Magnetic response to pedogenesis in aerobic soils of different weathering degree

Abstract

Abstract Magnetic parameters are widely used to indicate paleoclimatic and paleoenvironmental conditions in sedimentary records. However, the relations between magnetic parameters and paleoclimate conditions are very different depending on the weathering degree. This study explores various multidisciplinary parameters across an intensely weathered red soil section in the Bose Basin (the Damei sequence with layers of different weathering intensity) of subtropical southern China to understand the magnetic mineral response to pedogenesis in aerobic soils. We find that the only commonly used environmental magnetic parameter that correlates well with geochemical indices of weathering intensity, is the S-ratio (a measure of the relative amounts of high-coercivity remanence to low-coercivity remanence). HIRM (the absolute amount of high-coercivity remanence) and -IRM-0.3T (the absolute amount of low-coercivity remanence) are the main variable in S-ratio. Mineral magnetic analyses indicate that the main carriers of HIRM and -IRM-0.3T in the Damei red soils are respectively hematite and maghemite. Negative correlations between the S-ratio, the IRM-0.3T and the weathering intensity, and a positive correlation between the HIRM and the weathering intensity relates to the transformation of pedogenic maghemite into hematite which is favored in such a case of intense weathering. It seems that environmental interpretations from magnetic parameters are better constrained if the weathering stage can be first determined using the relation between concentration of hematite and ferrimagnetic minerals. A new magnetic weathering index is proposed for intensely weathered red soils based on the slope of the linear correlation between HIRM and -IRM-0.3T. These results open applications for assessing weathering and pedoclimate proxies using environmental magnetism.