Aeolian Deposits Along The Northeastern Edge Of Chinese Loess Plateau:Implications For Climatic And Environmental Changes During The Late Quaternary

The aeolian deposits in the Chinese Loess Plateau(CLP)are well-known for providing an important terrestrial record for broad paleoclimatic comparisons on various timescales.Until now,there are few studies concerning spatial variability of the CLP over geological times.However,the dynamic changes in the extent of loess deposition would directly affect the process of loess deposition on the CLP and paleoclimatic interpretations of loess sequences.Aeolian deposits in the desert-loess transitional belt are mainly composed of interbedded well-sorted sand,loess and/or paleosols.The aeolian deposits bear information on aeolian activity and dust accumulation,providing important stratigraphic sequences for understanding of variations in extent of loess depostion and dunefield evolution in northern China.In this study,two outcrop sections(Sections XS-A and XS-B)on the northern slope of the Xiangshan Mountain,southeast of Tengger Desert,and one section(Section KJ)in the downwind region of the Mugetan dune fields in the southeastern Gonghe Basin were investigated.Based on the optically stimulated luminescence(OSL)dating and the sedimentary stratigraphic sequences,combined with the proxy analyses,such as grain size,elemental compositions,magnetic susceptibility and loss on ignition,we attempted to discuss variations in the extent of loess deposition in northern China and those implications for regional environmental and climatic changes since the mid Plaeistocene.The main conclusions were drawn as follows:1.The characteristics of loess deposits in all of three sections are consistent with those of the typical loess on the CLP.However,the aeolian sand contains some amounts of silt fraction.The typical loess deposition occurred at least prior to 240 ka and lasted until 42.5 ka or even as late as25.4 ka at the Xiangshan site,northwest of CLP.In the southeast of Gonghe Basin,loess deposits occurred at least in the mid Plaeistocene and lasted until the Last Glacial Maximum(LGM),despite episodically intervened by layers of well-sorted sand.2.When the climate was warmer and wetter along the desert-loess transitional belt,and dry grassland or steppe vegetation developed in this region.The aeolian strata were relativelycontinuous,characterized by the gradual transition from aeolian sand to loess deposition,suggesting a northwestward expansion of the extent of loess deposition.On the contrary,when the climate was colder and drier,surface aeolian erosion increased,with disconformity surfaces in the aeolian sections,indicating a rapid transition from loess to aeolian sand and a southeastward retreat of the extent of loess deposition.3.At least since the mid Pleistocene,the northern boundary of loess deposition would be more northward compared to the current position of the CLP,which possibly reached the southeastern parts of the Tengger Desert and the Mu Us Sandy Land.In addition,the western boundary of loess deposition at least approached to Guinan County in the southeastern Gonghe Basin,and even further westward.Due to influence of wind or riverine erosion,aeolian sand covering the loess deposits suggests that the loess deposition may have been interrupted,representing shrinkages of the extent of loess deposition.4.The extent of Tengger Desert and the Mugetan dune fields may have held the current areas during the mid-to late-Pleistocene,although they had experienced several expansions.The expansion of desert/dunefields during the LGM in the study area basically formed the basic pattern of the modern desert/dunefileds.5.The dynamic changes of the loess depositional scope,accompanied by the wind erosion process,probably denote that airborne dust settled as loess deposition in the downwind areas is derived from the previously deposited loess itself,which may,to a great degree,reasonably explain a phenomenon that mass accumulation rates of loess deposition increase,while grain sizes remained less variable during some intervals of the last glacial.