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

Abstract

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 (Igyp) for semi-quantifying gypsum in sediments was further validated against powder X-ray diffraction results. Applying the new Igyp 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.