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Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils

Rasmussen, B. and Muhling, J. R. and Fischer, W. W. (2021) Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils. Astrobiology, 21 (6). pp. 655-672. ISSN 1531-1074. doi:10.1089/ast.2020.2376.

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The 1.88-billion-year-old Gunflint carbonaceous microfossils are renowned for their exceptional morphological and chemical preservation, attributed to early and rapid entombment in amorphous silica. The carbonaceous matter lining and partly filling filamentous and spherical structures is interpreted to be indigenous, representing thermally altered relicts of cellular material (i.e., kerogen). Here we show that stromatolitic black cherts from the Gunflint Formation, Schreiber Beach, Ontario, Canada, were saturated in syn-sedimentary oil. The thermally altered oil (pyrobitumen), which occurs in the stromatolites and intercolumn sediments, fills pores and fractures, and coats detrital and diagenetic grain surfaces. The occurrence of detrital bitumen grains in the stromatolites points to the proximity of shallow seafloor oil seeps and hence the possible existence of chemosynthetic microbes degrading hydrocarbons. We suggest that hydrocarbons that migrated through the silicifying stromatolites infiltrated semi-hollow microbial molds that formed following silica nucleation on the walls or sheaths of decayed cells. Upon heating, the hydrocarbons were transformed to nanoporous pyrobitumen, retarding silica recrystallization and enhancing detailed preservation of the carbon-rich microfossils. Hydrocarbon infiltration of silicified microbes offers a new explanation for the preservation of the Gunflint microfossils and may have played a role in the formation of some of Earth's oldest microfossils.

Item Type:Article
Related URLs:
URLURL TypeDescription
Rasmussen, B.0000-0001-5052-6797
Muhling, J. R.0000-0003-1890-9119
Fischer, W. W.0000-0002-8836-3054
Additional Information:© 2021, Mary Ann Liebert, Inc., publishers. Submitted 16 September 2020; Accepted 17 December 2020; Online Ahead of Print: March 8, 2021. The authors acknowledge support from Australian Research Council grants DP140100512 (B.R.) and DP190102237 (B.R. and J.R.M.), and Simons Foundation Collaboration on the Origins of Life (W.W.F.). SEM, FIB, and TEM analyses were performed at CMCA at UWA, a node of Microscopy Australia funded from university and government sources, and we thank A. Suvorova, S. Gain, and M. Saunders for technical support. We thank S. Sheppard and B. Krapez for their assistance in the field, and A.G. Dias and M.J. Lemos de Sousa (Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências da Universidade do Porto) for permission to reproduce images from Alpern et al. (1992). We thank four anonymous reviewers and the associate editor for their comments that helped improve the manuscript. No competing financial interests exist.
Funding AgencyGrant Number
Australian Research CouncilDP140100512
Australian Research CouncilDP190102237
Simons FoundationUNSPECIFIED
Microscopy AustraliaUNSPECIFIED
Subject Keywords:Gunflint; Microfossils; Precambrian; Carbonaceous matter; Oil; Fossilization; Chert
Issue or Number:6
Record Number:CaltechAUTHORS:20210316-084906579
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Official Citation:Ancient Oil as a Source of Carbonaceous Matter in 1.88-Billion-Year-Old Gunflint Stromatolites and Microfossils. B. Rasmussen, J.R. Muhling, and W.W. Fischer. Astrobiology. Jun 2021. 655-672;
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108449
Deposited By: Tony Diaz
Deposited On:19 Mar 2021 00:40
Last Modified:23 Jun 2021 22:11

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