【作者】 仵慧宁；

【导师】 马玉贞；

【作者基本信息】 兰州大学 ， 第四纪地质学， 2009， 博士

【摘要】 近25 ka以来跨越了末次冰盛期、末次冰消期及冰后期，是过去全球变化研究的一个重要时段。处于中纬度半干旱地区的陇中黄土高原，生态环境极为脆弱，是季风气候和全球变化的敏感区之一。本文以陇中黄土高原祖厉河流域中上游地区会宁硝沟剖面的711个样品的孢粉分析为基础，结合粒度及总有机碳（TOC）指标，在19个¹⁴C年龄支持下高分辨率地恢复了该区近25 ka以来的古植被及古气候演化过程，为区域环境变化研究提供了一个高分辨率的生物气候演化序列，并通过与区域及全球记录的详细对比浅析了研究区近25 ka以来的古植被及古气候演化机制。据野外地质调查分析和孢粉浓度与TOC的关系，初步认为孢粉谱中针叶树花粉含量及浓度变化实际上反映的是流域森林植被的变化，当其百分含量很高而浓度却极低时，并不代表当地有大面积针叶林／松林发育，相反往往反映当地植被覆盖度极低、冬季风相对强盛的冷干气候期。而后据百分比花粉谱中针叶树、阔叶树与灌木和草本植物各种类之间的相对含量关系及浓度花粉谱的变化特征，结合聚类分析谱系，将会宁硝沟剖面的孢粉谱自下而上划分为6个大带20个亚带。据此重建的古植被及古气候演化过程显示，研究区25 ka以来的环境变化明显可划分为末次冰期盛冰阶、末次冰消期、早中全新世、晚全新世四个时段，并揭示出多次冷干气候事件（H2、H1、YD及全新世系列冷干事件等）。1)约24．5～13．8 ¹⁴C kaBP，研究区处于末次冰期盛冰阶，气候以冷干与冷湿交替出现为特征，冷干期植被为荒漠或荒漠草原，冷湿期局部山地丘陵发育云冷杉属和松属为主的针叶林：其中20．0～17．1 ¹⁴C ka BP及15．1～13．8 ¹⁴C k aBP的极端冷干期分别对应于北大西洋冰筏事件H2及H1。2)末次冰消期气候逐渐回暖但波动很大：约13．8～13．3¹⁴C ka BP，植被为干草原，气候较冷干，与Oldest Dryas期对应；13．3～12．4¹⁴C ka BP，植被为针叶疏林草原，气候转温湿，相当于B（?）lling暖期；12．4～11．5¹⁴C ka BP，植被为荒漠草原，气候变冷干，与Older Dryas期对应；11．5～10．7¹⁴C ka BP，植被为针叶树为主的森林草原，气候较温湿，对应于Aller（?）d暖期；10．7～9．88¹⁴C ka BP，植被为干草原，气候再次变冷干，相当于Younger Dryas事件。3)早中全新世(9．88～4．37 ¹⁴C ka BP)，植被为针阔叶混交林为主的森林草原，气候总体暖湿适宜但波动频繁，其中8．06～7．12 ¹⁴C ka BP期间，研究区呈现针阔叶混交林景观，整个流域植被覆盖度最高，似为全新世气候最宜期。4)晚全新世以来(4．37～0．00 ¹⁴C kaBP)，气候在总体干旱的趋势下存在次一级的干湿波动：约4．37～3．27 ¹⁴C kaBP，植被为半干旱草原，气候明显变冷干；约3．27～1．45¹⁴CkaBP，植被覆盖度有所提高，为针叶疏林草原，气候总体温凉偏湿；1．45 ¹⁴C kaBP以来气候再次急剧变干，特别是约0．48 ¹⁴C kaBP以来，极低的花粉浓度显示植被覆盖度更低，呈现荒漠化草原景观，但这一时期的植被变化可能已受到人类活动的严重影响。研究区近25 ka以来的孢粉等高分辨率多指标记录揭示的多次冷干气候事件（H2、H1、YD及全新世系列冷干事件等）与北大西洋深海沉积、高纬格陵兰冰芯、南京葫芦洞石笋及贵州董歌洞石笋等记录具有良好可比性，说明中低纬地区与高纬地区气候变化的遥相关性，其气候冷暖干湿变化总体趋势与北半球夏季太阳辐射曲线基本一致，反映了万年尺度上东亚季风的源驱动力为太阳辐射。特别是孢粉、粒度及TOC多指标对比分析揭示，全新世期间研究区共出现12次显著冷干气候事件，其中有8次（E1-E8分别发生于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及6．42～5．81 cal kyBP）发生在气候总体暖湿适宜的早中全新世，这些事件大多与区域及全球性记录具有良好可比性，说明全新世气候的极端不稳定性及千-百年尺度的韵律变化在中纬度的陇中黄土高原地区同样存在，反映出研究区植被环境对全球气候变化的敏感性及区域古环境演化的复杂性。更多还原

【Abstract】 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.更多还原