Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments

Carbonate concretions accompanied by elemental sulfur are found in many upper Miocene marine successions across the Mediterranean area (e.g. SE-Spain, Sicily, Apennine, Cyprus). Most of these rocks are characterized by molds of evaporitic minerals (mostly gypsum) suggesting an early (syngenetic) or late (epigenetic) diagenetic origin. In contrast to these findings, a case study from the Ripa dello Zolfo area in northern Italy lacks evidence of carbonate and sulfur replacement of preexisting sulfate minerals. An integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses was used for the study of three main lithofacies: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The δ¹³C and δ¹⁸O values of authigenic carbonate minerals and δ¹³C of lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from a deeper zone was not only causing the brecciation of the stromatolites, but also delivered bicarbonate ions for the subsequent precipitation of additional, ¹³C-depleted calcite (δ¹³C values as low as -52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM within an SMTZ in a zone below the stromatolites and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ³⁴S and Δ³³S values close to coeval seawater sulfate. This study highlights that a multi-proxy approach has great potential for the reconstruction of spatially and temporarily separated biogeochemical processes in the shallow subsurface or at the seafloor (i.e., anaerobic oxidation of methane, sulfate reduction, sulfide oxidation) – processes that may induce the syngenetic formation of authigenic carbonate and sulfur deposits in marine sediments.