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

Vertical profiles of 210Pb and 137Cs 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 Lister Dyb tidal area (Denmark), at the BOA bridge in the mesotidal Dollard estuary (Netherlands), and at the Skeffling mudflat in the macrotidal Humber estuary (UK). Except for the Kongsmark site and site A in the Humber estuary, no accurate calculation of accumulation rates was possible for the investigated sites. The accumulation rate obtained for the Kongsmark site is confirmed by other independent data and shows that the accumulation is not supply limited at this site at present. For site A in the Humber estuary a modified CIC method was applied to the 210Pb-profile and the result agrees with results from the nearby Welwick Marsh. The salt marsh at Skeffling is advancing out onto the fringing mudflat with accumulation at the innermost part of the mudflat but with erosion taking place further offshore. The reasons for the generally poor dating results seems to be either erosion (two Humber sites) or postdepositional reworking of the deposited material (bioturbation and continued resuspension) and the data set suggests that the mixing depth may increase with tidal range and thereby hydrodynamic forcing.