Speleothem Paleoclimatology and Modern Speleochemistry Proxies

Modern (farmed) calcite in Hollow Ridge Cave, Marianna, FL, USA was cultivated seasonally and analyzed for isotopic composition (δ18O and δ13C). Simultaneous in situ cave-air chemistry, drip water composition, and micrometeorology were continuously monitored to unravel the connection between the depositional timing and isotopic composition of calcite and ventilation-driven changes in cave-air CO2. Multi-year collection of soil gas, atmospheric and cave-air CO2 samples and drip waters allow the prediction of dripwater calcite saturation state, and thus speleothem precipitation, as a function of cave-air pCO2. This study demonstrates that high summer-time cave air pCO2 inhibits calcite growth, and that deep interior Hollow Ridge Cave speleothems preserve winter-time paleoclimate signals. This work also demonstrates the existence of a relationship between ventilation, evaporation and rapid CO2 loss on modern Hollow Ridge Cave calcite. Furthermore oxygen isotopic composition of Hollow Ridge calcite exhibits a constant positive offset from both the O'Neil et al. (1969) and Kim & O'Neil (1997) inorganic calcite temperature equations, allowing the construction of a new temperature equation for speleothem calcite that describes both temperature and ventilation fractionation effects on speleothem precipitation. This study illustrates the value of investigating site-specific drip water and cave-air chemistry before making speleothem-based interpretations of paleoclimate. This study has also provided a valuable link between rainfall, epikarst saturation state, and modern drip water residence time and x/Ca element partitioning in both water and calcite. It was observed that each site of calcite growth inside HRC has a unique chemistry in terms of x/Ca vs. Mg/Ca ratios. The data at each site are representative of a two endmember mixing system; (1) calcite dissolution during wetter epikarst periods, and (2) rainfall dilution. Sites can clearly be chemically grouped which suggests that down-slope hydrology and water residence time partially control the chemistry of drip water at HRC. Distribution coefficients were established for HRC calcite, and by comparing x/Ca vs Mg/Ca in drip waters with x/Ca vs. Mg/Ca in calcite it was determined that Li, Sr, K, U and Si are suitable modern-day solid-phase indicators of the hydrological conditions in the epikarst above HRC. This study also established that Na, Mn and Ba are not suitable standalone paleohydrologic proxies, but may be useful indicators when used together.