Human Activities Over The Last 1000 Years Recorded By Gonghai And Sihailongwan Maar Lake Sediments

Since modern times,mankind has gradually mastered advanced industrial and information technology,and the ability to transform nature has been qualitatively enhanced,which has caused serious interference to the Earth system and resulted in negative impacts on human society.This makes the current Earth system significantly different from the Holocene.Based on this,some scholars put forward the concept of"Anthropocene".As a geological epoch,the Anthropocene must have a clear time boundary.Since the mid-20th century(ca.1950 CE),the Earth system and the global social economy have undergone accelerated changes.Through the comparison of global geological records,the international Anthropocene Working Group(AWG)suggests this time as the onset of the Anthropocene.However,there are still disputes about the onset time of the Anthropocene.The main problem is that mankind had put nonnegligible impacts on the earth environment through farming,mining,settlements and transportation network constructions before the mid-20th century.The key feature of the Anthropocene is that mankind has become one sort of geological force that is comparable to tectonic movement or climate change,which can greatly affect the geological environment.China has-20%of the world’s population and is the largest developing country.In historical times,there was a prosperous farming civilization in China,and human activities had significantly moved the geological environment from the natural state.Therefore,it is important to understand the evolution of human activities in China since historical times for the formal establishment of the Anthropocene.Through high-resolution historical reconstruction of geological records of human activities in the past 1000 years,this research aims to provide a reference for the establishment of the Anthropocene,understand the evolution of human-earth relationship during this period,and provide dating supports for responses to problems of human-earth relationship in China in the future.This research took the sediment cores of Gonghai Lake in Shanxi Province and Sihailongwan maar Lake in Jilin Province as the objects.Gonghai Lake is located in the eastern Chinese Loess Plateau,where human activities were relatively strong in historical times;Sihailongwan maar Lake is located in the cold Changbai Mountain area,where human activities were relatively weak in historical times.The cores were cut into 0.5 cm intervals to obtain high-resolution sedimentary sequences of various proxies.The evolutions of human activities around the two lakes were reconstructed on the basis of full consideration of climate change.The proxies measured are from three categories:(1)inorganic matter related proxies,including relative concentrations of partial major elements Si,Al,Fe,K,and Ca;(2)organic matter related proxies,including total organic carbon(TOC),organic carbon to nitrogen ratios(C/N),andorganic carbon isotope ratios(δ13Corg);and(3)weathering proxies including relative concentrations of Ti,rubidium to strontium ratios(Rb/Sr),calusium to titanium ratios(Ca/Ti),magnetic susceptibility and grain size.Through comparison of Rb/Sr curves of parallel cores,it is deduced that the cores were not obviously disturbed.The cores were dated from 137Cs,210Pb and 14C.The average resolution of Gonghai Lake and Sihailongwan maar Lake are～3 years and～10 years,respectively.The historical sequences of these proxies were compared with the historical sequence of climate changes in cold,warm,dry and wet,and the historical sequence of population reflecting the intensity of human activities.Results and main conclusions are as follows.1.By comparing the historical sequences of Al,Si,Fe,K,Ca concentrations with the Ti concentrations(Rb/Sr and Ca/Ti)that commonly used for indicating mechanical(chemical)weathering,it is deduced that the Al concentrations can reflect the silicate input and indicate mechanical weathering intensity,and the Ca concentrations can reflect the carbonate input and indicate chemical weathering intensity.2.Various proxies of Gonghai Lake/Sihailongwan maar Lake sediments indicate that human activities had significantly increased since～1550/～1850 CE.Since～1550 CE,changes in Al and Ca concentrations,TOC,C/N ratios,δ13Corg,grain size and magnetic susceptibility in Gonghai Lake sediments were not obviously related to climate change,but were closely related to human activities.In addition,the correlation of Al and Ca concentrations changed from the inverse correlation under the natural state to the positive correlation.Thus it is inferred that human activities around the Gonghai Lake were significantly intensified after～1550 CE.In terms of Sihailongwan maar Lake sediments,since～1850 CE,the Al centrations increased sharply,and Ca centrations,TOC contents,C/N ratios decreased significantly.After that,these proxies and grain size,magnetic susceptibility had no clear relationship with climate change,but were closely related to human activities.Thus it is inferred that human activities around Sihailongwan maar Lake significantly increased after～1850 CE.Before these two time points,changes in these proxies in the Gonghai Lake and Sihailongwan maar Lake sediments approximately corresponded to climate change.3.Although the transition from LIA to CWP spanned two centennial climate periods,the changes in various proxies during this period had no clear relationship with climate change.This can support that human activities became the dominant factor of changes in geological environment in this period.4.Three events before 1950 CE that related to human activities,including the border reclamation project in the middle of the Ming Dynasty(1500s～1560s),the population landslides in the terminal Ming Dynasty-early Qing Dynasty(1630s～1670s)and the late Qing Dynasty(1850s～1870s),were reconstructed from Al and Ca concentrations,TOC contents and C/N ratios of Gonghai Lake sediments.5.In the CWP,for Gonghai Lake sediments,Al and Ca concentrations,TOC contents,C/N ratios,δ13Corg,magnetic susceptibility and grain size changed significantly at 1950 CE,and further changed after 1980 CE;for Sihailongwan maar Lake sediments,C/N ratios,δ13Corg,magnetic susceptibility,grain size changed significantly at 1950 CE,and also further changed after 1980 CE.The changes at 1950 CE were mainly due to the rapid growth of the population and the initial development of agriculture and industry after the establishment of the People’s Republic of China;the changes after～1980 CE were due to the rapid industrialization,urbanization and agricultural modernization that put more profound impacts on the environment.6.Although the human activities around the two lakes had been relatively strong before 1950 CE,their changes were not synchronous.Synchronic changes in various proxies of the two lakes at 1950 CE can indicate that the environmental status around the two lakes changed synchronously at 1950 CE.Coupled with global comparable changes,the synchrony changes in proxies of the two lakes at～1950 CE can support the viewpoint that "the mid-20th century is the onset of the Anthropocene" from the AWG to some extent.7.The response mode of TOC to human activities was different before and after 1950 CE.The TOC of the two lake sediments before 1950 CE had a certain negative correlation with the intensity of human activities;after 1950 CE,the TOC contents of Gonghai Lake sediments increased with the intensification of human activities,and the TOC of Sihailongwan maar Lake sediments was almost unchanged due to the joint influence of many factors.The change in the mode is related to the popularization of chemical fertilizer that was resulted from the industrial development in the middle and late 20th century.8.Changes in δ13Corg and C/N ratios of Gonghai Lake sediments after 1550 CE are closely related to agricultural development,which could indicate the expansion of cultivated land area and crop selection,as well as the agricultural modernization after 1950 CE.