Fluvial Terrace Formation And Its Impacts On Early Human Settlement In The Hanzhong Basin,Qinling Mountains,central China

The evolution of sedimentary environments plays a critical role in early human occupation and settlement.As located in the climatic boundary between the temperate north and subtropical south of China,the intermontane basins of Qinling Mountains(QLM)are one of the most important areas for Paleolithic archaeology.The Paleolithic remains were generally unearthed from the silt layers in the middle and upper part of sediment sequence of fluvial terraces.These silt sediments show homogeneous structure because of intense weathering and then have been interpreted simply as aeolian deposits.In principle,however,it still remains unclear whether they are all aeolian sediments or other potential ones,such as fluvial or floodplain deposits.Meanwhile,the undeveloped chronological framework of sedimentary sequences in older terraces(except the first one)has caused some difficulties in revealing the paleoenvironments.Therefore,the studies of age and sedimentary formation of fluvial terraces in intermontane basins of QLM will benefit the exploration of the environment background of ancient human in central China,and further will be of great significance to reveal the scientific issues,such as the early human migration between southern and northern China and the human evolution.In this paper,the sedimentary characters and paleoenvironments are discussed through grain-size and grain-shape analyses of sediments from YFB and LGS4 Paleolithic archaeological sites in the second and the fourth terrace respectively,along the Hanjing River in the Hanzhong basin.Together with samples from the known depositional environments,different sedimentary environments have been distinguished.The OSL dating method is used to establish the chronological framework of the second terrace.End member modelling indicates that five end members can interpret all the grain size data in Hanzhong basin,with EM1 being the coarsest and EM5 the finest.EM1 and EM2 represent fluvial contribution,EM3 is formed by settling of aeolian silt in static water in a floodplain environment,and EM4 and EM5 are interpreted as loess and soil,respectively.Together with grain size and shape analyses can be powerful to distinguish disturbed and homogenious sediments.The fluvial terraces are comprised of three sedimentary units.The lower unit,i.e.unit 1,consists of gravelly sand mixed with fine silt.GSDs in this unit present bimodal character with a leptokurtic and coarse main mode at size 400-700 ?m.They are dominated by coarse end member EM1 and EM2.This unit is interpreted as shallowchannel-fill sediment deposited during the start of the transition from a channel to a floodplain environment.The middle unit comprises a fine-grained,gradually fining-upward sequence.Samples show bimodal GSDs as well,but the coarse peak is finer and disappears at the upper part,and the volume of silt particles increases upwards correspondingly.Both fluvial coarse sediments and aeolian fine sediments made a contribution to this unit.The fluvial hydrodynamic energy becomes weak and aeolian particles continuously increase,representative a floodplain sedimentary environment.The third,uppermost unit,is mainly comprised of silt particles which present unimodal GSDs with mode size 20-35 ?m.They are dominated by fine end member EM3-5.This unit represents aeolian loess interbedded with paleosol(s)and contains particles as the results of episodic surface runoff.The grain shape results show differences in these units as well.Grain shape aspect ratio(AR)value is remarkably low in unit 1,indicating the fluvial sediments are flat or elongated.However,AR value is relatively high in unit 3 which means the aeolian particles show more symmetrical shape.The deposits in middle unit 2 own an average AR value between the overlying and underlying units.All the evidence shows that the terrace sequences in Hanzhong basin,in order from the base to top,are channel gravel deposits,high-energy floodplain coarse sand deposits,low-energy floodplain fine sand and coarse silt deposits,aeolian medium and coarse silt deposits.A floodplain sedimentary environment can be discriminated from aeolian deposits,revealing that the fluvial terrace formation is continuous without a considerable age gap(no hiatus)between the fluvial-and aeolian sedimentary environments.The TT-OSL results show that the fluvial deposits and overlying aeolian sediments accumulated continuously during?25-220 ka.Besides,the age of floodplain and sand channel sediments in the first terrace is approximately 4 and 1 ka,respectively.In situ stone artifacts have been found in all three units,and the density increases from the base to top,which reveals the hominins occupied in the Hanzhong basin continuously and their activities have been gradually intensive.Artifacts unearthed from unit 2 reflects floodplain depositional environment is another crucial area for hominins occupation,besides the aeolian sedimentary environment.Gradual fluvial terrace formation benefits ancient human to adapt to environmental change step by step and thus they were able to live at the whole geomorphological surface in a long time.Overall,given the favorable sedimentary environments and possibly the relatively wet and warm climate,the intermontane basins of QLM have become one of the core areas of early human settlement in China,at least since the Middle Pleistocene.