Rare earth element dynamics in the deep subsurface of a high-energy beach

Subterranean estuaries of high-energy beaches are considered as dynamic biogeochemical reactors. The advective porewater flow and associated transport of organic and inorganic constituents reach deep into the subsurface. They, however, remain largely understudied particularly with respect to trace element transformations. Rare earth elements exhibit characteristic patterns of enrichment and depletion along the series and thus serve as sensitive tracers of scavenging intensity, redox conditions, and anthropogenic inputs. In this study, we show how conservative mixing and non-conservative redox and mineral formation processes influence rare earth element dynamics in the deep subsurface of the subterranean estuary on a high-energy beach on Spiekeroog Island, southern North Sea, Germany. The study is based on porewaters and sediments sampled to a depth of 24 meters below ground surface along a cross-shore transect of Spiekeroog beach. Additionally, a FTR experiment was conducted in order to observe rare earth element behaviour along the oxic-anoxic gradient under controlled conditions. We show that conservative mixing of seawater and the island’s fresh groundwater is the main driver of rare earth element cycling in the deep subsurface, due to rapid achievement of the maximum adsorption capacity of the sediments in this porous system. Additional non-conservative behaviour is related to iron-oxide reduction, particulate organic carbon remineralisation and potentially authigenic phosphate mineral formation. The FTR experiment demonstrates that the rare earth elements are mainly linked to iron-oxides in this system, and that anthropogenic gadolinium is accumulated in the sediments to be subsequently released under iron reducing conditions. Overall, this study shows that dissolved rare earth element concentrations in the deep subterranean estuary are largely controlled by conservative mixing between the terrestrial freshwater and seawater, and by different biogeochemical processes, particularly iron redox cycling, that partly overlap due to the dynamic conditions in this system.