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 (δC) and carbonate (δC) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δC 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 δC values and metamorphic grade indicates that variations in the δC 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 δC and δC 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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In marine ecology, dietary interpretations of faunal assemblages often rely on nitrogen isotopes as the main or only applicable trophic level tracer. We investigate the geographic variability and trophic level isotopic discrimination factors of bone zinc 66Zn/64Zn ratios (δ66Zn value) and compared it to collagen nitrogen and carbon stable isotope (δ15N and δ13C) values. Focusing on ringed seals (Pusa hispida) and polar bears (Ursus maritimus) from multiple Arctic archaeological sites, we investigate trophic interactions between predator and prey over a broad geographic area. All proxies show variability among sites, influenced by the regional food web baselines. However, δ66Zn shows a significantly higher homogeneity among different sites. We observe a clear trophic spacing for δ15N and δ66Zn values in all locations, yet δ66Zn analysis allows a more direct dietary comparability between spatially and temporally distinct locations than what is possible by δ15N and δ13C analysis alone. When combining all three proxies, a more detailed and refined dietary analysis is possible.

External 4He sources have been invoked to explain 4He concentrations in groundwater greater than those expected from in situ U and Th production. In a fractured aquifer of Ordovician age located in the St. Lawrence Lowlands (Quebec, Canada), 4He concentrations of up to 4.48 × 10−5 cm3 STP gnormalH2O−1 were measured. Such concentrations are ∼1000 times higher than would be expected from in situ production. A concomitant increase in 4He concentration and 234U/238U activity ratio is shown, suggesting a common release process in groundwater for 234U and 4He. This process has tentatively been identified as glaciation‐induced rock fracturing following the Laurentide Ice Sheet retreat. The resulting increase in exposed grain surface facilitates 234U release by α‐recoil and that of radiogenic 4He by diffusion. Using a model of helium diffusion from a spherical grain, it is shown that rock fracturing facilitated the release of accumulated 4He at rates ranging from 4.2 × 10−10 to 1.06 × 10−8 cm3 STP gnormalH2O−1 yr−1. These release rates are between 1000 and 30,000 times higher than the local U and Th steady state production rate, of 3.5 ± 1.4 × 10−13 cm3 STP grock−1 yr−1. Integration of 4He release rates over time yields a radiogenic 4He concentration of between 3.85 × 10−6 and 7.12 × 10−5 cm3 STP gnormalH2O−1, in the range of concentrations measured in the St. Lawrence Lowlands fractured aquifers. Results support the occurrence of a local radiogenic helium source to explain the excesses measured in groundwater without requiring a significant external He crustal flux.