Abstract
The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean1,2,3,4. This interpretation, however, has to be approached with extreme care5. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life1,2,3,4. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.
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Acknowledgements
We thank M. Wahlen and B. L. Deck for providing facilities for extraction of reduced carbon and subsequent isotopic measurement, D. R. Hilton for providing facilities for stepped-combustion extraction of reduced carbon, J. L. Teranes for measuring δ13C of carbonate phases, J. Finarelli for determination of cation composition of carbonate phases, and P. W. U. Appel for providing coordination and facilities for field work as part of the Isua Multidisciplinary Research Project. Support by the Marianne and Marcus Wallenberg Foundation (for A.L.) and NASA Exobiology is gratefully acknowledged. We thank L. P. Knauth, S. Moorbath and J. M. Hayes for their comments on this manuscript.
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van Zuilen, M., Lepland, A. & Arrhenius, G. Reassessing the evidence for the earliest traces of life. Nature 418, 627–630 (2002). https://doi.org/10.1038/nature00934
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DOI: https://doi.org/10.1038/nature00934
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