The onset of oxidative weathering traced by uranium isotopes

Abstract

The invention of photosynthesis was a key interval in Earth’s history, initiating major changes in the evolution of the oceans and atmosphere. Many studies suggest that oxygen levels were already enhanced before the Paleoproterozoic Great Oxidation Event (GOE, 2.45–2.32 Ga). However, the timing of the onset of photosynthetic oxygenation, as well as spatio-temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated. Here, we present a new and substantial uranium (U) isotope dataset comprising marine sediments, e.g. black shales, carbonates and iron-rich sedimentary rocks, from the Hamersley Basin (Western Australia), Kaapvaal and Zimbabwe Cratons (both southern Africa) with depositional ages ranging from 3.4 to 2.2 Ga. Sub-recent U mobilization was monitored by analyzing 234U along with δ238U, and detrital U contribution was monitored with [Al]/[U] or [Th]/[U] ratios. Samples with significant authigenic U enrichment (EFU > 2), relative to post-Archean Australian Shale (PAAS), exhibit authigenic δ238U ranging from −0.96 to −0.02‰ (±0.05‰, 2 s.d. long-term reproducibility), which deviates markedly from typical δ238U of the continental crust (−0.40 to −0.20‰). Remarkably, we find increased δ238U variability in the upper Ghaap Group (2.50–2.47 Ga, carbonates and black shales) and the lower Pretoria Group (2.42–2.32 Ga, Duitschland Formation, shales) of the Kaapvaal Craton. The predominantly light U isotope composition of those samples is likely explained by U isotope fractionation related to the onset of partial U mobilization during slow oxidative weathering of uraninite shortly before and during the GOE. The lack of low δ238U values in the Timeball Hill Formation (overlying the Duitschland Formation) indicate the onset of quantitative U mobilization during (modern-style) oxidative weathering of uraninite and other U-bearing minerals with limited U isotope fractionation. These limited U isotope variations measured in the Paleoproterozoic Timeball Hill Formation further suggest that, in contrast to modern oceans, in the period following the GOE, U was largely quantitatively reduced and removed from the water column of the Transvaal basin which remained mainly anoxic at depth.