Constraints on the evolution of sulfuric acid speleogenesis within carbonate rocks partially covered by evaporites (Sierra de Mollina, southern Spain)

Abstract

We investigated speleogenetical processes and the formation of unusual endo- and exokarstic features within carbonates rocks in the Sierra de Mollina mountain range (southern Spain), a sector affected by salt tectonics. Allochthonous Triassic evaporites partially overlie younger subsalt Jurassic limestones and dolostones. The carbonate beds show signs of karstification, including large collapse structures and a dense field of collapse sinkholes. We focus our study on the two deepest cavities (~100 m deep), namely Sima del Soldado and Órganos Cave, where significant secondary gypsum (CaSO4·2H2O) and traces of mineralogical associations (alunite, jarosite, barite and others) have been reported. We analyzed the sulfur (δ34S) and oxygen (δ18O) of the gypsum sulfate and the δ18O and δD of its structural water. Our results suggest that reduction of sulfates from the Upper Triassic evaporites generated hydrogen sulfide (H2S). Subsequently, sulfuric acid (H2SO4) was generated in oxic conditions, which triggered sulfuric acid speleogenesis (SAS). Our reconstruction of the isotopic composition of the paleo-aquifer using stable isotopes of gypsum hydration water (δ18O and δD) indicates that gypsum crystallized in a climatic setting colder than present. Gypsum precipitation included acid replacement of the carbonates and hydration of anhydrite (CaSO4) generated in previous stages. The overlying Triassic evaporites provided the hydraulic gradient needed to drive sulfate-rich water towards the deeper, O2-depleted carbonate aquifer. Sulfate reduction in anoxic conditions generated H2S that later mixed with O2-enriched waters in the upper carbonate aquifer, creating the sulfuric acid necessary for SAS. The stable isotopes of gypsum from Órganos Cave reveal that the vadose zone was not connected to the surface, probably being confined beneath Triassic evaporitic rocks, while the Sima del Soldado underwent a partial connection with atmospheric O2.