Abstract
The Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.
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Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding authors upon reasonable request.
Change history
29 November 2018
In Extended Data Fig. 1 of this Letter, the map showed the field-work location incorrectly; this figure has been corrected online.
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Acknowledgements
Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Funding for the work was provided by the NASA Mars 2020 PIXL flight project. We thank the Government of Greenland, Ministry of Mineral Resources for provision of access to the field sites, sampling permits and specifically geologist A. Juul-Nielsen for accompanying us on the field expedition, participating in discussions to determine an acceptable sampling strategy, and approving the final sample collection; T. Elam for providing the PIQUANT quantification code and descriptions of its architecture for the PIXL prototype; J. Thieme and E. Fogelqvist for assistance with data collection and reduction at the SRX beamline; T. Rasbury and K. Wooton for assistance with sample digestion and REE + Y analysis; I. Burch for field assistance including sample acquisition; K. Bourke for cutting and polishing the rock samples; and I. Fast for assistance in the field. J.A.H. acknowledges partial support from the Stony Brook University-Brookhaven National Laboratory Seed Grant program.
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Nature thanks M. M. Tice, M. van Zuilen and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Contributions
A.C.A. led the research and field expedition, determined analytical strategy for samples, interpreted data, analysed the structures and wrote the manuscript. M.T.R. coordinated field logistics, including sampling permits and inclusion of Greenland government staff in the fieldwork and sampling operations, made field observations, interpreted metamorphic history, provided regional geologic context and contributed to manuscript writing. J.A.H. performed synchrotron X-ray fluorescence analyses, REE + Y analyses and interpreted geochemical data, contributed to manuscript writing and helped to write the Methods. D.T.F. acquired and analysed PIXL maps, performed thin section petrography and contributed to manuscript revisions. C.M.H. processed the PIXL data used in elemental maps and helped to write the Methods.
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Extended data figures and tables
Extended Data Fig. 1 Satellite image showing the approximate outline and location of the Isua Structural Belt and the study area.
The satellite image of the study area. The image was obtained from Google Maps.
Extended Data Fig. 2 Putative stromatolites of the ISB at site A.
Yellow arrows point to triangular shapes with apices mostly pointing down relative to layering. Note, the stratigraphy was inverted, as the layers have been overturned1. Each of the triangles is approximately 4 cm across. The blue box shows the approximate outline of the sample acquired for the present study. The yellow box shows the area shown in Fig. 1.
Extended Data Fig. 3 Breccia at site C.
a, Close-up view of breccia, from the previous study1. Ch, chert; dol, dolomite. b, Larger field of view showing the same breccia block as in a, showing the location of the elongated rod-like fabric (rodding) on the upper right side of the rock. c, Top view of the breccia-containing block from a—note the contrasting appearance of the rock fabric.
Extended Data Fig. 4 Photographs of details from the deformation fabrics of site A.
The photographs show details of the observed deformation fabrics on the cut and polished faces of all three pieces of the columnar sample that we collected from site A. Each piece was cut to show rock fabrics on orthogonal faces. a, Largest piece, which includes one of the ‘stromatolites’ on the right face. Note contrasting fabrics on adjacent faces. b–e, Additional pieces of the rock sample, showing further examples of the contrasting rock fabrics. Green arrows on all images indicate the orientation of the spaced cleavage.
Extended Data Fig. 5 Petrographic context of samples that were digested for REE + Y analyses.
a, Slab sample before cutting. b, Slab sample after cutting. ‘M’ denotes the subsample from which analyses ‘M3M’ and ‘M3C’ were derived. ‘C’ denotes the subsample from which analyses ‘C3M’ and ‘C3C’ were derived. Scale bar, 10 mm.
Extended Data Fig. 6 Synchrotron XRF element maps of the ISB sample.
The distribution of trace elements relative to minerals is shown. a, Photograph of the sample. White squares show map locations. Scale bar, 10 mm. b, Distribution and X-ray intensity of detected elements for map 1. c, Distribution and X-ray intensity of detected elements for map 2. b, c, X-ray intensity variations were used to colour the element maps. Blue, zero X-ray intensity; red, maximum X-ray intensity. X-ray intensity ranges (counts per second (cps)) are shown beneath each map. All maps are for K-shell X-rays except for Ba, which was detected using L-shell X-rays.
Supplementary information
Supplementary Information
This file contains Supplementary Text: Interpretation of synchrotron micro-XRF element map data and Supplementary Table 1: Key evidence presented in support of biogenic interpretation of stromatolites in the 3,450 Ma Strelley Pool Formation.
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Allwood, A.C., Rosing, M.T., Flannery, D.T. et al. Reassessing evidence of life in 3,700-million-year-old rocks of Greenland. Nature 563, 241–244 (2018). https://doi.org/10.1038/s41586-018-0610-4
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DOI: https://doi.org/10.1038/s41586-018-0610-4
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