Magnetic susceptibility of the Quaternary Red Clay in subtropical China and its paleoenvironmental implications

Abstract

Magnetic susceptibility ( χ lf) and other magnetic parameters of 11 typical profiles of Quaternary Red Clay (QRC) in subtropical China were measured. χ lf curves of the QRC profiles show similar vertical variations: χ lf of the uppermost yellow-brown earth (YBE) and uniform red clay (URC) of the profiles is high, mostly ranging from 50 to 150 × 10 − 8 m 3 kg − 1 ; that of the reticulate red clay (RRC) sharply decreases downwards. Especially in the Lower RRC (LRRC) with dense reticulate pattern, χ lf is often less than 10 × 10 − 8 m 3 kg − 1 . Grain-size characteristics and geochemical properties of the QRC suggest that the sharp decrease of χ lf in the RRC is not related to sudden change of parent materials, but to the formation of the reticulate pattern. Further study indicates that pedogenic fine ferrimagnetic minerals (FM) (superparamagnetic particles (SP) and/or stable single domain (SSD)) are the main magnetic carriers of the QRC. The YBE and URC, with higher χ lf, show low coercivity and magnetic soft behavior; the RRC, especially the LRRC, with weak magnetism, shows high coercivity and magnetic hard behavior. The temperature-dependent susceptibility (TDS) of the QRC suggests that the dominant type of FM in the YBE and URC is pedogenic maghemite; the RRC, especially the LRRC, however, contains little maghemite but a trace amount of lithogenic multi-domain (MD) magnetite inherited from the parent rock. During post-depositional hydromorphic processes, pedogenic maghemite was more easily dissolved than hematite, which caused the sharp decrease of χ lf in the RRC. As χ lf of the QRC was mainly controlled by the intensity of hydromorphic processes, rather than the weathering degree, χ lf of the QRC in subtropical China cannot accurately indicate the paleoclimate during the Quaternary period, when the red clay was initially formed.