USING SULFUR AND OXYGEN ISOTOPES IN CAVE MINERALS TO TRACE SPELEOGENETIC PATHWAYS: A CASE STUDY FROM CERNA VALLEY (ROMANIA)

S-bearing minerals in the form of crusts, aggregates, and crystals from 7 caves in the region of Băile Herculane (SW Romania) were studied to achieve an improved understanding of S sources, as well as hydrological and sulfuric acid speleogenetic (SAS) processes.

The combined use of S and O isotopes with additional geochemical data allow to relate cave minerals to aqueous reactions in the thermo-mineral springs near Băile Herculane and to mineral sources such as diagenetic pyrite and primary sulfates from the limestone bedrock. Examination of δ³⁴S _SO4 and δ¹⁸O_SO4 values of 35 samples total reveals three distinctively different populations. P1 shows negative δ³⁴S values consistent with oxidation of dissolved sulfide produced during incomplete thermochemical sulfate reduction (TSR) (energy-limited); P2 has positive δ³⁴S values compatible with oxidation of dissolved sulfide produced during complete TSR (sulfate-limited), and P3 displaying intermediate δ³⁴S values may be consistent with several scenarios, including pyrite oxidation.

All cave sulfates δ¹⁸O values from the Cerna Valley fall between two theoretical limitations of sources of O: the local meteoric water, and atmospheric O₂. Sulfate minerals produced by incomplete (energy-limited) TSR in the caves upstream of Băile Herculane (P1) suggest oxidation under more oxic conditions than those downstream, the latter of which are produced by complete (sulfate-limited) TSR (P2). The correlation between δ³⁴S and δ¹⁸O values in the latter group of cave sulfates is consistent with increasing completeness of TSR downstream and progressively anoxic conditions downstream to the near complete exclusion of O derived from atmospheric O₂. δ¹⁸O values from the P3 are indicative of the majority of sulfate O derived from ¹⁸O-enriched atmospheric O₂, with little incorporation of O from aqueous solutions derived from local meteoric water.

Sulfate δ³⁴S values in cave minerals near Baile Herculane show that the cave sulfate isotope values from SAS depend not only on the source of the S, but also on the completeness of subsequent S species reactions producing a relatively wide range of δ³⁴S values and hence, S isotopic composition of the minerals alone does not provide enough information for clearly distinguishing between SAS or other more complex speleogenetic pathways.