Measuring sediment deposition and accretion on anthropogenic marshland - Part I: Methodical evaluation and development

The North Frisian Halligen (Northern Germany) are inhabited and highly anthropogenic modified coastal marshlands. Today a lack of knowledge about sedimentological parameters prevent for a predicated discussion on their adaptation capacity to recent and future sea-level changes. A combined field and laboratory method to calculate marshland accretion rates based on short-term (2010-2013) measurements of sediment depositions was developed. All studies were carried out at the marshlands of the Halligen Hooge, Langeness and Nordstrandischmoor. One litre LDPE bottles and small synthetic turf mats were used as simple but coast, time and quantity efficient sediment trap devices. Up to a deposition rate, of ≈ 2.0 kg/m2, both devices gained comparable results. Above this threshold the retention efficiency of the turf mats is decreasing compared to the LDPD bottles. The combined use of bottles and mats, especially when deposition rates are not exceeding the threshold, allows to (1) checking internal consistency of the data, (2) detecting outliers with respect to cattle- or man-made damage, and (3) estimating possible effects of post-storm sediment remobilization. To transfer sediment depositions into rates of vertical accretion, the bulk dry density as well as the organic matter concentration of the correspondent marsh soil was considered using data from shallow percussion cores. These parameters are different among all Halligen. Higher inundation frequencies cause lower soil organic matter concentrations, resulting in higher bulk dry densities (BDD) of the soil (Hooge 0.64 g/cm3, Langeness 0.67 g/cm3, Nordstrandischmoor 0.83 g/cm3). Autochthonous organic material (by source of the marshland vegetation) contributes by 9.0 ± 1.4 % (Hooge) to 21.4 ± 6.6 % (Nordstrandischmoor) to marshland accretion, for a correspondent time scale of 1915-2011. Average accretion rates (2010-2013) were calculated with 1.2 ± 0.8 mm/a for Langeness, 1.5 ± 0.9 mm/a for Hooge and 2.6 ± 0.9 mm/a for Nordstrandischmoor.