Carbonate shutdown, phosphogenesis and the variable style of marine anoxia in the late Famennian (Late Devonian) in western Laurentia

The shutdown of carbonate productivity and its replacement by black shale deposition is often observed in the geological record yet factors driving such a change are debated. latest Famennian western Laurentia (present day North America) provides good example limestone – transition that roughly contemporaneous with global Hangenberg Crisis series environmental biotic changes associated widespread development deposition. However, ceased began prior to this crisis. Examining type section Exshaw Formation at Jura Creek (Alberta) reveals declining seafloor oxygen levels were likely responsible for loss productivity, followed phosphogenesis youngest carbonates. top surface hard ground was reworked, possibly internal wave action, producing lag phosphatic clasts pyrite grains including giant framboids (reaching 100 ?m diameter) polyframboids. subsequent redox history lower shows substantial variations. Initially, occurrence small framboids, elevated FeHR/FeT Fepy/FeHR combined enhanced U, Mo Re concentrations indicates intense euxinia water column. These conditions replaced ferruginous anoxic recorded radiolarian-rich shales have moderate trace metals, low content, as confirmed both ratios scanning electron microscope (SEM) observation. Mo-U covariation trends suggest enrichment under promoted drawdown association Fe minerals precipitated return better ventilated around Devonian/Carboniferous boundary marked decrease metal content syngenetic pyrite. thus an improvement ventilation, from euxinic our study, evidence other basins America suggests even greater increase oxygenation time.