Sulfur and oxygen isotopes in the gypsum deposits of the Provalata sulfuric acid cave (Macedonia)

摘要

Sulfur stable isotopes from cave sulfates (mainly gypsum) have been used in a number of studies to trace the source of sulfur in caves formed by sulfuric acid, but only few studies apply combined use of sulfur and oxygen stable isotopes to further understand the processes operating in sulfuric acid speleogenesis (SAS). Here we present results of a detailed study of the distribution of sulfur and oxygen stable isotopes within the gypsum deposits formed during sulfuric acid speleogenesis of Provalata Cave (Macedonia). The delta O-18 (vs VSMOW) values range between -3.9 parts per thousand and +8.2 parts per thousand, and the delta S-34 (vs CDT) values between -7.5 parts per thousand and +0.7 parts per thousand. We found a strong positive correlation between the delta O-18 and delta S-34 values, with a 0.5 parts per thousand increase in the delta S-34 for every 1 parts per thousand increase in the delta O-18, indicating that both oxygen and sulfur isotopes were concurrently affected during the oxidation process. We attribute these effects to be either due to environmental control (concentrations of H2S and O-2) or due to isotope fractionation during multi-step microbial oxidation, also affected by the environmental conditions. Additionally a shift of +1.85 parts per thousand in the delta S-34 values prior to the H2S oxidation is found, indicating evolution of the H2S delta S-34 in the SAS system. The wide range of both delta O-18 and delta S-34 values in the gypsum deposits of the small Provalata Cave show that both the number of samples and their location can be an important factor for the understanding of sulfuric acid speleogenesis using stable isotopes. (C) 2018 Elsevier B.V. All rights reserved.