Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
Journal topic
Volume 7, issue 10
Biogeosciences, 7, 3123–3138, 2010
https://doi.org/10.5194/bg-7-3123-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 7, 3123–3138, 2010
https://doi.org/10.5194/bg-7-3123-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  13 Oct 2010

13 Oct 2010

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

B. van de Schootbrugge1, D. Harazim1,2, K. Sorichter1, W. Oschmann1, J. Fiebig1, W. Püttmann3, M. Peinl1, F. Zanella4, B. M. A. Teichert5, J. Hoffmann1, A. Stadnitskaia6, and Y. Rosenthal7 B. van de Schootbrugge et al.
  • 1Institute of Geosciences, Goethe University Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany
  • 2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NL A1B 3X5, Canada
  • 3Institute of Atmospheric and Environmental Sciences, Goethe University Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany
  • 4Institute of Neuroradiology, Goethe University Frankfurt, Schleusenweg 2–16, 60528 Frankfurt am Main, Germany
  • 5Institute of Geology and Paleontology, University of Münster, Corrensstraße 24, 48149 Münster, Germany
  • 6Royal Netherlands Institute for Sea Research, Landsdiep 4, 1797 SZ 't Horntje (Texel), The Netherlands
  • 7Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, USA

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

Publications Copernicus
Download
Citation