Nature | 30 Sep 2017
T Tashiro, A Ishida, M Hori, M Igisu, M Koike, P Méjean, N Takahata, Y Sano and T Komiya
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 (δ(13)Corg) and carbonate (δ(13)Ccarb) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ(13)Corg 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 δ(13)Corg values and metamorphic grade indicates that variations in the δ(13)Corg 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 δ(13)Ccarb and δ(13)Corg 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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