Salt-marsh stratigraphy of Ho Bugt, western Denmark

Salt-marsh stratigraphy and late Holocene sea-level changes in Ho Bugt, western
Denmark: implications for the ‘regional eustatic’ history of the North Sea
Roland Gehrels1, Katie Szkornik1, Jason Kirby2, Glenn Milne3, Jesper Bartholdy4,
Jørn-Torp Pedersen4, Jan Heinemeier5 and Wil Marshall1
School of Geography, University of Plymouth, UK
Department of School of Biological & Earth Sciences, John Moores University, Liverpool, UK
Department of Geology, University of Durham, UK
Institute of Geography, University of Copenhagen, Denmark
Aarhus Radiocarbon Laboratory, University of Aarhus, Denmark
The evolution of salt marshes around Ho Bugt, the northernmost part of the Danish
Wadden Sea, was investigated to establish a late Holocene sea-level history. This area
is of interest as it is located near the ‘hinge line’ of isostatic stability between zones of
isostatic uplift to the north and subsidence to the south. A sea-level history from this
coast, therefore, is relevant for testing geophysical and empirical models of glacioeustatic sea-level change in the late Holocene. This is important for modellers of the
glacio-isostatic adjustment process (i.e. Lambeck, Peltier and others) who are in
disagreement about estimates of late Holocene ice melt. Furthermore, Ho Bugt sealevel data can be compared with a nearby sea-level curve for the Kattegat (between
Denmark and Sweden) which has been proposed by Mörner (1969) to be
representative of the regional eustatic sea-level history of the North Sea.
Cores and exposed cliff sections were subjected to AMS14C dating and lithoand biostratigraphical (diatom) analyses to reconstruct the palaeoenvironmental
history of the Ho Bugt salt marshes. Diatoms were sampled from the modern
environment to establish their relationship with water levels (see Katie Szkornik’s
abstract – this volume). Four stages in the development of the salt marshes can be
identified: 1) groundwater table rise and growth of basal peat (from at least 4200 cal.
yrs. BP to 2000 cal. yrs. BP); 2) initial salt-marsh formation (2000?-1700 cal. yrs BP);
3) a freshening phase (1700-800? cal. yrs BP), culminating in drying out of the
marshes and producing a widespread very distinct black horizon; and 4) renewed saltmarsh deposition (800? cal. yrs BP to present), including a phase of aeolian sand
deposition possibly related to dune building in the 17th century.
From 18 AMS radiocarbon ages on fossil plant fragments we reconstructed a
preliminary local sea-level history. Sea level has risen 3 metres since ca. 4000 cal. yrs
BP and reached present mean sea level around 1500 cal. yrs BP. A sea-level fall of
several decimetres is not well constrained by dates but occurred after 1500 cal. yrs
BP. The Esbjerg tide-gauge record shows that sea level has been rising at a mean rate
of 1.1 mm/yr since 1890. The Ho Bugt sea-level history does not support the Kattegat
sea-level of Mörner (1969) and the late Holocene ‘regional eustatic’ significance of
this curve is questionable. When isostatic factors are taken into consideration, late
Holocene sea-level changes in Ho Bugt are in line with those documented in northern