Water circulation, redox, and productivity dynamics shaped late Ediacaran ecosystems: Insights from trace elements and combined Sr-Cr-Cd isotopes in the Corumbá Group, Brazil

The relationship between marine oxygenation and early animal evolution remains a highly debated topic. Recent research suggests that, rather than absolute atmospheric O₂ threshold levels, the main factor controlling early animal distribution was long-term local marine redox conditions. To explore this hypothesis, we present trace element data along with Sr, Cr, and Cd isotopes in carbonate rocks from the late Ediacaran Corumbá Group, Brazil, obtained from drill cores of the GRIND-ECT project. This multiproxy approach is designed to constrain basin water circulation, redox conditions, and bioproductivity levels from ca. 565 to 540 Ma, ultimately aiming to reconstruct the paleoceanographic scenario in which early animal colonization occurred in the Corumbá Basin. Redox-sensitive metal abundances indicate a contrasting redox scenario, with predominant anoxia in the Bocaina Formation and expanded oxia in the overlying Tamengo Formation. This interpretation is further reinforced by Cr isotopes, whereby δ⁵³Cr passes from low values in the range of Bulk Silicate Earth to positively fractionated values upsection. Two negative δ⁵³Cr excursions in the Tamengo Formation are interpreted as anoxic intervals. ⁸⁷Sr/⁸⁶Sr values decrease from 0.7100 in the Bocaina Formation to late Ediacaran values around 0.7085 in the Tamengo Formation. These radiogenic values in the Bocaina Formation are attributed to post-depositional diagenesis of Sr-depleted dolostones. Lastly, variable δ¹¹⁴Cd in the Bocaina Formation indicate contrasting productivity levels during highstand and flooding periods. The two anoxic intervals in the Tamengo Formation exhibit anomalous negative δ¹¹⁴Cd values, which may be linked to widespread eutrophication. Our study, combined with compilations of redox and fossil data, reveals that the Corumbá Basin evolved from a predominantly anoxic setting dominated by microfossils in the Bocaina Formation, to a connected marine setting in the Tamengo Formation. This later phase featured a deeper redoxcline and was dominated by fossils of biomineralizing animals, representing a near-optimal environment for early animal benthic colonization.