Response Of Soil Erosion To Environmental Changes And Human Activities Recorded By Sediments In Karst Depression

The ecological restoration and reconstruction of karst ecological fragile areas are a enormous systematic project,in which the prevention and treatment of soil erosion is a key link,so that effective soil erosion treatment requires a deep understanding of the laws and mechanisms for soil erosion in karst areas.As a“Reservoir”of watershed materials,karst depression detailedly records the change information of soil erosion.Therefore,a 325cm-deep ZLM sedimentary profile is collected from the Zhong Liang Mountain karst depression in Beibei District,Chongqing.firstly,we establish our age model based on the ¹³⁷Cs,²¹⁰Pb,AMS¹⁴C dating methods.Secondly,on the basis of reliable time series,a variety of indicators（including grain size,magnetic susceptibility,element Ti,chemical alteration index CIAm,Al/K）are used to study the effect of climate and environment,the climate and environment change history since 744 years was reconstructed by comparing with the regional geological record.Finally,combined with the heavy metals Zn,Pb,TOC,δ¹³C and historical records,the population and cultivated land area of Chongqing area are reconstructed,the possible impacts of climate change and human activities on soil erosion recorded by ZLM sediment profile are also discussed.The main conclusions are as follows:（1）The ²¹⁰Pb age of 0-55 cm above ZLM profile was calculated by Constant rate of supply mode model.A reliable dating sequence of ZLM profile was established by dating AMS¹⁴C with 7 charcoal at 325-60cm and calibrating AMS¹⁴C with calibration curve Int Cal13（error range is 2σ）.The results show that the ZLM sedimentary profile covering the last 744 years can be used to study soil erosion and environmental changes on the scale of decades to centuries.（2）The average soil erosion modulus recorded in the ZLM sedimentary profile is151.14 t km^-2yr^-1.According to the change of soil erosion modulus,the profile can be divided into seven stages,which are:1274-1410AD:25.98t km^-2yr^-1;1410-1640AD:177.04 t km^-2yr^-1;1640-1720AD:47.93 t km^-2yr^-1;1720-1890AD:273.97 t km^-2yr^-1;1890-1963AD:94.71 t km^-2yr^-1;1963-1993AD:349.30 t km^-2yr^-1;1993-2018AD:122.45 t km^-2yr^-1.The results showed that soil loss was serious in 1410-1640AD,1720-1890ad and 1963-1993AD,and reached the peak in 1963-1993AD.（3）In this paper,the contents of the grain size,χfd（frequency susceptibility）and Ti of sediments were selected to indicate precipitation change in the basin,the contents of Cu and Fe indicate the temperature change,the chemical alteration index and the ratio of Al/K indicate the chemical weathering intensity of the basin.Comprehensive comparative analysis of various proxy indicators was carried out,and then the evolution history of climate and environment was discussed.The results show that during the period of 1274-1483AD,all the proxy indexes of the ZLM profile showed that the monsoon showed an increasing trend as a whole,the temperature decreases and precipitation decreases during 1483-1530AD,and the precipitation increases during1530-1620AD,and the air temperature changes turn to 1575AD and decrease during the earlier period and later period;from 1620 to 1856AD,1620-1856AD,temperature increases and precipitation fluctuations increase;but the precipitation did not increase significantly since 1856AD.Since the 15th century,the research results of other geological carriers showed that the monsoon weakens,the precipitation decreases and the temperature decreases as a whole,which is significantly different from the climate change reconstructed in this paper,moreover,there is some uncoupling between climate change and the erosion rate recorded in the ZLM profile.This indicates that climate is not the main reason of soil erosion change in the study area.（4）The significant increase of TOC,heavy metal Zn and Pb in sediments of ZLM profile may be influenced by human activities.The change of agricultural activities,especially the planting of maize,may be the reason for the heavy variation ofδ¹³C value in the sediments of ZLM profile.The above indexes are consistent with the increase of population and the change trend of cultivated land index in Chongqing area,which further indicates that human activities have gradually become the main force of surface environment improvement in the study area in the past 700 years.The higher soil erosion modulus of 1410-1640AD(177.04 t km^-2yr^-1)and 1720-1890AD(273.97 t km^-2yr^-1)could be attributed to the“Migration from Hu Guang to Sichuan”.With the increase of population,the demand for food increased to sustain livelihoods,forcing people to reclaim land,destroying vegetation and causing soil erosion to intensify.In1963-1993AD,the soil erosion modulus increased sharply.According to the data from meteorological stations,there was no remarkable change in precipitation during this period,which further indicated that the soil erosion modulus might be controlled by strong human disturbance.In summary,soil erosion in karst areas might be a long-term land-surface process induced by human activity over timescales of>100 years rather than a short-term modern process occurring over a number of decades.