Specimens were collected by AGL with permission and assistance from J. Gehling in 2015. AGL is funded by the Natural Environment Research Council [grant number NE/L011409/2]. SM acknowledges support from the European Union’s Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie grant agreement 747877, and thanks M. Hall and A. McDonald for help with thin section polishing and Raman spectroscopy respectively. JJM recognises support from Mitacs, and all authors are grateful to N.J. Butterfield for constructive discussions during manuscript preparation. We thank R. Gaines, S. Darroch and J. Gehling for constructive reviews of this manuscript.Peer reviewedPostprin

Brasier, AT

Liu, AG

Marosi, D

Matthews, Jack

McMahon, S

Still, JW

Geology

English

Microbially mediated early diagenetic pyrite formation in the immediate vicinity of organic material has been the favoured mechanism by which to explain widespread preservation of soft-bodied organisms in late Ediacaran sedimentary successions, but an alternative rapid silicification model has been proposed for macrofossil preservation in sandstones of the Ediacara Member in South Australia. We here provide petrological evidence from Nilpena National Heritage Site and Ediacara Conservation Park to demonstrate the presence of grain20 coating iron oxides, framboidal hematite, and clay minerals along Ediacara Member sandstone bedding planes, including fossil-bearing bed soles. SEM and petrographic data reveal that framboids and grain coatings, which we interpret as oxidized pyrite, formed before the precipitation of silica cements. In conjunction with geochemical and taphonomic considerations, our data suggest that anactualistically high concentrations of silica need not be invoked to explain Ediacara Member fossil preservation: we conclude that the pyritic ‘death mask’ model remains compelling

Liu, Alexander G

McMahon, Sean

Matthews, Jack J

Still, John W

Brasier, Alexander T

Edinburgh Research Explorer

Petrological evidence in support of the death mask model for Ediacaran soft-bodied preservation in South Australia

Liu, Alexander G.

Matthews, Jack J.

Still, John W.

Brasier, Alexander T.

Aberdeen University Research

Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia

Oxford University Research Archive

Forum Reply  https://doi.org/10.1130/ G46519Y.1 © 2019 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. GEOLOGY FORUM | August 2019 | https://pubs.geoscienceworld.org/geology/ e474 Petrological evidence supports the death mask model for the preservation of  Ediacaran soft-bodied organisms in South Australia  Alexander G. Liu1, Sean McMahon2, Jack J. Matthews3,4, John W. Still5, Alexander T. Brasier5, and Diana Marosi6* 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 2UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK 3Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland  A1C 3X5, Canada 4Oxford University Museum of Natural History, Oxford OX1 3PW, UK 5School of Geosciences, University of Aberdeen, King’s College, Aberdeen AB24 3UE, UK 6NHS Scotland, Edinburgh EH12 9EB, UK  *Independent researcher  We gratefully acknowledge the Comment by Tarhan et al. (2019) in response to our paper (Liu et al., 2019) discussing preservation of Ediacara Member fossils. We agree that the mere presence of pyrite or iron oxides in these rocks does not establish either that pyrite was an agent of fossilization, or that authigenic marine silica was not. However, our observations were important in demonstrating both that early diagenetic pyrite cementation remains a plausible taphonomic mecha-nism in the Ediacara Member, and that the early silicification taphonomic model (Tarhan et al., 2016) is undermined by available petrological and geochemical data. We documented thin veneers of hematite occurring in abundance as grain coatings, together with clay minerals, along discrete horizons (i.e., not scattered), including fossiliferous bedding planes. The presence of framboidal aggregates within these Ediacara Member horizons implies that the observed hematite is likely to have replaced pyrite, which, as Tarhan et al. (2019) correctly note, is a common marine cement and microbial byproduct, and is thus not unexpected as an original mineralog-ical constituent. These observations are consistent with a potential role for pyrite as a cementing agent on these surfaces. Tarhan and colleagues convincingly establish that some iron oxides have been redistributed along exposed bedding surfaces in the Ediacara Member, forming as late-stage precipitates introduced by groundwater (Tarhan et al., 2018), including along microfractures (Tarhan et al., 2019, their figure 1). As we reported, iron oxides do occur at present-day grain boundaries (i.e., outside silica overgrowths) where microporosity has probably facilitated redistribution (e.g., Liu et al., 2019, our figures DR2D and DR2E; bright, non-arrowed patches). However, abundant hematite and some framboids are observed to directly abut original, rounded grain boundaries, and are encased wholly within syntaxial, grain-mantling silica, with little or no associated porosity (Fig. 1; our figure DR2). The observed silica cements must post-date these iron oxides. Furthermore, several of the observed hematite-rich horizons have never been exposed at the surface, and we interpret both their clays and the original iron minerals coating primary grain boundaries to be original or early diagenetic features. Even if the silica cement also formed early, it undoubtedly post-dates the clays and iron minerals embedded within it. We acknowledge that the extensive weathering history of the Australian continent could have introduced hematite at a late stage, but the same would then necessarily be true of the surrounding silica cement, critically undermining the petrological evidence offered by Tarhan et al. (2016) in support of their early silicification model. The suggestion of Tarhan et al. (2019) that a primary silica cement would form only on grain boundaries and exclude organic matter is implausible, and contrary to the evidence of much older silicified sandstones (e.g., Wacey et al., 2011). It represents a departure from the original silicification model of Tarhan et al. (2016, p. 953), which explicitly involved silica nucleation onto organic matter. Moreover, the geochemical evidence presented in that paper remains unconvincing. Al (ppm) and Ge (ppm) concentrations are weakly positively correlated in the combined data from sampled cements (e.g., Spearman’s  = 0.339; significant at 0.01 level), although this should be interpreted cautiously since the paired ppm values are not strictly mutually independent. The low Ge contents in detrital grains and high Ge contents in silica cements in some samples (e.g., ‘Aspidella 33’ of Tarhan et al., 2016) do not indicate anactualistic early silicification. High inter-sample variability in cement elemental concentrations, itself more consistent with late alteration than inheritance from Ediacaran seawater, raises further questions.  Figure 1. Backscattered electron micros-copy (BSE, left) shows hematite (white) embedded in silica cement (SC), where there is no porosity, unlike the modern grain boundary (M). Cathodolumines-cence microscopy (CL, right) reveals the original grain boundary (O). Our paper did not claim to present a mechanistic explanation for the role of pyrite in Ediacara Member preservation; such detailed explana-tions exist elsewhere (e.g., Gibson et al., 2018). We clearly outlined the future steps required to determine whether there is a systematic association between fossils in the Ediacara Member and continuous pyrite veneers. We agree with Tarhan et al. (2019) that our reliance on externally weathered samples is sub-optimal, but we share this weakness with every other taphonomic study of the Ediacara Member; fresh core material is currently unavailable. We also agree wholeheartedly with those authors that multiple taphonomic pathways operated during the late Ediacaran Period. While we remain open to the possibility that silica could have played a preservational role in the Ediacara Member, for example by favoring clay mineral authigenesis or consolidating pre-existing pyrite cements or rheological boundaries (cf. Bobrovskiy et al., 2019), we do not consider available petrological and geochemical evidence to support the early silicification model as originally articulated. REFERENCES CITED Bobrovskiy, I., Krasnova, A., Ivantsov, A., Luzhnaya, E., and Brocks, J.J., 2019, Simple sediment rheology explains the Ediacara biota preservation: Nature Ecology & Evolution, v. 3, p. 582–589, https://doi.org/10.1038/s41559-019-0820-7. Gibson, B.M., Schiffbauer, J.D., and Darroch, S.A., 2018, Ediacaran-style decay experiments using mollusks and sea anemones: Palaios, v. 33, p. 185–203, https://doi.org/10.2110/palo.2017.091. Liu, A.G., McMahon, S., Matthews, J.J., Still, J.W., and Brasier, A.T., 2019, Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia: Geology, v. 47, p. 215–218, https://doi.org/10.1130/G45918.1. Tarhan, L.G., Hood, A.V.S., Droser, M.L., Gehling, J.G., and Briggs, D.E.G., 2016, Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: Geology, v. 44, p. 951–954 https://doi.org/10.1130 /G38542.1, https://doi.org/10.1130/G38542.1. Tarhan, L.G., Planavsky, N.J., Wang, X., Bellefroid, E.J., Droser, M.L., and Gehling, J.G., 2018, The late-stage “ferruginization” of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes: Geobiology, v. 16, p. 35–48, https://doi.org/10.1111/gbi.12262. Tarhan, L.G., Hood, A.V.S., Droser, M.L., Gehling, J.G., Briggs, D.E.G., Gaines, R.R., Robbins, L.J., and Planavsky, N.J., 2019, Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia: Comment: Geology,  v. 47, p. exxx, https://doi.org /10.1130/G46326C.1. Wacey, D., Kilburn, M.R., Saunders, M., Cliff, J., and Brasier, M.D., 2011, Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia: Nature Geoscience, v. 4, p. 698–702, https://doi.org /10.1038/ngeo1238.Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/8/e474/4793759/e474.pdfby gueston 27 July 2019

Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia: REPLY

Specimens were collected by AGL with permission and assistance from J. Gehling in 2015. AGL is funded by the Natural Environment Research Council [grant number NE/L011409/2]. SM acknowledges support from the European Union’s Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie grant agreement 747877, and thanks M. Hall and A. McDonald for help with thin section polishing and Raman spectroscopy respectively. JJM recognises support from Mitacs, and all authors are grateful to N.J. Butterfield for constructive discussions during manuscript preparation. We thank R. Gaines, S. Darroch and J. Gehling for constructive reviews of this manuscript.Peer reviewe

We gratefully acknowledge the Comment by Tarhan et al. (2019) inresponse to our paper (Liu et al., 2019) discussing preservation ofEdiacara Member fossils. We agree that the mere presence of pyrite oriron oxides in these rocks does not establish either that pyrite was anagent of fossilization, or that authigenic marine silica was not. However,our observations were important in demonstrating both that earlydiagenetic pyrite cementation remains a plausible taphonomic mechanism in the Ediacara Member, and that the early silicification taphonomicmodel (Tarhan et al., 2016) is undermined by available petrological and geochemical data

Marosi, Diana

     Edinburgh Research Explorer                                      Petrological evidence supports the death mask model for thepreservation of Ediacaran soft-bodied organisms in SouthAustralia:REPLYCitation for published version:Liu, AG, McMahon, S, Matthews, JJ, Still, JW, Brasier, AT & Marosi, D 2019, 'Petrological evidencesupports the death mask model for the preservation of Ediacaran soft-bodied organisms in SouthAustralia:REPLY', Geology. https://doi.org/10.1130/ G46519Y.1Digital Object Identifier (DOI):10.1130/ G46519Y.1Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Peer reviewed versionPublished In:GeologyGeneral rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 12. Sep. 2019Forum  Reply  https://doi.org/10.1130/G4xxxxY.1	  © 2018 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. GEOLOGY FORUM | Month Year | https://pubs.geoscienceworld.org/geology/	  Petrological evidence supports the death mask model for the preservation of  Ediacaran soft-bodied organisms in South Australia  Alexander G. Liu1, Sean McMahon2, Jack J. Matthews3,4, John W. Still5, Alexander T. Brasier5, and Diana Marosi6 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 2UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK 3Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL A1C 3X5, Canada 4Oxford University Museum of Natural History, Oxford OX1 3PW, UK 5School of Geosciences, University of Aberdeen, King’s College, Aberdeen AB24 3UE, UK 6NHS Scotland, Edinburgh EH12 9EB, UK (independent researcher)  We gratefully acknowledge the Comment by Tarhan et al. (2019) in response to our paper (Liu et al., 2019) discussing preservation of Ediacara Member fossils. We agree that the mere presence of pyrite or iron oxides in these rocks does not establish either that pyrite was an agent of fossilization, or that authigenic marine silica was not. However, our observations were important in demonstrating both that early diagenetic pyrite cementation remains a plausible taphonomic mecha-nism in the Ediacara Member, and that the early silicification model (Tarhan et al., 2016) is undermined by available petrological and geochemical data.  We documented thin veneers of hematite occurring in abundance as grain coatings, together with clay minerals, along discrete horizons (i.e., not scattered), including fossiliferous bedding planes. The presence of framboidal aggregates within these Ediacara Member horizons implies that the observed hematite is likely to have replaced pyrite, which as Tarhan et al. (2019) correctly note, is a common marine cement and microbial byproduct, and is thus not unexpected as an original mineralog-ical constituent. These observations are consistent with a potential role for pyrite as a cementing agent on these surfaces.  Tarhan and colleagues convincingly establish that some iron oxides have been redistributed along exposed bedding surfaces in the Ediacara Member, forming as late-stage precipitates introduced by groundwater (Tarhan et al., 2018), including along microfractures (Tarhan et al., 2019, their figure 1). As we reported, iron oxides do occur at present-day grain boundaries (i.e., outside silica overgrowths) where microporosity has probably facilitated redistribution (e.g., Liu et al., 2019, our figures DR2D and DR2E; bright, non-arrowed patches). However, abundant hematite and some framboids are observed to directly abut original, rounded grain boundaries, and are encased wholly within syntaxial, grain-mantling silica, with little or no associated porosity (Fig. 1; our figure DR2). The observed silica cements must post-date these iron oxides. Furthermore, several of the observed hematite-rich horizons have never been exposed at the surface, and we interpret both their clay and the original iron minerals coating primary grain boundaries to be original or early diagenetic features. Even if the silica cement also formed early, it undoubtedly post-dates the clays and iron minerals embedded within it. We acknowledge that the extensive weathering history of the Australian continent could have introduced hematite at a late stage, but the same would then necessarily be true of the surrounding silica, critically undermining the petrological evidence offered by Tarhan et al. (2016) in support of their early silicification model. The suggestion of Tarhan et al. (2019) that a primary silica cement would form only on grain boundaries and exclude organic matter is implausible, and contrary to the evidence of much older silicified sandstones (e.g., Wacey et al., 2011). It represents a departure from the original silicification model of Tarhan et al. (2016, p. 953), which explicitly involved silica nucleation onto organic matter. Moreover, the geochemical evidence presented in that paper remains unconvincing. Al (ppm) and Ge (ppm) concentrations are weakly positively correlated in the combined data from sampled cements (e.g., Spearman’s ρ = 0.339; significant at 0.01 level), although this should be interpreted cautiously since the paired ppm values are not strictly mutually independent. The low Ge contents in detrital grains and high Ge contents in silica cements in some samples (e.g. ‘Aspidella 33’ of Tarhan et al., 2016) do not indicate anactualistic early silicification. High inter-sample variability in cement elemental concentrations, itself more consistent with late alteration than inheritance from Ediacaran seawater, raises further questions.       Our paper did not claim to present a mechanistic explanation for the role of pyrite in Ediacara Member preservation; such detailed explana-tions exist elsewhere (e.g., Gibson et al., 2018). We clearly outlined the future steps required to determine whether there is a systematic association between fossils in the Ediacara Member and continuous pyrite veneers. We agree with Tarhan et al. (2019) that our reliance on externally weathered samples is sub-optimal, but we share this weakness with every other taphonomic study of the Ediacara Member; fresh core material is currently unavailable. We also agree wholeheartedly with those authors that multiple taphonomic pathways operated during the late Ediacaran Period. While we remain open to the possibility that silica could have played a preservational role in the Ediacara Member, for example by favouring clay mineral authigenesis or consolidating pre-existing pyrite cements or rheological boundaries (cf. Bobrovskiy et al., 2019), we do not consider available petrological and geochemical evidence to support the early silicification model as originally articulated. REFERENCES CITED Bobrovskiy, I., Krasnova, A., Ivantsov, A., Luzhnaya, E., and Brocks, J.J., 2019, Simple sediment rheology explains the Ediacara biota preservation: Nature Ecology & Evolution, v. 3, p. 582–589, doi: 10.1038/s41559-019-0820-7. Gibson, B. M., Schiffbauer, J. D., and Darroch, S. A., 2018, Ediacaran-style decay experiments using mollusks and sea anemones: Palaios, v. 33, p. 185-203. Liu, A.G., McMahon, S., Matthews, J.J., Still, J.W., and Brasier, A.T., 2019, Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia: Geology, v. 47, p. 215–218, https://doi.org/10.1130/G45918.1. Tarhan, L.G., Hood, A.V.S., Droser, M.L., Gehling, J.G., and Briggs, D.E.G., 2016, Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: Geology, v. 44, p. 951–954, https://doi.org/10.1130 /G38542.1. Tarhan, L.G., Planavsky, N.J., Wang, X., Bellefroid, E.J., Droser, M.L., and Gehling, J.G., 2018, The late-stage “ferruginization” of the Ediacara Member (Rawnsley Quartzite, South Australia): Insights from uranium isotopes: Geobiology, v. 16, p. 35–48, https://doi.org/10.1111/gbi.12262. Tarhan, L.G., Hood, A.V.S., Droser, M.L., Gehling, J.G., Briggs, D.E.G., Gaines, R.R., Robbins, L.J., and Planavsky, N.J., 2019, Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia: Comment: Geology, v. XX, p. XX Wacey, D., Kilburn, M.R., Saunders, M., Cliff, J. and Brasier, M.D., 2011, Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience, v. 4, p. 698–702. Figure 1. Backscattered electron microscopy (BSE, left) shows hematite (white) embedded in silica cement (SC), where there is no porosity, unlike the modern grain boundary (M). Cathodolu-minescence microscopy (CL, right) reveals the original grain boundary (O). 

Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia:REPLY

Microbially mediated early diagenetic pyrite formation in the immediate vicinity of organic material has been the favoured mechanism by which to explain widespread preservation of soft-bodied organisms in late Ediacaran sedimentary successions, but an alternative rapid silicification model has been proposed for macrofossil preservation in sandstones of the Ediacara Member in South Australia. We here provide petrological evidence from Nilpena National Heritage Site and Ediacara Conservation Park to demonstrate the presence of grain-coating iron oxides, framboidal hematite, and clay minerals along Ediacara Member sandstone bedding planes, including fossil-bearing bed soles. SEM and petrographic data reveal that framboids and grain coatings, which we interpret as oxidized pyrite, formed before the precipitation of silica cements. In conjunction with geochemical and taphonomic considerations, our data suggest that anactualistically high concentrations of silica need not be invoked to explain Ediacara Member fossil preservation: we conclude that the pyritic ‘death mask’ model remains compelling

Apollo (Cambridge)

© 2019 Geological Society of America. Microbially mediated early diagenetic pyrite formation in the immediate vicinity of organic material has been the favored mechanism by which to explain widespread preservation of soft-bodied organisms in late Ediacaran sedimentary successions, but an alternative rapid silicification model has been proposed for macrofossil preservation in sandstones of the Ediacara Member in South Australia. We here provide petrological evidence from Nilpena National Heritage Site and Ediacara Conservation Park to demonstrate the presence of grain-coating iron oxides, framboidal hematite, and clay minerals along Ediacara Member sandstone bedding planes, including fossil-bearing bed soles. Scanning electron microscope (SEM), cathodoluminescence microscopy (CL), and petrographic data reveal that framboids and grain coatings, which we interpret as oxidized pyrite, formed before the precipitation of silica cements. In conjunction with geochemical and taphonomic considerations, our data suggest that anactualistically high concentrations of silica need not be invoked to explain Ediacara Member fossil preservation: We conclude that the pyritic death mask model remains compelling

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Petrological evidence supports the death mask model for the preservation of Ediacaran soft-bodied organisms in South Australia

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