Modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a Mesoarchaean marine oxygen oasis in Pongola Supergroup, South Africa

Abstract

There is growing evidence from Mo and Fe stable isotope tracers that shallow-water sediments of the 2.95 Ga Mozaan Group (Pongola Supergroup) in South Africa were deposited under oxidizing conditions, while mineralogical, elemental and S isotopic evidence favors the contemporaneous atmosphere to have remained reducing. In contrast, the first reported Cr-stable isotope data for White Mfolozi riverbed outcrop samples of the Ijzermijn iron formation (IF), Mozaan Group, and drill core samples of the underlying paleosol of the Nsuze Group were interpreted to require oxidative weathering conditions during the time of deposition. More recent Cr-isotope data of at least the Ijzermijn IF samples of the same White Mfolozi riverbed outcrop, however, suggested that modern weathering effects can superimpose oxidative signatures on paleo-redox proxies.This study compares (234U/238U) activity ratios along with stable Mo and Fe isotope data of corresponding drill core and White Mfolozi riverbed outcrop samples in order (i) to test (234U/238U) disequilibria as a potential tool to identify samples affected by modern weathering and (ii) to further investigate weathering effects on outcrop samples, particularly with regard to the stable Mo and Fe isotope systematics. All drill core samples from the Ijzermijn IF have (234U/238U) activity ratios that are within uncertainties of secular equilibrium (i.e. a value of 1), and thus were not significantly affected by modern weathering-related U gain or loss or other sub-recent rock-water interaction. By contrast, the outcrop IF and shale samples from the White Mfolozi riverbed show very strong deviations from the secular equilibrium (234U/238U) activity ratio. Furthermore, these deviations are accompanied by the observations that (i) outcrop samples have on average heavier δ56/54Fe values (−0.96‰) than drill cores (−1.59‰), and (ii) positive correlations in drill core samples between log[Fe/Mn] and δ98/95Mo or δ56/54Fe are disrupted by inclusion of the outcrop samples, as are negative correlations between log[MnO] and δ98/95Mo. However, δ56/54Fe and δ98/95Mo of our outcrop samples do not correlate with the extent of (234U/238U) disequilibria. As such, (234U/238U) activity ratios fail to act as a direct identification tool for modern weathering effects on stable Fe and Mo isotopes in outcrop samples. Overall, this study confirms the adsorption of isotopically light Mo onto MnO2 particles in oxic surface water conditions in the epicontinental Pongola Sea. Reduction of the Mn-oxides together with microbial Fe reduction in the reduced sediment pile of the Pongola basin led to the formation of Mn- and Fe-carbonates during early diagenesis storing the observed stable Fe and Mo isotopic signatures. We thus extend the so-called MnO2-shuttle first proposed for the Neoarchean 2.46 Ga old Koegas IF to late Mesoarchean shallow-marine depositional settings.