Climatic Records Of Insoluble Microparticles In An Ice Core From Miaoergou Glacier,Eastern Tien Shan

The arid and semiarid regions in central Asia are a major source of dust transported and deposited around the globe.Dust from central Asia can influence the global radiation budget by stimulating cirrus cloud formation and marine ecosystems by supplying nutrients to the open ocean.In addition,dust form central Asia can change mass balance of its surrounding glaciers by influencing its albedo.Therefore,it is important to understand spatial and temporal variation of atmospheric dust concentration in central Asia and its underlying driving forces.However,the sparse meteorological stations in the central Asia and extreme climate limit our understanding of variability of dust aerosol over central Asia and its relationship with climate change.Fortunately,central Asia holds the largest ice mass outside the polar regions,thus serve as an ideal "natural laboratory" for reconstructing high-resolution long-term climatic and environmental changes using ice core recordsTien Shan is located in a remote area of central Asia,surrounded by important dust source areas:the Taklimakan desert(the world's second largest shifting sand desert)in the south,the Southern Mongolian Gobi and the Northern Mongolian Plateau in the east,the Gurbantunggut desert in the northwest,and the Peski Sary-Ishikotrau desert in the west.Therefore,the ice core records from this region can offer an opportunity to understand the environmental changes over the most important dust sources in central Asia.In Tien Shan,previous studies on aeolian dust have been limited,with the longest available records representing only 88 years(1908-1995 A.D.).In this study,we present a 195-year atmospheric dust deposition record(1810-2004 A.D.)developed from a shallow ice core retrieved from the Miaoergou Glacier located in eastern Tien Shan.We first trace the potential sources of dust deposited at the Miaoergou Glacier.We then investigate the relationship between the Miaoergou dust record and regional climatic factors,including precipitation,soil moisture,and surface wind speed.Finally,we examine the impact of the antecedent winter North Atlantic Oscillation(NAO)on the Miaoergou dust record during the past two centuries.Main results of our study include:(1)Insoluble microparticle records of the Miaoergou ice core showed distinct seasonality,wither higher concentrations during winter/spring and lower concentrations during summer.The annual dust fluxes of the Miaoergou ice core range from 9.12 to 1581.91 ?g cm-2 a-1 with a mean value of 267.20 ?g cm-2 a-1 and a median value of 199.24 ?g cm-2 a-1,which are higher than those in the East Rongbuk ice core drilled from the central Himalayas,but lower than those in the Muztagata ice core drilled from the eastern Pamir.(2)The time series of dust flux for the past 195 years shows three periods of relatively low values(i.e.,1810-1829 A.D.,1863-1940 A.D.,and 1979-2004 A.D.)and two periods of relatively high values(i.e.,1830-1862 A.D.and 1941-1978 A.D.).Almost all the dust particle size distributions during the period 1810-1978 A.D.fit a lognormal distribution.The mode values between 1810 A.D.and 1978 A.D.range from 2.6 to 9.6?m with a mean value of 4.7 ?m and a median value of 4.1?m,which are typical of mineral particles deposited in alpine snowfields with a transport distance of several hundred kilometers but much coarser than those of Himalayan ice cores.In contrast,the dust particle size distribution during the period 1980-2004 A.D.does not fit a lognormal distribution well,mostly exhibiting a low positive slope with particles existing sparsely in higher channels.The mode value during the fifth stage(1979 A.D.to 2004 A.D.)is 7.9?m(Figure 3).The abnormal distributions during the period 1979-2004 AD may be associated with increased dust emissions from local sources.(3)Evidence from air mass back trajectories(HYSPLIT 4)suggests that insoluble microparticle reaching the eastern Tien Shan transported dominantly by westerlies during the dust storm season.The correlation coefficient with the Miaoergou dust flux is 0.46(p<0.01)for the Hami meteorological station close to our coring site in eastern Tien Shan and 0.48(p<0.01)for the averaged dust day records of the 16 meteorological stations around the Taklimakan desert.We therefore think that the Miaoergou ice core record could serve as a proxy for dust emissions over the deserts between Tien Shan and Kunlun Mountains.(4)To help understand the physical mechanisms responsible for the variability in atmospheric dust at Miaoergou,the annual Miaoergou dust flux was correlated to climate variables during high-frequency dust storm season(March-May).Possible regional factors controlling the dust flux,including antecedent precipitation,spring soil moisture,near-surface wind speed and severity of drought.(5)The high dust emissions over the northwestern China are closely associated with the negative 500 hPa geopotential height anomalies over this region,and vice versa.In the years with extremely high(low)dust fluxes,the negative(positive)geopotential height anomaly over the western and central part of Mongolia causes anomalous southerly(northerly)winds over the eastern part of China,and anomalous northerly(southerly)and westerly(easterly)winds over the northwestern China.This pattern tends to enhance(weaken)the intensity of northwest cold air flow from higher latitudes to the northwestern China.Therefore,the changes in the spring 500 hPa geopotential height may provide a large-scale dynamical condition favorable to the low dust emissions over the arid regions over the northwestern China since 1980s.(6)The Miaoergou dust flux is closely associated with the winter index of the North Atlantic Oscillation(NAO)over the past two centuries,with high(low)dustperiods coinciding with the negative(positive)phases of the NAO.The persistent relationship suggests that the NAO may have been a key driver on dust flux change over the arid regions between the Tien Shan and Kunlun Mountains.