This dataset compiles geochemical, mineralogical, and radionuclide measurements obtained from long sediment core PV1 and interface sediment core PV1i collected in the Loire estuary. The data include high-resolution depth profiles of porosity, solid-phase phosphorus concentrations derived from sequential extraction procedures, dissolved porewater phosphate, iron, and sulfate concentrations, as well as raw and corrected sediment depths. Radionuclide activities (210Pb, 137Cs, and 7Be) are reported for surface sediments to document recent sediment deposition and accumulation. Mineralogical data include scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses of vivianite. Spot EDS measurements are provided for both synthesized vivianite and vivianite identified within the PV1 sediment core, including quantitative elemental compositions from multiple analysis points. The dataset also contains corresponding SEM images, elemental maps, and associated text files documenting individual spectra, spatial distribution of elements, and particle morphology. Together, these datasets were generated to document phosphorus distribution, speciation, and burial in estuarine sediments and to constrain phosphorus cycling processes in the Brillantes mudflat in the Loire estuary.