OCEANIC REDOX CONDITIONS AT CA. 1.93 GA: A CASE STUDY FROM THE PILING GROUP, BAFFIN ISLAND, NUNAVUT (CANADA)

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the atmosphere-ocean system did not experience an irreversible transition to a well-oxygenated state. From ca. 2.22 to 2.05 Ga, during the Lomagundi carbon isotope excursion, oceanic levels of sulfate were high, but subsequently experienced a crash at ca. 2.05 Ga, which is thought to be the result of a decrease in the oxygen content in the atmosphere-ocean system. By ca. 1.88 Ga, major granular iron formations reappeared, signifying a return to a ferruginous oceanic redox state. The development of extensive euxinic conditions along continental shelves has been linked to an increased terrestrial flux of sulfate to the oceans, potentially causing the cessation of the deposition of major granular iron formation by ca. 1.84 Ga. However, the record of oceanic redox state for the period after ca. 2.05 Ga and prior to 1.88 Ga remains poorly resolved and is key to testing this hypothesis. We address this gap by presenting new iron-speciation, major and trace element, sulfur isotope, and organic carbon isotope data for the organic matter-rich shales of the ca. 1.93 Ga Bravo Lake Formation, Piling Group, Baffin Island that were deposited during this interval in Earth’s seawater redox history. The data suggest deposition of the Bravo Lake Formation under an euxinic water column, at least ca. 90 million yr before extensive euxinic conditions are inferred to have appeared in intracratonic basins and zones of upwelling on continental shelves.