High Resolution Microclimate Study of Hollow Ridge Cave

Long term, near continuous, in situ monitoring of cave meteorology, cave aerochemistry, and surface meteorology allows quantitative assessment of cave ventilation and the effect on CO₂ outgassing from cave systems. Because advances in mass spectrometry methods have lowered required sample sizes and increased accuracy for U-series age and carbonate stable isotope analyses, speleothems now have the potential to produce sub-annual paleoclimate records spanning tens of thousands of years (Dorale et al., 2002; Fairchild et al., 2006; Frappier et al., 2007; Lachniet, 2009). However, the need arises for long-term in situ monitoring of the cave environment to create multiple proxies that can be applied to speleothem geochemical records for more accurate interpretations of these records. Focusing on how the local precipitation signal transfers to the cave environment through the epikarst is essential to understanding how variations in precipitation affect drip water hydrochemistry, and ultimately the speleothems precipitated from these drip waters. However, an often overlooked subject is the role cave ventilation has on speleothem formation, and this research focuses on ventilation regimes and their affects on the CO₂ cycle in the cave system. Cave meteorology, cave aerochemistry, and surface meteorology were measured from October 2007 to March 2009 at Hollow Ridge Cave, FL. Cave meteorology follows a similar, but greatly damped, pattern of temperature and barometric pressure as surface meteorology. Continuous measurements of cave air radon-222 and CO₂ indicate that ventilation primarily occurs via gravitational overturn. Strong seasonal patterns were observed in cave air radon-222 and CO₂ concentrations, with decreased ventilation in the summer allowing concentrations to rise, while increased ventilation in winter keeps concentrations near outside atmospheric values. A model developed to estimate CO₂ outgassing indicates greater CO₂ outgassing in the summer and fall than the winter, primarily due to increased CO₂ transport to the cave environment from the soil zone, where rapid degradation of organics increases soil CO₂ production. These results have been submitted and accepted in the form of a manuscript (Chapter 3) to Earth and Planetary Science Letters (Kowalczk and Froelich, 2009). Continuous records of drip rates in the cave suggest the thin overburden results in water residence times of approximately two weeks in the epikarst. However, this residence time is short enough to ensure complete mixing of infiltration waters because analyses of drip waters reveal little variation in their isotopic composition (< 0.3‰ δ18O and < 8‰ δD), while isotopic variations of up to 6‰ δ18O are observed in local precipitation over similar periods. Also, the isotopic composition (δ18O and δD) of aquifer water sampled from Hollow Ridge Cave is lighter than drip waters, indicating present rainfall is isotopically heavier than rainfall over the past 30 years, the average age of water in the North Florida Aquifer (Davis and Katz, 2007). Isotopic, trace element, computed tomography, color, and age analyses of three speleothems collected from Brooks Quarry Cave, FL indicate little variation in the δ18O of precipitation from 70 ka to the present. However, poor chronology (radiocarbon dates) prevent comparison of isotopic, trace element, and color scan records from samples BC2 and BC3 to pollen records over the past 40 ka. Nevertheless, absolute U-series dating of sample BC1 suggest the approximate 3000 year isotopic record (69 Ka) may have recorded Dansgaard-Oeschger Event 19. The ä13C records from these speleothems suggest either large shifts in the overlying vegetation composition (possibly from forest type to grassland/prairie type) or variations in cave ventilation processes that in turn affect drip water CO₂ degassing processes. U-series dating of these samples will allow accurate comparison to pollen records from pond and lake sediment cores in north Florida, and will help construct a more accurate climate history of north Florida.