POSTGLACIAL RELATIVE SEA LEVEL HISTORY OF INGØYA, NORTHERN NORWAY

Reconstructions of postglacial relative sea level history in formerly glaciated terrains reflect the interaction of glacioisostatic and eustatic processes, providing insight into regional ice sheet dynamics, crustal rheology, and meltwater fluxes. Here, we study 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 eustatic changes from the late glacial through the late Holocene. To constrain the timing and rate of relative sea level change, we use 1) cosmogenic ¹⁰Be surface exposure dating of boulders on raised shorelines, 2) ¹⁴C dating of the marine-lacustrine transition in an isolation basin, and 3) 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 meters asl that record marine incursion during deglaciation and initial isostatic emergence. The average (n=6) ¹⁰Be age of schist-quartzite boulders on the marine limit shoreline at 30 m asl is 14.8 ± 0.5 ka (average, 1SD, n = 6, Northeastern North American production rate). A second, lower cobble shoreline across the valley is dated to 14.8 ± 0.3 ka (1 SD, n =2). 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 6.45 ± 0.40 cal ka 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 eustatic sea level rise outpaced glacioisostatic rebound. Sediment core ¹⁴C ages on plant macrofossils (analysis in progress) from an isolation basin at 6 m asl will further constrain the timing and extent of the mid-Holocene transgression. The relative sea level history from Ingøya sheds light on the timing of deglaciation of the north Finnmark coast, and the separation of BSIS and SIS as well as the timing and extent of the mid-Holocene transgression.