A TRANSIENT REDOX PERTURBATION IN THE EARLY SILURIAN

The Silurian Rhuddanian Stage directly follows both the Hirnantian glaciation and end-Ordovician mass extinction. Early Llandovery strata globally tend to record anoxic depositional environments with significant bathymetric heterogeneity produced by post-Hirnantian glacial erosion. The middle Rhuddanian of Northern Africa has been of significant economic interest for comprising an atypical “hot shale” hydrocarbon source rock. Here, we present iron, trace metal and carbon geochemistry across the “hot shale” strata of the Murzuq Basin, southern Libya. The E1-NC174 core analyzed spans the lower to upper Rhuddanian, from the Akidograptus ascensus-Parakidograptus acuminatus to Coronograptus cyphus global graptolite biozones (Normalograptus tilokensis to Neodiplograptus fezzanensis biozones in North Africa). Sequential iron geochemistry empirically confirms previous inferences that the Rhuddanian black shales of the Murzuq Basin were deposited under a consistently anoxic water column. Pyrite to highly reactive iron ratios further indicate that bottom waters were persistently euxinic. Published carbon geochemistry demonstrates a ~2‰ positive δ¹³C excursion associated with a >5 weight percent increase in organic carbon burial during the “hot shale” interval. These trends have previously been interpreted as recording glacioeustatic sea level fall in the Murzuq Basin, associated with increased paleoproductivity. Bulk trace metal analyses illustrate enrichments in molybdenum and uranium concentrations across the “hot shale” interval that are not fully explained by concomitant organic carbon enrichments. Redox sensitive element enrichment patterns in euxinic shale suggest an increase in Mo and U seawater concentrations due to global oxygenation or decreased basin restriction during the middle Rhuddanian. This evidence for transient global redox fluctuations in the early Silurian could be further supported by similar trace metal studies of contemporaneous basins.