Reevaluierung der frühesten Spuren irdischen Lebens
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
The nature of Earth’s earliest biosphere is largely unconstraint, yet one of the most fundamental evolutionary scientific questions. In the absence of unambiguous microfossils and with molecular clock estimates that are still burdened by significant uncertainties, the sedimentary hydrocarbon remnants of taxonomically diagnostic biological lipids may shed more light on the nature and diversity of early biology. In particular the question whether eukaryotes, whose evolutionary advent represents a first significant evolutionary step towards increasing complex life, had already existed during the Archean is repeatedly posed by geobiologists. Over the past decades, manuscripts repeatedly reported the presence of such biomarkers in Archean sediments. Despite the fact that their survival is unlikely in rocks >2.5 Gyr in age that experienced Prehnite-Pumpellyite metamorphic facies and despite increased awareness of contamination problems, indigenous and syngenetic hydrocarbons that may persist in rocks of this age could not be distinguished from potential contaminants. In this project we studied purpose-drilled ultra clean core samples from the Pilbara Craton, NW Australia, representing organic rich shales from the Jeerinah Fm. The overarching objective was to determine what unambiguously syngenetic fossil lipids exist in Archean sediments from the Pilbara Craton and to construct a more reliable picture of Archean biodiversity. This goal was successfully mastered, yet with an unexpected outcome. All samples were found to be devoid of any polycyclic terpenoid biomarkers, thereby implying that no biological information can be gained. These results were consistent amongst different laboratories that analyzed corresponding core halves (MIT and Macquarie University). The molecular inventory largely included diamondoids and PAH. Moreover, while tight contamination control during drilling (including organic-free negative core blanks: basalts and cherts) and sample workup implied these analytes to be indigenous, a stable carbon isotope mismatch to their associated kerogens implies that they are not syngenetic. Positive carbon isotope matches between kerogens and pyrolysates, and between extractable analytes and (likely-) pyrobitumen confirms this finding. The thermal maturity determined on the kerogens (H/C and Raman) suggest that even these samples that were counted amongst the thermally best preserved in the Pilbara Craton have experienced a thermal overprint too high for the survival of biologically diagnostic markers. With this information it is at present impossible to deduce Archean biological information and previous reports of Archean biomarkers should be treated with caution. Although negative, the results of this study provide important information on (i) the absence of knowledge of Earth’s earliest biosphere, (ii) the negative prospect of conducting further biomarker studies on Archean rocks as long as no better-preserved regions are found and (iii) provides a benchmark for future studies of exceptionally lean Precambrian sample material.
Projektbezogene Publikationen (Auswahl)
- (2015) Reappraisal of hydrocarbon biomarkers in Archean rocks. Proceedings of the National Academy of Sciences 112, 5915-5920
French, K.L., Hallmann, C., Hope, J.M., Schoon, P.L., Zumberge, J.A., Hoshino, Y., Peters, C.A., George, S.C., Love, G.D., Brocks, J.J., Buick. R., Summons, R.E.
(Siehe online unter https://doi.org/10.1073/pnas.1419563112) - (2016) Enhanced procedural blank control for organic geochemical studies of critical sample material. Geobiology 14, 469-482
Leider, A., Schumacher, T. C., Hallmann, C.
(Siehe online unter https://doi.org/10.1111/gbi.12183) - (2016) Rapid, direct and non-destructive assessment of fossil organic matter via microRaman spectroscopy. Carbon 108, 440-449
Ferralis, N., Matys, E. D., Knoll, A. H., Hallmann, C., Summons, R. E.
(Siehe online unter https://doi.org/10.1016/j.carbon.2016.07.039) - (2019) Low-femtogram sensitivity analysis of polyaromatic hydrocarbons using GC-APLI-TOF mass spectrometry: Extending the target window for aromatic steroids in early Proterozoic rocks. Organic Geochemistry 129, 77-87
Leider, A., Richter-Brockmann, S., Nettersheim, B.J., Achten, C.
(Siehe online unter https://doi.org/10.1016/j.orggeochem.2019.01.015)
