COMBINING HYPERSPECTRAL IMAGING, X-RAY DIFFRACTION, AND PETROGRAPHY TO INTERPRET STALAGMITES FOR PALEOCLIMATE RECONSTRUCTION

Stalagmites are excellent paleoclimate archives. Here, we use hyperspectral imaging (HSI), a non-destructive technique to identify mineralogy, organic matter content, and laminations in stalagmite collected from Anjokipoty Cave, NW Madagascar. It is the first high-resolution spectral analysis using Visible-Near infrared (VNIR) and Short-wave Infrared (SWIR) with wavelengths ranging from 400 to 2500 nm. Different false color composites, using red-green-blue combinations, were used to visualize changes across the stalagmite. HSI results were compared with spectral reflectance data using Analytical Spectral Devices (ASD) Spectroradiometer. Petrographic analyses of oversized thin sections along with X-ray diffraction (XRD) analyses of selected intervals were done to cross-validate the HSI variations.

HSI results suggest that aragonite is the dominant mineralogy of the upper part of the stalagmite (0-100 mm), while calcite is more prevalent in the bottom part (100-160 mm). These findings are in general good agreement with the XRD data. Analyses of the XRD pattern suggest that phase percentages of aragonite at depths 15 mm, 45 mm, 75 mm, and 85 mm are 89%, 52%, 96%, and 95%, respectively. XRD pattern at depths ranging from 105 mm to 135 mm shows that the mineral composition is 100% calcite. Petrographic analysis reveals that aragonite crystals change from needle-like radiating rays to a fan-like structure at the depth of 0 to 60 mm with few calcite mosaics at ~55 mm depth. At the depth of ~80–90 mm, a continuous growth of needle-like aragonite crystals was observed. Calcite crystal examined at the depth of 105 to 135 mm shows a wavy and irregular structure, possibly showing some signs of bacterial or microbial activities. This HSI method additionally allowed us to identify clay minerals, illite, which appear like clay mineralogy under the microscope. This method could be useful while developing a high-resolution non-destructive dataset for paleoclimate reconstruction and insights for future studies of stalagmite.