Salt-marsh growth and fluctuating sea level: Implications of a simulation model for Flandrian coastal stratigraphy and peat-based sea-level curves

Using a sea-level rise term composed of steady and periodic parts, a zero-dimensional, numerical model for the vertical growth of high tidal mudflats and marshes has been operated under varying conditions of sediment supply. The model simulates the creation of dynamically stable marshes beneath which form stratigraphic sections in which organic beds (silty peat and/or peat marshes) are intercalated with silts (mudflats or minerogenic marshes). Such successions are widely observed from Flandrian deposits in the coastal zone of northwest Europe. Partly because of a strong but variable lag effect, the experimental transgressive and regressive overlaps afford a registration of sea-level movements of limited reliability, in terms of chronology, amplitude of fluctuation and indicative meaning. In the model, implemented for conditions in the Severn Estuary (SW Britain), a regressive overlap arises closer to the maximum of the periodic component of sea level than does the corresponding transgressive overlap to follow. Overlaps arise at variable levels within the tidal frame, but generally well above the position of mean high-water spring tides. The results support the 'oscillatory' concept of sea-level movement but provide evidence and explanations for large intrinsic uncertainties in sea-level curves based on radiocarbon-dated peats.