Quantifying Soil Goethite/Hematite Ratios: A New Method Based on Diffuse Reflectance Spectra

Abstract

Goethite/hematite ratios in soils are widely used to reconstruct past climatic changes, but their accurate measurements have remained challenging due to the matrix effect. Here we present a new method for measuring soil goethite/hematite ratios by characterizing the band position of electron pair transition (EPT) based on the diffuse reflectance spectra processed by continuum removal. We introduce a “half-band-area wavelength”, λ1/2, to characterize the EPT band position and validate the method using synthetic standards made from mixing pure goethite and hematite and four sets of goethite- and hematite-free matrices derived from arid aeolian deposits and tropical saprolite. A consistent λ1/2-goethite/hematite relation across four matrices demonstrates that our method eliminates the matrix effect. We further validate our method by applying it to quantify goethite/hematite ratios in 180-Kyr loess-paleosol sequences and comparing the results to the reported paleoclimatic records. Our new method is promising for improving the measurement of goethite/hematite ratios in sediments.