SEASONAL FLUCTUATIONS OF SUBSURFACE CONDITIONS IN RUSHMORE CAVE, KEYSTONE, SOUTH DAKOTA

Air in cave systems experiences seasonal fluctuations in CO₂ concentration, temperature, and humidity—Rushmore Cave in Keystone, South Dakota is no different despite being heavily altered by tourism. It was hypothesized that the CO₂ concentration would decline in the winter due to increased ventilation efficiency between the surface and subsurface. For the same reason, temperature was expected to decrease during the winter. Humidity was anticipated to increase after precipitation occurred. Gas samples were collected in duplicate using a 30 mL syringe and injected into septa-capped vials twice per month from October 2019 through March 2020. Additional samples were collected in June and July 2020 when cave access was available during the Covid-19 pandemic. CO₂ concentrations were analyzed using an infrared gas analyzer. Kruskal-Wallis H tests and Friedman tests were done to analyze the variability in CO₂, temperature, and humidity. The CO₂ concentration within Rushmore Cave was found to decrease by 38.5% from October 2019 (3045 ppm) to a minimum of 1873 ppm in March 2020. CO₂ concentrations increased again to a maximum of 5201 ppm in July 2020. The combination of reduced ventilation efficiency and respiration of tourists during the summer likely increased the CO₂ concentrations. Temperature in the cave decreased in the winter and rose in the summer. Humidity was lowest in autumn and summer, while it was most variable and had the highest values in spring. Humidity was found to be higher in rooms further from the entrance and exit of the cave. Temperature appeared to decrease around one month after surface temperatures decreased during late autumn. Humidity increased several weeks after significant precipiation occurred. When cool air flowed into the cave during the winter, the air temperature was frequently below the dew point, which would increase humidity. CO₂ concentration, temperature, and humidity all had statistically significant variation with respect to time. These seasonal variations in the cave also impact speleothem precipitation. Carbonate precipitation will occur most readily when CO₂ concentration are low in the air. This means speleothem precipitation will primarily take place in the winter, making their use as a paleoclimate proxy biased towards winter temperatures rather than a year-round average.