Continuous record of Holocene sea-level changes and coastal development of the Kattegat island Læsø (4900 years BP to present)
Authors: Hansen Jens Morten , Stockmarr Jens , Møller Ingelise , Binderup Merete , Larsen Birger , Aagaard Troels , Nielsen Lars , Larsen Jan Hammer 
Published: December 2020

Læsø is the largest island of the Kattegat–Skagerrak region and exposes a vast array of relative sealevel (RSL) indicators, mainly raised beach ridges, swales, lagoons and saltmarshes.

The physical environment of continuous glacial rebound, excessive supply of sediment, shallow surrounding waters, low amplitudes of near-shore waves, and micro-tidal conditions produced numerous sealevel proxies of both barrier coasts and saltmarshes. About 1200 RSL/age index points reflect not only short-term sea-level highstands as in most other parts of Europe, but also short-erm sea-level lowstands, which in less regressive environments have normally been removed by coastal erosion or obscured by berms from subsequent highstands.

Based on a high-precision lidar digital terrain model, the beach ridges have been mapped, typified, levelled and correlated relative to their order of appearance. Transformation of this relative chronology to a robust absolute age model of the RSL changes has been made on the basis of 119 optically stimulated luminescence (OSL) datings, 14C datings, and tree-ring datings.

By ground penetrating radar (GPR) and terrain analyses, the height of the swash zone (run-up) has been determined in order to transform the ridge elevations to a detailed curve of the RSL/age relation. The curve reveals eight centennial sea-level oscillations of 0.5–1.1 m superimposed on the general trend of the RSL curve, including a Little Ice Age lowstand of 0.6 m at 1300 AD. The island grew from now eroded landscapes of Weichselian glacio-marine deposits, including the oldest known post-Weichselian forested area in Scandinavia.

During the last 4900 years new coastal landscapes have formed continuously, resulting in around 4000 km of still visible, raised palaeo-shorelines in mostly uncultivated landscapes. After formation of the oldest preserved beach-ridge complex, numerous sea-level proxies formed in a strongly regressive environment caused by glacial rebound supplemented with local uplift due to extensive erosion during Boreal and Atlantic time of the 1700 km2 glacio-marine platform upon which the island is still being built.

The combined uplift produced a relative sea-level fall of 10.3 m, corresponding to a mean vertical regression rate of 2.1 mm/year and a mean horizontal regression rate of 2 m/year, and formed eight distinct types of raised coastal landscapes where well separated beach ridges and saltmarshes developed continuously.
The oldest preserved part of Læsø appeared 4900 years BP as the eastern tip of a 10 km long barrier-spit system growing from a raised glacio-marine landscape, now represented only by boulder reefs west and north-west of the present island.

Around 4000 years BP another barrier-spit system appeared to the south, growing northwards from another raised glacio-marine landscape at the raised boulder reefs in the town of Byrum and the abrasion landscapes of Rønnerne.

Around 3000 years BP these two inital barrier-spit systems united and formed one major barrier between the present towns Vesterø and Byrum. To the north-east, a third glacio-marine landscape provided materials for the development of the eastern end of the island. Thus, around 2500 BP the island had become one triangular, completely detached island (’the old triangle’) between Vesterø, Byrum and Bansten Bakke. From this detached stage, nine subsequent barrier-spit systems grew to the east and formed the present Østerby peninsula, while a series of nine barrier-island complexes developed south-west of ’the old triangle’. To the south and south-east, low-energy coasts developed and formed low beach ridges and saltmarsh landscapes.