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Nature. 2017 Sep 27;549(7673):516-518. doi: 10.1038/nature24019.

Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada.

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Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan.
Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba 277-8564, Japan.
Institute for Excellence in Higher Education, Tohoku University, Sendai 9808576, Japan.
Department of Arts and Sciences, Osaka Kyoiku University, Osaka 582-8582, Japan.
Laboratory of Ocean-Earth Life Evolution Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa 237-0061, Japan.


The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible, and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ13Corg values of -28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

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