Mid-Late Holocene Environmental Changes In High-Low Elevation Lakes In Arid And Semi-Arid Regions Of Northern China Based On Biomarker Records

In the context of global warming,extreme climate events and frequent natural disasters have exerted a profound impact on human production and life,seriously threatened the security of human society and ecosystem.The study of past climate change is the key to predicting future climate,as well as the corrector to simulate and invert past climate.The Holocene,as the geological similar type of our modern period,still has many uncertainties about the evolution of natural climate.It is very important to study climate change and its driving mechanism in different regions since the Holocene for future climate prediction and to realize the link between record and simulation.The arid and semi-arid areas in northern China are vast,affected by monsoon and westerly winds,and their ecological environment is fragile and sensitive to climate change.At present,studies on climate evolution in this region are mostly based on traditional proxies such as palynology,particle size,magnetic susceptibility,TOC/TN（total organic carbon/total nitrogen）,and lack of reconstructed records of biomarkers with clear environmental indicators.In recent years,biomarkers,as a kind of emerging environmental proxy indicators,have great application potential due to their wide distribution and strong environmental indicator significance.However,the applicability of biomarkers in arid and semi-arid areas still needs further verification.Liupan Mountain Tianchi is located near the top of the southwest Loess Plateau,which is high in elevation and weak in human activities.Gouchi,located at the junction of Loess Plateau and Mu Us Sandy Land,is low in elevation and strongly influenced by human activities.A large number of scholars mainly focus on the study of climate change recorded in a single lake,but the comparative study of lakes in different geographical locations is slightly lacking.On this basis,this paper takes the core sediments of Tianchi and Gouchi Salt Lake in Liupanshan as the research object,uses AMS¹⁴C to establish an accurate dating frame,and on the basis of systematic modern process research,uses n-alkanes,glycerol dichylglycerol tetraethers（GDGTs）and other geochemical indexes to reconstruct the regional climate change since the Holocene.By comparing the climate change characteristics of two lakes with different elevations in arid and semi-arid areas,the different climate driving mechanisms of the lakes were preliminarily discussed.The main conclusions are as follows:（1）The results of modern process investigation show that Cren and GDGT-0 are the main isoGDGTs in the surface sediments of the surface soil and salt lake groups in the arid and semi-arid regions of northern China,and brGDGTs are mainly acyclic a series,in which the content of 6_ME-GDGTs is higher than that of 5_ME-GDGTs.The applicability test of the published temperature reconstruction indexes shows that the predicted temperature results can be closer to the measured values by using the multiple regression model of each component of brGDGTs,which emphasizes the importance of regional calibration.The applicability of existing salinity indices is also tested in salt lake groups.The results show that the published salinity conversion equation is still applicable in this region,but the fitting effect is not good.In this paper,by screening brGDGTs which are obviously related to salinity,the previous salinity conversion equation is improved,and the fitting effect is significantly improved.（2）The results of the n-alkanes index of Gouchi Salt Lake showed that the regional climate was relatively warm and humid during 8130-4500 cal BP,the terrestrial vegetation was mainly woody,and the nutrient level of the lake was also relatively stable.The climate gradually became arid after 4500 cal BP,but there was a period of warm and wet climate event during 2500 cal BP.The climate change in Gouchi area is mainly influenced by the solar radiation in the northern hemisphere in summer.Based on other indicators,it is found that the superior climatic conditions may promote the development of production technology,increase the population size,and thus create the prosperity of civilization in northern Shaanxi.Poor climatic conditions may have prevented the continuation of civilization.The results of regional summer temperature change since the Middle Holocene reconstructed by TEX₈₆index in Gouchi sediments show that when the climate becomes obviously arid,a large amount of gypsum in Gouchi Salt Lake precipitates out,which inhibits the activity of microorganisms and leads to a significant reduction of iso GDGTs content,which further indicates that the content of salt minerals affects the distribution of GDGTs.Lipid distribution is also affected when salt mineral content is extremely high.（3）Based on the comparison with the topsoil of the basin,it is found that br GDGTs in the Tianchi core of Liupan Mountain are mainly self-generated from lakes.In this paper,the temperature conversion equation established by Russell et al.（2018）using MBT’_5MEin East Africa lakes was used to quantitatively reconstruct the temperature changes in the region since the Middle Holocene,and the water level changes of Tianchi Lake in Liupan Mountain were reconstructed using%OH-GDGTs.The results showed that the average annual temperature decreased by about 2℃during 6200-3000 cal BP,and the water level continued to decrease.During the period of 3300-1500 cal BP,the annual average temperature increased from 11.5℃to12.5℃,and the water level showed an upward trend.Since 1500 cal BP,the temperature and water level of Tianchi Lake in Liupan Mountain have decreased significantly,from 14℃to 10℃,and the cooling range is larger than that from Middle Holocene to early Late Holocene.Generally speaking,the Tianchi Lake in Liupan Mountain records the climatic evolution history of warm and wet in the Middle Holocene and dry and cold in the late Holocene.Summer solar radiation is the most important climate driver in the northern hemisphere.With the weakening of the solar radiation in summer,ITCZ（Tropical complex belt）gradually moved southward,resulting in the westward migration of WPSH（Western Pacific Subtropical High）,which reduced the input of heat and water vapor from the Northwest Pacific to the southwest of the Loess Plateau,and finally reduced the precipitation in this region.Since the Late Holocene,the region has been subjected to increased human activities,which combined with the influence of ENSO and solar activity,has resulted in greater cooling.（4）Although the records of Tianchi and Gouchi in Liupan Mountain both show the climate characteristics of warm and wet in the Middle Holocene and dry in the late Holocene,and the arid climate events of about 3000 cal BP have been recorded,since2000 cal BP,Tianchi in Liupan Mountain has shown a cooling trend,while Gouchi has shown a warming trend.Under the warming effect of greenhouse gases,vegetation cover may be the main cause of regional climate differences.The vegetation cover in Gouchi area is poor,which is easier to absorb heat,and the regional temperature can rise rapidly,showing the climate characteristics of warming and drying.The lush vegetation in Tianchi area of Liupan Mountain is dominated by spruce forest landscape,which can absorb a large amount of CO₂and weaken the warming effect.In addition,transpiration can absorb a lot of heat,increase atmospheric humidity,conducive to the formation of clouds.Which greatly reduces solar radiation and cools the earth’s surface.In addition,topography and elevation may also be one of the factors affecting the regional climate.Gouchi has a lower elevation and is located in the basin topography,which is conducive to the inflow of dry westerly winds,making the Gouchi area drier than Liupan Mountain.