alexa An Akouemma hemisphaeria Organic Macrofossils Colony Hosting Biodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) dated at 2.2 Ga | Open Access Journals
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An Akouemma hemisphaeria Organic Macrofossils Colony Hosting Biodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) dated at 2.2 Ga

Edou-Minko A1*, Moussavou M1, Sato T2*, Tchikoundzi C1, Sawaki Y3, Ndong Ondo S1, Ortega R4, Maire R5, Kaestner A6, Mbina Mounguengui M1, Roudeau S4, Fleury G7, Carmona A4, de Parseval PH8, Makaya Mvoubou1, Musavu Moussavou B1, Ogandaga Agondjo M1, Sasaki O9 and Maruyama S2

1Université des Sciences et Techniques de Masuku, BP. 901 Franceville, Gabon

2Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, Japan

3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, Japan

4CENBG, Univ. Bordeaux, CNRS, IN2P3, UMR 5797, F-33170 Gradignan, France

5UMR 5319 Passages, Maison des Suds, CNRS-Université Bordeaux-Montaigne, France and Honorary Research Fellow, University of the Witwatersrand, Johannesburg, South Africa

6Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen - PSI, Switzerland

7Muséum d’Histoire Naturelle de Toulouse, 35 Allée Jules Guesde, 3100 Toulouse, France

8Géosciences Environnement Toulouse (GET), Observatoire de Midi-Pyrénées Université de Toulouse, CNRS, IRD, 14 Avenue Edouard Belin, F-31400 Toulouse, France

9Tohoku University Museum, Université de Tohoku, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Japan

*Corresponding Author:
Edou-Minko A
Université des Sciences et Techniques de Masuku, BP. 901 Franceville, Gabon
Tel: +24107173341
E-mail: edouminko@gmail.com
 
Sato T
Earth-Life Science Institute (ELSI), Tokyo Institute of Technology
2-12-1 O-okayama Meguro-ku, Tokyo 152-8551, Japan
Tel: +81357342283
E-mail: sato.t.bq@m.titech.ac.jp

Received date: January 19, 2017; Accepted date: February 06, 2017; Published date: February 10, 2017

Citation: Edou-Minko A, Moussavou M, Sato T, Tchikoundzi C, Sawaki Y, et al. (2017) An Akouemma hemisphaeria Organic Macrofossils Colony Hosting Biodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) dated at 2.2 Ga. J Geol Geophys 6: 281. doi: 10.4172/2381-8719.1000281

Copyright: © 2017 Edou-Minko A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

A colony of silico-carbonate Akouemma nodules found in Akou River sedimentary formations of the Palaeoproterozoic Okondja Basin consists of two groups, spheroidal (ovoid) nodules and elongated nodules. These nodules, which consist of two hemispheres separated by a median disc, are composed essentially of micro-quartz associated with calcite of extra-polymeric substance (EPS) type, clay minerals, organic carbon and oxides and sulphides of iron. They contain tubular microfossils, pluricellular clusters, microorganisms and vesicles, and have undergone considerable deformation by mutual lateral compression in tabular beds. They were interpreted as biogenic nodules hosting microorganisms.

We provide the following additional supporting evidence: Akouemma nodules exhibit internal fibro-radial fabrics initially composed of fibres and carbon particles; the initially well-organized structures are decaying and are in particles and fragments that are dispersed in the undeformed siliceous mass. These internal fabrics are strongly highlighted by Al- K- (Ti)-rich clay minerals that are often in close association with fibres and carbon particles. They had likely a vegetative reproduction by duplication.

We infer that the Akouemma nodules are macrofossils of sessile soft-plastic body organisms. These macrofossils, recently dated at 2.2 Ga, are designated “Akouemma hemisphaeria” and bring a new vision to the “large colonial organisms” found in the Franceville Basin.

Keywords

Macrofossils; Akouemma hemisphaeria; Internal fibroradial carbon; K-rich clay mineral; Syn-sedimentary deformation; Microorganisms; Palaeoproterozoic; Okondja basin; Gabon

Introduction

The sedimentary formations of the Francevillian Group of Gabon are very well preserved and are among the few non-hydrothermal, non-metamorphic Palaeoproterozoic formations in the world. The FB Formation of the Francevillian Group is characterized by smectite/ illite-rich clay minerals that are both randomly ordered and ordered, suggesting a low grade of diagenesis [1]. These formations contain a biodiversity of micro- and macrofossils [2-8], and current research shows macrofossil-like material distributed from FB1 to FD in the Francevillian Group (Franceville, Lastourville, Okondja and Booué Basins) including specimens from the Akou River in Okondja Basin (Figure 1).

geology-geosciences-Geological-context-fossiliferous

Figure 1: Geological context of fossiliferous Palaeoproterozoic sediments in Gabon, showing the location of four sedimentary basins and the stratigraphic columns of the Franceville and Okondja Basins (Akou sector). The stratigraphic level (red band) within Francevillian unit FB2 containing Akouemma hemisphaeria (Akouemma nodules (B)) from the Okondja Basin [8] is shown together with evidence from the Francevillian Group, which is known to contain stromatolites, microfossils ([2,3,6], large colonial organisms [5,7]. The current investigations have highlighted a variety of pyritized forms including vermiforms in FB1 (A); oval shapes in an upright position with a flattened base, showing in vertical tomographic section an internal symmetry and a higher density in edging, in FB and FC (C); and flattened shapes containing phosphorus and calcium and showing parallel thin strips in vertical section, in FD (D). Geochronological dating: 2910-2860 Ma basement, 2191+/- 13 Ma FB [9]; 2083+/- 6 Ma FD [71,72]. Scale bars (A-B-C-D): 1 cm.

The Okondja Basin is one of four Francevillian Basins (Figure S1) in which deposits are marked by a high abundance of organic matter. Its fossiliferous FB2b formations, dated at 2.2 Ga [9], are composed of repeated alternations of sandstones, basaltic lavas, siltstones, pyroclastic sandstones, black shales and mafic intrusions [10] (Figure 1). These basins exhibit highly negative δ13Corg values ranging between -21 and -46%ο [11-13] and δ13Ccarb values of diagenetic carbonate nodules and carbonate formations between -7 and 10%ο [6,14] with highly positive values being observed in the dolomitic carbonate formations of the Lastourville Basin [6].

We report remarkably preserved nodules (7-62 mm in length and 7-33 mm in diameter) localized near the top of subunit FB2 at the Akou River to northeast of Okondja (Figure S1). One hundred and ten specimens were the subject of a statistical, mineralogical and geochemical study. These nodules, consisting of two hemispheres with a median zone (Figures 2A-2k and 3A-3F) and designated Akouemma, have an essentially siliceous composition with more than 80% homogeneously distributed micro-quartz; other elements, such as Fe (9%) in the sulphated and oxidized form, Ca (3%) in the form of calcite, Al (7%) K (0.3%) and Ti (0.1%) of the clay minerals pole quite clearly highlight the internal fabric [8]. They present negative δ13Corg values ranging from -26 to -37%ο, negative δ13Ccarb values of -17 to -26%ο and δ34Spyr values of -7 to 18%ο, with most being higher than 7%ο [8]. These values suggest calcite precipitation and pyritization of organic matter via bacterial activity in a closed and reduced environment [15].

geology-geosciences-Complete-type-specimen

Figure 2: Morphology and internal structure of Akouemma nodules showing ovoid forms. (A) Complete-type specimen showing a nodule displaying a distinct median disc plus a surrounding mudstone matrix (green arrow). (B) Same specimen with the upper hemisphere raised to show the internal surfaces of the upper hemisphere and the median disc; note the central orifice on the median disc (white arrow). (C1) View of a sedimentary plate containing a nodule in the plane of the medial disc; note the central orifice in the median disc (white arrows) and the peripheral arrangement of repeated patterns (approximately 250 μm each) on the border of the nodule and in the sediment (probable form of microbially-induced sedimentary structures (MISS) red arrow). (C2) Profile view of the median disc; note its thickness, which decreases from the border towards the centre. (D) Internal surface of an upper hemisphere, showing radial sculpture and a central nipple-like feature. (E) Internal surface of a lower hemisphere, showing radial sculpture and a corresponding central depression. (F) Superposition of the two hemispheres at the central nipple-like feature and the corresponding central depression (likely communication point between hemispheres); note the empty space left by the median disc; (Figure S3E-S3K). (G-K) Vertical section views of five typical specimens consisting of two hemispheres and a median disc. (G) Polished plate; note the iron lines on the median disc (white arrow). (H) Thin section showing the median disc glued at the hemispheres; note the corrugated secondary crack. (I) Tomographic view showing the cracks delimiting the median disc. (J) Thin section containing two fairly similar undeformed specimens (1 and 2) with two hemispheres and a medial zone; the medial zones are very fragile, with lateral secondary cracks. (K) An isolated deformed nodule with a flattened and wave-like base (Figure S7); note the development of secondary cracks in the median disc, which was initially attached to the hemispheres (OKO 2). (L-N) Views of the top hemispheres of three nodules showing (L-M) depressions around the top (green arrows); (N) a wide dimple on the top (yellow arrows).

geology-geosciences-internal-median-surface

Figure 3: Morphology and internal structure of Akouemma nodules showing elongated forms. (A) View of an axial vertical section showing an altered border (white box 1), internal zone (white box 2) and a slight depression on the edge (white arrow); the blue arrow indicates the internal surface, and part of the median disc is indicated by the yellow arrow. (A1) Detailed view of white box 1 in ‘A’ showing a yellow lenticular form (probably clay) embedded in the nodule; note the deformed zone of the nodule under the lenticular body (green arrow). (A2) Detailed view of white box 2 in ‘A’ showing internal polygonal structure (red arrow) formed by abutting radial calcite bands; also seen are small radiating ‘channels’ filled with clay mineral and small secondary quartz grains. (B) View of a polished section showing an elongated nodule with a flattened upper hemisphere; the internal median surface is barely visible (AKOU-16). (B1) Thin-section detailed view of the white box in ‘B’ showing an internal polygonal structure (red arrow) and a slight depression on the edge (white arrow) on elongated nodule AKOU-16; for more details see Figure 11. (C-C2) View of an axial vertical section of the AKOU 14 nodule showing its internal polygonal structure (white box) and a median zone with curved lamellae. (C1) Detailed view of the white box in ‘C’ showing a more-or-less degraded internal polygonal structure (red arrow). (C2) Thin section showing internal polygonal structure formed by abutting clay mineral channels filled with opaque minerals of pyrite and iron oxides (the red dot serves as a reference). (D-D2) Different external and internal aspects of Akouemma elongated nodule in a single piece: (D) Top view of the hemisphere showing a slight depression on the edge (white arrow); (D1) Tomographic view of the vertical section of the same nodule showing two hemispheres separated by the median disc, note the communication point probable between the two hemispheres (yellow arrow) and the internal polygonal structure (red arrow); (D2) Tomographic view of the lateral section at the median disc of the same nodule showing a central depression on the cone (yellow arrow). (E-E2) Different external and internal aspects of Akouemma elongated nodules with two "segments". (E) Macroscopic view of the elongated nodule showing two hemispheres separated by a median surface; note the shallow furrow around the nodule (white arrow). (E1) Top view of the upper hemisphere clearly showing the furrow perpendicular to the elongated axis of the nodule. (E2) Tomographic view of lower hemisphere; view of the horizontal section parallel to the median surface showing the suture between two “segments” that connects the edge furrow, note a central depression in the left “segment” (yellow arrow), it is barely visible in the right “segment” (doted circle). (F-F1) Different external and internal aspects of Akouemma nodules with two "segments" or a "double nodule". (F) Macroscopic view of the top of the "double nodule" in the field showing two "attached nodules"; note the deep furrow that separates them. (F1) Vertical section of the same sample through two conjoined specimens showing weathered margin, radial fabric and median sutures; note the polygonal structures on either side of the suture. (G-G2) Different external and internal aspects of the elongated Akouemma nodule with three "segments". (G) Macroscopic view of the elongated nodule showing two hemispheres separated by a median surface (blue arrows); note the two furrows, one of which is shallow (furrow 1) and the other fairly deep (furrow 2) around the nodule (white arrows). (G1) Vertical polished section of the same nodule showing two sutures, one is slightly developed (suture 1) and draws an incomplete limit between the “segments”, and the other is more developed (suture 2) and shows a sharper limit between the “segments”; note the polygonal structures on either side of each suture. (G2) Tomographic view of the vertical section of the same nodule showing the sutures between the three “segments” with net separation of “Father” specimen on the right; see also Figures S2, S23.

Materials and Methods

The geological objects under study are Akouemma nodules from the Okondja Basin [8], and the main analyses of these nodules are described below.

SEM-EDS

Scanning electron microscopy-energy dispersion X-ray spectrometry (SEM-EDS) analyses were performed on Os-coated samples using a Hitachi S-3400N SEM with a Bruker Xflash 5010 at the Tokyo Institute of Technology to investigate the distribution of elements such as carbon, aluminum, potassium, and iron. Measurements were performed under an accelerating voltage of 15 kV. Complementary analyses were performed with a JEOL 6360LV SEM and a Bruker SDD XFLASH 5010 at the GET of Toulouse.

PIXE analysis

Nuclear microprobe analysis using PIXE (particle-induced X-ray emission) enables quantitative imaging of the distribution of chemical elements in various types of materials, including complex geological samples. In addition, the combination of PIXE and RBS (Rutherford backscattering spectrometry) allows full quantification of the concentrations of these chemical elements, expressed as grams of trace element per gram of sample at the micrometre level [16]. A thick section of sample AKOU16 was analysed using a PIXE microprobe at CENBG (Bordeaux Gradignan). A 1.775-MeV proton beam was employed not only to provide information on the distribution of chemical elements with atomic number >11 using PIXE but also to enhance the carbon RBS signal through carbon resonance at 1.75 MeV. The proton beam was focused down to 2 micrometres on the surface of the sample to achieve an intensity of 500 pA and then raster scanned to allow chemical imaging of 1.2 × 1.2 mm2 regions of the sample. Chemical maps were recorded in a 256 × 256 pixel matrix, resulting in a spatial resolution of 4.6 μm/pixel. In the case of the AKOU16 fossil, 393 square zones of 1.2 × 1.2 mm2 each were analyzed to cover the entire section of the specimen (approximately 576 mm2). Higher spatial resolution maps were also generated for specific areas such as carbonate-rich regions; in this case, a smaller beam (0.8 μm) with a reduced intensity (180 pA) was used for scanning, employing a 256 × 256-pixel matrix in regions with a maximum area of 0.2 × 0.2 mm2, thus achieving the highest spatial resolution of the setup (0.8 μm). Distribution maps for Al, Si, S, K, Ca, Ti, Mn, and Fe were obtained through PIXE analysis, and carbon and oxygen concentrations were determined simultaneously through RBS analysis. The quantification procedure was validated based on U.S. Geological Survey Geochemical Reference Materials BCR2 (Basalt, Columbia River) and BIR1 (Icelandic basalt).

Laboratory-based computed tomographic (CT) analysis

For sediment-enclosed nodules, data were acquired using a General Electric LightSpeed CT scanner with 0.5 mm thick slices. For single nodules and specific areas, data were acquired with a Carl Zeiss Nanotom and a Carl Zeiss Metrotom, with slice thicknesses varying from 17 μm to 70 μm depending on the size of the selected areas and the type of CT equipment. All CT and micro-CT datasets were exported using the DICOM export file format to ensure the best image quality and inter-operability, and the 2D and 3D images from the CT and micro-CT DICOM stacks were processed on a Dell Precision T7400 Dual Quad Core Intel Xeon 3.2 GHz workstation using 32 GB of DDR RAM and a Quadro FX 4600 graphics board running VG Studio Max 2.0 and 2.2 × 64 from Volume Graphics GmbH.

Complementary micro-CT analyses for X-strata were performed with a Micro-XCT-400 system at Tohoku University. All strata were photographed using a MOLCER 3D image viewer.

Neutron tomography

Neutron tomography is a transmission imaging technique in which neutrons are used as a radiation source to produce the radiographs required as projections to reconstruct a three-dimensional image of a sample [17]. Neutrons interact differently with matter than X-rays, and for many elements the two modalities provide completely different attenuation coefficients. Typically, neutrons are sensitive to low- atomic number elements such as hydrogen and lithium, whereas many highatomic number elements exhibit lower attenuation. Neutrons have the ability to discriminate between different isotopes of the same element. The difference in attenuation coefficients means that neutron imaging serves as a relevant probing method for palaeontological and geological samples [18]. However, neutron imaging is a less common imaging method than X-ray CT because few strong neutron sources are available worldwide; the neutron flux required to obtain a good image quality requires either a research reactor or a spallation neutron source. This investigation was performed at the Paul Scherrer Institut using the ICON cold neutron imaging beam line [19]. The CT data were acquired using the micro-tomography setup [20], which has a field of view of 27 × 27 mm2 and a pixel size of 13.5 μm. A 20-μm-thick Gadox scintillator was employed to convert the neutrons into visible light, which was captured using a cooled CCD camera (Andor DW436). The data were reconstructed using Octopus CT reconstruction software in parallel beam mode, which results in a valid approximation due to the high collimation of the neutron beam. This procedure provided CT datasets with 13.5-μm cubic voxels, which were saved as TIFF images. Kiptool software (Paul Scherrer Institut) was then used for 3D image processing, and final visualization, rendering and animation were performed with Drishti 2.5.1 software.

Results

Akouemma specimen morphology

The spheroidal (ovoid) specimen type measures an average of 2 cm in diameter and consists of two hemispheres separated by a discshaped central zone designated the "median disc" (Figures 2A-2C and 2G-2K). Each hemisphere has a central cone, and the median disc has a central orifice [8] (Figures 2B, 2C1 and 2D-2E and Figure S3IS3K 1). The internal surfaces of the upper hemisphere show a central nipple-like feature, and those of the lower hemisphere may show a corresponding depression (Figure 2D-2E and Figure S3E-S3K). The tripartite spheroidal shapes are similar and represent more than 60% of specimens. They sometimes have a flattened base (Figure 2K) with different structures at its edge (Figure S7B-S7I).

Elongated forms represent approximately 40% of the population. They have a diameter similar to that of the spheroidal shapes; the maximum length measured on the axis of elongation barely exceeds 6 cm (Figure 3). These specimens appear in the form of elongated nodules in a single piece (Figure 3A-3D) and in the form of elongated nodules with two or three segments [8] (Figure 3E-3G). They tend to have a bipartite structure with two hemispheres separated by a continuous median surface and a barely visible median disc at the ends of the nodule. Central nipple-like features or corresponding depressions are sometime visible on the median surface (Figures 3D2, E2 and 4D1). Their external cortex, which is generally continuous on each hemisphere, is often marked by one or two external furrows that are more or less deep and are oriented perpendicular to the direction of elongation (Figure 3A-3G). The transition from spheroidal to elongated forms is progressive (Figure S2) [8].

The nodules occur in isolation (Figures 2G-2I and 5), relatively distant from each other, near each other (Figures 2J, 4A and 5), or in groups (Figures 4 and 5). When present as a group, the nodules are deformed along a bedding plane (Figures 4A and 5). Some nodules exhibit strong volume reduction and lateral spreading of matter (Figures 4E-4I and 5E-5H), distortion and rotation of hemisphere and median disc (probably around of the ‘communication point’), micro-folding (Figures 4J-4K and 12C-12C3), and vertical flattening due probably to burial compaction (Figure 4B-4C). In AKOU 16 and AKOU 35 nodules only upper hemisphere is flattened, with deformation of the sediment under AKOU 35 nodule (Figures 3B, 11 and 12A-12A1).

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Figure 4: Morphology and internal structure of deformed Akouemma nodules. (A) View of a vertical section of three spheroidal nodules that are more or less weathered in sediment; two are adjoined with deformed contact surfaces and one is an isolated nodule; compacted sediment is visible between the nodules (yellow arrows). The green arrow shows a crushed contact surface; note the low ripple sediment below the nodules (dotted lines). (B-C) Macroscopic views of two joined and flattened specimens in (B) side view and (C) polar view; they seem to be covered by an ‘envelope’. (D) Macroscopic view of two mutually laterally compressed specimens with elongated (1) and spheroidal (2) shapes; note the deformed contact between them (yellow arrow) and the continuity of the cortex of the elongated shape, which is marked by a slight depression (white arrow); and in (D1) by an internal suture between two radial discs (red arrow); note the central nipple-like features on the elongated shape (doted circle); note the difference between these two adjoined nodules and the "triple nodule" (Figure 3G-G2). (E) Macroscopic view of five mutually laterally compressed and intensely compressed specimens contiguous with specimen (2) in the form of a ‘T’; note the aligned patterns on the sediment and the curve limit between sediment and nodule parallel to the aligned patterns (white arrow). (F) Three contiguous and deformed specimens; the medial zone of nodule 3 in the middle is strongly compressed (yellow arrows). (G-L) Three other contiguous and deformed specimens. (G) Together, the three nodules present deformed contact surfaces (dashed white line). (H) Vertical section of two specimens with a highly deformed contact surface; the blue arrows indicate the direction of deformation, note the folded and raised border of nodule 1 (clear blue arrow). (I) Tomographic view of the same specimens; note that the upper hemisphere of specimen 1 is pinched between specimens 2 and 3 (yellow arrow). (J) Vertical section of the same nodules (macroscopic view) showing the deformation of the medial disc and the two hemispheres; after deformation, the median discs, which are usually in a more or less horizontal position (yellow and blue bars), are offset downward toward specimen 1 (blue arrow) and upward toward specimen 2 (yellow arrow). (K) Detailed view of the white box showing the deformation of the medial disc and the lower hemispheres; note the bending (white arrow) and dissolution of the rods of the median disc (yellow arrow). (L) Detailed view of the yellow box showing a ferruginous lenticular body (yellow arrow) with a deformed band beneath it (green arrow), similar to Figure 2A1. The red arrows indicate the direction of mutual lateral compression.

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Figure 5: Undeformed tabular sedimentary plate containing undeformed isolated nodules, contiguous nodules, slightly deformed nodules and deformed nodules in a chain. (A) Macroscopic view of the tabular plate (planar, not deformed). (B) Transparent tomographic view of the same plate showing the chain of six joined and deformed nodules (1-6), undistorted isolated nodules (7, 10, 11), and contiguous and only slightly distorted nodules (8-9, 12-14), with a cutting line (ab) on the chain of nodules. (C-C1) Tomographic view of the lateral section, clearly showing the undeformed nodules and the deformed nodules in the chain and nearby (nodules 1-6, 10); note in (C1) the truncated median disc of nodule 6 and its central orifice showing the cone of the lower hemisphere with its central depression. (D) Tomographic view of the vertical section (ab) showing the more-or-less intense deformations of the nodules; note the significant reduction in volume of the lower hemisphere of nodule 2, the fan-shaped spreading upward of its upper hemisphere, and the fan-shaped spreading downward of lower hemisphere of nodule 4; much of the lateral constraint would come from nodule 3 (red arrows), well developed and showing no deformation. (E-J) Tomographic view of the lateral sections (top down) of the six nodules in chain and nodule 10; note the lateral spreading of matter within nodule 2 (yellow arrow); the nodule 10, adjoined to the deformed nodules in chain, does not present any deformation and highlights the median disc when moving from the upper hemisphere (E) to the lower hemisphere (J). Note the encrusted "oblong-shaped bodies" bordering the nodules (white arrows).

External and internal organization

The external organization around the Akouemma specimens is discrete but complex. Macroscopic observations indicate that the surfaces of these specimens are marked by changes in relief and by a number of depressions and grooves ranging from a few millimetres to approximately a centimetre in size (Figure 2L-2N and Figure S3AS3D). These surface structures do not exhibit a precise geometrical organization but are often aligned and rectilinear in form, and the alignments are formed by lenticular bodies ranging from a few tens of micrometres to >650 μm long. They can be siliceous, ferruginous, or formed from clay minerals, sometimes with relics of organic carbon (Figure 3A-3A1 and 4K-4L and Figures S5, S6A, 5A2, and 5A4-5A8). Four examples of peripheral ‘encrustation’ bodies are observed. Three similar examples are located on the surfaces of the upper hemispheres (Figure 3A-3A1 and Figures S5A-S5A1, S5B-S5B1 and S6A, S6A2), and one is observed at the periphery of a compressed medial disc (Figure 4K-4L), with local deformation of the nodule under the lenticular body. SEM-EDS analyses confirm also the existence of Al-K-rich lenticular bodies inside and along the edges of nodules and display their internal and peripheral alignment (Figure 9O4 and 9P4). Alignments of repeated patterns are also observed in the sediment and around nodules (Figures 2C1 and 4E).

The internal organization of the nodules is essentially radial. Macroscopic samples (Figures 2B-2F, 4D1 and 8A), scanned vertical sections Figures 2G-2H, 2J-2K, 3A-3C, 3E1, 3F1, 4J-4L and 9A1-9A2), optical microscopy (Figures S9A, S10, S11A), laboratory-based CT analysis (Figures 2I and 3D2), nuclear microprobe analysis, energydispersive X-ray spectrometry and scanning electron microscopy of the distribution of chemical elements (Figures 9B1-9S5, 11C-11H, 12B-12C, 13 and 14 and Figures S4, S5, S6, S15-S18 and S20-S22), neutron tomography (Figure 10) revealed a highly organized initial structure in the Akouemma nodules in the form of a centrifugal fibro-radial and fabric.

The relatively simple system of spheroidal forms consists of a radial fabric of filaments and particles arranged centrifugally downwards in the lower hemisphere, upwards in the upper hemisphere and laterally in the median disc [8] (Figures 2G-2K, 9,10 and Figure S4).

In detail, it consists of:

1. a more or less homogeneous cone that constitutes the origin of the fan-shaped radial system (Figure 9B1, 9C1 and 9D1);

2. a ramification zone (or development zone of the fan-shaped radial system) where the structuring of tufts begins, alternating with the zones of end tufts (Figure 9E2-9J3 and Figure S13);

3. a fan-shaped border zone (Figure 9K4-9P4);

4. a median zone consisting of rods arranged around the cones of the hemispheres to form the median disc, which is remarkable for its central orifice (Figure 9Q5, 9R5 and 9S5 and Figure S4).

The fibro-radial fabric of elongated forms is more complex. The single-piece elongated shapes exhibit a polyphase internal fibro-radial system containing a mostly internal polygonal structure that likely form a junction (suture) between two single radial fabrics (Figures 3A,3 A2, 3B-3B1, 3C-3C1, 3E1, 3F1-3F2; 11C-11H, 12A-12A1 and 12B-12B3). In elongated forms with two or three segments (double or triple nodules), each segment consists of an internal fibro-radial fabric (Figures 3F1, 3G1 and 12B-12B3). The elongated forms also display at their junctions a more-or-less-developed polygonal structure on either side of the median surface and a vertical suture that connects the outer furrow. We note that the more developed the polygonal structure, the deeper the outer furrow and the more obvious the internal suture that separates two 'segments' until individualization of the spheroidal bodies (Figure 3 and Figure S23). The polygonal structure, the outer furrow and the vertical suture would correspond to a phase of division.

Just as spheroidal nodules and undeformed nodules (in single, double or triple nodules) have non-deformed structures (fibro-radial fabrics) (Figures 3, 5, 9, 11 and 12B-12B3 and Figures S4, S23), the deformed and laterally compressed nodules have deformed internal structures (fibro-radial fabrics) (Figures 5, 12A-12A1, 12C-12C3, 13L3, 13P5, 14J8, 14N9 and Figure S5B1), essentially recorded in the distribution of Fe, (Ca), and (Ti)-K-Al-rich clay minerals.

Mineralogy and microorganisms

The Akouemma nodules exhibit a relatively simple mineralogy consisting mainly of micro-quartz and secondarily of oxides and sulphides of iron, organic carbon, calcite, and clay minerals including kaolinite clay, chlorite-vermiculite (K-rich clays), and lepidocrocite (Tirich clay) [8]. In non-deformed specimens, these minerals are arranged according to the radial system (Figures S6A-S6A3, S7B-S7C, S8B1-S8B2, S9A, S10 and S11A) and crystallize in the deformed zones according to those structures (Figure 4J-4L). Micro-quartz represents over 80% of the mass and is homogeneously distributed in the specimen as grains 2 to 5 μm in size (Figure S7C-S7E), which are locally associated with macro-quartz greater than 30 μm in diameter (Figure S10E, G, and H). Iron oxides and sulphides, calcite, clay minerals and organic matter make up less than 20% of the mass, and it is these “secondary” minerals that largely highlight the internal radial fabric of the nodules, showing its alignment, which may be continuous over a distance of 0.5 mm in the radial direction (Figures S10C and S11A) [8]. This orderly organization sometimes gives way to broken fragments dispersed in an undisturbed siliceous mass (Figure S11B). Other well-organized rows of Si-Al-Fe cell-like structures (2-5 μm in size) are disrupted with the dispersion of cell-like features in the homogenous siliceous mass (Figure S11C). Carbon particles (~5 μm) can be trapped in black “cubes” surrounded by iron in the oxidized form (Figure S7B, G-I); these iron oxides can completely replace the organic particles, and this association of iron and organic carbon may contain relatively high values of Al and Si (Figure S7I1-S7I3). Other carbon particles are trapped in siliceous “bricks” alongside red spherules (Figure S8G-S8H). Oxidation (and silicification) of “pyrite crystals” with bristling surfaces seems to lead to the formation of "cubes" and "bricks" with surfaces bristling with orange to orange-yellow particles, and sometimes spreading of particles (Figures S7K-S7K2, S8B1-S8H). Ferro-silico-aluminous spherules 2-5 μm in size and ranging in color from grey to yellow-orange to red are packaged in pockets with a thin film of silica (Figure S9C, S9D, and S9E) [8] or in ‘bricks’ with a siliceous outer wall, where they are arranged in a regular fashion (Figures S8E-S8H, S9F-S9N). The internal ‘brick’ walls are lined with a thin border of more ferruginous spherules (Figure S9I- 8L). The contact between the "brick" and the siliceous mass occurs in the form of a rectilinear net with silica platelets arranged parallel to the brick walls (Figure S9J, S9K and S9N), suggesting rapid crystallization of the silica and not crystallization via epigeny. This explains the nearly intact preservation of the internal structures of the nodules.

The Akouemma nodules contain a few cells and pluricellular clusters that are reminiscent of an algal prokaryote or eukaryotic organism, tubule (50 μm in diameter) and ring of yellow-orange particles (5-10 μm) ferro-bactéria like, small clusters of coccus-like bacteria, broken and degraded tubules with colours varying from green to red, oblong shape bodies, red cell-like spherules, carbon particles, and a large number of cyanobacteria that produce an EPS-type calcite [8], and tubular filaments similar to Neoarchean mineralized tubular microfossils (Figures 6, 7 and 8 and Figures S6A1, S7G-S7K2, S8F-S8H, S9A-S9N, S10E-S10H, S11 and S12) [21].

geology-geosciences-clusters-cell-like-structures

Figure 6: Elements of calcite biofilm, clusters of cell-like structures, and filaments in nodules. (A-B) Biofilm and amorphous calcite with small clusters of cocci-like bacteria (A, crossed nicols; B, parallel nicols, OKO-3b). (C) Example of biofilm with alignment of a continuous area of calcite micro-grains (AKOU-13). (D) Unjoined calcite grains, likely arising from the alteration of the calcite biofilm (crossed nicols, OKO 3b). (E) Close-up view showing the stack of cell-like structures infilled with red iron oxide (hematite) (crossed nicols). (F) Filament (50-μm diameter) and "ring" of ferrobacteria (3-10 μm in size) in a cluster (parallel nicols, OKO-13).

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Figure 7: Example of silicified filaments observed in the mass of the chert on four samples. (A) Thin section OKO 1 (not analysed; crossed nicols). (B) Thin section FRA 13 (not analysed; crossed nicols). (C) Thin section OKO 2, tubular filament (not analysed; crossed nicols). (D) Thin section OKO 3b, branched filaments. (not analysed; crossed nicols). (E) Polished surface OKO 3b (reflected light). (F) Polished surface OKO 3b (reflected light).

geology-geosciences-silicified-filaments-observed

Figure 8: Examples of filaments and silicified cells obtained by scraping in the siliceous mass (chert) of the OKO 26 fibro-radial fabric. (A) Vertical section of OKO 26 and the studied surface. (B) Branched siliceous filament embedded in the chert; note the sheath at one end (yellow arrow, analysed; crossed nicols). (C) Branched siliceous filament adorned with a cluster of ovoids (2-5 μm in size) (not analysed; crossed nicols). (D) Set of hollow filaments showing their tubular shape (diameter 5-8 μm) with a wall measuring 0.5 to 1 μm thick (not analysed; crossed nicols). (E) Detail of an extremity of a hollow filament showing a "double particle" and containing granules of a few μm (not analysed; crossed nicols). (F) Extremity of the same tube with a spherule-like content (not analysed; crossed nicols). (G) Other sheathed hollow filaments (not analysed; crossed nicols). (H) Fragment of large filament (tube) showing very clearly ovoid shapes resembling cells (white and yellow arrows) (not analysed crossed nicols). (I) Detail of a protist-shaped cell (white arrow) (not analysed crossed nicols).

geology-geosciences-silicified-filaments-observed

Figure 9: Internal fibro-radial system and chemical elements distribution map in Akouemma TS1. (A1) Thin axial vertical section of an Akouemma nodule showing the radial fabric of the tufted zones. Starting from the cones of the hemispheres, this radial fabric is upwardly centrifugal in the upper hemisphere and downwardly centrifugal in the lower hemisphere. (A2) Thin axial vertical section of another nodule and the analysed zones (1, 2, 3, 4, and 5). (B1-D1) Views of the lower hemisphere cone. (B1) BSE image showing the centrifugal fibro-radial fabric from the top of the cone, which is topped with a highly aluminous mass in (C1) that is positioned parallel to the border; the white arrow indicates the radial direction. (D1) Distribution of silica and iron showing the relatively homogeneous distribution of silica and the peripheral position of the iron surrounding the uppermost mass, which is rich in aluminum. Iron is also present in the form of edging with a continuous radial position; this border does not appear to be a fissure but a boundary between the two domains of the cone. (E2-G2) View of an internal fibro-radial branching zone (individualization of a ‘tuft’). The fibro-radial fabric is highlighted by the distribution of aluminum (clays), which indicates two intersecting directions. The image showing the superposition of silica, iron and carbon demonstrates the homogeneous distribution of silica, the radial arrangement of carbon particles and filaments (white arrow) in the branched area. Iron is in the form of particles and rods that are parallel to carbon in the branched area. (H3-J3) View of the end of a ‘tuft’ (yellow arrow) associated with several branches in a fan shape, with the entire structure highlighted by aluminum; carbon, again arranged in the radial direction, is concentrated in the central part of the tuft. Note the two carbon-rich patterns (white arrow). (K4-M4) View of the wavy border of the lower hemisphere, ending in a fan; the radial system is highlighted by aluminum and potassium. Note that the edging is composed of iron, aluminum and potassium, suggesting an envelope along the wavy edges of the nodule. Silica shows a fairly uniform distribution; iron is distributed either in fissures or particles according to the radial system or as a border at the wavy edges of the nodule. See the white boxes for details. (N4-P4) Details of the white boxes on the edge of (K4-M4). (N4) Overlay of the BSE image and iron showing the fissures (dark areas) and the form of a border around the nodules (white arrow). (O4) Distribution of aluminum, highlighting the radial and peripheral systems; note the aligned lenticular forms (more than 200 μm in length) within and around the nodule (white arrows). (P4) Superimposed images of silicon and potassium that clearly show lenticular forms (white arrows). (Q5-S5) View of the median zone showing rectilinear rods; iron is more concentrated at the borders of the rods.

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Figure 10: Neutron micro-tomography of Akouemma specimens. Neutron attenuation is globally more sensitive to light atoms (see Materials and Methods for details). Three-dimensional images of two reconstructed Akouemma specimens, FRA0213 (A, B, and C) and FRA0321 (D-H), obtained with 13.5-μm voxels. The data were filtered and thresholded to facilitate visualization of the internal structures, which are characterized by lower density and by the presence of iron particles and filaments that are generally arranged centrifugally downwards in the lower hemisphere, upwards in the upper hemisphere and laterally in the median disc (white arrow). Note the clearer internal partitions and the edges of the specimens, which are most likely composed of higher concentrations of iron oxide and/or manganese, as well as cracks (in transparency) at the median disc. The colour scale represents the neutron attenuation coefficients.

geology-geosciences-elongated-Akou-nodule

Figure 11: (A) Vertical section of elongated Akou nodule (AKOU 16). (B) Transect (a-b) showing the distributions of Fe and S. Sulphur is more concentrated in the ‘core’ of the specimen as pyrite. (C-H) Micro-PIXE images of a vertical section of AKOU 16 showing the distributions of silicon, iron, aluminum, potassium, titanium and calcium. The polygonal structure is clearly highlighted by Al, K and Ti of the clay minerals and by Ca.

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Figure 12: Examples of flattened elongated AKOU 35 nodule and two nodules joined together. (A-A1) Vertical section of flattened elongated AKOU 35 nodule showing in (A) a flattening of the upper hemisphere (yellow arrow) and a folding of sediments below (red arrow); note the internal polygonal structure (white arrow) (A1) the scan of same sample shows a distorted internal structure of the upper hemisphere, including the internal polygonal structure (arrow yellow); note the ‘duplication’ of nodule (red dotted line). (B-B3) Two non-deformed nodules (double nodule) in vertical section (B); with distribution maps of the elements Si, Al, Fe (B1-B3); note the median surface continuous on both nodules, also iron ring is continuous on the periphery of the nodules, and the internal fibro-radial fabrics of the two nodules form a polygonal structure (Box in dotted line) on either side of the suture, that is highlighted by Al. (C-C3) Two contiguous and deformed nodules, in vertical section (C); with distribution maps of the Si, K, Fe elements (C1-C3), which show an intense deformation of the hemispheres, with displacement of the median discs (yellow arrows), hemisphere rotation (red arrow) around a communication point probable (red circle), micro-folds (white arrow) and raised edge (green arrow); note encrusted oblong shape (red-white arrow). Clay minerals (represented by Al and K) clearly show the internal structures of undeformed nodules and deformed nodules. As the “triple nodule” (Figures 3G-3G2 and Figure S23), the non-deformed “double nodule” suggest a progressive evolution from the elongated forms into single piece (of Figures 3A-3D and 11 and AKOU 35 for example), via the intermediate step (Figure 3E-3E2), while the adjoining deformed nodules suggest a "confrontation" between two completely independent bodies.

Distribution of chemical elements

Organic carbon and calcium: Organic carbon is generally unevenly distributed both within single nodules and between different nodules, probably as a result of decomposition and mineralization. The maximum carbon content of the Akouemma specimens is 6.7%, and the average δ13Corg value is -26.9% [8].

Organic carbon occurs in several different forms: a flattened shape with a characteristic organization of repeated patterns and various forms (Figure 13E1 and Figure S14), including a massive form >500 μm in size (Figure 13G1 and Figure S14bis) and small encrusted oblong bodies (Figure 14D6 and 14E6 and Figure S19), as small-scale aligned carbon particles or filaments (Figures 13J2, 13N3, 13R5 and 14H7 and Figure S6A1 and S17C), and as particles with varying degrees of alignment and dispersion in the siliceous mass (Figures 13K2, 13O3, 13S5, 14I7, 14L8, 14M8, 14P9, and 14Q9 and Figures S13B, S13F, S16C, S16D, S17C, S18C, S20C, S20D, S21C, S21D, S22C and S22D). Some examples of these carbon masses are encrusted within the specimens and show fairly clean contact with them (Figures 13G1, 14A1, 14D6 and 14E6). In the first example, the carbon mass has a sinuous edge with a rounded contour in the nodule; this latter showing a raised edge at their contact (Figure 13G1 and Figure S14bis). The second example consists of a carbon mass with an oblong form (Figure 14A1, 14D6 and 14E6 and Figure S19A1, S19C, S19D and S19E), which is similar to encrusted "oblong-shaped bodies" tomographically and observed on the edges of nodules in chain (Figure 5C, 5F-5H, 5J), to the oblong form highlighted by the potassium distribution map (Figure 12C2), or to the oblong forms (20-100 μm) that are regularly observed microscopically, sometimes containing red spherules cell-like (Figures S8B2, S9A-S9E, S10A, S10C-S10D and S11K-S11L). In some of the compressed zones of the nodules, the linear carbon particles on which “filaments” of Al-Krich clay minerals are superimposed in the siliceous mass show lateral micro-shearing and lateral stretching of the carbon particles (Figure 13P5-13S5 and Figure S18C and S18E).

geology-geosciences-Internal-structures-contiguous

Figure 13: Internal structures of contiguous and deformed Akouemma 2nd1 nodules in a polished section, showing the distribution maps of aluminum and carbon. The internal structures of the nodules are clearly shown by aluminum and potassium neoformation clays and, more globally, by organic carbon. (A) Akouemma 2nd1 and the five zones analysed (1-5); for zone 4, see Figure S17. (B1-G1) Undeformed cone, zone 1. (B1) Distribution map of aluminum, showing a simple radial and centrifugal system originating from the cone; note the dark spots scattered throughout the cone. (C1) Distribution map of carbon, showing its presence in spots corresponding to the dark spots present in (B1); two zones (1 and 2) were chosen for magnification. (D1) Magnification of the aluminum image in box 1 of B1. (E1) Magnification of the carbon image in box 1 of C1; note the arrangement of the patterns in the image. For further details, see Figure S14. (F1) Magnification of the aluminum image in box 2 of B1. (G1) Magnification of the carbon image in box 2 of C1. The carbon is more massive and shows finer patterns, and the aluminum and carbon distributions do not overlap. For further details, see Figure S14bis. (H2-K2) Undeformed zone 2 highlighting the fibres and the limits of a tuft or ramp; the parallel fibres are visible and superimposable for the two elements, whereas the limit of the ramp is only made visible by aluminum. (I2) is the magnified image of (H2); (K2) is the magnified image of (J2). For further details, see Figure S15. (L3-O3) Arrowed zone 3 highlighting the filaments and curved tubules. Although the filaments and carbon particles follow the trend, aluminum clearly highlights these structures. (M3) is the magnified image of (L3); (O3) is the magnified image of (N3). For further details, see Figure S16. (P5-R5) Compressed zone 5 with straight filaments presenting lateral micro-shears that are clearly visible in (P5); carbon particles present lateral stretching in (O5). For further details, see Figure S18.

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Figure 14: Internal structures of deformed and contiguous Akouemma 2nd2 nodules. (A) Akouemma 2nd2 nodules showing the four areas analysed (6-9). (B6-E6) The lower hemisphere cone, showing no clear radial system but instead an indistinct structure and an oblong mass of organic carbon. (C6) is the magnified image of (B6); (E6) is the magnified image of (D6). For further details, see Figure S19. (F7 - I7) Zone presenting two superimposed ramps and a rectilinear structure with repeating patterns highlighted by aluminum. Note the boundary between the two ramps (yellow arrow); the carbon particles that are oriented in the same direction are mostly homogenous. (G7) is the magnification of (F7); (I7) is the magnified image of (H7). For further details, see Figure S20. (J8-M8) Zone with corrugated structures suggesting micro-folds (but comparison of images K8 and G7 suggests deterioration of the structure in K8). The structures are highlighted by aluminum and superimposed on those highlighted by carbon. (K8) is the magnified image of (J8); (M8) is the magnified image of (L8). For further details, see Figure S21. (N9-Q9) Zone with a disrupted structure that is more or less oriented. The Al distribution maps are superimposed on those of C; the size of the carbon particles is approximately 30 microns. (O9) is the magnified image of (N9); (Q9) is the magnified image of (P9). For further details, see Figure S22.

Calcium is also unevenly distributed between nodules, but it is sometimes uniform within a nodule. The calcium content is approximately 3%, and negative δ13Ccarb values (-23.87%ο on average) indicate an organic source for the carbon present in the EPS-type calcite [8] (Figure 6A-6D). This calcite is mainly found inside the nodules in the form of micron-sized grains and is relatively poorly correlated with the amount of carbon present (Figure S14, S14bis). Its distribution does not reveal the internal structure of the Akouemma specimens as well as the carbon (Figures S15H, S16H, S18G, S19J, S19K, S20J, S21J and S22J) or PIXE Ca images (Figure 11G), which sufficiently emphasize bundles in the internal structure that are probably cyanobacterial in origin and have been transformed into a calcite EPS [8].

Aluminum, potassium and magnesium

Aluminum and potassium (7 and 0.3% on average, respectively) and incidentally titanium [8] (Figure 11H), together with silica, are the principle elements found in clay minerals and are also the elements that most clearly highlight the internal structure of the nodules. These elements outline the external and internal structures of nodules, including fibres, tubules, oblong shape bodies, tuft boundaries, lenticular bodies forming repeated patterns that are aligned inside and around nodules (Figures 9C1, 9F2, 9I3, 9L4, 9M4, 9P4, 9O4, 9R5, 12B2, 13B1, 13H2, 13I2, 13L3, 13M3, 13P5, 13Q5, 14C6, 14F7, 14G7, 14J8, 14K8, 14N9 and 14O9 and Figures S5B2-S5B3, S13C-S13D, S13G-S13H, S15F-S15G, S16F-S16G, S17F-S17G, S18E, S19 F-S19G, S20 F-S20G, S20L-M, S21F-S21G and S22 F-S22G). They also are distributed according to the deformations experienced by the nodules (Figure 12C2, 13L3, 13M3, 13P5, 13Q5, 14J8 and 14K8, and Figures S5B1). Maps of the distribution of aluminum and potassium overlap fairly well with those of the distribution of carbon, and lesser in the areas with carbon masses >80 μm in size (Figures 13B1, 13D1, 13F1, 14B6). In most cases, aluminum and potassium overlay the particles and carbon filaments by highlighting their alignment (Figures 13H2, 13L3, 13M3, 13P5, 13Q5, 14F7, 14G7), their dispersion and distortion (Figures 14J8, 14K8, 14N9 and 14O9 and S17 F, S17G, S18E, S20C, S21 F, S21G, S22F and S22G).

Magnesium is found in trace amounts in the nodules; it is associated with aluminum and potassium, and outlines the internal structures of the nodules poorly (Figures S15I, S16I, S19H, S19I, S20I and S22I). Although infiltrating clays are often observed in the gangue in these specimens, the clay minerals in the Akouemma specimens are essentially authigenic clays that reveal their internal structure; these are lepidocrocite, vermiculite-kaolinite and chlorite (Figure S10) [8].

Iron

Iron, associated with cobalt or manganese, is present as both oxides and sulphides, and its distribution and concentration vary both within individual nodules and around nodules. Iron distribution in the nodules is radial and centrifugal in the form of particles and filaments (Figures 9 G2, 9J3, 10C, 10E-10H, 11D, 12B, 12B3 and Figures S4B, S4C and S5A-S5A3); sometimes a few particles intersect the radial direction (Figure S21M) or are dispersed within the siliceous mass (Figure S19L and S19M). Iron sometimes emphasizes micro-folds in the nodule (Figure 12C3). The sulphide form is found more in the “core” of the nodule (Figure 11B). Iron often forms a ring at the inner border and an external rim around the nodule (Figures 10A-10C, 10D-10E, 11D, 12B, 12B3) that is composed of a succession of lenticular bodies (Figures S5AS5A3, S6A, S6A4-S6A5). Iron is also concentrated in the cracks and in the interfaces between the internal structures of a specimen (Figures 9A1, 9D1, 9K4, 9N4, 9S5 and 12C3 and Figures S5A, S6A and S21H). Iron sulphide generally appears in the form of small crystals with a bristled surface. In its oxidized form, small cubic iron crystals exhibits fossilized clumps of 2-5 μm black carbon particles (Figure S7A, F-H in File S1) and red ferro-silico-aluminous spherules (Figure S7J-S7J2).

Silicon and radioactivity

Silicon is the most abundant element in the nodules (80%) [8]. It is homogeneously distributed in the nodules in the form of micro-quartz (chert). In Akouemma nodules, aluminum and potassium record in the detail the internal and external structures, whereas silica preserves in the whole their initial morphological (deformed and undeformed) states (Figure 11C, 12B1 and 12C1 and Figure S4G). The rapid crystallization of silicon allowed instantaneous preservation of various internal and external states of Akouemma nodules, thereby preserving the organic carbon, microorganisms and other biomorphs as well as the original structures and authigenic minerals. Locally, a few "bricks" with siliceous walls, observed in nodules, contain red Fe-Al-Si-rich spherules and black carbon-rich particles (Figure S8F-S8H, S9F-S9N).

An autoradiography was performed on the AKOU 14, AKOU 15, AKOU 16 nodules and on their gangue. The distribution of the radioactivity revealed a clear discontinuity between the non-radioactive nodules in the healthy part and the radioactive sediment. The altered edges of the nodules are contaminated with radioactivity (Figure S8A1, S8A2).

Discussion

Silica-carbonate nodules discovered in the FB2 formations of the Akou River were interpreted as biogenic nodules according to the following criteria:

1. Nodules with 2 hemispheres, 1 median disc, complex internal radial fabric, vertical and lateral symmetry, chains of joined nodules, indicating a complex organization;

2. Flexible deformations by lateral compression, indicating a ductile/plastic body;

3. Fibro-radial and polyphase internal complex fabric, suggesting growth;

4. Deformed sediments around and between specimens in tabular beds, suggesting growth in soft sediments;

5. 13C-depleted carbonaceous matter and 13C-depleted calcite, indicating precipitation of calcite by microbial metabolism and degradation of organic matter under anoxic photosynthesis conditions;

Radiating remnants of biofilms and calcite grains, suggesting an EPS calcite altered and replaced by silicification.

The ductile/plastic nodules with complex internal radial fabric containing linear and flattened clayey “channels”, cyanobacteria, many biomorphs (spherulites, filaments, vesicles) and multicellular clusters led to the conclusion that the Akou nodules are biogenic nodules hosting multicellular consortia; they are therefore designated Akouemma nodules.

The new criteria highlighted are:

1. Deformation and lateral spreading of matter;

2. An internal fibro-radial fabric consisting initially of carbon particles;

3. Decaying and dispersion of particles cell-like and broken tubules in the undeformed siliceous mass;

4. Close association between carbon and K-Al-(Ti)-rich clay minerals;

5. Likely vegetative reproduction by duplication.

The formation of silico-carbonate “abiotic” nodules by interactions between sediments and microorganisms lead to the formation of the accretion laminae observed in these nodules and in the stromatolites, from the Proterozoic [22-24] to modern analogues [25]; and microbially mediated calcification can be traced back for at least the Proterozoic [26-30]. Additionally, these “abiotic” concretions typically develop from a central point and present often a zonal concentric structure; they may contain concentric and lateral accretion laminae [31], characterized by variations in the isotopic and geochemical compositions of the different growth zones [32]. Other carbonate concretions are growing from inert clumps of organic matter, typically in fine sediments, probably as a result of the diffusion of carbon dioxide during the decomposition of organic matter through microbial processes [33-35]. These diagenetic nodules vary in shape and size within the same basin; they can be spherical, elongated, flattened and laterally extended; and can vary from a few millimetres to a few metres in size [36,37].

The preservation of organic micro- and macrofossils by clay minerals, which has received increased interest in recent years [38-44], has been widely reported from the Palaeozoic and terminal Neoproterozoic (Ediacaran) rock record [45-50]. Support for the role of authigenic clay precipitation in organism preservation comes both from experiments [51-54] and from field studies of modern environments [21,55-57]. These show that microbial mediation of clay minerals is common for the preservation of microorganisms and soft-body macroorganisms. Varieties of Fe-, Mg- and K-rich aluminasilicates have been found in close association with both well-preserved macrofossils [40,43,44,47,49] and, more rarely, microfossils [21,48,57].

Akouemma nodules contain EPS calcite as “abiotic” carbonate concretions that are characteristic of microbial activity. They differ in terms of their homogeneous form and size; they typically consist of two hemispheres with their central nipple-like feature of the upper hemisphere and corresponding central depression of the lower hemisphere. The central nipple-like feature and the corresponding central depression likely correspond to a “communication point” between the two hemispheres which are separated by the median disc with its central orifice. They differ by their centrifugal internal fabric radiates from the cones of each hemisphere in spheroidal nodules and a polyphase internal fabric with an internal polygonal structure in elongated forms. The presence of this radial internal fabric, highlighted by Al-K-(Ti)-rich clay minerals in close association with carbon particles and by Ca calcite and Fe oxides-sulphides shows that the clay minerals (and the other minerals) precipitated during early (pre-compaction) diagenesis, preventing compaction of nodules and enclosed microfossils. The absence of radioactivity in the unaltered silicified nodules, while sediments and altered edges of the nodules are radioactive probably confirms their very early silicification, would indicate a different origin between nodules and sediments. They differ also of their mutual flexible deformation which strongly affects nodules, including radial internal fabric. The deformations by mutual lateral compression of the grouped nodules, with spreading of matter (probably due to their growth), adjoined to other nondeformed nodules or by vertical compression affecting only the upper hemispheres of nodules, causing deformations of the surrounding sediment (compaction between two nodules or micro-folding under the nodules) in the bedding planes, indicate syn-sedimentary deformations of these nodules. These latter therefore would not have any direct link with the sediment, corroborating the arrangement of the repeated patterns around nodules and in the sediment. Similarly the nodules have various marks (depressions, grooves), repeated Fe-Alk- rich patterns in and around nodules, lenticular and oblong bodies that sometimes cause deformations on the nodules. Silica (microquartz) is uniformly distributed in all specimens (distorted or not) and does not present any deformation; therefore, we infer that silicification is an early process that fixes initial states, including deformations, degradations and internal degrees of dispersion and spreading of particles and fragments in the nodules. Furthermore, Neoproterozoic and Phanerozoic fossilizations are known to contain microorganisms and soft-bodied macroorganisms preserved by Fe-Mg-K-rich clay minerals [39,40,44-47]. In the Akouemma nodules, we highlight for the first time in the history of the earth sciences a close association of organic matter with Al-K-(Ti)-rich clay minerals in the presence of EPS-type calcite and pyrite in material from the Palaeoproterozoic at 2.2 Ga. The different associations of iron oxides with carbon particles in "bricks" with siliceous walls, and in “cubic crystals” containing carbon particles with a few grades of Si and Al, and undergoing epigenization by iron suggest that the red Fe- Al- Si-rich spherules which are very abundant in the nodules [8] arose from mineralization of the carbon particles.

Finally, the elongated Akouemma nodules show signs of division marked by the presence of external furrows that become increasingly deep with the progression from elongated shapes in single pieces to segmented elongated forms, with individualization of new spheroidal bodies. This process likely corresponds to a division phase, recalling the vegetative reproduction of unicellular micro-organisms [58,59] or primitive macro-organisms [60] including the enigmatic macrofossils of late Ediacaran [61].

Together with the criteria previously defined [8], the new biogenic criteria, which emphasize an initial fibro-radial internal fabric with carbon preserved by clay minerals, branched filaments and cells preserved in the siliceous mass, plastic deformation via mutual lateral compression, and “self-strangulation” similar to a cell division, support the integration of Akouemma specimens in the register of primitive organic macrofossils found in the Proterozoic.

Although these specimens exhibit different sizes, shapes, structures and habitats, they are added to those discovered in the Palaeo- and early Mesoproterozoic, including a eukaryotic Grypania spiralis found in banded iron formations in Michigan and in the Rohtas Formation (India) and dated approximately 1.6 Ga [62,64], colonies of Horodyskia fossils discovered in sedimentary rocks in China, North America, and Australia and dated to 1.5 Ga [65-68], and a colony of Discagma buttonii fossils discovered in the palaeosols of South African and dated to 2.2 Ga [69]. This discovery of Akouemma specimen colony in the FB2b formations of the Okondja Basin brings a new vision to the discovery of large colonial multicellular organisms in the FB2b formations of the Franceville Basin [5,7]. Otherwise, current investigations show macrofossil-like material, including pyritous vermiforms, distributed from FB1 to FD in the Francevillian Group (Figure 1) and would entirely change our understanding of life evolution on Earth.

Background

Since 1966, several studies have reported the presence of macrofossils [2], communities of microfossils and stromatolite [3,4,6] and abundant organic matter [12,70] in Francevillian sedimentary formations in Gabon. Unfortunately, these findings had little impact on the scientific world until the publication of an article on large colonial organisms [5] renewed interest in research on the origin of life in the Francevillian basin.

A team from the Research Unit in Geosciences and Environment (URESTE) of Masuku University had in recent years already accumulated observations of and collected various specimens, including vermiform pyritized in FB1, to more than 2.1 Ga. These include Akouemma nodules in FB, oval shapes along the transverse plane in FB-FC, and flattened shapes containing phosphorus and calcium in FD (Figure 1) dated at 2083+/- 6 Ma [71,72]. There was indeed a type of biological ‘proliferation’ at approximately this period that corresponds to a Great Oxidation Event (GOE), a period of increased oxygenation due in particular to cyanobacterial activity [73,74]. The first published results of the URESTE team following funding from the Ministry of National Education in December 2010 and from the Faculty of Sciences of Masuku University in 2014 described the Okondja nodules as biogenic nodules [8]. Further investigations of the same nodules, mainly financed by the Inter-University Cooperation (Tokyo Institute of Technology, Japan; CENBG, University of Bordeaux, France; Paul Scherrer Institute, Villigen PSI Switzerland and Geosciences Environment of Toulouse, GET, France) led to the elaboration of this paper. These remarkable results bring a new view of life as reflected by the characteristics of organisms that lived at least 2.2 Ga.

In the context of a Memorandum of Understanding between the Tokyo Institute of Technology and the “Université des Sciences et Techniques de Makusu” and with the support of partnerships with the Universities of Toulouse and Bordeaux and with the Paul Scherrer Institute; other investigations of pyritous fossils of FB1 and other specimens of FB, FC and FD (Figure 1) are under way. The Francevillian Group could be the oldest Konservat-Lagerstätten (a sedimentary deposit with exceptional fossil preservation), in which both micro- and macrofossils are preserved in silica, pyrite, clay minerals or phosphate. We remain open to all win-win collaborations.

Conclusion

Akouemma hemisphaeria, a colony of organic macrofossils

Although we cannot determine the precise modern analogues of the Akouemma specimens, we conclude that the Akouemma nodules formed an organic sessile macrofossil colony with a soft-plastic body, an internal carbon fibro-radial fabric and a likely vegetative reproduction by duplication. They hosted cyanobacteria, filamentous microfossils, green algae tubules, and other uni- and multicellular organisms on the seafloor of the Okondja basin at 2.2 Ga. These specimens are designated Akouemma hemisphaeria (1).

(1) Akouemma hemisphaeria: Fossile at two hemispheres of Akou River highlighted for the first time by Edou-Minko and Moussavou Mathieu.

Acknowledgements

We thank Harumasa Kano (Tohoku University Museum) for performing the micro-CT analysis and Tsuyoshi Komiya (The University of Tokyo) and Gérard Delorme (Comilog geologist, retired) for their helpful comments. This research was partly supported by a grant for “Hadean Bioscience (No. 26106002)” from the Japan Society for the Promotion of Science (JSPS).

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