| 一个新的划分第四纪的标志──高原季风演变的地质环境后果 |
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| 引用本文： | 汤懋苍,刘晓东.一个新的划分第四纪的标志──高原季风演变的地质环境后果[J].第四纪研究,1995,15(1):82-88. |
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| 作者姓名： | 汤懋苍 刘晓东 |
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| 作者单位： | 中国科学院兰州高原大气物理研究所 |
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| 基金项目： | 国家攀登A85－29项目 |
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| 摘 要： | 本文提出将深厚高原季风的开始形成作为第四纪开始的标志。它的形成产生了三个后果：１)全球性大变冷;２)非洲变干,人属始现;３)黄土高原的黄土开始堆积。三者几乎是同时发生在２．５—２．４ＭａＢ．Ｐ,它们都是青藏高原的隆升高度达到平均凝结高度后的必然产物。将此三者合起来作为第四纪的标志,显然比单提一种要合理得多。在青藏高原的扩展过程中,其水平尺度达到斜压大气地转适应的临界尺度时(约３７ＭａＢ．Ｐ．),浅薄高原季风开始出现,它伴随一次全球大变冷。在此之前地球上并无季风。
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| 关 键 词： | 高原季风 第四纪标志 全球大变冷 非洲变干 黄土堆积 |
A NEW MARK FOR DELIMITATION OF QUATERNARY: STABLE APPEARANCE OF PLATEAU MONSOON AND ITS ENVIRONMENTAL EFFECTS |
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| Institution: | Lanzhou Institute of Plateau Atmospheric Physics, Chinese Academy of Sciences |
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| Abstract: | A mark for delimitation of a geological period must simultaneously meet four prerequisites: (1) it must not be a local but a global event, (2) it must concur ultaneously on the globe, (3) it must be unique in a time series (not repeated at least in 107 years), and (4) it must be distinctly shown in a stratigraphy series.We have found that all existing marks for delimitation of Quaternary can not simultaneously meet these four prerequisites. For example, the first appearance of coldlike Hyalinen Baltica fossil (or other fossils) was taken as a mark, but the time of its first appearance were inconsistent all over the world so that the second item is not met. Another mark-global cooling didn't meet the third one.We consider delimltating of geological periods to systematically divide the evolution history of the whole Earth system but not to simply stratify geological sedifments. Thus, seeking of such marks should not be limited in strata but should face the whole Earth system. The uplift of the Tibetan Plateau has been undoubtedly the grestest event in the whole Earth system since Tertiary. It forced subsystems of the Earth system to make adjustment to adapt a new state. The atmosphere has the lightest quality among all subsystems. However it continuously covers and moves fast around the globe. So imformation transmits agiler in the atmosphere than in other subsystems. During the uplift and horizontal expansion of the Tibetan Plateau a heat low tpressure occurred in the summer and a cold high pressure in the winter in case the horizontal scale of the plateau was greater than the critical scale of geostrophic adjustment of the baroclinic atmosphere. At the same time the plateau monsoon occurred, resulting in reinforcing of the westerly jet to the North of the Plateau. The environmental effects of this pattern of circulation were that first severe detempera,ture appeared in high and middle latitude areas and the geographic distribution of precipitation came into the "monsoon pattern" -wet on the east side of the plateau and dry on the west side in the summer. This also forced the biosphere to make corresponding adjustement. The time of the first appearance of the plateau monsoon was about 37Ma B. P., corresponding to the beginning of Oligocene. The plateau monsoon at that time was a shallow and thin system with discontinuity of time and space, similar to the present plateau monsoon in western America.The mean height of the Tibetan Plateau rose to about 1500m at 2.4Ma B. P.This is a critical height that the air flow changed from mainly climbing mountants to mainly circling and maintained a quasi-geostrophic motion. It was the participation of latent heat of condensation that made the plateau monsoon a deep and thick system in the troposphere. They caused climatic and environmental affects below:(1) because zonal circulation intensified, the high and middle latitutde area continued getting cold and Great Ice Age appeared, (2) the intensification of the downdraft caused by the upper air convergence over the southwest of the plateau forced North Africa to get drier so that the forest climate there was turned into a prairie climate and the ancestor of human-Homo began to appear, (3) in the plateau monsoon a layer with maximum wind speed appeared at the altitude of about 2km above the ground so that yellow sands on the north side of the plateau were blowed up and subsided down in the Loess Plateau. The above three consiquences, is regarded to be the mark of Quaternary's beginning. |
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