Ca Isotope Fractionation During Gypsum Precipitation in a Sulfidic Cave

In sulfidic caves, limestone dissolution above the water table is assocated with sulfuric acid corrosion and attendant precipitation of CaSO4 crusts. Since sulfuric acid is produced by microbially-mediated sulfide oxidation, such systems present unique opportunities to study the effects of microbial processes on Ca isotope systematics. The current study presents preliminary measurements of the Ca isotopic composition of gypsum, calcite, and water samples collected in and around Grotta Bella cave in the Frasassi cave system (central Italy). The environment sampled in this limestone-hosted cave is situated close to flowing sulfidic groundwater (air [H2S]meas ~3 ppm) and is actively forming gypsum in close association with microbial communities ("snottites") dominated by Acidithiobacillus sp [1]. The pH ranges from >2.3 in gypsum crust accumulating on the cave walls to 0-2 at the surface of snottites, while pH in waters sampled outside the cave is between 7 and 8. The current rate of limestone dissolution is ~0.15 mmol CaCO3/cm2/a [2]. This study reports the Ca isotopic composition (δ44Ca, rel. to bulk Earth) of stream and seep water, limestone, and gypsum samples in and around Grotta Bella. Calcium isotopes were measured on a Finnigan Triton TI thermal ionization mass spectrometer at GEOMAR using a 43Ca-48Ca double spike. Field-acidified stream waters and dissolved gypsum were chromatographically purified using MCI Gel (Biorad) while bulk limestone samples were dissolved in nitric acid and loaded onto single Re filaments without additional purification. The δ44Ca values of stream waters and springs are relatively restricted (~0.2‰) and generally the same as bulk limestone. Actively-precipitating CaSO4 minerals, however, show a range of δ44Ca values from limestone-like to values almost 1‰ lighter than corresponding wallrock. Decreasing δ44Ca values in these gypsum minerals correspond to gradients in pH and mineral grain size. The smallest gypsum grains (typical diameter ~4 µm), which also occur closest to the dissolving limestone surface and are abundant in the active deposit, have isotopic compositions indistinct from wall rock. The δ44Ca of gypsum crystals decreases with increasing grain size and distance from the dissolving limestone surface, reaching δ44Ca values as light as -0.8‰. The lightest δ44Ca value also coincides with mineral that exists in close association with snottites. While it is clear that Ca isotopes fractionate during the precipitation of gypsum in the cave environment, it is not clear what controls this process. We investigate the extent to which the data from Grotta Bella is consistent with experimental studies of isotope effects during calcite precipitation. In addition, we will present sulfur isotope data to help constrain systematics and simple models to evaluate the extent to which microbial catalysis influences Ca isotope fractionation, either directly or indirectly. [1] Macalady et al. (2007) Environ Microbio 9(6): 1402-1414 [2] Galdenzi and Maruoka (2003) J Cave Karst Stud 65(2): 111-125