【作者】 韩文霞；

【导师】 方小敏；

【作者基本信息】 兰州大学 ， 自然地理， 2008， 博士

【摘要】 新生代以来地球上发生了一系列的重大地质事件,导致全球气候和环境相应发生了一系列巨大改变:全球变冷,亚洲内陆干旱化,青藏高原隆升、亚洲季风形成演化等。其中,全球干旱化是对人类生存环境影响最为深刻和最为广泛的一个重大科学问题,其发生和发展过程倍受国际社会的关注。揭示亚洲内陆干旱化的起始时间、演化过程和发展规律,并探讨其原因和预测其发展趋势,是目前急需解决的问题。目前解决这个问题的关键在于获取目前严重缺乏的亚洲内陆长序列气候变化记录。柴达木盆地作为高原北部的一个巨型山间盆地,发育了连续、完整的巨厚新生代沉积。本文主要基于在柴达木盆地中央北部马海一带发现的一个约5000米的连续地层剖面和靠盆地中央的一个约700米的鸭湖辅助剖面,展开对上述问题的初步探讨。通过这些剖面的系统采样和色度、氯离子含量、碳酸钙含量、元素分析和总有机碳以及有机生物标志化合物分析,率先获取了亚洲内陆约54Ma以来的几乎完整的新生代气候变化的多指标连续记录,结合谱分析技术和沉积相演化,初步探讨了新生代柴达木盆地的气候演化特征及其与亚洲内陆干旱化、青藏高原隆升、特提斯海退却和全球变冷的可能联系,得出了以下几点初步结论:1)柴达木新生代颜色红度记录与全球温度记录有很好的可比性,是灵敏的温度代用指标。2)柴达木气候代用指标的变化,大致可以44.6Ma和9.7Ma为界分成三大阶段:53.4～44.6Ma期间,剖面的碳酸钙、红度值和氯离子的含量都很高,红度还达到全剖面最大,且各个指标的波动都非常大,有机质含量相对较低。44.6～9.7Ma期间研究剖面中的气候变化指标发生了较始新世早期很大的变化:氯离子含量、碳酸钙含量以及红度值都较之前明显降低,而有机质含量比第一段明显增加。同时指标变化存在以33.5Ma、26Ma和14—15Ma为界的几个次级阶段的变化。9.7～1.83Ma,剖面的碳酸钙、氯离子含量和Sr/Ba比值总体比第二阶段急剧增加且一直呈增长趋势。红度的值也急剧减小,且一直呈减少趋势。有机质含量达到全剖面最低,且一直很稳定。而且从8Ma开始碳酸钙和氯离子含量增长趋势更加明显,相对应色度值也急剧降低。本文将上述指标的变化特征解释为约54～44.6Ma盆地气候特征以干热为主,长期趋势和次级波动均表现为相对暖湿和冷干的水热同向搭配变化:44.6～9.7Ma盆地整体相对湿润并且有显著的长期干湿波动,长期变化和次级波动均表现为相对暖干和冷湿的水热反向搭配变化。其中大致又可以33.5Ma、26Ma和14～15Ma为界分成几个次级阶段,反映出相对长期的干湿波动变化;9.7Ma以来,整体呈现显著的持续、阶段性变干趋势,长期变化和次级波动均表现为相对暖湿和冷干的水热同向搭配变化,显著的次级阶段性干旱事件分别发生在约8Ma、3.6Ma和2.6Ma以来。3)频谱分析揭示出柴达木盆地新生代气候变化受天文轨道变化的显著驱动,表现为受到2300kyr,1200kyr,400kyr等轨道偏心率和斜率长周期分量的控制,并且发现这些周期同时在在气候炎热干旱的新生代早期(53.4～44.6Ma)和中新世暖湿期(26～14Ma)分别达到最大化和次最大化,而在相对冷的时段(35～26Ma)渐新世南极大冰盖形成时期和15～14Ma中中新世大降温以来它们共同减小,而且从大约33.5Ma开始,几乎每次变冷事件都与1.2Ma周期的低幅段相对应。4)通过对青藏高原隆起过程、特提斯海退却过程的讨论及与全球和周边气候变化记录的对比,探讨了柴达木新生代气候形成的原因,提出了一个概念模型,认为约54～44Ma时,柴达木比现在靠南,可能完全处于当时宽广的亚热带副高的控制之下,盆地中气候非常炎热,其降雨可能主要通过副高北侧的西风从大西洋和北特提斯海带来的少量水汽造成。因而相对高温对应海洋上的相对高蒸发和空气中的高水汽,最后是柴达木盆地的相对湿润,反之,则少。从约44Ma开始,柴达木盆地可能在印度板块主碰撞期中开始向北明显移动。与此同时,全球温度从约50Ma开始一直下降,降温将导致赤—极温差变大,西风带将南移。两者可能共同导致柴达木盆地进入西风和亚热带副高的交汇地区及北特提斯海的东部方向。因而西风可将更多特提斯海水汽带到柴达木,且降温将使相对湿度增大,可能导致了此时柴达木盆地的明显湿润化,形成大湖。而天体轨道的周期性变化,则使西风和副高相应呈现周期性的南北移动,导致降温时西风南压,柴达木盆地变湿,升温则相反,副高北移,柴达木变干。从约9.7Ma开始,青藏高原开始的大规模隆起和特提斯海的快速消亡,不仅导致了西风地带水汽显著减少,而且西风被迫绕流波动和分叉,形成高原北侧反气旋性质的高压脊和低空西伯利亚高压,向我国境内倾泻西北干冷气流,西北地区急剧干旱化。并且越冷,高压越强,西北越干旱。与此相伴,青藏高原的强烈隆起激发或强化的亚洲季风,其前锋可能到达柴达木,形成温度越高,夏季风越强,带到柴达木的雨水也越多,可能形成了9.7Ma后的冷干和暖湿的季风型水热搭配格局和长期变化趋势。记录和模型表明,青藏高原的隆起、特提斯海的退却和全球气候变化均对柴达木盆地新生代气候变化产生了重要影响,但它们在各个时期的重要性不同。更多还原

【Abstract】 A series of significant geological events occurred in the Cenozoic which have led to enormous changes to the earth climatic and environmental system: the rapid stepwise cooling of the climate, the global aridification, episodic tectonic uplifts of Tibetan Plateau and the formation and evolution of Asian monsoon. Among them, the remarkable expanding of arid areas has attracted worldwide attentions due to its profound effects on human beings and scientifically linking with Tibet uplift, Paratethys Sea retreat and global cooling. To understand when and how the Asian inland aridification evolves is of great significance. However, up to now, the scarcity of high-resolution long continuous terrestrial climate records has hampered our insight into the above questions and land-sea comparison. Here we present firstly continuous Cenozoic climate record from the Qaidam Basin, Qinghai Province, NW China in the interior of Asian inland.The Qaidam Basin is a huge intermountain basin in the northern Tibetan Plateau and receives ultra-thick （up to over 10,000 m） and almost continuous Cenozoic sediments in response to the global climate change and uplift of the Tibetan Plateau. A～5000-meter-long continuous sediment outcrop at Lulehe from the north margin of the Qaidam Basin and a 700-meter-long supplementary outcrop at Yahu from the near center of the basin comprise almost a complete Cenozoic stratigraphic sequence （paleomagnetically dated between 53.4 Ma and 1.83 Ma）, thus providing a unique opportunity to study the above questions. We carried out detailed analyses of color, carbonate, chloric ion and TOC contents, biomarker and trace elements from these sediments. Through these multiple climatic proxy records and combining a framework research into the sedimentary facies and spectrum analysis, we obtain the following conclusions:1) The redness of color records of the Cenozoic Qaidam stratigraphy can be correlatedwell to the global temperature record, implying the redness can be served as asensitive alternative temperature proxy in Asian inland. 2) The characteristics of the Qaidam multiple climatic proxy records and thus the evolution of the Qaidam Cenozoic climate can roughly be divide as three large stages by two boundary ages at ca. 44.6Ma and 9.7Ma:During 53.4⁴4.6Ma: the records show a low TOC content, high contents of CaCO₃ and Cl^- and maximum redness value with large fluctuations.During 44.6⁹.7Ma: all the proxy records show an overall pattern of substantially lower values of redness and contents of CaCO₃ and Cl^- and higher TOC content, punctuated by some long changes of higher or lower values clearly subdivided as four sub-stages with boundary ages at 33.5Ma, 26Ma and 14¹5Ma.During 9.7～1.81 Ma: The records display a distinct fast persistent stepwise increasing of CaCO₃ and Cl^- contents and Sr/Ba ,and B/Ga ratios and decreasing of redness. The TOC content keeps low and reaches its minimum value.We interpret the above variations of our records as:i) a dry and hot climate dominated the Qaidam Basin during 54-44.6Ma, with a relative homodromous match of temperature and precipitation, i.e. warm vs. humid and cold vs. dry; ii) a relatively humid climate with large long fluctuations prevailed the period of 44.6-9.7Ma. The match of temperature and precipitation is just reversed, i.e., relative warm vs. dry and cold vs. humid; iii) a remarkably persistent and stepwise dry trends or drying of the Qaidam basin began ca. 9.7Ma, with a relative homodromous match of temperature and precipitation again. Further drying events occurred at ca. 8Ma、3.6Ma and 2.6Ma.3) Spectrum analysis of the records shows that Cenozoic climate in Qaidam basin was clearly forced by orbital parameters and modulated by long-term cycles such as 2.3Ma, 1.2Ma and 0.4Ma. Moreover, the amplitude of these periods achieved their maxima during the early Cenozoic （53.4～44.6Ma） and warm Miocene （26～14Ma） and decreased during the cold Oligocene （35～26Ma） and cold middle to late Miocene and thereafter （14～1.8Ma）. Detailed comparison between redness record and filtering results also indicates that nearly every cold event coincides with minimum amplitude of 1.2Ma long-term obliquity cycles since 33.5Ma.4) Based on detailed discussion on processes of Tibetan Plateau uplift and Paratethys retreat as well as comparison of our records with those worldwide, we proposed a simple conceptual model to illustrate our observations and interpretations above:A. The Qaidan Basin might be completely controlled by Subtropic High during 54～44Ma, leading to a dry and hot climate. The Westerlies carried limited vapor from Atlantic and Paratethys Sea to the Qaidam Basin and hence gave rise to a positive correlation between temperature and precipitation, i.e., hotter - more vapor - more precipitation in Qaidam Basin.B. The obvious northward move of Qaidam Basin since 44Ma driven by the collision of India with Asia and long global cooling since ca. 50Ma that will cause a southward move of westerlies due to enhanced equator - polar temperature gradient might jointly let the Qaidam Basin at the transitional area of the westerlies and Subtropic High. Consequently, cooling from periodic variations of Earth’s orbital parameters will increase relative humidity or reduce evaporation in one side and will force a southward move of the westerlies that carry more vapors from Paratethys Sea to the Qaidam Basin in other side, both will give rise to a humidification of the Qaidam Basin. In a reverse, periodic warming will drive northward move of the Subtropic High and retreat of the westerlies and increase evaporation, thus drying of Qaidam. This mechanism may be responsible for our observation of overall relative humid climate with a relative warm-dry and cold-wet match during this period.C. Since 9.7Ma, the dramatic uplift of Tibetan Plateau and rapid retreat of the Paratethys Sea has not only contributed to the tremendous decrease of vapors in westerlies over areas, but also forced the prevailing west-to-east flow of the westerlies to flow around the Plateau and formed Siberian High, leading to a rapid long aridification of Northwest China. This drying trend may be enhanced by a later rapid global cooling. At the same time, the enhanced Asian summer monsoon by the rapid uplift of Tibetan Plateau might also account for the increase of precipitation during the warm period. Hence, high temperature may also correspond to high precipitation during this period. In one word, both our preliminary records and supposed model indicate that the Tibetan Plateau uplift, Paratethys Sea retreat and global climate changes all have exerted strong impacts on Qaidam Cenozoic climate evolution with their different contributions at different stages, and a positive superimposing of all these three factors may have finally caused the aridification of Qaidam and Asian inland which resembles its present configuration since ca. 9.7 Ma.更多还原