Bacteria driven fossil ecosystems as indicator of active continental margins in the geological past. Usefulness of carbonate sediment hosted vent environments in geodynamic reconstructions

Abstract Continental rifting of the Tisza microplate has started during the Late Jurassic that resulted phreatic eruptions, peperites, and built up a volcanic edifice in the Early Cretaceous of the Mecsek Mountains (South Hungary). In SE direction from the volcanic edifice at Zengővárkony a shallow marine (depth 100 – 200 m) carbonate sediment hosted vent environment and iron-ore deposition occurred at end Valanginian too early Hauterivian hosting a diverse, endemic fauna of around 60 species. Detailed mineralogical analysis of the transport channels include Fe oxides (ferrihydrite, goethite, hematite, magnetite), quartz, mixed carbonate, pyrite, feldspar, Fe-bearing clay minerals, apatite, sulphates (barite, gypsum, jarosite) and native sulphur. Filamentous, microbially mediated microtexture with inner sequented, necklace-like spheric forms (diameter 1 μm) and bacterial laminae are observed inside the decapod crustacean coprolites (Palaxius tetraochetarius) and in the rock matrix, too. This complex, ecological and mineralogical analysis provided direct evidence for the presence of bacteria at fossil SHV environments on one hand, and the intimate connection between bacteria and decapod crustaceans at hydrothermal environments 135 Ma before. This observation completes this chemosymbiotic fossil food chain from the primary producers to the top carnivores reported for the first time from this locality.