Loess-palaeosol sequences in diverse environments: Aeolian accumulation identification and magnetic susceptibility models
Loess-palaeosol sequences in diverse environments: Aeolian accumulation identification and magnetic susceptibility models
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DOI：
10.1016/j.palaeo.2021.110683
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发表时间：
2021-12
期刊：
影响因子：
--
通讯作者：
Xiu-ming Liu;Xuegang Mao
中科院分区：
文献类型：
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作者：
Xiu-ming Liu;Xuegang Mao
中国黄土高原厚层黄土-古土壤序列是中中新世以来古气候和古环境变化的重要大陆档案。黄土磁化率记录常用于古气候重建。然而，两个磁气候学模型的黄土-古土壤序列的磁性增强-风力模型和成壤模型-仍然存在争议，没有一个模型单独足以解释不同环境中的黄土沉积磁化率地层。黄土的沉积环境会发生很大的改变，黄土的宏观特征可能与CLP记录良好的序列有很大的不同，这使得黄土识别及其古气候解释变得复杂。本文总结了不同环境下黄土沉积物的属性，并对识别风成堆积的可能方法进行了评述。总结和分类了从干旱到半湿润不同气候背景下的黄土-古土壤序列，并评价了磁化率与有效降水的关系。本文提出了一个由三个实例组成的磁化率模型，用于解释不同环境下黄土-古土壤序列的磁化率。在情况A（例如，在物理风化作用占主导地位、成土作用较弱的新疆黄土中，磁化率与成土作用之间不存在联系，可采用风力模型。在情况B（例如，CLP），其中，在主要氧化条件下，磁化率随成土强度增加，氧化成土模型是适用的。在情况C（例如，阿拉斯加和西伯利亚黄土），磁化率与成土作用之间存在负相关关系，这是由于还原条件下碎屑铁氧化物的溶解，还原成土模式是适用的。在情况C，成土作用（或化学风化/溶解）的行为，在很大程度上消除了碎屑磁化率信号。因此，与阿拉斯加和西伯利亚黄土磁化率的原始解释相反，风力模型不适用。在此基础上，提出了两个年平均降水量阈值，为不同环境下黄土磁化率记录的古气候解释提供了依据。
The thick loess-palaeosol sequences of the Chinese Loess Plateau (CLP) are valuable continental archives of paleoclimatic and paleoenvironmental changes since at least the mid-Miocene. The magnetic susceptibility record of loess sequences is frequently used for palaeoclimatic reconstruction. However, the two magnetoclimatological models for the magnetic enhancement of loess-palaeosol sequences—the wind vigour model and the pedogenic model—are still debated, and neither model alone is sufficient to explain the magnetic susceptibility stratigraphy of loess deposits in diverse environments. Loess can be substantially altered by its environment of deposition and the macroscopic characteristics of loess may differ substantially from the well-documented sequences of the CLP, which complicates loess identification and its palaeoclimatic interpretation. In the present study, we summarize the attributes of loess deposits in different environments and review possible approaches for identifying aeolian accumulation. Loess-palaeosol sequences in various climatic contexts, from arid to subhumid, are summarized and categorized and the relationship between the magnetic susceptibility and effective precipitation is evaluated. A model comprising three cases is proposed to interpret the magnetic susceptibility of loess-palaeosol sequences in a variety of different environments. In case A (e.g., the Xinjiang loess, NW China), where physical weathering dominates and pedogenesis is weak, there is no relationship between the magnetic susceptibility and pedogenesis, and here the wind vigour model is applicable. In case B (e.g., the CLP), where, under predominantly oxidizing conditions, the magnetic susceptibility increases with pedogenic intensity, and an oxidizing-pedogenic model is applicable. In case C (e.g., Alaskan and Siberian loess), there is a negative relationship between magnetic susceptibility and pedogenesis due to the dissolution of detrital iron oxides under reducing conditions, and a reduction-pedogenesis model is applicable. In Case C, pedogenesis (or chemical weathering/dissolution) acts to largely obliterate the detrital magnetic susceptibility signal. Hence, contrary to the original interpretation of the magnetic susceptibility of Alaskan and Siberian loess, the wind vigour model is not applicable. Two thresholds of mean annual precipitation between the three cases are proposed, which enable the paleoclimatic interpretation of loess magnetic susceptibility records in different environments.