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| Titel |
The enigmatic ichnofossil Tisoa siphonalis and widespread authigenic seep carbonate formation during the Late Pliensbachian in southern France |
| VerfasserIn |
B. Schootbrugge, D. Harazim, K. Sorichter, W. Oschmann, J. Fiebig, W. Püttmann, M. Peinl, F. Zanella, B. M. A. Teichert, J. Hoffmann, A. Stadnitskaia, Y. Rosenthal |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 10 ; Nr. 7, no. 10 (2010-10-13), S.3123-3138 |
| Datensatznummer |
250005013
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| Publikation (Nr.) |
 copernicus.org/bg-7-3123-2010.pdf |
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| Zusammenfassung |
| 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. |
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