Continuous record of Holocene sea-level changes and coastal development of the Kattegat island Læsø (4900 years BP to present) JENS MORTEN HANSEN, TROELS AAGAARD, JENS STOCKMARR, INGELISE MØLLER, LARS NIELSEN, MERETE BINDERUP, JAN HAMMER LARSEN & BIRGER LARSEN Hansen, J.M., Aagaard, T., Stockmarr, J., Møller, I., Nielsen, L., Binderup, M., Larsen, J.H. & Larsen, B. , 2016. Continuous record of Holocene sea-level changes and coastal development of the Kattegat island Læsø (4900 years BP to present). © 2016 by Bulletin of the Geological Society of Denmark, Vol. 64, pp. 1–55. ISSN 2245-7070. (www.2dgf.dk/publikationer/bulletin). https://doi.org/10.37570/bgsd-2016-64-01 Læsø is the largest island of the Kattegat–Skagerrak region and exposes a vast array of relative sea- level (RSL) indicators, mainly raised beach ridges, swales, lagoons and saltmarshes. The physical environment of continuous glacial rebound, excessive supply of sediment, shallow surrounding Received 30 June 2014 waters, low amplitudes of near-shore waves, and micro-tidal conditions produced numerous sea- Accepted in revised form level proxies of both barrier coasts and saltmarshes. About 1200 RSL/age index points reflect not 9 May 2015 Published online only short-term sea-level highstands as in most other parts of Europe, but also short-term sea-level 28 January 2016 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. Keywords: Læsø, Holocene, beach ridge, relative sea-level curve, isostatic rebound, Lidar DTM, opti- cally stimulated luminescence (OSL) dating. Holocene sea-level changes and coastal development of Læsø · 1 Jens Morten Hansen [[email protected]], Jens Stockmarr [[email protected]], Ingelise Møller, Merete Binderup, Birger Larsen, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Troels Aagaard [[email protected]], Lars Nielsen [[email protected]], Department of Geosciences and Natural Resource Management, Copenhagen University, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Jan Hammer Larsen [[email protected]], Coastal Museum of Northern Jutland, Bangsbo Division, Dronning Margrethesvej 6, 9900 Frederikshavn, Denmark. The island of Læsø covers an area of 118 km2 and is ern Baltic Sea, Læsø has no central or marginal core situated in northern Kattegat in the transition zone of an older and more elevated landscape of different between the Baltic Sea and the North Sea. The main origin than the marine foreland itself (cf. the general island and about 50 islets in its vicinity are situated hypothesis on the formation of marine forelands by on a platform with water depths less than 10 m, cover- Schou 1945, 1969a–g). Except for several residual boul- ing around 1700 km2. The area inside the 3 m isobath der reefs, no such xenomorphic landscapes of different covers around 400 km2. An area of about 208 km2 is origin and sediment composition have been preserved covered by less than 0.3 m of water and comprises the within the present coastline of the island. present islands and a 90 km2 temporarily dry sand The present vegetation and animal wildlife on platform south of the main island (Fig. 1, Fig. 2). Læsø possess many characteristics of remote islands In contrast to all other major marine forelands of (cf. EU’s Natura 2000 plan for Læsø at www.laesoe.dk the Skagerrak, Kattegat, Danish Straits and the south- and the proposal by Olesen (2005) of Læsø as Marine Jegens Hornex Bansten Flaget Bløden Hale Holtemmen Kærene Knogene National Vestre Nyland wildlife park Fjærene Fjærene Fjærene 5 km Fig. 1. Topographic map of Læsø. Map copyright Geodatastyrelsen 2006, with some place names added. Size of figure is 30×30 km. Geographical position: 11°00′ E, 57°10′ N (lower cross); 11°00′ E, 57°20′ N (upper cross). 2 · Bulletin of the Geological Society of Denmark National Park). However, some geological, archaeo- Pre-history of Læsø until the logical and biological studies indicate that the island was once attached to older, now completely eroded development of late Holocene landscapes and has a considerably longer history than coastal landscapes indicated from its present wildlife, outline, position and shoreline ages (Hansen 1995, 2015; Bradshaw et This chapter gives a brief review of the late Quater- al. 1999). nary history of the central northern Kattegat region. It Thus, a robust description of the coastal history of touches on some issues which have been clarified by Læsø should provide explanations for how detach- the present study and are presented later in the text, ment of the island from older and more elevated but which are needed in the context here. (xenomorphic) landscapes eventually took place, how initial stages of the outline and position of the island The early, middle and late Weichselian glacio- have been altered by erosion and redeposition, and marine environment of the Kattegat region how younger, detached stages of the island developed. Compared to other major parts of Danish waters Based on such reconstructions it is our intention the Quaternary geology of southern Skagerrak and to present a relative sea-level curve for the past 4900 northern Kattegat has been considerably less studied. years, which greatly extends the findings of Hansen Consequently, specific knowledge about the Quater- et al. (2012) for the past c. 900 years. nary structure is sparse around Læsø and to some 5 km Fig. 2. Topography of the seabed around Læsø. Blue: water depths of 10 to 3 m. Other colours: water depths of 3 to 0 m. White: land. Size of figure is 35×35 km. Compiled by the Geological Survey of Denmark and Greenland. Holocene sea-level changes and coastal development of Læsø · 3 extent rests with general conceptions derived from concept of extradomainal deformation constitutes an neighbouring regions. Until the Litorina transgression important basis for the glacial kineto-stratigraphy peaked in the Læsø region c. 6300 years BP (Chris- (Berthelsen 1978) and the general glacial stratigraphy tensen 1995), the landscape of northern Kattegat was of Denmark (Houmark-Nielsen 2004). probably characterized by landforms that had mainly The early, middle and late Weichselian glacio-ma- been formed by glacial load and some glacio-tectonic rine substrate of Læsø has been cut by a near-surface deformation (Larsen et al. 1986; Bahnson et al. 1986; fault with up to 4 m vertical throw that passes through Vangkilde-Pedersen et al. 1993) of the >180 m thick the southern part of the island (Hansen 1986); cf. Japsen deposits of early to middle Weichselian glacio-marine & Britze (1991), Vejbæk & Britze (1994) and discussion clays and silts (Bahnson 1986) which in this work are in Gregersen & Voss (2014). The fault is related to the OSL (optically stimulated luminescence) dated to the Fennoscandian Border Zone (or Sorgenfrei–Tornquist period 37000–19000 years BP.