Preprints
https://doi.org/10.5194/se-2020-187
https://doi.org/10.5194/se-2020-187

  25 Nov 2020

25 Nov 2020

Review status: this preprint is currently under review for the journal SE.

What makes seep carbonates ignore self-sealing and grow vertically? The role of burrowing decapod crustaceans

Jean-Philippe Blouet1,2,3, Patrice Imbert3,4, Sutieng Ho3,5, Andreas Wetzel6, and Anneleen Foubert1 Jean-Philippe Blouet et al.
  • 1Department of Geosciences, University of Fribourg, Chemin du Musée 6, 1700 Fribourg, Switzerland
  • 2Department of Geosciences, Environment and Society, Université Libre de Bruxelles, Avenue Franklin Roosevelt 50, 1050 Bruxelles, Belgium
  • 3Fluid Venting System Research Group, Nancy 54000, France
  • 4Université de Pau et des Pays de l’Adour, 64000 Pau, France
  • 5Ocean Center, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, 10671, Taipei, Taiwan
  • 6Department of Environmental Sciences – Geology, University of Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland

Abstract. We investigated the mechanisms that govern vertical growth of seep carbonates by studying the sedimentary architecture of a 15-m-thick, 8-m-diameter column of limestone encased in a deep-water marl succession in the Middle Callovian interval of the Terres Noires Formation in the SE France basin. The limestone body, a.k.a. pseudobioherm is characterized by intense bioturbation, with predominant burrows of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation is organized in three tiers. The upper tier corresponds to shallow homogenization of soupy sediment and the second one to pervasive burrowing dominated by Thalassinoides passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range −5 to −10 ‰. The deepest tier in contrast is filled by diagenetic cements arranged in two phases. The first cement phase makes a continuous rim coating the burrow wall, in which carbon isotope data show consistent 13C depletion near −8 ‰ to −12 ‰, indicating precipitation by anaerobic oxidation of methane in the sulfate-methane transition zone. In contrast, the second cement phase is dominated by saddle-dolomite indicating precipitation at a temperature > 80 °C, largely post-dating the burial of the pseudobioherm. The late final blocking of the burrows means that vertical fluid communication was possible over the whole thickness of the pseudobioherm up to the seabed during its active growth. Vertical growth is related to the presence of this open burrow network, providing a high density of localized bypass points across the intra-sediment calcite precipitation zone in the sulfate-methane transition zone and preventing self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key characteristic that prevented passive fill of the burrows is their geometric complexity with numerous subhorizontal segments that could trap sediment shed from shallower bioturbation tiers.

Jean-Philippe Blouet et al.

 
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Jean-Philippe Blouet et al.

Jean-Philippe Blouet et al.

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Short summary
Biochemical reactions related to hydrocarbon seepage are known to induce carbonates in marine sediments. Seep carbonates may act as seals and force lateral deviations of rising hydrocarbons. However, crustacean burrows may act as efficient vertical fluid channels allowing hydrocarbons to pass through upward, hence allowing the vertical growth of carbonate stacks over time. This mechanism may explain the origin of carbonate columns in marine sediments throughout hydrocarbon provinces worldwide.