Multicellular Consortia Preserved in Biogenic Ductile-Plastic Nodules of Okondja Basin (Gabon) by 2.1 Ga

Abstract

We highlight complex ductile nodules found in the FB2 formation of the Okondja francevillian Basin (Gabon) dated to 2.1 Ga, during the Great Oxygenation Event (GOE). More than 500 specimens were collected from unmetamorphosed pelites in an excellent state of preservation. These nodules are divided into two groups: globular and elongated forms, one to three cm in diameter and over six cm long along the axis of elongation. They are characterized by two hemispheres separated by a central zone. They have a remarkable radial structure in spheroidal forms, and often have a polyphased structure in elongated forms. Chemical and microscopic analyses indicate that these nodules are formed over 80% of a fabric of micro-quartz, fossilizing calcite grains (rounded grains and remnants of biofilms) and clay channels. These slightly pyritized nodules also contain iron in the form of hematite and goethite. There are several types of micro-organisms, several biomorphic iron particles (generally less than 300 μm in size) among which are preserved some multicellular clusters measuring between 50 and 250 microns, and some larger biomorphs. Their organization is very complex with fibro-radial and polyphased internal fabric and a discrete external peripheral system. The very low values of δ13C carb in calcite (-17 and -26‰) suggest precipitation of calcite from decomposition of organic matter in anoxic photosynthesis conditions. The morphological, petrographic, geochemical and isotopic characteristics of these nodules indicate a biological origin such as microbial/algal consortia, associated with eukaryotic organisms. They lived on the seafloor, buried just below the surface of the sediment, in calm, shallow and oxido-reducing environment.