Long term variations in backbarrier salt marsh deposition on the Skallingen peninsula – the Danish Wadden Sea
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.
Section snippets
Study area
The Skallingen salt marsh is one of the largest undiked salt marsh areas in Europe. It is located in the Grådyb tidal area on the east (lagoonal) side of the Skallingen barrier spit (Fig. 1) which was formed during the last 400 years (Jacobsen, 1937). The foredunes on the west coast have regularly been breached during storms, forming prominent washover channels and washover fan deposits across the barrier (Davis et al., 2001). Since the beginning of the 20th century attempts have been made to
Materials and methods
The sea level database used in the present study, recorded at Esbjerg since 1888, consists of hourly measurements until 1978 conducted by the Danish Meteorological Institute. Since 1978, the harbor authorities in Esbjerg have also built up a database with four measurements an hour. The recorded levels in both sets of data are relative to DNN. Both databases have been used in order to minimize the length of unrecorded periods. The data treatment has solely been based on hourly records in order
Analysis of over-marsh high tides
Since the measurements of salt marsh accretion started at Skallingen in 1931, the yearly mean sea level at Esbjerg (Fig. 2A) has increased. Mean sea level, described by the traditional 19-year-running mean, showed a moderate increase until 1958, a slight decrease between 1958 and 1972, followed by a rapid rise after 1980. This curve bears a striking resemblance to the observed global temperature curve (e.g. Mikkelsen and Kuijpers, 2001, Thejll and Stendel, 2001). The mean water level variations
Deposition at test site 5/31
The sediment core from test site 5/31 shows in photography as well as in X-ray radiography (Fig. 3, left) that a salt marsh deposit with a well-defined horizontal lamination overlies a wash-over sand with inter-laminated small clay horizons in its top. Occasional roots are visible (i.e. 7–8 cm), but as a rule the horizontal bedding is preserved and no bioturbation occurs. The bare sand flat, which formed the upper surface of the backbarrier area at the beginning of the 20th century (Nielsen,
Deposition as a function of high tide level
Based on 10 precise measurements of accretion at test site 5/31 from 1931/32 to 1958 (Jakobsen, 1958), salt marsh accretion rates have been related to the numbers and levels of high waters between the measurements.
Measurements in the creeks of Skallingen near the test site show that during moderately windy and calm weather periods, the concentration of suspended matter in the tidal creeks is only a few mg l−1. During storms, the concentration can increase to the order of 200 mg l−1 (Bartholdy,
Past and present accretion
More sophisticated corrections of the modeled accumulation rates across the backbarrier, incorporating distance to the salt marsh creeks (e.g. French and Spencer, 1993) would most likely improve the result. Such corrections, however, are not possible on the basis of the available data. Nevertheless, as it appears from Fig. 6, the combined result of the calibrated , gives a reliable picture of the overall mean accretion on the salt marsh. The correlation coefficient between measured and
Conclusions
(1) The number of over-marsh high tides at the Skallingen backbarrier can be described as an exponentially decreasing function with increasing high water level. The constants that control this function are directly related to variations in the mean water level.
(2) A model based on three independent variables (high water level, distance to salt marsh edge and initial salt marsh level) was found to be able to describe long term variations in the salt marsh accretion. The model is based on a
Acknowledgements
This study was made possible by grants from the Danish Science Research Council (SNF-9701836 and 21-01-05-13). J.B. wishes to thank M. Jespersen for placing the late Dr. B. Jakobsen’s field books at his disposal when she retired. Ditte, Mette and Anders are thanked for indefatigable assistance in the field, not least in the search for 60-year-old pegs in the dense salt marsh vegetation of Skallingen. This thank also goes to Johannes Jensen who with his hawk’s eye found the key peg of Line III.
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