NHMFL_Presentation-4697 by zhangyun


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

                            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

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.

                                                                                                                                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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