The enigmatic ichnofossil Tisoa siphonalis and widespread authigenic seep carbonate formation during the Late Pliensbachian in southern France

摘要: Abstract. Tubular carbonate concretions of up to 1 m in length and perpendicular bedding, occur abundantly the Upper Pliensbachian (upper Amaltheus margaritatus Zone, Gibbosus Subzone) outcrops (Fontaneilles section) vicinity Riviere-sur-Tarn, southern France. Stable isotope analyses these show negative δ13C values that decrease from rim center −18.8‰ −25.7‰ (V-PDB), but normal marine δ18O (−1.8‰). Carbon Late bulk (matrix) samples Fontaneilles section clearly decreasing C-isotope across A. +1‰ −3‰ (V-PDB). Isotope coeval belemnite rostra do not document such a trend with remaining stable around +2‰ Computer tomographic (CT) scanning tubular multiple canals are lined or filled entirely pyrite. Previously, formation one, two, more central tubes, has been ascribed activity an enigmatic organism, possibly annelid arthropod affinities, known asTisoa siphonalis. Our results suggest tisoan structures abiogenic. Based on our geochemical sedimentological observations we formed as combination anaerobic oxidation methane (AOM) sulfate reduction within sediment. Fluids rich and/or hydrocarbons likely altered local rock carbon records, did affect global cycle. Interestingly, Tisoa siphonalis described many locations Grands Causses Basin France, northern France Luxemburg, always occurring at same stratigraphic level. authigenic carbonates thus cover area thousand square kilometers. Greatly reduced sedimentation rates needed explain stabilization sulfate-methane transition zone sedimentary column order for form. cooling, reducing run-off, influx colder water vigorous circulation could be responsible halt sedimentation. At time (thermogenic) may have destabilized during major phase sea level fall. As is than geological curiosity, its further study prove pivotal understanding Early Jurassic paleoenvironmental change.