黄土高原S5古土壤磁性特征空间变化及其机制

In Chinese Loess Plateau (CLP), the study of loess deposits has made great achievements in the East Asian monsoon evolution and global change, and demonstrated that there is a general positive relationship between magnetic susceptibility and pedogenic development of loess-paleosol sequences. But in the southernmost CLP, where is warmer and wetter, it shows non-positive correlation between magnetic susceptibility and the degree of pedogenesis, such as S5 paleosol in Baoji. The S5 is the most prominent and well-developed paleosol unit, and can be distinguished by its great thickness and darkest reddish color. The climate during S5 formation was much warmer and wetter than today, and has been interpreted as representing a climatic optimum for the last 2.5 Ma. Generally,the S5 paleosol corresponds to the maximum magnetic susceptibility, which is one of the summer monsoon proxies. However，in Baoji,the annual rainfall is more abundant than that in the central Loess Plateau, the magnetic susceptibility value of the S5 unit is not the highest in the profile, and is even lower than that of Xifeng and Lingtai in central Loess Plateau. The reason for the low magnetic susceptibility value of S5 unit on the southern edge of CLP has not yet been fully known. This study is mainly based on rock magnetism and geochemical methods, through the spatial comparison of environmental proxies and magnetic characters of S5 paleosol samples from Xifeng, Lingtai, Linyou and Baoji sections from north to south, to analyse the change of magnetic mineral types,concentration, and grain sizes during different pedogenic process, and the transformation of magnetic minerals under oxidizing and reducing conditions.So that to know about the scope and conditions of magnetic susceptibility as one of the summerm monsoon proxies,and to reveal the main reason about the spatial difference of S5 magnetic properties. This can provide an environmental magnetic basis for understanding the weak relation between magnetic susceptibility and the degree of pedogenesis in the southernmost CLP.

黄土高原黄土在东亚季风和全球变化研究方面取得了极大成就，也揭示了黄土地层磁化率与古气候温湿程度（成土强度）基本成正相关关系。然而，在年均降雨量更多、年均温度更高的黄土高原南缘地区，却不再遵循着这种关系。如宝鸡S5古土壤磁化率值却是S5以来6个土壤层中较低的一层，是什么原因造成的，当前认识并不一致。本项研究拟选取西峰、灵台、麟游和宝鸡S5古土壤为研究对象，以岩石磁学和地球化学方法为依据，通过对西峰-灵台-麟游-宝鸡断面各环境代用指标与磁学性质详细对比，分析S5古土壤磁性矿物类型，含量和磁性颗粒大小在成壤过程中的变化，分析随着成土环境氧化还原条件的变化磁性矿物之间的转化。通过空间对比分析，认识磁化率作为夏季风代用指标的适用范围和条件,揭示影响S5古土壤磁学性质空间差异的主要原因。这为气候较为暖湿的黄土高原南缘地区磁化率与成壤程度弱相关的变化特征提供环境磁学依据。

黄土高原黄土在东亚季风和全球变化研究方面取得了极大成就，也揭示了黄土地层磁化率与古气候温湿程度（成土强度）基本成正相关关系。S5是成壤作用最明显、最强的古土壤层，代表过去2.5Ma期间气候最适宜（暖、湿）时期。然而在年降雨量比中部更充沛的黄土高原最南部宝鸡地区，S5 古土壤层的磁化率却是S5以来的较低值，产生这种现象的原因是什么？这是本项目“黄土高原S5古土壤磁性特征空间变化及其机制”的研究重点。 本项研究通过对黄土高原野外考察和样品采集获取了大量的第一手材料。考察采样包括西峰剖面（980个样品）、灵台剖面（900个样品）、麟游剖面（800个样品）和宝鸡剖面（870个样品），共采集了3550个黄土古土壤样品。首先运用岩石磁学和地球化学相结合的方法对宝鸡剖面S5和S3古土壤的磁性特征进行了详细对比分析，提出磁性矿物的转化可能导致了成壤强的S5古土壤超顺磁亚铁磁性矿物含量的减少和磁化率的降低。进一步对西峰-麟游-宝鸡断面S5-1古土壤层空间对比分析，确定了高的降水量使部分细粒强磁性矿物（主要是氧化的磁赤铁矿和磁铁矿）通过成壤作用转化为弱磁性矿物（赤铁矿和针铁矿）导致了SP和SSD亚铁磁性矿物含量减少和磁化率由北向南的降低。