Uranium isotopes (δ238U values) are an emerging paleoredox proxy that can help to better understand the redox evolution of Earth’s surface environment. Recently, uranium isotopes from marine sedimentary rocks have been used to reconstruct ocean and atmospheric redox conditions. However, the δ238U values of U inputs to the ocean over time have not sufficiently been determined so it is unclear whether δ238U values from marine sediments track marine processes (i.e. biotic U(VI) reduction) or varying δ238U values of ocean inputs over time and place. In this study, we present the U isotope record of a temporally and geographically expansive suite of paleosols and modern soils to constrain the δ238U values of inputs to the ocean in both modern and ancient systems as well as track the redox conditions of Earth’s early atmosphere.
U isotope measurements of both modern soils and paleosols are predominantly unfractionated (relative to igneous rocks), which is evidence that the U isotope values of inputs to the ocean have remained constant and crustal (unfractionated compared to igneous rocks) over Earth’s history. This determination reaffirms recent U isotope work from marine sedimentary rocks which suggests U isotope variations in marine rocks track marine processes and not just varying U inputs to the oceans. Furthermore, the few fractionated paleosol samples suggest an intermittently oxygenated atmosphere leading up to 2.4 Ga and the onset of biological oxygen production by at least 3.0 Ga, which is consistent with recent geochemical studies (Crowe et al., 2013; Wille et al., 2013; Planavsky et al., 2014; Satkoski et al., 2015; Wang et al., 2016).
De Corte, Brian P., "Uranium isotope ratios in modern and Precambrian soils" (2016). Lawrence University Honors Projects. Paper 99.