『Abstract
　The redox-sensitive geochemical behavior of uranium permits the use of Th/U ratios as a geochemical proxy for the oxidation state of the atmosphere during deposition. Due to the effects of post-depositional uranium mobility on Th/U ratios during events involving oxygenated fluids, direct measurements of Th/U ratios are often misleading even for drill core samples. Because both of these elements radioactively decay and produce lead isotopes, the Pb isotope composition may reflect the depositional Th/U ratio, although the Th/U ratios induced by changes shortly after deposition may not be distinguished from the true depositional Th/U ratios. In order to effectively evaluate the time-integrated Th/U ratio (κa), values for the initial depositional Pb isotope composition must be determined or accepted from the models for the whole Earth.
　While the timing for the rise of atmospheric oxygen is reasonably well constrained now, its effect on continental weathering and ocean redox state remains poorly constrained and debated. The ca. 2.15 Ga Sengoma Argillite Formation of Botswana contains organic-rich shales deposited during the Great Oxidation Event. The slope of the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb array of shales from the Sengoma Argillite Formation corresponds to a Pb-Pb age that is within analytical error of the depositional age and is, therefore, inferred to be the time by which the time-integrated thorogenic and uranogenic lead growth started. The time-integrated lead growth corresponds to an average κa of 2.63 (±0.62, 1σ) for the organic-rich shales of the Sengoma Argillite Formation . This is lower than Th/U ratios measured in Archean shale suites or estimated for the Archean-Proterozoic average upper continental crust [Taylor,S.R. and McLennan,S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312pp.], which indicates that these samples were enriched in uranium with respect to thorium (and perhaps lead) at the time of deposition. In the modern ocean, sediments are enriched in uranium under reducing conditions by reduction of the water-soluble uranyl ion, which is delivered to the ocean by oxidative weathering of continental crust. To evaluate the potential mobility of Th, U, and Pb during post-depositional processes, the concentrations of the rare earth elements (REE) were also determined. Interelement ratios of the largely immobile REE (in this study, La/Nd and Gd/Er) can be used as a proxy for the Th/U ratio, as the geochemical behavior of the lanthanide and actinide elements is similar under a variety of conditions. Furthermore, close similarity in the chondrite-normalized REE patterns and small range in La/Nd and Gd/Er ratios in studied samples indicate that variations in κa values are not likely to have been controlled by mixing of one or more REE-, Th-, and U-rich heavy minerals from the multiple detrital sources. Our study of shales from the ca. 2.15 Ga Sengoma Argillite Formation indicates that decoupling of U from Th, most likely related to the oxidative continental weathering, began by 2.15 Ga, at the latest.

Keywords: Pb isotopes; Atmospheric oxygen; Great Oxidation Event; Oxidative continental weathering; Rare earth elements; U-Th decoupling』

1. Introduction
2. Geochemical background
　2.1. U-Th-Pb geochemistry
　2.2. Rare earth element geochemistry
　2.3. Th-U-REE compatibility
3. Application of Pb isotopes and REE systematics to resolve between redox decoupling of U from Th and multiple provenance sources
　3.1. Pb isotope ratio interpretations
　3.2. REE pattern interpretation
4. Regional geology and stratigraphy of the Sengoma Argillite Formation
5. Analytical methods
　5.1. Whole-rock Pb isotope analysis
　5.2. Sequential acid Pb leaching
　5.3. Whole-rock REE analysis
6. Results
　6.1. Whole-rock Pb isotope ratios
　6.2. Leach step Pb isotope compositions
　6.3. REE data
7. Discussion
　7.1. Implications from U-Th-REE data
　7.2. Comparison to Archean and Paleoproterozoic record
8. Conclusions
Acknowledgements
References