Localized and Transient Oxygenation of Shallow Oceans of Southwestern Laurentia at the Ediacaran–Cambrian Boundary

The Ediacaran-Cambrian boundary, which precedes one of the most significant biotic diversification events in Earth's history, is associated with a global negative carbon isotope excursion termed the BAsal Cambrian carbon isotope Excursion (BACE). Late Ediacaran and early Cambrian changes in shallow marine oxygenation have been proposed to relate to the BACE as well as metazoan extinction and radiation. However, reconstructing paleoredox conditions at the Ediacaran-Cambrian boundary is limited by challenges in correlating carbonate strata due to sparse stratigraphic markers and non-unique chemostratigraphic correlations. These imprecise correlations have led to uncertainty in how redox changes across the BACE should be interpreted in relation to broader regional and global environmental patterns. Here, we present redox reconstructions from southwestern Laurentian carbonate successions that record the BACE, including the limestone-dominated Deep Spring Formation, southwestern USA, and the dolostone-dominated La Ciénega Formation, northern Mexico. We combine local (carbonate-bound iodine, I/(Ca + Mg) and cerium anomaly, Ce/Ce*) and global (carbonate-associated uranium isotopes, δ²³⁸U_carb) redox proxies to investigate marine oxygenation in relation to the BACE. Contrary to previous suggestions that a global ocean oxygenation event coincided with the BACE, we do not observe a shift in δ²³⁸U_carb concurrent with the carbon isotope excursion in either section. The δ²³⁸U_carb values differ between two sections, likely reflecting distinct diagenetic offsets attributed to different diagenetic U reduction, but together provide a minimal constraint on the carbonate δ²³⁸U value and suggest a more anoxic ocean compared to today. The local proxy results at both sites suggest widespread low-oxygen surface waters with a transient and localized interval of shallow marine oxygenation at one site that coincides with the nadir of the BACE. Persistently low I/(Ca + Mg) ratios, below values observed in today's oxygenated oceans, suggest a broadly redox-stratified surface ocean. Negative Ce anomalies in the La Ciénega Formation were recorded during the BACE nadir, suggesting a short-lived interval of local oxygenation within otherwise low-oxygen conditions. In sum, we do not find evidence for major, widespread oxygenation coincident with the BACE, but a continuation of low-oxygen conditions punctuated by a short-lived oxygenation event in the shallow oceans. These brief fluctuations in oxygen levels, in turn, may have played a role in the onset of behavioral complexity among bilaterian invertebrates during this critical transition.