| The nature of the relationship between humans and climate is a core issue in geographical research.The most recent glacial-interglacial cycle in the history of geology occurred in the late Pleistocene.The global climate has experienced significant and rapid changes,such as H events and D-O cycles during the last glacial period.The demise of Neanderthals,the global expansion of modern humans and the technological revolution of stone tools happened at the meantime.Therefore,the late Pleistocene became a hot-spot interval to study climate change and human-land relationship.Some previous studies have shown that the climate in the late MIS3 is humid and accompanied by high lake level in the northern arid regions of China and the Qinghai-Tibet Plateau.However,this conflicts with the glacial climate revealed by the marine oxygen isotopes curve and the high sedimentation rate of loess.With the application of the new dating method,the uncertainty of time interval makes this problem more complicated.The western Loess Plateau is located in the fringe of Asian monsoon,a sensitive zone to climate change.The deep loess accumulation and the obvious loess-paleosol sedimentary sequence on the Loess Plateau make it a good region for reconstructing paleoclimatic changes.However,in the study of loess,one key issue whether the natural vegetation of the Loess Plateau is steppe or forest is still controversial.The past studies of the relationship between human activities and environmental changes were focused on direct comparison between changes in climate elements(such as temperature and precipitation)and human activities.The lack of research on the impact process has hindered the in-depth understanding of this issue.This paper selected two sites(Loufangzi site and Dadiwan site)located in the western Loess Plateau and a salt lake(Jilantai Salt Lake)located in the monsoon edge region,to reconstruct the history of vegetation evolution,lake evolution and climate change in the late Pleistocene,and further more,to research the relationship between human activities and environmental changes in comparison with regional archaeological data.The main results are as follows: 1)Vegetation history in the late PleistoceneThe region vegetation was dominated by steppe during the late Pleistocene;the dominant taxa include Artemisia,Asteraceae,Amaranthaceae,Ephedra,Picea and Pinus.The vegetation evolution history as follows: Loufangzi site,the vegetation changed from the forest steppe during the MIS5 to desert steppe/desert during the MIS4,and changed to steppe during MIS3;Dadiwan site,the vegetation changed from forest steppe during the early MIS3(62.6-46.7 ka)to steppe during the mid-late MIS3(46.7-27.3 ka);Jilantai Salt Lake,the vegetation was steppe during the interval(85.8-83.4 ka),and desert(83-79.1 ka),steppe(79.1-72.1 ka),desert steppe(71.9-68.1 ka),steppe/desert steppe(67.8-53.9 ka),steppe(53.5-52.7 ka),steppe/desert steppe(51-42.7 ka),and then steppe(42.6-9.4 ka).2)Variations of effective moisture during the late Pleistocene and the control factorsThe overall moisture change trend in the study area was fluctuant,decreasing from the MIS5 to MIS4,increasing from the MIS4 to the early MIS3,decreasing from the early MIS3 to middle MIS3,increasing from the middle MIS3 to the late MIS3,and decreasing from the late MIS3 to early MIS2.The specific changes go as follows: around the Loufangzi site,the effective moisture was highest during the MIS5,lowest during the MIS4,middle during the MIS3;around the Dadiwan site,the effective moisture was highest during the early MIS3,lowest during the middle MIS3,middle during the late MIS3;around the Jilantai Salt Lake,the effective moisture decreased from the interval(85-72 ka)to the late interval(72-67 ka),increased in the interval(67-59 ka),increased significantly in the interval(59-49 ka),decreased significantly in 49 ka,increased slowly in the interval(49-42 ka),decreased significantly in 42 ka,increased significantly in the interval(42-26 ka),decreased in the interval(26-12 ka),and increased a little in the interval(12-9 ka).In addition,the variations of effective moisture are in good agreement with the changes of solar insolation and East Asian summer monsoon.Therefore,we suggest that the effective moisture change in the study area is mainly controlled by the East Asian summer monsoon.3)Nature vegetation on the Loess Plateau and high lake level during the late MIS3The pollen results of Loufangzi site and Dadiwan site show that the vegetation types were mainly steppe from MIS5 to MIS3,which also support the previous conclusion drawn from data in the Holocene that forest and forest steppe can grow in the gully and the vegetation types are mainly affected by East Asian summer monsoon.The results coming from the synthesis of pollen,grain size and shoreline data show that there lay a stable deep-water lake in the Jilantai Basin in the interval(36-29 ka).This conclusion supports the existence of deep-water lakes in late MIS3.4)The relationship between climate change and human activitiesIn the late Paleolithic period,the stone tool types of northern China changed from core/flake technology to microlith.During the core/flake technology stage(MIS5-early MIS2 in this study),the research results show that the changes in regional vegetation types caused by climate change are in good agreement with the intensity changes of human activities.We suggest that the change in vegetation type cause change in above-ground biomass in steppe ecosystem.As above-ground biomass increase,human activities increase,and vice versa.During the late MIS2(20-10 ka),the distribution of microlith was mainly concentrated in northern China.Comparing the distribution of vegetation types of the same period,we suggest that the distribution of vegetation types affects the spread of microlith.The steppe with relatively low above-ground biomass is beneficial to applying the advantages of microlith,which include easy carrying and standardized production.In the interval(20-10 ka)the vegetation type is mainly steppe in northern China. |
