The Pronounced Variations Of Atmospheric Circulation Indicated By The Loess Records Since 1.0 Ma In Chifeng,Northeastern China

Since the early Pleistocene,the climatic variations,alternated of glacial and interglacial periods,were characterized by drastic fluctuations.And they are closely related to the variations of atmospheric circulation on regional or global scales.The loess-paleosoil series record in northeastern China,another loess deposits area,located on the margin of the modern monsoon influencing,which is comparatively sensitive to the climate change,is an ideal site to study the prevailing wind shifts in different epochs associated with the atmospheric circulation adjustment.The study site is situated at Sishijiazi section,southern Chifeng City,northeastern China.Here we combine the anisotropy of magnetic structure?AMS?with the environmental magnetism and magnetostratigraphic records to reconstruct the atmospheric circulation evolutions,and discuss its forcings to better understand the variability of deposition source.According to the isothermal remanent magnetization?IRM?record,magnetic minerals embedded in the loess are comprised of the magnetite and maghemite.The magnetic anisotropy mainly reflects the direction of the magnetite,and the macro-axis of K_max indicates the orientation of particles.Comparisons of polar column built by Paleomagnetic thermal demagnetization method to the standard one demonstrate that the Jaramillo Subchron is correlated to the bottom of the deposition section.Therefore,it is likely based on deposition rate record that the loess deposited in Chifeng is earlier than 1.0 Ma BP,which is consistent with that of the MPT in terms of timing.The AMS record indicates that the influence of pedogenic process and compaction on the Chifeng loess series is limited,preserving the original information of the AMS.Therefore,the magnetic declination(K_1-dec)of the largest principal axis of magnetic susceptibility can be used as an indicator for the direction of wind when the loess deposited on.Based on the palaeo-wind direction record indicated by the K_1-dec,we find there are two prominent changes in atmospheric circulation since late early-Pleistocene.The first one occurred near the B/M transition,with the prevailing wind direction shifted from the N-S to the NNW-SSE,which is concordant with that of records from the Loess and Qinghai-Tibet Plateaus,suggesting that monsoon had been a component of atmospheric circulation,and had stretched into northeastern China.Although the wind direction of the loess and paleosoil is same with a NNW-SSE,the westerly wind was also imprinted on the loess series,indicating the controlling of the westerlies on this area of the loess deposition.The magnetic susceptibility record implies another readjustment of atmospheric circulation,characterized with a cooling event which occurred at 0.3 Ma BP.Two prevailing wind directions,the one is NW-SE indicated by the loess layer,and the other is NE-SW recorded by the paleosoil layer,whose discrepancies of wind direction are increased.The loess record manifests the stability of the NW-wind,and differs from that of the paleosoil indicated an ameliorating climate.It therefore suggests that the drastic fluctuations of climate oscillated between cold and warm states,and the disparities between summer and winter monsoon are increasing.In sum,the changes of the main wind direction in northern China generally display an anti-clockwise since 1.0 Ma BP.And monsoon circulation was enhancing in northern China,prone to be stable,and became a significant part of the whole atmospheric circulation.The above changes of atmospheric circulation likely result from the orbital forcing,possibly resulting in the expansion of global ice volume,so that the climate tends to be cold and dry,and winter monsoonal circulation was strengthened.Meanwhile,the climatic magnification effect of the periodic uplift of the Qinghai-Tibet Plateau on it as well should be taken into consideration.