Cycling of redox-sensitive elements in a sandy subterranean estuary of the southern North Sea

Abstract

The cycling of metals and sulfur plays an important role in beach sediments. We studied the redox-sensitive metals Mn, Fe, U, Re, Mo, and V, in relation to salinity, organic matter-, and sulfur cycling within the subterranean estuary of a barrier island sand beach on Spiekeroog Island, Northern Germany. Pore water samples were obtained along a cross-shore transect down to 5m 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 demonstrate 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 SO42−. The deviations from conservative mixing between terrestrial and marine endmembers influence trace metal fluxes into the ocean via submarine groundwater discharge. Redox reactions may be accelerated by the presence of dissolved reactive Mn, which we detected in shallow suboxic pore waters. Changes in the O isotope composition of SO42− indicate a cryptic S cycle in the suboxic zone that is fueled by the cycling of Fe and Mn.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. Net flux rates from the subterranean estuary into the ocean are estimated to be 185mmol for Fe, 48mmol for Mn, 8.3μmol for Mo, −24.7nmol for Re, −8.6μmol for U, and −24μmol for V per day and meter of shore line. These fluxes include freshwater flux from the island's freshwater lens and seawater circulation through the beach face. The subterranean estuary therefore serves as V, U, and Re sink, but as a source for Fe, Mn, and Mo to adjacent seawater.A first comparison of net fluxes of dissolved metals from the Spiekeroog beach system with the island's backbarrier tidal flat margins indicates that the tidal flat area might be of greater importance for the supply or removal of metals to/from the North Sea.