Molecular Signatures from Kerogens Preserved in 3.42 Ga Microbial Mats (Buck Reef Chert, Barberton Greenstone Belt, South Africa)

2020 | conference paper. A publication with affiliation to the University of Göttingen.

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​Molecular Signatures from Kerogens Preserved in 3.42 Ga Microbial Mats (Buck Reef Chert, Barberton Greenstone Belt, South Africa)​
Reinhardt, M. ; Thiel, V. ; Drake, H.; Goetz, W.   & Reitner, J. ​ (2020)
pp. 2185​-2185. ​Goldschmidt2020​, Online. DOI: 

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Reinhardt, Manuel ; Thiel, Volker ; Drake, Henrik; Goetz, Walter ; Reitner, Joachim 
The 3.42 Ga Buck Reef Chert (Barberton Greenstone Belt, South Africa) provides a rare sequence of exceptionally well preserved silicified microbial mats, containing abundant kerogen [1] . We investigated this macromolecular organic material (cherts from drill cores, Barberton Drilling Project - Peering into the Cradle of Life) on structural (microscopy, FTIR/ATR, Raman), and molecular level (HyPy followed by GC-MS). Kerogen is solely associated with microbial mat structures and not entrapped in any post-depositional veins or microfractures. Raman confirmed the regional peak metamorphic temperatures (greenschist facies) and therefore supports the syngeneity of the kerogens. While FTIR/ATR mostly indicated an overall graphitic structure, GC-MS after HyPy treatment revealed robust above-blank-concentrations of aliphatic and aromatic hydrocarbons. In the two-step heating approach used (330 °C / 520 °C), these compounds exclusively occurred in the high-T HyPy runs, thus pointing to cracking of covalent bonds in the kerogens. Detected n- alkane homologues (C12–C26) showed a noticeable decrease in abundance after n-C16 and n-C18. Further, isomeric mixtures of monomethyl alkanes (C 12–C21) and low amounts of PAHs were found. Preferences in chain-length of n-alkanes are not known from abiotic organic matter (FTT and extraterrestrial). The idea that these distributions instead may represent a syngenetic biological signal, is supported by (i) the thermal stability of n-alkanes [2] (ii) the careful state-of- the-art kerogen isolation, including extensive extraction, swelling, and blanks, (iii) the two-step HyPy approach used, (iv) similar findings in hydropyrolysates from >3.4 Ga chert kerogens of the Pilbara Craton [3],[4] , and (v) the occurrence of kerogen exclusively in microbial mat structures. To further decode the origin of these molecular fingerprints, compound specific stable carbon isotopes will be analyzed.
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Abteilung Geobiologie ; Max-Planck-Institut für Sonnensystemforschung 
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