Tracing paleoredox conditions across the Devonian–Carboniferous boundary event: A case study from carbonate-dominated settings of Belgium, the Czech Republic, and northern France

Abstract

The Devonian–Carboniferous boundary was coeval with the Hangenberg Crisis, which is regarded as a first-order mass extinction event related with large sea-level changes and widespread anoxia. This study aims to trace the geochemical paleoproductivity and paleoredox proxies across the Devonian–Carboniferous carbonate-dominated successions of the Namur-Dinant Basin (Belgium, northern France) and the Moravo-Silesian Basin (Czech Republic), located in the Variscan Rhenohercynian Zone. The research was focused on the distribution of oxides (SiO2, Al2O3, Fe2O3, CaO, Na2O, K2O, TiO2, MnO) and trace elements (Th, U, V, Zr, Mo, Zn, Pb, Ni, Cu) measured by ICP-OES and ICP-MS, respectively. The enrichment factors of Mo and U and their ratios were used as the main paleoredox proxies, whereas the enrichment factors of Zn, Cu, Pb, and Ni were employed to trace the paleoproductivity changes. High values of Zr/Al2O3 (>0.001), TiO2/Al2O3 (>0.06), and Fe/Ti (>20) and low values of Al/(Al + Fe + Mn) (<0.35) coinciding with an increase of paleoredox proxies suggest a possible volcanic and hydrothermal source of nutrients, related to Late Devonian and Early Carboniferous extensional magmatism in the Rhenohercynian domain, although a relationship between higher paleoproductivity and enhanced continental runoff of volcanic material cannot be excluded. The studied Devonian–Carboniferous boundary sections from the Namur-Dinant Basin (Gendron-Celles, Ardennes quarry) reveal three levels with similar vectors of Mo and U enrichments. The pre-crisis Famennian pattern is typical for weakly restricted basins with Fe-Mn redox cycling accompanying vertical fluctuations of the oxic/anoxic chemocline close to the sediment/water interface with the influence of particulate shuttle. The MoEF and UEF patterns corresponding to the unrestricted or weakly restricted basin with alternating suboxic to anoxic conditions are considered to represent the transgressive Hangenberg Black Shale Event. The lower Tournaisian MoEF and UEF patterns fall along a vector in the direction of the strongly restricted marine conditions with prevailing suboxic conditions. In the Moravo-Silesian Basin (Lesní lom section), the pre-crisis Famennian MoEF and UEF vector indicates an unrestricted marine trend which converges with that of restricted systems. Redox conditions range from suboxic to euxinic. In the Tournaisian, MoEF and UEF oscillate between oxic and anoxic conditions and fall along a vector of strongly restricted marine conditions. Increased isolation of both studied basins in the Early Tournaisian is regarded as a reflection of a significant eustatic fall during the Hangenberg Crisis, linked to a severe episode of the Gondwana glaciation. These paleohydrographic patterns have correlative potential that should be considered for a revised Devonian-Carboniferous Boundary definition.