| 黄土剖面中古气候变化的地球化学记录 |
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| 引用本文： | 文启忠, 刁桂仪, 贾蓉芬, 周厚云. 黄土剖面中古气候变化的地球化学记录[J]. 第四纪研究, 1995, 15(3): 223-231. |
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| 作者姓名： | 文启忠 刁桂仪 贾蓉芬 周厚云 |
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| 作者单位： | 中国科学院广州地球化学研究所 |
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| 摘 要： | 作者通过对陕西洛川黄土剖面中碳酸盐总量、氧化物比值和元素演化周期性的分析，讨论了黄土高原地区2.4Ma以来古气候的演化模式。对渭南黄土剖面高密度连续采集的样品，着重分析了CaCO3、Fe2O3/FeO值和有机碳的高分辨记录。指出它们的变化彼此同步（但趋势不尽相同）,与黄土一古土壤系列变化一致，井与深海沉积物氧同位素可进行良好的对比。它们综合反映了该区134500年来有五次万年级的暖湿（或凉湿）一冷干的气候波动，其中各期又有若干次级的小颤动。它们的变化与季风影响强弱程度明显有关。
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| 关 键 词： | 黄土 古气候 地球化学标志 |
GEOCHEMICAL RECORDS OF PALEOCLIMATE CHANGE IN LOESS SECTIONS |
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| Wen Qizhong, Diao Guiyi, Jia Rongfen, Zhou Houyun. GEOCHEMICAL RECORDS OF PALEOCLIMATE CHANGE IN LOESS SECTIONS[J]. Quaternary Sciences, 1995, 15(3): 223-231. |
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| Authors: | Wen Qizhong Diao Guiyi Jia Rongfen Zhou Houyun |
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| Affiliation: | Guangzhou Institute of Gcochemistry, Chiners Academy of Sciences |
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| Abstract: | The research of geochemical characteristics of environment-instructing elements and isotopes in loess will give a important foundation to the reconstruction of paleoclimate since Quaternary. We made detail geochemical studies of typical loess sections of Louchuan and Weinan in Shaanxi Province. The Luochuan section has a total thiGhness of 132.5 m and consists of alternative loesses and paleosols, reflecting frequent fluctuations of cold and dry climate to warm and humid climate since 2.4Ma B. P. The upper part (about 14 m thick) of the Weinan section emphatically analyzed consists of the Malan Loess intercalated with two weakly developed layers of paleosol and offers high resolution geological record for approaching climatic changes of the last 150 000 a. This paper mainly discusses some geochemical records in the two sections and their indicated paleoclimate changes.The research results obtained are as follows:(1) The variations in CaCO3 content and oxide ratios in the Luochuan loess section coincide with Quaternary loess-paleosol alternations, which reflects repeated fluctuations of dry and cold climate to warm and humid climate. The CaCO3content rhythmically increases from low to up and so do some oxide ratios. It could be related to that the climate during loess accumulation gradually became more arid.Furthermore, their variations show that the climatic changes before the B/M boundary were many in frequency but smaller in strength, whereas, the changes after the B/M boundary are relatively few in frequency but larger in strength.(2) The variations of element compositions and ratios sensitive to climate show cycles, which are keeping pace with the loess-paleosol alternations. There are four cycles to be identified: 364 000-222 000 a, 141 000-84 000 a, 45 000-34 000 a and 25 000-14 000 a. The four cycles are also coincident both with the periods calculated by Milankovich in terms of astronomical factors and with the periods based on the variation of 818O in deep-sea sediments. It is certain that the climatic changes during Quaternary were mainly controlled by astronomical factors.(3) In the Weinan Loess section, the carbonate index has a high resolution thathigh CaCO3 contents correspond to loess and low CaCO3 contents correspond to paleosol. Three and a half cycles can be recognized from bottom to top (from S1 to S0). And in this section, the variations of Fe2O3/FeO and TOC are also notable.The highest value of Fe2O3/FeO is in S1. Upward to L(1-5), the oxidizing degree of Fe decreases obviously. The three layers of L(1-4), to L(1-2), are very close in average Fe2O3/FeO and their oxidizing degrees increase slightly. Further upward to L(1-1), Fe has the lowest oxidizing degree, implying the coldest period in the last 150 000 a.The trend of changes of TOC is similar to that of Fe2O3/FeO and opposite to that of CaCO3.The variation of the three geochemical indexes not only coincides with loess-paleosol alternations, but also are correlatable with the 5 stages of δ18O based on deep-sea sediments. There were five over-ten-thousand-year-lasting fluctuations of warm-humid (or cool-humid) climate to cold-dry in Loess Plateau since 134 500aB. P. Every fluctuation includes several small climatic vibrations, which are related to the influence of monsoon. Besides, the variations of TOC and CaCO3indicate that precipitation changed abruptly in the transformation of cold to warm and gradually in transformation of warm to cold. But temperature always changed gradually. Both are not completely synchronous.(4) The variation of CaCO3 content is mainly sensitive to the changes of precipitation, that of Fe2O3/FeO to temperature and that of TOC to humidity. Synthetical analyses of the three geochemical indexes indicates that S0 and S1 correspond to periods of relatively strong summer monsoon and that the upper and lower parts of the Malan Loess correspond to periods of very weak summer monsoon but very strong winter monsoon and its middle part corresponds to a period of relatively weak summer monsoon, which is characterized by obviou |
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