Authigenic phosphate nodules (Late Cretaceous, northern Ireland) as condensed succession microarchives

This study uses a range of proxy sedimentological data in order to elucidate what information might be interpreted from the phosphate nodules at a condensed, transgressive horizon at the base of the Cretaceous succession (Coniacian–Santonian) in Northern Ireland. It comprises phosphate pellets and nodules, quartz, glauconite and occasional metamorphic and igneous clasts. The phosphate pellets and nodules contain an admixture of carbonate fluorapatite, calcite, glauconite, pyrite and quartz. The formation of glaucony abstracted Mg from the seafloor environment, aiding the precipitation of phosphate as nodules. Some nodules exhibit concentric lamination with borings both on the outer surface and within, where they are covered by successive layers of phosphate. Such truncated borings suggest reworking with subsequent re-phosphatisation on hiatus concretions. Concentrically-laminated hiatus concretions are enriched in heavy metals and rare earth elements. The phosphates developed during a period of transgression and onlap that promoted sediment starvation in a succession of limestones and greensands that were already deficient in clastic supply. Simple (internally homogenous) P2O5-enriched phosphate nodules probably replaced calcite nodule precursors in suboxic conditions. The overall scenario is of oceanic upwelling conditions, exhumation and burial coupled with alternating oxic to suboxic conditions.