长江中下游地区第四纪红土磁学特征及其环境意义

通过采集长江中下游湖南长沙(CS)、岳阳(HN)、江西南昌(NC)、新余(XY)和浙江金华高村(GC)5个剖面不同层位的红土样品,对其粒度和多种磁学参数进行测量和分析,探讨了红土剖面随深度变化的成土作用强弱以及多磁学参数在揭示我国南方红土第四纪气候演化信息的价值。结果表明:①粒度分析显示,红黄土交接带(约28°~31°N)的CS、HN、NC、GC 4个剖面的红土粒度与北方黄土和下蜀黄土有很好的可比性,具有明显的风成特性;位置更偏南的XY剖面红土颗粒较粗,10~50 μm粒组富集不明显,具有典型的冲积相特征。②各红土剖面的多磁学参数分析表明,成土过程中产生的磁铁矿、磁赤铁矿和赤铁矿是红土磁性的主要载体。③同一红土剖面内,不同沉积结构的磁性矿物含量和组成具有很大差异。下部网纹红土中赤铁矿的含量较高,磁赤铁矿的含量较上覆均质红土或黄棕色土低,揭示了网纹红土形成于中国南方一个极端湿润期,长期剧烈的水分活动导致均质红土中磁赤铁矿的溶解和铁质的流失,同时较强的成土作用使强磁性的磁赤铁矿向弱磁性的赤铁矿转化,从而导致网纹红土的磁化率比上覆均质红土或黄棕色土低了一个数量级。④具有冲积相特征的XY剖面磁学特征与其他风尘成因红土(特别是NC剖面)差异甚大,可能成为它具有不同物质来源的一个佐证。

Magnetic Properties of Quaternary Red Earth Profile in Yangtze River Valley and Its Paleo-environmental Implications

The Quaternary red earth is widespread in the present Yangtze River Valley. Like the Loess-palesol sequences in north of China, the red earth provides detailed records of Quaternary environmental and climatic change. The typical red earth profiles usually consists of homogenous red earth range in the upper part and reticular red earth horizons in the lower part. In this study, we have investigated magnetic properties and particle-size distribution of two red earth sections (CS and HN) at Hunan Province, two sections (NC and XY) at Jiangxi Province, and a section (GC) at Zhejiang Province, with the purpose to investigate the feasibility of magnetic properties in paleo-climate reconstruction. The results suggest:1) The Quaternary red earth in CS , HN, NC and GC, located in the transitional zone between red earth and Xiashu loess, was aeolian origin, as it shared many aeolian features with Xiashu loess and Chinese North loess; however, the Quaternary red clay in Xinyu is significantly different from the other four areas in particle-size composition. It often has gravels and a relatively high content of sands, demonstrating diluvial or alluvial properties.2) Multiple magnetic parameters show that maghemite, hematite and goethite of pedogenic origin make dominant contributions to the enhanced magnetic susceptibility of the red earth.3) The relative content of these minerals are various in different regions and different weathering stages. However, magnetic susceptibility and S-ratio declines while Hard Isothermal Remanent Magnetization (HIRM) increases significantly in the lower strongly weathering reticulate red clay layer, which results from the enrichment of antiferromagnetic minerals and the dissollution of the maghemite. Therefore, we conclude that the anomalous decrease of the magnetic susceptibility is the result of the reducing action, which might be the production within a period of enhanced precipitation.4) Magnetic characteristics of Xinyu far different from other four sections may be a reason that material source is not identical with other four dust caused clay. Further work is needed to provide more evidence.