Trace element and Nd isotope composition of shallow seawater prior to the Great Oxidation Event: Evidence from stromatolitic bioherms in the Paleoproterozoic Rooinekke and Nelani Formations, South Africa

Abstract

Pure, stromatolitic limestones from the ∼2.44 to 2.43 Ga old Rooinekke Formation, Transvaal Supergroup (South Africa), provide unique insight into the geochemistry of Paleoproterozoic shallow seawater and, therefore, into the evolution of ambient redox-levels of the Earth’s atmosphere-hydrosphere system. In contrast to the stromatolitic Klipput dolostones from the overlying Nelani Iron Formation (IF), the stromatolitic Rooinekke limestones do not show post-depositional alteration and are virtually free of detrital contamination. This makes them excellent geochemical archives of proxies of ambient shallow-marine Transvaal seawater. Shale-normalized (SN) rare earth element and yttrium (REY) patterns of pure Rooinekke limestones show seawater-like REY distributions with positive LaSN and GdSN anomalies, super-chondritic Y/Ho ratios and a depletion of the light (LREY) relative to the heavy REYSN (HREY). However, the lack of negative CeSN anomalies implies that redox conditions at the Earth’s surface were not oxidizing enough to stabilize significant amounts of Ce(IV). In contrast to the majority of Archean marine chemical sediments, the pure Early Paleoproterozoic Rooinekke limestones lack positive EuSN anomalies, suggesting that the fraction of REY derived from high-temperature (>250 °C) hydrothermal fluids in shallow Rooinekke seawater was negligible. The predominance of continental sources in the geochemical REY budget of Rooinekke seawater is corroborated by unradiogenic εNd(2.44 Ga) values close to those of contemporaneous upper crustal material. The Rooinekke limestones, therefore, reveal the absence of a high-temperature hydrothermal component from shallow seawater already at least 100 million years prior to the Great Oxidation Event. In contrast, older chemical sediments from the Transvaal Supergroup, such as the Campbellrand carbonates and the Penge and Kuruman IFs, show positive EuSN anomalies and more radiogenic εNd(t) values, indicating a significant contribution of mantle-derived REY via high-temperature hydrothermal fluids to the late Neoarchean shallow marine REY inventory on the Kaapvaal Craton. The declining impact of high-temperature hydrothermal fluids on surface seawater, that is recorded by the late Neoarchean to Paleoproterozoic shallow water Transvaal carbonates, is similar to the general trend of decreasing positive EuSN anomalies with decreasing depositional age, that is observed in Eoarchean to Proterozoic IFs which recorded the composition of deeper marine waters.