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 (δ53Cr) 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 δ53Cr 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 pO2 was probably higher than 1% PAL during this period. The lack of Ce anomalies (0.90 < Ce/Ce∗ < 1.02) suggests that during this period the seawater in which the BIFs were deposited was at a redox state under which the oxidation of Fe2+ to Fe3+ could occur but the oxidation of Ce3+ to Ce4+ could not. A compilation of published δ53Cr 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 O2 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.