Epigenic vs. hypogenic speleogenesis governed by H2S/CO2 hydrothermal input and Quaternary icefield dynamics (NE French Pyrenees)

Abstract

Abstract Hypogenic caves, linked to carbonate rock dissolution due to CO2- and H2S-rich ascending deep waters, represent more than 10% of karstic networks worldwide; a proportion that increases as these systems are better constrained. However, interaction between hypogenic and epigenic processes is still poorly understood, especially since the subsequent invasion of surface water often obliterates the morphological and mineral markers of hypogenic activities. The Ariege Valley (French Pyrenean foothills) hosts significant karstic networks epigenically reworked by several episodes of glacier meltwater penetration during the successive coverage of Quaternary icefields. Among these karstic systems, the Vapeur and Ermite caves were probably initiated by a hypogenic component during the Miocene. In particular, multiple-S, Sr, H, C, and O isotopes of thermo-mineral waters and calcite-sulfate speleothems confirm that hydrothermal fluids reached the caves, and subsequently interacted with Quaternary glacial epigenic phases. Deep fluids conveyed CO2 and H2S, both produced from the thermochemical reduction of Triassic evaporites at depth. H2S oxidation and CO2 hydration in the cave atmosphere, above the water table, created sulfuric and carbonic acids responsible for an intense karstification. Interpretation of isotopic data, together with a geomorphological, mineralogical and textural study of cave minerals, allow us to propose a speleogenetic model in which the respective impact of epigenic and hypogenic processes was driven by base-level changes during successive Quaternary glacial/interglacial epochs: (i) during glacial periods, invasion of glacier meltwater within the karst led to the dilution of the hydrothermal water, and was responsible for an “epigenic mechanical-dominant” speleogenesis through water-related abrasion; (ii) interglacial epochs were marked by base-level drops and the establishment of a vadose domain in caves, favoring the widening of karstic conduits through carbonic and sulfuric acid condensation-corrosion during thermal water degassing. This “hypogenic chemical-dominant” speleogenesis was active until a new advance of glaciers, and this cycle occurred several times.