Climate And Environmental Change Recorded By Lacustrine Sediments From Gucheng Lake Since Last Deglaciation In Jiangsu Province

Gucheng Lake is located in Gaochun District,Nanjing City,Jiangsu Province,at the border with Wuhu City,Anhui Province,belonging to the Qingyi River and Shuiyang River basins in the lower reaches of the Yangtze River.It is located in the southern edge of Jianghuai River basin in the north-south transition zone of the eastern monsoon region of China,and belongs to the semi-closed lake in the climate transition zone.The sediments recorded abundant paleoclimate environment information and presented the process of climate and environment change in this region completely.In this paper,we extracted the information of many environmental proxy indexes,such as chronology,grain size,magnetic susceptibility and productivity index of 22 m core sediments from GZK1804,Gucheng Lake.Then,the high resolution lacustrine sedimentary records from nearby and other areas or from other carriers were combined with those.In addition,the results from environmental archaeological data,archaeological stratigraphy,and historical document records were taken into consideration.Based on the situations,the process of climate and environment change since the last deglaciation in this region has been reconstructed.The results show that the process of climate and environment change since the last deglaciation in Gucheng Lake area can be divided into seven stages(I-VII).Stage I(14500-12200 cal.a BP),in this period of time,the climate conditions gradually became warm and humid,and then tended to cold and dry for a long time,and there were two dry-wet alternation changes in the climate.The silt content of sediments increased,while the clay content decreased.The volume content of average particle size and median particle size decreased first and then increased from bottom to top.The overall particle size was fine,and the magnetic susceptibility fluctuated greatly.Stage II(12200-11000 cal.a BP),the climate in this stage was very cold and arid,the silt and clay content of sediments was decreasing,the sand content was increasing,and the magnetic susceptibility value was low and stable.Stage III(11000-10500 cal.a BP),the climate tended to be moist and the humidity was increasing,the sediment composition was mainly silt and sand,and the proportion of sand content was higher than that of the previous stage,the coarse grains were increasing,and the magnetic susceptibility curve fluctuated frequently.Stage IV(10500-9500 cal.a BP)was a wet period,mainly composed of silt.The contents of median and average particle sizes increased significantly compared with the previous stage,and the intensity of magnetic susceptibility gradually decreased.Stage V(9500-7700 cal.a BP),the climate characteristics of this stage were warm and humid as a whole,and there were several obvious climatic fluctuations.The sediment composition gave priority to silt and clay,and the proportion of sand content was going up,the grain size took the coarse grain as the main body,and the fluctuation of magnetic susceptibility value was decreasing.Stage VI(7700-1200 cal.a BP),climate change had experienced a warm period(7.6-3.5 cal.a BP)after a cold dry period(3.5-2.6 cal.a BP),and then reached to 1.1 cal.a BP fluctuations in climate change process of the partial cold-dry gradually,sediment composition was given priority to with silt and clay,sand content was low,the median diameter and average particle size than the last one stage partial coarser,C/N curve changed from high value gradually reduced to minimum,magnetic susceptibility value change was bigger.Stage VII(1200-300 cal.a BP),when the climate conditions became cold and arid,the composition was mainly silt and clay,the sand content was very low,the C/N was very low,the precipitation decreased,the lake water level decreased,and the sorting was poor.In this stage,the deposition rate was too fast and the magnetic susceptibility curve fluctuated sharply.Combined with the historical records,we can know that there was a strong disturbance of human activities in this stage.The abrupt climatic events since the last deglaciation have been completely recorded in Gucheng Lake sediments,which are B/A warm period,YD cold event,9.3ka BP cold event,8.2 ka BP cold event,Middle Holocene optimum period,4.2 ka BP cold event,Ming and Qing Little Ice Age.The climate change pattern in eastern China is consistent with that in other regions,which is driven by solar radiation,solar activity and human activity.Meanwhile,the climate and environmental change process in this region has unique regional characteristics,and the time and characteristics of climate change are different from those in other regions.The YD cold event and the Holocene drought event recorded in Gucheng Lake correspond well with the North Atlantic ice raft event.The early Holocene climate in Gucheng Lake region was in a warm stage,with more precipitation.In the Middle Holocene,the solar radiation reached the maximum value in summer at 30°N,the number of sunspots was higher,and the solar activity level was stronger.In the Late Holocene,the climate tended to be cold-dry,and human activities,such as dam construction,lake reclamation,river excavation and other projects,affected the sedimentary environment and lake area of Gucheng Lake Basin,and changed the local water environment,which was the main influencing factor of the sedimentary environment change of Gucheng Lake region during this period.The changes of lake area,water environment and lake level in Gucheng Lake area also affect local people's living environment,labor production mode,planting and aquaculture type and scale.Reasonable use of water and soil resources in Gucheng Lake area and protection of the environment in Gucheng Lake area are the keys to the harmonious coexistence between man and nature and the sustainable development of the basin in the future.