Establishment of euxinic oceanic conditions following the Lomagundi Event

Following the great oxidation event (GOE), the isotope record of marine carbonate rocks suggests a massive burial of organic carbon in an event known as Lomagundi excursion (LE). It is estimated that during this period atmospheric oxygen attained levels much higher than during GOE [1]. It is proposed that as the LE declined, oxygen dropped to lower levels [1, 2], but the direct geochemical evidence for the state of atmospheric and ocean oxygenation both during and after the LE is missing. This time window is also represented by the 2.2-2.0 Ga Francevillian group of Gabon, well known for the earliest large colonial organisms [3]. In order to investigate the nature of marine water-column chemistry, samples representing the entire section were subjected to multielement (C, S, Fe, Mo, U) biogeochemical study. Geochemical data show deep water oxic conditions during deposition of the lower part of the section in agreement with the LE. Nevertheless the interlayered Mn deposits occured in ferruginous anoxic conditions reflecting sea-level changes. The upper part of the section reflect euxinic conditions, and together with #98Mo values, these data confirm a significant decrease in the oxygenation of ocean water in the aftermath of the Lomagundi Event.