Redox-sensitive trace metal mobility in the deep subsurface of a high-energy beach
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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 Co, along redox and salinity gradients in the deep STE of a high-energy beach on Spiekeroog Island (Germany). Groundwater down to 24 m was sampled from permanent wells and by additional direct push sampling to obtain a high resolution cross-sectional view on the deep STE biogeochemistry between 2022 and 2024. Sediments were collected during the well installation in 2022. Co behavior revealed a specific mobilization horizon, which was predominantly related to zones of incipient Mn reduction. Fe reducing conditions resulted in Co removal to the solid phase, possibly due to accompanying slight sulfate reduction and the formation of Co-rich Fe sulfides or CoS. Solid phase analyses indicated that the potential for metal oxide 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 removal behavior observed in the field could be reproduced in a laboratory incubation experiment illustrating high release and removal rates of Co along a redox gradient of oxic to Fe/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 favour Co mobilization due to high electron acceptor supply resulting in less reducing conditions and spatially extended intermediate redox levels. Furthermore, discharge of elements like Co, Mn, and Fe may be enhanced by advective groundwater flow in contrast to diffusion-dominated systems where the exfiltration is more likely to be limited by reoxidation processes.