1. High Resolution Time Series Cave Ventilation Processes and the Effects on Cave Air Chemistry and Drip Water: Speleoclimatology and Proxy Calibration ( # PP51C-1521) A. Kowalczk P. N. Froelich, D. Tremaine, C. Gaffka Department of Oceanography, NHMFL-Geochemistry, Florida State University, E Paul Dirac Dr, Tallahassee, FL 32310
Speleometeorology is essential to understanding speleothem paleoclimate records. The study of modern cave atmospheric processes and chemistry and their effects on calcite deposition allows for calibrations that can be applied for more accurate interpretations of geochemical proxies from ancient speleothems. Radon-222 can be used as a monitor of physical cave air exchange while high cave air CO2 inhibits calcite precipitation. Cave ventilation is required to form speleothems. Knowledge of seasonality in calcite deposition links climate to speleothem isotope records because seasonal variations in temperature, cave air ventilation, CO2 concentrations, and the isotopic composition of precipitation control the composition of precipitated calcite.
Hollow Ridge Cave, a Southeastern Cave Conservancy (SCCi) owned wild cave preserve in Marianna, FL, was instrumented in October 2007 to continuously monitor cave meteorology and aerochemistry at high resolution time intervals (sub-hourly). This time-series has been supplemented with bi-weekly drip and aquifer water sampling, seasonal cave air grab sampling, and the deployment of quartz slides under drips to capture calcite precipitation (see Figure 1 for cave layout). Connecting modern calcite growth to speleometeorology will enable better interpretation of a Holocene stalagmite from Hollow Ridge Cave and potentially offer multiple long-term speleothem paleoclimate records from the North Florida region.
Introduction
Fall
Winter
Spring
Summer
Fall
Figure 4. Cave Ventilation Processes as a Function of Weather. Episodes of rapid air flushing (low 222Rn) occur with low CO2 and likely coincide with episodes of calcite precipitation. Left: Passage of a strong cold front on January 20, 2008 resulted in a major flushing of Hollow Ridge Cave with atmospheric air via a strongly positive air density difference. Right: Passage of TS Faye over Marianna on August 23rd, 2008 resulted in a similar major flushing of Hollow Ridge Cave. Sustained strong N winds across the entrances drove atmospheric air into the cave system, lowering 222Rn activities. Note y-axis scale differences in 222Rn activities and density differences between left (cold front) and right (TS Faye) panels.
Figure 1. Map of Hollow Ridge Cave near Marianna, Jackson County, FL (3046’58.17”N, 8512’13.15”W, 30 m asl), showing sampling locations. A map of Florida and the location of Hollow Ridge is in the upper left. The green square is the micro-meteorology station above the cave. The red triangles are Cave Stations 1 and 2. Cave Station 1 was redeployed in June 2008 from the Ballroom after a flooding event in February Dark blue diamonds are water sample locations. The blue octagon is the air flow sensor. The red pentagon is the drip logger and calcite growth site. The green crosses are cave air grab sampling locations. 1 2
LEGEND
(Above Cave)
MET Station (Above Cave)
Cave Stations 1 & 2 (Continuous)
Drip & Sump Water Sites
Air Flow Sensor
Drip Rate & Calcite Growth
Cave Air Grab Samples
Hollow Ridge Cave
Tape and Compass Survey
By Paul Boyer
Feb 10, 1974
3400 ft Cumulative Passage Length
Jackson County, FL
Figure 6. Hollow Ridge Cave Air Keeling Plot. The relationship between δ13CO2 and inverse CO2 of cave air for Hollow Ridge (FL- Red) and Obir (Austria- Blue) caves (Spotl 2005) represents a Rayleigh mixing line between the atmosphere (-7 to -10 ‰ d13C) and soil gas (-22 ‰ d13C) CO2 and should be valid for most caves.
Figure 2. Time Series of Meteorological and Cave Aerochemical Data. 13 month time-series of A: Daily Total Rain; B: Air Density Difference between Atmosphere and Cave; C: Daily Average Atmospheric and Cave Temperatures; D: Daily average 222Rn activities; and E: Daily Average CO2 concentrations. Seasonal ventilation regimes are color-coded: Winter-blue; Spring-Green; Summer-Yellow; Fall-Red. Winter ventilation is dominated by air density differences; spring and fall ventilation, by both winds and density differences; and summer, by winds. Missing data are from power loss, removal of sensors for calibration, and loss of equipment from flooding.
Figure 3. Drip Rates and Rainfall June-November Time series of drip rates in the Ballroom (see Figure 1) and daily rainfall. The passage of Tropical Storm Faye (August 23) recharged the epikarst above Hollow Ridge, increasing drip rates and discharge. A decay constant calculated from slowing drip rates suggests a water response time of about 14 days during normal summer conditions. Increased evapotranspiration (ET) during summer may inhibit the epikarst from reaching saturation except during periodic high intensity precipitation events (tropical storms). Rainfall-to-Drip-Delay after TS Faye was about 15 days (double-ended red arrow).
Figure 5. Hollow Ridge Drip Water Isotopes. Hollow Ridge drip waters (Red) fall close to Tallahassee Rainfall (Black), the Global (Blue) and Florida (Green) Meteoric Water Lines. This agrees with a drip water study at Florida Caverns State Park, 2 km NW of Hollow Ridge (Onac 2008).
Drip Delay
Tropical
Storm
Faye
Conclusions
Strong positive density differences drive ventilation in winter, while winds have a greater effect during the summer. Density differences and winds both have a significant effect on ventilation in the spring and fall.
Drip water isotopes at Hollow Ridge are consistent with the local MWL. The small isotopic variation likely reflects a well mixed pool in the epikarst above the cave.
A Keeling plot of cave air shows a linear mixing relationship between atmosphere and soil gas CO2 (Figure 6).
Ongoing analyses of cave meteorology, drip waters, and farmed calcite will determine whether a seasonal cycle is present in calcite precipitation and should reveal seasonal relationships between drip hydrochemistry and speleothem geochemistry.
Results
Fall cave ventilation is driven by diurnal winds while winter ventilation is driven by strongly positive density differences between atmospheric and cave air masses. Spring and summer ventilation regimes are driven by a combination of winds and density differences.
Drip rates are dependent upon water storage in the epikarst and display an exponential decay during periods of light precipitation and strong ET during summer and late fall (Figure 3).
Decreased ventilation in summer allows buildup of 222Rn and CO2, while increased winter ventilation results in lower radon activities and CO2 concentrations. Radon-222 and CO2 follow similar seasonal patterns.
Strong diurnal ventilation in Fall results in large amplitude high-frequency CO2 and 222Rn cycles at Hollow Ridge. Similar cycles have also been found at two Irish caves (Baldini 2008).
References
Baldini, J. et al. (2008). Very high-frequency and seasonal cave atmosphere PCO2 variability: Implications for stalagmite growth and oxygen isotope-based paleoclimate records. Earth and Planetary Science Letters 272:
Onac, B., Pace-Graczyk, K., Atudirie, V. (2008). Stable isotope study of precipitation and cave drip water in Florida (USA): implications for speleothem- based paleoclimate studies. Isotopes in Environmental and Health Studies 44:2,
Pflitsch, A. and Piasecki, J. (2003). Detection of an airflow system in Niedzwiedzia (Bear) Cave, Kletno, Poland. Journal of Cave and Karst Studies 65(3):
Przylibski, T. (1999). Radon concentration changes in the air of two caves in Poland. Journal of Environmental Radioactivity 45: 81-94
Sharp, Z. (2007). Principles of Stable Isotope Geochemistry. Pearson Prentice Hall, Upper Saddle River, NY. Pg. 70f.
Spotl, C., Fairchild, I.J. and Tooth, A.F., Cave air control on dripwater geochemistry, Obir Caves (Austria): Implications for speleothem deposition in dynamically ventilated caves. Geochimica et Cosmochimica Acta, 69(10):
Methods
Sub-hourly monitoring of the weather above the cave: temperature (T), relative humidity (RH), barometric pressure (BP), air density (calculated - ρ), wind speed and direction, and rainfall. Sub-hourly monitoring at Cave Station 1 of T, RH, BP, [CO2], air flow and direction, ρ (calculated) and hourly monitoring of 222Rn. Sub-hourly monitoring at Cave Station 2 of T and [CO2] and hourly monitoring of 222Rn.
Hourly monitoring of drip rates and bi-weekly collection of drip waters from the Ballroom and aquifer waters from the Sump for isotopic (d18O and dD) and trace element analyses.
Seasonal collection of cave air for analysis of [CO2] and d13CO2 from five grab sample locations.