High Resolution Time Series Cave Ventilation Processes and the Effects on Cave Air Chemistry and Drip Waters: Speleoclimatology and Proxy Calibration
Abstract
Continuous high resolution (sub-hourly), long-term (Nov 2007-present) monitoring of cave air chemistry (Temperature, Relative Humidity, Barometric Pressure, Radon-222, CO2, Air flow, Wind speed and direction) in a shallow subtropical cave (Hollow Ridge) in N Florida reveals two major ventilation mechanisms: 1) ventilation driven by winds across the cave entrances, and 2) ventilation driven by density differences between atmospheric and cave air. The degree and type of ventilation strongly influence the 222Rn and CO2 of cave air, which in turn affects the timing and extent of calcite deposition in speleothems. The degree of ventilation is estimated using a cave air CO2-δ13CO2 Keeling Plot, or a simple radon deficiency model. Results show cave air has an atmospheric component ranging from 10-90%. During fall and winter, average CO2 (700 ppmv) and 222Rn (50-100 dpm/L) are lower than in spring and summer (CO2 = 1200 ppmv; 222Rn = 1000 dpm/L) due to increased winter ventilation. Decreased ventilation during the summer allows CO2 and 222Rn levels to rise. Winter daily ventilation is primarily a function of density gradients between cave air and atmospheric air, while summer daily ventilation is primarily a function of late morning NW-NE winds above the cave. Stable isotope analyses of drip water (fracture drip and pore flow drip) and aquifer water from Hollow Ridge agree with previous isotope studies of drip water at Florida Caverns State Park, 2 km to the NE. During summer, isotopic composition of pore flow drip water (δ18O -3.8 to -4.0 per mil; δD -17.3 to -20.2 per mil VSMOW) and aquifer water (δ18O -4.0 per mil; δD -18.0 to -21.1 per mil) are similar to average annual weighted isotopic composition of precipitation (δ18O -3.6 per mil) while fracture drip waters (δ18O -3 to -3.4 per mil; δD -11.9 to -14.3 per mil) likely reflect the isotopic composition of individual precipitation events. Pore flow drip waters δ18O are weakly correlated with drip rates (enriched δ18O during periods of higher drip rates) but show no correlation to precipitation amount. Knowledge of the type of drip flow is important when considering stalagmites for paleoclimate studies. A significant decrease in drip rate was observed from June (1034 drips/hour) through August 2008 (34 drips/hour). Higher water demands during summer months with increased evapotranspiration may be responsible for this decrease. A semi-diurnal drip rate cycle, negatively correlated with barometric pressure, is also observed throughout the period. This strong negative correlation is hypothesized to be controlled by atmospheric tidal oscillations. Observations into the fall and winter seasons should reveal seasonality, if any, and if there is an evapotranspiration effect present in the water cycle. High resolution studies of cave air chemistry and ventilation processes will enhance knowledge of the timing, extent, and isotopic and chemical composition of calcite deposition. When combined with drip water and precipitation isotope analyses, these studies will improve the understanding and interpretation of high- resolution (sub-annual) speleothem paleoclimate records.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP51C1521K
- Keywords:
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- 3344 Paleoclimatology (0473;
- 4900);
- 4914 Continental climate records;
- 4958 Speleothems;
- 4994 Instruments and techniques