Records Of Paleoenvironment By Loess-paleosol Sequence Since MIS Ⅱ In Nuanquan Mountain,Longxi

The environment and climate are intimately linked to human life,and since the20^th century human activities have caused the atmosphere,oceans,polar regions and land masses to warm to an extent never before seen in human history,and many climatic events have occurred since MIS II,and even during the Holocene,which is considered to have been warm and humid,there were a number of transient climatic events.There have been many climatic events since MIS II,and even during the Holocene,which is generally regarded as a warm and humid period,there have been a number of transient climatic events which,together with the long-term climatic context,have had an impact on the palaeoenvironment of the Chinese Loess Plateau.Loess-palaeosol sequences are widely distributed and easily accessible in the Chinese Loess Plateau region.Therefore,they are a good tool to study the palaeoenvironmental record.A comprehensive analysis of the climate proxies recorded in the loess-paleosol vectors and a reconstruction of the palaeoenvironmental processes since MIS II will help us to understand the mechanisms of climate change and thus provide a scientific basis for the guidance of future human social development.The Longxi Nuanquanshan（NQS）section is located in the southeast of Gansu Province,between the Loess Plateau and the Qinling Branch,and is influenced by the Tibet Plateau and the East Asian Monsoon,with complex climatic genesis,and the Holocene stratigraphy also observing Qijia culture sites and frequent human activities.In this paper,the loess-paleosol sequence since MIS II at Nuanquanshan,a Qijia culture site in Wenfeng Town,Longxi County,was selected for this study.,and the age framework of the NQS loess-paleosol sequence was established by AMS ¹⁴C dating and the climatic proxies such as total organic carbon（TOC）,organic carbon isotope(δ¹³Corg),carbonate carbon,oxygen isotope,calcium carbonate content and colourimetry were comprehensively analysed.This paper reconstructs the paleoenvironment of the NQS loess-paleosol sequence in Longxi and investigates the impact of the 5.5 cal ka BP event on human activities and the response of environmental changes to human activities in the middle Holocene.In this paper,the loess-paleosol samples from the NQS loess-paleosol sequence in Longxi are analysed and the following conclusions are drawn from them:（1）This paper investigates the applicability and climate-indicative significance of various climate proxies in the Longxi NQS loess-paleosol sequence.The variation characteristics of TOC,δ¹³Ccarb and L*differ from other proxies and typical loess climate because the Longxi NQS loess-paleosol sequence is located at the sampling site on the Weihe terrace,which has the same characteristics of near-source sediment contribution as the river valley zone,making the climatic proxies complex.In addition,Qijia culture remains have been found in the study area,and human activity may have influenced the climatic significance of some of the proxies.Although proxies may be influenced by multiple factors,palaeoenvironmental reconstructions of loess-palaeo-soil sequences should be combined with multiple proxies for a comprehensive analysis,as only one palaeoenvironment existed at the same time at the same site.（2）The palaeoenvironment of the profile is divided into six stages based on the variation of multiple proxies and their response to climatic parameters such as temperature,monsoon and precipitation:21.7～19.6,19.6～10.8,10.8～6.1,6.1～4.0,4.0～1.8,1.8～0 cal ka BP,combining the climatic significance of the proxies and the actual field conditions in the study area.It is reconstructed that the Megathermal of the NQS loess-paleosol sequence in Longxi lasted until the end of about 6.0 cal ka BP,and the climate since MISⅡhas undergone from cold,toward wet and warm,wet and warm,cold and dry,wet and warm,the alternation of cold and warm,six stages in total.（3）The cold-dry climate during the 6.0-4.0 cal ka BP NQS loess-paleosol sequence in Longxi responded to the 5.5 ka BP event,with changes in solar radiation and the El Ni（?）o-Southern Oscillation（ENSO）as one of the drivers of this event.This event occurred during the post-Pleistocene development phase of human civilization,where declining temperatures and precipitation reduced the natural resources available for human production and subsistence,disrupting the normal development of human processes and having a major influence on the of ancient civilizations.