MINERALOGY AND MICROBIAL ECOLOGY OF SEDIMENTS FROM LEHMAN CAVE, GREAT BASIN NATIONAL PARK

Some of the world’s largest and most spectacular limestone caves, including Carlsbad Cavern and Lechuguilla Cave, formed by sulfuric acid speleogenesis (SAS). These caves develop in carbonate-hosted voids where hydrogen sulfide (H₂S)-rich ground waters interact with oxygen in recent surface recharge or cave air. Although “active” near-surface sulfidic caves are rare and are limited to a handful of sites worldwide, “fossil” sulfidic caves that contain morphological and mineralogical evidence for past sulfidic acid speleogenesis are more common. Ancient sulfidic caves are of interest in part because relict gypsum deposits are a target of astrobiological focused biosignature research.

Lehman Caves in Great Basin National Park, NV, is a hypogenic cave with some passages that preserve features consistent with a sulfidic past (Hose and Havlena, 2019). Until recently, however, little was known about the geological history and evolution of the cave system, and the geomicrobiological processes that affect the modern cave have not yet been explored. Here we used microbiological and geochemical analyses to evaluate the origin of gypsum and other sediments within the cave and characterize the extant microbial communities. We used X-ray diffraction and electron microscopy to map the mineralogical character of floor and wall deposits in the cave, including allochthonous clastic sediments, ferromanganese wall crusts, and white floor and wall deposits from the Gypsum Annex region. The white deposits are composed of microcrystalline calcite, dolomite, quartz, and gypsum, where gypsum was found only in certain sections of the Gypsum Annex. Microbial biomass in most of these cave sediments is very low, although enrichment culturing on R2A media indicates that some organoheterotrophic populations are present. High-throughput 16S rRNA sequencing showed that several potential N cycling taxa were among the most abundant community members, indicating that reduced N compounds support microbial growth in this environment. We will present additional results from ongoing work using rRNA methods, as well as new isotopic and geochemical evidence for how these sediments compare to similar deposits from better-studied ancient sulfidic caves like Lechuguilla and Carlsbad Cavern.

Hose, L.D., and Havlena 2019, Speleogenesis and Geology of Lehman Caves, Great Basin National Park, Eastern Nevada USA. Geological Society of America. https://doi.org/10.1130/abs/2019am-341141