Centennial to millennial-scale paleoenvironmental record from a coastal iron-manganese concretion

Abstract Coastal sea ecosystems are increasingly threatened by global change and human activities. Our understanding of these changes is limited, especially in dynamic coastal regions lacking thick sediment sequences. Iron-manganese concretions, biogeochemical precipitates on the seafloor, grow in non-depositional coastal areas and record numerous marine processes and environmental variability. Establishing reliable geochronology for these precipitates has been challenging. We combine anthropogenic lead accumulation, cobalt chronometry, and radiocarbon dating to develop a high-resolution Bayesian age model for a rapidly growing iron-manganese concretion from the Baltic Sea. The concretion core formed ca. 11,000 cal. BP, with overgrown material showing successively younger ages from 7500 years ago to recent decades. Analyses of microstructure, magnetic microscopy, and bulk mineral magnetic properties, trace elements, and iron isotopic composition reveal that the concretion records environmental variability over the past 7000 years. Our study may serve as a benchmark for paleoenvironmental records from coastal iron-manganese concretions.