TY - JOUR
T1 - Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada
AU - Tashiro, Takayuki
AU - Ishida, Akizumi
AU - Hori, Masako
AU - Igisu, Motoko
AU - Koike, Mizuho
AU - Méjean, Pauline
AU - Takahata, Naoto
AU - Sano, Yuji
AU - Komiya, Tsuyoshi
N1 - Funding Information:
Acknowledgements This research was supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan (grant numbers: 23253007, 26220713 and 24221002) and the Mitsubishi Foundation. We thank K. D. Collerson and B. Ryan for sharing their geological information. We are grateful to W. Broomfield, Parks Canada, Labrador Inuit Development Corporation (LIDC) and many bear monitors who assisted with our geological fieldwork at the Saglek Block.
Publisher Copyright:
© 2017, Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
PY - 2017/9/28
Y1 - 2017/9/28
N2 - 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 possible1,2,3, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt4,5,6,7. 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 Labrador8,9. 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.
AB - 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 possible1,2,3, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt4,5,6,7. 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 Labrador8,9. 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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U2 - 10.1038/nature24019
DO - 10.1038/nature24019
M3 - Article
C2 - 28959955
AN - SCOPUS:85031853963
SN - 0028-0836
VL - 549
SP - 516
EP - 518
JO - Nature
JF - Nature
IS - 7673
ER -