1. Microstructure-specific carbon isotopic signatures of organic matter from ∼3.5 Ga cherts of the Pilbara Craton support a biologic origin
- Author
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Kevin Lepot, Pascal Philippot, Christophe Thomazo, Kouki Kitajima, Martin J. Van Kranendonk, Kenneth H. Williford, Navot Morag, John W. Valley, 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), Centre National de la Recherche Scientifique (CNRS)-Université de La Réunion (UR)-Université Paris Diderot - Paris 7 (UPD7)-IPG PARIS-Institut national des sciences de l'Univers (INSU - CNRS), School of Biological, Earth and Environmental Sciences [Sydney] (BEES), University of New South Wales [Sydney] (UNSW), Géosystèmes - UMR 8157, Centre National de la Recherche Scientifique (CNRS)-Université de Lille, Sciences et Technologies, Biogéosciences [UMR 6282] [Dijon] (BGS), Centre National de la Recherche Scientifique (CNRS)-Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement, 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), NASA Astrobiology Institute ( NAI ), University of Wisconsin-Madison [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 ) -IPG PARIS-Université Paris Diderot - Paris 7 ( UPD7 ) -Université de la Réunion ( UR ) -Centre National de la Recherche Scientifique ( CNRS ), School of Biological, Earth and Environmental Sciences ( UNSW - BEES ), University of New South Wales [Sydney] ( UNSW ), Laboratoire Géosystèmes (Lille, France), Université de Lille, Sciences et Technologies, Biogéosciences [Dijon] ( BGS ), AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), ANR-11-IDEX-0005-02/10-LABX-0023,UnivEarthS,Earth - Planets - Universe: observation, modeling, transfer ( 2011 ), Géosystèmes UMR 8157, Université de Lille, Sciences et Technologies-Centre National de la Recherche Scientifique (CNRS), Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS), and ANR-11-IDEX-0005-02/10-LABX-0023,UnivEarthS,Earth - Planets - Universe: observation, modeling, transfer(2011)
- Subjects
Carbon isotope ratio ,010504 meteorology & atmospheric sciences ,Metamorphic rock ,Pilbara Craton ,Mineralogy ,010502 geochemistry & geophysics ,[ SDU.STU.ST ] Sciences of the Universe [physics]/Earth Sciences/Stratigraphy ,01 natural sciences ,Microfossils ,Pilbara craton ,Geochemistry and Petrology ,[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry ,Dresser Formation ,Organic matter ,Lithification ,Ankerite ,0105 earth and related environmental sciences ,chemistry.chemical_classification ,δ13C ,Geology ,15. Life on land ,[ SDU.STU.GC ] Sciences of the Universe [physics]/Earth Sciences/Geochemistry ,chemistry ,13. Climate action ,Stylolite ,[SDU.STU.ST]Sciences of the Universe [physics]/Earth Sciences/Stratigraphy ,Vein (geology) ,SIMS - Abstract
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 is supported by the abundance of OM in sub-seafloor hydrothermal chert veins. The range of δ13C(OM) values observed in the samples may therefore represent the remains of different organisms utilizing different C-fixation pathways. Other biologic effects, such as the growth rate and density of microbial communities, and further heterotrophic overprinting of the autotrophic biomass may have also contributed to the observed range of δ13C(OM) values.
- Published
- 2016
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