SEDIMENT ACCUMULATION RATES, HISTORICAL INPUT, POSTDEPOSITIONAL MOBILITY AND RETENTION OF MAJOR ELEMENTS AND TRACE-METALS IN SALT-MARSH SEDIMENTS OF THE SCHELDT ESTUARY, SW NETHERLANDS

The distribution of major elements and trace metals has been studied in two radiodated salt marsh sediment cores (core K and core E) of the Scheldt estuary, in order to determine the recent pollution history and to assess the impact of diagenetic processes. The sedimentation rates, based on Pb-210 and Cs-137 inventories, were 0.84-0.90 cm/yr in core E and 1.3-1.7 cm/yr in core K. The sediments show a vertical redox zonation with an oxic upper layer, where trace metals are mobilized, overlying a reduced layer where the metals are precipitated. Though these diagenetic processes have greatly modified the original trace metal distribution, anthropogenic maxima could be identified for Cd, Cr, Cu, Pb, and Zn during the mid-sixties. A second pollution maximum was found at the end of the seventies for Cd, Cu, Pb, and Zn. The down-core distributions of Mn, Fe, total S, CaCO3, Sr, Co, and Ni are controlled by diagenetic processes, whereas the P profiles reflect both anthropogenic and diagenetic impacts. Rapid remobilization of trace metals occurs after their deposition on the marshes due to the occurrence of oxic conditions in the upper sediment layer, preventing precipitation of metal sulfides from the pore waters. Though the metals are partly reprecipitated in the reduced sediment layer (e.g. as sulfides), the inventory of the cores suggests that Cr is the only metal which is completely retained by the sediments. Slight losses are observed for Fe, Co, and Pb (11-14%), moderate losses for Zn, Cu, and Ni (19-25%), and high losses for P, Mn, and Cd (40-50%). Similar mobilization of trace metals is to be expected in other salt marshes if a distinct oxic sediment layer is present.