Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event

The ca. 2.1 Ga Francevillian Group of Gabon was deposited in the aftermath of the Great Oxidation Event (GOE) and records the Lomagundi Event (LE), which is the most pronounced and long-lived carbon isotope excursion in the geologic record. Moreover, the sedimentary succession contains putative evidence for the earliest appearance of macro-eukaryotes. An emerging paradigm is that the end of the LE was accompanied by a deoxygenation event that preceded the apparent stability of environmental and redox conditions as well as the carbon cycle characteristic of the Mesoproterozoic. However, the processes that led to deoxygenation some 300 to 200 Ma after the beginning of the GOE are not well understood. Here we present a multi-proxy stable isotope (δ34S, Δ33S, Δ36S, δ98Mo, δ13Corg, δ13Ccarb, and δ18Ocarb) study of the Francevillian Group. We suggest that sedimentation of the lower part of the Francevillian Group took place during the LE in oxygenated shallow waters with elevated sulfate concentrations. Two episodes of anoxic water shoaling during deposition of the upper Francevillian Group correspond with broader marine deoxygenation and a contraction of the seawater sulfate reservoir. This shoaling of anoxic conditions may be linked to intense submarine hydrothermal and volcanic activity that led to sedimentary manganese deposits. We propose that increased concentrations of aqueous, hydrothermally sourced reductants drove oxygen consumption during the first deoxygenation event and established a sulfidic oxygen-minimum zone at the margin of the shallow shelf. Carbonates with positive δ13Ccarb values characteristic of the LE precipitated during this first stage of deoxygenation. The second deoxygenation, separated from the previous event by a period of well-oxygenated conditions, was marked by a stronger contraction of the seawater sulfate reservoir and coincided with the end of the LE. During this time, widespread euxinic conditions were established in shallow (above storm wave base) marine environments. The presence of a shallow-water redoxcline points to a generally low-oxygen atmosphere-ocean system. Further, the negative co-variation between δ34S and δ13C values in sediments of the Francevillian Group and other sedimentary successions of similar age worldwide suggests that the inferred two-step deoxygenation corresponding to the end of the LE reflects global rather than local events that likely occurred between ∼2.1 and 2.05 Ga ago.