Vertical Growth of a Young Back Barrier Salt Marsh, Skallingen, SW Denmark
Keywords:Salt marsh, rate of sediment accretion, sea-level rise, storm surge frequency, Danish Wadden Sea
The spatial and temporal distribution of the sediment accretion upon a young (less than 100 years old), predominantly inorganic back barrier salt marsh has been studied based on leveling in 1973, a re-survey in 1998 and observations from 1931 to 1954 of the same area. The surface of the back barrier of Skallingen is not sloping gradually, from the dune ridges to the marsh edge, but is separated into three zones: an inner and an outer marsh at a level of about 1 m DOD (Danish Ordnance Datum), and a lower section in between at 0.9 m DOD. During the last 25 years the rate of sedimentation has been 3 mm a 1 on the inner and outer part and about 2 mm a-1 on the lower. The accretion on recently developed salt marsh areas along the marsh edge has been 8 mm a-1, a rate which looks characteristic for new marsh building on Skallingen as early investigations in the 1930s indicate a similar magnitude. This relatively high rate may be partly a result of sediment imported by sea ice rafting. Different methods used to measure the rate of sedimentation (marker layers, profiling and 210Pb isotope dating) all agree quite well. The average rate of sedimentation since 1931 on established marsh surfaces has almost been uniform, 3 mm a-1 which is well above the average sea-level rise of 1.3 mm a-1 during the last century. However, concerning the last 25 years, the hydrographical conditions have change dramatically. There has been an increased frequency of inundation, more storm surges, and the sea-level rise has increased to 4.2 mm a-1. But the accretion rate has not change, it is still 3 mm a-1. No evidence of stagnation or degradation of the salt marsh, however, has been observed yet.