A Novel Approach of Semi-Quantifying Gypsum in Sedimentary Rocks by Visible and Near-Infrared Diffuse Reflectance Spectroscopy

Gypsum is one of the major evaporite minerals that can be used as a sensitive archive for past climate change, revealing variations in the hydrological cycle and dry-wet climatic alternations. Measuring variations in gypsum content, rapidly and with high accuracy, will contribute to more extensive paleoclimate reconstructions. However, a fast, convenient, and non-destructive method for quantifying gypsum in sedimentary rocks is still lacking. Diffuse reflectance spectroscopy (DRS) is a commonly employed technique for rapidly identifying and quantifying minerals; however, its application to semi-quantifying gypsum in paleoclimate studies has received comparatively less attention. Here, we synthesized artificial and natural samples with precisely controlled gypsum content and analyzed the visible and near-infrared DRS characteristics of the gypsum crystals. We report a linear correlation between gypsum content and the intensity of negative peaks in the DRS spectra at 1,540 nm (I₁₅₄₀) and 1,942 nm (I₁₉₄₂), converted by the second derivative of the Kubelka-Munk (K-M) remission functions. The robustness of this I₁₅₄₀/I₁₉₄₂-based DRS proxy (I_gyp) for semi-quantifying gypsum in sediments was further validated against powder X-ray diffraction results. Applying the new I_gyp proxy to the Eocene gypsum sequence from the Xining Basin (China) shows a strong correlation with calibrated gypsum contents, demonstrating its potential for the identification and quantification of gypsum in sedimentary rocks. This method offers a promising new approach for gypsum analysis in paleoclimate studies.