Transient and stepwise ocean oxygenation during the late Ediacaran Shuram Excursion: Insights from carbonate delta U-238 of northwestern Mexico

The Ediacaran Shuram Excursion (SE) was a key episode in the history of atmospheric and oceanic oxygenation that has also been linked to contemporaneous bioevolutionary events. However, key aspects of the redox state of Ediacaran oceans during the SE remain in doubt. Here, the uranium isotope compositions (delta U-238) of marine carbonates from Sonora, northwestern Mexico were measured to investigate ocean oxygenation during the SE. We found that the SE in Sonora is characterized by a shift toward higher delta U-238 values (from - 0.7 parts per thousand to - 0.3 parts per thousand), consistent with a major oxygenation event that has now been documented in widely separated coeval upper Ediacaran sections including the Doushantuo Formation in South China, the Bol'shoy Patom Formation in Siberia, and the Johnnie Formation in California. In the Sonora Succession, this delta U-238 shift exhibits a similar to 0.8-Myr lag relative to the delta C-13(carb) excursion that defines the SE. To account for these observations, we propose a twostep oceanic oxygenation model for the SE: (1) initial oxidation of an aqueous reductant (e.g., dissolved organic carbon) that resulted only in a shift of delta C-13(carb) (from + 5 parts per thousand to - 3 parts per thousand); and (2) subsequent oxidation of methane or particulate organic matter that caused a continuation of the negative shift in delta C-13(carb) (from - 3 parts per thousand to -12 parts per thousand) as well as a positive excursion of delta U-238 due to progressive oceanic oxygenation. This transient period of enhanced oceanic oxygenation of the late Ediacaran ocean, which may have promoted the evolution of early metazoans, terminated after similar to 5.0 to 6.5 Myr.