Paleoclimatic Significance Indicated By The Magnetic Susceptibility Of Loess Over The 2.2Ma At Xijin,Lanzhou

Accumulation of eolian loess is regarded as one of the remarkable characteristics of globally climatic deterioration since the late Cenozoic.A large amount of research showed that the formation of Quaternary loess was tightly related to the aridification of Asian inland.During the process of arid intensification,dust was carried to the periphery of deserts even the North Pacific and Greenland by monsoon and westerlies,then having a great impact on climate throughout the world via umbrella effect,iron fertilization hypothesis,erosion weathering and other physical and chemical mechanisms.The Loess Plateau is situated in the junction of eastern humid region,northwestern arid region and Qinghai-Tibet Plateau and is the pivot of tectonic-climatic interactions among the three natural regions.Since the mid-20th century scholars from home and abroad have carried out a lot of investigations about the paleoenvironmental evolution in this area.However,up till now,we still know little about the northwestern Loess Plateau and how loess-paleosol reflects the strength of East Asian winter and summer monsoon,the uplift of the Tibetan Plateau,the desertification in Central Asia and the variation of global climate has yet to be further understood.The Lanzhou Basin,adjacent to its dust provenance--the Tengger Desert and located in the western Loess Plateau,is the depositional centre of dust in China,where the sedimentation rate and depth is unusually high and huge.Moreover,lying at the northern edge of the summer monsoon,this basin is extremely sensitive to any abrupt variability in climate system,whilst the loess-paleosol developed there contains plentiful paleoclimatic archive with high-resolution.In 2012,by implementing a drilling project at Xijin Village,the suburbs of Lanzhou City,we successfully obtained a loess core with 416.2 metres long,which provided an invaluable opportunity to indicate the evolvements of paleoclimate and paleoenvironment in Lanzhou Basin,the western Loess Plateau even the whole East Asian monsoon region and their relationships with Tibetan uplift and global change.Based on paleomagnetic dating,experiments of high-and low-frequent magnetic susceptibility(MS)and some grain size data with high-precision,our new findings are concluded as follows:1.Sequence of loess-paleosol over the 2.2Ma,which is the oldest in the western Loess Plateau,has been reconstructed.There are 30 layers of loess and 29 layers of paleosol,respectively,thus compositing twenty-nine and a half cycles.Furthermore,according to the chronological frameworks of stalagmite and marine oxygen isotope,age model for boreholes at Xijin during the past 630ka has preliminarily been established as well.2.By the detailed analysis of the MS of loess-paleosol sequence at Xijin,our results suggest that on the whole,MS of loess before L9 was stable whereas it gradually increased after L9,which may be attributed to the input of coarser magnetite particles and imply that winter monsoon was stronger while aridification in Central Asian was intensified since then.Besides,MS of paleosol prior to L15 presented a trend with high frequency and low magnitude,and the differences of values between crests and troughs were tiny,which revealed the dominance of obliquity about 41ka.Additionally,due to the enhancement of summer monsoon resulted by Kunhuang Movement and mid-Brunhes transition,two stepwise increments of MS appeared after S10 and S4,separately.3.By synthesizing the loess records from the eastern and western Loess Plateau and worldwide multi-proxies,it is clear that under the tectonic scale,paleoclimatic variation in Lanzhou Basin was fundamentally consistent with the pace of global fluctuant cooling,ice sheet expansion and sea level descent in Pleistocene,hence realizing the coupling of lithosphere,hydrosphere,atmosphere and cryosphere and accurate comparisons trans-regionally.Under the orbital scale,MS differences between loess and paleosol were more significant,inferring that the climate system was amplified by the periodicity of eccentricity.Meanwhile,the existence of mid-Brunhes event was also proved by other proxies.Under the millennial scale,climate instability during the last interglacial-glacial cycle was exhibited in the core at Xijin,and the winter and summer monsoon of the Olduvai subchron had a phase difference,but not agreed with their macro regularity of waxing and waning.