The record of changing hematite and goethite accumulation over the past 22 Myr on the Chinese Loess Plateau from magnetic measurements and diffuse reflectance spectroscopy

摘要

High-field isothermal remanence (HIRM) and diffuse reflectance spectroscopy (DRS) were used to characterize hematite and goethite in a suite of bulk and particle-sized fractioned samples from the Chinese Loess Plateau, representing the main changes in magnetic mineralogy over the past 22 Myr. The record of DRS-defined hematite largely reflects variations in the pedogenic fraction (<2 pm). By contrast, the detrital fraction (>8 pm) makes the main contribution to goethite concentrations in the bulk samples. Pedogenic goethite concentrations generally vary inversely with those of hematite. The lack of consistent correlations between HIRM-inferred mineral content and DRS measurements casts doubt on the validity of using HIRM to characterize the concentrations and relative proportions of hematite and goethite in sediments representing a long time span or a wide grain size distribution. The progressive decline in the coercivity of the antiferromagnetic minerals in bulk samples throughout the whole period is mainly an expression of changes in the detrital fraction, reflecting changes in the source region, possibly involving increased aluminum substitution. The good correlation between DRS-defined hematite and frequency-dependent magnetic susceptibility in several chronologically defined subgroups of samples is consistent with the hypothesis that maghemite forms a transitional phase in a weathering sequence from ferrihydrite to hematite. The results shed new light on using HIRM to characterize hematite and goethite in sediments and on the history of weathering and climate change over the past 22 Myr in the dust source regions and on the Loess Plateau.