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

[摘要] Tubular carbonate concretions of up to 1 m in length and perpendicular tobedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus margaritatus Zone,Gibbosus Subzone) in outcrops (Fontaneilles section) in the vicinity ofRivière-sûr-Tarn, southern France. Stable isotope analyses of theseconcretions show negative δ¹³C values that decrease from therim to the center from −18.8‰ to −25.7‰ (V-PDB), but normal marine δ¹⁸O values (−1.8‰). Carbon isotope analyses of Late Pliensbachian bulkcarbonate (matrix) samples from the Fontaneilles section show clearlydecreasing C-isotope values across the A. margaritatus Zone, from +1‰ to −3‰ (V-PDB).Isotope analyses of coeval belemnite rostra do not document such a negativeC-isotope trend with values remaining stable around +2‰ (V-PDB). Computertomographic (CT) scanning of the tubular concretions show multiple canalsthat are lined or filled entirely with pyrite. Previously, the formation ofthese concretions with one, two, or more central tubes, has been ascribed tothe activity of an enigmatic organism, possibly with annelid or arthropodaffinities, known asTisoa siphonalis. Our results suggest tisoan structures are abiogenic.Based on our geochemical analyses and sedimentological observations wesuggest that these concretions formed as a combination of the anaerobicoxidation of methane (AOM) and sulfate reduction within the sediment. Fluidsrich in methane and/or hydrocarbons likely altered local bulk rock carbonisotope records, but did not affect the global carbon cycle. Interestingly,Tisoa siphonalis has been described from many locations in the Grands Causses Basin insouthern France, and from northern France and Luxemburg, always occurring atthe same stratigraphic level. Upper Pliensbachian authigenic carbonates thuspossibly cover an area of many thousand square kilometers. Greatly reducedsedimentation rates are needed to explain the stabilization of thesulfate-methane transition zone in the sedimentary column in order for thetubular concretions to form. Late Pliensbachian cooling, reducing run-off,and/or the influx of colder water and more vigorous circulation could beresponsible for a halt in sedimentation. At the same time (thermogenic)methane may have destabilized during a major phase of Late Pliensbachian sealevel fall. As such Tisoa siphonalis is more than a geological curiosity, and its furtherstudy could prove pivotal in understanding Early Jurassic paleoenvironmentalchange.

[学科分类] 地球化学与岩石