Redox condition in the Nanhua Basin during the waning of the Sturtian glaciation: A chromium-isotope perspective

Abstract The Neoproterozoic oxygenation event (NOE) is often considered to be synchronously and possibly causally associated with the Cryogenian glaciations. Reconstructing early Cryogenian redox conditions is therefore essential for understanding the onset of the NOE. We analyzed Cr-isotope compositions (δ 53 Cr) as well as major and rare earth element (REE) concentrations of the banded iron formations (BIFs) in the basal Fulu Formation of the Xiajiajiang section (Nanhua Basin), which are thought to have been deposited during the waning stage of the Sturtian glaciation. Strongly positively fractionated authigenic δ 53 Cr values were obtained after detrital correction, ranging from 0.64 to 1.22‰ and averaging at 0.91 ± 0.40‰ (2σ, n = 10) or after detrital evaluation, ∼0.71‰. This suggests extensive oxidative Cr cycling occurred at the Earth’s surface and implies that the p O 2 was probably higher than 1% PAL during this period. The lack of Ce anomalies (0.90  ∗ 2+ to Fe 3+ could occur but the oxidation of Ce 3+ to Ce 4+ could not. A compilation of published δ 53 Cr data from BIFs, ironstones, and shales deposited from the late Paleoproterozoic to early Cryogenian (1.8–0.7 Ga) suggests that the early increase in atmospheric O 2 related to the NOE probably first took place during the late Tonian Period (ca. 800–720 Ma). This was temporally coincident with the first appearance of several eukaryotic groups, and the estimated initial divergence of early animals.