Testing the geological fate of abiogenic carbonaceous matter
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Ancient organic matter (OM) has long been in the focus of science, as it may preserve a direct remnant of once extant biomass and is a recorder of biogeochemical pathways even in Precambrian settings. However, OM can not per se been taken as evidence of life, because abiotic organic synthesis reactions, possibly fueled by hydrothermal energy, impacts, lightning, or UV-radiation, are a potential source of OM on Earth and elsewhere in the universe. Such abiotic reactions may have proceeded to a much greater extent on the early Earth than today. As the distinction of biogenic vs. abiotic OM solely based on biomarkers and δ13C-signatures is problematic, particularly the origin of Archaean OM is notoriously ambiguous. The validity of OM as a proxy for early life has been critically discussed, and many authors have suggested an abiotic Fischer-Trospch-type-synthesis (FTT-) origin of at least part of the OM enclosed in Archaean rocks. This project was aimed at providing experimental data to enable a better distinction between biogenic and abiotic OM in Archaean rocks and elsewhere. Abiotic FTT OM was successfully and reproducibly produced under hydrothermal conditions (175°C) using a custom-designed device based on Morey-type reactors. Organic extracts of the FTT products generated were characterized in detail using gas chromatography–mass spectrometry (GC–MS), combustion infrared detection, attenuated total reflectance Fourier transform infrared spectroscopy, and pyrolysis GC–MS. This revealed a number of lipids, including more complex compounds previously undescribed as FTT products that may potentially mimic biological signals, such as methyl-branched alkanes. No cyclic or aromatic compounds were observed in the freshly produced FTT material. Likewise, no ‘classical’ kerogen was observed as a product of the FTT experiments, but extraction residues of FTT products contained macromolecular organic material in the form of a polymer comprised of low-molecular weight compounds, most probably oxalic acid and diol species. FTT reactions a priori produce unimodal chain length distributions and isomeric mixtures of methyl-branched compounds, as opposed to biolipids, which typically show preferences of individual homologs and/or isomers. Experimental maturation of an immature kerogen demonstrated the pathway along which these biological signatures may pass into unimodal distributions similar to FTT reaction products by thermal overprint. Therefore, discrimination of biologically derived compounds from FTT organics may become increasingly problematic with ongoing thermal maturation.
Publications
- (2016): Testing the preservation of biomarkers during experimental maturation of an immature kerogen. International Journal of Astrobiology 15, 165-175
Mißbach H., Duda, J.-P., Lünsdorf, N.K., Schmidt, B.C., Thiel, V.
(See online at https://doi.org/10.1017/S1473550416000069) - (2018): Assessing the diversity of lipids formed via Fischer-Tropsch-type reactions. Organic Geochemistry 119, 110-121
Mißbach H., Schmidt B.C., Duda J.-P., Lünsdorf N.K., Goetz W., Thiel V.
(See online at https://doi.org/10.1016/j.orggeochem.2018.02.012)
