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

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} m³ 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} m³ 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.