Temporal and spatial variability of isotopic and hydrochemical parameters in cave drip-water feeding stalagmites: a case study from SE Slovenia

The accuracy and uncertainty of paleoclimate interpretations of geochemical and isotopic proxies from stalagmites depend critically on how accurately isotopic signals are transmitted through the vadose zone of the aquifer and how the elemental composition of the groundwater feeding the stalagmite changes as it passes through the aquifer.

Results of the first year (2021) of drip-water monitoring at 15 drip sites in the Jama v Dovčku Cave in SE Slovenia are presented. Cumulative monthly samples were analysed for δ¹⁸O of water, δ¹³C of dissolved inorganic carbon (DIC), elemental composition of drip-water and concentration and δ¹³C of CO₂ in the cave atmosphere. While the seasonal variability of δ¹⁸O of precipitation in 2021 at the nearest meteorological station exceeded 10 ‰ (from –15.42 ‰ in January to –5.28 in June), the intra-annual variability of δ¹⁸O of drip-water was reduced to 0.18–1.28 ‰ and showed no correlation with the thickness of the roof, which varied between >1 and 49 m. A discernible annual cyclicity was observed at some drip sites, with the highest δ¹⁸O values determined in winter and the lowest in late summer. Dissolved inorganic carbon (DIC) δ¹³C values exhibited a wide range (between –15.5 and –5.0 ‰), and drip sites could be divided into two groups: some drip sites exhibited large seasonal variability (up to 9.9 %) with low values in the warmer season, while the others varied within <3 ‰ with no apparent seasonality. The CO₂ concentration in the cave atmosphere was significantly higher from May to October, about 4000 to 8000 ppm, while it fluctuated between 700 and 1500 ppm in the colder part of the year. The δ¹³C values of CO₂ varied between –23.8 and –15.2 ‰ and decreased exponentially with CO₂ concentration. Similar to CO₂, the δ¹³C values of DIC also decreased exponentially with increasing DIC concentration. The δ¹³C value of “added” CO₂ in the atmosphere obtained from the δ¹³C x (C/C₀-1) vs. (C/C₀-1) plot (Sayles &% Curry, 1988) was –23.4 ± 2.6 ‰, which is almost identical to the CO₂ added to the drip-water estimated from DIC concentration and δ¹³C values of DIC (–23.9 ± 3.4 ‰, considering the isotopic fractionation factor between CO₂(g) and HCO₃^- from Mook et al. 1974). Chemical analysis showed that drip sites with a large seasonality of δ¹³C values have significantly lower saturation indices with respect to calcite in the warmer part of the year and that earlier calcite precipitation is most likely to occur at drip sites with lower drip rates.

 

Reference:

Mook et al., 1974, Earth Planet. Sci. Lett. 22,169–186.

Sayles & Curry, 1988, Geochim. Cosmochim. Acta 52, 2963–2978.

 

ACKNOWLEDGEMENT: We acknowledge the financial support of Slovenian Research Agency (J1-2478).