Geochemical behavior of trace elements in sub-tidal marine sediments of the Belgian coast

High resolution profiles of trace elements (Fe, Mn, Co, As, Cu, Cr, Ni and Pb) were assessed using the DET (Diffusive Equilibrium in Thin films) and DGT (Diffusive Gradients in Thin films) techniques in silty, organically enriched, sub-tidal sediments of the Belgian coast during late winter and spring 2008. The general chemical properties of the sediments such as dissolved oxygen, pH. Eh and sulfide profiles. controlling precipitation/mobilization reactions. were determined with electrodes (pH and Eh) and microelectrodes (oxygen) and Agl-DGT probes (sulfide). Most trace elements show subsurface maxima and low concentrations beneath 8 cm of depth. The main physicochemical parameters controlling the vertical concentration profiles are dissolved oxygen and redox potential in the surface sediment and sulfide in the deeper sediment layers. Thermodynamic equilibrium calculations have been carried out verifying which solid phases can explain the dissolved trace metal concentrations. Seasonal variations of trace elements have been observed during the sampling period and sedimentation of fresh particulate organic matter (POM) derived from phytoplankton blooms appear to be the main cause of this temporal variability. Flux calculations based on DGT profiles (these fluxes are minimum ones) show that exchange fluxes of trace metals in February are slightly higher than in April. In addition, "DGT pistons" were deployed at the sediment water interface (SWI) to accumulate labile ions from below. This way all labile ions, binding onto the DGT Chelex resin, are pumped out of the pore waters and the solid sediment phase (only the mobile fraction). These results are a direct estimation of the amount of trace elements that can be released from the upper sediment to the water column (in the range of 4.4 . 10(-5) to 0.10 mmol.m(-2).d(-1) for Co, Pb, Cr, As, Cu, Ni, Fe and Mn). (C) 2009 Elsevier B.V. All rights reserved.