Redox heterogeneity of the Ediacaran ocean constrained by chromium isotopes

Abstract

Ocean oxygenation during the Ediacaran Period (635–541 Ma) is thought to be a potential stimulus for the emergence and diversification of early animals. However, the atmospheric and oceanic redox states during this period remain controversial. The chromium (Cr) stable isotopic system is an emerging proxy for constraining the redox states of the ocean and atmosphere over the geological time. Here, we present systematic Cr isotopic (δ53Cr) data for two Ediacaran successions in the Yangtze Block of South China: the Wangji section deposited in shallow water and the Lantian section deposited in deep water. The positively fractionated δ53Cr values at the Wangji section (up to 1.15‰ for carbonates and up to 1.11‰ for shales) imply that atmospheric oxygen had risen to a relatively high level to allow the oxidative weathering of Cr(III) during the Ediacaran. Additionally, a peak of δ53Cr values is closely correlated to the δ13C positive excursion in the upper Doushantuo Formation, likely because of the enhanced biological productivity at that time. In contrast, positively fractionated δ53Cr values are suppressed (from −0.11‰ to 0.05‰) in the shales at the Lantian section, indicating that the contemporary deep water might have still been extensively anoxic, such that the terrestrial Cr(VI) with positive δ53Cr values would have been quantitatively reduced and deposited before being transferred to the deeper settings. A comparison of these two sections of different depositional environments and their δ53Cr profiles suggests that the redox conditions of the Ediacaran ocean were highly stratified and heterogeneous.