A High Resolution Record Of Vegetation And Environmental Variation Through The Last ～25,000 Years In The Western Part Of The Chinese Loess Plateau

The past～25,000 years,which covers the Last Glacial Maximum,the last deglaciation andHolocene,witnessed the most dramatic climatic changes of global scale during recentgeologic history.The western part of the Chinese Loess Plateau has been demonstrated to beone of the most sensitive areas to large-scale climate changes because of the interactionbetween the winter and summer monsoons.Therefore,we present a high-resolutionreconstruction of environmetnal variations and associated vegetation changes,based on 711pollen samples,grain size,Total Organic Carbon and 19 ¹⁴C dates,from a 40-m thick sectionat Xiaogou (36.1 o^N and 104.9 o^E) in the Zuli River drainage basin on the Loess Plateau.It isexpected that this research will add a piece of valuable information to improving ourunderstanding of the large-scale mechanisms that control or modulate the climate changesduring recent geologic history.Based on the field survey for geology and relationship between pollen concentration andthe Total Organic Carbon in Xiaogou section,we considered that the existence of coniferousforest may be indicated not only by the high percentage of coniferous pollen (e.g.,Pinus andPicea) but the rather high concentration.It is subsequently demonstrated that a relatively highpercentage of coniferous pollen in a terminal lake sediments of any arid or semiarid basinindicates that the coniferous pollen grains were fluvially transported from the montaneconiferous forests within the catchment.Six pollen assemblage zones and twenty sub-zones from the bottom to the top wereidentified based on the pollen concentration and the percentages of coniferous trees,temperate deciduous trees,and herbs,and CONISS analysis.The reconstructed vegetation andclimate show that the environmetnal variations maybe include the Last Glacial Maximum,thelast deglaciation,early-mid Holocene and late Hoiocene,and reveal cold and dry events,suchas H2,H1 and YD.1) A desert steppe occupied the landscape between 24.5 and 13.8¹⁴C kaBP (the Last GlacialMaximum).Two episodes of further vegetation deteriorations occurred between 20.0 and 17.1¹⁴C kaBP and between 15.1 and 13.8 ¹⁴C kaBP,corresponding to the Heinrich events 2 and 1. 2) The climate gradually warmed up but more secondary changes between 13.6 and 9.9 ¹⁴CkaBP.During the period of 13.8～13.3 ¹⁴C kaBP,a arid steppe landscape occupied under amoderately dry condition,corresponds to the Oldest Dryas period.The second sub-stage(13.3～12.4 ¹⁴C kaBP) represents a coniferous woodland grassland,corresponds to the B(?)ilingwarm period.The third sub-stage (12.4～11.5 ¹⁴C kaBP) represents a Desert steppe,corresponds to the Older Dryas cold period.The fourth sub-stage (11.5～10.7 ¹⁴C kaBP)represents a forest steppe,corresponds to the Aller(?)d warm period,and the fifth Sub-stage(10.7～9.9 ¹⁴C kaBP) represents a arid steppe landscape,corresponds to the Younger Dryascold period.3) The early-mid Holocene (9.9～4.4 ¹⁴C kaBP) was a period when a forest steppedominated the landscape under a warm and wet climate.Specifically,a mixed forest occupiedthe landscape from 8.1 to 7.1 ¹⁴C ka BP with the densest vegetation comparing with theHolocene Optimum (warm/humid climate).4) After the wet regime,the climate has significantly fluctuated in drought conditionsduring later Holocene (4.4～0.0 ¹⁴C ka BP).A semi-arid steppe and cold and dry climateoccurred between 4.4～3.3 ¹⁴C ka BP.The study area experienced improved moistureconditions between 3.3～1.5 ¹⁴C ka BP (i.e.,coniferous woodland grassland),followed by apersistent deterioration of vegetation under a drying climate during the past～1500 years.Above climatic events (i.e.,H2,H1,YD) documented by pollen data through the last～25,000 years appear to be chronologically comparable with those recorded in Greenland icecore,deep-sea sediment,and Chinese stalagmite,Those suggest that the climate change instudy region was droved by east Asian monsoon controlled by solar insolation during recentgeologic history.It should be particularly noted that our pollen data documented as many aseight cold and dry spells (i.e.,11.1～11.00 cal kyBP,10.7～10.6 cal kyBP,10.0～9.84 cal kyBP,9.65～9.58 cal kyBP,9.49～9.39 cal kyBP,9.20～8.94 cal kyBP,8.01～7.47 cal kyBP and6.42～5.81 cal kyBP) during the early-mid Holocene when the climate was generally warmand wet,and as many as four cold and dry spells during later Holocene,suggesting that theHolocene climate has been extremely unstable in the western part of the Chinese LoessPlateau.