A rock magnetic study on red palaeosols in Yun-Gui Plateau (Southwestern China) and evidence for uplift of plateau

Abstract

Magnetic signals of red palaeosols from the Yun-Gui Plateau (YGP), southwestern China, are studied using rock magnetism, selective chemical dissolution, differential X-ray diffraction (DXRD) and high-resolution transmission electron microscopy (HRTEM) in order to explore the potential of red palaeosol as a proxy indicator of paleogenesis and uplift of plateau. Red palaesols are characterized by highly magnetic signals and dark red color with a hue of 5 YR (yellow-red). The low-frequency magnetic susceptibility (χlf) of topsoils is shown to vary from 1500 × 10-8 to 2500 × 10-8 m3 kg-1 in a decreasing pattern from the top to bottom of the profile. Magnetic profiles reveal that the red palaeosols contain significant amount of fine-grained superparamagnetic (SP) grains, which is attributed to the higher concentration of pedogenic SP maghemite. The dithionite-citrate-bicarbonate (DCB) procedure can selectively dissolve ultrafine pedogenic magnetic minerals in the red palaeosols, as evidenced by the highly correlation between mass-specific frequency-dependent magnetic susceptibility (χfd) and χlf loss of DCB treatment. The magnetic loss after DCB treatment accounts for 87-95 per cent of the original susceptibility. Rock magnetism and DXRD reveal that the main magnetic mineral in the red palaeosols is the pedogenic SP/stable single domain (SP/SSD) maghemite. These pedogenic maghemites account for about 1 per cent of free iron oxides. HRTEM observations show the evidence of pedogenic SP (