【作者】 张科；

【导师】 赵艳；

【作者基本信息】 兰州大学 ， 自然地理学， 2010， 博士

【摘要】 过去2000年气候变化研究是认识气候变化的自然规律和机制,揭示人类活动的影响程度及预测未来气候变化趋势的基础,从不同的气候敏感区域获取精确年代控制的高分辨率且环境指示意义明确的自然记录已成为2000年研究的重要手段之一。目前中国的2000年高分辨率的气候变化研究主要局限在树轮、冰芯、石笋以及历史文献记录等方面且取得了很大的进展,但由于冰芯及树轮的分布受到明显的地域限制(主要在高海拔地区),历史文献记录则主要集中在中国东部地区,这对全面揭示和理解我国过去2000年来气候变化的区域特征、变化模式、驱动反馈机制还远远不够。青藏高原东部和黄土高原西部地区位于季风边缘区,生态环境极其脆弱,受季风和西风的共同影响,这些地区对气候变化反映敏感,是研究过去2000年环境变化的理想场所。但限于研究记录的分辨率和代用指标的敏感性,目前在该区域关于过去2000年的气候变化和人类活动研究还相对匮乏。具有年纹层的湖泊沉积能够提供精确定年及高分辨率多指标的综合分析,与树轮、冰芯及石笋记录在揭示过去2000年环境变化方面都起着关键的作用。我们选取具有纹层沉积的苏干湖(青藏高原东北部柴达木盆地)和关山天池(黄土高原西部六盘山地区)为研究对象。柴达木盆地的苏干湖流域历史时期受人类干扰较小,气候是区域环境变化的主控因子,我们通过苏干湖高分辨率的孢粉分析(分辨率10年)重建该区域过去2700年的植被和气候变化,并通过与其它多种代用指标的综合分析及区域对比,系统认识了柴达木盆地过去2700年气候的干湿波动规律及其变化机制；而黄土高原西部地区关山天池周围历史时期人类活动较强,区域环境变化受气候和人类活动共同影响,通过对天池沉积物中的孢粉分析(分辨率20年),结合炭屑、气孔器及盘星藻等指标重建过去3200年来植被变化历史,并通过与区域气候指标及历史文献记录等的对比揭示研究区过去气候变化和人类活动对当地环境的影响。获得以下初步结论：(一)苏干湖：(1)苏干湖湖泊表层花粉和盆地内表土花粉分析表明,湖泊沉积物花粉来源于整个苏干湖盆地,其花粉组合可以较好的代表区域植被状况。蒿属/藜科花粉的比值(A/C)能很好地区分荒漠植被和草原植被,可以用作该区域古环境重建中有效湿度指标。(2)化石孢粉分析表明研究区过去2700年来以荒漠植被为主,主要有藜科、蒿属、麻黄、禾本科及菊科等草本植物。A/C比值显示出过去2700年湿度波动变化较大,波动分为以下几个主要阶段：700BC-300AD气候干旱且相对稳定；300-1200 AD之间气候相对湿润；1100-1200 AD之间A/C比值由0.9迅速升高到3.9,代表一种快速的气候变化过程；1200AD后,气候剧烈波动,其中在1250-1400AD和1700-1800AD两个阶段较高的A/C值表明当时气候湿润；1800AD之后气候趋于干旱。(3)多指标对比分析揭示出苏干湖孢粉记录重建结果与其它指标(碳酸盐百分含量、碳酸盐氧同位素、摇蚊、介形虫氧同位素等)的重建结果表现出相反的湿度变化模式。即当孢粉重建的气候“干旱”时,其它指标反映出却是一种“湿”的信号。这种差异可能分别反映的是区域有效湿度与湖泊系统内部水文沉积环境的变化。温度升高引起周围高山冰雪融水增多,最终导致入湖水量增多而引起湖泊水文性质及沉积环境环境发生变化(水位升高、盐度降低),此时碳酸盐氧同位素、摇蚊等指标均指示出较“湿”的特征；而整个盆地内由于温度的升高而导致蒸发增强,有效湿度降低,植被反映出较“干旱”的气候特征。这一发现在干旱半干旱地区以冰川融水和山地降水补给为主的湖泊古环境重建中具有很重要的现实意义和参考价值。(4)通过区域气候对比分析揭示出苏干湖盆地与周围高海拔地区存在相反的气候变化模式：即盆地内干—湿的气候,对应于周围高海拔地区的湿—干气候。区域地形差异引起的上升和下沉气流可能是导致这种现象的主要原因。当季风增强时,高海拔山地降水增多湿度增加,而低海拔盆地盛行的下沉气流却导致盆地内气候干旱；当夏季风减弱时,这种上升-下沉气流机制变弱,盆地内气候相对湿润。(5)苏干湖多指标分析揭示出研究区1200 AD左右存在着气候突变事件,气候由之前较长时间的相对稳定状态转化为之后的剧烈变化。通过对比分析发现全球范围内在这一时期普遍存在类似气候转型事件,但引起此突变事件的原因和机制还有待于进一步的研究。(二)天池：(1)高分辨率孢粉记录(结合针叶树气孔器分析)揭示出天池附近过去3200年植被演化过程：由最初3200-2200cal yr BP之间的针阔混交林(桦、栎、冷杉)过渡到2200-1100cal yr BP之间的森林草原(桦、栎、蒿),最后1100cal yr BP以来以草原植被类型(蒿、禾本科、莎草科)为主。(2)天池多指标分析结合区域气候及历史文献记录对比研究发现：1100cal yr BP为研究区植被演化的重要转折点。1100cal yr BP之前气候是影响植被变化的主要决定因素,此阶段逐渐减少的乔木花粉与季风衰退具有很好的对应关系,表明植被对季风气候变化的敏感性；1100cal yr BP之后人类活动成为当地植被变化的主导因素,人类活动孢粉指示体、炭屑及盘星藻的增多均表明人类活动在此阶段在天池附近增强。(3)根据孢粉资料重建的森林变化过程,与历史文献记录的人类活动导致的森林砍伐具有很好的一致性。其中孢粉记录揭示的1100-750cal yr BP和350-0cal yr BP两个森林退缩阶段,分别对应于唐朝晚期、宋朝和明清三个时期,人口的急剧膨胀,农业的快速发展及局部的战争冲突是引起当地森林衰退的主要原因；700-350cal yr BP之间天池附近森林植被略有恢复,对应于元朝及明朝早期人口的减少和农业的废弃。本研究为黄土高原西部地区首个利用高分辨率湖泊沉积物记录重建研究区过去植被的变化,重建结果与历史文献记录互为验证和补充,为充分了解历史时期黄土高原森林变化动态过程和机制及人类活动对植被的影响提供了很好的证据支持。(三)苏干湖和天池晚全新世环境对比苏干湖重建结果主要反映自然背景下晚全新世以来的气候和环境变化,其中季风、西风和区域地形对当地气候的调节起着重要的作用。天池记录主要揭示出晚全新世以来气候变化和人类活动共同影响下当地的植被和环境变化。我们的研究结果表明两个研究区在晚全新世截然不同的环境变化过程,揭示出环境变化的区域性特点。更多还原

【Abstract】 Knowledge of climate change during the last two millennium can both improve our understanding of natural climate variability and help address the question of whether modern climate change is unprecedented in a long-term context, which also be of great significance in predicating future climate change. Well-dated, high-resolution records from different key regions in the world are crucial to reconstruct the climate change during the last two thousand years. A great number of fine-resolution paleoclimate studies from China using tree rings, ice cores, speleothem as well as historical documents have revealed significant climate variability over the last two millenniums. However, most of these high quality proxy climate records (such as tree rings and ice cores) have been collected at high elevations, which may show different climate respond to large-scale climate forcing compared to low elevations due to the topographic effects; moreover, historical document based reconstructions mainly came from East China and few studies are related to West China. Therefore, much more work is still needed in order to identify and understand the main patterns and modes of climate variability, their teleconection as well as driven and feedback mechanism in the past 2000 years. The northeastern Tibetan Plateau and western Loess Plateau are both located at the monsoonal boundary areas, so they are very sensitive to climate and environment changes. Unfortunately, high-resolution and well-dated climate records covering the last 2000 years in these two regions are still scarce.Annually laminated sediments are the ideal deposits for fine-resolution and multi-proxy studies, so they play a key role in last 2000 years environment reconstruction together with tree ring, ice core and speleothem. We selected the varved Sugan Lake (Qadam basin) and Tianchi Lake (West Loess Plateau) to reveal the environmental changes during the last 2000 years. The Sugan lake drainage is located at the remote areas with little human disturbance during the historical time, so the climate variation might be the main controller of the environment change. We conducted a 2700-year high resolution (ca.10-yr) pollen record extracted from Sugan Lake in order to reconstruct the regional vegetation change and climate variability; Moreover, multi-proxy analysis method were used together with the comparison with regional climate records to understand the wet-dry fluctuation mechanism in the last 2700 years in the Qadam Basin. The vegetation pattern on the Loess Plateau during the late Holocene, when vegetation was probably greatly affected by human activities, remains poorly understand. We present a high-resolution (ca.20-yr) pollen record together with charcoal, conifer stomata and Pediastrum data from Tianchi Lake in the western Loess Plateau, in order to reconstruct the paleovegetation change in the study areas and to investigate the human impacts on forest change by comparing with other regional paleoclimatic records from speleothems, archaeological records of human habitation and historical documents. We draw the following conclusions:1. Sugan Lake(1) Both the surface samples from lake sediment and the Sugan Basin suggested that pollen in the lake are mainly from the whole Sugan Basin, the pollen assemblages from the lake sediment could well represent the regional vegetation. The desert and steppe vegetation type could be distinguished by different A/C (Artemisia/Chenopodiaceae) ratio, and A/C ratio could be used as an effective moisture index in the paleclimate reconstruction in our study region.(2) Pollen data shows that the Sugan Basin was constantly covered by open desert-steppe vegetation dominated by Chenopodiaceae, Artemisia, Poaceae and Ephedra. However, large variations in Artemisia/Chenopodiaceae (A/C) ratios suggest regional moisture fluctuations over the last 2700 years, including a dry and relatively stable climate prior to 300 AD, relatively wet climate from 300-1200 AD with variability during 1100-1200 AD, and unstable climate since 1200 AD with relatively moister climate during 1250-1400 AD and 1700-1800 AD.(3) The pollen-based moisture change shows a generally opposite phase relationship with that of other proxies (varve thickness, low-salinity chironomid taxa percentage, oxygen isotope of precipitated carbonate) from Sugan Lake. Our hypothesis is that fresh water input from ice melting on the surrounding mountains might be responsible for low values of carbonate,δ18O and salinity, while low effective moisture inferred from A/C ratio in the Basin is probably due to the high evaporation. As a result, carbonate, chironomids, and isotopes in the lake convey a "moister" climate signal and contradict moisture patterns in the basin inferred from pollen A/C ratio, which reflects regional environment. This finding has great significance in paleolinimology studies in arid and semi-arid regions, where the melt water from mountain glacier is the main supply of the lakes.(4) The comparisons of pollen data and other regional paleoclimatic proxies suggest the contrast of moisture change in the Basin and surrounding mountains: dry-wet climate in the basin corresponded to the wet-dry climate in the high elevation, respectively. The regional topography may be important in controlling this regional moisture patterns as mediated by rising and subsiding air masses at regional scale.(5) Pollen A/C ratio shows a big shift at ca.1200 AD, from stablely moist to drier and variable conditions. This climate shift has the nature of widespread global occurrence; however, more evidence for this transition is needed.2. Tianchi Lake(1) The pollen record from the Tianchi Lake indicates that the vegetation changes from mixed deciduous-coniferous forest dominated by Betula, Quercus, Abies and Pinus at 3200-2200 cal yr BP, to forest steppe co-dominated by trees (Betula, Quercus) and herbs (mostly Artemisia) at 2200-1100 cal yr BP and to steppe-like vegetation in the present.(2) Distinct vegetation type shift occurred at ca.1100 cal yr BP, accompanied by the increase of anthropogenic indicators (cereal-type pollen, Humulus-type pollen and Pediastrum). We propose that a critical threshold of the forest changes must have been crossed in this region at ca.1100 cal yr BP. Climate is probably the main controlling factor on the vegetation before ca.1100 cal yr BP, while the vegetation is mainly affected by the anthropogenic activities since then. The enhanced human activities are also indicated by a generally increasing trend of the micro-charcoal abundance during the last 1000 years.(3) Our results reveal several distinct deforestation phases which show good correlation with historical human activities. The first significant deforestation occurred at 1100-750 cal yr BP, correlating with the late Tang-Song dynasty, a period with dense population, flourishing agriculture as well as plenty of border conflict. Forest expansion during 700-350 cal yr BP corresponds to the Yuan and the early Ming dynasty, with sharply decreasing population and many abandoned agriculture land. The final substantial forest clearance occurred at ca.350 cal yr BP. Vegetation history inferred from pollen record generally agrees with the deforestation records based on the historical documents in the Loess Plateau. 3. Comparsions between the two lakesOur study suggests that the environment change in the Sugan Lake drainage during the late Holocene is mainly caused by the natural variability, and the SE Asian Monsoon, Westerlies and regional topography paly a key role; while the environment change around the Tianchi Lake during the late Holocene is resulted from the combintion of climate change and human activities. Our results reveal two different environmental change patterns during the late Holocen in Western China, which indicates the temporal and spatial difference of the environmental change.更多还原