Farmed calcite δ13C at Ascunsă Cave, Romania, and its relation with CO2 outgassing and drip rate

When calcite precipitates in caves, its carbon stable isotope signature can be modified by the CO2 outgassing gradient between drip water and cave atmosphere. This effect is modulated by the water residence time in the cave, from its emergence in the cave until the deposition of calcite. Moreover, CO2 solubility, calcite precipitation rate, and isotopic fractionation are controlled by temperature. Here, we present up to date results of an ongoing monitoring study at Ascunsă Cave (Romania), exploring the relationship between farmed calcite δ13C, drip rate, and CO2 outgassing. In addition to measuring CO2 concentration in cave air, we also measured the CO2 concentration in the headspace of a water-air equilibrator that collects drip water without exposing it to cave atmosphere, preventing outgassing. δ13C from calcite farmed at two neighboring stalagmites with different drip rates was also measured. Although caves have generally stable temperatures, we show here that temperature inside Ascunsă and Isverna caves has risen by more than 2°C over the course of a year, bearing important implications for stable isotopic fractionation equations and CO2 dynamics. Our results show that δ13C of farmed calcite has a strong relationship with drip rate at the slow dripping site, but no correlation at the faster dripping site. These two sites are also different when δ13C is compared to the outgassing gradient. At the slower drip site, δ13C and the outgassing gradient are directly correlated, whereas at the faster drip site their correlation is inverse. Our study brings new light onto speleothem δ13C behavior in general, and at Ascunsă Cave in particular, which is crucial for understanding the paleoclimate information captured by speleothems from this cave or elsewhere.