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Abstract:

Neoproterozoic iron formations record an unusual and apparently final recurrence of this sediment type after a hiatus of more than one billion years. Despite the unusual environmental conditions that led to their formation, specifically their association with glaciogenic deposits, Neoproterozoic iron formations have strongly influenced models for the Precambrian Earth’s surficial evolution and iron formation in general. A suite of high-quality trace element data for 42 samples from the Rapitan iron formation in northwestern Canada were used to reconstruct the configuration and redox evolution of the Rapitan Basin. Complete rare-earth element and yttrium (REE+Y) patterns demonstrate that the Rapitan Basin was hydrologically connected to the open ocean, but that local catchments of an evolved, possibly granitic composition supplied dissolved REE+Y, suggesting partial basin restriction. Molybdenum and U systematics are consistent and indicate a partly restricted, or “silled” basin. In contrast to modern analogues for such basins, such as the Cariaco Basin, the stratigraphic association with glaciogenic clastic rocks requires ice cover to be considered in basin reconstruction. Accordingly, the Rapitan iron formation was deposited through a complex interplay of processes: during intervals of ice cover, glacially sourced iron oxyhydroxides were bacterially reduced to dissolved ferrous iron, which was subsequently oxidized to ferric iron following ice withdrawal. During this time, extreme primary productivity in the shallow water column drove eutrophication at middle water depths and the production of a three-tiered stratified water column with ferruginous deep water, a thin euxinic wedge at middle depths, and oxic surface water. Regardless of the basinal redox conditions and depositional constraints, the absence of a positive Eu anomaly in the Rapitan iron formation suggests that the global ocean was fully oxygenated by the Neoproterozoic.