Deposition and mixing depths on some European intertidal mudflats based onPb-210 and Cs-137 activities

Vertical profiles of Pb-210 and Cs-137 have been collected at micro-, meso- and macrotidal mudflats in order to calculate present accumulation rates. Sediment cores were taken at the mudflat at Kongsmark in the microtidal Lis ter Dyb tidal area (Denmark), at the BOA bridge in the mesotidal Dollard es tuary (Netherlands), and at the Skeffling mudflat in the macrotidal Humber estuary (UK). Except for the Kongsmark site and site A in the Humber estuar y, no accurate calculation of accumulation rates was possible for the inves tigated sites. The accumulation rate obtained for the Kongsmark site is con firmed by other independent data and shows that the accumulation is not sup ply limited at this site at present. For site A in the Humber estuary a mod ified CIC method was applied to the Pb-210-profile and the result agrees wi th results from the nearby Welwick Marsh. The salt marsh at Skeffling is ad vancing out onto the fringing mudflat with accumulation at the innermost pa rt of the mudflat but with erosion taking place further offshore. The reaso ns for the generally poor dating results seems to be either erosion (two Hu mber sites) or postdepositional reworking of the deposited material (biotur bation and continued resuspension) and the data set suggests that the mixin g depth may increase with tidal range and thereby hydrodynamic forcing. (C) 2000 Elsevier Science Ltd. All rights reserved.