· 25 Holocene development of the Arresø area, Denmark Holocene development of the Arresø area, north-east Sjælland, Denmark OLE BENNIKE, PERNILLE PANTMANN & ESBEN AARSLEFF Bennike, O., Pantmann, P. & Aarsleff, E. 2017. Holocene development of the Arresø area, north-east Sjælland, Denmark. © 2017 by Bulletin of the Geological Society of Denmark, Vol. 65, pp. 25–35. ISSN 2245-7070. (www.2dgf.dk/publikationer/bulletin) https://doi.org/10.37570/bgsd-2017-65-02 The Arresø area in north-east Sjælland, Denmark, was deglaciated about 18,000 to 16,000 years ago. In the Holocene it was probably a land area until it was transgressed by the sea c. 8500 years BP. During a first marine phase the area housed a species-rich marine fauna that included the oyster Ostrea edulis, the salinity and water temperatures were higher than at present, and there was a wide connection to the Kattegat sea. At about 6500 years BP there was a short-lived lake or brackish-water phase, but marine conditions were soon re-established with a fauna less diverse than before, and both salinity and water temperatures decreased. The present lake Arresø became isolated from the sea about 2500 years BP. The transition from brackish water to fresh water was rapid; the lake developed from shallow alkaline waters to deeper more acidic waters and finally to eutrophic waters. Keywords: Holocene, Arresø, Arrefjord, Denmark, Littorina Sea, relative sea-level changes, Ostrea. Ole Bennike [obe@geus.dk], Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Pernille Pantmann [ppa@museumns.dk] and Esben Aarsleff [eaa@museumns.dk], Museum Nordsjælland, Frederiksgade 11, DK-3400 Hillerød, Denmark. Corresponding author: Ole Bennike Denmark and the surrounding seas have undergone a dynamic and complex history after the last deglacia- tion. The region experienced isostatic uplift that was greatest in the north, where the ice was thickest. At the same time as the land was uplifted, global sea level increased by about 130 m because melt water from the shrinking ice sheets ended up in the oceans (Lambeck et al. 2014). The interplay between the rising sea level and regional uplift has determined the relative sea level history. In addition, storminess and shoreline dynamics have influenced the local palaeogeographi- cal evolution. The natural and archaeological history of Denmark after the last deglaciation have been studied for two centuries, but many details are still unknown, and the timing of events is poorly constrained. For example, the timing of the early Holocene marine transgres- sion has been debated (Mörner 1969). Detailed, high- resolution studies of continuous records are crucial for understanding the late Quaternary palaeogeographi- cal evolution of southern Scandinavia. The aim of this paper is to report on detailed studies of sediment cores from the lake Arresø, Denmark. We studied macro- scopical remains of plants and animals to assess the long-term environmental changes of the area. The Arresø area Arresø (Fig. 1) is the largest lake in Denmark, with an area of 3987 ha. The lake is shallow with a mean water depth of 3.1 m and the greatest depth is 5.6 m (Fig. 1B). The eutrophic lake is exposed to wind, and during stormy weather surface currents up to 30–50 cm/sec can be created. Deep areas are confined to nar- row troughs that are probably kept free of sediments due to bottom currents. The lake level is 4.0 m above present sea level (Høy 1996; Fronval et al. 2012). The tidal range in the region is about 10 cm at spring tide (Leppäranta & Myrberg 2009). In the Middle Holocene the Arresø area was a ma- rine bay or fjord called Arrefjord by Rørdam (1892). Figure 2 shows the palaeogeography of Arrefjord ac- cording to Andersen & Houmark-Nielsen (2014). Two older palaeogeographical maps were published by Rørdam (1892) and Milthers (1922). Rørdam showed a larger extent of Arrefjord than later reconstructions, including a strait from Arrefjord to Roskilde Fjord to the south. Coring is needed to show if such a strait existed. Several small shallow inlets were situated along the shores of Arrefjord (Fig. 2). The fjord was connected to the Kattegat sea to the north-west via Received 30 January 2017 Accepted in revised form 6 March 2017 Published online 5 April 2017