Co-evolution of atmosphere-hydrosphere redox states and life at the end of the Lomagundi-Jatuli Event: A failed natural experiment for a prelude of the Cambrian Explosion of life?
Mineralogy, Petrology and Geochemistry
Final Report Abstract
In this project we identified the sediments that formed the Francevillian FA sandstones, which host the 2.1 Ga old U deposits of Mounana, Boyindzi, Oklo-Okelobondo, Mikouloungou, and Bangombé, to originate from the reworked material of the Congo craton with minor contributions of material from the East Gabonian block, using a combination of zircon U-Pb age dating and Hf isotope data. Furthermore, we found that different sources of hydrocarbons responsible for the accumulation of U in these massive ore deposits may explain why only two of these U-deposits developed into natural nuclear reactors (those at Oklo-Okelobondo and Bangombé). Those U deposits that did not become critical are associated with vanadium (V)-rich hydrocarbons originating from black shales of the Francevillian FC and FD formations, whereby V acted as a strong nuclear poison adsorbing vast amounts of neutrons liberated during 235U fission reactions. The deposits at Oklo-Okelobondo and Bangombé, on the other hand, are associated with V-poor hydrocarbons from FB formation black shales. Our combined N and Se isotope data of chemical sediments deposited during the 2.15- 2.08 Ga Lomagundi Event support an extensive ocean oxygenation during that time, but the oxygen inventory was not significant enough to build up an oceanic nitrate reservoir capable of changing the N isotopic composition of the global ocean. This implies that O2 saturation levels in the atmosphere-ocean system across the GOE and related Lomagundi Event remained much lower than during the NOE, and Paleoproterozoic biogeochemical cycles are very far from being comparable with the Neoproterozoic or modern styles. The putative 2.1 Ga old Francevillian fossils of large multicellular organisms show high Zn enrichment and are isotopically depleted relative to their host rocks and non-related fossil pyrites of various ages. A high demand for zinc marked by high Zn enrichment, and preferential utilization of lighter zinc isotopes of Francevillian fossils are defining characteristics of eukaryotes. We thus provide compelling geochemical evidence to support biological zinc utilization by eukaryotes c. 2.1 Ga ago. Our findings establish a critical calibration point for eukaryogenesis, and suggest that this major evolutionary innovation occurred in the Paleoproterozoic, c. 400 million years earlier than is currently widely accepted.
Publications
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(2020) Constraining provenance for the uraniferous Paleoproterozoic Francevillian Group sediments (Gabon) with detrital zircon geochronology and geochemistry, Precambrian Research, 343
Ossa Ossa, F.G., Hofmann, A., Ballouard, C., Vorster, C., Schoenberg, R., Friedrich, A., Mayaga- Mikolo, F., Bekker, A.
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(2021) Limited expression of the Paleoproterozoic Oklo natural nuclear reactor phenomenon in the aftermath of a widespread deoxygenation event-2.11-2.06 billion years ago, Chemical Geology 578
Ossa Ossa, F.G., Bekker, A., Hofmann, A., Poulton, S.W., Ballouard, C., Schoenberg, R.
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Increase in ecosystem complexity aided organic carbon sequestration during the c. 2.22‑2.06 Ga Lomagundi carbon isotope excursion (2021), Conference Proceedings, EGU
Ossa Ossa, F., König, S., Hofmann, A., Bekker, A., Spangenberg, J. E., and Schoenberg, R.
