Speleoclimatology of a Wild Florida Cave: the Present is Key to the Past
Abstract
Speleoclimatology is an emerging field of cave paleoclimate research. Understanding carbon dioxide degassing, calcite precipitation, and how chemical and isotopic information transfers from the atmosphere to drip water to cave calcite is critical to interpretations of paleorecords in dripstones. We have instrumented multiple micro- meteorological time-series stations in a wild cave in Northwest Florida (Hollow Ridge) near Marianna and the Chipola River. Each station continuously records temperature, barometric pressure, relative humidity, 2D acoustic airflow, drip rates, air radon and carbon dioxide. Hollow Ridge Cave is a Southeastern Cave Conservancy, Inc. (SSCi) preserve with 4 entrances and 3370 feet of mapped passage located at the contact of the early Oligocene Marianna Limestone and Bumpnose member of the Crystal River formation. In May 2007 carbon dioxide concentrations ranged from 556 ppm 50 feet inside the main entrance (site 1) to 827 ppm 500 feet into the cave (site 2), with δ13C values of -13.2 ‰ to -15.6 ‰, respectively. In August 2007 carbon dioxide ranged from 734 ppm (site 1) to over 13,500 ppm (1.35%) (site 2), with δ13C values of -14.9 ‰ to -21.5 ‰. Higher rainfall in July resulted in elevated CO2 input to the cave from soils and drip waters than in May. Estimates of the endmember δ13C values of the air and soil gas data predict a CO2 soil gas at -22 ‰, as expected from decay of overlying C3 plants. 222Rn and CO2 data from air samples at sites 1 and 2 display a positive correlation. 222Rn activities reach a maximum of 134 dpm/L in May but increased to 1358 dpm/L in August at site 2, presumably due to slower air flushing rates. We plan to seal the cave to allow radon and carbon dioxide to grow into steady-state (while continuously monitoring) in order to estimate exchange rates directly from 222Rn models, and thus estimate CO2 exhalation and calcite precipitation rates. δ13C data reveals air masses throughout the cave are mixtures ranging from 27% soil / 73% atmospheric (site 1) in May, whereas a ratio of 42% / 58% was present in August. At site 2 the composition increased from 47% / 53% in May to soil gas dominated 96% / 4% in August. The mixing ratios appear to be a function of distance from entrance, but data collected during the entire seasonal cycle will detect longitudinal flow-through and exchange with the atmosphere and surrounding soil. These initial data show similar patterns in 222Rn activities and CO2 concentrations in cave air. Further data will reveal the significance of the positive correlation. These speleoclimate studies, when combined with isotope and chemical data from contemporaneous drip water and dripstones, will help connect climate variations to paleoclimate interpretations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMPP43A0999K
- Keywords:
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- 4914 Continental climate records;
- 4924 Geochemical tracers;
- 4958 Speleothems;
- 4994 Instruments and techniques