Holocene Pollen Record And Reconstruction Of Palaeovegetation And Palaeoclimate In The Zoige Basin,Tibetan Plateau

The climates on the eastern Tibetan Plateau are strongly influenced by direct insolation heating as well as monsoon-derived precipitation change.Vegetation succession in northeastern Tibetan Plateau where locates the transitional zones of alpine meadow,alpine conifer forest and temperate steppe is sensitive to climate change.In this dissertation,we present a well dated and high-resolution loss-on-ignition,peat property and fossil pollen record over the last 11,400years from a sedge-dominated fen peatland in the southern Zoige Basin on the eastern Tibetan Plateau and discuss its ecology and climatic interpretations.Lithology results indicate that organic matter content is low at 0-20%between 11.4 and 10.6 ka BP,the sedimentary environment experienced changes from a shallow pond to peat bog.Organic matter content is high at 60%-80%between 10.6 ka and 3 ka and shows large-magnitude fluctuations in the last 3000 years.Ash-free bulk density,as a proxy of peat decomposition and peatland surfacemoisture conditions,oscillates around a mean value of 0.12 g/cm3,with low values at 9-6.3 ka,reflecting a wet interval,and an increasing trend after 6.3 ka,suggesting a drying trend.The time-averaged mean carbon accumulation rates are 49.8 gC/m2/yr for the last 11,400 years,higher than that from many northern peatlands.Due to the different sedimentary environment,between the period of 11.4-10.6 ka BP and 10.6 ka BP-present,pollen assemblages reflect a smaller range of vegetation condition after 10.6 ka BP.Tree pollen(mainly from Picea),mostly reflecting temperature change in this alpinemeadow-forest ecotonal region,has variable values(from 5 to 30%)during the early and middle Holocene,reachesthe peak value during the mid-Holocene at 6.3 ka,and then decreases until present.The combined peatproperty and pollen data indicate that a warm and wet climate prevailed in the early and middle Holocene(9.6-3.9 ka),representing a monsoon maximum or "optimum climate" for the region.The timing isconsistent with recent paleo-monsoon records from southern China and with the idea that the interplaysof summer insolation and other extratropic large-se boundary conditions,including sea-surfacetemperature and sea-level change,control regional climate.This could reflect an influence directly from solar insolation on the climate and vegetation succession in the Tibetan Plateau.The cooling and drying trend since the mid-Holocene likely reflects the decrease in insolation heating and weakening of summer monsoons.The peatland and vegetation changes in the lateHolocene suggest complex and dramatic responses of these lowland and upland ecosystems to changesin temperature and moisture conditions and human activities.The vegetation successions can response the rapid climate change during Holocene.It records 8.2 ka,5.5 ka,4.2 ka,and 2.8 ka events in our record,which can correlate with ice rafting events from North Atlantic.