Trace metal distribution in muddy-patch sediments from the northern Portuguese Shelf

Abstract

Three sediment cores were collected in the muddy patches of the northern Portuguese Shelf located offshore the Douro and Minho rivers. Bivalves and worms were found in the upper and sub-surface layers of two cores. Vertical distributions of dissolved manganese and iron, iron oxides and acid volatile sulphide (AVS) in the bioturbated cores differ dramatically from those in the core where macrofauna was not observed. Also, Fe, Zn, Cu, Pb and Cd simultaneously extracted with AVS (1 M HCl) showed pronounced increases in concentrations deeper in the sediments. Where bioturbation was not visible the profiles of extractable Cu, Pb and Cd are consistent with the precipitation of downward diffusing dissolved metals. However, this redistribution did not affect the metal-total vertical profiles. Reactive metals exhibited multiple peaks in the irrigated sub-surface layers where AVS and dissolved Fe showed successive maxima and minima with the depth. We interpreted these profiles as the net result of dissolved oxygen being pumped into the sediment and the formation of several oxic/anoxic interfaces. At these depths trace metals are mobilized in the oxidized layers, diffused a few millimetres towards the layers where sulphide is present, and precipitated. The consequence of this internal redistribution was the broad maxima of total metal concentrations in irrigated sediment depths.