Knowledge Resource Center for Ecological Environment in Arid Area
| 青藏高原中部晚渐新世至早中新世植被、气候与环境 | |
| 其他题名 | The vegetation, climate and environment in central Tibetan Plateau during the Late Oligocene to Early Miocene |
| 谢淦 | |
| 出版年 | 2018 |
| 学位类型 | 博士 |
| 导师 | 王宇飞 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 白垩纪以来，印度次大陆从南半球向北一路漂移，穿过赤道，在晚白垩-古新世之交约60至50 Ma和亚欧大陆发生碰撞，逐步俯冲插入至亚欧大陆之下。两大陆板块之间的碰撞导致碰撞区域的地壳收缩、增厚，并抬起隆升形成青藏高原。新生代青藏高原的隆升事件不仅直接塑造了今天亚洲的地形和地貌，而且由隆升带来的生态学效应体现在一方面阻挡了南亚夏季风北上，加强了东亚夏季和冬季风强度。另一方面，隆升的高原阻挡并分隔了西风急流，造成了南北两支分流，从而改变了周边大气环流的模式。这些生态学效应引发的水热组合变化，带来了一系列生物多样性分布的改变，最终形成了今天的生态系统格局。近年来，科学界积累了来自地质学、地球化学以及生物学的越来越多的证据正在逐步揭开青藏高原抬升历程的神秘面纱。不过，目前科学界对青藏高原中部抬升历史的认识仍有很大争议，流行的高原原型说认为青藏高原中部为高原的原型，其广大区域在始新世约40 Ma之前就已经抬升到今天的高程；而另一些科学家则主张高原中部抬升到今天高程发生在中新世约18 Ma之后。另外，目前对高原抬升导致的生态学效应的认识多是基于模型模拟，尚缺乏生物学实证；同时，对亚洲季风从晚始新世显现雏形，到中、晚中新世现代季风系统基本成型，其间的演变过程，由于缺乏气候数据关联，目前仍一无所知。为了解答上述科学问题，本文针对青藏高原中部伦坡拉盆地上渐新统至下中新统丁青组地层中的植物大化石和孢粉作了系统研究，并以植物为气候与环境的代用指标，从定性到定量地重建了当时青藏高原中部的植被、气候和海拔。具体结果和认识如下：1、植被演替：青藏高原中部伦坡拉盆地晚渐新世至早中新世的孢粉组合共有40个孢粉类型，包含：被子植物25科29个花粉型，相对丰度为14.4%，其中乔木占5.6%，以壳斗科、桦木属为主，灌木占2.6%，草本占6.2%；裸子植物3科7个花粉型，相对丰度为64.9%，其中松属占19.2%，云杉属占37%；蕨类植物4科4个孢子型，相对丰度为8.1%。按照生态需求分类，孢粉组合中含有喜暖的类群1个，中温偏暖类群4个，中温类群12个，中温偏凉类群和偏凉类群各2个。该孢粉组合指示当时当地的植被为暖温带针阔混交林。进一步的分析显示孢粉相对丰度在时间序列上的变化可以被划分为三个阶段。第一阶段涵盖27.1至24 Ma，该针阔混交林以松属和壳斗科占优势；第二阶段涵盖23.9至20.0 Ma，此时的森林以松属、桦木属、壳斗科占优势，相较上一阶段针叶树分布范围收缩、阔叶树分布范围扩张；第三阶段涵盖19.9至18.0 Ma，此时的森林以松属、锥属为优势类群，相较第二阶段针叶树扩张、阔叶树收缩。2、气候变化：通过孢粉组合的共存分析，获得青藏高原中部晚渐新世至早中新世的气候参数变化范围如下：年均温为7.7至15.6 °C，最热月均温为24至26.1 °C，最冷月均温为-9至6.2 °C，年降水为660至1050 mm，夏季最大三月降水为280至610 mm，冬季最小三月降水为29至74 mm。该结果反映青藏高原中部晚渐新世至早中新世远比今天温暖、湿润。进一步和欧洲中部以及深海氧同位素反映的全球海洋温度变化进行对比，发现青藏高原中部约27至20 Ma间的年均温变化和欧洲中部以及全球海洋同时期的温度变化大体一致。本文首次在青藏高原中部检测到了两次明显的降温事件，似乎对应了深海氧同位素反映的约24 Ma的Mi-1和约22 Ma的Mi-1a两个降温事件，这可能意味着这一时期青藏高原中部和欧洲中部的气候变化都受到全球气候变化调控。约20至18 Ma间，青藏高原中部地区的年均温呈现降温趋势，迥异于欧洲中部以及海洋。该时期青藏高原中部的降温可能源于青藏高原中部在23.7 Ma至18 Ma间的抬升和亚洲冬季风的加强。同时，青藏高原中部地区的年降水变化和欧洲中部同样在约27至20 Ma相似，从约20 Ma开始变化出现差异。3、海拔估测：基于发现自伦坡拉盆地约24 Ma前的小型挺水植物苹属化石萍（Marsilea cf. quadrifolia）和该化石同层位围岩中的孢粉组合数据分析，本文首次估测了当时青藏高原中部伦坡拉古湖平面的海拔范围在3100至3500 m，指示渐新世之后该古湖平面还有约1000至1500 m的抬升，不支持高原中部在始新世就已经达到今天高程，以及晚渐新世时尚低于1000 m的说法。4、景观重建：结合沉积学和地质学证据，本文图形化再现了约24 Ma时，青藏高原中部伦坡拉古湖周围以松属、桦属、栎属为主的暖温带针阔混交林生态景观。该植被景观迥异于同时期高原以南印度的热带阔叶林植被、和高原以北塔里木盆地的塔克拉玛干沙漠，指示此时青藏高原抬升的生态学效应已经显现。这一时期青藏高原抬升导致的对季风阻挡效应可能导致了青藏高原南坡、高原中部以及高原北坡之间的生态系统和气候格局的分异。5、季风活动：降水组合数据显示，以夏季和冬季三月降水差值为标识的亚洲季风在晚渐新世至早中新世不仅已经存在，而且其强度渐强。频谱分析揭示亚洲季风呈约1.5 Ma的周期变化，似乎对应了同期地球轨道偏心率约0.4 Ma的周期和地轴倾角约1.2 Ma的调制周期变化，这种关联暗示地球轨道驱动力有可能通过某种机制参与了这一时期亚洲季风的演化。地球轨道偏心率和/或地轴倾角的增大，可能导致了高纬度太阳辐射量增加，从而引发冰盖消融、全球升温。而升温时，陆地的温度上升快于海洋，这将导致夏季海、陆温差加大、夏季风增强。本项研究首次量化了青藏高原中部晚渐新世到早中新世的海拔高程和气候变化，填补了亚洲季风从肇始到定格的演化缺环。这些发现无疑拓展和丰富了科学界对青藏高原抬升历史的理解，为其产生的生态学效应提供了重要的生物学实证，有助于科学界完整地理解亚洲气候和生态系统格局的演化。 |
| 英文摘要 | The Indian continent, moving from the South Hemisphere to the North Hemisphere since the Late Cretaceous, collided and under-thrust the Eurasian continents during 60 to 50 Ma. The collision, subduction and thickening between the Indian and Eurasian continents caused a momentous orogeny and finally formed the Tibetan Plateau during the Cenozoic Era. The uplift of the Tibetan Plateau shaped the topography and landform in Asia. The ecological impacts of the Tibetan Plateau uplift changed the pattern of the climate system and related ecosystems in Asia by influencing atmospheric circulation around the plateau, such as blocking and enhancing the South Asian Summer Monsoon, enhancing the East Asian Summer Monsoon and Winter Monsoon, and dividing the Westerly Jet into northern and southern distributaries.Nowadays the uplift history of the Tibetan Plateau is being revealed by more and more evidences from geophysical, geochemical and biological research. However, the uplift process of central Tibetan Plateau is still unclear. Prevailing hypotheses considered that central Tibetan Plateau reached its modern elevation in the Eocene, while others proposed that this event occurred after the Early Miocene. At the same time, the onset and development of Asian Monsoon under the influences of uplifting Tibet are poorly understood. Some evidences indicated that proto Asian Monsoon appeared in the Late Eocene, and the modern monsoon system occurred in the Miocene under the background of Tibetan Plateau uplift. The lack of climate data prevent us to understand the evolution of Asian Monsoon during these time intervals.In the present work, we have for the first time reconstructed the vegetation, climate and elevation of Lunpola Basin (at 4627 m a. s. l. today), central Tibetan Plateau during the Late Oligocene to Early Miocene based on fossil plants and pollen found in the Dingqing Formation. The detailed results are listed below.1. Vegetation successions. The palynoflora included 40 palynomorphs assigned to 20 families, including angiosperms (25 families), gymnosperms (three families) and pteridophytes (four families). The relative abundance (RA) of angiosperm pollen in this palynoflora was 14.4%, of which trees composed 5.6% (Betula 0.8%, Castanopsis 1.2% and Fagaceae 1.3%), shrubs 2.6%, and herbs 6.2%; gymnosperms 64.9% (Pinus 19.2%, and Picea 37%), and pteridophytes 8.1%. The palynomorph taxa were grouped by their temperature requirements as one megathermic element, four mega-mesothermic elements, 12 mesothermic elements, two meso-microthermic elements, and two microthermic elements. The pollen assemblage suggested a mixed deciduous broad-leaved and coniferous forest around Lunpola Lake. Cluster analysis on relative abundance of pollen assemblage reveals three periods of vegetation succession in the central Tibetan Plateau. The mixed deciduous broad-leaved and coniferous forest was dominated by Pinus and Fagaceae during the first period, 27.1 to 24 Ma. During 23.9 to 20.0 Ma, the number of broad-leaved trees increased and conifers decreased, Pinus, Betula and Fagaceae dominated the second period forest. The third period covered 19.9 to 18.0 Ma. Pinus and Castanopsis dominated the forest, with a decreasing number of broad-leaved trees and increasing conifers.2. Climate changes. Using the Co-existence Approach analysis on the pollen assemblage, we reconstructed the climatic parameters in the Lunpola Basin during the Late Oligocene to Early Miocene as follows: the mean annual temperature ranged from 7.7 to 15.6 °C, the mean warmest monthly temperature was 24 to 26.1 °C, the mean coldest monthly temperature was -9 to 6.2 °C, the mean annual precipitation was 660 to 1050 mm, the mean three consecutive highest monthly precipitation was 280 to 610 mm, the mean three consecutive lowest monthly precipitation was 29 to 74 mm. The climatic data indicate a warm and moist climate. The temperature curve showed similar fluctuations in central Tibetan Plateau, central Europe and deep-sea with warming and cooling cycles during 27.1 to 20 Ma. Two cold events were firstly detected in central Tibetan Plateau, corresponding to the Mi-1 and Mi-1a glaciation reflected by heavy oxygen isotope in deep sea. These findings implied that the temperature in central Tibetan Plateau and central Europe was strongly constrained by global climate. During ca. 20 to 18 Ma, unlike the warming in central Europe and deep-sea, cooling occurred in central Tibetan Plateau. The stronger Asian Winter Monsoon and uplift of central Tibetan Plateau probably contributed together to this cooling in central Tibetan Plateau. On the other hand, the precipitation curves in central Tibetan Plateau and central Europe display similar fluctuations during ca. 27.1 to 20 Ma, but opposite during 20 to 18 Ma.3. Elevation. We reconstructed the elevation of Lunpola Lake, central Tibetan Plateau as 3100 to 3500 m a. s. l. at ~24 Ma, and the nearby landscape with a mixed deciduous broad-leaved and coniferous forest by crosschecking the elevation and environmental data extracted from a small aquatic fossil fern (Marsilea) found in the Lunpola Basin and the pollen assemblage in the same fossil bed at 4627 m a. s. l. today. These data do not support those hypotheses that central Tibetan Plateau had reached its current elevation during the Eocene and was lower than 1000 m in the Late Oligocene.4. Landscape. By combining fossil plant, pollen, animal and sedimentary evidences, we reconstructed the ecosystem with a mixed deciduous broad-leaved and coniferous forest near the paleolake. A comparison of the landscapes on the southern and northern slopes of the Tibetan Plateau during the same time interval, demonstrates that differences in the ecosystem had already emerged by the time the central Tibetan Plateau uplifted to 3500 m. The pattern divergence of these ecosystems was probably triggered by a blocking of the Asian Monsoon by the Tibetan Plateau.5. Asian Monsoon. We detect a crescendo Asian Monsoon marked with upsurge precipitation difference between summer and winter. Spectral analysis reveals a 1.5 Ma cycle available in the Asian Monsoon during that time, coupling with the ca. 1.2 modulation cycles of the Earth orbital obliquity and 0.4 Ma cycle of eccentricity. These results might indicate that orbital forcing had somehow contributed to the evolution of Asian Monsoon. Higher obliquity and eccentricity probably enhance the insolation in high latitude regions, leading ice sheet melt and global warming. The land warming is faster than that of sea due to their different specific heat capacities. So increasing land-sea temperature difference might induce a stronger monsoon.In the present dissertation we have for the first time quantitatively depicted climate changes in central Tibetan Plateau, fill the gap of Asian Monsoon development during the Late Oligocene to Early Miocene. These findings not only expand and enrich our knowledge of Tibetan Plateau uplift history, but also provide solid biological evidence for understanding ecological impacts of the uplift, which in turn shed light on the evolution of the climate system and ecosystem pattern in Asia. |
| 中文关键词 | 青藏高原中部 ; 古植被 ; 古气候 ; 古环境 ; 晚渐新世至早中新世 |
| 英文关键词 | Central Tibetan Plateau Vegetation Climate Environment Late Oligocene to Early Miocene |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 植物学 |
| 来源机构 | 中国科学院植物研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/288219 |
| 推荐引用方式 GB/T 7714 | 谢淦. 青藏高原中部晚渐新世至早中新世植被、气候与环境[D]. 中国科学院大学,2018. |
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