Accretion on a natural backbarrier salt marsh was modeled as a function of high tide level, initial salt marsh level
and distance to the source. Calibration of the model was based on up to ca 80 year old marker horizons, supplemented
by 210Pb/137Cs datings and subsequent measurements of clay thickness. Autocompaction was incorporated
in the model, and shown to play a major role for the translation of accretion rates measured as length per unit time
to accumulation rates measured as mass per area per unit time. This is important, even for shallow salt marsh
deposits for which it is demonstrated that mass depth down core can be directly related to the bulk dry density
of the surface layer by means of a logarithmic function. The results allow for an evaluation of the use of marker
horizons in the topmost layers and show that it is important to know the level of the marker in relation to the salt
marsh base. In general, deeper located markers will indicate successively smaller accretion rates with the same
sediment input. Thus, stability analysis made on the basis of newly established marker horizons will be biased and
indicate salt marsh stabilities far above the correct level. Running the model with a constant sea level revealed that
balance between the inner and the outer salt marsh deposition can not be achieved within a reasonable time scale.
Likewise it is shown that only one specific sea level rise provides equilibrium for a given location on the salt marsh.
With a higher sea level rise, the marsh at the specific location will eventually drown, whereas - with a sea level
rise below this level – it will grow towards the top of the rising tidal frame. The short term variation of salt marsh
accretion was found to correlate well with variations in the North Atlantic Oscillation - the NAO winter index.
Comparisons between the geomorphological development of wind tide affected salt marshes, like those present
on the Danish North Sea coasts, and primary astronomically controlled tidal marshes like those in the Georgian
Bight, USA showed that the former - when first established - relatively quickly grow above the level of the highest
astronomical tide, whereas this - in practice - will never happen for the latter.