CONSTRAINING THE MOLYBDENUM ISOTOPE BUDGET OF 2.63 – 2.50 GA OCEANS

Molybdenum (Mo) concentrations and isotopic compositions from ancient organic-rich marine sedimentary rocks can track redox changes at Earth’s surface over geologic time. If deposition of organic-rich shales took place under locally anoxic and sulfidic (euxinic) conditions with [H₂S]_aq > 11 μM, then information about the availability of Mo in overlying waters and the global seawater Mo isotope signature can be derived. If local deposition was characterized by non-euxinic conditions, however, the Mo concentrations of shales can erroneously imply a low seawater Mo inventory and yield lighter Mo isotope compositions than global seawater. Almost none of the previous Mo isotope studies on sedimentary rocks deposited before the ~2.5 Ga Mt. McRae Shale has complementary Fe speciation data, meaning it is difficult to constrain local bottom water redox conditions. Hence, it is challenging to interpret previous Mo isotope data.

To address this gap, we measured Mo abundances and isotopic compositions for multiple marine shales deposited between 2.63 and 2.50 Ga under both non-euxinic and euxinic conditions, as delineated by correlative iron speciation analyses. The oldest euxinic samples, from the 2.63 Ga Jeerinah Formation (Fm), have δ⁹⁸Mo as heavy as 1.2‰ (relative to NIST SRM 3134 + 0.25‰) after correction for detrital content. Non-euxinic samples from the 2.60 Ga Wittenoom Fm and 2.58 Ga Nauga Fm yield lower values, likely due to non-euxinic local depositional conditions, and thus do not directly constrain seawater Mo isotope compositions. The youngest euxinic samples, from the 2.50 Ga Klein Naute Fm, have heavier Mo isotope compositions up to 1.5‰, similar to values reported for the roughly coeval Mt. McRae Shale.

Our data from the euxinic Jeerinah Fm provide evidence for a low seawater Mo reservoir and isotope composition, suggesting low environmental O₂ levels at 2.63 Ga. In contrast, higher Mo abundances and heavy isotope values in the 2.5 Ga euxinic shales from the Klein Naute Fm and Mt. McRae Shale are most likely a response to a higher seawater Mo reservoir and heavier global Mo isotope signature. Consistent with previous arguments, a “whiff” of atmospheric O₂ at this time most likely enhanced delivery of Mo to the ocean and increased the area of oxic seafloor where isotopically light Mo is preferentially buried.