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

Abstract

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.