Trace-metal hyper-enrichment in Tremadocian black shales of the Baltic Palaeobasin linked to transgression and ultra-slow sedimentation rates

Determining the local versus global influence on the metallogeny of redox-sensitive trace-metal-enriched black shales remains challenging despite extensive geochemical research in related topics. A set of local triggers or modifiers of syngenetic metal enrichment, still poorly understood, impedes the ability to adequately predict the potential distribution of metal resources. This study of the Lower Ordovician Türisalu Formation examines the stratigraphic trends of Mo and U isotopes in Mo-U-V hyper-enriched thin transgressive black shales and associated beds from the inner shelf of the Baltic Palaeobasin (Aseri PH012B drillcore, NE Estonia) to reveal underlying connections between isotopic fractionation, palaeoenvironmental changes, and metal enrichment processes. Twenty samples from the basal 28-cm-thick Unit I and overlying 66-cm-thick Unit II were analysed by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and interpreted within a recently developed high-resolution chemostratigraphic framework. The tested samples yielded variable δ⁹⁸Mo (from −0.89 to +2.03 ‰) and δ²³⁸U values (from −0.27 to +0.52 ‰). Unit I, which is Mo-enriched, yielded the highest δ²³⁸U and lowest δ⁹⁸Mo values, whereas Unit II, which is U-enriched, is marked by sharply lower δ²³⁸U values. This black shale succession was deposited under a prolonged sea-level transgression. Variations in δ⁹⁸Mo and δ²³⁸U suggest that the mechanism of Mo and U sequestration changed depending on facies during the transgression. Metal enrichment was influenced by a combination of ultra-slow sedimentation rates (0.6–1.0 mm kyr⁻¹), relative sea-level changes, and shallow redox stratification maintained in part by surface heating under a super-greenhouse climate. Extremely slow sedimentation prolonged seawater-sediment interactions, facilitating diffusion-controlled uptake of U and Mo under euxinic conditions. Landward expansion of the subpycnoclinal oxygen-depleted watermass and related redox shifts from suboxic to euxinic conditions near the seafloor drove the selective hyper-enrichment of Mo, and U. Isotopic trends highlight efficient Mo sequestration in euxinic settings likely aided by recycling of FeMn near redoxcline and U hyper-enrichment under Mo-depleted watermasses.