Long term variations in backbarrier salt marsh deposition on the Skallingen peninsula – the Danish Wadden Sea

Abstract

Accretion on a natural backbarrier salt marsh was determined and modeled as a function of high tide level, initial salt marsh level and distance from the salt marsh edge. Accretion measurements were based on up to 67-year-old marker horizons, supplemented by 210Pb/ 137Cs datings. The salt marsh is situated on the backbarrier of the Skallingen peninsula in the northern part of the Danish Wadden Sea. The tidal range (mean 1.5 m) is strongly affected by wind tide which occasionally adds up to about 3 m to the astronomical high tide level. Accretion is restricted to a narrow vertical band from about 0.1 m below to about 0.7 m above the mean high water level. In the outer part of the backbarrier (close to the tidal flat) mean accretion is about 4 mm yr −1 and in the inner part it is about 2 mm yr −1. The decrease takes place in a similar manner as the decrease across a flood plain. The major part of salt marsh accretion is associated with high water levels corresponding to weather conditions characterized by gales. The long term variation of salt marsh accretion correlates with variations in the North Atlantic Oscillation winter index. The number of over-marsh high tides decreases exponentially with high tide level. The function constants controlling this distribution vary in direct ratio to the mean sea level. Plausible near future scenarios of tidal development were obtained by extrapolating this relation. Three sea level scenarios were tested: (i) sea level rise continues at a constant (long term) rate of 2.3 mm yr −1, (ii) sea level rise continues at a constant (short term) rate of 4.2 mm yr −1 and (iii) sea level rise accelerates to a constant rate of 6.4 mm yr −1. In the first case accretion on the salt marsh will keep pace with the high water level rise. In the second case deposition in the inner part of the salt marsh will lag behind while that of the outer part of the salt marsh will keep pace with the rising high water level. In the third case, corresponding to a ‘worst case’ scenario for the 21th century, the salt marsh will gradually drown.