Identification Of Human Activities By Paleoenvironmental Proxies In The Qilian Mountains And Their Surrounding Areas

The coupling process and mechanism of human activities and environmental changes have become a research hotspot since the Holocene in the field of Earth Sciences.Sediment proxy indicators contain information on human activities.The study of the paleoenvironmental proxies of surface sediments can help to excavate the traces of human activities in the sediment proxy indicators,and provide a basis for identifying and quantifying past human activities.Lacustrine sediments store a wealth of environmental information and become a good material for retrieving past climate changes.Numerous studies have shown that some paleoenvironmental proxies in lacustrine sediments can indicate human activities.The purpose of this study is to understand the coupling process and mechanism of human activities and Holocene environmental changes in the Qilian Mountains and surrounding areas,and to provide a scientific basis for identifying human activities in the reconstruction of the paleoclimate in this region in the future.In this study,466 surface sediments collected in the Qilian Mountains and surrounding areas were selected and analyzed by total organic nitrogen(TN),total organic carbon(TOC),carbon to nitrogen ratio(C/N),organic carbon isotope(?¹³C_org),carbonate carbon isotope(?¹³C_carb)and oxygen isotope(?¹⁸O).We established the relationship between the paleoenvironment proxies and factors such as altitude,latitude,longitude,temperature,and precipitation,and explored the impact of modern human activities on the paleoenvironment proxies of the surface sediment.On the basis of surface sediment research,we analyzed the lithology,age,and paleoenvironmental proxies data of 14 terminal lakes in the Qilian Mountains and surrounding areas,reconstructed the climate change since the Holocene,and explored the coupling relationship between climate change and human activities since the Holocene.Moreover,combined with the characteristics of environmental changes and human activities in the paleoenvironmental proxies since the Late Holocene,this study proposed the possible beginning of the Anthropocene in this region.The preliminary results are as follows:(1)TOC,TN,C/N and?¹³C_orgindexes in surface sediments had a certain correlation with altitude,mean annual temperature(MAT),and mean annual precipitation(MAP),among which the correlation with MAP was the most significant,indicating that precipitation is the main influencing factors.?¹³C_carband?¹⁸O were mainly correlated with MAT and MAP.The change of paleoenvironmental proxies with latitude and longitude indirectly indicated the influence of precipitation and temperature on the paleoenvironmental proxies.The effect of temperature on the environmental proxies in surface sediments is weaker than that of precipitation,so the environmental proxies have certain indicative significance in reconstructing the paleoclimate,especially the change of precipitation.According to the 30 m human activity parameter data of the Qilian Mountains,the human activity area was divided.The study found that the paleoenvironmental proxies of the surface sediments fluctuated significantly in the human activity area,and the correlation was greatly reduced compared with the non-human activity area,indicating that the influence of human activities can change the indicative meaning of the paleoenvironmental proxies.(2)The organic matter was mostly a mixed source of aquatic and terrestrial plants in the terminal lakes of the Qilian Mountains and the surrounding areas since the Holocene.Furthermore,the organic matter was mainly derived from aquatic plants due to less human activities in some high-altitude lakes.The climates in the early and middle Holocene were characterized by warm and humid.Since the late Holocene,the climate has featured as significantly arid.The paleoenvironment proxies fluctuated greatly,which may be related to the intensity of human activities especially since 2 cal ka BP.(3)The change of the paleoenvironment proxy is closely related to the change of sedimentary facies.The changes of the paleoenvironment proxies of the aeolian sediments in the sediments of the terminal lake and the changes of the paleoenvironment proxies of the lacustrine sediments are mostly in the opposite trend.The sedimentation rate of lake sediments in the same period is relatively small,and the rate of aeolian sediments is relatively large.Since the Late Holocene,the climate has become arid,the lake has retreated gradually,and the sedimentary facies has undergone rapid changes,resulting in abnormal fluctuations of paleoenvironment proxy.To sum up,the fluctuation of the paleoenvironment proxy since 2 cal ka BP is obviously related to the change of sedimentary facies.(4)Since the Late Holocene,the Qilian Mountains and surrounding areas have become significantly arid,and the rapid retreat of lakes is closely related to the intensification of human activities.In addition to being affected by regional aridification since the Late Holocene,human activities are also one of the factors that have contributed to the change of sedimentary facies since 2 cal ka BP.The period when human activities began to affect the regional environment has been since 2 cal ka BP.