Speleothems in Utah caves record past dynamics of mountain permafrost

Abstract Mountain permafrost is a climatically sensitive but poorly constrained component of the terrestrial cryosphere. We use 108 U-Th dates from speleothems in two limestone caves in the Uinta Mountains of northern Utah, USA, to reconstruct a 600,000 year history of permafrost presence and absence in this alpine setting. Speleothem growth in both caves is confined almost entirely to discrete intervals that align with interglacial conditions of Marine Isotope Stages (MIS) 5e, 7e, 9e, 11c, and 13. In contrast, growth hiatuses correspond to glacial periods, when permafrost apparently inhibited infiltration of liquid water. Regional lapse rates indicate that cooling of ∼3°C to 5°C relative to present would be sufficient to generate permafrost above the caves. The persistence of age clustering across multiple speleothems in two independent cave systems suggests that mountain permafrost repeatedly formed and degraded in concert with orbitally paced climate cycles. Comparison with nearby paleoclimate records confirms that these changes reflect regional-scale climate forcing. Our results provide rare empirical evidence for the extent of alpine permafrost over Quaternary timescales. As mountain permafrost degrades under modern warming, such records are essential for refining climate models, assessing geomorphic and hydrologic risks, and placing recent changes within the context of natural variability.