Sulfur isotopic composition and the source of dissolved sulfur species in thermo-mineral springs of the Cerna Valley, Romania

Abstract

Documenting the source and processes controlling dissolved sulfur (S) mineralization in thermo-mineral waters of the Cerna Valley, Romania is important to understanding speleogenesis in this karst region, in addition to understanding hydrogeological controls, therapeutic qualities and sustainability of the region's historic spas. Stable S and carbon (C) isotopic results reported here elucidate controls on redox processes, the source of dissolved S mineralization, and sulfur-bearing mineral precipitation in this unique karst hydrothermal system. At reservoir temperatures that occur in the Cerna Valley aquifers, it is likely that thermochemical sulfate reduction (TSR) is the dominant S reduction pathway. However the apparent isotope enrichment that we observed between coexisting dissolved sulfate and sulfide is higher than normally associated with TSR—a fact that likely reflects rapid redox cycling at low grade hydrothermal temperatures. δ13C values of dissolved inorganic carbon (DIC) are consistent with TSR using methane as an electron donor. δ34S values of total dissolved S (sum of sulfide and sulfate) in all springs sampled and particularly in those for which closed-system conditions can be demonstrated, is greater than + 16‰, consistently pointing to dissolved S that derives from marine-derived sulfate mineral sources. To this combined S–C isotope data set, we apply a model of Rayleigh distillation which describes exponentially increasing δ34S values of a diminishing sulfate reservoir during TSR, and linearly decreasing δ13C values of DIC indicating mixing of C from the electron donor involved in TSR. Comparison of our results to this model shows two distinct stages of TSR during transport of fresh water from karst aquifers towards the local geothermal anomaly. In an up-gradient group of springs and wells, incomplete TSR progress that is limited by energy from electron donors is evident from: low concentrations of dissolved sulfide with low δ34S values (as low as − 21.9‰), a large balance of remaining as SO42− similar in isotopic composition to its source (∼ + 17.4‰), and δ13C values showing little methane-derived DIC. Conversely, in a downstream group of springs and wells, excess concentration of methane provides abundant energy for near-complete TSR, and this near complete reaction progress is evident from: high δ34S values of remaining SO42− (up to + 71.8‰), high dissolved sulfide concentrations (> 32 mg/L as S2−) with δ34S values that take on the approximate isotopic signature of the total dissolved S (mean + 17.4‰), and low δ13C values of additional DIC derived from methane (as low as − 30‰). Thus the unique hydrogeology of the Cerna Valley allows the observation of two end-members of TSR (energy- and sulfate-limited) demonstrating wide boundary conditions of stable isotopic composition of dissolved S and C produced by TSR in a single natural system.