Coupled vanadium and thallium isotope constraints on Mesoproterozoic ocean oxygenation around 1.38-1.39 Ga.

The oxygenation state of the Mid-Proterozoic (1.8 – 0.8 Ga) ocean and atmosphere is heavily debated and has implications for the relationship between environmental O 2 and the emergence of complex life. While recent geochemical proxy studies of Mesoproterozoic marine sedimentary rocks inform a picture of deep water redox dominated by widespread ferruginous (iron-rich and anoxic) conditions, a growing number of datasets indicate that short-lived ocean oxygenation events were recurrent features of this time interval. The spatial extent of previously reported Mesoproterozoic oxygen pulses is often unclear as they are predominantly observed via localized environmental proxies. In this study, we use the vanadium (V) and thallium (Tl) isotope paleoredox proxies to provide a global marine redox perspective during a short interval of the Mesoproterozoic. We reconstructed seawater V (δ 51 V SW) and Tl (ε 205 Tl SW) isotopic compositions from shales from Unit 2 of the 1.38-1.39 Ga Xiamaling Formation (North China craton) that show an up-section shift in average δ 51 V SW values from 0.02‰ to 0.15‰ and a short-lived perturbation in ε 205 Tl SW from −2.5 to −4.2. Mass-balance models for both isotopic systems are consistent with these isotopic shifts representing an expansion of oxic water column and porewater conditions from a more anoxic (ferruginous) baseline ocean state. The short-lived nature of the ε 205 Tl SW perturbation versus the shift in δ 51 V SW suggests that environments sufficiently oxygenated to support widespread Mn oxide burial were only established for a fraction of this overall oxic expansion. The ocean residence time of V and Tl requires that this oxygenation event was global in extent and may reflect either a purely oceanic phenomenon such as global deepening of the oxycline, or alternatively, global ocean-atmosphere equilibration during a short-lived episode of elevated atmospheric oxygenation. • We report V and Tl isotopic systematics for Unit 2 of the Mesoproterozoic Xiamaling formation. • An up-section shift to higher δ 51 V and lower ε 205 Tl values is observed. • These shifts are interpreted as recording a change in global marine V and Tl mass balance. • Mass balance calculations suggest an expansion of oxic V and Tl sinks in the upper section. • We infer an episode of expanded global oxic V and Tl sinks in an otherwise anoxic regime. [ABSTRACT FROM AUTHOR]