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

Key words: 
• red earth /
• grain-size /
• magnetic minerals /
• environment

Abstract: 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.