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 delta U-238 of most samples is 0.24 +/- 0.16 parts per thousand (2SD; relative to standard CRM145), which is slightly higher than the average delta U-238 of 0.02 +/- 0.12 parts per thousand for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled delta U-238 value of 0.2 parts per thousand for open ocean euxinic sediments in the modern well-oxygenated oceans. Because U-238 is preferentially removed to euxinic sediments compared to U-235, expanded ocean anoxia will deplete seawater of U-238 relative to U-235, ultimately leading to deposition of ORM with low delta U-238. Hence, the high delta U-238 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) > 11 mu M) 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, delta Mo-98 approaches the modern seawater value of 2.34 +/- 0.10 parts per thousand. High delta Mo-98 points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O-2-rich compared to O-2-deficient marine environments. However, the delta Mo-98 value for most Member IV ORM is near 0 parts per thousand(relative to standard NIST SRM 3134 = 0.25 parts per thousand), suggesting extensive anoxia. The low delta Mo-98 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 delta U-238. Hence, we propose that the low delta Mo-98 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 delta Mo-98 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 O-2-deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans. (C) 2015 Elsevier Ltd. All rights reserved.