Microstructure-specific carbon isotopic signatures of organic matter from ∼3.5 Ga cherts of the Pilbara Craton support a biologic origin.

• Published in: Precambrian Research
• Main Authors: Morag, Navot, Williford, Kenneth H., Kitajima, Kouki, Philippot, Pascal, van Kranendonk, Martin J., Lepot, Kevin, Thomazo, Christophe, Valley, John W.
• Other Authors: NASA Astrobiology Institute (NAI), Department of Geoscience Madison, University of Wisconsin-Madison, Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS), School of Biological, Earth and Environmental Sciences Sydney (BEES), University of New South Wales Sydney (UNSW), Géosystèmes - UMR 8157, Université de Lille, Sciences et Technologies-Centre National de la Recherche Scientifique (CNRS), Biogéosciences UMR 6282 (BGS), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Funding for this study provided by the NASA Astrobiology Institute nd the Labex Uni-vEarths program of Sorbonne Paris Cité., ANR-11-IDEX-0005,USPC,Université Sorbonne Paris Cité(2011)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: Dresser Formation; Organic matter; Carbon isotope ratio; SIMS; Microfossils; Pilbara craton; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDU.STU.ST]Sciences of the Universe [physics]/Earth Sciences/Stratigraphy; Calvin; North Pole
• Online Access: https://hal.science/hal-01297306; https://doi.org/10.1016/j.precamres.2016.01.014

21 pages International audience The ∼3.5 Ga Dresser Formation from the North Pole Dome of the Pilbara Craton (Western Australia) contains some of the oldest evidence for life on Earth. Here, we present a detailed study of microstructure-specific carbon isotopic composition of organic matter (OM) preserved in Dresser Formation bedded cherts and hydrothermal chert vein using in situ Secondary-Ion Mass Spectrometry (SIMS). The OM in these rocks occurs mainly as clots that, together with minor fine OM layers and laminae, are considered primary textures formed prior to host rock lithification. Other than rare OM-rich stylolites, no evidence was found for later OM migration beyond the micrometer scale. Average δ13C(OM) values in specific microstructural types range between −33.6‰ and −25.7‰. No correlation is seen between measured δ13C values and H/C ratios in the studied OM microstructures. This lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of OM isotopic composition by later metamorphic alteration. It is thus concluded that the range of δ13C values found in the samples represents primary OM isotopic variability. Within some individual samples variable δ13C(OM) values are correlated with specific microstructural types. This observation is not consistent with solely abiotic OM formation via Fisher-Tropsch type reactions. When compared with associated δ13C(ankerite) values, average δ13C(OM) values indicate C isotopic fractionation [Δ13C(Ank–OM)] of 25–33‰, which translates to dissolved CO2–OM isotopic fractionation [Δ13C(CO2–OM)] of 20–30‰. This range of Δ13C(CO2–OM) is consistent with enzymatic C fixation via the Calvin cycle utilized by photoautotrophs and the reductive acetyl-CoA pathway utilized by chemolithoautotrophs. Photosynthetic OM formation is supported by the relatively shallow water depth inferred for the Dresser environment and the restricted occurrence of stromatolites to shallow water deposits in this unit, whereas chemolithosynthesis ...