WIDESPREAD AND PERSISTENT EUXINIA IN FORTUNIAN (EARLY CAMBRIAN) OCEANS: URANIUM ISOTOPES AND CERIUM ANOMALIES IN CARBONATES FROM THE IGARKA UPLIFT, SIBERIA

The Ediacaran-Cambrian transition arguably marks the most fundamental geobiological transformation in Earth history, yet it occurred against the backdrop of low and unstable O₂ levels in the global oceans. Uranium isotope evidence from Namibia, South China, and Siberia all indicate widespread euxinia in the global oceans during the terminal Ediacaran Period, and uranium isotopes also indicate expansive ocean anoxia during Cambrian Stage 2. Relatively limited constraints on ocean redox exist for the intervening Fortunian Stage, however. Here, we present new carbon, oxygen, strontium, and uranium isotope, as well as cerium anomaly data from Fortunian marine carbonates of the Sukharikha River succession, Igarka Uplift, Siberia. The Sukharikha River section begins with a negative δ¹³C excursion across the Ediacaran-Cambrian boundary (the BACE event), after which a broadly climbing δ¹³C trend is exhibited with six discrete and progressively rising δ¹³C maxima interspersed with five short-lived δ¹³C minima. The well-preserved nature of the succession is further supported by a remarkably coherent decreasing trend in ⁸⁷Sr/⁸⁶Sr values from ~0.7090 to 0.7082, matching global trends. δ²³⁸U values begin near –0.7‰ at the base of the succession and remain low (mostly between –0.7 and –0.5‰) over >600 m of section, indicating widespread removal of isotopically heavy ²³⁸U from the global oceans—a process that occurs most efficiently in sediments underlying a euxinic water column. Furthermore, Ce/Ce* values consistently >1 indicate persistent local anoxia in the Igarka Uplift at the time of deposition. The exception to these trends is a brief positive δ²³⁸U excursion associated with the BACE event, where δ²³⁸U values reach –0.17‰, similar to carbonates deposited in the well-oxygenated modern ocean. Thus, the BACE event, which is associated with major sulfate evaporite deposits in Siberia and elsewhere, could have been associated with a brief ocean oxygenation event. Overall, these data suggest that important geobiological events of the terminal Ediacaran, Fortunian, and Cambrian Stage 2 all occurred against the backdrop of widespread ocean euxinia, minimizing the potential role of sustained ocean oxygenation in early animal evolution.