Late Devensian and Holocene Records of Relative Sea-Level

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 described 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. the present tide level represented the index northwest well constrained records can be obtained and point, interpreted from the litho- and bio-stratigraphic there is a range of palaeoenvironments preserved that data. These data also show the tendency of sea level, record relative sea-level change. Comparisons of obser- negative for a reduction in marine in#uence and positive vations and model predictions of relative sea-level for an increase, from which a relative fall or rise of sea change suggest that there is uncertainty over the dimen- level may be indicated. The indicative meanings allow sions of the Late Devensian ice over northwest Scotland direct comparison of di!erent types of index points by (Lambeck, 1995; Shennan et al., 1995a; Ballantyne et al., standardising the altitude to relative sea level (RSL) 1998). change from present (Table 2), with an error term that I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1105

Fig. 1. Location map of the "eld study area, with insets of the sites from which new sea-level data are described. includes the sum of all the quanti"ed height errors, in- 2.1. Kentra cluding "eld levelling, present tide heights, and interpretation of the indicative meaning (total error" The only new information is re-dating of the isolation ( # #2 (e e eL)). contact for the basin at Ardtoe. The original radiocarbon 1106 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

nges during the Holocene and back to approximately 12 kyr C BP (c. 14 kyr cal BP) in the Late Devensian, with the highest index point from the isolation basin at Rumach Meadhonach, sill at 17.8 m OD. Further "eld surveys revealed a series of isolation basins up to Upper Loch Dubh, sill at 36.5 m OD (Fig. 4). At Lochan a Chleirich, sill just above 40 m, diatom analyses record no marine sedimentation. Therefore, we infer that the marine limit in this area lies between these two altitudes. This "ts well with the terraces and banks of shingle and sand, interpreted as raised beaches, at 41 m OD just north of Arisaig (Peacock, 1970). All of the new data come from basins that show very similar lithostratigra- phy, comparable with that from Upper Loch Dubh which, from base to top comprises: (1) basal blue grey clay silt with a trace of sand; (2) green grey silty organic Fig. 2. Observations of relative sea-level (RSL) change for the Kentra limus; (3) thin blue grey silt with sand; (4) green brown } Arisaig area provides an independent test of models of Late Deven- sian and Holocene isostasy, ice loads and relative sea-level change. The organic limus with herbaceous detritus; (5) brown Sphag- observations are from a number of recent publications (summarised by num peat; (6) dark brown herbaceous peat with wood. In Shennan et al., 1995a, b, 1999a) and were not used in the calibration of addition, the original cores from the basins at Loch nan the models used for the predictions (Lambeck, 1995). Predictions and Eala and Rumach were resampled and re-dated using the observations show good agreement in the general form of relative AMS radiocarbon technique. sea-level change through time, but with signi"cant di!erences in the magnitude and age of the turning points and the age of the rapid Late Diatom analyses for each of the new basins, and the Devensian relative fall in sea level. The predicted altitude of the mid- foraminifera and charophyte oogonia where present, Holocene maximum is in close agreement with the observations, show that in each case the transition between lithostrati- though the latter record a much #atter peak, i.e. of longer duration. The graphic units 1 and 2 represents part of the isolation di!erences between the predictions and observations could indicate process (Fig. 5). The samples dated come from the lithos- a larger ice mass, by approximately 15%, in the area to the NW of the Great Glen at the last ice maximum, in comparison to current ice sheet tratigraphic boundary (i.e. the lowest level within reconstructions (Lambeck, 1995). A larger ice mass could also explain su$cient organic material to date). Variations in basin the di!erences between the predictions and observations from Fear- hydrology, balancing tidal input against freshwater dis- nbeg and around Inverness (Firth and Haggart, 1989; Lambeck, 1995; charge, and organic productivity will in#uence both the Shennan et al., 1996a). age and the indicative meaning of the index point. How- ever, the altitude parameter is well constrained by the staircase of basins. Where the lithostratigraphic bound- date was for a 3 cm slice directly above the isolation ary, and therefore the dated sample, is coincident with contact analysed using conventional radiocarbon the decline from marine-dominated diatom assemblages, methods (Shennan et al., 1996b), whereas the new date is the radiocarbon age is always the oldest for the cluster of an AMS date for a 1 cm thick sample on the isolation basins at similar altitudes, for example Lochan nan Tri contact. This provides a better age, older by over 700 yr Chriochan, Loch Dubh and Loch a Mhuilinn (Fig. 5). (Table 3), for the relative fall of sea level across the sill of These date an earlier stage in the isolation process com- the basin because the original sample obviously includes pared to the other basins. The scatter of sea-level index more post-dating the isolation of the basin. points (Fig. 6) is consistent with these variations, allow- Apart from the relative fall of sea level recorded at ing for indicative meanings from mean high water of Ardtoe, and a limiting date from Kentra Moss core 32 spring tides for those samples recording the later stage of (Shennan et al., 1995b) the relative sea-level record is the isolation process, and mean high water of neap tides sparse until the fall from the mid Holocene maximum, for those relating to the earlier stage. after which it is well constrained. (Fig. 3). In addition, the oldest dates from the highest basins, Upper Loch Dubh and Lochan nan Tri Chriochan, con- 2.2. Arisaig strain the ice sheet reconstructions, giving a minimum age (c. 15.5 to 16.25 kyr cal BP) for deglaciation in the The rocky glacially eroded landscape of the Arisaig area. Freshwater, including halophobic species charac- area provides a series of rock-lipped depressions that terise the higher clastic layer, unit 3. This accumulated have accumulated shallow marine, intertidal, lacustrine during the Younger Dryas (Loch Lomond) Stadial when and terrestrial sediments since the time of deglaciation. an ice sheet reached to within 6 km of the site. Previous studies (Shennan et al., 1993, 1994, 1995a, b, The relative sea-level plot for the Arisaig area (Fig. 6) 1996a, b, 1999a) securely constrain relative sea-level cha- also includes seven replacement dates obtained using I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1107

Table 1 Published studies and sites presented in this paper with radiocarbon-dated reconstructions of relative sea-level change in north and northwest Scotland

Area and sites Type of palaeoenvironment Summary of relative sea-level record Reference

Kentra Kentra Moss Tidal marshes Late Holocene RSL fall Shennan et al. (1995b) Ardtoe Isolation basin Late Devensian RSL fall Shennan et al. (1996b) Arisaig Loch nan Eala Isolation basins Late Devensian RSL fall, early Holocene rise, Shennan et al. (1994) late Holocene fall Glenancross Wetland and barrier Mid Holocene RSL maximum Shennan et al. (1995a) Mointeach Mhor Wetland and barrier Mid Holocene RSL maximum Shennan et al. (1995a) Gartenachullish Wetland and barrier Early Holocene RSL rise Shennan et al. (1999a) Rumach Isolation basins Late Devensian RSL fall Shennan et al. (1993, 1996a) 7 new sites Isolation basins Late Devensian RSL fall This paper Kintail Loch nan Corr Isolation basin Early Holocene RSL rise through to Late This paper Holocene RSL fall Kirkton Tidal marsh Late Holocene RSL fall This paper Applecross Fearnmore Tidal marsh Late Holocene RSL fall This paper Fearnbeg Isolation basin Late Devensian RSL fall Shennan et al. (1996a) Coigach Dubh Lochan Isolation basin Early Holocene RSL rise through to Late This paper Holocene RSL fall Loch Raa Tidal marsh Late Devensian RSL fall This paper Badentarbat Wetland and barrier Mid Holocene barrier evolution and sea-level This paper trend Wick River Estuarine tidal marshes Mid Holocene RSL rise, late Holocene Dawson and Smith (1997) maximum then fall Dornoch Firth Estuarine tidal marshes RSL fall to early Holocene minimum, rise Smith et al. (1992) to mid-Holocene maximum then fall Beauly Firth Estuarine tidal marshes RSL fall to early Holocene minimum, rise Firth and Haggart (1989) to mid Holocene maximum then fall Scapa Bay, Orkney Wetland and barrier Holocene barrier evolution and sea-level trend De La Vega and Smith (1996)

AMS rather than conventional radiocarbon techniques level and mean high water of spring tides. Numerous (Table 3). Although not dated directly, biostratigraphic authors support a model of rapid coastal erosion during data constrain the altitude of the Late Devensian / early the Younger Dryas (e.g. Dawson, 1988, 1994; Stone et al., Holocene sea-level minimum. Dino#agellate cyst and 1996). Fig. 6 shows relative sea level within this height diatom evidence from the Rumach VI basin (Shennan range for a long period: while gradually falling through et al., 1999a) show that highest tides and surges crossed the Younger Dryas (c. 11.7 to 13 kyr cal BP), through the the sill throughout the accumulation of the organic sedi- early Holocene minimum, and for much of the rise to the ment from which the lower boundary dates the relative mid-Holocene maximum. This allows for a model with fall of sea level and the upper boundary the rise (Table 1). rapid coastal erosion during the Younger Dryas followed The microfossil assemblages constrain the altitude of the by a long period of intertidal erosion and transport early Holocene relative sea-level minimum to c. 2.5 to processes under interglacial conditions. 3.0 m above present. These data help resolve the debate regarding the age and mode of formation of the `Main 2.3. Kintail Rock Platforma (Main Lateglacial Shoreline). Although this fossil shore platform is better developed at sites Loch nan Corr and Kirkton provide complementary further south, there are numerous fragments between and contrasting evidence of Holocene relative sea-level Kentra and Arisaig around 5 m OD (Dawson, change. Bennett and Boulton (1993) record the Younger 1988,1994). For comparison with the relative sea-level Dryas (Loch Lomond) Stadial ice limit between the two plot (Fig. 6) this represents 2.6 } 4.7 m above present sites, although May et al. (1993) suggest a possible limit assuming the shoreline was formed between mean tide further west. Loch nan Corr is an isolation basin at the 1108

Table 2 Sea-level index points

Labora- C age BP$1p Calibrated age yr cal BP Altitude and thickness Indicative meaning, to OD and Tendency RSL change from tory code relative to tide level of sea level present

max. mean min. m OD m m OD m m$error

Kentra Kentra Moss 21-1 SRR4722 1080 40 1063 966 927 3.87 0.03 2.88 MHWST #0.50 } 0.99 0.21 Kentra Moss 21-2 SRR4723 1370 45 1336 1288 1182 3.75 0.02 2.73 MHWST #0.35 } 1.02 0.21 .Senne al. et Shennan I. Kentra Moss 21-3 SRR4724 1480 40 1413 1345 1295 3.66 0.03 2.58 MHWST #0.20 } 1.08 0.21 Kentra Moss 3-1 SRR4732 2255 45 2346 2251 2139 5.05 0.02 2.88 MHWST #0.50 } 2.17 0.21 Kentra Moss 3-2 SRR4733 2410 45 2711 2358 2340 5.00 0.03 2.58 MHWST #0.20 } 2.42 0.21 Kentra Moss 5-1 SRR4735 3065 45 3368 3296 3089 6.39 0.02 2.88 MHWST #0.50 } 3.51 0.21 Kentra Moss 5-2 SRR4736 3195 40 3471 3388 3346 6.35 0.03 2.58 MHWST #0.20 } 3.77 0.21

Kentra Moss 39-1 SRR4730 3435 45 3827 3668 3569 6.87 0.03 2.88 MHWST #0.50 } 3.99 0.21 / Kentra Moss 38-1 SRR4728 3515 45 3891 3772 3639 7.20 0.02 2.88 MHWST #0.50 } 4.32 0.21 Re Science Quaternary Kentra Moss 38-2 SRR4729 3730 45 4227 4039 3924 7.14 0.03 2.58 MHWST #0.20 } 4.56 0.21 Kentra Moss 37-2 SRR4727 3860 45 4411 4264 4093 7.69 0.03 2.58 MHWST #0.20 } 5.11 0.21 Kentra Moss 37-1 SRR4726 3880 45 4416 4321 4099 7.76 0.03 2.88 MHWST #0.50 } 4.88 0.21 Kentra Moss 39-2 SRR4731 3940 45 4513 4408 4236 6.65 0.03 2.18 MHWST !0.20 } 4.47 0.21 Kentra Moss 32-1 SRR4725 8320 45 9435 9311 9055 9.45 0.04 4.08 MHWST #1.70 L 5.37 0.21 Ardtoe 94-1 AA28095 12540 90 15109 14710 14329 20.60 0.01 1.19 MHWNT } 19.41 0.40 v Arisaig 1103 (2000) 19 iews Mointeach Mhor SRR4855 2565 45 2758 2737 2484 7.33 0.03 2.88 MHWST #0.50 } 4.45 0.21 Rumach VI Be91304 2670 50 2856 2763 2739 4.80 0.01 1.98 MHWST !0.40 } 2.82 0.40 Mointeach Mhor SRR4856 3005 45 3340 3188 3021 7.23 0.03 2.73 MHWST #0.35 } 4.50 0.21 Rumach VI SRR5487 3350 70 3811 3574 3399 4.80 0.02 1.18 MHWNT } 3.62 0.40 Loch nan Eala 1B SRR4737 3440 50 3831 3689 3566 5.20 0.04 1.58 MHWNT #0.40 } 3.62 0.57 Loch nan Eala 29B SRR4740 3600 45 4064 3885 3728 6.64 0.04 2.73 MHWST #0.35 } 3.91 0.41 } Loch nan Eala 16B SRR4738 3745 50 4237 4088 3926 5.20 0.04 1.18 MHWNT } 4.02 0.57 1135 Loch nan Eala 92-65 AA28099 4245 50 4868 4832 4579 6.30 0.01 1.18 MHWNT } 5.12 0.40 Mointeach Mhor 45-1 SRR4895 4640 45 5560 5319 5287 8.88 0.03 2.58 MHWST #0.20 } 6.30 0.21 Rumach VI Be91305 4760 40 5592 5527 5327 4.80 0.01 !1.02 MLWNT !0.50 } 5.82 0.40 Glenanacross 2-2 SRR4858 5285 45 6185 6101 6005 9.19 0.04 2.88 MHWST #0.50 } 6.31 0.21 Rumach VI Be91306 5690 50 6632 6467 6348 4.80 0.01 !1.52 MLWST #0.40 } 6.32 0.40 Glenanacross 2-1 SRR4859 5805 50 6736 6638 6479 9.03 0.04 2.58 MHWST #0.20 } 6.45 0.21 Mointeach Mhor 45-3 SRR4896 6625 45 7539 7495 7389 8.61 0.03 2.58 MHWST #0.20 # 6.03 0.21 Loch nan Eala 66B-3 SRR4863 6630 50 7543 7495 7388 6.27 0.05 0.58 MTL #0.30 ! / # 5.69 0.60 Gartenachullish UB4031 7270 80 8170 8035 7908 7.99 0.02 2.58 MHWST #0.20 } 5.41 0.21 Loch nan Eala 92-65 AA28100 8115 75 9251 8990 8716 6.30 0.01 0.93 MHWNT !0.25 # 5.37 0.40 Loch nan Eala 67-2 SRR4864 8310 45 9433 9312 9051 6.27 0.04 1.18 MHWNT # 5.09 0.40 Rumach VI SRR5488 8790 70 9962 9750 9532 4.80 0.03 1.18 MHWNT # 3.62 0.50 Loch nan Eala 92-200B AA28097 8925 65 10013 9925 9689 5.20 0.01 1.18 MHWNT # 4.02 0.40 Rumach VI SRR5489 9005 55 10040 9980 9899 4.80 0.03 1.78 MHWST !0.60 } 3.02 0.50 Loch nan Eala 92-200B AA28096 10250 70 12347 12060 11347 5.20 0.01 1.18 MHWNT } 4.02 0.40 Rumach VI Be91307 10290 60 12378 12140 11748 4.80 0.01 1.18 MHWNT } 3.62 0.50 Loch nan Eala 67-1 SRR4865 10500 90 12645 12420 12117 6.27 0.04 0.93 MHWNT !0.25 } 5.34 0.40 Rumach Iochdar 92-5B AA28092 10980 85 13086 12900 12705 9.30 0.01 1.18 MHWNT } 8.12 0.40 Rumach V SRR5169 11894 65 14151 13870 13621 16.30 0.02 1.18 MHWNT } 15.12 0.40 Rumach V SRR5168 11895 95 14201 13870 13580 16.30 0.02 1.18 MHWNT } 15.12 0.40 Loch A'Mhuilinn 95-2 AA22339 12390 85 14882 14490 14153 15.50 0.005 1.18 MHWNT } 14.32 0.40 Rumach Meadhonach 92-12 AA28093 12605 85 15193 14804 14419 17.80 0.01 1.18 MHWNT } 16.62 0.40 Loch Torr A'Bheithe 95-5 AA22346 12630 95 15251 14843 14435 24.00 0.01 2.38 MHWST } 21.62 0.40 Rumach IV 93-F5 AA28094 12730 85 15378 15001 14585 16.30 0.01 1.18 MHWNT } 15.12 0.40 Torr A'Bheithe 95-35 AA22345 12750 95 15429 15033 14595 33.20 0.005 2.38 MHWST } 30.82 0.40 Allt Achadh Na Toine 95-1 AA22342 12890 100 15646 15259 14797 22.50 0.005 1.78 MHWNT #0.60 } 20.72 0.40 Loch Dubh 95-3 AA22341 13080 100 15924 15563 15132 20.36 0.005 1.18 MHWNT } 19.18 0.40 Upper Loch Dubh 95-4 AA22343 13280 110 16220 15859 15458 36.48 0.005 2.38 MHWST } 34.10 0.40

Lochan nan Tri Chriochan 95-52 AA22344 13320 100 16251 15916 15554 33.40 0.005 1.18 MHWNT } 32.22 0.40 al. et Shennan I.

Kintail Loch nan Corr 96-100 AA23868 710 60 725 660 552 2.70 0.01 2.60 MHWST } 0.11 1.00 Kirkton, KT96-6 AA27216 880 70 934 774 665 3.28 0.01 3.10 MHWST 0.50 } 0.19 0.21 Kirkton, KT96-11 AA27217 1150 110 1288 1059 794 3.88 0.01 3.10 MHWST #0.50 0.79 0.21 /

Kirkton, KT96-6 AA28091 1495 45 1504 1352 1297 3.05 0.01 2.80 MHWST } 0.25 0.21 Re Science Quaternary Kirkton, KT96-11 AA27215 1730 70 1816 1658 1503 3.76 0.01 2.80 MHWST #0.20 } 0.96 0.21 Loch nan Corr, LC96-2 AA23869 3950 80 4787 4410 4146 2.70 0.01 !0.19 MTL !0.50 } 2.89 1.00 Loch nan Corr, LC96-2 AA23870 6085 75 7169 6905 6752 2.70 0.01 !0.94 MTL !1.25 } 3.64 1.00 Loch nan Corr, LC96-2 AA25605 7250 60 8131 8031 7916 2.70 0.01 !1.19 MTL !1.50 # / ! 3.89 1.00 Loch nan Corr, LC96-2 CAM38851 7280 40 8129 8036 7947 2.70 0.01 !1.19 MTL !1.50 # / ! 3.89 1.00 Loch nan Corr, LC96-2 AA23871 8370 80 9490 9406 9050 2.70 0.01 !0.19 MTL !0.50 # 2.89 1.00 Loch nan Corr, LC96-2 AA23872 8480 95 9641 9450 9253 2.70 0.01 !0.19 MTL !0.50 # 2.89 1.00 v es1 20)1103 (2000) 19 iews Applecross Fearnmore, FM96-52 AA27587 3415 50 3824 3664 3479 4.30 0.01 2.78 MHWST #0.20 } 1.52 0.41 Fearnmore, FM96-52 AA27220 3425 75 3858 3664 3471 4.38 0.01 2.93 MHWST #0.35 } 1.45 0.41 Fearnmore, FM96-18 AA27214 3855 70 4433 4252 3996 5.17 0.01 3.08 MHWST #0.50 } 2.09 0.41 Fearnmore, FM96-18 AA27219 4145 65 4839 4692 4444 5.01 0.01 2.93 MHWST #0.35 } 2.08 0.41 Fearnmore, FM96-18 AA27218 4330 80 5243 4864 4649 4.91 0.01 2.78 MHWST #0.20 } 2.13 0.41

Fearnbeg, FB93-1 AA28102 11980 80 14288 13970 13690 5.70 0.01 1.23 MHWNT } 4.47 0.54 } Fearnbeg, FB93-1 AA28101 12280 75 14714 14340 14037 5.70 0.01 !0.25 MTL !0.50 } 5.95 0.70 1135

Coigach Loch Raa,LR96-4 AA27222 4020 55 4804 4477 4354 5.09 0.01 2.58 MHWST #0.20 } 2.52 0.25 Loch Raa, LR96-1 AA27221 4030 110 4834 4478 4153 4.16 0.01 2.78 MHWST #0.40 } 1.39 0.25 Loch Raa, LR96-8 AA27223 4235 50 4866 4829 4575 3.62 0.01 2.58 MHWST #0.20 } 1.05 0.25 Dubh Lochan, DHL96-17 AA23873 4250 60 4961 4833 4574 3.69 0.01 2.38 MHWST } 1.32 0.43 Dubh Lochan, DHL96-17 AA23874 5265 60 6192 6009 5913 3.69 0.01 1.93 MHWNT #0.80 } 1.77 0.43 Dubh Lochan, DHL96-17 CAM38852 7500 50 8370 8250 8135 3.69 0.01 1.53 MHWNT #0.40 # 2.17 0.43 Dubh Lochan, DHL96-17 AA23875 8915 75 10030 9920 9661 3.69 0.01 2.97 'HAT L 0.72 0.52 Badentarbat SRR5486 5035 50 5910 5836 5652 0.9 0.03 0.23 MTL } 0.67 1.50 1109 1110 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

cies Cibicides lobatulus and Elphidium macellum; a lagoonal assemblage dominated by Haynesina germanica; saltmarsh species Jadammina macrescens and Miliammina fusca; freshwater thecamoebians dominated by Cen- tropyxis aculeata. The lower boundary of the latter assemblages is dated 0.55}0.725 kyr cal BP. From the microfossil assemblages we attribute di!erent indicative meanings for the dated samples (Table 2), ranging from below mean sea level, when nearshore shelf species dom- inate, gradually through the stages of isolation to the present, where the sill is just above mean high water of spring tides. The two dates from 678 cm are signi"cant. The ages are identical, yet one is a date an a sample of the bulk Fig. 3. Relative sea level (RSL) observations from the Kentra area. sediment, an organic limnic mud, and the other on tests Data from Table 2, giving altitudes standardised to change relative to of the calcareous foraminifera Cibicides lobatulus. These present. The ᭿ symbol indicates a maximum limit for RSL based on the base of a peat layer from which microfossil data show that it formed suggest that in shallow marine marginal environments above sea level at Kentra Moss (Shennan et al., 1995b). such as isolation basins, the mixing of sea water is such that the marine reservoir e!ect is negligible. Alterna- tively, the reservoir e!ect may be o!set by old carbon in eastern end of Loch Duich (Fig. 7), whereas Kirkton is the limnic mud, but we have no evidence to discriminate a raised tidal marsh 12 km to the northwest on the north between these explanations. shore of Loch Alsh, approximately 3 km west of an exposure of clay, with a Holocene marine fauna, approx- 2.3.2. Kirkton imately 1.5 m above high water mark described by A series of 12 cores (Fig. 7) extending almost 500 m Baden-Powell (1937). landward from the current tidal marsh shows a stratigraphy directly comparable to that described at Kentra 2.3.1. Loch nan Corr Moss (Shennan et al., 1995a, b). All cores "nished in sand The rock sill of the basin is at #2.70 m OD, only or gravel or were stopped on rock. Above the sand or 0.08 m above present mean high water of spring tides. gravel there is a partly organic partly minerogenic unit. Core LC96-2, from the edge of the lake, provides a com- This has a variable grain size distribution, ranging from prehensive record of relative sea-level change well illus- clay to "ne gravel in di!erent proportions in di!erent trated by the foraminiferal and thecamoebian cores. The minerogenic component "nes and decreases biostratigraphy (Fig. 8). The site lies within the Younger up core to a humi"ed surface peat with herbaceous root- Dryas ice limit (Bennett and Boulton, 1993; May et al., lets and Sphagnum macrofossils. Peat cutting over recent 1993), and therefore we should expect only a Holocene centuries has removed much of the surface peat and this record. The date from the lowest organic deposit, restricts the choice of cores for biostratigraphic and 9.25}9.65 kyr cal BP (Table 2), agrees with this but we radiocarbon analysis. Cores KT96-6 and KT96-11 have infer that the complete record is not present in core su$cient undisturbed surface peat and together cover LC96-2 because there is no record for the period back to most of the altitudinal range of the transition from the the opening of the Holocene and no bio-stratigraphic peat to the underlying minerogenic sequence, between evidence of a transgression into the basin following ice approximately 3.0 and 3.8 m OD. Current MHWST is retreat. A core from the centre of the lake is more likely to 2.57 m OD. include sediments covering this part of the sequence but No diatoms or foraminifera are preserved in either we have yet to obtain one. core but the pollen assemblages provide a comprehensive From 770 cm to 746 cm the transition from mainly record of relative sea-level fall in both cores (Fig. 9). lagoonal foraminifera to nearshore shelf species indicates Comparison with the assemblages from Kentra Moss an increase in tidal input to the basin, representing a rela- (Shennan et al., 1995a,b; Innes et al., 1996) provides the tive rise in sea level. The maximum tidal input, and indicative meaning for the four radiocarbon dated sea- therefore maximum sea level, occurs at 678 cm level index points (Table 1). The lowest dated sample in (7.9}8.1 kyr cal BP), indicated by minimum frequencies of each core records a regressive sequence from minerogenic saltmarsh and lagoonal species and a diverse assemblage tidal #at sedimentation to salt marsh, typi"ed by of nearshore shelf species (Fig. 8 shows only the two most abundant Plantago maritima pollen, at &20 cm above abundant species). After this the basin becomes slowly MHWST. The upper index point from KT96-11, with isolated from tidal input, illustrated by the following a pollen assemblage dominated by Gramineae, rising sequence of dominant assemblages: nearshore shelf spe- Alnus, and Plantago maritima at low and declining I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1111

Fig. 4. Arisaig site map, showing the locations of isolation basins sampled and their sill altitudes (m OD). The insets show for one basin, Upper Loch Dubh, the reconstruction of the basin stratigraphy from a transect of boreholes. One location, in this case core 4, was then selected to represent the sequence and resampled to provide sediment for further analyses. The same approach was used for each basin. frequencies, represents a level &50 cm above MHWST is 12 km northwest of Loch nan Corr there is likely to be (approximately HAT). The uppermost pollen sample in- some di!erential glacioisostatic movement between the dicates the continued regression with the "rst sign of two sites but this may lie within the error range of the raised Sphagnum bog. The sequence from KT96-6 shows reconstructed relative sea levels. a comparable regressive succession, with dominant Gramineae and Cyperaceae, Plantago maritima at low 2.4. Applecross and declining frequencies, followed by the transition directly to a Calluna mire which represents approximately At the northern end of the Applecross peninsula two HAT and is dated by the upper index point. sites, Fearnbeg and Fearnmore (Fig. 11), provide a record The ten sea-level index points from Loch nan Corr and of Late Devensian and Holocene relative sea-level Kirkton show a consistent record (Fig. 10). Since Kirkton change. 1112 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 5. Summary of microfossil (diatom, foraminifera and charophyte), radiocarbon and altitude data for seven isolation basins from the Arisaig area. The altitude refers to the rock sill of the basin. Calibrated ages are shown in Table 2. In each case the date comes from the transition from clastic sediment to organic limus. Diatom salinity classes, based on % total diatoms counted at each level, show trends from the most marine species (polyhalobian class), through brackish (mesohalobian), to more freshwater (oligohalobian) and salt-intolerant species (halophobe). Foraminifera and charophyte oogonia indicate respectively marine and freshwater conditions in the basin. The microfossil data help de"ne the indicative meanings (method described in Shennan et al., 1999a) used to calculate the changes in relative sea level shown in Table 2 and Fig. 6. I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1113

cutting of the surface peat limits the choice of cores for microfossil and radiocarbon analyses. Core FM96-18 records the highest altitude of the sandy silty peat unit (Fig. 11). Samples for pollen and diatom analyses show that in the cores where the peat lies directly on a coarse sand or sand and gravel the peat formed in a freshwater bog environment. In core FM96-18 pollen, diatom and foraminifera data (summarised in Fig. 12) record a regressive sequence from tidal #at, indicative of the contemporaneous MHWST, through salt marsh to raised bog, at about contemporaneous HAT, comparable with those described from Kentra Moss and Kirkton. Regular tidal inundation, indicated by the upper limit of marsh foraminifera and transition to freshwater diatoms, ceased Fig. 6. Relative sea level (RSL) observations from the Arisaig area. c. 4.4}4.8 kyr cal BP (Fig. 12). Occasional inundation by Data from Table 2, giving altitudes standardised to change relative to extreme tides continued until the development of raised ᭿ present. The symbol indicates a minimum for RSL based on bio- bog, illustrated by the rise in Sphagnum spores and stratigraphic evidence from the lowest isolation basin, Rumach VI # (Shennan et al., 1999a). halophobic diatoms at 157 cm, 5.17 m OD. These data concur with the observation from Fearnbeg, that the Holocene relative sea-level maximum was less than 5.70 m OD. 2.4.1. Fearnbeg Below the altitude of FM96-18 pollen and diatom Fearnbeg is an isolation basin with a sill at 5.7 m OD samples from the base of the peat show that a compara- (Fig. 11b). Diatom analyses show gradual isolation ble regressive sequence occurs in the cores down to the through more than 20 cm of clastic and then more present shore. Only core FM96-52, from an adjacent organic sediments (Shennan et al., 1996a). Two AMS transect in the small valley adjacent to the one shown in radiocarbon ages replace the original conventional date, the transect, had su$cient surface peat remaining to which had a large standard error (Tables 2 and 3). These provide an uncontaminated radiocarbon date. Two sam- two dates record a relative fall in sea level of about 1.5 m. ples from the regressive sequence in FM96-52 give dated The older sample (c. 14.0}14.7 kyr cal BP) has a diatom sea-level index points for the transition from tidal #at assemblage which indicates that the sill of the basin was through salt marsh (Table 2). below MTL, whereas the assemblages from the younger The six sea-level index points from Fearnmore and sample (c. 13.7}14.3 kyr cal BP) indicate that the sill was Fearnbeg record Late Devensian and late Holocene rela- around MHWNT. There is no evidence for a Holocene tive falls in sea level but no direct evidence from the transgression back into the basin. This suggests the mid- intervening period during part of which sea level would Holocene relative sea-level maximum lies below 5.7 m have risen (Fig. 13). OD, an observation supported by the evidence from Fearnmore (below) where the highest level sea-level index point is at 5.17 m OD (core FM96-18, Table 2 and 2.5. Coigach Fig. 11). Shoreline fragments to the east, at the head of Loch Torridon (Fig. 1) lie at 7.07 m OD (Robinson, Although much of the coastline of Coigach comprises 1977). Assuming they formed at a level at or above mean low cli!s there are a number of sheltered embayments high water of spring tides the di!erence in height is with accumulations of unconsolidated sediments. Field compatible with estimates of the shoreline gradient (e.g. investigations reveal three main types of sites: an isola- Firth et al., 1993). tion basin; a raised tidal marsh; and a number of wet- lands behind gravel barriers. While the "rst two usually 2.4.2. Fearnmore provide excellent sea-level index points, the latter fre- At Fearnmore surface peat overlying organic sands quently reveal complex interrelationships between sedi- occurs in small valleys between low rock ridges. Core ment supply, coastal processes and relative sea-level transects show that close to the present shoreline the change (e.g. Shennan et al., 1999a). The sequences at surface herbaceous peat grades downward through Dubh Lochan, Loch Raa, and Badentarbat record a his- a sandy, silty peat with abundant herbaceous rootlets, to tory of Holocene relative sea-level change (Fig. 14). In a slightly organic sand. The altitude of the peat-sand addition, a well-de"ned raised shoreline occurs at c. 5.2 m transition increases away from the shore until the sandy OD around Achnahaird Bay. Current MHWST at Ul- silty peat unit disappears and the herbaceous peat rests lapool some 30 km to the southeast is 2.45 m OD and directly on a coarse sand or sand and gravel. Recent 2.30 m OD at Loch Nedd, 30 km to the north}northeast. 1114 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

the bay via a stream that cuts through a small vegetated gravel ridge down to a rock sill visible at 3.69 m OD. A stratigraphic survey reveals a basin behind the rock sill (Fig. 14a). In the deepest part of the basin there is a sequence that comprises a basal sand, a lower limus with a 2 cm intercalated organic silt horizon, a grey sand with abundant gastropods and foraminifera, a brown limus with a silt component that decreases upcore, and "nally a surface herbaceous peat. Pollen, foraminifera and thecamoebian assemblages record the following sequence of environmental changes (Fig. 15). Above the unfossiliferous basal sand the lower limnic unit contains freshwater microfossils, especially Pediastrum, Myriophyllum alterniyorum and charophyte oogonia. Changes in the abundance of Cyperaceae, Em- petrum and Juniperus pollen suggest that the intercalated organic silt represents the Younger Dryas Stadial. The top of the lower limnic unit is abrupt in all cores in which it is overlain by grey sand. None of the microfossils in this limnic unit suggest any saline input into the lake at that time. In contrast the overlying grey sand contains a rich assemblage of gastropods and calcareous foraminifera, especially Cibicides lobatulus. We interpret these data as indicative of a rising relative sea level that led to the construction of a small gravel barrier across the outlet. As sea level continued to rise the barrier was broken around 8.1}8.4 kyr cal BP, and tidal waters entered the basin, eroding the upper surface of the freshwater limus that is dated 9.7}10.0 kyr cal BP (Table 2 and Fig. 15). A regression is well underway by 5.9}6.2 kyr cal BP, the transition from the grey sand to the overlying silty limus which contains abundant marsh foraminifera. The regression continues until c. 4.6}5.0 kyr cal BP when the basin is isolated from tidal input across the sill and freshwater thecamoebians replace the marsh foraminifera. As with Loch nan Corr, we interpret the microfossil assemblages to assign the indicative meanings for the dated samples, in this case from below mean high water neap tides, following the breakthrough of the barrier, to mean high water of spring tides by the "nal isolation (Table 2). Late Devensian marine features around Little Loch Broom, around 27 km south, at 15.5 } 17 m OD (Sissons and Dawson, 1981) apparently contrast with the sequence from Dubh Lochan. Allowing for their `provis- ional gradient of 0.33 } 0.39 m/kma (p. 123), there ought to be pre-Younger Dryas shallow marine or inter-tidal Fig. 7. Kintail sites, showing details of Kirkton and Loch nan Corr sediments in the basin. In order to test this hypothesis (locations shown in Fig. 1). (a) transect of cores at Kirkton, stratigraphy of the raised marsh sequence, and location of cores sampled for further further, the basal sand sequence will require further in- analyses, 6 and 11. (b) location details for the isolation basin at Loch nan vestigation since no microfossils have been observed in Corr. Soft ground conditions and open water prevented sampling of a the samples collected so far. transect of cores, so only one entire core, LC96-2, was completed (lithology shown in Fig. 8). The out#ow stream crosses the rock sill of the basin. 2.5.2. Loch Raa At the south end of Achnahaird Bay, Loch Raa and 2.5.1. Dubh Lochan Loch Vatachan lie in the low area below the coll to Dubh Lochan is a small lake in a small embayment on Badentarbat Bay. Transects of cores at the southern end the east side of Achnahaird Bay. The lake discharges into of Loch Raa reveal a sequence of peat above minerogenic I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1115

Fig. 8. Summary microfossil (foraminifera and thecamoebians) diagram from Loch nan Corr. Only those species reaching '10% total count are shown. Lithology modi"ed from Troels-Smith (1955): cross-hatching * organic limus;LLL* silt and clay; horizontal dashes * dark, highly humi"ed organic deposit; dots * sand. Details of the radiocarbon dates, shown as calibrated age ranges and taken at the levels indicated, in Table 2. The sill of the basin is at 2.70 m OD.

sediments directly comparable with the previously de- a similar tidal range to today. It is also at the same scribed sites at Kentra Moss, Kirkton and Fearnmore. At altitude at the raised shoreline around Achnahaird Bay higher elevations, above c. 5.20 m OD, the surface peat for which a similar age and origin is therefore assumed. lies directly on sand. Single pollen and diatom samples As the altitude of the peat-sand boundary decreases a from the peat-sand boundary above this altitude (cores transitional organic silt occurs between the peat and LR96-6 and 7) reveal freshwater environments. This pro- the sand (Fig. 14b). In cores LR96-4, !1 and !8 the vides a limiting altitude for the mid-Holocene relative pollen and diatom assemblages show regressive se- sea-level maximum, about 2.6 m above present assuming quences from tidal #at through tidal marsh to freshwater 1116 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 9. Summary microfossil (pollen, spores and dino#agellate cysts) diagram from Kirkton, core 96-6 (Fig. 9a) and core 96-11 (Fig. 9b). Showing taxa reaching '10% total land pollen and additional taxa indicating the transition from intertidal to saltmarsh and bog environments. Below 80 cm (Fig. 9a) coarse sand prevented further sampling. Lithology modi"ed from Troels-Smith (1955) L L L * silt and clay; dots * sand; horizontal dashes * dark, highly humi"ed organic deposit; vertical dashes * herbaceous root macrofossils. Details of the radiocarbon dates, shown as calibrated age ranges and taken at the levels indicated, in Table 2.

show a transition from a low energy marine/brackish lagoon to a freshwater environment, and "nally a succession to bog (Fig. 17). The calibrated age, 5.7}5.9 kyr cal BP, records the end of the brackish phase. The dated sediment accumulated in an unknown depth of water. The age most probably relates to the "nal stage of barrier migration, closing o! a tidal connection to the back- barrier environment. Although barrier migration is in- #uenced by sea-level change there are other controlling processes, such as sediment supply. The dated sample cannot be accurately related to the contemporaneous tide level. The sea-level index points from Loch Raa and Dubh Lochan (Fig. 18) and the shoreline around Achnahaird Fig. 10. Relative sea level (RSL) observations from the Kintail area. Bay constrain the mid-Holocene sea level to a maximum Data from Table 2, giving altitudes standardised to change relative to of no more than&2.5 m above present with the regression present. The four index points from the raised tidal marsh at Kirkton underwayby5.9}6.2 kyr cal BP. The date from Baden- are those with the small vertical error bars. tarbat suggests that barrier migration was closely related to relative sea-level rise and the Holocene maximum. bog c. 4.2}4.8 kyr cal BP (Fig. 16 and Table 2), with sea level then falling to present. The height } age scatter they 2.6. Further sites in north and northwest scotland reveal cannot be resolved without further investigations. Previous studies from the Beauly Firth (Firth and 2.5.3. Badentarbat Haggart, 1989), the Dornoch Firth (Smith et al., 1992), This coastal wetland with a small lagoon lies behind Wick River (Dawson and Smith, 1997) and Scapa Bay, a gravel barrier that extends across the small valley at the Orkney (De La Vega and Smith, 1996) provide radiocar- head of Badentarbat Bay (Fig. 14c). Diatom assemblages bon dated relative sea-level index points (Table 4, and I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1117

Fig. 11. Detail of the Fearnmore and Fearnbeg sites in Applecross (general locations, Fig. 1). (a) Fearnmore, showing the location of cores, those samples for further analyses, 18 and 52, and a reconstructed stratigraphic section of the raised tidal marsh. (b) Fearnbeg, showing the location of cores, core 1 sampled for further analyses, and a reconstruction of the basin stratigraphy for the east-west transect. The sill of the basin is at 5.70 m OD at core 22, east of which the rock surface drops to the intertidal zone.

Fig. 19) that constrain model predictions (e.g. Lambeck, constraints on model predictions come from the limited 1995). In addition, morphological shoreline data provide Holocene evidence available from the Shetland Isles (e.g. further constraints, although they have not been directly Hoppe, 1965; Birnie et al., 1993) and the outer Hebrides dated and are not detailed here. The Beauly Firth (e.g. Ritchie, 1966, 1985; Gilbertson et al., 1996). Both radiocarbon-dated index points record a Late Devensian areas record relative submergence. relative fall in sea level, an early Holocene rise to a mid- Holocene maximum around 7 m above present. The height of the mid-Holocene maximum decreases north to 3. Models below 5 m in the Dornoch Firth, and at the Wick River there is no clear maximum but relative sea level was just High resolution glacio-isostatic rebound models have above present for some of the mid- to late-Holocene. In previously been developed for the British Isles and com- Orkney it is di$cult to eliminate the in#uence of barrier pared with a large data set of relative sea-level indicators development on sedimentation but nevertheless the data for England, Wales and Scotland (Lambeck, 1993a, b) reveal no Holocene sea level above present. Further and for Ireland (Lambeck, 1996). Comparisons of the 1118

Table 3 Sea-level index points re-dated using AMS methods. Details in italics for the original samples published in (Shennan et al., 1994, 1995b, 1996a, b).

Laboratory C age BP$1p Calibrated age yr cal BP Altitude and thickness Indicative meaning, to OD and Tendency RSL change from code relative to tide level of sea level present

max. mean min. m OD m m OD (m) m$error .Senne al. et Shennan I. Kentra Ardtoe 94}1 AA28095 12540 90 15109 14710 14329 20.60 0.01 1.19 MHWNT } 19.41 0.40 Ardtoe SRR5167 12040 110 14429 14040 13707 0.03

Arisaig

Loch nan Eala 92}65 AA28099 4245 50 4868 4832 4579 6.27 0.01 1.18 MHWNT } 5.12 0.40 / utraySineRe Science Quaternary LochnanEala66B}1 SRR4741 4010 50 4787 4479 4353 0.04

Loch nan Eala 92}65 AA28100 8115 75 9251 8990 8716 6.27 0.01 0.93 MHWNT }0.25 # 5.37 0.40 LochnanEala66B}2 SRR4742 8195 45 9362 9147 9067 0.04

Loch nan Eala 92}200B AA28097 8925 65 10013 9925 9689 5.20 0.01 1.18 MHWNT # 4.02 0.40 Loch nan Eala 01 UB3634 8743 149 9996 9747 9440 0.04 v es1 20)1103 (2000) 19 iews Loch nan Eala 92-200B AA28096 10250 70 12347 12060 11347 5.20 0.01 1.18 MHWNT } 4.02 0.40 Loch nan Eala 01 UB3633 10060 86 12109 11340 11001 0.04

Rumach Iochdar 92}5B AA28092 10980 85 13086 12900 12705 9.30 0.01 1.18 MHWNT } 8.12 0.40 Rumach Iochdar 5 SRR4862 10755 90 12885 12690 12462 0.04

Rumach Meadhonach 92}12 AA28093 12605 85 15193 14804 14419 17.80 0.01 1.18 MHWNT } 16.62 0.40 } Rumach Meadhonach UB3643 11820 145 14037 13905 13437 0.03 1135

Rumach IV 93}F5 AA28094 12730 85 15378 15001 14585 16.30 0.01 1.18 MHWNT } 15.12 0.40 Rumach IV SRR5170 11940 105 14137 13975 13817 0.03

Applecross Fearnbeg, FB93}1 AA28101 12280 75 14714 14340 14037 5.70 0.01 1.23 MHWNT } 4.47 0.54 Fearnbeg SRR5171 11920 125 14293 13900 13556 0.02 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1119

Fig. 12. Summary microfossil (pollen, spores, foraminifera and diatoms) diagram from Fearnmore, core 96-18 (Fig. 12a) and summary pollen diagram, core 96-52 (Fig. 12b). Showing taxa that indicate the transition from intertidal to saltmarsh and bog environments. Frequencies are calculated as % total land pollen, % total foraminifera or % total diatoms counted. Lithology modi"ed from Troels-Smith (1955) L L L * silt and clay; dots * sand; horizontal dashes * dark, highly humi"ed organic deposit; vertical dashes * herbaceous root macrofossils. Details of the radiocarbon dates, shown as calibrated age ranges and taken at the levels indicated, in Table 2.

observations with predictions has led to estimates of the that signi"cant discrepancies between observations and rheological response parameters for the mantle (Lam- model predictions were noted for northern Scotland, beck et al., 1996) as well as to estimates of the ice thick- particularly in the Beauly Firth area, where the model ness and the extent of the ice margins (Lambeck, predictions underestimated the rebound. This led to the 1991, 1995). The resulting models give a satisfactory suggestion that the ice thickness in the north of the description of sea-level change across the region except adopted model were inadequate although the dearth of 1120 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

for Scandinavia is based on the recent analysis by Lambeck et al. (1998) and the models for the other major ice sheets are the same as discussed in Lambeck (1993b). The ice loads over the British Isles and Fennoscandia are de"ned on a 25;25 km grid, whereas the more distant ice sheets are de"ned on a 100;100 km grid, mostly at intervals of 1000 yr since the time of the Last Glacial Maximum (LGM) and at longer intervals for the earlier period. The melt water from the global ice sheets is distributed into the oceans and the concomitant loading e!ects take

Fig. 13. Relative sea level (RSL) observations from the Applecross area. Data from Table 2, giving altitudes standardised to change relative to present. Absence of any marine sediments in the Fearnbeg basin after the isolation dated by the two Late Devensian index points indicates that even the highest tides during the period of the Holocene maximum did not cross the sill of the basin at #5.70 m OD (Table 2), which suggests that mean sea level was no higher than &3.0$0.5 m above present. The sediments at Fearnmore, and their biostratigraphy, suggest that the upper limit of Holocene saltmarsh sediments is at core 18 (Figs. 11a and 12a), where the regressive sequence from intertidal #at to raised bog occurs between #4.91 and #5.17 m OD. These raised marshes indicate a Holocene maximum mean sea level approximately 2.1$0.4 m above present (Table 2).

quantitative sea-level information for northern and northwestern Scotland did not make it possible to be precise about this de"ciency. Earth models comprising three mantle layers give a satisfactory description of the rebound and a greater strati"cation is not required to explain the observational evidence for the relative sea-level change of Britain (Lam- beck et al., 1996). The chosen earth model comprises an elastic lithosphere of e!ective thickness H , an upper mantle extending from the base of the lithosphere to the major 670 km seismic discontinuity with an e!ective vis- g cosity of , and a lower mantle with an e!ective viscos- g ity of . Within each of these three layers the density and elastic moduli vary with depth in accordance with seismic models of the mantle. Phase boundaries and other discontinuities within the mantle are assumed to behave as material boundaries on the time scales in question but models with isobaric boundary conditions (Johnston et al., 1997) lead to essentially the same solutions for the three mantle parameters H , g , g . Only Fig. 14. Coigach site map, showing details for Dubh Lochan, Loch models with lateral homogeneous viscosity and elastic Raa, and Badentarbat (locations shown in Fig. 1). (a) Dubh Lochan, parameters are considered. showing the location of cores, core 17 sampled for further analyses, and The ice model, BR-D, for the British Isles is based on a reconstructed stratigraphic section of the basin. (b) Loch Raa, show- that developed in Lambeck (1993b) but in which the ice ing the location of cores, those samples for further analyses, 1, 4 and 8, thickness north of the Great Glen has been increased and a reconstructed stratigraphic section of the raised tidal marsh. (c) Badentarbat, showing the location of cores, 1R sampled for further by about 10% because of the discrepancy between analyses, and a reconstructed stratigraphic section of the back barrier sea-level model predictions and observations in the sediments (it was not possible to di!erentiate whether cores ended on Beauly Firth area (Lambeck, 1995). The ice model rock or large gravel or boulders). I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1121

Fig. 14. Continued.

into account the time dependence of the shape of these increase in ocean volume over the past 6000 yr so as to basins, as well as the condition that the ocean surface is increase eustatic sea level by 3 m, with most of this an equipotential surface at all times. The total ice and change occurring between 6 and 2 ka BP. In the prelimi- water mass is conserved throughout. Both the ice and nary comparisons of model predictions with observa- water loads are expressed with a spatial resolution of tions discussed in the following section, this increase for harmonic degree 256, higher degree expansions not being the past 6000 yr has not been included. required. Changes in the equipotential surface, and hence The preliminary model predictions presented in the sea level, caused by changes in the Earth's rotation dur- following section are all based on the radiocarbon time ing the deglaciation and post-glaciation stages (Milne scale, the time scale also used to de"ne the history of the and Mitrovica, 1996) are not included since this e!ect ice sheets. The mantle viscosity estimates are therefore introduces minimal spatial variation in the sea-level re- given in units of Pa C seconds, a not wholly trivial sponse over the small area under consideration. The distinction since the time scale during the past 20 000 or eustatic sea-level function adopted is from Nakada and so years di!ers from the calendar time scale by about Lambeck (1988) and recent analysis (Fleming et al., 1998) 15% (e.g. Stuiver and Reimer, 1993; Hughen et al., 1998). has shown that this provides an adequate approximation Provided that the radiocarbon time scale di!ers from the to the time-dependence of the land-based ice volumes calendar time scale only in a linear way, and provided since the time of the LGM. This function includes an that the same time scale is used throughout, the use of the 1122 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 15. Summary microfossil (pollen, freshwater algae, foraminifera and thecamoebians) diagram from Dubh Lochan, showing taxa that illustrate the changes in saline water input into the basin. Pollen taxa Empetrum and Juniperus corroborate the age of the lowest radiocarbon date. Chenopodiaceae and Plantago maritima pollen indicate salt marsh environments fringing the basin. Pollen of Myriophyllum alterniyorum, colonies of the freshwater alga Pediastrum and freshwater thecamoebians indicate two episodes of freshwater environments. Foraminifera illustrate changes in the degree of marine incursion into the basin. Frequencies are calculated as % total land pollen and % total foraminifera#thecamoebians counted. Lithology modi"ed from Troels-Smith (1955): cross-hatching - organic limus; L L L * silt and clay; dots * sand. Details of the radiocarbon dates, shown as calibrated age ranges and taken at the levels indicated, in Table 2. The sill of the basin is at 3.69 m OD.

former does not change the results. Any non-linearities in BR-D, and without the eustatic sea-level correction. The the radiocarbon time scale do not introduce signi"cant data are divided into sub-regions corresponding to the errors into the analysis (Lambeck, 1998), provided that four main localities for the observational data, (i) Kentra realistic error estimates are assumed. Di$culties arising and Arisaig are shown on the same "gure because the from possible `plateaua e!ects of the radiocarbon time di!erences are small and give a longer time series, (ii) scale, in for example, earliest Holocene time (e.g. Stuiver Kintail, (iii) Applecross, and (iv) Coigach. Within each and Reimer, 1993; Hughen et al., 1998) are not resolved if locality the predictions are made for the speci"c site all ages are converted to the calendar time scale because co-ordinates corresponding to each observation such all the observational evidence is given with respect to that the spatial variation in the rebound across the local- radiocarbon time. ity is re#ected in a loss of smoothness in the height}age Where direct comparisons are made between the ob- function particularly for Late Devensian time (e.g. for the servations and predictions, the latter are for the exact Kentra and Arisaig sites). p geographic co-ordinates and times of the observed data The observational accuracies of the shoreline indi- points, even though observations from a given locality cators are estimated according to may be plotted on the same age}height diagrams. p "+p#p# f p, M F P (d /dt) R 3.1. A preliminary comparison of predicted and observed p p sea levels where F is the height-measurement variance, P is the variance associated with the uncertainty in the reduction Fig. 20 illustrates the comparisons of the predicted and of the sea-level indicator to mean sea level at the epoch in observed values for the new data from the northwest question, df/dt is the predicted rate of change in sea-level p coast of Scotland. In these preliminary comparisons be- change at the epoch in question and R is the variance of p tween predictions and observations, the former are based the age determination. For R a value of twice the formal on the &best-"tting' earth-model for the British Isles of standard deviation of the radiocarbon age determination p Lambeck (1998) (model E-0, Table 5), on the ice model is adopted. F is usually small but should include any I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1123

Fig. 16. Summary microfossil (pollen, spores and foraminifera test linings preserved in the pollen preparations) diagram from Loch Raa, core 96-1 (Fig. 16a), core 96}4 (Fig. 16b) and core 96}8 (Fig. 16c). Showing taxa that indicate the transition from intertidal to saltmarsh and bog environments. Frequencies are calculated as % total land pollen. Lithology modi"ed from Troels-Smith (1955) L L L * silt and clay; dots * sand; horizontal dashes * dark, highly humi"ed organic deposit; vertical dashes * herbaceous root macrofossils; diagonal dashes * wood root macrofossils. Details of the radiocarbon dates, shown as calibrated age ranges and taken at the levels indicated, in Table 2. 1124 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 17. Summary diatom diagram from Badentarbat showing species '10% total diatom valves. The individual species shown are included within the summary graphs of salinity classes, from the most marine species (polyhalobian class), through brackish (mesohalobian), to more freshwater (oligohalobian) and salt-intolerant species (halophobe). Lithology modi"ed from Troels-Smith (1955): cross-hatching * organic limus; L L L * silt and clay. Details of the radiocarbon date, shown as calibrated age range and taken at the level indicated, in Table 2.

uncertainty arising from the relation between the levell- sistent with the observational evidence. Similar Late De- ing datum and mean sea level, and a value of 0.5 m has vensian discrepancies have been previously noted for p been adopted. P takes into consideration any uncertain- other localities in the Wester Ross area (Lambeck, 1993b) ties arising from di!erences in tidal conditions at the past although the available observational evidence then was epoch from present conditions and a value of 2 m has limited to nominal ages for some of the raised shorelines been adopted here. (e.g. Sissons and Dawson, 1981). Such discrepancies for For Kentra-Arisaig and Kintail (Fig. 20a and b) agree- the more western of the localities, lying near the ice ment between predictions and observations is satisfac- margin during Late Devensian time, could, in principle, tory except that the predicted highstands at 6 ka CBP be a consequence of the adopted earth-model parameters exceed the observed values by a few metres, a discrepancy being inappropriate or of the ice-load model being inad- that would be largely removed if the eustatic correction equate. Thus the earth-model and ice-model depend- term is applied. At the other two localities, the agreement encies of the predictions will "rst be examined. between observations and predictions is unsatisfactory in that the observed Late Devensian data points lie consis- 3.2. Earth-model dependence tently above the predicted values. At both Applecross and Coigach the predicted Late Devensian and early The parameters E-0 summarised in Table 5 yield pre- Holocene levels are below present-day sea level, incon- dictions of sea-level change that give the best overall I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1125

discrepancy there is attributed to an inadequate choice of earth-model parameters. Such low H models do yield predictions that are inconsistent with the observations at Kentra, Arisaig and Kintail (Fig. 21). Also it is the Late Devensian and early-Holocene part of the observational sea-level record for localities elsewhere in northern Brit- ain that constrains the value for H adopted in model E-0. No combination of earth-model parameters, at least not within a range that is consistent with the data from the British Isles in general, leads to improved predictions for the northern sites of Coigach and Applecross. A general inference that can be drawn from both Fig. 18. Relative sea level observations from the Coigach area. Data from Table 2, giving altitudes standardised to change relative to pres- Figs. 20 and 21 is that the discrepancies between observa- ent. The data record a rise in relative sea level during the early Holo- tions and predictions are greater for the northern sites of cene to a maximum that is represented by both the morphological Coigach and Applecross, than for the more southern evidence, the raised shoreline around Achnahaird Bay, and the strati- localities of Kentra, Arisaig, and Kintail. This trend is graphic evidence shown here by the dated index points. The mor- similar, but more accentuated, to that previously noted phological shoreline is not dated directly, but shown on the graph with a minimum age taken from the evidence for falling sea level in the basin mainly for northeastern Scotland in the models of at Dubh Lochan. Lambeck (1993b, 1995). This points to inadequacies in the ice-sheet model used rather than to major inadequacies in the earth models. In particular, because the magnitude of the discrepancies change over rela- agreement with the observational data for the British tively short distances, this points to the limitations being Isles, but their uncertainties remain relatively large. Thus in the British ice sheet rather than in the more distant ice to examine whether the discrepancies between observa- sheets. tions and predictions noted above may be attributable to the choice of earth-model parameters, predictions are 3.3. Ice sheet model dependence also made for a series of parameters that encompass the optimum values (models E-1 to E-6, see Table 5). The The ice model BR-D used in the above predictions results are illustrated in Fig. 21 for the above four subsets of relative sea-level change is based on: (i) empirical of the observational data. All predictions are based on observations of the ice margin at the time of maximum the BR-D ice model for the British Isles (see above). glaciation and during the subsequent retreat, (ii) a few At all localities, the dependence of the predicted sea estimates of ice thickness at the time of maximum gla- levels on the lower-mantle viscosity is relatively minor ciation, and (iii) the assumption that the ice-height pro- (models E-5 and E-6; Fig. 21) and any variation in sea- "les can be characterised by quasi-parabolic functions level prediction within the range of 5;10}3;10 Pa (Lambeck, 1993b). Considerable uncertainty remains in C s does not lead to an improved comparison with the the ice model. In the context of the present investigation Late Devensian data from Applecross, nor with the early questions remain about the limits and thickness of the ice Holocene data from Coigach. Likewise, within the range over northern Scotland, including the extent of ice over of uncertainties in the estimate of the upper-mantle vis- the Orkney Islands and northeastern Caithness, the loca- cosity (models E-3 and E-4; Fig. 21) the predictions do tion of the ice margins to the west and northwest of not lead to much improvement in the comparisons for Scotland, and the ice thickness over the Outer Hebrides. the Applecross and Coigach localities. Some of these are resolved by Ballantyne et al. (1998) but The dependence of the predictions on the value of the similar data are needed for a much larger area. Other ice lithospheric thickness (H ) is more signi"cant. Thus mod- models proposed for the British Isles di!er considerably els with a low H (model E-1) leads to a Late Devensian from BR-D in terms of the volumes of ice contained prediction for Applecross (Fig. 21) that is consistent with within them and the main purpose of this section is to the observed value, but it does lead to a prediction for the examine the sensitivity of the sea-level predictions to the mid-Holocene highstand that much exceeds the observed choice of ice model and to determine whether inferences value. In addition to the observations shown in Fig. 21, can be drawn from the observations of sea-level change the fact that there is no mid-Holocene transgression into about the past ice volumes. the basin at Fearnbeg (see discussion above) the high- Several alternative ice models are examined. A model stand must be below the altitude of the single Late that can be considered to be a maximum reconstruction Devensian observation which is from the same basin. For is that of Boulton et al. (1977). In this model the ice sheet Coigach an even lower value for H is required if the extends across the North Sea to adjoin the Scandinavian 1126 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Table 4 Relative sea-level observations from Wick River (Dawson and Smith, 1997), Dornoch Firth (Smith et al., 1992), Beauly Firth (Firth and Haggart, 1989), and Scapa Bay, Orkney (De La Vega and Smith, 1996)

Laboratory C age BP$1p Calibrated age yr cal BP Altitude and Tendency RSL change from code thickness of sea level present

max. mean min. m OD m m$error

Wick River Wick River 53 B81152 6770 80 7693 7560 7439 !3.11 0.03 # 4.60 0.21 Wick River 10 B81155 7070 80 7992 7855 7672 !1.02 0.02 # 2.51 0.21 Wick River 10 B81157 5940 60 6893 6757 6661 1.05 0.02 !!0.67 0.21 Wick River 23 B81156 6830 70 7738 7622 7488 1.14 0.02 #!0.35 0.21 Wick River 10 B81163 2160 80 2342 2138 1937 1.30 0.01 !!0.42 0.21 Wick River 87 B81154 2130 100 2344 2114 1870 1.64 0.02 !!0.08 0.21 Wick River 23 B81159 4400 50 5248 4924 4852 1.71 0.02 G 0.02 0.70 Wick River 87 B81153 1110 70 1171 983 917 1.72 0.02 ##0.23 0.21 Wick River 23 B81162 1130 50 1165 1036 936 1.85 0.02 ##0.36 0.21 Wick River 30 B81161 1490 60 1287 1350 1520 2.3 0.03 !!0.58 0.21 Wick River 31 B81160 970 50 744 918 961 2.44 0.2 ##0.95 0.21 Wick River 10 B81158 4420 80 4835 4985 5300 1.23 0.2 #!0.26 0.21

Dornoch Firth Creich!1, HB36 SRR3690 9560 55 11149 10994 10687 !2.09 0.03 !!4.49 0.21 Creich-2, HB36 SRR3691 7860 55 8948 8566 8439 !1.12 0.03 #!3.22 0.21 Creich-3, HB11 SRR3692 7930 55 8982 8686 8545 2.65 0.03 ##0.55 0.21 Creich-4, HB7 SRR3693 6950 55 7895 7713 7628 5.37 0.03 ##3.27 0.21 Creich-5, HB7 SRR3694 6930 55 7888 7690 7616 5.63 0.03 !!3.23 0.21 Creich-6, HB16 SRR3695 7055 50 7929 7860 7715 5.21 0.03 ##3.11 0.21 Dounie-2, HB56 SRR3787 5190 65 6171 5930 5757 6.29 0.01 !!3.89 0.21

Beauly Firth Barnyards 3B HV10010 9200 100 10566 10307 10163 1.99 0.05 !!0.65 0.21 Barnyards 14B BIRM1123 9610 130 11217 10944 10444 6.64 0.05 ! 3.99 0.21 Barnyards 14B BIRM1122 5510 80 6449 6297 6113 8.81 0.05 ! 6.17 0.21 Moniak 4B BIRM1127 7430 170 8500 8167 7848 6.82 0.05 # 4.18 0.21 Moniak 4B BIRM1126 7270 90 8179 8035 7847 7.23 0.05 ! 4.59 0.21 Moniak 4B BIRM1125 7100 110 8115 7905 7653 7.40 0.05 # 5.05 0.21 Moniak 4B BIRM1124 4760 90 5656 5527 5299 8.76 0.05 ! 6.12 0.21

Scapa Bay, Orkney Scapa Bay SB23 not known 4820 90 5732 5590 5319 !0.32 0.02 Limiting !0.95 0.88 Scapa Bay SB23 not known 5140 90 6170 5910 5665 !1.08 0.02 !? !1.71 0.88 Scapa Bay SB23 not known 5730 60 6706 6500 6407 !1.40 0.02 #? !3.33 0.50 Scapa Bay SB23 not known 6940 60 7894 7700 7616 !3.48 0.01 !? !5.08 0.50 Scapa Bay SB23 not known 8540 80 9785 9490 9382 !5.10 0.01 Limiting !5.73 0.88 Scapa Bay SB23 not known 9860 80 11502 11000 10948 !5.69 0.02 Limiting !6.32 0.88 Scapa Bay SB33 not known 6720 60 7631 7540 7431 !2.27 0.01 !? !3.87 0.50 Scapa Bay SB33 not known 6950 50 7891 7713 7632 !2.36 0.01 !!3.96 0.50

ice sheet as well as extending out to the edge of the basal conditions were assumed such that the ice thickness continental shelf to the west and north of Scotland. increases less rapidly with distance in from the ice margin Frozen-bed basal conditions were assumed such that the than is the case for BR-A. The ice sheet in this case does ice thickness, following quasi-parabolic functions, in- not extend across the North Sea to the Norwegian ice creases rapidly with distance inwards from the ice mar- sheet, nor does it extend as far north onto the continental gin. The maximum ice thickness attained in this model is shelf as BR-A. The maximum ice thickness is also much about 1900 m. This model is denoted here as BR-A and reduced, to about 1100 m at the time of the maximum details about the assumed time-dependence of the ice glaciation. This model, denoted here as BR-B, corres- retreat are discussed in Lambeck (1993a); Fig. 10). In ponds to a minimum reconstruction of the ice sheet over a second model by Boulton et al. (1985) more mobile the British Isles (see Fig. 8 of Lambeck, 1993a). The ice I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1127

script e.g. BR-A), illustrated in Fig. 22c and d indicate that models BR-A and BR-C contain substantially more ice over western Scotland at the time of the LGM than do the other two models. In particular, the ice in the models BR-A and BR-C extends much further west- wards than in the other two models, with model BR-A extending to the edge of the continental shelf (because model BR-C at the glacial maximum has the same ice thickness as model BR-A at 16 000 BP, by de"nition of the former model, the scaled BR-C model actually contains more ice in Late Devensian time than the scaled BR-A model, even though both have the same ice thickness at 18 000 BP). Fig. 19. Relative sea level observations from four locations in northern Fig. 23 illustrates the predictions for the four localities, Scotland (locations on Fig. 1). Data from Table 4, giving altitudes all based on the earth-model E-0 but di!ering in the standardised to change relative to present. Table 4 gives the original data sources. The Beauly Firth and Dornoch Firth record a fall in choice of ice model. In all cases the scaled models BR-A relative sea level from the Late Devensian to the early Holocene, then and BR-C overestimate the rebound, for both the Holo- a rise to a mid-Holocene maximum and a fall to present. The data from cene and the Late Devensian parts of the observational Wick River show a Holocene rise to just above present in the late record, and the ice volumes contained in these models are Holocene. The earliest part of the Orkney record comprises two limit- excessive. If a thick ice sheet extended as far west and ing dates, i.e. MSL was below the levels indicated, followed by a series of index points that illustrate a generally rising trend but not reaching north as assumed in model BR-A then this would only above present. be consistent with the rebound evidence if the retreat occurred much earlier and faster than proposed in the isochrone reconstructions by Andersen (1981), such that much of the concomitant relaxation was completed by retreat assumed that this model is similar to that adopted the time of the oldest sea-level data at about 12 000 for BR-A, being based on the ice margin isochrones of C BP. Predictions based on the minimum model BR-B Andersen (1981). underestimate the Late Devensian rebound and the sea Because the model BR-A was found to yield rebound levels occur above present level only for a short interval predictions that were inconsistent with most of the obser- before about 12 500 C BP at Kintail and Kentra vational evidence (Lambeck, 1991, 1993a), a third model } Arisaig. At Applecross and Coigach, this model pre- BR-C was developed in which the ice margins at the time dicts no raised shorelines during any part of the Late of the maximum glaciation were assumed to coincide Devensian. The Late Devensian observation from with Andersen's ice limits at 16 000 BP such that the Applecross and the early Holocene observations from central North Sea was ice free and the northern and Coigach are more consistent with predictions based on northwestern ice margins are closer to shore than is the a larger ice model, between BR-D and BR-A and BR-C. case for BR-A. The maximum ice thickness of this inter- Thus ice volumes in the minimum ice model are inad- mediate model is about 1500 m. The fourth model BR-D equate throughout, with the discrepancy between obser- is that previously discussed. All models have the same vation and prediction being greatest for the northern loading history prior to the LGM and the distant ice localities. sheets are the same in all cases. Predictions of Late Devensian relative sea level based Fig. 22a compares the ice thickness pro"les for the four on model BR-D gives, as previously noted (Fig. 21), models along an east}west section at latitude 573N and satisfactory agreement with observations at the Arisaig this illustrates well the large range of the estimates of ice and Kintail localities, whereas the di!erences become volumes contained within each of these models. Earlier larger for the more northern sites. This model also fails to studies have indicated that models such as BR-A and predict Late Devensian highstands at Coigach, a clear BR-C lead to a gross overestimation of the rebound contradiction with the observations of Sissons and Daw- across the British Isles, irrespective of the choice of earth son (1981). Di!erences in predictions, at all localities, model, and that the ice thicknesses have to be substan- between the two models BR-B and BR-D are substantially reduced in order for the rebound predictions to be tially greater than the observational accuracies of the sea comparable with the observed values. Thus, for present levels, despite the relatively minor di!erences in the re- purposes only, the three models BR-A, BR-B and BR-C, spective ice pro"les as illustrated in Fig. 22b. This indi- have been scaled such that they have the same ice thick- cates the considerable sensitivity of the rebound ness as BR-D of about 1200 m at the point in the pro"le parameters to the choice of ice model and that accurate (about 43W, Fig. 22b) where the maximum ice thickness sea-level observations can provide signi"cant constraints is achieved. These scaled models (denoted by the sub- on ice models provided that (i) the data are spatially 1128 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 20. Comparison of predicted (solid line) with observed (open circles with error bars) sea-level change for (a) Kintail, (b) Kentra } Arisaig, (c) Applecross, (d) Coigach. The predictions are based on earth model E-0 (Table 5), ice model BR-D, with a nominal eustatic sea-level function in which all melting had ceased by 6000 BP.

Table 5 C and BR-D. A simple upwards scaling of the ice thick- Earth model parameters ness in model BR-D does not, however, lead to much improvement between observations and predictions be- Earth model Lithosphere Upper mantle Lower mantle thickness viscosity viscosity cause the northen sites lie close to the model ice margin g g in early Late Devensian time and hence near the position H (km) Pa Cs Pa Cs where the rebound signal is close to zero. To increase E}0654;10 10 further the rebound at these sites requires that the ice ; E}150410 10 margins extended beyond the limits assumed in model E}2 100 4;10 10 E}3653;10 10 BR-D (e.g. Stoker et al., 1993; Ballantyne et al. (1998)). E}4655;10 10 Models with extended ice limits, such as BR-C, and in E}5654;10 5;10 which the ice thickness is reduced further than the factor E}6654;10 3;10 0.8 assumed above, do lead to better agreement between predictions and observations. This is illustrated in Fig. 24 for the two northern localities, for here the sites lie well within the maximum ice margin of this model. The well-distributed, and (ii) the dependence on the rheologi- scaling required is, however, not uniform, being about 0.5 cal parameters can be resolved. for Kentra } Arisaig, 0.6 for Applecross and 0.7 for The model predictions for Applecross and Coigach Coigach, indicating that proportionally more ice is re- indicate that the ice volumes are likely to have been quired to the north of Coigach than to the south. somewhere between those described by the models BR- A simple scaling of the model BR-C by these factors I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1129

Fig. 21. Predicted sea-level change for (a) Kintail, (b) Kentra } Arisaig, (c) Applecross, (d) Coigach, based on the di!erent earth models (E-1 } E-6) summarised in Table 5. Models E-1 and E-2 illustrate the marked dependence of the predictions on lithospheric thickness; models E-3 and E-4 indicate the e!ect of increasing the upper-mantle viscosity from 3;10 Pa C s to 5;10 Pa C s. Dependence on lower-mantle viscosity (compare E-5 with E-6) is less. Open circles with error bars show the observed sea-level values. suggests that ice thickness may have been as much as remain uncertain and have not been used here). Agree- 300 m greater over the Coigach area than over the ment with the observed values is improved although the Kentra } Arisaig } Kintail region. mid-Holocene observations still lie below the predicted At all four localities, the model predictions, irrespective values at all localities. This is unlikely to be a conse- of the choice of ice model, indicate a well developed and quence of the corrective term being too small in ampli- short duration highstand at about 6000 yr BP, a result of tude because, if larger, the ubiquitous highstands the assumption that all deglaciation ceased globally at observed along most continental margins that lie far that time and that no further increases in ocean volume from the former ice sheets would vanish (e.g. Lambeck et occurred. A number of studies have shown, however, that al., 1990). Alternatively, it could be a consequence of this is unlikely to have been the case, that a small amount inappropriate earth-model parameters. From Fig. 21, for of melting may have continued into more recent times example, it can be seen that a small increase in lithos- (e.g. Nakada and Lambeck, 1988; Lambeck, 1998; Flem- pheric thickness or a small decrease in upper mantle ing et al., 1998) with the consequence that the mid- viscosity would su$ce to reduce the mid-Holocene high- Holocene highstand is reduced in amplitude, less sharply stand by a few meters. This is further illustrated in Fig. 25 de"ned, and occurs earlier than otherwise predicted. by the curves (ii) corresponding to a model in which the Fig. 25 illustrates the predictions for the four localities. lithospheric thickness has been increased from 65 to The predictions marked (i) refer to the earth model E-0, 70 km and the upper mantle viscosity has been reduced the ice model BR-D and the eustatic correction applied from 4;10 Pa C s to 3.5;10 Pa C s, parameters for the past 7000 yr (The corrections for the earlier period that lie within the uncertainty estimates of the earth 1130 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 22. Ice thickness pro"les across northwestern Scotland along latitude 573 north, (a) and (c), and longitude 43 west, (b) and (d), for the four ice models discussed in text: (a) and (b) original ice thickness; (c) and (d) scaled ice thickness such that ice heights are the same at 573N, 43W.

model E-0. Disagreement between the observations and (Lambeck, 1993b). Thus the assumption made in con- predictions for the Late Devensian at Kentra } Arisaig, structing the ice model BR-D, that the northeastern however, is now increased. This could be accounted for part of Caithness was ice free at the time of the LGM, by an increase in ice volume as is further illustrated in based on the geomorphological interpretations of Fig. 25 by the curves (iii) in which the predictions are Sutherland (1984) and Bowen et al. (1986) is unlikely to based on the modi"ed earth model used for (ii) and an ice be valid and a substantial thickness of ice must have model that lies midway between BR-D and BR-C. existed over both Caithness and the Orkney Islands Agreement with observed sea levels is improved for Kin- at that time. Numerous studies now suggest more ex- tail for these model predictions. The rebound is overes- tensive ice limits, beyond Caithness and Orkney, onto timated at Kentra } Arisaig and Applecross but it is still the Hebridean Shelf and West Shetland Shelf (e.g. underestimated at Coigach, indicating again the need to Hall and Bent, 1990; Peacock et al., 1992; Stoker et al., introduce more ice into the model for northernmost 1993; Hall, 1995, 1996; Ballantyne et al., 1998). A combi- Scotland. nation of more extensive ice limits and slightly reduced These various combinations of earth and ice-model thicknesses over the mainland indicated in the ice sheet parameters illustrate the non-uniqueness of the solution reconstructions of Ballantyne et al. (1998) may pro- even when the data set is spatially limited. But if unique duce the overall increase in ice mass to model the Late solutions for the ice distribution cannot be inferred, Devensian and Holocene sea-level observations de- the inference that the ice mass of the model BR-D needs scribed above. to be increased in northernmost Scotland is consistent Some limits may be placed on the extent of the ice with the discrepancies between observed and predicted to the north by the limited observations of sea-level sea levels previously noted for the Beauly Firth region change in the Outer Hebrides and Shetland. At the former I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1131

Fig. 23. Predicted sea-level change for (a) Kintail, (b) Kentra } Arisaig, (c) Applecross, (d) Coigach based on di!erent ice models. The earth model in all cases is E-0. Open circles with error bars show the observed sea-level values.

Fig. 24. Predicted sea levels for (a) Applecross and (b) Coigach based on earth model E-0 and the scaled ice model b * BR-C with b "0.8, 0.6 and 0.4. b " Model BR-C in Figs. 22 and 23 is BR-C scaled with 0.8. Open circles with error bars show the observed sea-level values. 1132 I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135

Fig. 25. Predicted sea-level change at (a) Kintail, (b) Kentra } Arisaig, (c) Applecross and (d) Coigach for (i) earth-model E-0 and ice model BR-D, (ii) " " the same as (i) but with the modi"ed earth model (H 70 km; h 3.5x10 Pa C s), (iii) same as (ii) but with an ice model that is intermediate b" between BR-D and BR-C ( 0.8). Open circles with error bars show the observed sea-level values.

locality raised shorelines are absent (Ritchie, 1966, 1985; than the northern part of the ice model are essential if Gilbertson et al., 1996). Models such as BR-A and BR-C previously noted discrepancies for other parts of the predict elevated mid-Holocene sea levels throughout the British Isles (Lambeck, 1995) are to be solved. For Outer Hebrides and this was one of the reasons for example, initial analyses, based on ice model BR-D, earth favouring smaller ice models in Lambeck (1993b). Model model E-0 and deglaciation globally continuing after BR-D does not predict raised shorelines for the Outer 7000 yr ago, now predict Holocene sea levels above Hebrides. Raised shorelines have not been observed in present in Northumberland that are consistent with re- Shetland either, where sea levels appear to have been cent observations (Shennan et al., 1999b, c, Lambeck, rising throughout the Holocene (Flinn, 1964; Hoppe, 1995). Changes to ice sheet extent, thickness and pattern 1965; Firth and Smith, 1993; Smith, 1993). Quantitative of retreat modify sea-level predictions for locations dis- data are limited but the evidence does favour models in tant from the areas where these changes are made. Inter- which the ice thickness over these islands was relatively dependencies between earth model, ice model and minimal. eustasy model parameters can only be resolved with Improved evidence for past sea-level change, parti- analyses covering areas larger than those described here cularly for the Late Devensian period, for localities in and there is little gain to our understanding of these northern Scotland is clearly desirable if improved con- interdependencies in producing a model solution that straints are to be placed on the northern limits of the incorporates only the sea-level observations described British ice sheet, but they are only one element of the here and the new ice sheet parameters for the same broader solution sought. Improvements relating to more geographical area (e.g. Ballantyne et al., 1998). I. Shennan et al. / Quaternary Science Reviews 19 (2000) 1103}1135 1133

The improved spatial and temporal distributions of Baden-Powell, D.F.W., 1937. On a marine Holocene fauna in north- sea-level index points resulting from research over the western Scotland. Journal of Animal Ecology 6, 273}283. last "ve years, in particular for Scotland and northern Ballantyne, C.K., McCaroll, D., Nesje, A., Dahl, S.O., Stone, J.O. 1998. The last ice sheet in North-West Scotland: reconstruction and England, and the recent information on ice sheet dimen- implications. Quaternary Science Reviews 17, 1149}1184. sions usher in the next phase of research. New, quantitat- Bennett, M.R., Boulton, G.S., 1993. Deglaciation of the Younger Dryas ive ice models for the whole British Isles ice sheet should or Loch Lomond Stadial ice-"eld in the northern Highlands, Scot- include realistic sub-glacial topographies, to resolve the land. Journal of Quaternary Science 8, 133}145. di!erences between models based on ice thickness and Birnie, J., Gordon, J., Bennett, K., Hall, A. (Eds.), 1993. The Quaternary of Shetland. Quaternary Research Association, London. pp. 140. reconstructions of ice-sheet altitudes. This is critical be- Boulton, G.S., Jones, A.S., Clayton, K.M., Kenning, M.J. 1977. A Brit- cause the parameter sought is ice mass for each location ish ice-sheet model and patterns of glacial erosions and deposition and time interval, yet ice thickness, ice-sheet altitude and in Britain. In: Shotton, R.W. (Ed.), British Quaternary Studies: land surface altitudes are of the same order of magnitude Recent Advances. Clarendon Press, Oxford, pp. 231}246. for northern Britain at the LGM. These ice models and Boulton, G.S., Smith, G.D., Jones, A.S., Newsome, J., 1985. Glacial geology and glaciology of the last mid-latitude ice sheets. Journal of new sea-level data from critical areas and time periods Geophysical Research 142, 447}474. are necessary to progress further the integration of di!er- Bowen, D.Q., Rose, J., McCabe, A.M., Sutherland, D.G. 1986. Correla- ent linked earth } ice } ocean models (e.g. Lambeck et al., tion of Quaternary glaciations in England, Ireland, Scotland and 1998; Peltier, 1998) with reliable "eld and laboratory Wales. Quaternary Science Reviews 5, 299}340. observations. Dawson, A.G., 1988. The main rock platform (Main Lateglacial Shore- line) in Ardnamurchan and Moidart, western Scotland. Scottish Journal of Geology 24, 163}174. Dawson, A.G., 1994. The Main Lateglacial Shoreline (Main Rock Acknowledgements Platform). In: Shennan, I. (Ed.), ICGP Project 367 `Late Quater- nary Coastal Records of Rapid Changea, Field Guide, Dunblane and Fort William, Scotland, 13}20th September 1994. Environ- The major part of this work was completed under mental Research Centre, University of Durham, Durham, pp. NERC grant GST/02/761, part of the Land}Ocean Inter- 13}15. face Study (LOIS). Radiocarbon dates with AA and Dawson, S., Smith, D.E., 1997. Holocene relative sea-level changes on CAM laboratory codes were provided under the LOIS the margin of a glacio-isostatically uplifted area: an example from project. This is LOIS Publication number 603 and also is northern Caithness, Scotland. The Holocene 7, 59}77. De La Vega, A.C. , Smith, D.E., 1996. Holocene relative sea-level and a contribution to IGCP Projects 367 and 437. coastal changes in Scapa Bay, Orkney. In: Hall, A.M. (Ed.), The Additional information was obtained from projects Quaternary of Orkney Field Guide. Quaternary Research Associ- funded by the Commission of the European Communi- ation, London, pp. 69}83. ties, Directorate General for Science, Research and De- Firth, C.R., Haggart, A., 1989. Loch Lomond Stadial and Flandrian velopment (DG XII), Environment Programme, as part Shorelines in the Inner Moray Firth area, Scotland. Journal of ` a Quaternary Science 4(1), 37}50. of Climate Change and Coastal Evolution in Europe Firth, C.R., Smith, D.E., 1993. Sea-level changes and coastal develop- (EV5V-CT94-0445), `Impacts of Climate Change and ment on Shetland. In: Birnie, J., Gordon, J., Bennett, K., Hall, A. Relative Sea-Level Rise on the Environmental Resources (Eds.), The Quaternary of Shetland. Quaternary Research Associ- of European Coastsa (EV5V-CT93-0258), `Relative Sea- ation, London, pp. 15}16. Level Changes and Extreme Flooding Events around Firth, C.R., Smith, D.E., Cullingford, R.A., 1993. Late Quaternary a ` glacio-isostatic uplift patterns for Scotland. In: Owen, L.A., Stewart, European Coasts (EV5V-CT93-0266) and Climate I., Vita-Finzi, C. (Eds.), Neotectonics: Recent Advances, Quaternary Change, Sea-Level Rise and Associated Impacts in Euro- Proceedings, Vol. 3, Quaternary Research Association, Cambridge, pea (EPOC CT-90-0015). The authors thank Frank pp. 1}14. Davies, Derek Coates and Brian Priestley for sample Fleming, K., Johnston, P., Zwartz, D., Yokoyama, Y., Lambeck, K., preparations, Niamh McElherron for help in producing Chappell, J., 1998. De"ning the eustatic sea-level curve since the last glacial maximum using far and intermediate-"eld sites. Earth and the manuscript, and Steven Allan and David Hume Planetary Science Letters, 163, 327}342. for cartographic work. Frances Green, Antony Long, Flinn, D., 1964. Coastal and submarine features around the Shetland Ian Sproxton and Kevin Walker provided signi"cant Islands. Proceedings of the Geologists' Association 75(3), 321}339. help with "eldwork. Finally we thank Callum Firth Gilbertson, D., Grattan, J., Pyatt, B., Schwenninger, J.L.., 1996. The and Douglas Peacock for their excellent suggestions for Quaternary geology of the coasts of the islands of the Southern Outer Hebrides. In: Gilbertson, D., Kent, J., Grattan, J. 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