High-resolution Luminescence Dating Of Loess Records From Tianshan Mountains And Junggar Basin And Its Revealed Climatic Changes At Westerlies Dominated Central Asia During Late Quaternary

Arid Central Asia（ACA）,as one of the driest areas of the world,constitutes one of the most important sources of atmospheric dust in the Northern Hemisphere.This region has an arid and semi-arid environment with sparse vegetation and fragile ecosystems that are vulnerable to climate change.Over the past 50 years,the extent of desertification in ACA has gradually expanded due to global warming and population expansion,which leads to further degradation of the ecosystems in ACA.In addition,ACA is affected by the Westerly and East Asian summer monsoon（EASM）,which are the two main components of the global climate system.Generally speaking,the Westerly dominates the climate from Europe eastward to the ACA,while EASM dominates the semi-arid and humid East Asia.Apart from that,the boundary changes of the Westerly and EASM on different time scales significantly affect the climate in ACA.However,the differences in spatial distribution and interpretation of climate signals recorded from loess,lakes,and stalagmites in different regions in ACA make it difficult to understand the pattern of climate change and the possible driving mechanism in ACA.At present,the process and mechanism of climate changes in the arid Central Asia（ACA）remain unclear.ACA,as one of the main loess regions in the world,provides important information for the understanding of paleoclimate and paleoenvironmental changes.However,dating of loess records remains controversial in ACA.Besides,organic material and shells from loess may be influenced by a reservoir effect or by old carbon contamination that limits the use of ¹⁴C dating.Moreover,the method is limited to dating samples less than 30-40 ka.For quartz Optically Stimulated Luminescence（OSL）dating,the quartz OSL signals of some loess samples are dim and insensitive,which renders them unsuitable for age determination.Furthermore,the majority of published ACA loess records are limited to the last Glacial,with few well-dated sections extending into the glacial-interglacial cycles.The fully independent high-resolution age models for loess records spanning past 200-300 ka have been developed by using a combination of K-feldspar Post-IR Infrared Stimulated Luminescence（p IRIR）dating.In this study,five loess-paleosol sequences had been collected at different altitudes in the Tianshan-Junggar Basin.To be specific,the SG18 section（526 m above sea level,23.8 m thick）is located in the Southwest Junggar Basin,and the QS16 section（909 m above sea level,44 m thick）is located in the northern slope of the Tianshan Mountains.At the same time,there are three section in the Yili Basin from west to east,including the XY17 section（956 m above sea level,29 m thick）,ZD17 section（1000 m above sea level,28.3 m thick）,and NLT17 section（1396 m above sea level,4 m thick）.In addition,luminescence dating samples were collected at 40 cm intervals from Pleistocene sections SG18,QS16,XY17 and ZD17,and were collected at 10 cm intervals from Holocene section NLT17.The researches on dating methodology of quartz OSL dating and K-feldspar p IRIR dating were conducted in Tianshan-Junggar Basin.For quartz samples from QS16 section and NLT17 section,the quartz dose recovery test and preheat plateau test were performed firstly.For K-feldspar samples,the prior temperature Plateau tests,the dose recovery test,and sunlight bleaching test were conducted in each section.Through the internal test of the luminescence characteristics of quartz OSL signal and K-feldspar p IRIR signal,the reliability of the two dating methods was verified.Furthermore,the reliability of K-feldspar ages was confirmed by comparing with reliable quartz OSL ages in QS16 section and NLT17 section.Overall,this study established laboratory protocols for quartz OSL dating and K-feldspar p IRIR dating in ACA.Based on the Bacon-age-depth model,the mass accommodation rates were calculated and spatial differences of loess deposits in different regions were discussed.Finally,combining with the analysis of paleoclimate proxy（magnetic susceptibility,grain size,color and carbonate）,this study reconstructed the precipitation and effective humidity changes in Tianshan-Junggar Basin since 250 ka.Additionally,moisture variation between lowland and high mountain areas of ACA during past two glacial-interglacial cycles was analyzed.In addition,this study summarized Holocene records in Tianshan-unggar Basin and compared whether there are differences in Holocene paleosol formation ages in different regions.In combination with climatic modeling using a Flexible Global Ocean-Atmosphere-Land System model Gridpoint Version 2（FGOALS-g2）,the spatial distribution of precipitation in Central and East Asia of MIS 5（130 ka-110 ka）,LGM（21 ka-PI）and MIS 1（6 ka-PI）was analyzed.What’s more,the driving mechanism of climate change at different time scales in the arid region of Central Asia since 250 ka was discussed.Based on the above research,the following new understandings have been gained.（1）Based on the luminescence characteristics and comparison of quartz OSL ages and K-feldspar p IRIR ages,it was observed that the upper limit of quartz OSL dating was～40 ka.For the p IRIR dating of K-feldspar,this study found that ages for loess samples less than 20 ka and 250 ka could be measured by using the p IR₅₀IR₁₇₀and p IR₂₀₀IR₂₉₀ signals,respectively.（2）Based on 30 quartz ages and 166 K-feldspar ages,a high-resolution chronology had been established in ACA over the past～250 ka by using Bacon age modelling.It was found that there were depositional hiatuses on the scale of millennium to ten thousand years in loess deposition in ACA.Apart from that,loess deposition rates in Central Asia and East Asia were consistent with high glacial and low interglacial features,but there were significant spatial differences in loess accumulation rates within glacial and interglacial periods.This suggests that deposition,erosion,and preservation of loess in ACA is a dynamic process,which may be a result of multiple factors driven by variation of climate,wind strength,vegetation,and local geomorphological context.（3）Based on the high-resolution chronology,this study synthesized the loess climate proxy records from five loess sections and reconstructed moisture variations in Tianshan Mountains during the past 250 ka in ACA.The lowχ_fd%values of sections SG18 in Junggar Basin and QS16 in the north slope of Tianshan Mountains indicated that the overall precipitation and the intensity of pedogenesis were continuously low over the last glacial-interglacial cycle.Besides,theχ_fd%values of sections XY17,ZD17 and NLT17 in Yili Basin revealed that precipitation obviously increased during the last interglacial and Holocene after drier Last Glacial and Penultimate Glacial periods.All these results suggested that the precipitation increasing during interglacial at central Asia mainly occurred at high mountain area,and the precipitation showed an obvious spatial difference between low lands area of Junggar Basin,northern slope of Tianshan Mountains,and high mountain area of Tianshan Mountains.（4）During the last interglacial,three palaeosols with highχ_fdvalues in sections XY17 and ZD17 from Yili Basin attested to relatively moist climate conditions during cold MIS 5d,5b and the MIS5/4 transition in the region.During Holocene,theχ_fd%of NLT17 sections from high elevation of Yili Basin presented a distinctive increasing trend from early Holocene to late Holocene.As shown by these results,the climate characteristics of the last interglacial period and modern interglacial period in ACA are cold-wet and warm-dry,the change of effective humidity and the East Asian summer monsoon showed an opposite phase change model,and the precipitation was mainly controlled by the change of Westerly intensity.（5）Based on the analysis of paleoclimate simulations,it was further confirmed that both ACA and EASM showed the cold-dry glacial and warm-wet interglacial climate pattern on the glacial-interglacial cycle scale.Apart from that,the glacial-interglacial cycle timescale moisture changes in ACA were dominated by variation in atmospheric humidity in response to the variation of coupled ice volume,similar to that in East Asia.By contrast,moisture changes in ACA during interglacials were dominated by the intensities of Westerlies,which was directly out of phase with the moisture changes of East Asia.The out of phase relationship between the intensity of the Westerlies and East Asian monsoon systems during interglacial periods indicates that the precessional forcing is the reason for differences in moisture patterns between ACA and East Asia.