Tracing the Sources of Cave Sulfates: A Unique Case from Cerna Valley, Romania

In order to reliably distinguish between different genetic processes of cave sulfate formation and to quantify the role of thermo-mineral waters on mineral deposition and cave morphology, it is critical to understand sulfur (S) sources and S transformations during hydrological and speleogenetic processes. Previous work has shown that sulfuric acid speleogenesis (SAS) often produces sulfate deposits with ³⁴ S-depleted isotopic signatures compared to those of the original source of S in sulfate rocks. However, ³⁴ S-depleted isotopic composition of S-bearing minerals alone does not provide enough information to clearly distinguish SAS from other speleogenetic processes driven by carbonic acid, geothermal heat, or other processes. The isotopic composition (δ ¹⁸ O and δ ³⁴ S) of sulfate minerals (mainly gypsum) from seven caves of the Cerna Valley (Romania) defines three distinct populations, and demonstrates that the δ ³⁴ S values of SAS-precipitated cave sulfates depend not only on the source of the S, but also on the H ₂ S:SO ₄ ²⁻ ratio during aqueous S species reactions and mineral precipitation. Population 1 includes sulfates that are characterized by relatively low δ ³⁴ S values (− 19.4 to − 27.9‰) with δ ¹⁸ O values between 0.2 and 4.3‰ that are consistent with oxidation of dissolved sulfide produced during methane-limited thermochemical sulfate reduction (TSR) that presently characterizes the chemistry of springs in the upper Cerna Valley. Population 2 of cave sulfates has ³⁴ S-enriched δ ³⁴ S values (14.3 to 19.4‰) and more ¹⁸ O-depleted δ ¹⁸ O values (from − 1.8 to − 10.0‰). These values argue for oxidation of dissolved sulfide produced during sulfate-limited TSR that presently characterizes the chemistry of springs further downstream in the Cerna Valley. The δ ¹⁸ O values of cave sulfates from Population 1 are consistent with oxidation under more oxic aqueous conditions than those of Population 2. δ ³⁴ S values of cave sulfates within Population 3 (δ ³⁴ S: 5.8 to 6.5‰) may be consistent with several scenarios ( i.e. , pyrite oxidation, oxidation of dissolved sulfide produced during methane-limited TSR coupled with O ₂ -limited oxidation during SAS). However, comparatively ¹⁸ O-enriched δ ¹⁸ O _SO4 values (11.9 to 13.9‰) suggest the majority of this sulfate O was derived from atmospheric O ₂ in gas-phase oxidation prior to hydration. Thus, the combined use of oxygen- and sulfur-isotope systematics of sulfate minerals precipitated in a variety of cave settings along Cerna Valley may serve as an example of how more complex cave systems can be deconvoluted to allow for more complete recognition of the range of processes and parameters that may be involved in SAS.