Seasonal variations of cave conditions and drip water stable isotopes from a monitoring study of Raccoon Mountain Caverns, Tennessee, and its implications in interpreting speleothem record

Abstract:

A two-year monitoring study at Raccoon Mountain Caverns near Chattanooga, Tennessee was carried out in an attempt to establish quantitative relationships between climate signals and drip water stable isotopes for interpreting speleothem paleoclimate records from the cave. Eight field trips were made from Jan. 2014 to Jun. 2015, during which cave meteorological conditions (RH, temperature and cave air CO₂ concentration) and drip rate were measured for 5 sites inside the cave. 63 cave drip and pool water samples were collected and analyzed for oxygen and hydrogen isotope compositions (δ¹⁸O and δD values). Cave air temperature varied throughout the study period, the temporal variations ranged at different sites from 2 to 8.4 °C (the greatest variation was observed at sites that are closer to the entrance or surface). These are significantly less than outside temperatures range of 24 °C, but more than observed in other monitored caves. Elevated cave-air CO₂ concentration (3200 ppm) and slow drip rate during the summer indicated slowed or stalled growth of calcite. The overall range of δ¹⁸O values were -7.1‰ to -4.5‰. A δD vs δ¹⁸O diagram yields a slope of 6.1, which falls within the normal range of 6-8 for local Meteoric Water Line. The value is slightly above Global Meteoric Water Line, indicating lack of evaporative effect. Throughout the study period, the δ¹⁸O values varied from 0.6 ‰ at some sites to 1.9‰ at others. The largest changes were likely due to the close proximity of collection sites to the surface precipitation. Spatially, for samples collected at each cave trip, different sites displayed variations of δ¹⁸O values from 0‰ to 1.7‰. The difference could be attributed to different type of drip sites with varying types of flow paths rainwater takes to the drip sites. The significant seasonal shift of drip water δ¹⁸O values and growth conditions indicate importance of consideration of seasonality in interpreting speleothem δ¹⁸O record from this cave.