Chapter 3 Measuring sediment deposition and accretion on anthropogenic marshland
4.5 Results
4.5.1 RADIOMETRIC MEASUREMENTS
In each of the 12 sediment cores, two distinct peaks of 137Cs activity were found in the upper 20 cm (fig. 4-3). The radiation intensity of those peaks is in a range between 35.4 to 170.2 Bq/kg.
137Cs peaks at Nordstrandischmoor occur deeper under the surface due to higher yearly sedimentation rates. Below depths of 12 cm (Hooge and Langeness) to 20 cm
Figure 4-3: Excess 210Pb (black line), 137Cs (grey line) and 210Pb age (dashed black line) versus depth in all 12 sediment cores. Horizontal error bars are one sigma errors resulting from the gamma ray measurements. Depths are medium depth of the measured core increment, rectified for the core compaction rate, resulting from percussion coring. The upper 137Cs peak is supposed to be a result of the Chernobyl nuclear accident (1986). The lower peak to nuclear weapon tests before the
comprehensive nuclear-test-ban treaty (1963).
(Nordstrandischmoor), where marsh sediments were deposited before the nuclear fission age, the 137Cs activity decreases below the detection limit. The excess 210Pb curves are mainly decreasing from surface with depth (fig. 4-3). At Hooge and Langeness, the excess 210Pb age versus depth line drops steeply by depth what is equivalent to lower accretion rates. For Nordstrandischmoor the depth versus time line and therefore accretion rates are more stable. It is also visible from figure 4-3 that the agreement between lead and cesium ages is mostly adequate. However, 137Cs ages especially for cores Lan01 Lan02 and Lan03, tend to younger sediment ages compared to the 210Pb dates. This is contrary to previous studies dealing with 137Cs and 210Pb data (Schuerch et al. 2012) who supposed a downwards relocation of 137Cs to be responsible for older ages compared to 210Pb data. For Hooge as well as Nordstrandischmoor a better agreement has been achieved.
4.5.2 SHORT TO LONG-TERM DEPOSITION AND ACCRETION RATES
Inundation events mainly occur during the winter months from October to March. Therefore, winter sediment deposition based on sediment trap measurements can be regarded as representative for the whole year (e.g. summer 2012 to summer 2013). Average annual marshland accretion rates between 2010 to 2013 vary from 0.7 to 4.4 mm/a depending on the individual Halligen as well as frequency of storm surge events (i.e. storm activity) of the respective winter season (fig. 4-4). In years of high storm activity such as 2011 to 2012, marshland accretion rates at Nordstrandischmoor are more than twice the rates as at Langeness and Hooge.
Figure 4-4: Annual average marshland accretion rates during the observation period (October to March 2010 to 2013) in comparison to the inundation frequency. For reason that inundations mostly occur during the winter month, accretion rates can be regarded as representative for the whole year.
CHAPTER 4 In years with lower storm activity (i.e. 2010 – 2011, 2012 – 2013) accretion rates at Nordstrandischmoor are still the highest, but the difference to the bigger Halligen is smaller. In all three years the values for accretion rates are lowest at Langeness, moderate at Hooge and highest at Nordstrandischmoor. Even though a higher storm surge activity at Langeness compared to Hooge would suggest for higher accretion rates at Langeness.
Figure 4-5: Comparison of vertical accretion rates at (a) Hooge, (b) Langeness and (c)
Nordstrandischmoor with the storm surge frequency. Accretion rates are based on the constant rate of supply (CRS) method for excess 210Pb according to Appleby and Oldfield (1978, 1983). All 12 cores and all depth down to the dating horizon are shown. The storm surge frequency is calculated as a 19 year running mean based on the yearly number of gauge levels exceeding an threshold of 1.54 m above MHW at the gauge station Wyk on the island Föhr.
Long-term accretion rates derived from the CRS dating model have been found variable in space (different locations on different Hallig marshlands) and time as well (fig. 4-5a,b,c). Nevertheless, the development of accretion rates on different Halligen obviously shows some similarities.
During the first half of the 20th century accretion rates don not exceed 1 mm/a at Hooge, 2 mm/a at Langeness and 3.5 mm/a at Nordstrandischmoor. Approximately 1960 rates start to increase and reach the highest values between 1980 and 2000. For some periods at Hooge and Langeness accretion rates are more than double compared to dates before 1960. During the first decade of the 21st century, rates decline to values slightly above the level before 1960. The development of the accretion rates seems to follow the development of the storm surge frequency that had likewise a maximum during the eighties of the 20th century. The 19 year moving average of the storm surge frequency was calculated by the number of gauge levels (station Wyk on Föhr, 1951 – 2011) exceeding a threshold of 1.54 m above MHW level. The threshold represents the average summer dyke height of Hallig Hooge and is nearly consistent with the administrational definition of a storm surge which is defined as MHW + 1.50 m. Combining excess 210Pband 137Cs data (by calculating average values of 137Cs and 210Pb data of an identic time scale) (tab. 4-1) confirms that marshland accretion during the second half of the 20th century (1963 – 1986 and 1986 – 2011) was above average compared to a time span of 1915 to 2011. Comparing 1963 to 1986 against 1986 to 2011, all investigated Halligen show stagnant accretion rates.
Table 4-1: Comparison of former marshland accretion rates according to 137Cs and 210Pb datings with recent field measurements.
137Cs/210Pb 137Cs/210Pb 210Pb sediment trapping 1963 – 1986 1986 – 2011 1915 – 2011 2010 – 2013
(mm/a) (mm/a) (mm/a) (mm/a)
Hooge 1.3 ± 0.7 1.4 ± 0.6 1.0 ± 0.3 1.5 ± 0.9 Langeness 1.9 ± 0.9 1.6 ± 0.7 1.2 ± 0.3 1.2 ± 0.8 N. Moor 3.1 ± 1.9 3.2 ± 1.6 2.6 ± 0.9 2.6 ± 0.9
CHAPTER 4 4.5.3 SPATIAL DISTRIBUTION OF ACCRETION RATES (2010 – 2013)
Sediment distribution maps visualize the spatial variability of sediment deposition and illustrate contrasting distribution patterns between the individual Halligen (fig. 4-6). Accretion values are calculated as three year averages (2010 – 2013) on both sediment traps type A and B. Hooge shows increasing accretion rates from the windward southwest side to the leeward north and northeast side of the marsh island. Eye-catching is a band of observation points located at the southwestern marsh edge where accretion rates are lowest (0.7 – 1.1 mm/a). At Langeness, in the eastern and western part of the Hallig sediment accretion rates are highest near the marshland margin. Points of highest accretion are located close to the two biggest channel outlets
"Osterwehl" (3.8 mm/a) and "Der Jelf" (3.4 mm/a). With increasing distance to the edge, accretion rates are decreasing to values < 1 mm/a. Also at the mid part of the Hallig, where the distance from southern to northern edge is smallest, rates are very low (< 1 mm/a). Due to its small extend, Nordstrandischmoor has only 12 grid (400 x 400 m) observation points. Accretion data shows a more or less random spatial distribution.
Figure 4-6: 2D contour maps of accretion rates. The displayed map is calculated by average values of a three year lasting field campaign (2010 – 2013) via linear point kriging interpolation without drift.
Average lateral distance between the observation points is 400 m.