Holocene Climate Change And Fire Activity Recorded By Lake Sediments From Central Yunnan

Climate change and human activities have significantly impacted various spheres of the Earth system at an accelerating rate in the context of current global warming.Lakes,interconnecting different spheres of hydrosphere,geosphere,atmosphere,biosphere in Earth system,are the important ecosystems in the world.The similarities and differences between responses of lakes to these two stressors remains unclear.Thus an improved understanding of the long-term influences of climatic and anthropogenic disturbances is essential for the management of lakes and watersheds.As one of the regions with high biodiversity and population density in the world,the Yunnan Plateau in the typical Indian summer monsoon（ISM）region is a typical fire-prone area in subtropical East Asia.There are numerous lakes of tectonic origin on the plateau and they are valuable archives for studying past climate changes,human impacts and environment responses.In order to address these issues,several studies of climate change and lake environment have been conducted on lake sediments from Yunnan,and significant progresses in the investigation has been made during the last few decades.However,the history and driving factors of the ISM during the Holocene are still poorly understood,and the effects of long-term anthropogenic disturbance on the environment are also unclear.Thus high-resolution reconstructions of paleoclimate and paleoenvironment in the region is still needed.Here we presented multi-proxy records from Lake Yilong as the archives of regional climatic and environmental changes,and then made a comparison of the relative impacts of natural climatic event and human activity on the lake ecosystem.In addition,we presented and analyzed Holocene fire history documented by sedimentary black carbon records from four lakes in the Yunnan Plateau:lakes Dian,Xingyun,Qilu,and Yilong,together with an observational dataset of fire for the Yunnan Plateau for the past 20 years.Our aims were to explore the linkage between wildfires and potential natural and anthropogenic variables driving wildfire occurrence on multiple timescales.The research has the following results and new findings:The chronology of core Yilong C-19 was based on AMS ¹⁴C dates from 13 samples of plant macrofossils and charcoal,which show that the record spans the last～12,000yr.Geochemical indices,including the carbon and oxygen isotopes of carbonate(δ¹³C andδ¹⁸O),elements,total organic carbon（TOC）and total nitrogen（TN）,were used to reconstruct regional climatic change,hydrological variations and lake ecosystem responses.From the late Pleistocene to the beginning of the Holocene,the dominant mineral in the core was terrigenous quartz,and the TOC and carbonate contents were low,indicating a cold and humid climatic condition and low primary productivity.During the middle and late Holocene,calcite precipitated in the lake waters and theδ¹⁸O records of the calcite indicate a consistent trend with gradually increasing values,consistent with the decreased index I and TOC content.All of the evidence suggest that the soil erosion and primary productivity followed the declining of precipitation and experienced a gradual weakening trend.During the early Holocene,the humid climate was interrupted by an abrupt decrease in precipitation at the interval from 9.7 to 8.7 cal kyr BP（corresponding to the 9.3 kyr event）,as indicated from the strongly positiveδ¹⁸O values of endogenetic siderite.On the other hand,we revealed a rapid increase in the input of terrestrial material since～1500 cal yr BP because of increasing anthropogenic soil erosion around the watershed,and a decrease in C/N ratios after～600 yr BP suggests a rise of the productivity of lake water caused by cultural eutrophication.A comparison of the effects of a centennial-millennial natural climatic event and human disturbance reveals contrasting lake ecosystem responses.The lake productivity declined slightly and subsequently recovered from the impact of this environment perturbation,indicating that the lake ecosystem was resilient to the 9.3 ka event;however,long-term human activity in the watershed,including deforestation and cultivation,reduced the stability of the lake ecosystem and positive feedback effects were strengthened,leading to the deviation of the system far from its previous stable state.Based on a 20-years observational dataset of fire counts for the Yunnan Plateau,combined with data on the natural driving factors,the impacts of weather and vegetation on the fires are assessed.The results reveal that that humidity（and hence biofuel flammability）in the winter and spring was the main factor driving the occurrence of regional wildfires from monthly to annual timescales in this biofuel-rich ecosystem.We then presented the sedimentary black carbon records from the four lakes in the Yunnan Plateau,and combined with AMS ¹⁴C dating results to reconstruct the integrated fire history in central Yunnan during the Holocene.The results show that low severity wildfires occurred during the relatively humid early Holocene（11.7–6 cal kyr BP）,and severe wildfires occurred during the humidity-decreased middle Holocene（6–2.8 cal kyr BP）.These observations indicate that fire activity is also controlled by the humidity on a longer timescale,against the background of the abundance of biofuel.The lowest degree of biomass burning occurred after～2.8 cal kyr BP,and was linked to the anthropogenic reduction of biomass availability,despite an increase in drought.The finding suggests the importance of active fire management and prescribed clearance measures,especially given projections of continued drought and increased vegetation cover.