PROBING THE ORIGIN AND MODERN MICROBIAL COLONIZATION OF GYPSUM SEDIMENTS IN LEHMAN CAVES, GREAT BASIN NATIONAL PARK, NV, USA

Lehman Caves is set in mylonitic marble in the Southern Snake Range of Eastern Nevada, USA, and is part of Great Basin National Park. Recent morphological evidence described by Hose et al.¹ indicate abundant features within the Gypsum Annex consistent with a hypogenic and sulfuric acid origin, contrasting earlier and limited descriptions of the cave. Hose et al. propose a two-stage speleogenetic model with sulfuric acid speleogenesis (SAS) as the initial dissolution event followed by a later stage of meteoric water overprinting most of the cave. In this scenario, gypsum deposits in the cave would have been hotspots for microbial life at the time they formed, as is seen in active sulfidic caves such as Frasassi Caves, Italy and Cueva de Villa Luz, Mexico. We sought to determine if sediments in the Gypsum Annex are part of the proposed sulfuric acid origin versus a later-phase feature, and whether they serve as a repository for past or present microbial life. We first characterized the mineralogy of the sediments throughout Lehman using powdered X-ray diffraction spectroscopy (pXRD), and for select samples, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). To determine if the white sediments (gypsum and calcite) found in the Gypsum Annex were related to an SAS phase or if they precipitated during a later phase of speleogenesis, we examined the δ³⁴S values of gypsum and carbonate associated sulfate. We then sought to determine if these sediments host microbial life and how they compare to microbial communities observed in active SAS systems. Microbial biomass in most of the sediments is very low, although 16S rRNA gene libraries suggest segregation of microbial species within different passages of the cave, and that inorganic N compounds are important energy resources for extant cave communities. Petrographic analysis of thin sections (in progress) will be used to evaluate diagenetic processes in the white deposits and look for potential traces of past life.

1. Hose, L.D., DuChene, H.R., Jones, D., Baker, G.M., Havlena, Z., Sweetkind, D., and Powell, D., 2021, Hypogenic karst of the Great Basin, in Florsheim, J., Koeberl, C., McKay, M.P., and Riggs, N., eds., 2021 GSA Section Meeting Guides: Geological Society of America Field Guide 61.