Speleothem Paleoclimatology and Modern Proxies: Calcite Farming In a Continuously Monitored Cave PP41B-1513 Darrel Tremaine (email@example.com), P. N. Froelich (firstname.lastname@example.org), B. Kilgore, A. Kowalczk Department of Oceanography, NHMFL - Geochemistry, 1800 E Paul Dirac Drive, Tallahassee, Florida, 32310. United States. Introduction Cave Micrometeorology – Ventilation Regimes 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 ρATM > ρCAVE (Degrees) ventilation is necessary for speleothem deposition. High cave air ρCO2 ρCAVE > ρATM 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. Fig. 2: Winter 2009 ventilation systematics at both Cave Stations Hollow Ridge Cave record atmospheric CO2 levels. ΔDensity (Atm-Cave) is positive 3 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. 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. CS2 Calcite Farming – A Seasonal Venture Fig. 4: Calcite growth rates during different seasons. Colored circles represent an average cave air 5 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, - BALLROOM - SMITH & JONES - BR Sea Salt - S&J Sea Salt resulting in little or no calcite precipitation. Fig. 7: Time series of major and minor cations and silicon in Ballroom CS1 (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 Fig. 1: Hollow Ridge Cave Research: Water sampling locations, calcite 4 event. Data were lost after flooding events in December 2008 and April 2009. Sea salt corrections calculated using chloride as a conservative farming locations, drip logger, acoustic anemometer, and micrometeorology Fig. 5: Calcite farming plates mounted atop tracer in drip waters – data not shown. stations: RH, Barometric Pressure, Temp, ρCO2, and 222Rn activity. Map actively growing stalagmites by a flexible adapted from Boyer, 1974. wire mesh. Future Research Goals Samples and Methods • Farmed calcite crystals will be analyzed for chemical and isotopic 6 composition to determine if calcite is in equilibrium with drips and cave • Air temperature, ρCO2, relative humidity, barometric pressure, and 222Rn air. δ13C analyses will allow us to verify our hypothesis that calcite δ13C is activity are continuously monitored at both Cave Station 1 (CS1) and Cave a function of endmember mixing and ventilation. This data will be used as Station 2 (CS2). An acoustic anemometer captures air flow velocity and a site-specific calibration of calcite as a paleoclimate proxy. direction through Entrance A, allowing characterization of cave ventilation • Measurements of pH, carbonate alkalinity, and DIC of dripwaters to dynamics; either breathing in, breathing out, or stagnant. Drip rate is establish the saturation state of aqueous calcium carbonate. These data continuously monitored in the Ballroom (Figure 1). will allow us to predict the likelihood of precipitation from dripwater as a • The calcite farm consists of Pyrex™ slides (2” x 1”) installed atop function of cave-air ρCO2, and thus seasonality and storm events. actively growing stalagmites under active drip sites throughout HRC. Each plate is attached to a stalagmite with a flexible wire mesh. Slides are 250 μm References visually inspected bi-weekly, and replaced with clean plates after a Spotl, Fairchild, Tooth 2005 Edwards, et al. 2005 sufficient amount of calcite has been “grown”. Fig. 6: Low-magnesium calcite crystals Baldini, McDermott, Fairchild 2006 Kowalczk & Froelich 2009 Treble, et al. 2003 Fairchild, et al. 2000 precipitated on glass slides. Photo captured Roberts, et al. 1998 Banner, et al. 2007 • Drip water has been collected from Hollow Ridge Cave farming sites with Nikon OPTIPHOT2-POL microscope. Van breukelen, et al. 2008 Baldini 2008 Bar-Mathews, et al. 1996 Baker, Genty, et al. 1998 monthly since June 2008 and analyzed for major cations with an Agilent Baker, et al. 2007 Kowalczk & Froelich 2008 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.
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