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Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks

Abstract

Some graphite contained in the 3.7-billion-year-old metasedimentary rocks of the Isua Supracrustal Belt, Western Greenland1, is depleted in 13C and has been interpreted as evidence for early life2. However, it is unclear whether this graphite is primary, or was precipitated from metamorphic or igneous fluids3,4. Here we analyse the geochemistry and structure of the 13C- depleted graphite in the Isua schists. Raman spectroscopy and geochemical analyses indicate that the schists are formed from clastic marine sediments that contained 13C-depleted carbon at the time of their deposition. Transmission electron microscope observations show that graphite in the schist occurs as nanoscale polygonal and tube-like grains, in contrast to abiotic graphite in carbonate veins that exhibits a flaky morphology. Furthermore, the graphite grains in the schist contain distorted crystal structures and disordered stacking of sheets of graphene. The observed morphologies are consistent with pyrolysation and pressurization of structurally heterogeneous organic compounds during metamorphism. We thus conclude that the graphite contained in the Isua metasediments represents traces of early life that flourished in the oceans at least 3.7 billion years ago.

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Figure 1: Geologic maps and photos of the study area in the ISB, West Greenland.
Figure 2: REE patterns in the examined graphite-rich schist.
Figure 3: Carbon stable isotope compositions of graphite in the ISB.
Figure 4: Transmission electron microscopy images of graphite.

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Acknowledgements

We thank E. Aoyagi for technical assistance with STEM and HRTEM observations. The isotope ratio mass spectrometer (infrared-MS) analyses were carried out with support from T. Watanabe and F. W. Nara. The manuscript was improved by discussions with Y. Furukawa and T. Otake. This study was supported by the Japan Society for the Promotion of Science (grants 17403011 and 21403009).

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Y.O., T.K. and M.T.R. conducted the geological surveys and collected rock samples. Y.O. carried out the geological and petrographical analyses, carbon stable isotope analyses of graphite using the graphite combustion method, XRD analyses, HRTEM observations and thermodynamic/isotopic calculations. A.I. and T.N. contributed to sample preparation and HRTEM observations. T.K. carried out carbon stable isotope analyses of graphite using the in situ Nd–YAG laser microprobe technique, STEM observations and Raman microspectroscopic analyses.

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Correspondence to Yoko Ohtomo.

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Ohtomo, Y., Kakegawa, T., Ishida, A. et al. Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks. Nature Geosci 7, 25–28 (2014). https://doi.org/10.1038/ngeo2025

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