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The late Cambrian Steptoean positive carbon isotope excursion (SPICE) is a distinct chemostratigraphic feature of the Paleozoic, marked by a 4–5‰ shift in carbonate δ13C that has been recognized across the globe during the Paibian Stage. The SPICE may be related to enhanced burial of organic matter and pyrite during the expansion of marine euxinia, which as a source of O 2 also results in a pulse of atmospheric oxygen. However, geochemical proxies have not clearly illustrated how the ocean redox evolved with atmospheric oxygen changes during the SPICE. This study presents new carbonate I/Ca data, a redox proxy for the upper ocean, from three basins. I/Ca values are low at Great Basin and South China from early into the peak of SPICE, indicating generally anoxic conditions in shallow waters. The overall increasing trend in I/Ca through the peak and recovery phase of the SPICE roughly correlates with the previously modeled rise in atmospheric oxygen. Spatially, the Georgina Basin (Mt. Whelan) might have recorded a relatively more oxic upper ocean compared to the Great Basin and South China. Earth system model simulations also demonstrate the importance of paleogeographic and oceanographic settings on local redox conditions, highlighting the redox heterogeneity during the SPICE. • I/Ca proxy was used for reconstructing redox condition during SPICE. • An oxygenation event in the upper ocean was found during the peak and recovery phase of SPICE. • The ocean redox changes varied in different paleogeographic settings. [ABSTRACT FROM AUTHOR]

• Detailed I/(Ca + Mg) profiles from a shallow-to-deep transect of Doushantuo cap carbonates. • Low I/(Ca + Mg) in calcites with extremely low δ13C. • Spatial I/(Ca + Mg) patterns may support stronger fluid-buffered diagenesis in slopes. • Multiple events may have influenced I/(Ca + Mg) patterns on a temporal scale. Late Neoproterozoic cap carbonates formed following the Cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the Snowball Earth events. Large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post-Marinoan cap carbonates across the globe, but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations. In this study, we present I/(Ca + Mg) profiles of four sections from a shallow-to-deep water transect of the basal Doushantuo cap carbonates, one of the most extensively studied Ediacaran units in South China. Most samples have I/(Ca + Mg) values lower than 1 μmol/mol with a few exceptions in the shelf sections up to ~2.5 μmol/mol. All calcite samples with extremely low δ13C have iodine signal below detection limit, suggesting carbonate precipitation from anoxic seawater or diagenetic fluids. Among the four sections, the slope section has the lowest I/(Ca + Mg) values, suggesting that it may have experienced more extensive fluid-buffered diagenesis and more severe loss of IO 3 –. In combination with available major, trace, rare earth element and isotope data, the temporal I/(Ca + Mg) variations support freshwater mixing and deep-water upwelling in proximal sites and transient oxidation during deposition of the Doushantuo cap carbonates. [ABSTRACT FROM AUTHOR]