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 possible1,2,3, 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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Figure 1: Geological maps and sample localities in Saglek Block.
Figure 2: Correlation between TOC contents and δ13Corg values.
Figure 3: Comparison between the δ13Corg values of pelitic rocks and metamorphic grades.

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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.

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T.K. designed the study and Y.S. designed the geochemical study. T.K., T.T., A.I., M.H., M.I., M.K., P.M., N. T., and Y.S. conducted geochemical analyses. T.K. and T.T. collected samples in the field. T.K. wrote the manuscript with important contributions from all co-authors.

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Correspondence to Tsuyoshi Komiya.

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Reviewer Information Nature thanks A. Polat and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Detailed geological maps of four areas in the Saglek Block.

a, A geological map of St. John’s Harbour South area (SJHS). The area is composed of the supracrustal rocks, Iqaluk-Uivak Gneisses, Saglek dykes, young granite intrusion and the Proterozoic mafic dikes. The supracrustal rocks form a NS-trending belt, and are intruded by around 3.95 Ga Iqaluk-Uivak Gneisses. The pelitic rocks are predominant in the supracrustal rocks. b, A geological map of Big Island area. The area is subdivided into two parts by a NS-trending fault. The eastern side is composed of the supracrustal rocks, Iqaluk-Uivak Gneisses, Saglek dykes, young granite intrusion and the Proterozoic mafic dykes. The western side is predominant in pelitic rocks, and contains ultramafic and mafic rocks, and carbonate rocks. c, A geological map of a small point of the western coast of the Shuldham Island. The area is characterized by ultramafic rocks with large olivine-needle structures. The ultramafic rock-bearing body consists of harzburgitic ultramafic rocks, olivine-clinopyroxene rocks, clinopyroxene-hornblendite, gabbroic rocks, fine-grained amphibolite and pelitic rocks, in ascending order. d, A geological map of St. John’s Harbour East area (SJHE). A supracrustal belt is composed of some fault-bounded blocks from ultramafic rocks through mafic rocks to sedimentary rocks of pelitic rocks, carbonate rocks and cherts in ascending order. The figures are modified from Figs 2, 3, 5 and 7 of Komiya et al.8 with permission.

Extended Data Figure 2 Photos of outcrops and thin sections of the metasedimentary rocks.

a, An outcrop of pelitic rocks (LAA269, LAA270) at St. John’s Harbour South. b, An outcrop of a conglomerate (LAD849A, LAD849B, LAD849C, LAD852) at St. John’s Harbour East. The photo was taken from the east. A large siliceous clast in the conglomerate displays north to south extension. c, A carbonate rock (LAA742) with chert nodules at St John’s Harbour East. d, An outcrop of the chert at St. John’s Harbour East. e, A representative microscopic image of a pelitic rock (LAF492), containing biotite (Bt), garnet (Grt), quartz (Qtz), pyrrhotite (Po) and graphite. The most graphite grains have elongated shapes and occur along biotite grains and within garnet grains. f, Another representative microscopic image of a pelitic rock (LAF491). The graphite occurs along the biotite grains, forming bedding planes, or along the cleavages of the biotites. g, A microscopic image of a carbonate rock (LAA766). The needle-like mineral is serpentine (Srp), and sparry carbonate consists of calcite (Cal) and dolomite (Dol). Magnetite (Mgt)-rich rings are present in the fine-grained carbonate (Cal). h, A microscopic image of a chert nodule (LAA760) in the carbonate rock (c). The graphite grains have globular shapes, and form an aggregate.

Extended Data Figure 3 Carbon isotope values of individual graphite grains and rare earth element+Y patterns of carbonate rocks.

a, Carbon isotope values of individual graphite grains in a pelitic rock (LAF491) and a chert nodule of carbonate rock (LAA760). The graphite grains in the LAF491 range from −19.3 to −30.8‰ in δ13Corg values, whereas those in the LAA760 vary from −26.1 to 33.6‰ in δ13Corg values. The formers are consistent with the whole-rock carbon isotope ratio (−28.2‰) but the latter is much lower than the whole-rock value (−10.3‰). b, Post-Archean-Australian-shale-normalized rare earth element + Y diagrams of carbonate rocks with low Y and Zr contents. The carbonate rocks show diagnostic Eu and Y anomalies in the St. John’s Harbour South (A), Big Island (B), St. John’s Harbour East (C), and Pangertok Inlet (D).

Extended Data Figure 4 The comparison between metamorphic temperature and crystallization temperature of graphite.

The metamorphic temperatures were estimated from mineral parageneses of metabasaltic rocks and compositions of garnet and biotite in pelitic rocks, whereas the crystallization temperatures were estimated from Raman spectra of graphite. In the case of absence of D1 bands, the estimated crystallization temperature is over 650 ± 50 °C. The estimated crystallization temperatures of graphite are consistent with the metamorphic temperatures except for those from a chert nodule.

Extended Data Figure 5 The distribution of the δ13Corg values in Saglek Block and Isua supracrustal belt.

The δ13Corg values in Isua supracrustal belt range between −28 and −6‰28,29,30,31,32,33,34,35. The lower column shows variations of carbon isotope fractionation in four different carbon fixation pathways by modern autotrophic bacteria36.

Extended Data Table 1 Total organic carbon contents and carbon isotope values of graphite
Extended Data Table 2 Carbon isotope values of individual graphite grains
Extended Data Table 3 Representative compositions of garnet and biotite
Extended Data Table 4 Summary of Raman spectrum parameters of graphite and estimated temperatures

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Tashiro, T., Ishida, A., Hori, M. et al. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada. Nature 549, 516–518 (2017). https://doi.org/10.1038/nature24019

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