264-9 Sustained ocean oxygenation in the early Tonian Period? New carbonate uranium isotope records from Siberia and Australia

Abstract:

Although much debate has centered on the relationship between ocean oxygenation and the evolution of complex life during the Cryogenian and Ediacaran periods, comparatively few constraints currently exist on ocean redox conditions during the preceding Tonian Period (1000 to 720 Ma). To fill this gap, here we present new carbonate uranium isotope (δ²³⁸U) data from two sections that span negative δ¹³C excursions in the early Tonian. The Upper Turukhansk Uplift succession from Siberia contains a discrete negative δ¹³C excursion in the upper Shorikha Formation, whose age is constrained by a newly published volcanic zircon U-Pb age of 963 ± 6 Ma at the base of the formation, ~500 m conformably below the δ¹³C excursion. Thus, the negative δ¹³C excursion in the upper Shorikha Formation is tentatively correlated to the ~940 Ma Majiatun anomaly identified in North China. In our section from Siberia, carbonate δ²³⁸U values are consistently very low prior to the δ¹³C excursion (approximately –0.8‰), but rise sharply to higher values (~–0.2‰) coincident with the negative δ¹³C shift, and remain elevated for the remaining ~300 m of section. We preliminarily interpret this positive δ²³⁸U shift as a global ocean oxygenation event, although effects of carbonate saturation state and ferruginous ocean conditions on the δ²³⁸U signal cannot be ruled out. We also present stable strontium isotope (δ⁸⁸Sr) data from this Siberian section, which show a lack of change across the δ¹³C excursion, potentially arguing against carbonate saturation control on U isotopes, thus supporting an oxygenation event. We also present δ²³⁸U data from the ~880 Ma BRISA anomaly (formerly the ‘Bitter Springs’ anomaly) in the Amadeus Basin, Northern Territory, Australia. Here, δ²³⁸U values are uniformly high (~–0.4‰ to –0.1‰) in the pre-excursion Gillen Formation and through the negative δ¹³C excursion in the Loves Creek Formation, suggesting a potential continuation of well-oxygenated oceans through the early Tonian Period. Thus, our preliminary δ²³⁸U data suggest that the early Tonian Period may represent an overlooked early Neoproterozoic interval of well-oxygenated oceans, which is consistent with the widespread presence of sulfate evaporites in Tonian successions. In addition, carbonate δ²³⁸U data suggest that the early Tonian oceans may have been better-oxygenated than the later Ediacaran oceans that witnessed the diversification of complex, macroscopic organisms.           

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-22161

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