Late Devensian and Holocene Records of Relative Sea-Level
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Quaternary Science Reviews 19 (2000) 1103}1135 Late Devensian and Holocene records of relative sea-level changes in northwest Scotland and their implications for glacio-hydro-isostatic modelling Ian Shennan! *, Kurt Lambeck", Ben Horton!, Jim Innes!, Jerry Lloyd!, Jenny McArthur!, Tony Purcell", Mairead Rutherford! !Environmental Research Centre, Department of Geography, University of Durham, Durham DH1 3LE, UK "Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia 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-hydro- isostatic rebound model parameters. Earth models comprising three mantle layers, with lateral viscosity and elastic parameters, give " g " ; a satisfactory description of rebound. The parameters H (lithosphere thickness) 65 km, (upper mantle viscosity) 4 10 Pa g " C seconds and (lower mantle viscosity) 10 Pa C 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 di!erent 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. ( 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction present. The main features are a rapid, c. 9 mm/C yr, fall of sea level before 10 kyr C BP, an almost station- Previous studies from northwest Scotland provide ary level in the early Holocene, a rise to a mid-Holocene a rich record of Late Devensian and Holocene relative maximum and then falling in the Late Holocene (e.g. sea-level changes from around 12 kyr C BP to the Shennan et al., 1995a). This general form is the result of the interplay of isostatic rebound and eustatic sea-level rise and this interplay has been quantitatively modelled * Corresponding author. Tel.: 0044-191-374-2496; fax: 0044-191-374- for di!erent parts of the Earth (e.g. Lambeck, 1993a, b; 2456. Peltier and Andrews, 1976; Peltier, 1998). Observations E-mail address: [email protected] (I. Shennan). of relative sea-level change provide constraints on both 0277-3791/00/$- see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 8 9 - X 1104 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 components, "rstly on the isostatic rebound and there- From initial "eld and laboratory investigations of fore on the mantle rheology model and ice model para- some 50 sites we selected those sites that were likely to meters, and secondly on the eustatic change, that is, on provide precise sea-level index points. Details of the the magnitudes of grounded ice volumes and rates of methods employed for establishing the age-height rela- melting of the global ice sheets. tionships of the sea-level indicators have been previously Observational constraints from much of northwest published (references in Table 1) and we present here only Scotland (Fig. 1) on past models have been limited to the summary litho- and bio-stratigraphy. Full details will a few isolated data points and on assumed ages for some be published elsewhere. All of the new sea-level index of the principal shorelines, in particular the `Main points (Table 2) are from either isolation basins or from Rock Platforma and the `Main Postglacial Shorelinea now elevated tidal marshes. Table 2 shows the vertical (Peacock, 1970; Robinson, 1977; Sissons and Dawson, relationship of each index point to the estimated tide 1981; Dawson, 1988; Lambeck, 1993a, b, 1995 and refer- level at which it formed and the di!erences for such levels ences therein; Stone et al., 1996). Broad agreement between the various sites. between the general form of observed and predicted Descriptions of sediments in the "eld followed Troels- relative sea-level changes has been achieved but signi"- Smith (1955), but those shown on the "gures are modi"ed cant discrepancies also have been identi"ed (Fig. 2) to include our interpretation of the sequences following and indicate that more precise observations could pro- biostratigraphic analysis and simpli"ed for the sake of vide important constraints on the Late Devensian ice clarity of reproduction. Preparation of samples for sheet over Scotland. More observations are now avail- microfossil (pollen, dino#agellate cysts, diatoms, able, based on investigations of a range of palaeoenviron- foraminifera and thecamoebians) analysis follows stan- ments that record relative sea-level changes (Fig. 1 and dard methods (Moore et al., 1991; Palmer and Abbott, Table 1). From this range of palaeoenvironments the 1986; Scott and Medioli, 1980) and, unless indicated in tidal marshes and isolation basins provide the most re- the text or "gures, the data shown are percentages based liable, precise sea-level index points. In contrast, wetland on a minimum count of 200 individuals per level. environments that formed behind coastal sand dunes or Our "eld sites fall into "ve areas: Kentra, Arisaig, gravel barriers contain important evidence of coastal Kintail, Applecross, and Coigach (Fig. 1). For each area evolution, but the sea-level change component is not we present new data and a reconstruction of relative easily separated from the e!ects of storms or variations in sea-level change based on the new and previously pub- sediment supply. lished data. Table 2 includes all the sea-level index points The aims of this paper are to compare these observa- from the "ve areas. All dates are listed with both conven- tions with the model predictions, to test the validity of the tional radiocarbon ages and calibrated ages based on latter and then infer from any discrepancy improvements Stuiver and Reimer (1993), their method A, using 95% to the model parameters. Finally we assess the implica- con"dence limits. We also applied the revised calib- tions of the results for palaeocoastline reconstructions ration of Hughen et al. (1998) for 8.9 to 12.7 kyr C BP. beyond the immediate area of NW Scotland. While the two calibrations match at 8.9 kyr C BP, and 9.9 kyr cal BP, they show a 400 yr di!erence for a sample dated 12.7 kyr C BP. This makes no fundamental 2. The 5eld data modi"cation to the interpretations of the sequences de- scribed the following sections. All radiocarbon-dated The area of northwest Scotland examined lies between samples mentioned in the text and shown on microfossil Ardnamurchan and the Beauly Firth (Fig. 1), with new diagrams are abbreviated to the minimum and maximum "eld data collected from "ve areas on the west coast. This calibrated ages shown on Table 2. The preliminary model selection was based on the following reasons. Over much predictions presented in later sections are all based on of Scotland there are well developed morphological the radiocarbon time scale, the time scale also used to shoreline features whose ages are poorly constrained and de"ne the history of the ice sheets. Use of the radiocarbon in particular there is a lack of radiocarbon dated relative time scale for the model predictions is discussed later. sea-level data of Late Devensian age. Recent studies from Table 2 also shows the indicative meaning of each index the Arisaig area (Fig. 2 and Table 1) show that in the point, i.e.