Tracing the sources of cave sulfates: a unique case from Cerna Valley, Romania

► Sulfuric acid speleogenesis can be diagnosed using S & O isotopes in cave sulfates. ► S & O isotopic composition of cave sulfates in Cerna Valley defines 3 populations. ► Population 1 includes S from incomplete thermochemical sulfate reduction, methane-limited. ► Population 2 includes S from complete TSR, sulfate-limited, in anoxic conditions. ► S of Population 3 is consistent with several sources; O indicates oxidation in gas phase.