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

1. Speleothem Paleoclimatology and Modern Proxies: Calcite Farming In a Continuously Monitored Cave PP41B-1513 Darrel 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. Cave Micrometeorology – Ventilation Regimes Introduction Dripwater Chemistry 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. (Degrees) ρATM > ρCAVE ρCAVE > ρATM 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. Hollow Ridge Cave 3 N 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. Calcite Farming – A Seasonal Venture CS2 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 - BALLROOM - S&J Sea Salt - BR Sea Salt - SMITH & JONES 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. CS1 4 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. Fig. 5: Calcite farming plates mounted atop actively growing stalagmites by a flexible wire mesh. Future Research Goals Samples and Methods • 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. 6 • 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). • 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. • 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”. References 250 μm Edwards, et al. 2005 Kowalczk & Froelich 2009 Fairchild, et al. 2000 Banner, et al. 2007 Baldini 2008 Baker, Genty, et al. 1998 Kowalczk & Froelich 2008 Spotl, Fairchild, Tooth 2005 Baldini, McDermott, Fairchild 2006 Treble, et al. 2003 Roberts, et al. 1998 Van breukelen, et al. 2008 Bar-Mathews, et al. 1996 Baker, et al. 2007 Fig. 6: Low-magnesium calcite crystals precipitated on glass slides. Photo captured with Nikon OPTIPHOT2-POL microscope. • 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. This research is conducted in collaboration with Southeastern Cave Conservancy, with support from Dawn Baker, Plum Creek Timber, Seattle, WA.