Devonian Upper Ocean Redox Trends Across Laurussia: Testing Potential Influence of Marine Carbonate Facies on Bulk Rock I/Ca Signal

Based on geochemical proxy and modeling data, atmospheric oxygen levels are interpreted to have risen to near-modern levels during the Devonian Period, while the exact timing is uncertain. Frequent deposition of black shales and anoxic events are also observed in the Middle and Late Devonian. Previous Devonian seawater redox studies focus on short time intervals associated with abrupt biotic crises and rapid 13C excursions rather than evaluating long-term background Devonian redox conditions. In addition, many geochemical proxy redox studies do not take into account observed lithologic variations to address potential influences of rock types/stratigraphy on measured redox signals. This study presents new lithology-specific I/Ca data from well-dated Lower Devonian through Upper Devonian limestone-dominated sections from Nevada (western Laurussia) and the Illinois Basin (eastern Laurussia). Our results indicate that lithologic changes did not exert control on bulk carbonate I/Ca trends at either location, while diagenesis might have overprinted some trends. The Nevada I/Ca signal is more likely to reflect global upper ocean redox conditions based on its proximity to the Panthalassa Ocean and its temporal covariation with global environmental variability trends that were based on increased frequency of 13C variations, biotic crises, sea-level cycles, and conodont biozone durations (Brett et al., 2020). The Nevada I/Ca trends suggest local upper oceans were relatively more reducing in the early Early Devonian (Lochkovian), more oxic in the late Early Devonian (Emsian), and more reducing in the early Middle Devonian (Eifelian) to increasingly more oxic in the late Middle Devonian and early Late Devonian (Givetian-Frasnian).