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

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, ?(1/2), 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 ?(1/2)-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.

Automated summary

A new method for measuring soil goethite/hematite ratios is presented, which utilizes the band position of electron pair transition (EPT) in diffuse reflectance spectra processed by continuum removal. The method is validated and shown to effectively eliminate the matrix effect. Application of the method to loess-paleosol sequences confirms its potential for improving the measurement of goethite/hematite ratios in sediments.