Clumped-Isotope Thermometry and Oxygen Isotope Systematics in Speleothem Calcite From a Near Cave-Entrance Environment

Abstract:

Speleothems that grow in well-ventilated zones of caves have not been widely used in paleoclimate studies, yet may provide paleotemperature records. These zones are characterized by low CO₂ concentrations year-round and, in temperate climates, large seasonal temperature fluctuations. They are typically avoided for paleoclimate reconstruction due to concerns about kinetic isotope effects (KIE). However, speleothems in general seem to be sensitive to KIE, even in non-ventilated areas and can nonetheless provide useful paleoclimate records. At Westcave Preserve (Westcave), a shallow, well-ventilated cave in central Texas, we have found seasonal temperature differences recorded in both the oxygen isotope and clumped isotope compositions of speleothem calcite grown on glass-plate substrates harvested from active drips.

Although growth rates in this cave are relatively rapid, speleothem analogs in Westcave are growing near oxygen-isotopic equilibrium with their drip waters (between the calibrations of Kim and O’Neil, 1997 and Coplen, 2007). We have tested the compatibility of the Zaarur et al. (2013) clumped isotope bulk solution thermometer calibration to glass-substrate calcite in the cave collected during various months. This technique can provide absolute temperatures, but is sensitive to kinetic isotope effects, often significantly overestimating growth temperatures of speleothems. When this thermometer was applied to calcite collected from near where the plates were impacted by drip water, it overestimated measured temperatures by 7.7 ± 4.3°C, showing moderate KIE. When applied to calcite away from the drip impact, it overestimated temperatures by 18.7 ± 4.2°C, showing KIE increasing away from the drip. Measured monthly average temperatures in the cave ranged seasonally between 8 and 28°C, and daily temperatures vary significantly. At Westcave, calcite growth rates increase with temperature, and the calcite may therefore preferentially record warmer daily or hourly temperatures. In temperate regions, seasonal temperature records are lacking. Our results suggest near-entrance speleothems may be a valuable proxy for seasonal temperatures, but may overestimate mean seasonal temperatures in seasonally resolved records due to KIE and/or temperature-dependent calcite growth rates.