Minerals and Microbes in Lehman Caves

This article was originally published in The Midden - Great Basin National Park: Vol 24., No.1, Summer 2024.

The Gypsum Annex section of Lehman Caves is known for its very white walls, ceiling, and floors. This section of the cave holds some of the answers to the speleogenesis (cave formation) of Lehman Caves.

NPS/G. Baker

By Zoë Havlena, researcher, New Mexico Institute of Mining and Technology

Lehman Caves is world famous for its many shields and other spectacular speleothems. But it also has other, less conspicuous minerals that hold important clues about the history of the cave. In 2018 and 2019, we sampled numerous mineral deposits as part of a collaboration with cave expert Dr. Louise Hose (University of Nevada, Reno). Dr. Hose recently showed that many of the morphological features of Lehman Cave, especially those in the protected Gypsum Annex passage, indicate Lehman Caves originally formed through a rare process known as hypogene speleogenesis. This means the cave was made from deep groundwater rising up in the subsurface and “hypogene” refers to this bottom-up process (“speleogenesis” means cave formation). When these ancient waters have high concentrations of hydrogen sulfide, we call this “sulfuric acid speleogenesis” (SAS). Among other corrosion features and evidence, sulfuric acid speleogenesis often leaves behind an abundance of the sulfate mineral gypsum (CaSO₄•2H₂O). Dr. Hose wrote an article in the Summer 2018 article of The Midden which describes the SAS and Lehman Caves development in greater detail.

Although the Gypsum Annex is named because of the white-colored minerals that line the passage, no one had previously measured the amount of gypsum in these deposits or characterized the other minerals in this part of the cave. We wanted to understand if this gypsum was formed during early sulfuric acid speleogenesis, which Dr. Hose hypothesizes may have happened over 10 million years ago, or if it was deposited more recently.

Using mineralogical techniques such as powdered X-ray diffraction, electron microprobe analysis, and scanning electron microscopy, we were able to measure gypsum and other minerals throughout the Gypsum Annex. While many of the Gypsum Annex mineral samples contained gypsum, not all of it did. We also used a method called stable sulfur isotope analysis to further analyze the sulfur in the minerals. Gypsum and other sulfur minerals have a mixture of different types of sulfur atoms called “isotopes”, and because different biological and physical processes can cause change the amount of one stable isotope over another, the ratios of isotopes can help show the source of the sulfur. This analysis told us that the gypsum does not appear to be from the earliest phase hypogenic cave formation, but more likely was deposited later in time during one or more periods of infiltration from waters on the surface.

A gypsum flower in the Gypsum Annex of Lehman Caves, Great Basin National Park.

NPS/G. Baker

We also discovered other unexpected minerals, such as trace amounts of metatyuyamunite, which is a yellow, uranium-vanadium (Ca(UO₂)₂(VO₄)₂•3-5H₂O) mineral crust seen in other ancient hypogene caves around the world.

Modern techniques like DNA sequencing help us learn about microorganisms inhabiting an environment like caves. Not much is known about who might be living on mineral surfaces in systems such as Lehman Caves, because Lehman and other arid caves are very nutrient limited (“oligotrophic”). All of our samples from the Gypsum Annex have very low biomass, meaning that it is hard even for microbes like bacteria and archaea to find enough energy to survive in that part of the cave. These samples, which include gypsum and some of the other mineral types like dark-colored “corrosion residues” and soft “punk” rock had different microorganisms compared to surfaces along the tour route in Lehman Caves—perhaps because the latter is closer to the entrance and more human-impacted. We also found some unusual taxa of bacteria and archaea such as Nitrosococcales and members of Thaumarchaeota that might be using inorganic nitrogen compounds as their main energy source.

This project was a valuable contribution to our knowledge of ancient hypogenic caves and microbial colonization of cave-hosted minerals. It highlights the importance of keeping systems like Lehman Caves protected for future scientific discoveries.

These results were recently published in the journal Geobiology (Havlena et al., 2024 DOI: 10.1111/gbi.12594).
Havlena, Z. E., Hose, L. D., DuChene, H. R., Baker, G. M., Powell, J. D., Labrado, A. L., Brunner, B. & Jones, D. S. (2024). Origin and modern microbial ecology of secondary mineral deposits in Lehman Caves, Great Basin National Park, NV, USA. Geobiology, 22(3), e12594. https://www.biorxiv.org/content/10.1101/2023.08.15.553329v1.full.pdf