A strategy towards estimating a sediment budget for the Baltic Sea coastline of Schleswig-Holstein, Germany

Abstract

<p>Sea level rise along with the changing climate leads to severe enhancement of hydrodynamic impact to coastlines worldwide. Along the Baltic Sea coast of Schleswig-Holstein (Germany), this leads to the erosion of exposed glacial cliffs (up to 30 % of the coastline) and abrasion platforms (unknown extend). Irreversible land loss and seafloor deepening are the consequences, causing socio-economic and environmental concerns in affected areas. However, the adjacent coastal sections benefit from the development as the mobilized material constitutes the main sediment source to the nearshore bar and beach systems. Here, temporal built up of nearshore bars and the deposition at sandspits and beaches functions as natural shore protection.</p><p>The heterogenous and dynamic morphology, exposition and geology of the cliff sections and their offshore continuation complicates system understanding and management of the Schleswig-Holstein coastline. The availability of coarse-grained sediments (sand, gravel, stones) from the poorly sorted glacial till, forming the cliffs, is comparatively low. This lack of obtained material suitable to build up a coastal morphology attributes a central role to the source areas and the quantification of the sediment budget regarding coastal preservation.</p><p>On this account we attempt to develop a strategy towards a classified coastal sediment budget, which is based on a comprehensive field and literature data base, addressing the highly variable character of the observed coastline described in morphological, morphodynamic, geological, sedimentological, hydrodynamic and anthropogenic parameters.</p><p>The coastline of Schleswig-Holstein is structured into 58 active cliff sections for individual description via categorized cliff profiles. Furthermore, 22 abrasion platforms are defined in the offshore region and characterized by descriptive summaries. The data summary reveals well investigated zones (e.g. Schönhagen, Stohl, Heiligenhafen, Brodten), serving as potential pilot areas for complementary studies, but also identifies study areas which require further research.</p><p>The literature values for past cliff retreat and eroded sediment volumes bear high uncertainties. This is due to the fact that former studies are based on unequal spatial extend of cliff sections, variable time intervals and differing methods. Further, computation of eroded material volumes is lacking important input parameters, e.g. the degree of compaction and the grain size distribution. This is considered for budget calculations and their confidence for individual coastal units in template form.</p><p>The current study compiles and visualizes the heterogenous data for further scientific applications. The project aims to support future studies on the sediment availability and transport in the near-shore system using hydrodynamic modelling and thus creates a sound scientific base for system understanding and new governmental regulations concerning coastal protection measures at the Schleswig-Holstein Baltic Sea.</p>

Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

“…The total CaCO 3 flux from coastal and seabed erosion would amount to 2 -94 Tg yr -1 for till-CaCO 3 contents of 3 -27 wt-% CaCO 3 (Averes et al, 2021). Dissolution of the eroded carbonate would yield Ca fluxes of 20 -940 Gmol yr -1 that may be high enough to explain the O-I-Ca value derived above (122 Gmol yr -1 , Table 1).…”

“…The erosive coast is characterized by a series of sandy barrier islands and dunes connected with till cliffs (Weisse et al, 2021). The till contains about 3 -27 wt-% CaCO 3 (Averes et al, 2021) with a mean CaCO 3 content of about 17.5 wt-% in the southern Baltic Sea (Healy and Werner, 1987). Studies on coastal cliff erosion indicate that about 2 -7 m 3 of till are eroded along the German coast of the Baltic per meter cliff length per year (Schwarzer et al, 2003;Averes et al, 2021).…”

“…The till contains about 3 -27 wt-% CaCO 3 (Averes et al, 2021) with a mean CaCO 3 content of about 17.5 wt-% in the southern Baltic Sea (Healy and Werner, 1987). Studies on coastal cliff erosion indicate that about 2 -7 m 3 of till are eroded along the German coast of the Baltic per meter cliff length per year (Schwarzer et al, 2003;Averes et al, 2021). The total length of cliffs bordering Germany is 276 km corresponding to 27% of the coastline (Łabuz, 2015).…”

“…Assuming that 20% of the 2000 km long erosive southern coastline of the Baltic Sea is composed of till (Averes et al, 2021) and that the till is eroded at an average rate of 5 m 3 m -1 yr -1 , the total till erosion rate can be estimated as 2 x 10 6 m 3 yr -1 . Applying a till bulk density of 2 tons m -3 (Healy and Werner, 1987), the mass flux amounts to 4 Tg yr -1 .…”

“…For a clay/silt content of 50 wt-% (Healy and Werner, 1987), the flux of fine sediment results in 17 -170 Tg yr -1 . Applying the full range of observed carbonate contents in till (3 -27 wt-% CaCO 3 , Averes et al, 2021), results in possible CaCO 3 fluxes of 1 -92 Tg yr -1 for seabed erosion.…”