中欧黄土中古土壤S5的磁性特征对比

Comparison of the magnetic properties of paleosol S5 between Chinese and Serbian loess

关键词:  环境磁学  磁化率  成壤作用  古气候变化

Key words:  environmental magnetism  magnetic susceptibility  pedogenesis  paleoclimate change

摘要:

S₅古土壤是中国黄土剖面中的重要标志层，对中国黄土高原宝鸡剖面和塞尔维亚北部Stari Slankamen剖面的古土壤S₅及其下覆黄土层L₆进行多种环境磁学参数的对比分析，探讨二者磁学性质的异同点。结果表明：两个古土壤层磁性增强的主导因素是成壤作用，所含磁性矿物有亚铁磁性的磁铁矿和磁赤铁矿，以及反铁磁性的赤铁矿和针铁矿。湿润的成壤环境可能是两个古土壤层中针铁矿含量较多和磁赤铁矿含量较少的原因，也是导致二者低磁化率现象的原因之一。综合古土壤形态和磁学性质的多方面差异，Stari Slankamen剖面S₅古土壤的磁性增强属于通常认为的氧化环境下的成壤作用所致，而宝鸡剖面S₅古土壤则是过度湿润的成壤环境导致的潜育化作用的结果。

Abstract:

Background, aim, and scope Rock-magnetic research laid the foundation for paleoclimate studies from Chinese loess over the past forty years; particularly the parameter of magnetic susceptibility made tremendous contributions. Existing studies show that the magnetic susceptibility behaviors of the loess deposits in the Vojvodina region in northern Serbia exhibit a similar pattern with those of the Chinese Loess Plateau. Moreover, detailed stratigraphic comparisons and adequate paleoclimatic reconstructions have confirmed the loess-paleosol sequences in the Vojvodina region have an essential link with those of the Chinese Loess Plateau in pedostratigraphy, magnetic susceptibility behaviors, and relevant Pleistocene environments. The paleosol S₅ in the Chinese loess profiles was considered as an important marker layer owing to strong pedogenesis and thick horizon. This study aims to investigate the magnetic properties of paleosol S₅ of the Baoji loess in northern China and the Stari Slankamen loess in northern Serbia to compare the mechanisms of the variations in magnetic enhancements. Materials and methods Nine samples from each paleosol S₅ and the underlying loess layer L₆ were respectively taken from the Baoji profile and the Stari Slankamen profile for study. For all 18 samples, we measured or calculated low- and high-frequency magnetic susceptibility, frequency-dependent susceptibility, percentage frequency-dependent susceptibility, anhysteretic remanent magnetization, saturation isothermal remanent magnetization, remanent coercivity, hard isothermal remanent magnetization, hysteresis loops, and thermomagnetic curves. Results The results showed a dominating cause of pedogenesis in the enhancements of magnetic properties of the two paleosols, both comprising ferrimagnetic magnetite and maghemite, and antiferromagnetic hematite and goethite. Relative to the Baoji S₅, the paleosol S₅ of the Stari Slankamen profile bears higher proportion of fine-grained magnetic minerals. Discussion The causes of relatively lower content of thermally unstable maghemite and higher content of goethite in both of the paleosols are analyzed based on existing studies. Combining the natures of minerals maghemite and goethite, we consider that, wetter environment of the soil developments perhaps is the reason of above difference, and hence is one of the causes of the lower magnetic susceptibility values. The morphological feature of the Baoji S₅ is characterized by abundant iron-manganese coatings and strong eluviation, which meaning very higher soil moisture. However, these features are absent in the paleosol S₅ of the Stari Slankamen profile. Moreover, taken the difference of magnetic properties of the two paleosols into account, these discrepancies imply different mechanisms of magnetic enhancements in the two paleosols. Conclusions We conclude the pedogenesis under an oxidation condition was the cause of the magnetic enhancement of the paleosol S₅ in the Stari Slankamen loess profile, and the lower magnetic susceptibility of the Baoji S₅ was the result of gleying from excessively wet pedogenic environment. Recommendations and perspectives This study provides a reference for understanding the difference in magnetic characteristics of Chinese and Serbian loess, and for the pedogenic mechanism of magnetic enhancement in loess studies.