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

Abstract

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 possible1-3, and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt4-7. Here we present the total organic carbon contents and carbon isotope values of graphite (δ¹³C_org) and carbonate (δ¹³C_carb) in the oldest metasedimentary rocks from northern Labrador8,9. Some pelitic rocks have low δ¹³C_org 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 δ¹³C_org values and metamorphic grade indicates that variations in the δ¹³C_org 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 δ¹³C_carb and δ¹³C_org 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.