EVALUATING THE NATURE OF THE RISE OF ATMOSPHERIC OXYGEN USING LEAD ISOTOPE COMPOSITIONS FROM PALEOPROTEROZOIC SHALE SUITES

A study into using lead isotopes to evaluate the oxidative part of the uranium cycle (Pollack et al., 2006) supported the model proposing that the rise of atmospheric oxygen level above 10^-6 PAL is observed in the Rooihoogte-Timeball Hill Formation, South Africa.  Here, we present results of an expanded whole-rock lead isotope study and rare earth element (REE) concentrations of the same shale samples from the Rooihoogte-Timeball Hill Formation, with additional samples from the 2.5 Ga Mount McRae Shale (Australia), ca. 2.2 Ga Sengoma Argillite (Botswana), and ca. 2.1 Ga Ludikovian Series (Scandinavia).  The ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb array from each of these suites corresponds to a slope age that is slightly younger than the published radiometric age for the units and is inferred to be the time by which the time-integrated thorogenic and uranogenic lead growth was established.  The time-integrated lead growth shows that the organic-rich shales had an average ²³²Th/²³⁸U ratio (κ) of about 2.36 for all of the organic-rich suites.  This is lower than Th/U ratios measured in Archean shale suites or estimated for the Archean-Proterozoic average upper crust (Taylor and McLennan, 1985), which indicates that these samples were enriched to some extent in uranium with respect to thorium and lead at the time of deposition.  In the modern ocean, uranium enrichment of sediments in reducing environments corresponds to uranyl ion reduction from seawater, which is delivered to the ocean by oxidative weathering of continental crust.  Furthermore, strong similarities in the chondrite-normalized REE patterns of each shale suite indicate that variations in κ are not due to mixing of multiple detrital sources.  Our data indicate that the oxidative part of the uranium cycle started by 2.5 Ga and suggests that, by then, sufficient oxygen levels existed in the atmosphere for oxidative weathering of continental crust.