CAVE MONITORING AND DRIPWATER SAMPLING TO SUPPORT INTERPRETATION OF A LONG SPELEOTHEM RECORD FROM WHITEROCKS CAVE, UTAH

Whiterocks Cave, with a mapped length of 800 m and a vertical extent of 20 m, is a solution cave developed in Mississippian-age Madison Limestone in the Uinta Mountains of Utah. Air temperature and relative humidity have been monitored in the cave for several years, and a specially designed carousel sampler was installed to collect bi-weekly dripwater samples in 2018. Air temperature in the cave averages 6 °C and reaches an annual minimum in late February/early March. Relative humidity is steady at 100%. Drip rates are low and stable for much of the year, with values <200 per minute, but feature a dramatic increase at the beginning of May to 1000 drips per minute (or higher) before dropping back down in mid-summer. Values of δ¹⁸O_vsmow in dripwater samples average -15.5‰, reaching -15.8‰ in late winter, and -15.3‰ in mid-summer. These results are useful in considering long stable isotope records developed for speleothems from this cave. Preliminary age models for 3 of these features, established using 31 ages, range from 118 ± 0.6 ka BP to the limit of U-Th dating (~600 ka BP). Five ages demonstrate that one stalagmite grew during Marine Isotope Stage (MIS)-5 (118-126 ka BP). In contrast, the uppermost part of another accumulated during MIS-11 (or possibly MIS-9) and lower parts of this sample represent deposition during MIS-13 and MIS-15. The third sample grew intermittently between MIS-5 and MIS-11. Despite the preliminary nature of the age control, this pattern strongly suggests that these speleothems only grew during the six interglaciations from MIS-5 to MIS-15. Given the elevation of the cave and geomorphic evidence for extensive alpine glaciation nearby, it is likely that permafrost existed above the cave for most of the past 600,000 years, blocking water infiltration and limiting speleothem growth to peak interglacial conditions. The average modern δ¹⁸O_vsmow of dripwater (15.5‰) at a temperature of 6 °C would yield a calcite δ¹⁸O_vpdb of around -13‰, which is similar to the values for MIS-5, 7, 9, and 11, but higher than the average value of -14‰ in MIS-13, suggesting a difference in air temperature or moisture source during that interglacial. Future efforts will focus on refining the age models for these speleothems to better constrain the connection between interglacial conditions and stalagmite growth.