Coupled Cr and Mo isotope investigations of post-GOE Paleoproterozoic sediments: Testing the hypothesis of an oxygen overshoot in atmospheric O2 levels

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Coupled isotope investigations of the redox-sensitive elements Cr and Mo in 2.5 – 2.0-Gyr-old sediments of the Pechenga Greenstone Belt (PGB) and the Onega Basin (OB) in Russia will be used to reconstruct the redox evolution of the atmosphere and oceans postdating the Great Oxidation Event (GOE). The GOE marks the first significant increase in atmospheric oxygen levels and defines the transitions from anoxic to oxic environmental conditions. The aftermath of the GOE is characterized by major perturbations of the oxygen, carbon, and sulfur cycle - probably global - during the 2.22 – 2.05-Gyr-old Lomagundi and the following Shunga events. While the post-GOE sediments archive evidence for the burial of vast amounts of organic carbon, the expansion and contraction of the seawater sulfate reservoir, a proposed oxygen overshoot and the onset of effective continental weathering, the timing and response between the oxygenation of the atmosphere and oceans is poorly constrained. Moreover and more puzzling, even though the GOE marks the onset of oxygen-rich atmospheric conditions that was probably followed by an oxygen overshoot, Cr isotopes that are suggested as a tracer for free oxygen fail to identify oxygen-rich conditions in the Paleoproterozoic. This project seeks to perform detailed coupled Cr and Mo isotope investigations on various near-shore sedimentary environments from the Kuetsjärvi, Kolosjoki (PGB) and Zaonezhskaya (OB) formations. The first part of the project will characterize the paleoenvironmental conditions by major and trace elements and carbon isotopes, whereas the second part focuses on the combined Cr and Mo isotope investigations. Complementary geochemical investigations will be carried out on near-shore sediments from the 2.2 – 2.1-Gyr-old Pretoria Group (Botswana), because these sediments also capture the Lomagundi Event and will help to provide a more detailed, and probably global, understanding of the Paleoproterozoic atmosphere-ocean system.