A Long-term Speleothem-Based Record of Permafrost Presence-Absence in the Mountain Critical Zone

Caves provide unique access to deeper levels of the Critical Zone (CZ), and speleothems formed within caves preserve dateable records of past CZ process. This study used a combination of modern cave climate monitoring, dripwater sampling, and geochemical investigation of speleothem records to consider how the mountain CZ in part of Utah, USA has changed over time. The study site, known as Whiterocks Cave, is a solution cave 800 m long developed in Mississippian-age Madison Limestone in the Uinta Mountains. At its deepest accessible point, the cave is ~150 m below the ground surface. Air temperature and relative humidity have been monitored in the cave for many years, and custom-built sequential samplers have been deployed to collect dripwater. Results reveal that air temperature in interior parts of the cave averages 5.5 °C, varies only a few tenths of a degree over the course of a year, and reaches an annual minimum in late winter/early spring. Relative humidity is steady at 100%. Drip rates are low and stable for much of the year, with values ~20 drips per hour, but exhibit a dramatic increase at the beginning of May before dropping back down in mid-summer. The timing of this annual peak in drip rate matches the main pulse of snowmelt on the upload above the cave. Values of δ 18 O vsmow in dripwater average -14.6‰ ( d -excess of 8.2‰) and exhibit only minor variability through the year, implying a well-mixed aquifer in the epikarst above the cave. These results are useful in considering stable isotope records developed for 11 speleothems collected from the cave. Dating of these features yielded ages (n=78) ranging from ~80 ka to the limit of U/Th dating (~600 ka). Notably, ages are strongly clustered within interglacial stages, implying that speleothems only grow in Whiterocks Cave under interglacial climate conditions. Given the elevation of the cave, a plausible explanation for this pattern is that permafrost existed in the CZ above the cave for most of the past 600,000 years, blocking water infiltration and prohibiting speleothem growth. Only under peak interglacial conditions, such as those characterizing the Holocene, is this permafrost absent, allowing snowmelt to recharge the aquifer and deliver dripwater to the cave passages. This study provides an example of how cave science can inform understanding of past CZ process.