Preservation of Organic Carbon in Dolomitized Cambrian Stromatolites and Implications for Microbial Biosignatures in Diagenetically Replaced Carbonate Rock

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Authors

Ashley Elizabeth Murphy , Scott T. Wieman, Juliane Gross, Jennifer C. Stern, Andrew Steele, Mihaela Glamoclija

Abstract

Stromatolites have been a major focus in the search for ancient microbial biosignatures, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized and correlated with their dolomitizing conditions. The lack in detailed dolomitic stromatolite studies is likely because dolomitization rarely preserves microbial morphology, which hampers the characterization of fossils’ biogenicity, syngenicity and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite is near- to stoichiometric, and poorly to well ordered. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) The microspar dolomite formed by early diagenetic replacement at or near the surface, that produced finely crystalline dolomite, (2) The zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) The saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high temperature phase. Organic carbon, characterized using confocal Raman microscopy, has exclusive distribution within the microspar dolomite, and the D and G bands’ characteristics reveal similar style thermal alteration as host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars.

DOI

https://doi.org/10.31223/osf.io/e3ktj

Subjects

Biogeochemistry, Earth Sciences, Geochemistry, Geology, Physical Sciences and Mathematics, Planetary Geology, Planetary Sciences

Keywords

biosignature, burial diagenesis, Cambrian stromatolites, dolomitization, organic carbon

Dates

Published: 2020-05-26 02:02

Last Updated: 2020-08-24 17:29

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License

CC BY Attribution 4.0 International

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