Mid-Late Holocene Climate Change And Its Impact On The Agriculture-Nomadism Evolution In The West Liaohe Basin

The Holocene,is the most recent and important geological age for the development of early civilization.The Holocene climatic evolution and the associated mechanism of human response and adaptation became a research focus for geo-archaeological communities.During the early-mid Holocene,the warm and wet monsoon climate would have promoted the development of agricultural civilization,however,the decline of the global climate during mid-late Holocene has made early civilization struggled,especially those in the marginal region of monsoon climate.The West Liaohe Basin,one of the most densely occupied regions by Neolithic cultures,is located on the northern edge of the East Asian summer monsoon(EASM)where the ecological environment is relatively sensitive to the Holocene climatic changes.During the middle to late Holocene,the Neolithic subsistence showed the pattern of interlaced agriculture and nomadism under the background of the decline of EASM.The research on the environmental background and driving mechanism of subsistence changes is of great value for deepening the understanding of the climate change and socioeconomic responses in the marginal region of the Holocene summer monsoon.In this study,a natural sediment profile near the Xinglongwa archaeological site located in the upstream of West Liaohe was obtained.Multiple analyses including environmental magnetism,grain size,loss on ignition(LOI)and geochemical elements were applied to reveal the past climate change in West Liaohe Basin,with a reliable chronology that mainly emphasize the past 5000 years.Published records of climate change and human activities in the study area were also collaborated to discuss the relationship between climate change and agriculture-nomadism succession.The main conclusions are drawn as follows:(1)Accelerator mass spectrometry radiocarbon(AMS-¹⁴C)and optically stimulated luminescence(OSL)dating results reveal that the bottom of XLW profile contained fluvial deposit of the late Pleistocene(40 cm),but most of the profile contains successive sedimentary records between 5000-2000 cal yr BP(390 cm).From ca.5000-3500 cal yr BP,the sedimentation rate was 0.75 mm/yr,which is relatively low.From3500-2000 cal yr BP,the sedimentation rate increased to 1.68 mm/yr.(2)The climate indexed(grain size,magnetic susceptibility,LOI and element geochemistry)indicate that from 5000 to 2000 cal yr BP,the West Liaohe Basin has experienced a cool/dry-warm/wet-cool/dry climate evolution process.Stage I:From5000 to 4200 cal yr BP,low values of magnetic susceptibility and LOI indicated the climate in the study area turned cooler/drier.Stage II:During 4200-2800 cal yr BP,high values of magnetic susceptibility,LOI and Fe₂O₃,Al₂O₃ with low values of Na₂O,Si O₂and grain size suggested an enhanced precipitation and a warmer climate comparing to previous stage.However,the high values of Ca O and Mg O also indicated that the increase of precipitation was still very limited.Stage III:After 2800 cal yr BP,a decreasing and fluctuating pattern of magnetic susceptibility and LOI indicated a climate deterioration at this time and gradually turned cool and dry.(3)By collaborating the climatic records of XLW profile with 30 other records collected from north and northeast China,it is indicated that the monsoon precipitation in NE China showed a decreasing trend landward(from east to west).During ca.4000-3000 cal yr BP when the Lower Xiajiadian Culture flourished,most of the collected records indicated a warm and wet climate condition in NE China.These records were coeval with climate information revealed by XLW profile(stage II),implying the climate condition at West Liaohe Basin was mainly influenced by the EASM,which showed a strengthening trend during stage?.(4)On the basis of the reconstructed climate condition,the evolution sequence of the Neolithic human subsistence in the middle and late Holocene was further established through collaborating published records.It was found that sites decreased in numbers significantly during ca.5000-4000 cal yr BP,and showed a southward spatial distribution.This may be related to the environmental deterioration in stage I in the northern part of the West Liaohe Basin under the background of weakening EASM.During the Lower Xiajiadian Culture(ca.4000-3000 cal yr BP),high proportion of crop seeds and high content of pig bones were found in come representative sites,reflecting an agricultural-based subsistence in this period.The the spatial distribution expanded significantly,indicating the reviving EASM climate would have promoted demographic and agricultural development in stage II.After ca.3000 cal yr BP,the change of crop to a more drought-tolerant type,and the appearance of abundant horse bones and bronze weapons with nomadic characteristics showed that agriculture was gradually replaced by nomadism.The spatial distribution of sites shifted southward again,reflected the process of human subsistence change,in adaptation to the weakening EASM in stage ?.