Hydrographic architecture of the Late Devonian Illinois Basin, North America

The hydrographic architecture of an aqueous basin is strongly influenced by thermal- and salinity-induced stratification that can lead to isolation of a deep anoxic watermass. Salinity is, therefore, a crucial factor shaping Earth’s marine systems, but despite its significance, it is relatively understudied in deep time, particularly in tandem with redox conditions. This study focuses on the Upper Devonian black shale succession of the Illinois Basin, North America (i.e., the New Albany and Chattanooga shales), presenting a novel temporal and spatial reconstruction of the redox, salinity, and productivity conditions that focuses on the hydrodynamics of this epeiric sea. Our findings indicate that the basin was highly heterogeneous in space and time with respect to redox and salinity conditions, with strong redox and salinity gradients existing over relatively short lateral and depth transects, as well as the basin experiencing a fundamental hydrographic reorganization across the Frasnian-Famennian boundary. During the Frasnian, transiently oxygenated conditions existed in the basin center outside of a discrete anoxic pulse associated with the Rhinestreet Event, whereas strong anoxia persisted in shallow-water environments along the Cumberland Sill, with shallow-water anoxia likely driven by enhanced nutrient loading and eutrophication. The basin was highly restricted in the Frasnian, leading to higher-than-marine salinities in the basin center. In the Famennian, however, the basin developed strong vertical salinity stratification driven by enhanced freshwater runoff, which may have resulted from an increase in land plant abundance or a shift in the latitudinal position of the basin toward the Intertropical Convergence Zone. The development of salinity-driven stratification led to a major expansion of euxinia in the Illinois Basin across the Frasnian-Famennian boundary, coincident with the Upper Kellwasser Event. This study highlights that salinity-related processes and evolving basin hydrography can control redox and productivity changes within epicratonic basins, influencing the expression of major biological transitions in Earth’s history.