【作者】 陈建徽；

【导师】 陈发虎； Stephen J Brooks；

【作者基本信息】 兰州大学 ， 自然地理学， 2009， 博士

【摘要】 过去千年是古气候研究中具有特殊意义的时间段，湿度变化对干旱区生态环境状况和社会经济发展具有尤为显著的影响。我国内陆干旱区缺乏年代可靠、指标意义明确的高分辨率记录，不仅制约了对过去千年干湿变化过程和规律的理解，也阻碍了对(数)百年尺度上西风影响区湿度变化与亚洲季风演变历史关系的认识。本论文选择柴达木盆地苏干湖为主要研究点，在SG03I孔纹层年代序列提供的可靠年代框架下，利用对环境响应敏感的摇蚊指标，结合现代过程研究，高分辨率(约10年)地重建苏干湖过去千年盐度变化，进而讨论苏干湖流域过去千年湿度变化趋势和特征。在此基础上，综合前人的研究成果，重建西风影响的我国内陆干旱区以及中东亚干旱区近千年湿度演变序列。在(数)百年尺度上对西风区湿度与亚洲季风演化格局进行了对比研究，并探讨了可能的驱动机制。另外，对新疆博斯腾湖BSTC001孔岩芯进行摇蚊分析，初步重建了博斯腾湖晚全新世以来的水位波动和气候变化历史。获得了以下初步结论：1．摇蚊记录的苏干湖过去千年有效湿度变化(1)青藏高原摇蚊—环境因子数据库研究结果、苏干湖湖盆内部摇蚊组成格局特征、以及已有的相关属种生态学资料共同表明，苏干湖SG03I孔沉积岩芯四个摇蚊属种可以分为高咸水属种Pb和Oc，以及低咸水属种Pr和Ps，沉积序列高／低咸水属种相对丰度的变化可用于反映苏干湖水体的盐度波动。(2)苏干湖过去千年摇蚊盐度记录与同岩芯反映有效湿度变化的其他代用指标在波动格局和变率两方面均显示出较好的一致性，与平行岩芯介形虫壳体Sr／Ca记录的水体盐度变化趋势相符，表明以摇蚊重建盐度为依据重建湖区过去千年有效湿度变化具有较高置信度。(3)苏干湖流域近千年来有效湿度变化框架：990～1550 AD，高咸水属种占绝对优势，低咸水属种零星出现，重建水体盐度处于高值段，反映了较低的入湖水量／蒸发量之比，流域气候干旱；1550～1840 AD，低咸水属种显著增加，重建水体盐度总体为过去千年最低时段，有效湿度增加，流域气候相对湿润；1840～2000 AD，高咸水属种恢复绝对优势地位，重建水体盐度处于较高水平，流域气候再次变干。(4)苏干湖流域近千年来典型气候事件：990～1550AD整体干旱时段内，出现1200～1230AD间30年气候湿润，低咸水属种丰度突然升高，重建水体盐度显著降低；整体湿润的小冰期内部出现1590～1700 AD间110年的相对干旱，高咸水属种优势地位有所恢复，重建水体盐度升高。区域气候记录对比显示，苏干湖摇蚊记录的年代际～百年尺度干／湿事件可能主要是局地性的气候变化，与补给流域关系密切。(5)苏干湖流域近千年来有效湿度变率特征：相对湿润的小冰期阶段摇蚊种群组合及相应的盐度变化具有更大的变幅，并表现出明显的高频振荡，指示小冰期时段气候不稳定性增加。这种特征在我国内陆干旱区、东部季风区、以及全球许多地质记录中都有显示，可能代表了在大尺度气候背景下(例如低温)区域气候运行规律的改变。2．西风影响的我国内陆干旱区以及中东亚干旱区过去千年湿度变化摇蚊记录的苏干湖有效湿度变化具有较大范围代表性，与来自我国内陆干旱区不同地点、由各种代用指标重建的过去千年湿度(降水)变化序列在整体趋势上有很好的一致，以“中世纪暖期整体干旱—小冰期显著变湿—近百年来重新回复干旱气候”为主要特征。西风影响的中东亚干旱区高分辨率记录建立的过去千年湿度变化集成曲线表明，1000～1350AD研究区湿度水平低于平均值；1350～1500AD湿润程度有所增加，接近平均水平略低；1500～1850AD是显著的湿润时段；1850AD之后区域气候回复干旱状态，但自上世纪中叶起有小幅上升。3．近千年来西风影响的中东亚干旱区湿度变化与亚洲季风演变历史对比(1)在数百年尺度上，近千年来西风影响的中东亚干旱区湿度变化与亚洲季风演变格局存在显著差异，表现出“错位相”特征。中世纪暖期时，西风区大多数研究点记录了相对干旱的气候，尤以我国内陆干旱区最为明显；与此相对，季风区记录以强季风或者中等强度的季风为主。小冰期时，西风影响区普遍表现出整体湿润的状况；而季风区各研究点大多记录了偏弱或者中等季风强度，与西风影响区明显不同。(2)在年代际尺度上，树轮PDSI记录和冰芯积累量记录表明，我国西风影响区和季风影响区湿度变化格局在最近两百年表现出“错位相”甚至“反位相”特征。西风区湿度自19世纪初起即持续下降，直至20世纪初开始回升，尤其从20世纪中期起升高显著；而季风区湿度则从20世纪中期开始持续下降。4．可能的驱动机制西风影响的中东亚干旱区小冰期气候相对湿润(降水增加)可能主要是中东亚干旱区低温减少蒸发蒸腾量，以及北半球低温和NAO偏负导致区域降水增加等因素共同作用的结果，可能最终代表了对当时较弱太阳活动(辐射)和较频繁火山活动的响应。在千百年～年代际尺度，太阳活动(辐射)是亚洲季风的重要驱动因素，太阳活动(辐射)强时，亚洲季风较强盛；太阳活动(辐射)弱时，亚洲季风衰退。西风影响的中东亚干旱区湿度(降水)与亚洲季风强度变化对外部强迫，尤其是太阳强迫的响应方式不同，是造成近千年来(数)百年尺度上西风区湿度(降水)与亚洲季风强度错位相甚至反位相变化的主要原因。5．摇蚊记录的博斯腾湖晚全新世水位和气候变化(1)博斯腾湖BSTC001孔岩芯摇蚊种群包含浅水型(Po、Cr、Ps、Dt)和深水型(Ch、Mi、Cc)属种，利用其相对丰度变化初步重建博斯腾湖晚全新世水位变化历史：4100～2700 cal a BP湖泊水位较低，区域气候以干旱为主。2700～190 cal a BP期间湖泊水位较高，区域气候偏湿；但在1700～990 cal a BP博斯腾湖可能经历了一次数百年尺度的水位回落和气候干旱时期。190 cal a BP以来环境状况发生较大改变，人类活动影响明显。(2)博斯腾湖晚全新世摇蚊记录和硅藻记录在变化趋势上有很好的一致性，摇蚊记录揭示的高(低)水位阶段与硅藻记录揭示的低(高)盐度阶段相对应。博斯腾湖摇蚊记录的晚全新世气候变化框架还得到了西风影响区其他代用资料的支持，具有较好的区域代表性。更多还原

【Abstract】 Knowledge of past climate variability over the last millennium （late Holocene）provides perspective for understanding and modeling current and future states ofregional and global climate. Hydrological changes are especially relevant toecosystem dynamics and human well-being in arid regions. However, obvious lack ofhigh-resolution unambiguous proxy records with reliable chronology in aridNorthwestern China not only hamper attempts to investigate moisture variability andits possible mechanisms in the last millennium, but also prevent us from exploring therelationship between moisture variability in westerly dominated arid Central Asia andAsian summer monsoon history on a （multi-）centennial time scale.The annually laminated sediments of Sugan Lake, a hydrologically-closed salinelake in the northern part of the Qaidam Basin, northem Tibetan Plateau, afforded us aunique opportunity to reconstruct the moisture changes in arid Northwestern Chinaover the last 1000 years at a relatively high resolution, using sub-fossil Chironomidae（Insecta: Diptera） as an indicator of past water salinity. Then, based on ourreconstruction and previous proxy records, moisture variability in westerly-dominatedarid Northwestern China and central Asia was systematically studied, allowing acomparison with the Asian monsoon history during the last millennium.In addition, chironomid analysis was also carried out in a sediment coreBSTC001 retrieved from Bosten Lake, Xinjiang, climatically controlled by westerlies,and the late Holocene lake level fluctuations and climate changes were preliminarilyreconstructed.Main conclusions of this dissertation are outlined as follows:1. Moisture variability in Sugan Lake over the last millennium reconstructedusing subfossil chironomid remains（1） Salinity. optima of taxa in the Tibetan Plateau chironomid training set, spatialpatterns of chironomid assemblage within Sugan Lake, and relevant ecological knowledge from the literature together suggest that the four major chironomid taxaidentified from SG03I can be divided into two groups, Pb and Oc indicative ofrelatively high-saline-waters, and Pr and Ps indicative of relatively low-saline-waters.Therefore, the changes in percentage abundances of high-saline-water taxa (HSW_ch)and low-saline-water taxa (LSW_ch) throughout the sequence are suitable forpalaeosalinity reconstruction from Sugan Lake.（2） Chironomid-based salinity records are consistent with other palaeo-moistureproxy data from the same core in terms of general trends and the magnitude andfrequency of the variability, and in broad agreement with the ostracod-shell Sr/Caratio inferred salinity variability derived from parallel core SG00C, confirming thereliability of the effective moisture reconstruction by chironomid analysis for theSugan Lake catchment.（3） The chironomid-based salinity records revealed the effective moistureevolution in the catchment of Sugan Lake during the past millennium. From 990 to1550 AD, the climate was generally arid, as suggested by the. predominance ofhigh-saline-water taxa and the high value of chironomid-inferred salinity (Sal_ch).During the period 1550～1840 AD, LSW_ch considerably increased and Sal_chdramatically declined, indicating relatively humid conditions in the Little Ice Age（LIA）. After 1840 AD, the HSW_ch and Sal_ch both rose to a high level, implying thatthe dry climate prevailed in this area once again.（4） On the decadal to centennial time scale, the chironomid-based salinityrecords document two remarkable climatic events. A relatively wet oscillationoccurred in 1200～1230 AD, interrupting the generally dry period （990～1550 AD）.And a comparatively dry oscillation occurred in 1590～1700 AD, punctuating thegenerally humid LIA. Regional climate comparison shows that the decadal tocentennial wet and dry events revealed by our chironomid-based salinityreconstruction are just of local climatic significance, and could be related tomoisture anomalies in the headwater area of Sugan Lake drainage basin.（5） As far as the trends in the inferred salinity time series are concerned, it isevident that the magnitude of salinity fluctuation was greater, and the frequencyhigher, in the relatively humid LIA （1550～1840AD） than in the preceding andfollowing dry periods, implying a highly unstable LIA climate in the catchmentarea of Sugan Lake. The highly unstable LIA climate had its fingerprint in a lot ofproxy records from arid Northwestern China, monsoonal China and other parts of the world, and probably represented a shift of regional climatic regimes duringtimes characterized by large-scale climatic anomaly.2. Moisture evolution in westerly-dominated arid Northwestern China andarid central Asia over the last millenniumThe 1000-year moisture variability recorded by chironomids in Sugan Lakewas generally consistent with palaeo-moisture data from other proxy sites in aridNorthwestern China, demonstrating a dry Medieval Warm Period （MWP）, arelatively humid LIA, and a dry period during the last 150 years. Moisture recordsynthesized by combining five high-resolution proxy time series from arid centralAsia shows that climate was drier than average from 1000 AD to ca. 1350 AD,moderate from 1350 to 1500 AD, wetter than average from 1500 to 1850 AD, anddrier than average once again after 1850 AD with a slightly wetting trend since themid-twentieth century.3. Comparison of millennium moisture variability in westerly-dominated aridcentral Asia with Asian summer monsoon history（1） On multi-centennial time scale, moisture evolution in westerly-dominatedarid central Asia has an out-of-phase relationship with Asian summer monsoonhistory. Most proxy records from arid central Asia, especially in arid NorthwesternChina, show that there was a relatively dry MWP, contrasting to the moderate tostrong Asian summer monsoon during that period. In LIA, climate of arid centralAsia was generally humid, whereas a weaker or moderate monsoon prevailed from1400 to 1900 AD.（2） On decadal time scale, two records of tree-ring reconstructed PalmerDrought Severity Index （PDSI） and three records of ice core net snowaccumulation indicate that there is an out-of-phase or anti-phase relationship ineffective-moisture or precipitation changes between arid Northwestern China andmonsoonal China during the recent 200 years. Moisture decreased from earlynineteenth to early twentieth century, in arid Northwestern China, and has anincreasing trend since 1920s, especially since 1950s. On the other hand, climate ofmonsoonal China has become progressively drier in the recent fifty years.4. Possible mechanismsThe relatively humid LIA in the westerly-dominated arid central Asia mayhave resulted from combining effects of decreased evapotranspiration caused bythe cooling of arid central Asia, and increased precipitation caused by cooling of the Northern Hemisphere and persistently negative North Atlantic Oscillation（NAO）, all of which are plausibly associated with a generally weak solar activity（radiation） and strong volcanic activity during that time. In contrast, on the decadalto （sub-）millennium time scale, the relationship between solar activity （radiation）and Asian monsoon can be generalized as follows: weak summer monsoon broadlycorrelated with weak solar activity （radiation）, and vice versa. Therefore, thedifferent response of moisture in arid central Asia from that of Asian summermonsoon to the external forcings, particularly solar forcing, probably results in theout-of-phase or anti-phase relationship between moisture evolution in arid centralAsia and Asian summer monsoon history on the multi-centennial time scale.5. Late Holocene lake-level and climate changes recorded by chironomids inBosten Lake（1） Based on variations in percentage abundance of littoral （Po, Cr, Ps, Dt）and profundal （Ch, Mi, Cc） taxa in the upper 383 cm of a sediment core BSTC001from Bosten Lake, late Holocene lake level fluctuations and thus climate changeswere preliminarily recovered. From 4100 to 2700 cat a BP, littoral chironomid taxadominated the assemblage, indicating a low lake level and dry climate. From 2700to 990 cal a BP, the generally increased profundal taxa demonstrates a relativelyhigh lake-level period. However, this ca. 2500-years-long highstand wasinterrupted by a regression during 1700～990 cal a BP. From 190 cal a BP onward,chironomid communities of Bosten Lake experienced remarkable changes andprobably reflected the eutrophication process in the recent past as a result ofintensified human activity.（2） The chironomid-based lake level reconstruction correlates well with thediatom-based salinity reconstruction from the same core, high （low） lake levelscorresponding to low （high） water salinities and thus indicating wet （dry） climateconditions. The late Holocene lake level and climate evoluation recorded bychironomids at Bosten Lake was in broad agreement with other proxy records fromarid central Asia, showing a regional palaeoclimatic significance.更多还原