Karstic calcite (CaCO3) cave formations, or speleothems, incorporate and preserve climate signals within their crystalline matrix. Stable isotopes and elemental compositions of speleothems have become robust fields of paleoclimate research. Although many caves in China, Brazil, Austria, and UK have been studied for regional and global climatic variation, there have been comparatively few compositional analyses of modern calcite (farmed in situ), modern dripwater, and cave ventilation processes. Cave air ventilation is necessary for speleothem deposition. High cave air ρCO2 kinetically restricts dripwater degassing rates, effectively inhibiting calcite growth.

This study presents a direct seasonal compositional comparison between dripwater and modern calcite, in conjunction with high-resolution micrometeorological time series of the cave system. Long-term goals include elemental and isotopic analyses of Hollow Ridge Cave speleothems, using modern data from “farmed” calcite as a direct calibration of the paleoclimate signals contained within in situ dripstones.

Speleothem Paleoclimatology and Modern Proxies: Calcite Farming In a Continuously Monitored Cave

PP41B-1513Darrel Tremaine (tremaine@magnet.fsu.edu), P. N. Froelich (froelich@magnet.fsu.edu), B. Kilgore, A. Kowalczk

Department of Oceanography, NHMFL - Geochemistry, 1800 E Paul Dirac Drive, Tallahassee, Florida, 32310. United States.

Introduction

Calcite Farming – A Seasonal Venture

Dripwater Chemistry

References

Hollow Ridge Cave

Cave Micrometeorology – Ventilation Regimes

Samples and Methods

Spotl, Fairchild, Tooth 2005Baldini, McDermott, Fairchild 2006Treble, et al. 2003Roberts, et al. 1998Van breukelen, et al. 2008Bar-Mathews, et al. 1996Baker, et al. 2007

• Air temperature, ρCO2, relative humidity, barometric pressure, and 222Rn activity are continuously monitored at both Cave Station 1 (CS1) and Cave Station 2 (CS2). An acoustic anemometer captures air flow velocity and direction through Entrance A, allowing characterization of cave ventilation dynamics; either breathing in, breathing out, or stagnant. Drip rate is continuously monitored in the Ballroom (Figure 1).

Fig. 3: Biweekly sampling of soil gas and atmospheric (above cave, under tree canopy) CO2, combined with intensive spatial grab sampling transects has improved our understanding of how ventilation effects in situ air chemistry and isotopic composition. If speleothem formation waters are in equilibrium with cave air, then this mixing diagram illustrates that carbonate δ13C must be a function of seasonal ventilation.

Edwards, et al. 2005Kowalczk & Froelich 2009Fairchild, et al. 2000Banner, et al. 2007Baldini 2008Baker, Genty, et al. 1998Kowalczk & Froelich 2008

Fig. 1: Hollow Ridge Cave Research: Water sampling locations, calcite farming locations, drip logger, acoustic anemometer, and micrometeorology stations: RH, Barometric Pressure, Temp, ρCO2, and 222Rn activity. Map adapted from Boyer, 1974.

• Drip water has been collected from Hollow Ridge Cave farming sites monthly since June 2008 and analyzed for major cations with an Agilent Quadrupole ICP-MS and major anions by Ion Chromatography.

• The calcite farm consists of Pyrex™ slides (2” x 1”) installed atop actively growing stalagmites under active drip sites throughout HRC. Each plate is attached to a stalagmite with a flexible wire mesh. Slides are visually inspected bi-weekly, and replaced with clean plates after a sufficient amount of calcite has been “grown”.

Fig. 2: Winter 2009 ventilation systematics at both Cave Stations record atmospheric CO2 levels. ΔDensity (Atm-Cave) is positive causing cooler outside air to flood the lower levels of the cave. Summertime ventilation is a function of wind speed across cave entrances, and results in incomplete ventilation.

Fig. 5: Calcite farming plates mounted atop actively growing stalagmites by a flexible wire mesh.

Fig. 4: Calcite growth rates during different seasons. Colored circles represent an average cave air ρCO2 for each season (½ [CS1+CS2]). Fall and winter ventilation regimes result in complete ventilation of the entire cave, while summer ventilation results in periods of stagnation and higher (soil gas) cave air ρCO2. From the general lack of calcite growth during the summer, except near the entrance, we infer that ventilation-driven ρCO2 levels significantly reduce dripwater degassing rates, resulting in little or no calcite precipitation.

5

6

Fig. 6: Low-magnesium calcite crystals precipitated on glass slides. Photo captured with Nikon OPTIPHOT2-POL microscope.

Fig. 7: Time series of major and minor cations and silicon in Ballroom (BR) and Smith & Jones Room (SJ) drip waters plotted below cumulative rainfall and drip rate. Tropical Storm Fay rained 22 cm in late August, recharging the epikarst and increasing the drip rate. Aqueous cation concentrations peaked approximately 3-4 weeks after the intense rainfall event. Data were lost after flooding events in December 2008 and April 2009. Sea salt corrections calculated using chloride as a conservative tracer in drip waters – data not shown.

• Farmed calcite crystals will be analyzed for chemical and isotopic composition to determine if calcite is in equilibrium with drips and cave air. δ13C analyses will allow us to verify our hypothesis that calcite δ13C is a function of endmember mixing and ventilation. This data will be used as a site-specific calibration of calcite as a paleoclimate proxy.

• Measurements of pH, carbonate alkalinity, and DIC of dripwaters to establish the saturation state of aqueous calcium carbonate. These data will allow us to predict the likelihood of precipitation from dripwater as a function of cave-air ρCO2, and thus seasonality and storm events.

Future Research Goals

3

250 μm

4

N

ρATM > ρCAVE

ρCAVE > ρATM

(Degrees)

CS2

CS1

This research is conducted in collaboration with Southeastern Cave Conservancy, with support from Dawn Baker, Plum Creek Timber, Seattle, WA.

- BALLROOM - SMITH & JONES - BR Sea Salt - S&J Sea Salt