Sulfuric acid speleogenesis in the North Caucasus: Sharo-Argun valley Caves (Chechen Republic, Russia)

Abstract

Geological and hydrogeological conditions in the North Caucasian monocline allow ascending cross-formational flow and discharge of artesian waters from Upper Jurassic and Lower Cretaceous strata, thus enabling hypogene karst. The presence of evaporites (gypsum and anhydrite) in the deep-seated Mesozoic deposits promotes the sulfate-reduction processes, which supply H2S in the waters, thus enabling the sulfuric acid speleogenesis (SAS). Here we report on four hypogene caves of the Sharo-Argun valley. Three caves, Sheki, Magomed, and Kamila are presently active; H2S waters discharge through them. The fourth cave, Said-Emin is in the fossilized state. The active caves are characterized by high H2S contents in the air (up to 60 ppmv), high acidity of condensation solutions (pH 0.5–3.0) and abundant bacterial biofilms and mats dominated by sulfur-oxidizing bacteria. Morphological features of the caves (feeders, condensation-corrosion domes, horizontal water table notches and slots, gypsum replacement pockets, etc.) and mineral deposits (replacement gypsum, native sulfur, minerals of the alunite group) are typical of the cases formed by SAS. Sulfates and native sulfur of the caves have ‘light’ δ34S isotope compositions, consistent with sulfate-reduction in deep seated conditions and subsequent H2S oxidation in the near-surface conditions.