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 × 10−8m3 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 (<∼20–30 nm) and pure maghemite. These magnetic particles vary from several to tens of nanometres in size and exhibit typical crystallochemical characteristics of this mineral. Magnetic evidence suggests that the red palaeosols experience a strongly pedogenic processes. Pedogenic processes result in the neoformation of hematite and maghemite, and causes a substantial increase in the magnetic susceptibility and other magnetic signals. Therefore, the rock magnetism of red palaeosols potentially yields significant palaeopedogenic information and evidence on the neotectonic movement of plateau. Results of the present work suggest that the red palaeosols could be a good tool for understanding the amplitude and the age of uplift in the YGP.