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

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 t
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 delta 13C values that decrease from the rim to the center from - 18.8‰ to - 25.7‰ (V-PDB), but normal marine delta 18 O 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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Author:Bas van de Schootbrugge, Dario Harazim, Katrin Sorichter, Wolfgang Oschmann, Jens Fiebig, Wilhelm Püttmann, Mark Peinl, Friedhelm Zanella, Barbara M. A. Teichert, J. Hoffmann, Alina Stadnitskaia, Yair Rosenthal
URN:urn:nbn:de:hebis:30-83780
URL:http://www.biogeosciences.net/7/3123/2010/bg-7-3123-2010.html
DOI:http://dx.doi.org/doi:10.5194/bg-7-3123-2010
ISSN:1726-4189
ISSN:1726-4170
Parent Title (English):Biogeosciences
Publisher:Copernicus-Ges.
Place of publication:Katlenburg-Lindau
Document Type:Article
Language:English
Date of Publication (online):2010/10/26
Year of first Publication:2010
Publishing Institution:Univ.-Bibliothek Frankfurt am Main
Release Date:2010/10/26
Volume:7
Pagenumber:16
First Page:3123
Last Page:3138
Source:Biogeosciences, 7, 3123-3138, 2010 ; http://www.biogeosciences.net/7/3123/2010/bg-7-3123-2010.html ; doi:10.5194/bg-7-3123-2010 ; Biogeosciences Discuss., 7, 4181–4222, 2010 ; http://www.biogeosciences-discuss.net/7/4181/2010/bgd-7-4181-2010.html ; doi:10
HeBIS PPN:229792928
Institutes:Geowissenschaften
Medizin
Dewey Decimal Classification:560 Paläontologie; Paläozoologie
Sammlungen:Universitätspublikationen
Licence (German):License Logo Veröffentlichungsvertrag für Publikationen

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