Last Millennium Vegetation History And Climate Variability Reconstructed From High-resolution Multiproxy Analysis Of Subalpine Peat Sediment In JiangXi Province,Southeast China

Climate change during the last millennia is one of the most important focuses of the"Past Global Changes"(PAGES)initiative.Recently,the improved climate change prediction methods and high-resolution techniques have led to increased interest in climate change studies during the last millennia.Medieval Warm Period(MWP),Little Ice Age(LIA)and Current Warm Period(CWP)are three important periods during the last millennia.However,the starting and ending time,temperature humidity configuration and driving mechanism still need to be further explored.In this study,a sediment core was constructed by merging two peat samples(XYH-14 and XYH-15)obtained from the central part of the swamp in Siyao Lake(Jiangxi Province,China).Firstly,we utilized 210Pb,137Cs and AMS 14C dating methods to determine the sediments' buried time and built the age framework on the basis of Bacon model accordingly.Next,we conducted the peat pollen analysis every 2-cm to reconstruct the vegetation history succession pattern.To further find depositional environment evolution,we also measured Humification Degree(HD),Loss-on Ignition(LOI),XRFScan elements and grain-size distribution.The main results and conclusions are listed as follows.We think these results of the study could help to reveal the climatic variation features of unique periods during the last millennia.1.During 637-800 AD,Siyao Lake was in bedrock weathering stage and waterlogged time.The majority of the sediments are gray granite weathering crust with almost no peat deposition.The sediment records indicated that the vegetation combination consists of upland herbs taxa and scattered Quercus(E).However,the pollen concentration was very low,and plant genera were seldom.Since harsh environment is not conducive to pollen storage,vegetation condition reconstructed by pollen information cannot reflect real climate change.2.Medieval Warm Period(MWP,800-1250 AD),vegetation is abundantthrough the entire period,Quercus(E)is the building group of the forest,Pinus and Castanopsis aresporadic.Upland herbs grew up vigorously in the lower part of forest.Peat began to accumulate in the basin high terrain,where wetland herbs grew vigorous.The climate during MWP was characterized by warm and wet,inside there were obvious secondary fluctuations.In details,800-890 AD and 1050-1125 AD were warm with a bit dry,890-1050 AD was warm humid climate,the climate became cool and wet from 1125 to 1250 AD.In summary,MWP temperature underwent a warm turn cool process and dry wet fluctuation was the main feature in precipitation change.3.Dramatic vegetation changes were recorded during the LIA(1340-1870 AD).The vegetation community was primarily dominated by Castanopsis,upland land herbs thrive;wetland herbs were sparse with great fluctuations depending on changes in the humidity.The sediment types vary considerably and silty clay was the main sediment type during 1340-1430 AD.Brownish peat instead of clay became the main sediments in 1430-1870 AD.Obvious fluctuations were recorded in the total pollen concentration and charcoal concentration.LIA was characterized by cold and dry,the temperature stayed low,but the humidity fluctuated dramatically.Changes in temperature were volatile during 1340-1750 AD,generally speaking,climate tend to be cold-dry during 1340-1425 AD,1475-1575 AD and 1625-1750 AD,temperature recovered a little in 1425-1475 AD and 1575-1625 AD.During 1780-1870 AD,humidity increased with temperature rose,which was still characterized by cold-wet.Overall,during LIA,temperature pattern was featured by"four cold caught three warm",humidity condition was experienced a process from drought to wet.4.High resolution pollen analysis indicated unique climate period fluctuate severely,MWP was warm-humid,LIA was cold-dry.On centennial or multi-decadal scale,precipitation variation and temperature reconstruction are positive correlated and warm-humid/cold-dry was the main climate pattern in lastmillennia.Solar activity was the fundamental force that drove the same-phase variation of the temperature and precipitation in East Asia monsoon region.Besides,temperature was also affected by strong volcanic activity,and the East Asian monsoon changes would result in precipitation fluctuation.5.Periodic analysis of the moisture proxy(PCA 1)and temperature indicator(E/D)showed cyclic fluctuations of?150 years in the temperature and precipitation for the Siyao Lake region,which corresponded to historical document records.Both the comparison between the Total Solar Irradiance(TSI)and E/D and the comparison between the stalagmite oxygen isotope ?18O from Wanxiang Cave and PC A 1 indicated that a?150-year solar active period was the trigger factor for the climate change during LIA.The TSI controls the amount of surface radiation,which led to shifts between the cold and warm periods.Asia Monsoon(AM)precipitation is driven by the thermal contrast between Asia and the tropical Indo-Pacific,which is controlled by the regional temperature.Therefore,the TSI influences temperature changes directly and precipitation changes indirectly.After the 19th century,AM precipitation showed significant variability which was hardly correlated with the TSI.Therefore,we could infer that anthropogenic forcing may play a key role in the East Asian Monsoon precipitation.