Acta Scientiarum Naturalium Universitatis Pekinensis ›› 2019, Vol. 55 ›› Issue (3): 422-436.DOI: 10.13209/j.0479-8023.2019.033

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Interpreting the Contrasted Magnetic Susceptibility Records for the Upper and Lower Parts of a Loess Section in the Southern Tajikistan, Central Asia

SUN Sijia, REN Xiaohuan, SONG Jingjing, ZHOU Liping†   

  1. College of Urban and Environment Sciences, Peking University, Beijing 100871
  • Received:2018-06-09 Revised:2019-03-09 Online:2019-05-20 Published:2019-05-20
  • Contact: ZHOU Liping,E-mail:lpzhou(at)pku.edu.cn

中亚塔吉克斯坦南部黄土剖面上下部地层磁化率明显差异原因浅析

孙思佳, 任小换, 宋晶晶, 周力平†
  

  1. 北京大学城市与环境学院, 北京 100871
  • 通讯作者: 周力平,E-mail:lpzhou(at)pku.edu.cn
  • 基金资助:
    国家自然科学基金(49925307)资助

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Abstract:

Loess sequences in southern Tajikistan, central Asia are characterized by the occurrence of alternating loess and pedocomplexes, a feature which is similar to that in the Loess Plateau of north-central China. Previous studies of Tajik loess revealed that below the third pedocomplex (PC3), the loess units and pedocomplexes have distinctly lower magnetic susceptibility than the upper part. No conclusive consensus has been reached on the paleoclimatic interpretation of such a pattern. Here, results of a systematic mineral magnetic study in combination with spectrophotometric measurements on selected samples from Darai Kalon loess section in the southern Tajikistan, central Asia are presented. The mineral magnetic properties suggest increasing concentrations of the ferromagnetic minerals as well as the superparamagnetic and single domain magnetic grains, together with the decreasing hard magnetic components (i.e. contents of hematite or goethite), from the lower part to the upper part of the sequence, which would point to enhanced pedogenesis through time. Such paleoclimate inference is supported by the higher content of hematite in the lower part of the sequence on the assumption that higher temperature and lower precipitation favor the formation of hematite. In this case, the dry climate in the lower part leads to weak pedogenesis. Therefore, there are less superparamagnetic and single domain magnetic grains, which are responsible for the reduced magnetic susceptibility in the lower part. However, the upwards decreasing goethite content revealed by the measurements of diffuse reflectance spectrophotometric data suggests a change towards a drying climate, as formation of goethite requires wet conditions. In this case, the lower magnetic susceptibility below the PC3 may be manifestation of dissolution of superparamagnetic and single domain magnetic grains due to a prevailing humid environment. While the high field (up to 2 Tesla) isothermal remanence shows that the upper part has a lower content of hematite-dominated hard magnetic components, the poor correlation between redness (a*) and the content of hematite in the upper part indicates the strong influence of the matrix effect in the diffuse reflectance spectrophotometric data. Hence, there exists a major uncertainty in the interpretation of the spectrophotometric result of hematite. The overall good negative correlation between the content of goethite and magnetic susceptibility may be interpreted as indicating a trend of drying climate from the lower part to the upper part of the sequence. Such an interpretation is compatible with previous palynological and geochemical studies in the region. This study highlights the complication in the paleoclimate interpretation of the magnetic susceptibility and even mineral magnetic data for loess-pedocomplex sequence in southern Tajikistan. It calls for caution in the application of the models based on the magnetic investigations of Chinese loess and indicates the need of the use of the multiple proxies.

Key words: central Asia, loess sequences, mineral magnetism, spectrophotometric measurement, paleoclimate

摘要：

中亚塔吉克斯坦南部的黄土序列与中国黄土高原相似, 呈现黄土?古土壤交替的特征, 前人研究发现塔吉克斯坦黄土序列中以PC3为界的下部黄土与古土壤单元磁化率明显低于上部地层, 目前对这一现象尚无合理的古气候解释。本文对中亚塔吉克斯坦南部Darai Kalon黄土剖面的典型样品进行系统的矿物磁学研究以及漫反射光谱测量, 结果表明, 从黄土序列的下部到上部, 亚铁磁性矿物的总量以及超顺磁与单畴磁性颗粒的含量均呈增长的趋势, 与此同时, 硬磁性组份(如赤铁矿或针铁矿)的含量逐渐减少, 这可能指示成壤作用强度随时间增强。赤铁矿的适宜生长条件为高温干燥环境, 因此黄土序列下部的较高赤铁矿含量可能支持气候逐渐变湿的推论, 即下部地层沉积时期气候偏干, 成壤作用较弱, 古土壤中含较少的亚铁磁性矿物, 超顺磁与单畴磁性矿物颗粒较少, 故下部地层的磁化率较低。然而, 光谱测量数据显示, 下部地层针铁矿含量高于上部地层, 由于针铁矿的适宜生长条件为湿润环境, 其含量向上逐渐降低的趋势当指示气候逐渐变干。因此, PC3以下地层的较低磁化率也可能是过于湿润的环境溶解超顺磁和单畴磁性矿物颗粒造成的。尽管高场(达2 T)等温剩磁结果显示上部地层中以赤铁矿为主的硬磁性矿物含量较少, 但上部地层红度(a*)与赤铁矿含量之间较弱的相关性指示基体效应可能对光谱测量结果产生强烈的影响, 导致对赤铁矿光谱数据的解释存在很大的不确定性。针铁矿含量与磁化率的良好负相关性的确指示从下至上气候变干的趋势, 这一趋势与该地区前人的孢粉学和地球化学研究结果一致。以上研究结果表明, 对于中亚塔吉克斯坦南部黄土?古土壤序列的磁化率乃至矿物磁学数据的古气候解释非常复杂, 在中亚黄土研究中应用基于中国黄土磁学研究的模式需要谨慎, 有必要结合多指标进行综合分析。

关键词: 中亚, 黄土序列, 矿物磁学, 漫反射光谱, 古气候

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