EGU22-955, updated on 27 Mar 2022
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Taphonomy of early life: Role of organic and mineral interactions

Julie Andréa Ngwal Ghoubou Ikouanga1, Claude Fontaine1, Olabode M. Bankole1, Claude Laforest1, Armelle Riboulleau2, Alain Trentesaux2, Celine Boissard1, Andrea Somogyi3, Alain Meunier1, and Abderrazak El Albani1
Julie Andréa Ngwal Ghoubou Ikouanga et al.
  • 1UMR 7285-IC2MP, University of Poitiers, Poitiers, France
  • 2UMR 8187-LOG, University of Lille, Poitiers, France
  • 3Synchrotron-Soleil, Paris Saclay, Paris, France

Biogenicity and taphonomy of the early life fossil records are debated as most of the previous studies focussed mainly on isotopes geochemistry. The non-metamorphosed Paleoproterozoic (~2.1 Ga) sedimentary succession of the Francevillian Basin (Gabon) contains the oldest complex multicellular organisms embedded in black shale facies. Several studies have confirmed the biogenicity of these soft-bodied organisms. Here, we used multi-proxy techniques to show that the preservation of these macro-organisms happened in a close system that limits interaction with their host rocks, which leads to their good preservations. The macro-organisms are present in different shapes and sizes: lobate (L), elongate (E), tubular (T), segmented (S), and circular (C), and are often associated with bacterial mats. Except for the C form, most of the other specimens are pyritized. Sulfur isotopes data confirms that pyritization occurred by bacterial sulfato-reduction during early diagenesis. We compare the clay mineral assemblages between the pyritized specimens and the late-diagenetically formed pure pyritized concretions in the sediments because the early pyritization process could not explain the taphonomic preservation alone. Our clay mineralogical data show that the specimens are dominated mainly by randomly mixed layer Illite-smectite (IS MLMs), illite, and chlorite relative to the host rocks. The abundance of IS MLMs indicates incomplete illitization of smectite, potassium deficiency, and limited mineral reactions in a semi-close local chemical system within the fossils.  In addition, the authigenic chlorites are more iron-rich and show vermicular habitus. By contrast, the pyritized concretions mainly consist of well-crystallized illite and less iron-rich chlorite, while the smectite phases are absent. These results confirmed that the diagenetic reaction is controlled by interaction with an open late diagenetic system. We concluded that taphonomic preservation of the ancient fossil record resulted from the early diagenetic growth of pyrite crystals during bacterial sulfato reduction in the fossils, which creates a semi-closed system that drastically reduced fluid-rock interactions with the host sediments.

How to cite: Ngwal Ghoubou Ikouanga, J. A., Fontaine, C., M. Bankole, O., Laforest, C., Riboulleau, A., Trentesaux, A., Boissard, C., Somogyi, A., Meunier, A., and El Albani, A.: Taphonomy of early life: Role of organic and mineral interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-955,, 2022.