地理学专场
青藏高原研究
2014年04月02日

  一、引言

  进入21世纪,国际地质学界正在经历着一场新的变革,即由过去单一的学科研究发展向多学科交叉的“地球系统科学”发展,而且这样的融合并不仅仅局限于地质学的各分支学科,而是包括了与大气科学、地理学、海洋学、生物学等众多学科的交叉发展。事实上,发生在地球上的许多重大地质或气候事件,特别是新生代以来青藏高原的隆升,更是与岩石圈、水圈、大气圈、生物圈的圈层相互作用密切相关。

  青藏高原以其最复杂的形成机制、最高的海拔、最大的面积、最重要的环境效应、最脆弱的生态环境成为全球地学关注的焦点,也是开展地球系统科学研究最理想的实验室。新生代以来印度板块的北向俯冲导致了新特提斯洋的消亡以及印度板块与欧亚板块的碰撞。这一岩石圈的构造变动进一步影响了北半球乃至全球尺度的大气环流,高原浅表层的剥蚀风化、地貌分异、水系调整、动植物演替也影响到矿产资源的形成演化。因此,青藏高原是开展新生代岩石圈-水圈-大气圈-生物圈各圈层相互作用的关键地区。

  在基础理论层面上,青藏高原研究是我国地球系统科学研究取得重大突破,占据国际学术制高点的关键领域,有望对国际地球系统科学研究起到引领作用。在国家需求层面上,针对青藏高原隆升与资源环境关系的研究有助于理解人类面临的气候变化问题中的诸多不确定性,并对青藏地区资源合理利用与社会可持续发展发挥重要的科学支撑作用。

  二、本学科近两年最新研究进展

  近年来,青藏高原研究在大陆碰撞、高原隆起与变形、资源形成与环境效应方面取得了一系列进展,主要概括为:

  印度与欧亚大陆的碰撞是青藏高原最基础、最重大的科学问题。在特提斯喜马拉雅带的北部亚带萨嘎地区识别出一期周缘前陆盆地沉积,结合蛇绿岩仰冲时间,将板块碰撞时间限制在沉积环境发生明显改变的白垩纪/第三纪界限(65Ma)。

  青藏高原岩石圈构造、性质及热状态认识是探讨高原形成演化动力学的深部观测约束证据。通过对青藏高原中北部和东部40Ma以来的新生代钾质岩浆岩研究,认为确实存在岩石圈减薄,在青藏高原(南部:~15km;北部:20-50km)发现有低速高导层。这些低速高导层对理解青藏高原的隆起模式具有非常重要的意义。

  在青藏高原成矿理论模式研究上,近年来伴随着增生造山和地壳生长而发生的流体活动与成矿作用得到了详细研究。聚焦于青藏高原碰撞造山带,目前发现的大规模成矿作用主要形成于后碰撞时期,发育在地壳伸展环境。

  青藏高原的古高度变化是讨论高原隆起过程及其气候、环境效应的关键。近年来,利用动植物化石、同位素高度计等对高原不同时期的高度进行了广泛的研究,尽管目前仍存在较大争论,但无疑为厘清其变化提供了明确的思路与方向。

  青藏高原的隆升过程存在明显的区域分异,高原南部的喜马拉雅山在碰撞初期(50Ma)就已隆起,东南部外围的水系与高山变化反映该地区的隆起可能不晚于22Ma,而北部则在5-6Ma时仍表现出明显的抬升。

  以青藏高原为主体的我国西部新生代的剥蚀风化强度和过程不仅与全球变化和亚洲季风气候演化密切相关,也对全球气候产生重要的影响,而后者反过来又作用于亚洲季风。青藏高原的隆升可能是其中多个链接的驱动力。

  青藏高原的隆升和副特体斯海共同影响了东亚季风的形成,且副特体斯海的作用甚至比青藏高原的隆升更为显著。除了上述的构造因素,新生代的全球气候变冷同样会影响到环境空间格局的改变。

  青藏高原的过去气候环境变化研究得以不断深化。以树轮、冰芯和湖芯为载体的过去环境变化,在重建的时间精度和定量程度方面取得了较大进展。在短时间尺度上,为恢复青藏高原地区的气候环境变化提供了高分辨率的连续记录,在较长时间尺度的环境重建研究中,探讨了季风西风两大环流系统的发展及其相互作用。

  青藏高原地区大气降水具有季风型、过渡型和西风型的三种模态,在季风减弱和西风增强的趋势下,高原不同圈层间的水体相态转化过程加剧。冰川变化存在明显的空间分异,表现为:高原东南部和南部冰川退缩程度最强,中部冰川退缩程度稍弱,喀喇昆仑山地区冰川略有前进。高原的湖泊整体表现为扩张的趋势,但是尽管冰川退缩与湖泊扩张在时间上是同步的,其相互关系仍需要进一步的研究。

  青藏高原的气候变暖、生长季延长有利于植被生产力的提高,温暖化效应使落叶灌木和草本的高度和盖度增加,垫状植物、苔藓和地衣的盖度减小,物种多样性和均匀度减少。高原森林和农田生态系统为碳汇,高寒草甸和高寒草原生态系统为弱碳汇。但降水格局的潜在变化也会不可避免地对生态系统生产力和碳交换量产生影响。人类不合理的土地利用方式,尤其是过度放牧等是导致高原目前草地退化等重要土地覆被变化的主要原因。人类活动排放的重金属和有机污染物可以通过大气环流跨境乃至全球传输,在青藏高原地区发现了印迹。

  三、本学科国内外研究进展比较

  通过对全球青藏高原研究的论文发表量和内容统计分析发现,2009-2012年全球青藏高原论文发表量为2350篇/年,大大高于2003-2008年和1982-2002年的年均发表量(1068篇/年和275篇/年),体现了青藏高原研究在国际上越来越得到关注的事实。

  在国际青藏高原研究的发文国家中,从发文总量和总被引频次看,中国均位居第一,继续显示了中国作为青藏高原研究大国的地位。从篇均引用次数看,中国的排名已经明显上升,代表了青藏高原研究整体水平得到不断提升。从高被引论文的发表情况分析,在58篇高被引论文中,有60%都有中国参与,其中中国作为第一发文国家的占43%,显示了中国已经初具引领国际青藏高原研究的态势。从研究的热点和主题看,青藏高原的形成演化模式与气候环境变化是目前两大研究热点。

  四、本学科发展趋势和展望

  根据对国内外青藏高原研究的基础、成果和迫切需要解决的理论与实际应用问题分析,青藏高原研究目前的科学前沿与社会需求主要集中在以下领域:

  1.印度与欧亚大陆碰撞时限的时空特征。大陆初始碰撞及封闭过程是青藏高原地区海陆格局大转换的起点,是正确认识高原古高程变化的前提,是研究大陆岩石圈变形过程及机制的先决条件,是正确开展高原气候环境影响研究的首要因素,是评价和寻找大陆俯冲成矿及碰撞成矿的基础。在印度与欧亚大陆长达2500km的碰撞带上,要回答碰撞是否存在均一性,不同地区碰撞的时间和特点是什么等问题。

  2.高原深部结构与隆升机制的关系。青藏高原南北部的岩石圈厚度与结构具有明显差异,但地表均表现为平坦的高原。青藏高原的岩石圈是世界上最年轻而成分富集的高温岩石圈,这也是青藏高原变形、隆升和成矿的基础,也与高原边界扩展隆升机制密切相关。

  3.陆内俯冲与大陆变形的理论。从板块汇聚的形式来看,陆陆碰撞带是解决板块构造“如何登陆”的理想场所。应加强不同类型地貌区深部岩石圈结构对比,如羌塘-可可西里岩石圈深部结构探测,青藏高原-伊朗高原深部过程对比,以及青藏高原-印缅碰撞侧向造山带深部过程对比等。

  4.巨型岛弧与造山后斑岩Cu-Mo矿的联系。通过冈底斯弧岩浆-构造-成矿作用的研究,查明具有世界特色的俯冲碰撞造山带叠加成矿的特色。建立造山带中与大洋俯冲相联系的斑岩Cu-Mo矿床的成矿理论,完善后碰撞斑岩型矿床的成矿模型,并指导这类矿床的找矿勘探。

  5.青藏高原何时达到其隆升的最大高度。青藏高原的隆升高度是评价其环境效应及其剥蚀风化的关键,更是联系深部岩石圈地球动力学与浅表层演化的纽带,只有准确重建古高度才能正确评价高原隆升与扩展过程对区域与全球气候的影响。

  6.高原隆升的远程变形效应。印度与亚洲板块的碰撞效应,不仅使靠近板块碰撞边界的喜马拉雅山系剧烈抬升,成为全球海拔最高的造山带,同样导致了其周缘造山带的复活,特别是高原东北缘在新生代的快速隆升。

  7.高原隆升对亚洲宏观地貌格局与水系演化的影响。中国大陆的地貌格局在高原隆升前后有重大差异,自印度和亚洲板块发生陆-陆碰撞以来,中国大陆原来东高西低的地形逐渐演变为向东倾斜,而源自青藏高原的长江、黄河的形成与演化历史是探讨这一巨型地貌演变的关键。

  8.高原隆升对风化剥蚀速率的影响。新生代青藏高原的隆升和扩展是地球上最为显著的构造运动,隆起后的高原其浅表层经历了剥蚀风化。因此,定量估算高原周边新生代剥蚀量以及剥蚀速率的变化对于正确评价高原的隆升幅度与历史是十分重要的。

  9.高原隆升对亚洲腹地干旱化和季风演化的影响。中国东部及西南地区为季风区,而西北内陆盆地则为西风环流控制下的干旱区,这显然不同于高原隆起前行星风系控制下的纬向环流。解释这个变化的关键在于了解高原隆升如何影响亚洲腹地干旱化,以及亚洲季风演化与高原隆升和全球变化存在怎样的动力学关联。

  10.高原地表各个圈层对全球气候变化的响应。冰冻圈是受气候变化影响最为直接与显著的圈层,冰冻圈的变化改变了地表下垫面状况和水热交换条件,地表温度与水分的变化又进一步影响了区域的生态环境,通过大气环流影响到周边地区的气候与环境。

  11.青藏高原生态系统对气候变暖的响应过程与机理。青藏高原是我国乃至全球气候变暖最为强烈的地区之一,生态系统对气候变暖的响应是否也更为强烈、更为敏感;反过来生态系统的变化又进一步影响气候。研究全球变化条件下青藏高原生态系统变化,将有助于进一步理解青藏高原-东亚区域气候系统的相互影响,也为国家实施大型生态工程、构建青藏高原生态安全屏障提供理论基础。

  12.全球变暖和人类活动对高原生态安全屏障的影响。青藏高原是土地利用/土地覆被类型多样的地区,也是国家生态安全屏障建设的重要地区。青藏高原的草地退化十分严重,气候环境变率的增加导致了自然灾害的频发。了解人类土地利用活动与气候变暖对地表景观和灾害发生的影响,将为青藏高原的环境保护提供科学依据和对策建议。
 

  Tibetan Plateau Research
 

The Tibetan Plateau is a focus and an ideal natural laboratory of earth system sciences researches in the world, due to its complex formation mechanism, the highest elevation, the largest area, the most important environmental effect and the most fragile ecological system. It is a key region for multi-spheres interaction researches, including lithosphere, hydrosphere, and atmosphere.

The basic theory research of the Tibetan Plateau is a platform for China to obtain research breakthroughs and occupy international academic dominant height in earth system science, and further more become a leadership. For national strategy, from the researches on the relations between the Tibetan Plateau uplift and resource and environment, it helps to understand many uncertainties faced by human beings in climatic changes, and play an important role in supporting resources rational utilization and sustainable development of society.

Recently, some academic developments are achieved from the following points.

The collision of India and Eurasia plates is a basic and important scientific question for the Tibetan Plateau research. From the sediments in the foreland basin near Saga of the Tethys-Himalaya, and combined with the evidence of the ophiolite obduction time, the collision time between the two plates is defined as 65 Ma, when the sedimentary environment performed apparent changes.

Lithosphere structure, properties and thermal state of the Tibetan Plateau provide observational evidence in deep processes to explore the dynamics of the formation and evolution of the Plateau. Studies on Cenozoic potassic magmatic rock in the north, central and eastern Tibetan Plateau since 40 Ma confirmed the existence of lithospheric thinning. In addition, the discoveries of low-speed and high-conductivity layer in the Tibetan Plateau (southern part: ~ 15 km; northern part: 20~50 km) has a very important significance for understanding the uplift model of the Plateau.

In recent years, the theoretical models of mineralization in the Tibet areas have been achieved. Fluid activity and mineralization resulting from orogenic activities and crustal growth have been studied in detail. Focusing the orogeny belts on the Tibetan Plateau, large-scale mineralization has been found mainly to be formed in the post collision period and developed in crustal extension conditions.

The paleo-elevation of the Plateau is a key point for discussing the uplift process and its climatic and environmental effects. Along with debate, the Plateau elevations in different periods are estimated by using animal and plant fossils and isotope altimeter, providing a clear route and direction for understanding its changes.

There are obvious regional differences of the uplift processes of the Tibetan Plateau. The Himalayas in the southern part uplifted in the early collision period (50 Ma). The uplift of the southeastern part was probably not later than 22 Ma, which was revealed by the changes of the surrounding drainages and mountains. The northern part possessed significant uplift around 5~6 Ma.

The intensity and process of Cenozoic weathering and erosion in Western China (mainly on the Tibetan Plateau) is not only closely related to global change and Asian monsoon evolution, but also deeply influence the global climate. The latter, in turn, affects the Asian monsoon. Thus, the uplift of the Tibetan Plateau is probably the driving factor of many processes.

Both the uplift of the Tibetan Plateau and the Paratethys Ocean have affected the formation of the East Asia monsoon, and the influence derived from the latter should be more intensive than the former. Beside these tectonic factors, the Cenozoic global cooling had also influenced the change of environmental spatial pattern.

Researches of the past environmental changes on the Tibetan Plateau have been deepened continuously. Significant progresses have been made in the temporal resolution and quantitative study when reconstructing the environmental change history using the proxies of tree-ring, ice core and lacustrine core. This provides continuous high resolution records for the reconstruction of environmental change in Tibetan Plateau in short time scales, and is also significant in discussing the change of monsoon and the westerlies systems and their interactions in long time scales.

There are three patterns of precipitation in the Tibetan Plateau, which are monsoonal, transitional and westerlies patterns. Water phase transformation among different spheres of the plateau is being intensified, along the tendency of the weakening monsoon and strengthening westerlies. The changes of glaciers are showing obvious spatial differentiation, e.g. the intensive retreating in the southeastern and southern parts, less retreating in the central part, and stable or light advancing in the Karakoram region. Lakes on the Plateau have generally expanded, however, despite the glacier retreat and lake expansion are temporally synchronized, their relationship still needs to be clarified in further study.

Climatic warming on the Tibetan Plateau is propitious to the extension of growing season and increasing of vegetation productivity. This also increases the height and coverage of deciduous shrubs and herbs, decreases the coverage of cushion plants, mosses and lichens, and reduces the biodiversity and species evenness. Forest and farmland ecosystems on the Tibetan Plateau are carbon sinks, and alpine meadow and alpine grassland ecosystems are weaker carbon sinks. However, the potential change of precipitation pattern will definitely influence the productivity and carbon exchange of the ecosystem.

Irrational land use, especially overgrazing, has been the main reason for the major vegetation cover changes on the Plateau, such as grassland degradation. Heavy metals and organic pollutants emitted by human activities could be transported cross-border even globally through the atmospheric circulation. This has disturbed the environmental background values in the polar areas, and has also been found in the Tibetan Plateau.

According to the amount and subject analyses of the publications related to the Tibetan Plateau research, China is undoubtedly a big country on the Tibetan Plateau studies, because it possesses the first position in the total publications’ amount and their total citations. According to the numbers of citations per paper and highly-cited papers, the status of China has significantly risen, which means a continuous improvement of the Tibetan Plateau research. The formation and evolution model and environmental changes of the Tibetan Plateau are hot issues according to the research focuses and themes.