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

Subterranean estuaries (STE) of high energy beaches are important coastal reactors that can alter elemental fluxes to the sea. The advective flow of pore water in these STE systems has rapid transport rates due to the high permeability of medium to coarse grained sediments. This flow is controlled by the inland hydraulic gradient, density differences caused by different matrixes (fresh and saline), and oceanic forces (tides and waves). Rare earth elements (REEs) are useful tracers for biogeochemical processes like scavenging, redox changes and provenance. Therefore, a better understanding of their dynamics in different environments is required. In order to investigate REE cycling in the deep subsurface of a sandy beach on spatial and temporal scales, groundwater REE concentrations were analysed along a cross-shore transect down to a depth of 24 m below the sediment surface over the timespan of a year. Together with other trace element, nutrient, and dissolved organic carbon concentrations as well as environmental data (salinity, temperature, pH) from the same stations and depths, the results provide first insight into spatio-temporal variations of biogeochemical processes and changes that lead to the retention or mobilisation of REEs in the deep STE of a high-energy beach.