【作者】 杨美临；

【导师】 陈发虎；

【作者基本信息】 兰州大学 ， 自然地理学， 2008， 博士

【摘要】 全新世气候变化研究是预测人类未来生存环境演变的重要依据,因而得到了国际学术界的广泛关注。随着具有精确测年的高分辨率记录（如石笋和泥炭等）的日益丰富,亚洲季风区全新世气候演化的模式已经逐渐为人们所认知。但对于降水过程主要受西风影响的亚洲内陆西风区而言,高分辨率的全新世气候变化记录还相对匮乏,而且各记录的测年精度、年代时标和指标分辨率也各不相同,使得人们对该区整体全新世气候演化模式至今未能达成共识。其实近年来西风区一些最新的研究记录已经揭示,该区与亚洲季风区的全新世气候变化在大趋势上可能具有显著差异,但这种差异是否普遍存在还有待于更丰富、可靠的研究记录对其予以验证。此外,限于研究记录的分辨率和代用指标的敏感度,在季风区和季风边缘区的全新世记录中所广泛探讨的千百年尺度的快速气候变化事件在西风区却鲜见报道。因此,选择西风区的理想研究点,在可靠的年代框架下进行高分辨率的敏感代用指标分析,对正确认识西风区全新世气候变化过程及判断快速气候变化事件在该区是否发生均具有重要意义。博斯腾湖是我国最大的内陆淡水湖,位于亚洲内陆西风区中部,主要受西风带来降水的影响,是研究西风区全新世气候变化过程的理想地点。此外,已有工作表明博斯腾湖的沉积速率较高,对湖泊沉积物开展高分辨率的代用指标分析可能检测出短尺度气候变化事件是否发生,因此选择该湖作为本文的研究点具有一定的理论研究价值。而湖泊中大量生长的硅藻由于对环境变化具有较强的敏感性,且其遗骸在沉积物中保存相对完好,能较好地记录短尺度气候变化事件,在全新世古气候、古环境重建中应用广泛,因而成为本研究的首选气候代用指标。本文首先对博斯腾湖现生硅藻进行调查,共鉴定现代湖泊表层沉积物中的硅藻样品35个（每个样品统计400～600个硅藻壳面）,以了解现代环境状态下硅藻的主要类型和分布特征。然后从9.52m长的BSTC001孔和9.25m长的平行钻孔XBWu46中挑选出12个测年材料,利用AMS ¹⁴C技术对其进行年代测定。根据磁化率值进行两根钻孔深度序列的对比,将全部的年代数据转至同一深度坐标系之下。由于钻孔位置靠近河口区,水体交换较为迅速,老碳效应并不明显,并且选用的测年材料大都为陆生植物残体,所以碳库效应可以被忽略。经筛选后,利用其中7个层位的AMS ¹⁴C年龄的校正结果进行内插计算沉积速率,而钻孔顶部样品的沉积速率则由¹³⁷Cs时标年控制,最终建立BSTC001孔的年代模式。然后以硅藻分析为主（共分析样品179个,每个样品统计400～600个硅藻壳面,分辨率约为45年）,以碳酸盐（共分析样品456个,分辨率约为18年）、粒度（共分析样品421个,分辨率约为19年）、磁化率（共分析样品451个,分辨率约为18年）、烧失量（共分析样品443个,分辨率约为18年）和C/N（共分析样品263个,分辨率约为30年）等常规分析为辅的多指标分析方法,重建了博斯腾湖区早中全新世以来的古环境和古气候。主要研究结论如下:1、对湖泊现生硅藻进行CA分析发现,矿化度是影响博斯腾湖硅藻种群组成最为重要的环境因子,根据硅藻对矿化度的耐受程度的不同,可以大致将其分为两个组合:喜低矿化度、广布型的Fragilaria组合和喜高矿化度、沿岸型的Mastogloia smithii-elliptica组合。这与利用PCA分析钻孔硅藻组合的结果基本相同。2、以BSTC001孔硅藻化石的鉴定结果为主,结合其他代用指标的分析结果,重建了过去8000多年以来博斯腾湖区的古环境和古气候的演化过程,结果显示早中全新世有效湿度较低,而中、晚全新世后有效湿度较高,可具体分为以下六个阶段:8.3～7.8 cal ka BP,湖相沉积物开始出现,硅藻组合以喜冷水、淡水的Fragilaria属的一些种为主,且碳酸盐处于低值,表明气候寒冷湿润;7.8～5.9 cal ka BP,硅藻组合中淡水种的含量逐渐减少,淡水—半咸水种和半咸水—咸水种的含量逐渐增加;暖水种的丰度相对较高;碳酸盐含量、粒度和C/N比值亦处于相对高值;但磁化率值整体相对较低;这些指标的变化均表明气候由冷湿转为暖干;5.9～4.8 cal ka BP,硅藻组合以境遇浮游性淡水种为主,碳酸盐、粒度和烧失量都处于相对低值,而磁化率处于相对高值,表明湖泊水位升高、气候湿润;在5.6 cal ka BP之后冷水种硅藻含量增加,指示湖泊水温降低、湖区气候寒冷;4.8～2.7 cal ka BP,淡水种硅藻的丰度较低,淡水—半咸水种和半咸水—咸水种硅藻的丰度较高,碳酸盐含量也较高,所以整体来看气候以干旱为主。该阶段冷水种和淡水种硅藻群集的丰度频繁波动且变化幅度较大,指示该阶段温度和湿度变化都比较剧烈,例如在4.0 cal ka BP左右就出现以冷水、淡水硅藻丰度增高为代表的冷湿气候波动;2.7～0.12 cal ka BP,淡水种硅藻占绝对优势,冷水种硅藻的含量也较高,同时磁化率处于相对高值,而碳酸盐含量、粒度和C/N比值均较低,这些指标表明气候条件以冷湿为主;0.12 cal ka BP以来,淡水种硅藻含量下降,淡水～半咸水种和半咸水～咸水种硅藻含量上升,表明湖水矿化度升高、湖泊水位下降。但该阶段湖泊水体的变化除了与气候变化相关外,还受到人类活动的影响。3、博斯腾湖BSTC001孔的化石硅藻群集记录了8000多年来七次快速气候变化事件,先后出现于ca.7.9 cal ka BP、ca.6.8 cal ka BP、5.7～4.8 cal ka BP、3.1～2.9cal ka BP、ca.4.0 cal ka BP、2.0～1.6 cal ka BP和0.7～0.4 cal ka BP,分别对应于北半球降温、冰川前进和北大西洋全新世冰筏事件的发生期;并与亚洲内陆西风区其他古气候研究重建的（冷）湿时段相一致。此外,通过多窗谱分析,发现这种快速气候变化事件具有2500 a和500 a的周期,与太阳活动和北大西洋温盐环流的周期非常接近。4、通过区域对比发现,博斯腾湖多指标记录的早中全新世相对干旱,中、晚全新世相对湿润的气候演化模式,与新疆塔里木盆地,乃至整个亚洲内陆西风区的许多研究结果都比较一致,并表现出与亚洲季风区近乎反相的气候演化模式,即在夏季风减弱时西风区的有效湿度趋于增加,反之降低。5、通过对驱动机制的探讨,我们认为博斯腾湖BSTC001孔多指标记录反映的全新世以来亚洲内陆西风区的气候演化过程,在长时间尺度上受到北半球夏季太阳入射量变化的驱动以及北半球温度变化的调控还有ENSO活动的影响,而千百年尺度的快速气候变化事件则可能受到太阳活动和北大西洋温盐环流变化的影响。由北大西洋涛动异常所导致的西风强度和位置的改变可能是这些影响因子实现其驱动作用的途径。另外,冷期时亚洲内陆干旱区较低的地表和水面蒸发量可能对区域相对湿度的增加也有所贡献。更多还原

【Abstract】 As one of the most vitally important principles to predict the future environment what we are depending closely,the understanding of Holocene climate changes has attached extensive attentions of the world.With the increasing wealth of high-resolution and precisely dated climatic proxy records such as oxygen isotopic data from speleothem and peat,the consensus on Holocene climate evolutionary pattern of the Asian Summer Monsoon（ASM）domains has been reached gradually. Comparatively speaking,as to the study of the westerly dominant area of inland Asia （namely,Arid Central Asia or ACA）,high-resolution climatic proxy records are still scare.Furthermore,dating accuracy,timing scales and time resolution of those records are different.All of the inferior status quo of study would inevitably make it difficult to get a coherent knowledge of how climate changed in the ACA during the Holocene.In recent years latest studies have revealed that the climate evolutionary pattern of the ACA might show significant discrepancy with that of the ASM domains. However,either this discrepancy represents a local phenomenon or has a regional linkage with other areas under the same atmospheric circulation,which is still need to be tested and confirmed by more convincing climate records.Moreover,rapid climate changes with periodicity have been extensively explored in the Holocene studies on the ASM domains and the ASM marginal areas,but they have been rarely elaborated in study sites of the ACA.Therefore,to carry out some high-resolution analyses on well dated,highly sensitive proxy records in an selected ideal site would significantly help us to improve our understanding of the Holocene climate evolutionary process of the ACA,to evaluate whether rapid climate changes occurred in the ACA or not and to calculate the periodicity of these rapid climate changes if they did occur.Bosten Lake,the largest inland freshwater lake in China,locates at the middle part of the ACA.Controlled by the westerlies,Bosten Lake is an ideal site to understand the Holocene climate evolutionary process of the ACA.Furthermore,the high sedimentation rate of Bosten Lake during the Holocene has been shown in previous studies,which makes it possible to exact information from lake sediments to examine the pattern and process of rapid climate changes in the ACA.Diatoms,single-celled algae,are extremely abundant in many lake ecosystems. They are made of opaline silica,well preserved in sediments and highly sensitive to environmental changes.Therefore,shifts in diatom abundance and community composition in lake sediments can reflect rapid climate changes of short duration.Up to now,diatom fossil records have been performed for many lakes around the world and thus help to gain valuable information on Holocene palaeoenviromental and palaeoclimatic reconstructions in those lake areas.Judging from above evidence, diatoms are selected as the main proxy recorder of past environmental changes in our study.In order to get knowledge of the composition and distribution of modern diatom assemblages in Bosten Lake,a diatom survey was carried out systematically.A total of 35 diatom samples from surface lake sediment were collected and analysed and at least 400 diatom valves were counted under microscope for each sample.Then,ASM ¹⁴C dating was carried out on twelve dating materials picked out from BSTC001 of 9.52m in length and a 9.25m long parallel core,XBWu46.By the correlation of magnetic susceptibility between these two cores,all ASM ¹⁴C dates were aligned along a unified depth coordinate.It was expected that the carbon reservoir effect was of minor significance as the location of our coring site is near to the estuary of Keidu River where water circulation works efficient and fast.Besides,most of the dating materials are composed of terrestrial plant debris,which confirm the carbon reservoir effect can indeed be neglected.By comparison and analysis,seven ASM ¹⁴C dates from respective depths were screened out to develop the final age-depth model and calibrated into calendar years.Subsequently,sedimentation rates of BSTC001 were calculated by interpolating of these seven calendar ages,except for the uppermost section whose sedimentation rates were determined by the ¹³⁷Cs dating results.On the basis of sedimentation rates,a robust age-depth sequence for BSTC001 was eventually developed.Multiple proxy indicators were analyzed including diatoms（a total of 179 samples,400～600 valves per sample,with time resolution of 45 a）, carbonate content（a total of 456 samples,with time resolution of 18 a）,grain size（a total of 421 samples,with time resolution of 19 a）,magnetic susceptibility（a total of 451 samples,with time resolution of 18 a）,loss-on-ignition（a total of 443 samples, with time resolution of 18 a）and C/N ratio（a total of 263 samples,with time resolution of 30 a）,which can provide help information of palaeoenvironment and palaeoclimate of the lake drainage during the past ca.8.0 cal ka BP.The main results of this study include the followings:1.By Correspondence Analysis（CA）of modern diatom assemblages,it shows that salinity/conductivity was the most influential environmental factor on the components and distribution of diatom assemblages in different lake zones. According to their different tolerances of high conductivity,two diatom assemblages can be divided,which are Fragilaria group preferring lower conductivity/salinity and indifferent to water depth and Mastogloia simtii-elliptica group preferring higher conductivity/salinity and low water depth.This result of CA consists with that of Principal Correspondence Analysis（PCA）.2.Evolution of palaeoclimate and palaeoenvironment in the Bosten Lake drainage during the past ca.8.3 cal ka BP was successfully established mainly on the basis of diatom and other multiple proxy indicators.The results show that the climate was dry before the early middle Holocene and relatively wet during the rest of Holocene.Six evolutionary stages could be segmented as follows:From 8.3 to 7.8 cal ka BP,lacustrine deposits began to present in the sediment core which indicate stable standing water gradually covered the drilling site.A wet and humid climate phase can be inferred by the low carbonate content and sparse diatom assemblages mainly composed of Fragilaria sp.preferring cold and fresh water during this period.From 7.8 to 5.9 cal ka BP,generally warm and dry conditions were shown by the increase of freshwater-brackish and brackish-saline diatom taxa,high abundance of mesoeuthermal diatom taxa and the decrease of freshwater diatoms;relatively high value of carbonate content,grain size and C/N ratio,but low magnetic susceptibility.From 5.9 to 4.8 cal ka BP,lake level increased under humid climate conditions which could be deduced by the high abundance of facultative planktonic freshwater diatom;low percentage of carbonate content,finer grain size and low value of loss-on-ignition,but high value of magnetic susceptibility.After 5.6 cal ka BP,the high abundance of cold-water diatom taxa can be linked with the decrease of water temperature due to cold climate conditions.From 4.8 to 2.7 cal ka BP,a generally dry climate prevailed in the lake drainage,which is indicated by the increase of abundance of freshwater-brackish and brackish-saline diatom taxa as well as high carbonate content.Superimposed on this broad trend of climate changes, some strong oscillations in temperature and humidity are suggested by frequent fluctuations in the abundance of cold-water diatom taxa and freshwater diatom taxa in a large scale.For instance,a pronounced cold-humid at 4.0 cal ka BP can be inferred by the increase of freshwater diatom taxa preferential of cold water conditions.From 2.7 to 0.12 cal ka BP,freshwater diatoms became the dominate taxa of Bosten Lake associated with high abundance of cold-water diatom taxa,low carbonate content,finer grain size,low C/N ratio as well as high magnetic susceptibility,which indicate cold and wet climate conditions in the study area.After 0.12 cal ka BP,the rapid increase of freshwater-backish and brackish-saline diatom taxa would be linked with falling of lake level under arid climate conditions or/and intense human impact.3.Seven rapid climate changes were clearly documented in diatom assemblages from BSTC001 core,namely,ca.7.9 cal ka BP,ca.6.8 cal ka BP,5.7～4.8 cal ka BP,ca.4.0 cal ka BP,3.1～2.9 cal ka BP,2.0～1.6 cal ka BP and 0.7～0.4 cal ka BP. All these climate events broadly correspond to the cooling of Northern Hemisphere,the advance of glaciers and the outbreak of ice-rafting events in North Atlantic during the Holocene,and consist with humid（cold）periods inferred from other records from the ACA.In addition,by MTM-Multitaper Method of Spectrum Analysis,it can be detected that these rapid climate changes own cycles of 2500a and 500a approximating to the periodicity of solar activity and North Atlantic Thermohaline Circulation.4.The climate evolution pattern of dry early Holocene,mild middle Holocene with rapid climate changes and wet late Holocene as shown in multi-proxies of BSTC001 from the Bosten Lake corresponds with the results from other archives of the Tarim Basin,Xinjiang and even wider range of the ACA.The quasi-antiphase pattern of climate evolutionary pattern of the ACA against the ASM domains exhibits that the effective moisture increased in the ACA with the decreased strength of ASM and vice versa.5.The Holocene climate evolutionary process of the ACA reflected by multi-proxies of BSTC001 was primarily driven by the change of summer insolation of Northern Hemisphere.In addition,it was strongly regulated by the temperature changes of Northern Hemisphere and the variation in the frequency/intensity of ENSO activity on longer timescale.As to the rapid climate changes of millennial and centennial scale,solar activity and North Atlantic Thermohaline Circulation should be the main driving force.The variation of the strength of westerly and displacement of its path due to the NAO anomaly may help these factors to perform their functions on the climate evolution pattern of ACA.Additionally,the decrease of land and surface water evaporation during the cold phase might contribute to increase the effective humidity in the ACA.更多还原