Climate And Environmental Changes In Erxianyan Area Recorded By Lake Sediments In The Past 2000 Years

This paper based on the variation characteristics of grain size,magnetic susceptibility and TOC and other substitute indicators of a lake sediment profile in Erxianyan Wetland Nature Reserve,the climatic and environmental changes in the past 2000 years of the Erxianyan area were discussed.To analyze whether there were consistent cold and warm periods in different parts of China in the past 2000 years,and try to explain the mechanism of climate and environment change in Erxianyan area.According to this study,the climatic and environmental changes recorded by the sediment profile of the Erxianyan lake can be divided into four stages:Stage ?(194BC?320AD):The sediments in the study area are mainly silt,with low magnetic susceptibility and high TOC content.The climate is warm and humid,which is equivalent to the Qin-Han warm period.At this time,the surface runoff in the study area is well developed,the lake water level is high,and the frequency of heavy precipitation is relatively large.Stage?(320?820AD):The clay content in the sediment increases,so the sediment particles are finer,the magnetic susceptibility increases,and the TOC content begins to decrease.The temperature and humidity in the study area have decreased,so the development of surface runoff has weakened,the water level of the lake has declined,and the climate is cool and wet.Stage ?(820?1518AD):There are three major fluctuations in the climate.In the early stage(820?1010AD):The clay content increased sharply and the sediment particles were fine.Therefore,the magnetic susceptibility increased sharply and the TOC decreased rapidly.The surface runoff in the study area is not developed,the lake water level continues to decrease,and the area shrinks.The climate is cold and dry.Mid-term(1010?1283AD):The silt content increases,the sediment particles are coarse,the magnetic susceptibility begins to decrease,the TOC content increases,and the climate shows a short period of warmer and humid.The surface runoff in the lake area is well developed,and the lake water level has been raised to a certain extent,which is equivalent to MWP.Late period(1283?1518AD):The content of silt and sand is reduced,the clay content is increased,the surface runoff development is weakened,and the sediment particle size becomes thinner.The magnetic susceptibility increased,and the TOC content decreased rapidly.The climate is cold and dry,corresponding to the early period of the LIA,the lake water level began to decrease again,and the area shrank.Stage ?(1518?1975.5AD):The clay content and magnetic susceptibility are higher,indicating less precipitation.The TOC content is very low and the temperature is also low.The climate is still cold and dry,corresponding to the late period of the LIA.In general,the Erxianyan area has experienced warm and humid-cold and wet-cold and dry-relatively warm and humid-cold and dry in the past 2000 years,showing a gradually drying trend.During the Qin-Han Warm Period,the MWP and the LIA,they showed generally consistent changes with other regions in southern China,indicating that there were indeed consistent warm and cold periods in China in the past 2000 years.At the same time,the research in this paper also found that the warmth and humidity of the medieval warm period in the Erxianyan area did not exceed that of the Qin-Han dynasties,indicating that the medieval warm period was not the warmest and humid period in the past 2000 except the modern warm period.In the past 2000 years,the environmental change process in the Erxianyan area was basically dominated by natural changes,and the impact of human activities was weak.During the period of total solar radiation and 33°N summer solar radiation energy is high,the climate in Erxianyan is warm and humid,and the vegetation is lush.The surface runoff is well developed,with strong carrying capacity,and can carry more distant land-based detrital materials to the lake basin for sedimentation.Therefore,the sediment is coarse,the magnetic susceptibility is low,and the TOC content in the sediment is high.The lake has a high water level and a large area.Conversely,in the period when the total solar radiation and the summer solar radiation energy at 33°N are relatively low,the climate in Erxianyan is relatively cold and dry.There is less precipitation in the basin,the surface runoff is poorly developed,and the carrying capacity is weak.Therefore,the lake water level is low,the area is small,the TOC content in the sediment is low,the sediment is fine,and the magnetic susceptibility is high.