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| 中国黄土和阿拉斯加黄土磁化率气候记录的两种模式探讨 | |
| 引用本文： | 刘秀铭,夏敦胜,刘东生,丁仲礼,陈发虎,J.E.Begét.中国黄土和阿拉斯加黄土磁化率气候记录的两种模式探讨[J].第四纪研究,2007,27(2):210-220. |
| 作者姓名： | 刘秀铭 夏敦胜 刘东生 丁仲礼 陈发虎 J.E.Begét |
| 作者单位： | 1. 兰州大学资源环境学院西部环境教育部重点实验室,兰州,730000;Department of Physical Geography,Macquarie University,NSW 2109,Australia 2. 中国科学院寒区旱区环境与工程研究所沙漠与沙漠化重点实验室,兰州,730000;兰州大学资源环境学院西部环境教育部重点实验室,兰州,730000 3. 中国科学院地质与地球物理研究所,北京,100029 4. 兰州大学资源环境学院西部环境教育部重点实验室,兰州,730000 5. Department of Geology and Geophysics,University of Alaska,Fairbanks,AK 99775,USA |
| 基金项目： | 国家自然科学基金 , 澳大利亚Macquarie大学资助项目 |
| 摘 要： | 在中国和中欧黄土-古土壤研究中发现的磁化率与成壤作用(或古气候温湿程度)的正相关性已被第四纪科学家广泛认识,并应用于古气候研究中.成壤过程形成的亚铁磁性矿物被认为是古土壤磁化率增加的主要原因;然而,这一模式并不一定适用于其他黄土沉积地区,如阿拉斯加和西伯利亚黄土沉积显示了一个完全相反的磁化率行为,即在黄土层获高磁化率值而在古土壤层获低磁化率.这种相反的关系过去被解释为磁化率反映的是与风动力吹来的亚铁磁性矿物的含量,即与风速或风力大小有关.本研究发现阿拉斯加黄土与古土壤的磁性矿物性质有明显差异,不仅仅是粒径的大小,还有磁性矿物的种类即矿物相的差别.这一证据很难单纯以风力强度的大小来解释,意味着阿拉斯加古土壤的低磁化率至少部分是在成壤过程中亚铁磁性矿物发生改变(如溶解)而造成,表明阿拉斯加黄土和中国黄土的磁化率与古气候记录可能存在两种模式,即氧化和还原条件下的成土模式.黄土磁化率在不同的气候(温度湿度)条件下有着不同的对应关系:在低降水量、高蒸发量的干旱氧化成壤条件下,利于亚铁磁性矿物的生成,其磁化率与古气候的关系呈正相关,如中国和中亚黄土;在高纬高湿的还原成壤条件下,亚铁磁性矿物会被破坏被分解,其磁化率与古气候呈负相关关系,如阿拉斯加黄土.如果成壤条件在氧化和还原之间来回变换,那么就很难找到两者之间的联系.因此,将磁化率应用于古气候的重建时要加倍的小心. |
| 关 键 词： | 中国黄土 阿拉斯加黄土 磁化率 古气候 磁学特性 成壤作用 岩石磁学中 |
| 文章编号： | 1001-7410（2007）02-210-11 |
| 收稿时间： | 2006-12-05 |
| 修稿时间： | 2007-01-10 |
DISCUSSION ON TWO MODELS OF PALEOCLIMATIC RECORDS OF MAGNETIC SUSCEPTIBILITY OF ALASKAN AND CHINESE LOESS |
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| Liu Xiuming,Xia Dunsheng,Liu Tungsheng,Ding Zhongli,Chen Fahu,J.E.Begét.DISCUSSION ON TWO MODELS OF PALEOCLIMATIC RECORDS OF MAGNETIC SUSCEPTIBILITY OF ALASKAN AND CHINESE LOESS[J].Quaternary Sciences,2007,27(2):210-220. | |
| Authors: | Liu Xiuming Xia Dunsheng Liu Tungsheng Ding Zhongli Chen Fahu JEBegét |
| Abstract: | The positive correlation between magnetic susceptibility and pedogenic degree or weathering intensity, observed in the loess-palaeosol sequences from China and Central Europe, has been widely used by Quaternary scientists for palaeoclimatic studies. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However this pattern of high magnetic susceptibility in palaeosols, and lower values in loess, is not replicated in some loess deposits. Alaskan and Siberian loess deposits display a completely opposite susceptibility behaviour: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Alaskan samples in this paper suggests that there are notable differences in magnetic properties between Alaskan loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to fully explain through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Alaskan palaeosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. Magnetic susceptibility has variable behaviour corresponding to different temperature-moisture environments. Ferrimagnetic minerals are produced (positive correlation) under pedogenic conditions resulting from low precipitation and high evaporation but will be destroyed (negative correlation) under high moisture (waterlogged) pedogenic conditions. If pedogenic development occurs under conditions which oscillate between ferrimagnet formation and destruction then it may be difficult to find a correlation between them. Therefore great care should be taken when using susceptibility values for palaeoclimatic reconstruction. |
| Keywords: | Chinese loess Alaskan loess magnetic susceptibility palaeoclimatology magneticproperties pedogenesis rock magnetism |
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