Work

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

Year: 2025

Type: article

Abstract: 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 we evaluate the potential for Co release from such systems to the coastal ocean. 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 deep STE biogeochemistry. Deep sediments collected from cores were used to determine the sedimentary basis for trace metal cycling. Co concentrations 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 reduction and the formation of Co-bearing Fe sulfides or 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 meters 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 SO4 reducing conditions in comparison to the redox-related reactivity of U, Mo, and Re. With regard to coastal element budgets, 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.

Source: Marine Chemistry

DOI: 10.1016/j.marchem.2025.104583