Daniel Stubbs

The concentration of dissolved oxygen in the deep oceans has varied over Earth History, but the timing of the transition from anoxic to oxic deep oceans is debated. Under modern-day, oxic, deep ocean conditions, alteration of the upper sections of mafic oceanic crust with U-rich seawater leads to U enrichment, low Th/U ratios, and heterogeneous 238U/235U ratios relative to fresh mid-ocean ridge basalt (MORB). Given the solubility behaviour of U, its uptake into altered oceanic crust (AOC) is expected to be smaller and less isotopically fractionated when deep oceans were anoxic and thus U-poor. Determining when, in the geological record, the U elemental and isotopic systematics of ancient oceanic crust first resemble modern day AOC should indicate when deep oceans became oxic. We provide U concentration, Th/U, and U isotopic data on upper-crustal sections of three ophiolites from 750 to 480 Ma, spanning the period inferred for deep ocean oxygenation (~ 850 to 400 Ma). The ophiolites at 480 and 540 Ma have high U contents, low Th/U ratios, and variability in 238U/235U ratios like modern-day AOC, reflecting seawater alteration of oceanic crust under oxygenated seawater conditions. In contrast, the 750 Ma ophiolite does not show the distinctive decreasing Th/U with increasing U concentrations trend of modern AOC and has fewer samples with 238U/235U ratios perturbed from mantle values, reflecting alteration under largely anoxic deep ocean conditions. This is also supported by Fe3+/FeT ratios in these samples that are like unaltered modern MORB. Thus, our data suggest oxygenated deep oceans at some time between 750 and 540 Ma, either reflecting a full transition or intermittent deep ocean oxygenation events within an otherwise anoxic deep ocean.