Stable isotope imprint of hypogene speleogenesis: Lessons from Austrian caves

Hypogene speleogenesis involves the formation of solution-enlarged permeability structures driven by ascending water acquiring its acidity from deep-seated sources. This process occurs in a wide range of geological settings and in different lithologies. Although hypogene speleogenesis has received increasing attention in the last two decades, the identification of former hypogene activity in caves remains challenging, unless unambiguous morphological, mineralogical and/or hydrogeological evidence is available. Here, we document that geochemical analyses (mainly stable oxygen and carbon isotopes) of cave wall rock samples may fill this methodological gap and provide a useful tool to identify hypogene karstification through geochemical fingerprinting of water-rock interaction. Stable isotope profiles of a comprehensive set of wall rock cores from eight hypogene caves in Austria in both limestone and dolomite settings were investigated. Alteration halos typically a few centimeters wide and characterized by up to 12‰ and 15‰ depletion in δ13C and δ18O, respectively, were identified in limestone and calcite marble caves. Besides isotopic alteration, bleaching and/or staining of the wall rock was observed due to oxidizing conditions of the paleowater. No isotopic change of the wall rock was identified in hypogene caves carved in dolomite, which may be attributed to low Ca/Mg ratios of the paleowater and/or aggressive fluids leading to fast wall rock retreat. On the other hand, dedolomitization was observed in some wall rocks implying the presence paleowaters with elevated Ca/Mg ratios. Similarly to dolomitic caves, no isotopic alteration front was observed in a cave formed by sulfuric acid speleogenesis. A likely explanation is the high rate of wall rock retreat outpacing the rate of isotope exchange in the wall rock. Our study also shows that water-rock interaction during epigene speleogenesis does not lead to isotopic halos. This geochemical fingerprinting approach offers a powerful tool to test the hypogene origin of caves and may provide additional insights into the temperature and provenance of the paleowater and the source(s) of its dissolved inorganic carbon.