séminaire – jeudi 27 février 2014

Isotopic and nanoscale textural evidences for the biogenicity of the oldest cellular structures (3.4 billion years old)

Kevin Lepot, université de Lille 1, laboratoire Géosystèmes, UMR 8217, Villeneuve d'Ascq

jeudi 27 février à 11 heures (amphi à préciser)



The oldest evidences of life, ca. 3.5 billion years old, come from sulfur isotope ratio indicating sulfur metabolism, and bedded deposits named stromatolites. Finding diagnostic geochemical (isotopic and molecular) biosignatures in organic matter preserved in such ancient rocks remains difficult due to 1) the metamorphic alteration caused by deep burial and 2) the abundance of seafloor hydrothermal systems that may have produced abiogenic organic compounds. Because the earliest microorganisms likely had simple morphologies, cellular imprints in organic matter are difficult to distinguish from abiogenic microstructures such as migrated hydrocarbons precipitated along grain boundaries of variable shapes. Abundant cell-like organic microstructures have been recently reported in the 3.4 Gyrs old Strelley Pool Formation from Western Australia. We measured carbon isotope ratio at the microscale (15 µm spot) in these organic microstructures using Secondary Ion Mass Spectrometry (SIMS). We characterized the texture and structure of organic matter using Raman spectromicroscopy, Scanning Transmission X-ray Microscopy, Scanning Electron Microscopy, Focused Ion Beam and Transmssion Electron Microscopy. Textural analyses revealed strong similarities with younger microfossils down to the sub-micrometer scale and distinguished spherical and lenticular cell-like structures from other, non-biogenic textures. Raman spectromicroscopy and carbon isotope ratios distinguished the indigenous cell-like structure and kerogen from late migrated bitumen. SIMS revealed C-isotopes heterogeneities between spherical and lenticular cell-like structures and kerogen clots, and internal heterogeneties in lenticular structures. The heterogeneities can be explained by selective diagenetic preservation of the distinct isotopic fractionations inherited from different precursor biomolecules. Altogether, our data argue for the biogenicity of cell-like structures and for the preservation of cellular morphologies.