Cobalt mobility in high-energy beach sediments – a view on the redox cascade of trace metals combining in-situ observations and flow-through experiments

Intertidal sandy high-energy beach subterranean estuaries (STE) represent complex and dynamic biogeochemical reactors. The aim of our study was to assess trace metal mobility, with a special focus on cobalt (Co), along redox and salinity gradients in the shallow to deep STE of a high-energy beach on Spiekeroog Island (Germany). Co is a micronutrient for phytoplankton growth and characterizing its transformation behavior in such systems contributes to the overall understanding of coastal Co cycling. Groundwater down to 24 m was sampled ∼every 6 weeks between 2022 and 2024 to obtain a high spatial and temporal resolution cross-sectional view on the STE biogeochemistry. Sediments collected from cores were used to determine the sedimentary basis for trace metal cycling. Co concentrations in groundwater revealed a specific mobilization horizon, which was predominantly related to zones of incipient manganese (Mn) reduction. Iron (Fe) reducing conditions resulted in Co fixation in the solid phase, possibly due to accompanying slight net microbial sulfate (SO 4 2− ) reduction and the formation of Co-bearing Fe sulfides or Co sulfide (CoS). Solid phase analyses indicated that the potential for reactive Fe, Mn, and Co mobilization was highest in a former tidal flat sediment layer, which is currently covered by ∼5 m of beach sands and underlain by Pleistocene glacio-fluvial deposits. Co mobilization and fixation behavior observed under in-situ conditions was reproduced in a laboratory flow-through experiment illustrating high release and removal rates of Co along a redox gradient of oxic to Fe and potentially slightly SO 4 2− reducing conditions in comparison to the redox-related reactivity of uranium (U), molybdenum (Mo), and rhenium (Re). With regard to coastal element cycling, high-energy beach STEs may favor Co mobilization due to high electron acceptor supply resulting in less reducing conditions and spatially extended intermediate redox levels. Furthermore, discharge of elements like Fe, Mn, and Co may be enhanced by advective groundwater flow in contrast to diffusion-dominated systems, where the exfiltration is more likely to be limited by re-oxidation processes. • Incipient manganese reduction induced high cobalt release to beach groundwaters. • Slight iron and sulfate reduction limited cobalt mobility. • Cobalt reactivity towards redox changes was high compared to U, Re, and Mo. • Stretched redox gradients below high-energy beaches may facilitate cobalt release.