Chemostratigraphy of the Shaler Supergroup, Victoria Island, NW Canada: A record of ocean composition prior to the Cryogenian glaciations

•The Bitter Springs negative carbon isotope excursion is reproducible across the Minto Inlier.•The excursion is attributed to local overprinting from authigenic carbonate cement.•We document basin euxinia and anoxia during the same time interval.•Mo, V, and U concentrations suggest this was a transitional period in Earth's redox evolution.

A new δ13Ccarb curve combined with δ13Corg values is presented for the upper Shaler Supergroup (∼900 to ∼720 Ma), Amundsen Basin, northwestern Canada. The dataset fills gaps in the existing stratigraphic record and makes correlations with adjacent basins more robust. There is a pronounced negative δ13C excursion in the Wynniatt Formation that can be correlated with a putative worldwide negative carbon isotope excursion, namely the Bitter Springs stage. However, in the Amundsen Basin, the δ13Ccarb excursion drops to anomalously negative values (−14‰), which we attribute to local overprints wherein isotopically light carbon in pore waters, released by oxidation of methane and organic matter during sulphate and iron reduction, was incorporated into authigenic carbonate cement. We document basin euxinia and anoxia during the same time interval using a multi-proxy approach; specifically, Fe-speciation and redox-sensitive trace metal data. Patterns of pronounced enrichment in Mo, V, and U concentrations in euxinic black shales suggest that the Bitter Springs stage was a transitional period in Earth's redox evolution, from the more reduced global oceans during the mid-Proterozoic to the more oxygenated oceans during the Phanerozoic.