PhD ceremony: A.V. de Groot, 14.45 uur, Academiegebouw, Broerstraat 5, Groningen
Thesis: Salt-marsh sediment.
Natural γ-radioactivity and spatial patterns
Promotor(s): prof. R.J. de Meijer, prof. J.P. Bakker
Faculty: Mathematics and Natural Sciences
Salt marshes, vegetated areas periodically flooded by the sea, are of interest because of their contribution to coastal defence and biodiversity. To manage these dynamic areas, insight is necessary into the long-term sedimentation and development of salt marshes. Salt marshes on the North-Sea barrier islands consist of a clayey layer on top of sand. The composition and thickness of these deposits contain information on the development of the salt marsh. In this thesis, the spatial patterns of thickness and composition are described and interpreted. Next to the use of shallow soil cores, Alma de Groot investigated whether in-situ measurements of natural
γ
-radiation of sediment (40K, 232Th en 238U) can yield more insight into the spatial composition of salt-marsh sediment.
First, she modelled the effect of sediment water content and bulk density on in-situ measurements of
γ
-radiation, resulting in a correction for water content. On the studied island of Schiermonnikoog, the measured radiation intensity further mainly depends on sediment grain size. This result was applied by mapping the intertidal flats on mud content and identifying the thickness of the marsh deposits based on radiation patterns.
The cores show that there are spatial variations in deposition caused by the dynamics of wind and water during the development of a salt marsh. Approximately every decade, storms deposit sand on the clayey salt marsh. The contribution of this sand to the marsh deposits is around ten percent. The spatial patterns in total long-term sedimentation are complex and often change during marsh development.
