The postglacial sea level history of Ingøya, northern Norway

• Main Author: Rickerich, Sam
• Format: Text
• Language: unknown
• Published: SCARAB 2018
• Subjects: sea level; Ingøya; Norway; deglaciation; cosmogenic nuclide; Barents Sea; Raised Beach; Storegga; Finnmark; Ice Sheet; Modiolus modiolus; Northern Norway; Sea ice
• Online Access: https://scarab.bates.edu/honorstheses/258; https://scarab.bates.edu/cgi/viewcontent.cgi?article=1276&context=honorstheses

Reconstructions of postglacial relative sea level history in formerly glaciated terrains reflect the interaction of glacioisostatic processes and ocean water volume changes, providing insight into regional ice sheet dynamics, crustal rheology, and meltwater fluxes. This study focuses on the relative sea level history of Ingøya, an island in the Finnmark region of northern Norway, which during the Last Glacial Maximum was located near the confluence of the Scandinavian Ice Sheet (SIS) and Barents Sea Ice Sheet (BSIS). Previously undated raised beach sequences on Ingøya record isostatic and ocean water volume changes from the late glacial through the late Holocene. A combination of field, analytical and geochronogical approaches were utilized Tto constrain the timing and rate of relative sea level change. These include cosmogenic 10Be surface exposure dating of boulders on raised shorelines, 14C dating of the marine-lacustrine transition in an isolation basin, and 14C dating of marine bivalves found in late Holocene raised beach deposits. The uppermost raised marine shorelines on western Ingøya are a series of boulder beaches from 20 to 30 m asl that record the marine limit during deglaciation and initial isostatic emergence. The average (n=6) 10Be age of schist-quartzite boulders on the highest shoreline at 30 m asl is 14.8 ± 0.5 ka (average, 1 SD, n = 6, North East North American production rate). A second, lower cobble shoreline across the valley at 20 m asl has a similar age of 14.8 ± 0.3 ka (1 SD, n =2), indicating rapid isostatic uplift following deglaciation. Sediment core AMS 14C ages on plant macrofossils from an isolation basin at 6 m asl further constrain the timing and extent of the mid-Holocene transgression and the 8.2 ka Storegga Tsunami. Maximum ages of 7590 ± 20 cal yr BP and 5318 ± 5 cal yr BP from woody macrofossils define the timing of inundation and isolation of the 6 m asl basin. The stratigraphic relationship between the coarse gravel unit below the 7590 ± 20 cal yr BP age suggests that this basin was inundated by the Storegga Tsunami. Projections of tsunami run-up of 3 – 5 m constrains relative sea level at 8.2 ka to range from 1 – 3 m above modern day levels. On southern Ingøya, a flight of lower elevation gravelly raised beaches that contain abundant bivalve shells, mainly Modiolus modiolus, extends from near sea level to a prominent bench-like terrace at 9 m asl. A 14C age of 6,450 ± 40 cal yr BP was obtained on a Modiolus shell from a trough directly below the highest terrace. The age of the upper terrace corresponds to similarly dated deposits in coastal northern Norway related to the mid-Holocene Tapes Transgression, during which sea level rise outpaced glacioisostatic rebound. The relative sea level history from Ingøya sheds light on the timing of deglaciation of the north Finnmark coast, the separation of BSIS and SIS, and the extent of the mid-Holocene transgression.