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Metamorphic evolution of carbonate-hosted microbial biosignatures

C.R. Cousins1,

1School of Earth and Environmental Sciences, University of St. Andrews, Irvine Building, North Street, St. Andrews, Fife, UK

S. Mikhail1,2,

1School of Earth and Environmental Sciences, University of St. Andrews, Irvine Building, North Street, St. Andrews, Fife, UK
2Geophysical Laboratory, Carnegie Institute of Washington, Broad Branch Road, Washington D.C., USA

F. Foucher3,

3CNRS, Centre de Biophysique Moléculaire, UPR 4301, Rue Charles Sadron, CS80054, 45071 Orléans Cedex 2, France

A. Steele2,

2Geophysical Laboratory, Carnegie Institute of Washington, Broad Branch Road, Washington D.C., USA

F. Westall3

3CNRS, Centre de Biophysique Moléculaire, UPR 4301, Rue Charles Sadron, CS80054, 45071 Orléans Cedex 2, France

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Cousins, C., Mikhail, S., Foucher, F., Steele, A., Westall, F. (2020) Metamorphic evolution of carbonate-hosted microbial biosignatures. Geochem. Persp. Let. 12, 40–45.

Royal Society of Edinburgh Research Fellowship

Geochemical Perspectives Letters v12  |  doi: 10.7185/geochemlet.2002
Received 12 August 2019  |  Accepted 06 December 2019  |  Published 16 January 2020
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Starting material for experiments showing (a) Six high pressure, high temperature (HPHT) experimental conditions investigated and their corresponding metamorphic grades spanning lower and upper blueschist (BS), greenschist (GS) and amphibolite (Amph.). Grey depicts HPHT conditions investigated previously (Schiffbauer et al., 2012

Schiffbauer, J. D., Wallace, A.F., Hunter, J.L., Kowalewski, M., Bodnar, R.J., Xiao, S. (2012) Thermally‐induced structural and chemical alteration of organic‐walled microfossils: an experimental approach to understanding fossil preservation in metasediments. Geobiology 10, 402–423.

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). (b) Si-rich microbial filamentous material and coccoidal cell structures are observed under SEM in the the starting material, shown in (c)  by a view ~ 1 cm across; forming Si-rich biofilms around calcite grains (Fig. S-1a,b).
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Figure 2 Evolution of D1 (~1360 cm-1) and G (~1610 cm-1) Raman peaks from experimental samples, and the increasing 467 cm-1 quartz peak at 425, 500, and 550 °C (box inset); and corresponding SEM images showing the textural evolution of OM mineralisation within the carbonate (CaCO3) matrix and siliceous (Si) phases Individual cells in the amorphous organic matrix can be seen in the starting material (arrows). Unprocessed Raman spectra are given in Figure S-3.
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Figure 3 BSE+EDS elemental maps or plain BSE image (left), where red = Ca and green = Si, and corresponding Raman maps (right) of the same region, where fuchsia = calcite (1006 cm-1 band), orange/yellow = quartz (465 cm-1 band), green = carbon (combined D1 and G bands), light pink = resin, black = masked fluorescence. (a) Starting material; (b) 800 MPa, 300 °C; (c) 500 MPa, 200 °C; (d) 800 MPa, 425 °C; (e) 500 MPa, 350 °C; (f) 800 MPa, 550 °C; (g) 500 MPa, 500 °C; (h) 800 MPa, 550 °C. EDS spectra are provided in Figure S-5.
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Figure 4 (a – g) BSE+EDS elemental maps (red = Ca, green = Si, yellow = S) and SEM images of sulfur globules in (a, b) Starting material; (c) 800 MPa, 425 °C; (d, e) 800 MPa, 300 °C; (f) 800 MPa, 550 °C; and (g) 500 MPa, 500 °C; (h) BSE+EDS elemental map showing the co-location of Fe (Fe = purple) with S (S = yellow) within a silica-rich (Si = red) fabric (i) in the 800 MPa, 425 °C experiment.
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