Holocene shoreline displacement and the impact of the Storegga tsunami on Hinnøya, northern Norway

The Holocene relative sea‐level (RSL) history of Norway's largest island, Hinnøya, has been investigated in detail, using sediment records from 25 isolation basins. The sediments were analysed for macrofossil and phytoplankton content, which served as the basis for identifying marine–lacustrine transitions, that is isolation contacts. Terrestrial plant remains from the transitions between depositional environments were radiocarbon dated to obtain ages of palaeo‐sea levels corresponding to the basin threshold elevations. We also mapped morphological traces of the 7–8 ka, diachronous “Tapes shoreline”, which becomes younger from southeast to northwest across the field area. The regional Tapes shoreline isobase direction is 45° northeast, and the shoreline tilt is 0.37 m km −1 . RSL curves have been constructed for two sites located 12.5 km apart across the isobase direction: Øksnes on southern Hinnøya, and Tjeldsundet east of the island. Postglacial land emergence at rates of ~20 mm a −1 was interrupted by the Tapes transgression, which started around 9.3–9.0 ka and lasted until a highstand was reached at 8.0–7.5 ka. RSL has since fallen, at a mean rate of ~3 mm a −1 . A shoreline diagram for a 25 km long transect across Hinnøya shows details of the changing shoreline geometry through time, and extrapolated shorelines provide insights into the regional extent and magnitude of the Tapes transgression. Five of the basin records hold traces of the ~8200‐year‐old Storegga tsunami, which had a vertical run‐up of at least 4 m on southern Hinnøya and propagated more than 25 km northward, into the narrow marine strait of Tjeldsundet. The results are compared with recent modelling of the postglacial RSL history for the same region, demonstrating that the modelling fails to capture the details and variability of RSL change, at a relevant scale for environmental reconstruction and chronological use. The modelled RSL curves will often be misleading on a local scale and should not be used for geological or archaeological interpretations.