Distinguishing Calcite With and Without Biomarkers Using Laser-Induced Breakdown Spectroscopy (LIBS), Guadalupe Mountains, New Mexico

Abstract

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 can then be analyzed to determine both the type and abundance of elements present in that sample. 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 transported to New Mexico State University and examined using a Hitachi T-1000 scanning electron microscope (SEM) to look for microbial textures such as rods, filaments, cocci, and biofilms. One-hundred shots per sample were taken using LIBS. The spectra collected using LIBS were used to construct and train a model using the multivariate technique of partial least regressive squares (PLSR) and the SEM data were used to validate this model. Using these techniques, we were able to successfully discriminate samples with microbial textures vs. inorganic textures with a 78% success rate for Cottonwood Cave and a 90% success rate for Black Cave.