Microbially induced sedimentary structures in fluvial settings: the gas domes from the Bolzano Megacaldera (Permian, Italy)

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Abstract
Gas domes serve as some of the earliest and most persisting indicators of life on Earth, yet their documentation in continental environments remains sparse. This study aims to bridge this gap by examining gas domes within the Permian fluvial succession of Monte Luco, located in the caldera of the Bolzano Supervolcano. These structures occur as circular elevations on bedding planes. They are interpreted as resulting from gas accumulation beneath microbial mats, a hypothesis supported by comparisons with modern gas domes from both continental and intertidal environments. This microbial mediation highlights the astrobiological potential of gas domes as indicators of ancient life, although morphologically similar features are observed in non‐microbial sand substrates along the Atlantic Coast of Portugal, where they result from water expulsion processes. At Monte Luco, gas domes exhibit a rhythmic stratigraphic occurrence consisting of (1) a basal sandstone with planar lamination and trace fossils ( Planolites , Cylindrichnus ), (2) an overlying cross‐laminated sandstone and (3) a thin, uppermost horizon containing the gas domes. This upper horizon occasionally preserves tetrapod footprints, horizontal undermat‐mining burrows ( Helminthoidichnites ) and arthropod trails ( Diplopodichnus ). The lower intervals are interpreted as being formed in subaqueous conditions, while subsequent subaerial exposure facilitated microbial mat development and rapid colonisation by terrestrial fauna. The Monte Luco gas domes and published examples indicate that gas dome formation is restricted to wet, subaerially exposed substrates with microbial mats and minimal bioturbation. These findings enhance the application of gas domes as palaeoenvironmental proxies and underscore the complex interplay among microbial, sedimentary and hydrological processes in ancient volcanic landscapes.

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