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