Late Devensian and Holocene records of relative sea-level changes in northwest Scotland and their implications for glacio-hydro-isostatic modelling

Abstract

Raised tidal marshes and isolation basins (lakes that were once connected to the sea) in northwest Scotland record changes in relative sea level following deglaciation during the Late Devensian to the present. The Kentra to Arisaig area, which was covered by relatively thick ice (c. 900 m) at the Last Glacial Maximum (LGM), shows a regression from a marine limit between 36.5 m OD and 40 m OD at c. 15.9 kyr cal BP (range 15.6 16.3 kyr cal BP) through to an early-Holocene minimum. A range of sites in the same area record a mid-Holocene maximum, indicative of mean sea level c. 6.5 m above present. The maximum is not a well-developed and short duration highstand as predicted by a number of models, but is an extended period, ≃8.0 - 5.0 kyr cal BP, with sea level within c. 1 m of the maximum. Sites to the north, in Kintail, show no Late Devensian record because much of the area lies within the Younger Dryas ice limit. The altitude of the mid-Holocene maximum in Kintail is not well constrained, but occurred 7.9 - 8.1 kyr cal BP. Further north, sites on the Applecross peninsula record a Late Devensian fall in sea level and a Holocene maximum for mean sea level no higher than c. 3.0 m above present. In Coigach, the furthest north of the new sites and well outside the Younger Dryas Ice limit, there is no evidence recorded of Late Devensian sea levels above present. The Holocene maximum here was around c. 2.5 m above present. These observations of sea-level change, all standardised to change in mean sea level relative to present, constrain the glacio-hydroisostatic rebound model parameters. Earth models comprising three mantle layers, with lateral viscosity and elastic parameters, give a satisfactory description of rebound. The parameters H1 (lithosphere thickness) = 65 km, η(um) (upper mantle viscosity) = 4 x 1020 Pa14C seconds and η(1m) (lower mantle viscosity) = 1022 Pa14C seconds give the best overall agreement but discrepancies between observations and predictions remain. An increase of 10% in ice thickness north of the Great Glen, compared to the previous optimum ice model, provides good agreement for many sites but important discrepancies remain for the northern sites and indicate inadequacies in the model of the British ice sheet. Several alternative ice models are examined but the various combinations of earth and ice-model illustrate the non-uniqueness of the solution. A combination of more extensive ice limits, especially onto the Hebridean Shelf and West Shetland Shelf, and some changes to ice thicknesses over the mainland should produce a better agreement, but the spatial coverage of observations remains a limitation to producing a unique solution. The characteristics of the Holocene highstand, age, duration and amplitude, at the different sites refutes the assumption that globally deglaciation ceased abruptly 7000 yr ago. The observations are consistent with an ice model that includes c. 3 m of melting over the last 7000 yr. (C) 2000 Elsevier Science Ltd.