DISTINGUISHING CALCITE SAMPLES WITH AND WITHOUT BIOMARKERS USING LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS), GUADALUPE MOUNTAINS, NEW MEXICO

In a cave environment, speleothems may form by inorganic precipitation of calcite or by precipitation through microbial action. Laser-Induced Breakdown Spectroscopy (LIBS) is an analytical technique that may be used to differentiate between these two types of speleothems. LIBS is a type of atomic emission spectroscopy that uses a high-powered laser to generate an ablation plasma on a sample surface. The emissions from the ablation plasma contain information about the concentrations of all elements, several isotopic ratios, and structural state of the material. These data can be used to differentiate between organic and inorganic calcite samples. LIBS has several advantages over traditional analytical techniques in that it offers a rapid, portable, and in-situ method of analyzing samples in the field.

This study uses LIBS to analyze a total of 40 samples collected from two caves in the Guadalupe Mountains of southeastern New Mexico. Under permit from the U.S. Forest Service 20 samples were collected from Cottonwood Cave and another 20 samples from Black Cave. Samples were selected to include speleothems suspected of being both biologic and non-biologic in origin and include stalactites, soda straws, calcite gravels, cave popcorn, pool fingers, and u-loops. Samples were examined by SEM to look for microbial textures such as rods, filaments, spheres, and biofilms. SEM observations and LIBS spectra are used to train a model using the multivariate technique of Partial Least Squares Regression (PLSR). This model can then be used to differentiate biologic from non-biologic samples. Test-set model validation is performed using samples that were reserved and not used in training.

This study is a follow up to a 2013 study (Chavez et al., GSA Abstracts with Programs, v., 45, no. 7, p. 777) that used LIBS to distinguish between calcite samples with and without biofilms from Ft. Stanton cave, southeastern New Mexico. The Fort Stanton cave project achieved a success rate of 92%, indicating the potential use of LIBS in this application. The current study addresses the question of the influence of groundwater on speleothem chemistry and if the calibrations for one cave will work correctly with other caves. This method could be deployed to map locations of fossil microbial colonies.