HOLOCENE CHANGES IN WINTER SNOWPACK SUBLIMATION RECORDED BY A SUBTERRANEAN CRYOSPHERE ARCHIVE IN NORTHERN UTAH, USA

Bear Ice Cave (BIC) is entered through a collapse doline at an elevation of 2900 m asl on the south flank of the Uinta Mountains in Utah, USA. An average air temperature of 2.5 °C was measured outside the cave between October, 2022 and September, 2024; however due to cold air trapping, temperatures on the floor of the cave are perennially subzero. As a result, the cave contains an accumulation of perennial ice more than 3 m thick containing breakdown and organic debris, with numerous unconformities. In 2022, wood fragments and Neotoma fecal pellets were radiocarbon dated to constrain the age of the ice, and 125 ice samples were collected to span the entire top to bottom extent of the accessible ice mass. Samples were also collected from icicles on the cave ceiling in June, 2024. Melted ice samples were filtered before analysis for stable isotopes with a cavity ring-down spectrometer and hydrochemical characterization with ICP-MS. Radiocarbon results reveal that the oldest accessible ice accumulated before ~5500 BP, and the upper meter of ice accumulated ~700 BP. Accumulation was not constant, rather the prominent unconformities testify that accumulation was interrupted repeatedly by episodes of ablation that nonetheless failed to eliminate the ice body entirely. Data from a nearby SNOTEL site reveal that the soil in this area is only saturated for a few weeks in May each year. This means that appreciable amounts of water can only transit the epikarst and reach the cave as a pulse associated with annual snowmelt, and the ice within the cave represents an archive of winter precipitation. However, values of δ¹⁸O in the ice (mean of -15.4‰) and the modern icicles (-14.2‰) are notably higher than winter precipitation at the cave site estimated from the Online Isotopes in Precipitation Calculator (-22‰) and measured in snow samples (-19‰) collected during the spring of 2022. That offset suggests that sublimation from the snowpack during winter shifts the integrated stable isotope signature of the snow before it melts into the ground. Furthermore, values of δ¹⁸O are generally higher in the younger ice, and d-excess correspondingly lower. This transition suggests that the influence of sublimation became more important later in the Holocene. Sublimation is enhanced by lower relatively humidity and warmer temperatures. The ice in BIC may record, therefore, a trend in the late Holocene toward colder winters with lower humidity and extended snow cover duration, allowing for greater cumulative sublimation effects.