AN OCEAN VENTILATION EVENT RECORDED BY URANIUM ISOTOPES IN THE TONIAN SHORIKHA FORMATION (BITTER SPRINGS-EQUIVALENT), SIBERIA

The Neoproterozoic Era is characterized by large swings in the δ¹³C composition of marine carbonates, often associated with extreme changes in global climate, ocean redox, and biological innovation. Uranium isotopes (δ²³⁸U) provide a promising tool to link the δ¹³C record to changes in ocean oxygenation because of the sensitivity of the δ²³⁸U composition of seawater (which can be recorded in marine carbonates) to the global extent of anoxic and/or euxinic marine environments. Uranium isotopes have previously provided evidence for ocean ventilation coincident with the Ediacaran Shuram negative δ¹³C excursion, with associated increases in complexity of the marine biosphere. Little is known, however, about the redox context of earlier Neoproterozoic δ¹³C excursions, which also may have had profound biological consequences. Here, we present new δ²³⁸U data from the late Mesoproterozoic to early Neoproterozoic Turukhansk Uplift succession in Siberia, with particular focus on the Tonian-aged Shorikha, Miroyedikha, and Turukhansk formations. Newly measured sections and δ¹³C profiles reveal a major negative δ¹³C excursion from ~0‰ down to −5‰ in the Shorikha Formation, which is interpreted as equivalent to the Tonian Bitter Springs anomaly previously identified in Australia, Canada, Svalbard, and Ethiopia. This is followed by a recovery to δ¹³C values between 0 and +4‰ in the overlying Miroyedikha and Turukhansk formations. Our δ²³⁸U profile shows evidence for strongly anoxic oceans during deposition of the lower Shorikha and underlying formations (median δ²³⁸U value = −0.74‰), analogous to low δ²³⁸U values recently reported in early Tonian carbonates from North China. This is followed, however, by a shift to near-modern carbonate δ²³⁸U values in the upper Shorikha and lower Miroyedikha formations, coincident with the Bitter Springs-equivalent negative δ¹³C excursion. This can be interpreted as a major Tonian ocean ventilation event associated with the first significant Neoproterozoic carbon cycle perturbation. Ocean ventilation associated with the Bitter Springs anomaly adds to a growing body of evidence that the Neoproterozoic oceans were characterized by pulsed oxygenation events against a backdrop of widespread anoxia, and that this redox pattern is a primary cause of observed δ¹³C variability.