title: The Raman-Derived Carbonization Continuum: A Tool to Select the Best Preserved Molecular Structures in Archean Kerogens.
abstract: The search for indisputable traces of life in Archean cherts is of prime importance. However, their great age and metamorphic history pose constraints on the study of molecular biomarkers. We propose a quantitative criterion to document the thermal maturity of organic matter in rocks in general, and Archean rocks in particular. This is definitively required to select the best candidates for seeking non-altered sample remnants of life. Analysis of chemical (Raman spectroscopy, (13)C NMR, elemental analysis) and structural (HRTEM) features of Archean and non-Archean carbonaceous matter (CM) that was submitted to metamorphic grades lower than, or equal to, that of greenschist facies showed that these features had all undergone carbonization but not graphitization. Raman-derived quantitative parameters from the present study and from literature spectra, namely, R1 ratio and FWHM-D1, were used to draw a carbonization continuum diagram showing two carbonization stages. While non-Archean samples can be seen to dominate the first stage, the second stage mostly consists of the Archean samples. In this diagram, some Archean samples fall at the boundary with non-Archean samples, which thus demonstrates a low degree of carbonization when compared to most Archean CM. As a result, these samples constitute candidates that may contain preserved molecular signatures of Archean CM. Therefore, with regard to the search for the oldest molecular traces of life on Earth, we propose the use of this carbonization continuum diagram to select the Archean CM samples.
  
  
  
  Archean-Early life-Kerogen-Raman spectroscopy-Carbonization. Astrobiology 16, 407-417.
authors.affiliation: 1 IMPMC Sorbonne Universités-MNHN , UPMC Univ Paris 06, UMR CNRS 7590, IRD UMR 206, Paris, France .
authors.forename: Frédéric
authors.initials: F
authors.lastname: Delarue
authors.affiliation: 2 Laboratoire de Géologie de l'ENS , UMR CNRS 8538, Paris, France .
authors.forename: Jean-Noël
authors.initials: JN
authors.lastname: Rouzaud
authors.affiliation: 3 Sorbonne Universités , UPMC Univ Paris 06, CNRS, UMR 7619 METIS, CC 105, Paris, France .
authors.forename: Sylvie
authors.initials: S
authors.lastname: Derenne
authors.affiliation: 3 Sorbonne Universités , UPMC Univ Paris 06, CNRS, UMR 7619 METIS, CC 105, Paris, France .
authors.forename: Mathilde
authors.initials: M
authors.lastname: Bourbin
authors.affiliation: 4 Centre de Biophysique Moléculaire , UPR CNRS 4301, Orléans, France .
authors.forename: Frances
authors.initials: F
authors.lastname: Westall
authors.affiliation: 5 Institute of Paleobiology , Polish Academy of Sciences, Warszawa, Poland .
authors.forename: Barbara
authors.initials: B
authors.lastname: Kremer
authors.affiliation: 6 Department of Environmental Engineering and Architecture, Graduate School of Environmental Studies, Nagoya University , Nagoya, Japan .
authors.forename: Kenichiro
authors.initials: K
authors.lastname: Sugitani
authors.affiliation: 2 Laboratoire de Géologie de l'ENS , UMR CNRS 8538, Paris, France .
authors.forename: Damien
authors.initials: D
authors.lastname: Deldicque
authors.affiliation: 1 IMPMC Sorbonne Universités-MNHN , UPMC Univ Paris 06, UMR CNRS 7590, IRD UMR 206, Paris, France .
authors.forename: François
authors.initials: F
authors.lastname: Robert
mesh_terms: D001105:Archaea / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D066241:Carbon-13 Magnetic Resonance Spectroscopy; D018559:Exobiology; D019015:Geologic Sediments / Q000737:chemistry*; D013059:Spectrum Analysis, Raman / Q000379:methods*; D013696:Temperature
chemical_list: D002244:Carbon