EGU21-13722, updated on 04 Mar 2021
https://doi.org/10.5194/egusphere-egu21-13722
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Isua (Greenland) relict stromatolites cannot be confidently interpreted as original sedimentary structures

Mike Zawaski1, Nigel Kelley1,2, Phil (Omero) Orlandini3, Claire Nichols4,5, Abigail Allwood6, and stephen Mojzsis1,7
Mike Zawaski et al.
  • 1University of Colorado at Boulder, Geology, Boulder, United States of America
  • 2Bruker Nano Analytics, 415 N. Quay Street, Kennewick, WA 99336, USA
  • 3Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712 – 1722, USA
  • 4Department of Earth Sciences, South Parks Road, Oxford OX1 3AN UK
  • 5Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307. USA
  • 6NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109. USA
  • 7Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary (stephen.mojzsis@colorado.edu)

The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. Our 2020 publication argues against biogenicity for the proposed stromatolites. The subsequent Comment to our work challenged some of our fundamental arguments for a tectonic origin to the structures. This Comment has been an opportunity for us to elaborate on these structures and further refine and solidify our initial conclusion that they represent the expected outcome of the tectonic deformation displayed in the ISB. This dialogue between groups is essential as the consequence of these structures being biogenic would move the date for complex microbial communities 200 million years closer to Earth's formation, to a time when Earth’s surface would have been even less habitable. Here we reexamine our four key observations that support our tectonic origin. First, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they were never primary linear, deformation-parallel stromatolites or deformed conical stromatolites. Second, our combined major element (e.g., Ca, Mg, Si) scanning μXRF maps fail to reveal internal laminations for the cores of these structures, but other authors argue layers are present. In the instance where layers appear to be preserved, we argue that an amorphous core is still present.  Also, layering on its own is inconclusive of a biogenic origin as relict internal laminations could be preserved. Third, the gross morphology of these structures being nearly identical in morphology and dimensions to clearly tectonic structures only tens of meters away is a more reliable indicator of a tectonic versus biogenic origin than internal laminations. Lastly, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results reinforce that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.

How to cite: Zawaski, M., Kelley, N., Orlandini, P. (., Nichols, C., Allwood, A., and Mojzsis, S.: The Isua (Greenland) relict stromatolites cannot be confidently interpreted as original sedimentary structures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13722, https://doi.org/10.5194/egusphere-egu21-13722, 2021.

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