Molybdenum isotope evidence for mild environmental oxygenation before the Great Oxidation Event

We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O₂ in the environment ~50Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ~2.5Ga Mt. McRae Shale. The molybdenum isotope compositions (δ^98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ^98/95Mo compared to average upper continental crust. In addition, the upper (S1) and lower (S2) black shale units within the Mt. McRae Shale exhibit systematic differences in average isotopic compositions and distinct patterns of δ^98/95Mo variation. Heavier δ^98/95Mo values occur in the S1 unit, where δ^98/95Mo correlates with Mo enrichments. In the S2 unit, δ^98/95Mo is not as heavy and is relatively invariant.Based on sedimentary Fe proxies we infer that S1 sediments were deposited under euxinic conditions, so that Mo removal from bottom waters was likely quantitative. Thus, δ^98/95Mo in this interval likely records coeval seawater. The lighter δ^98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ^98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ^98/95Mo values in the S2 unit represent a lower limit on the value in contemporaneous seawater.After evaluating a range of hypotheses, we conclude that the isotopically heavy δ^98/95Mo values seen throughout the Mt. McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces on land or in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ^98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.