Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late ediacaran period

To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average δ²³⁸U of most samples is 0.24±0.16‰ (2SD; relative to standard CRM145), which is slightly higher than the average δ²³⁸U of 0.02±0.12‰ for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled δ²³⁸U value of 0.2‰ for open ocean euxinic sediments in the modern well-oxygenated oceans. Because ²³⁸U is preferentially removed to euxinic sediments compared to ²³⁵U, expanded ocean anoxia will deplete seawater of ²³⁸U relative to ²³⁵U, ultimately leading to deposition of ORM with low δ²³⁸U. Hence, the high δ²³⁸U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago.The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H₂S]_aq >11μM) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV, δ⁹⁸Mo approaches the modern seawater value of 2.34±0.10‰. High δ⁹⁸Mo points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O₂-rich compared to O₂-deficient marine environments. However, the δ⁹⁸Mo value for most Member IV ORM is near 0‰ (relative to standard NIST SRM 3134=0.25‰), suggesting extensive anoxia. The low δ⁹⁸Mo is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high δ²³⁸U. Hence, we propose that the low δ⁹⁸Mo of most Member IV ORM was fractionated from contemporaneous seawater. Possible mechanisms driving this isotope fractionation include: (1) inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater-like δ⁹⁸Mo signature in sediments or (2) delivery of isotopically light Mo to sediments via a particulate Fe-Mn oxyhydroxide shuttle.A compilation of Mo isotope data from euxinic ORM suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late Neoproterozoic and early Paleozoic time. Hence, Member IV does not capture irreversible deep ocean oxygenation. Instead, complex ocean redox variations likely marked the transition from O₂-deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans.