The loess in the northeast of the middle subregion of the CLP,located at the desert boundary,had a depositional discontinuity of 30 to 60 ka during the last glacial age.The loess in the southeastern region of the central subregion of the CLP and the Luzhong mountain area,which is far away from the dust,was deposited the fastest in MIS3 period,and the deposition rate was lower in MIS2 and MIS4 period.（5）There are differences in the development time of S₀ paleosol in different regions of northern China.Under the influence of westerlies intensity,the paleosol developed from about 6 ka to now in Tianshan region of Xinjiang.Paleosol was developed from 18-16 ka in some areas of Gonghe Basin,and from 10-8 ka to 2 ka in Gonghe Basin and Qinghai Lake Basin.However,the development of paleosol in some sections was interrupted by drought events for many times.The paleosol began to develop in the Holocene on the CLP roughly from 11 to 8 ka,but generally speaking,the development time of paleosol in the central region was earlier than that in the western region.S₀ paleosol was developed in Shandong Peninsula from the last deglaciation to the Early and Middle Holocene.The paleosol appeared in the Middle Holocene in northeastern China and its development period was relatively short.In the last glacial period,the loess of Yili Valley and CLP developed weak pedogenesis during MIS3,but there is no report of weak pedogenesis in other regions.Within the last interglacial period,the profiles located on the north slope of the Tianshan Mountains and the west side of the Junggar Basin did not develop obvious paleosol during the last interglacial period.The Yili Valley and the NETP both developed three layers of paleosol,but the development period of paleosol was not consistent.Paleosol development occurred in the CLP,Shandong Peninsula and northeastern China during the last interglacial period,but the development of paleosol in Shandong Peninsula and northeastern China during this period may be discontinuous.（6）Based on the multi-proxy indexes of loess profiles in the Tianshan Mountains and NETP,and the integration results of effective moisture,on the scale of glacial-interglacial cycles,the Tianshan region in the westerly region and the NETP and the CLP in the East Asian monsoon region both present a dry-glacial and wet-interglacial pattern.In the last glacial period,the CLP was moist during MIS3 and MIS4.In the last interglacial period,the effective humidity in the westerly and monsoon regions showed an approximate inverse phase change pattern.During the Holocene,the variation of effective humidity in the Tianshan Mountains was controlled by the change of westerly intensity,showing a pattern of drought in the early Holocene and continuous wetness in the middle and late Holocene.The effective humidity in the NETP,CLP and northeastern China in the monsoon area was controlled by the East Asian summer monsoon intensity,showing a trend of gradually wetness in the early and middle Holocene and gradually drying in the late Holocene.