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.

A New Perspective on the Applicability of Diffuse Reflectance Spectroscopy for Determining the Hematite Content of Fe-Rich Soils in the Tropical Margins of China

“…Long et al [6] accurately estimated the content of natural hematite using an equation based on the reflectance percentage of the red band, eliminating the need for time-consuming CBD treatment. Additionally, spectral experiments with a wider range of wavelengths (400-2400 nm) have been applied to loess [22], and, recently, Meng et al [23] proposed a new method for determining the ratio of goethite to hematite that avoids the influence of soil matrix compositions and applied it to the Luochuan loess sequence. Different soil matrices represent different mineral categories and contents of non-measured target minerals.…”