Holocene Dust Variability Inferred From Peat Deposits In Northern Xinjiang

Atmospheric mineral dust is not only sensitive to climatic changes,but it also has significant impacts on the global climatic system.Dust processes,i.e.emission,transport and deposition,are closely associated with the climatic and environmental conditions in dust source areas,wind strength and atmospheric circulation patterns.Therefore,the variability of atmospheric dust in the past is crucial for understanding of changes in regional climates and environments and those potential forcing mechanisms.The Arid central Asia is one of the most important source areas for atmospheric dust over the Northern Hemisphere,and thus this area is an ideal choice to study the history of atmospheric dust on various timescales,based on geological archives.Peats have the nature of continuous deposition and high-temporal resolution for climatic reconstructions;the peat matters are abundant in plants remains,which are good materials for radiocarbon dating.In addition,mineral debris input into peatlands is less affected by diagenesis and post-deposition migration.Thus,peat deposits have been gradually recognized as one of important archives in investigating variability of atmospheric dust in geologic times.In this study,the different peat deposits from Tianshan Mountains(cores CWP and JBLK-B)and Altai Mountains(core ATM10-C7)were employed to investigate Holocene dust variability.24 samples of sedge or sphagnum plant remains derived from the different peat layers were used for radiocarbon dating using accelerator mass spectrometer(AMS).Grain size and elemental compositions of detrital particles from peat deposits were analyzed for reconstructing dust variability.The specific indicators for dust variability were determined by understanding the input process of mineral debris to peatlands and the effect of post-deposition on the detrital component.In combination with the AMS dating results,Holocene changes in dust was reconstructed.The potential forcing factors,such as regional climatic and environmental changes,were thoroughly understood.The main conclusions were as follows:1.Mineral debris is primarily transported to peatlands by winds.The detrital particles are less affected by post-deposition processes.Thus variations in elemental compositions and ash flux in peat deposits could be used as indicators for changes in atmospheric dust loadings;Grain size of the detrital particles in peat deposits indicates changes in wind strength.2.Due to the differences in the altitude and latitude of the studied sites,the reconstructed dust variability during the Holocene is characterized with an apparent spatio-temporal differentiation.As recorded by core ATM10-C7 in the Altai Mountains,the high level of atmospheric dust occurred during the Holocene(11.2-7 cal ka BP)and after 4 cal ka BP;the maximum dust loading occurred since 1.5 cal ka BP.Furthermore,the high-resolution of dust variability during the late Holocene is derived from cores JBLK-B and CWP in the northern piedmonts of Tianshan Mountains.Low levels of atmospheric dust loadings recorded by core JBLK-B occurred from 2.1 to 1.3 cal ka BP and form 0.9 to 0.1 cal ka BP.In contrast,between 1.3 and 0.9 cal ka BP,the atmospheric dust input gradually increased,accompanied by a gradual increase in mean grain size of detrital particles.Since 0.1 cal ka BP,the mineral debris contents in peats at the Jiangbulake site increased rapidly.A high level of dust loading is observed at the Chaiwobu site from 3.3 to 2.7 cal ka BP,and the dustiness overall decreased between 2.7 and 1.1 cal ka BP.Nonetheless,the dustiness slightly increased in the spells of 1.7-1.4 cal ka BP,1.1-0.5 cal ka BP,and 0.2 cal ka BP.3.In the light of the implications of applied proxies for dust variability,the variations in ash flux and grain size in core CWP may have suggested either the aridity and/or the strength of near-surface winds in the Junggar Basin.However,the variations in atmospheric dust loadings recorded by cores ATM10-C7 and JBLK-B form the subapline regions,in particular the difference between variations in ash flux and grain size in core JBLK-B,perhaps reflect changes in source environments(e.g.central Asia)and in strength of the Westerlies.4.By comparing to other climatic records from the study area and the adjacent regions,the regional climate that affect the changes in atmospheric dust were elucidated.The high level of dustiness during the early Holocene(11.2-7 cal ka BP)and the late Holocene(since 4 cal ka BP),as recorded by core ATM10-C7 from the Altai Mountains,may have resulted from intensive dust emission in the source area of central Asia,modulated by the aridity there.The mid-Holocene low level of dustiness represented humid environments in central Asia.As recorded by core JBLK-B,the low level of dustiness between 2.1 and 1.3 cal ka BP was probably in response to the weakening Westerlies.However,the high dust loadings at the spells of 1.3-0.9 cal ka BP and after 0.1 cal ka BP were associated with the dry climate in Central Asia and the strengthened westerly circulation.During the period of 0.9-0.1 cal ka BP,although the Westerlies enhanced,the low atmospheric dust loadings likely represented the wet climate in the Central Asia.As for the Chaiwobu peatlands,the maximal dust input from 3.3 to 2.7 cal ka BP was likely associated with a dry climate and enhanced winds across the Junggar Basin.A similar scenario occurred from 1.1 to 0.5 cal ka BP,despite of a less intense extent compared to the former.The low dustiness form 2.7 to 1.1 cal ka BP and from 0.5 to 0.2 cal ka BP may have resulted from wet climate and weak wind strength at those times.A rapid increase in dust input to the peatland probably represents intense human activity over the past 0.2 ka.