Redox-sensitive metal concentrations and isotopic composition of dissolved sulfate in a sandy subterranean estuary of the southern North Sea (Spiekeroog Island, May 2014)

The redox-sensitive metals Mn, Fe, U, Re, Mo, and V and sulfur cycling were studied within the subterranean estuary of a sand beach on Spiekeroog Island, Northern Germany. Pore water samples were obtained along a cross-shore transect down to 5 m below the sediment surface. Redox conditions range from oxic, close to the base of the dune, to sub- to anoxic towards the low water line. We could show that biogeochemical processes alter not only the pore water concentrations of the trace metals, but are also reflected by the O isotope- and to a minor degree the S isotope composition of dissolved sulfate. Seawater circulation through sediments of the upper beach (duneward part of the intertidal zone) removes U and V from solution, but serves as a source for Mn, Fe, Re, and Mo to the ocean. Pore water discharging from a berm close to the low water line exhibits lower U, V, and Re concentrations than adjacent seawater. This part of the beach thus serves as a sink for U, V, and Re, but as a source for Mn, Fe, and Mo. No significant Mo depletion is found in the pore water, due to the lack of dissolved sulfide.