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.

Introduction

Representing the borderline between land and sea, salt marsh areas have long been of key interest for sedimentological as well as environmental researchers (e.g. Wolff et al., 1979, Allen and Pye, 1992, Boorman, 1999, Van Wijnen and Bakker, 2001, Pye and Allen, 2000). A growing awareness of salt marsh areas as fragile early warning indicators of consequences of climate change is now fully recognized (Orson et al., 1985, Stevenson et al., 1986, Christiansen et al., 2001).

Fine-grained deposition and salt marsh development have been studied by numerous authors in many parts of the world (e.g. Steers et al., 1979, Letzsch and Frey, 1980, Pethick, 1981, Reed, 1988, Reed, 1995, Allen, 1990, Allen, 2000, Allen and Pye, 1992, French and Spencer, 1993, Cahoon et al., 1996, Bartholdy, 1997, Kirchner and Ehlers, 1998, Van Proosdij et al., 2000, Davidson-Arnott et al., 2002). A large number of salt marsh areas exhibit a positive sediment balance and keep pace with rising sea levels. In this context, they represent a valuable archive of historical information of sedimentological (e.g. Madsen, 1981, Allen and Rae, 1998, Christiansen et al., 2002) as well as archaeological interest (e.g. Mellalieu et al., 2000, Gonzales et al., 2000, Petzelberger, 2000, Bell, 2000, Firth, 2000, Van de Noort and Ellis, 2000). Human interference has been suggested to have a negative feedback on the positive sediment balance which leads to salt marsh formation along the Wadden Sea coast (e.g. Flemming and Bartholomä, 1997, Flemming, 2002).

In order to understand observed changes in salt marsh development, and correctly interpret possible consequences of climate change, it is essential to understand natural long term trends in salt marsh development, and to explore relations between observed depositional patterns and coastal dynamics. Long term observations of both sediment deposition and dynamics are thus essential.

In 1931 colored sand was spread out on salt marsh test sites established on the backbarrier of the Skallingen peninsula in the northernmost part of the European Wadden Sea (Fig. 1). The purpose was to investigate salt marsh deposition, and the sand functioned as a recognizable marker horizon for many years (Nielsen, 1935, Jakobsen, 1953). These test sites were surveyed until 1958, and the results provide a precise log of the accretionary history of the backbarrier area (Jakobsen, 1958). With concurrent water level information from a nearby tide gauge at the town of Esbjerg, which has been recording since the last part of the 19th century, these data provide an excellent opportunity to interpret interactions between water level variations and salt marsh growth.

In 1998 one of these test fields and one of the associated survey lines across the backbarrier area were rediscovered. The colored sand was still visible and the depositional record was supplemented by ²¹⁰Pb dating. The primary objectives of the present paper are: (1) to analyze and quantify the spatial and time dependent variations in salt marsh accretion across the backbarrier, (2) to combine the results into a simple model, based on the sea level record and (3) by means of the model, to describe possible future scenarios for the development of the backbarrier area and relate salt marsh accretion to climate variations.