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?

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The Paleoproterozoic Francevillian sedimentary successions of the Republic of Gabon, West Africa were described to record the most prominent aftermaths of the Great Oxidation Event (GOE) constrained at ca. 2.46-2.32 Ga ago. These include (1) the Lomagundi-Jatuli Event (LE) ─ the most positive carbon isotope excursion of the Earth’s history, and (2) the evidence of putative colonies of large eukaryotes associated with a well-oxygenated surface ocean. However, a number of geochemical-based studies shown that the upper part of this ca. 2.1 Ga old Francevillian Group, overlaying the lower fossil-bearing part, was deposited under anoxic water column which also coincided with the end of the LE. Such an upper Francevillian ocean anoxia is particularly consistent with an emerging paradigm arguing for a global deoxygenation event into our planet some 200-300 Ma after the beginning of the GOE. Moreover, recent studies investigated Se isotope systematics for deep marine organic-rich shales and inferred that surface oceans were only suboxic during the onset of the GOE and thus O2 levels were not high enough to allow the development of eukaryotic life. Here we intend to investigate (i) the C and O isotopic compositions of organic- and carbonate carbon for bringing further insights on the global average fractionation between C-org and C-carb likely contributing to improve our understanding on the amount of oxygen released in the atmosphere-ocean system during the late stages of the GOE and the LE, (ii) the major and trace elements variations and Fe-Mo isotopic compositions of Francevillian sediments for a better understanding of the water column chemistry and associated redox states with particular emphasis on what controlled the cycling of transitional metals in the oceans during the late stages of the GOE and the LE, (iii) detailed stable Se isotope systematics to further test whether shallow oceans were oxygenated enough to allow the development and proliferation of complex life. The ultimate goal of this study however is to test the attempt of marine environmental conditions at creating higher life forms during the onset of the GOE and LE as well as why it probably did not work and what ended the LE.