Trimlines, blockfields, mountain-top erratics and the vertical dimensions of the last British–Irish Ice Sheet in NW Scotland

Quaternary Science Reviews 55 (2012) 91e102

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Trimlines, blockfields, mountain-top erratics and the vertical dimensions of the last BritisheIrish Ice Sheet in NW Scotland Derek Fabel a, Colin K. Ballantyne b, *, Sheng Xu c a

School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK School of Geography and Geosciences, University of St Andrews, Fife KY16 9AL, Scotland, UK c Scottish Universities Environmental Research Centre (SUERC), East Kilbride, Scotland, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 June 2012 Received in revised form 12 August 2012 Accepted 2 September 2012 Available online 2 October 2012

Periglacial trimlines separating glacially eroded lower slopes from blockfield-covered plateaus on British and Irish mountains have been interpreted either (1) in terms of representing the maximum altitude of the last ice sheet during the Last Glacial Maximum (LGM), or (2) as a thermal boundary separating wetbased ice at pressure melting point from cold-based ice on summit plateaus. We test these competing hypotheses through 10Be exposure dating of high-level erratic boulders above trimlines on five mountains in NW Scotland. Nine out of 14 erratics yielded post-LGM exposure ages ranging from 14.0
0.7 ka to 16.5
0.9 ka or from 14.9
0.9 ka to 17.6
1.1 ka, depending on the 10Be production rate employed in exposure age calculation. These ages refute hypothesis (1) as they imply that the last ice sheet overtopped the mountains. Preservation of apparently intact blockfields on the summits implies cold-based ice cover, supporting hypothesis (2). As altitudinally consistent high-level trimlines extend from our sampled sites across much of NW Scotland and the Hebrides, our conclusions apply to all trimlines in this broader area, and probably to all high-level trimlines elsewhere in the British Isles. Preservation of blockfields under cold-based ice is consistent with blockfield evolution on plateaus throughout much or all of the Quaternary. Averaged exposure ages of w15e16 ka for plateau-top erratics implies nunatak emergence from the downwasting ice sheet prior to a regional readvance of the ice margin (the Wester Ross Readvance) and before rapid warming at w14.7 ka at the onset of the Lateglacial Interstade, but after the timing of ice-sheet thinning as retrodicted by recent proxy climate-driven thermo-mechanical coupled models. Our findings provide an additional constraint on the future development of such models by implying that high-level trimlines represent the altitude of a former transition zone between ice at pressure-melting point and ice below pressure melting point. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Trimlines Blockfields Erratics BritisheIrish Ice Sheet Deglaciation chronology Surface exposure dating

1. Introduction Research based on offshore bathymetry, sediment cores and seismic studies (Sejrup et al., 2005; Bradwell et al., 2008) and supported by onshore and offshore dating (Phillips et al., 2008; Scourse et al., 2009; Ballantyne, 2010a; Clark et al., 2012) indicates that the last BritisheIrish Ice Sheet (BIIS) was confluent with the Fennoscandian ice sheet in the North Sea basin, and extended west to the Atlantic shelf edge during the last global glacial maximum (LGM; 26e21 ka; Peltier and Fairbanks, 2006) during Marine Isotope Stage (MIS) 2. Constraining the vertical dimensions of the BIIS has proved problematic. Inverse models based on

* Corresponding author. Tel.: þ44 0 1334 463907; fax: þ44 0 1334 463949. E-mail address: [email protected] (C.K. Ballantyne). 0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2012.09.002

inferred ice-sheet extent or glacial isostatic adjustment (inferred from relative sea-level data) have produced widely different altitudinal outcomes (e.g. Boulton et al., 1991; Lambeck, 1993; Brooks et al., 2008). Thermo-mechanically coupled (TMC) numerical models driven by proxy climate parameters suggest a low-profile ice sheet comprising a cold-based upland core periodically downdrawn by high velocity ice streams (Boulton and Hagdorn, 2006; Hubbard et al., 2009). Critical to verifying these models is the interpretation of high-level trimlines that mark the upper limit of glacially eroded terrain and lower limit of blockfields on the higher mountains of the British Isles. Such trimlines were first interpreted as representing the LGM upper limit of the last BIIS (Ballantyne et al., 1998; and references therein), but extension of the ice sheet westwards to the Atlantic shelf break throws doubt on this interpretation as it implies very low ice surface gradients and correspondingly low driving stresses (Bradwell et al., 2007). The

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relatively thin ice cover implied by this interpretation also appears incompatible with recent models of glacial isostatic adjustment based on relative sea-level data for NW Scotland (Kuchar et al., 2012). An alternative hypothesis is that the last ice sheet buried mountain summits and that blockfields above trimlines indicate subglacial ‘cold patches’, where basal ice remained frozen to underlying terrain for prolonged periods, so that evidence for glacial modification of pre-existing regolith cover is lacking (Kleman and Glasser, 2007). Verification of this alternative model would permit trimlines to be used to define frozen-to-bed zones within the former ice sheet and thus to constrain ice-sheet thermal regime and calibrate parameters within TMC models. Relaxation of trimline-based ice thickness constraints also has implications for models coupling differential ice loading, glacial isostatic adjustment and sea-level change (e.g. Shennan et al., 2006; Brooks et al., 2008; Bradley et al., 2011; Kuchar et al., 2012). Exposure dating of bedrock surfaces above and below trimlines using terrestrial cosmogenic nuclides has produced results compatible with both hypotheses. Outcrops below trimlines have almost invariably produced post-LGM ages (<21 ka), indicating removal of rock by glacial erosion and exposure during shrinkage of the last ice sheet (Ballantyne, 2010a). Above-trimline bedrock outcrops, however, have consistently yielded exposure ages >21 ka (and usually >30 ka) implying either (1) continuous exposure of high ground on former nunataks that remained above the level of the last BIIS, or (2) a complex exposure history that includes preLGM exposure, burial by passive (cold-based) ice cover, then reexposure during ice-sheet downwastage (Ballantyne, 2010a; Ballantyne et al., 2011). Here we test these two competing hypotheses of trimline interpretation through cosmogenic 10Be exposure dating of high-altitude erratic boulders resting on summit blockfields above trimlines in NW Scotland, as these provide conclusive evidence of former ice cover on high ground, well above the upper limit of evidence for glacial erosion. Such erratics have previously been interpreted as deposited by a thicker, earlier ice sheet (Ballantyne et al., 2008). In terms of our hypothesis test, above-trimline erratic boulders deposited on blockfields by an earlier (pre-MIS 2) and thicker ice sheet should record exposure ages markedly greater than 26 ka; erratics deposited by the last BIIS (implying glacial over-riding of summits during the LGM) should yield exposure ages younger than w21 ka. We then explore the implications of our results for ice-sheet thickness, blockfield evolution, TMC modelling and the pattern and chronology of regional deglaciation. 2. Study area Sampling of high-level erratics was carried out on summit blockfields in the Wester Ross area of NW Scotland (57 240 e 58 000 N; 05100 e05 540 W; Fig. 1). This area was selected for three reasons. First, it has been subject to detailed mapping of trimlines, which reach a maximum trimline altitude of w900 m in the vicinity of the sampled summits, declining gently westwards or northwestwards (Ballantyne et al., 1997, 1998; Fig. 1). It should be noted, however, that mapped trimlines are not associated with a break of slope and are rarely represented by a sharp upslope boundary between ice-moulded bedrock and regolith cover; most have been identified from the contrasting terrain characteristics of level ground on cols, shoulders and summit plateaus where postglacial mass movement has been limited (Ballantyne et al., 1998). Only a single trimline is represented on high ground. Second, it is an area of contrasting bedrock geology (British Geological Survey, 2007), and erratics of distinctive lithology have been documented in blockfields above the mapped trimlines on several mountains (Peach et al., 1913; Ballantyne et al., 1987, 1998), though lack of large

erratics suitable for sampling at most high-level sites precluded a more extensive dating programme than that undertaken here. Finally, the timing of ice-sheet deglaciation of low ground in Wester Ross has been established through cosmogenic 10Be exposure dating of boulders on moraines deposited by a late-stage icemargin readvance (the Wester Ross Readvance) when large outlet glaciers terminated in the fjords along the western seaboard of Wester Ross (Ballantyne and Stone, 2012; Fig. 1), so it is possible to compare exposure ages relating to deglaciation of high ground with the extent of ice cover on low ground during the Wester Ross Readvance. During the LGM, glacier ice crossed Wester Ross in a general westerly to northwesterly direction (Peach et al., 1913; Ballantyne et al., 1997) and extended offshore into the Minch, where it joined ice from the Outer Hebrides to form a major ice stream that drained northwestwards towards the Atlantic shelf break (Stoker and Bradwell, 2005; Bradwell et al., 2007, 2008). 10Be exposure ages obtained for the Wester Ross Readvance moraines imply that the ice-sheet margin had retreated to the mouths of fjords in the area by ca 15.1e14.3 ka (Ballantyne and Stone, 2012), and the pollen stratigraphy at a site on the present drainage divide at Loch Droma in northern Wester Ross implies that glacier ice had disappeared from most or all low ground in the area by ca 14.0 ka (Kirk and Godwin, 1963; Finlayson et al., 2011). Following complete or nearcomplete deglaciation during the Lateglacial Interstade (zGreenland Interstade 1 of Lowe et al. (2008); ca 14.7e12.9 ka), the area experienced a limited readvance of glacier ice during the Loch Lomond (Younger Dryas) Stade (zGreenland Stade 1 of Lowe et al. (2008) ca 12.9e11.7 ka). Glaciation during this final glacial episode was limited to outlet glaciers draining icefields centred near the present drainage divide and small independent corrie glaciers (Sissons, 1977; Bennett and Boulton, 1993). 3. Sampling and sample preparation We sampled 14 erratic boulders at altitudes of 883e989 m asl from plateau blockfields in NW Scotland that lie above trimlines mapped by Ballantyne et al. (1998) on five mountains (Fig. 1): Maol Chean-dearg (933 m), Slioch (980 m), Beinn Liath Mhór (925 m), Stob Bán (989 m) and Beinn Eighe (1010 m). In all cases the erratics are distinctly different from the underlying bedrock, comprising white Cambrian quartzite boulders deposited on blockfields of pink Torridon Sandstone or vice versa, or gneissic boulders deposited on Torridon sandstone or Cambrian quartzite blockfields (Fig. 2). We also sampled for comparison two large boulders and two plucked bedrock surfaces at 834e839 m asl immediately downslope from the trimline on Maol Cheann-dearg (Table 1). Sample locations and altitude were recorded using GPS and checked on Ordnance Survey 1:25,000 topographic maps. Above-trimline samples were chiseled from the horizontal upper surfaces of erratic boulders 0.6e2.0 m long and >0.3 m thick that rest on or are embedded within blockfields, and corrections for topographic shielding were calculated from skyline surveys. Sample thicknesses were measured and samples were crushed and sieved. Quartz was separated from the 250 to 500 mm fraction using magnetic and chemical techniques. The isolated quartz was cleaned in 2%HF/HNO3 in an ultrasonic bath to remove remaining contaminants (mainly feldspar) and meteoric 10Be, following modified procedures adopted from Kohl and Nishiizumi (1992). Quartz purity was assessed by measuring the amount of native aluminium in the quartz sample using flame atomic absorption spectrometry. Al concentrations in the samples ranged from 50 to 132 mg/g. Beryllium extraction from purified quartz was carried out at the University of Glasgow Cosmogenic Isotope Laboratory at the

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Fig. 1. Location of the five above-trimline erratic sampling sites, showing the generalised trimline altitudes mapped by Ballantyne et al. (1997) and the Wester Ross Readvance moraines mapped by Robinson and Ballantyne (1979). Four samples for 10Be exposure dating were obtained for above-trimline erratics on both Maol Chean-dearg and Slioch, three from Beinn Eighe, two from Beinn Liath Mhór and one from Sgurr Bàn. Samples were also analysed from boulders and plucked bedrock surfaces 30e50 m below the inferred trimline on Maol Chean-dearg. Figures on the map border are Ordnance Survey 10 km northings and eastings.

Scottish Universities Environmental Research Centre (SUERC). BeO targets were prepared for 10Be/9Be analysis using procedures modified from Child et al. (2000). Between 233 and 256 mg Be was added as carrier and between 20 and 26 g of each sample was dissolved. The 10Be/9Be ratios were measured with the 5 MV accelerator mass spectrometer at SUERC (Xu et al., 2010). Ion currents of 9Be16O were typically 4 mA, and no correction for 10B count rates was necessary. 10Be/9Be ratios were normalized to NIST SRM 4325 with a 10Be/9Be ratio of 2.79 * 1011 (in agreement with Nishiizumi et al., 2007).

Process blanks prepared with the samples yielded an average Be/9Be ratio of 5.1  1015. Blank-corrected 10Be/9Be ratios of the samples ranged from 1.9 to 24.4  1013. Total one-sigma uncertainties for the concentrations determined at the SUERC-AMS Laboratory include the one-sigma uncertainty of the AMS measurement and a 2% uncertainty as a realistic estimate for possible effects of the chemical sample preparation which includes the uncertainty of the Be concentration of the carrier solution. Exposure ages were calculated the CRONUS-Earth online calculator (Developmental version; Wrapper script 2.2, Main

10

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Fig. 2. Examples of sampled above-trimline erratic boulders. A. Sample DF0803, Cambrian quartzite boulder resting on a Torridon sandstone blockfield, Maol Cheann-dearg. B. Sample DF0804, Torridon sandstone boulder resting on a Cambrian quartzite blockfield, Beinn Liath Mhór. C. Sample DF0812, gneiss boulder embedded in a Torridon sandstone blockfield, Slioch. D. Sample BE-04, Torridon sandstone boulder resting on a Cambrian quartzite blockfield, Beinn Eighe. All of these erratic boulders yielded post-LGM 10Be exposure ages.

Table 1 Sample locations and analytical details. Altitude (m)

Thickness (mm)

Horizon correction

Maol Chean-dearg Above-trimline samples DF0801 57.4913 DF0803 57.4923 DF0804 57.4917 DF0805 57.4912

5.4641 5.4631 5.4628 5.4637

930 924 927 930

10 10 20 10

0.9999 0.9956 0.9982 0.9999

60.106 15.404 15.228 49.460







1.701 0.464 0.490 1.263

Below-trimline DF0806 DF0807 DF0808 DF0809

samples 57.4890 57.4890 57.4889 57.4888

5.4638 5.4638 5.4630 5.4621

838 838 839 834

50 50 30 20

0.9496 0.9235 0.9963 0.9965

11.112 12.069 34.550 12.489







0.439 1.109 0.965 0.467

57.6652 57.6668 57.6678 57.6676

5.3446 5.3444 5.3429 5.3381

978 969 964 940

50 20 50 30

0.9997 0.9994 0.9998 0.9999

17.260 16.276 15.864 15.533







0.643 0.568 1.409 0.752

Beinn Liath Mhór DF0814 57.5122 DF0815 57.5123

5.4002 5.3985

916 920

40 30

0.9967 0.9999

15.683
0.553 14.915
0.512

Stob Bàn SB-03

57.7180

5.2672

989

30

1.0000

24.965
0.829

Beinn Eighe BE-02 BE-03 BE-04

57.5837 57.5830 57.5790

5.4391 5.4293 5.4155

970 912 883

19 42 24

1.0000 1.0000 1.0000

118.789
2.733 140.029
4.062 13.081
0.462

Slioch DF0810 DF0811 DF0812 DF0813

Latitude ( N)

[10Be]a (104 atoms g1 SiO2)

Longitude ( W)

Sample

a Isotope ratios normalized to NIST SRM 4325 with a value of 2.79  1011 (Nishiizumi et al., 2007). Uncertainties are propagated at the 1s level and include all known sources of analytical error (blank, carrier mass and counting statistics). A density of 2.7 g cm3 is assumed for all samples. All samples are from the upper surfaces of glacially deposited boulders, except for samples DF0806 And DF0807, which are from the lee sides of roches moutonnées.

D. Fabel et al. / Quaternary Science Reviews 55 (2012) 91e102

calculator 2.1, constants 2.2.1, muons 1.1; Balco et al., 2008) and calibrated using four locally derived 10Be production rates. Local production rates (LPRs) were employed because scaling uncertainty is minimised (e.g. Balco et al., 2009; Kaplan et al., 2010; Balco, 2011), so the precision of derived exposure ages is significantly improved. Three LPRs (NWH LPR11.6, NWH LPR11.9 and NWH LPR12.2) were derived from data produced for the CRONUS-Earth production-rate calibration project: the 10Be calibration data can be accessed at http://depts.washington.edu/ cosmolab/cronus/cronus_cal.html, and further site and analytical details are given in Ballantyne and Stone (2012). These three LPRs are based on samples from glacially deposited rockfall boulders and bedrock surfaces inside the limits of small glaciers that formed in Wester Ross and the neighbouring island of Skye during the Loch Lomond (Younger Dryas) Stade of w12.9e11.7 ka. The differences between them reflect the exposure ages assigned to sampling surfaces, which range from 11.6
0.3 ka (NWH LPR11.6), through 11.9
0.3 ka (NWH LPR11.9) to 12.2
0.3 ka (NWH LPR12.2), yielding reference 10Be production rates (assuming a sampling surface erosion rate of 1 mm ka1 for calibration samples) of 4.20
0.14 atoms g1 a1, 4.09
0.14 atoms g1 a1 and 3.99
0.13 atoms g1 a1 respectively. The fourth LPR employed here (LL LPR) is based on 10 Be concentration in samples from erratic boulders on the terminal moraine of the Loch Lomond glacier advance (Fabel et al. in prep). Radiocarbon ages of macrofossils associated with Table 2 10 Be exposure ages (ka) calculated for four local

95

a varve sequence deposited in the glacial lake created by the advance provide independent age control (MacLeod et al., 2011). The calculated 10Be concentrations from the moraine boulders include an estimated 3 mm ka1 surface erosion rate, based on relief of quartz veins on the moraine boulders, and resulted in a reference 10Be production rate of 3.92
0.18 atoms g1 a1. As the calculated exposure ages of samples are inversely related to the assumed 10Be production rate, NWH LPR11.6 yields the youngest exposure ages for samples and LL LPR yields the oldest ages. We assume that the four LPRs collectively bracket the range of plausible exposure ages for our samples, and in the text that follows we cite two ages for individual samples or sample means: a ‘youngest age’, calculated using NWH LPR11.6, and an ‘oldest’ age, calculated using LL LPR. For consistency with previously reported 10Be exposure ages obtained for boulders on Wester Ross Readvance moraines, the exposure ages reported here (Table 2) are based on the timedependent Lm scaling scheme of the CRONUS-Earth online calculator (Lal, 1991; Stone, 2000), and assumption of a sampling surface erosion rate (3 ) of 1 mm ka1. For exposure ages <20 ka, the other scaling schemes (the St, Du, De and Li schemes) available via the online calculator produce ages that differ on average from the Lm scheme by less than 1% of sample age. Similarly, for ages <20 ka, assumption of 3 ¼ 0 reduces our calculated exposure ages by 1.1e 1.4%, and assumption of 3 ¼ 2 mm ka1 increases the exposure ages by a similar margin.

10

Be production rate models.

Production rate model

NWH LPR11.6

NWH LPR11.9

NWH LPR12.2

LL LPR

Assigned calibration age (ka)

11.60
0.30

11.90
0.30

12.20
0.30

11.67
0.14

Reference 10Be production rate (atoms g1 a1)

4.20
0.14

4.09
0.14

3.99
0.13

3.92
0.18

Maol Chean-dearg Above trimline samples DF0801 DF0803 DF0804 DF0805

58.97 14.59 14.81 47.96







2.82 0.68 0.71 2.18

60.58 14.97 15.20 49.25







2.87 0.69 0.72 2.21

62.24 15.36 15.59 50.59







2.93 0.71 0.74 2.26

63.32 15.61 15.85 51.46







3.63 0.86 0.90 2.84

Below trimline samples DF0806 DF0807 DF0808 DF0809

12.52 14.03 37.41 13.14







0.67 1.39 1.75 0.68

12.85 14.40 38.41 13.48







0.68 1.43 1.76 0.69

13.18 14.77 39.43 13.83







0.69 1.46 1.80 0.71

13.40 15.01 40.10 14.06







0.81 1.56 2.23 0.84

Slioch DF0810 DF0811 DF0812 DF0813

16.46 15.24 15.29 15.02







0.85 0.76 1.48 0.91

16.88 15.64 15.69 15.41







0.87 0.78 1.52 0.93

17.32 16.05 16.09 15.81







0.89 0.79 1.56 0.95

17.61 16.31 16.36 16.07







1.05 0.95 1.62 1.09

Beinn Liath Mhór DF0814 DF0815

15.68
0.79 14.69
0.73

16.09
0.80 15.07
0.74

16.50
0.82 15.46
0.76

16.78
0.98 15.57
0.91

Sgurr Bàn SB-03

23.35
1.15

23.96
1.17

24.60
1.20

25.01
1.45

124.23
5.94 163.04
8.85 13.95
0.68

127.89
6.06 168.05
9.08 14.31
0.69

131.66
6.23 173.24
9.36 14.68
0.71

134.14
7.93 176.66
11.6 14.92
0.85

15.08
0.84

15.47
0.86

15.87
0.88

16.12
1.02

Beinn Eighe BE-02 BE-03 BE-04 Mean of above-trimline Post-LGM ages

Samples DF0806 and DF0807 were obtained from high-level bedrock surfaces below the trimline on Maol Chean-dearg. All other samples were obtained from boulder surfaces. Production rate models NWH LPR11.6, NWH LPR11.9 and NWH LPR12.2 (see text) assume an erosion rate (3 ) of 1 mm ka1 for the calibration dataset (Ballantyne and Stone, 2012). Production rate model LL LPR assumes 3 ¼ 3 mm ka1 for the calibration data. Calculated ages are scaled using 3 ¼ 1 mm ka1 and the Lm scheme of the CRONUS online calculator (Balco, 2007; Balco et al., 2008), wrapper script version 2.2, main calculator version 2.1, constants version 2.2.1, muons version 1.1, with a 10Be half life of 1.387  106 years (Chmeleff et al., 2010; Korschinek et al., 2010).

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D. Fabel et al. / Quaternary Science Reviews 55 (2012) 91e102 10

Be exposure ages

Exposure ages for our 14 above-trimline erratic samples are summarised in Table 2 and Fig. 3. Nine samples produced post-LGM (<21 ka) ages. Calculated using NWH LPR11.6, these span the range 14.0
0.7 to 16.5
0.9 ka, with a mean age of 15.1
0.8 ka. Calculated using LL LPR, the nine-post-LGM ages range from 14.9
0.8 ka to 17.6
1.1 ka, with a mean age of 16.1
1.0 ka. The remaining 5 samples (DF0801, DF0805, SB-03, BE-02 and BE-03) produced much greater (apparent) exposure ages ranging from w23 ka to w177 ka with no overlap at
1s (Fig. 3). As all samples producing pre-LGM ages (>26 ka) were obtained from blockfields where other erratics yielded post-LGM ages, we interpret all >21 ka exposure ages as influenced by nuclide inheritance due to complex histories involving at least one cycle of exposure, burial by glacier ice then re-exposure during the final deglaciation of the sampled plateaus. Of the four below-trimline samples obtained from bedrock outcrops and quartzite boulders at altitudes of 834e839 m on a broad bedrock bench on Maol Chean-dearg, three yielded postLGM exposure ages as expected, but one boulder (sample DF0808) produced an apparent 10Be exposure age of w37e40 ka, probably reflecting complex exposure history. The ages of remaining three samples average 13.2
0.9 ka (NWH LPR11.6) to 14.2
1.1 ka (LL LPR), markedly younger than the equivalent average ages of the two above-trimline erratics (14.7
0.7 to 15.7
0.9 ka) sampled less than 100 m higher on the summit plateau of the same mountain. We note that our combined sample of 12 above- and below-trimline post-LGM ages shows a general increase in age with altitude (Fig. 4). This relationship suggests exposure of higher sites earlier as the last ice sheet downwasted, but in view of the distance between sampling sites, the likelihood of complex ice-surface configuration and the large uncertainties associated with individual ages it cannot be regarded as conclusive. Probability density distributions (PDDs) generated for the four sites that yielded above-trimline post-LGM erratic exposure ages (Fig. 5A and B) illustrate slight differences in average age from site to site, with the oldest mean ages (15.5
1.0 to 16.6
1.2 ka) being recorded for the four samples from Slioch, and the single post-LGM

sample from Beinn Eighe yielding the youngest age (14.0
0.7 to 14.9
0.9 ka). The other two sites record intermediate mean postLGM ages of 14.7
0.7 to 15.7
0.9 ka (Maol Chean-dearg) and 15.2
0.8 to 16.2
0.9 ka (Beinn Liath Mhór). Despite these differences, the nine post-LGM ages collectively define nearsymmetrical probability density distributions (Fig. 5C) with an arithmetic mean ages of 15.1
0.8 to 16.1
1.0 ka. The reduced chisquare values of the aggregated age distributions in Fig. 5C are all <0.9, consistent with sampling from a single age population (Balco, 2011), and implying that the range of post-LGM exposure ages may be attributable to sampling and analytical variance (i.e. no significant differences in the timing of erratic exposure), though ‘real’ differences in exposure ages for samples at different sites is to be expected, given the differences in their location and altitude. However, irrespective of the LPR used in the age calculations, all nine individual ages and their collective mean ages are significantly younger (p < 0.005) than (1) the assumed termination of the (global) LGM at w21 ka (Fig. 3; Peltier and Fairbanks, 2006), (2) the onset of ice-margin retreat from a position at or near the Atlantic shelf edge at w19.4 ka, as retrodicted by TMC modelling (Hubbard et al., 2009) and indicated by dating and morphological evidence (Clark et al., 2012). Collectively, the nine post-LGM exposure ages therefore demonstrate (1) deposition of erratics on blockfields near mountain summits (and well above mapped trimlines) by the last BIIS, implying that the ice sheet overtopped mountain summits in Wester Ross, and (2) widespread emergence of summit plateaus from under glacier ice cover at w16e15 ka, several millennia after the onset of ice margin retreat. 5. Implications 5.1. Interpretation of trimlines Trimlines delimiting a marked contrast between glacially scoured valley-side slopes and blockfield-mantled summits and plateaus have been observed on many of the higher mountains of the British Isles, notably in NW Scotland and the Hebrides (Ballantyne et al., 1998; and references therein), the English Lake District (Lamb and Ballantyne, 1998), Snowdonia (McCarroll and

Fig. 3. 10Be exposure age ranges (horizontal lines) of all 14 above-trimline erratics, together with those of the four below-trimline samples obtained from boulder and bedrock surfaces at 834e839 m altitude on Maol Chean-dearg. The age range for each sample represents the range between the youngest calibrated age (calculated using NWH LPR11.6) minus 1s and the oldest calibrated age (calculated using LL LPR) plus 1s. The shaded vertical bar represents the assumed duration (26e21 ka) of the LGM. Nine of the above-trimline erratic exposure ages and three of the below-trimline ages post-date the LGM. The widely dispersed apparent ages of the remaining six samples are interpreted as reflecting nuclide inheritance due to complex exposure histories.

D. Fabel et al. / Quaternary Science Reviews 55 (2012) 91e102

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A

B

Fig. 4. Post-LGM 10Be exposure ages (calculated using NWH LPR11.6) plotted against sample altitude for above- and below-trimline sites (n ¼ 12). Exposure ages calculated using other LPRs are almost identical in form. All plots show a general increase in exposure age with altitude, significant at p < 0.001.

Ballantyne, 2000) and various mountains in Ireland (e.g. Rae et al., 2004; Ballantyne et al., 2008, 2011). Early researchers viewed such trimlines as representing the maximum thickness of Pleistocene ice sheets (e.g. Geikie, 1873; Wright, 1927). More recently, such trimlines have been interpreted as representing the maximum altitude achieved by the last ice sheet during the LGM (the nunatak hypothesis), an inference consistent with pre-LGM apparent exposure ages obtained on bedrock outcrops above trimlines and postLGM exposure ages on outcrops below trimlines (Fig. 6). During the past decade the validity of this interpretation has been questioned, notably in view of offshore evidence for a much more extensive ice sheet than previously envisaged (Sejrup et al., 2005; Bradwell et al., 2008; Scourse et al., 2009; Clark et al., 2012), incompatibility of implied maximum ice-surface altitudes with TMC models and relative sea-level data (Boulton and Hagdorn, 2006; Hubbard et al., 2009; Kuchar et al., 2012) and inconsistencies in trimline altitudes (Ballantyne and Hall, 2008). The post-LGM ages we obtained for nine above-trimline erratic boulders (Figs. 3e5) demonstrate that the last ice sheet must have overtopped mountain summits at our sampling sites and thus conclusively refute the nunatak hypothesis for the LGM in this area. As there is no evidence of glacial erosion above trimlines, the post-LGM erratic exposure ages therefore imply that the trimlines in Wester Ross represent a former englacial transition within a thick ice sheet, from erosive warm-based ice at lower altitudes to ‘passive’ cold-base ice that formerly covered and preserved pre-existing blockfields on high ground. Moreover, because trimline altitudes in NW Scotland and the Hebrides decline in a fairly consistent manner along former ice flowlines (Ballantyne et al., 1997, 1998; Stone and Ballantyne, 2006, Fig. 1), we infer that our results invalidate the LGM nunatak hypothesis for all ice-sheet trimlines in this broader area. Furthermore, we suggest that it is likely that ice-sheet trimlines identified on mountains elsewhere in Britain and Ireland also reflect an englacial thermal transition rather than a ‘palaeonunatak’ interpretation. This suggestion is consistent with evidence of glacial modification of tors that surmount blockfields on the high plateau of the Cairngorm Mountains in NE Scotland (Hall and Phillips, 2006), post-LGM 10Be exposure ages obtained on some tors in the same area (Phillips et al., 2006), modelling of ice thickness over blockfield-mantled mountains in SW Ireland (Ballantyne et al.,

C

Fig. 5. Probability density distributions (PDDs) of above-trimline post-LGM erratic exposure ages. A: PDDs for individual sites, scaled by sample size using NWH LPR11.6 (youngest ages). B: PDDs for individual sites, scaled by sample size, calculated using LL LPR (oldest ages). C: Aggregated PDDs for all samples calculated for all four locally derived 10Be production rates (LPRs).

2011) and retrodiction of former ice-sheet thickness by TMC modelling (Boulton and Hagdorn, 2006; Hubbard et al., 2009). More generally, it accords with evidence for emergence of glacially unmodified blockfields from under the retreating margins of coldbased plateau ice caps (Whalley et al., 1981; Rea et al., 1996) and a substantial body of evidence demonstrating that plateau blockfields in Scandinavia and North America survived, apparently intact, under a ‘protective’ cover of cold-based glacier ice during the LGM (e.g. Kleman and Stroeven, 1997; Fabel et al., 2002; Hättestrand and Stroeven, 2002; Briner et al., 2003; Marquette et al., 2004; Staiger et al., 2005; Fjellanger et al., 2006; Kleman and Glasser, 2007). Indeed, although trimlines certainly represent the former upper altitudinal limit of LGM icefields or valley glaciers

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Fig. 6. Above-trimline erratic ages plotted on an ageealtitude diagram that also depicts above- and below-trimline bedrock exposure ages for various British and Irish mountains (from Ballantyne, 2010a). For the bedrock samples, horizontal bars indicate
1s external (total) uncertainties. The age range (horizontal line) for each erratic sample represents the range between the youngest calibrated age (calculated using NWH LPR11.6) minus 1s and the oldest calibrated age (calculated using LL LPR) plus 1s. The above-trimline bedrock ages all exceed w21 ka (and all but one exceed w30 ka), but 9 of the 14 above-trimline erratic ages are younger than 19 ka. This contrast implies erratic emplacement by the last BIIS and exposure of erratics since ice disappearance from high ground in NW Scotland, and suggests limited or negligible erosion of above-trimline bedrock surfaces by the last ice sheet. Ages >21 ka are only ‘apparent’ exposure ages, as pre-LGM bedrock and erratic ages probably reflect a complex exposure history.

elsewhere (Ballantyne, 2007) and some blockfields apparently occupy plateaus that escaped glacier ice cover during the LGM (e.g. Small et al., 1999; Landvik et al., 2003; Paasche et al., 2006), our reinterpretation of trimlines in the British Isles contributes to a growing body of evidence that trimlines representing the altitudinal transition between ‘erosive’ and ‘passive’ ice cover within a former thick ice sheet represent the norm rather than the exception. 5.2. Implications for blockfield evolution The term blockfield describes a bouldery regolith mantle, usually developed on plateau surfaces in mid- or high-latitude mountains. Because there is only very limited evidence for weathering of icemoulded bedrock on high ground since Late Pleistocene or Early Holocene deglaciation (Ballantyne, 1998; André, 2002, 2003; Nicholson, 2009; Glasser et al., 2012) it is generally accepted that blockfields in such areas are relict (Boelhouwers, 2004). Many blockfields have been interpreted as remnants of Neogene regolith covers modified by frost action during the Quaternary (e.g. Nesje, 1989; Rea et al., 1996; André, 2003; Marquette et al., 2004; Whalley et al., 2004), an interpretation apparently consistent with the association of some blockfields with pockets of saprolite representing in situ chemical weathering of bedrock (Paasche et al., 2006). Studies of blockfield structure and sediments in Scotland and Sweden, however, have concluded that blockfields are essentially Quaternary periglacial landforms, produced mainly by frostwedging operating on stress-release joints (Ballantyne, 1998; Goodfellow et al., 2009). Moreover, research on rates of tor emergence from blockfield-covered plateaus has demonstrated that such supposedly ‘preglacial’ surfaces have undergone significant lowering during the Quaternary (Phillips et al., 2006; Goodfellow,

2007; Darmody et al., 2008). This finding prompted Ballantyne (2010b) to propose a model of long-term blockfield evolution in which an initial Neogene regolith cover is gradually removed by subaerial surface lowering during ice-free periods and progressively replaced by the products of frost-wedging under periglacial conditions, so that traces of Neogene inheritance may or may not be preserved, depending on the original regolith depth and the amount of surface lowering. This blockfield evolution model implies that some blockfields may have escaped significant glacial erosion throughout the Quaternary. In Scotland, the occurrence of erratics in blockfields above trimlines has hitherto been explained in terms of (1) erratic emplacement by a thick, erosive ice sheet that over-rode mountain summits and (2) subsequent formation of blockfields on palaeonunataks by frost-wedging of bedrock under severe periglacial conditions (Ballantyne, 1998; Ballantyne et al., 1998). Our findings, however, demonstrate blockfield survival on Scottish mountains under cold-based ice during the LGM. They are therefore consistent with the possibility that blockfields on British mountains evolved during successive periglacial periods when plateaus were free of glacier ice, with preservation of blockfield debris under cold-based ice cover during recurrent periods of ice-sheet glaciation. This interpretation implies the possibility of blockfield evolution over a much longer timescale (potentially spanning the entire Quaternary), and is consistent with evidence of progressive tor emergence from blockfield-mantled plateaus in the Cairngorm Mountains since the Early Pleistocene (Phillips et al., 2006) and preservation of pockets of saprolite of possible Neogene origin on high ground in Scotland (Hall and Mellor, 1988; Phillips et al., 2006). It does not, however, exclude the possibility that some blockfields on British or Irish mountains have developed during the Late Pleistocene on plateaus previously eroded down to sound bedrock by glacier ice.

5.3. Implications for deglacial chronology Fig. 7 depicts the youngest (NWH LPR11.6) and oldest (LL LPR) mean age of above-trimline erratics plotted against the NGRIP d18O Greenland ice-core data (Rasmussen et al., 2006) and mean July temperatures for SE Scotland inferred from chironomid assemblages (Brooks and Birks, 2000). Irrespective of which LPR is used to calibrate our data, both mean ages pre-date the rapid warming at w14.7 ka evident in both the ice-core record and the chironomidbased temperature record. This warming has been interpreted as representing resumption of thermohaline circulation in the North Atlantic region and resultant northwards migration of the oceanic polar front (Ruddiman and McIntyre, 1981) bringing relatively warm surface waters to the latitude of Scotland. Chironomid assemblages at Whitrig Bog in SE Scotland imply a rapid rise in mean July temperatures from w6  C to w12  C at this time (Brooks and Birks, 2000), and although this rise is not captured in chironomid assemblages from Skye and NE Scotland, these also confirm that the highest mean July temperatures in these areas (w12.5  C and w13.6  C respectively) occurred early in the Lateglacial Interstade (Brooks et al., 2012). The mean exposure ages obtained for high-level erratics on mountains in Wester Ross (15.1
0.8 ka to 16.1
1.0 ka) provide the first evidence that at least some mountain summits emerged from the ice sheet prior to 14.7 ka, indicating substantial reduction in the volume of the downwasting ice sheet before the rapid warming at the Dimlington Stade e Lateglacial Interstade (zGS2/GI-1e) transition (Fig. 7). The strongest evidence for plateau emergence prior to 14.7 ka is for our highest sampling sites (940e978 m) on Slioch; even if the oldest exposure age from this site is excluded, the remaining three tightly clustered ages average 15.2
1.1 to 16.3
1.2 ka. As several summits in Wester

D. Fabel et al. / Quaternary Science Reviews 55 (2012) 91e102

99

Fig. 7. Mean exposure ages (vertical dashes) and
1s external uncertainties (horizontal bars) for (1) above-trimline erratics that yielded post-LGM exposure ages (n ¼ 9); (2) belowtrimline boulder and bedrock surfaces on Maol Chean-dearg that produced a post-LGM exposure age (n ¼ 3); and (3) boulders on three of the Wester Ross Readvance moraines depicted in Fig. 1 (the Applecross, Redpoint and Gairloch moraines; n ¼ 11), recalibrated from the data in Ballantyne et al. (2009) using the same LPRs and erosion rates employed in this study. In each case the upper vertical dash (a) represents the mean of exposure ages calculated using NWH LPR11.6 (i.e. ‘youngest’ ages) and the lower dash represents the mean age calculated using the LL LPR (‘oldest’ ages). The mean ages and associated uncertainties are plotted against NGRIP ice core d18O data for 11e17 ka (Rasmussen et al., 2006), the ice core stages and events proposed by Lowe et al. (2008), and mean July temperature data inferred from chironomid assemblages in SE Scotland (Brooks and Birks, 2000), visually matched to the NGRIP ice core data.

Ross exceed 1000 m altitude, it is likely that they also emerged from the downwasting ice sheet prior to 14.7 ka. The mean exposure ages for high-level erratics exceed those for boulders sampled on the nearest dated Wester Ross Readvance (WRR) moraines (the Applecross, Redpoint and Gairloch Moraines in Fig. 1) by w1.0 ka, implying that the highest mountain summits in Wester Ross formed nunataks rising above the downwasting ice sheet before the WRR occurred, and remained free of glacier ice during the readvance. Intriguingly, however, the mean of the three exposure ages obtained for below-trimline sites at 834e838 m altitude on Maol Chean-dearg (13.2
0.9 to 14.2
1.1 ka) postdate the timing of the WRR by 1.1e1.2 ka, and appear to suggest persistent ice cover at this site long after the onset of the Lateglacial Interstade at w14.7 ka. This may indicate that a thin cover of ice survived on the gently sloping shoulder of Maol Chean-dearg during the earlier part of the interstade, but this explanation is difficult to reconcile with the two above-trimline erratic dates (mean exposure age 14.7
0.7 to 15.7
0.9 ka), which suggest that the broader and higher summit plateau was ice-free w1.5 ka earlier. Finally, it is notable that with the possible exception of belowtrimline samples DF0806 (12.5
0.7 ka to 13.4
0.8 ka) and DF0809 (13.1
0.7 to 14.1
0.8 ka), all of our samples produced ages much older than the Loch Lomond (Younger Dryas) Stade of w11.7e12.9 ka. Various authors have argued that glacier ice may have survived throughout the Lateglacial Interstade in the Scottish Highlands in favourable locations such as high corries or as plateau ice caps then expanded as the climate cooled after w12.9 ka, though definitive evidence for this has proved elusive (e.g. Sutherland, 1984; Stone and Ballantyne, 2006; Hubbard et al., 2009). Irrespective of the LPR employed in their calculation, the 14.0 ka exposure ages obtained for all high-level erratics on the summit plateaus at our study sites (Table 2) appear to rule out the possibility of ice persistence on these plateaus throughout the

Lateglacial Interstade. It remains possible, however, that plateau ice caps survived the interstade on higher and/or broader plateaus, as argued by Finlayson et al. (2011) for the Beinn Dearg massif in northern Wester Ross. 5.4. Implications for TMC models Thermo-mechanically coupled (TMC) ice sheet models driven by proxy climatic inputs such as the Greenland ice-core oxygen isotope records have demonstrated great potential for understanding the growth, dynamic behaviour and demise of Late Quaternary ice sheets. Such models can be tuned by varying climatic, rheological and mechanical variables through iterative experiments designed to obtain a ‘best fit’ with the geological evidence for former ice extent, directions of movement and evidence of streaming behaviour. Of the two such models hitherto proposed to describe the history and behaviour of the last BIIS, the earlier (Boulton and Hagdorn, 2006) demonstrated the difficulty of generating a physically viable ice sheet under the assumption that high-level trimlines represent the upper surface of the last ice sheet at its maximum thickness, but also introduced a two-dimensional analysis (Boulton and Hagdorn, 2006, figure 15) showing that the altitude of trimlines in NW Scotland is compatible with their model as a transitional zone separating warm-based ice in valleys from cold-based ice on high ground. Our conclusion that cold-based ice occupied high plateaus (Section 5.2) provides empirical support for this conclusion. The optimal higher-resolution model (experiment E109b8) of Hubbard et al. (2009) is based on rather different assumptions and parameters, and yields a remarkably good fit to the geological evidence. Our finding (Section 5.2) that cold-based ice persisted on high plateaus throughout the LGM is entirely consistent with retroduction of persistent cold-based ice cover on high ground in NW

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Scotland in Hubbard et al. (2009, figure 4). Moreover, as generalised trimline altitudes have been established for NW Scotland (Fig. 1; Ballantyne et al., 2008), areas of apparently undisturbed blockfields above trimlines in this area (and probably those elsewhere in the British Isles) indicate the location of persistent ‘cold patches’ within the former ice sheet (Hall and Glasser, 2003; Kleman and Glasser, 2007), providing an additional constraint on future models: models that retrodict warm-based, potentially erosive ice in such areas cannot be valid. Additionally, our data appear to confirm that trimline altitudes (Fig. 1) represent, at least approximately, the maximum altitude of ice at pressure melting point, imposing a thermal constraint on future TMC models. Although geologically based spatial constraints on the extent of the last BIIS are relatively well established (e.g. Bradwell et al., 2008; Clark et al., 2012), temporal constraints are limited. The optimal-fit model of Hubbard et al. (2009) retrodicts major volume loss in the last BIIS between 19.4 ka and 17.2 ka, with retreat of the northwestern margin of the ice front from near the Atlantic shelf edge to the present coastline of Wester Ross by the latter date. A more detailed temporal reconstruction (available at http://www. aber.ac.uk/en/iges/research-groups/centre-glaciology/researchintro/britice-model/) suggests that by w15.0 ka ice cover was restricted to an ice cap over the Scottish Highlands, with its margin along the western seaboard of Wester Ross, reasonably consistent with the mapped limits of the WRR. However, ice-surface altitudes derived from the model (Henry Patton, personal communication, 2012) imply ice surface altitudes < 500 m at our sampling sites for the period 16e15 ka, suggesting that the model under-represents coeval ice thickness in Wester Ross (cf. Kuchar et al., 2012). An alternative explanation is that as the ice sheet downwasted, thin cold-based plateau ice caps continued to occupy summit plateaus, so that our exposure ages reflect the timing of disappearance of local plateau ice caps rather than exposure from under the downwasting ice sheet. The TMC model of Hubbard et al. (2009) also successfully retrodicts very rapid ice wastage after w14.7 ka, and suggests that glacier ice was restricted to high ground in southern Wester Ross by w14.0 ka, consistent with widespread deglaciation of northern Wester Ross prior to w14.0 ka (Finlayson et al., 2011; Ballantyne and Stone, 2012). Our data therefore demonstrate the possibility of combining exposure ages relating to ice extent with those relating to deglaciation of high ground to obtain a threedimensional picture of the nature of ice wastage that may be used to constrain or validate TMC models. 6. Conclusions 1. Trimlines marking the boundary between glacially eroded lower slopes and blockfield-mantled summit plateaus have traditionally been interpreted as marking the upper limit of the last ice sheet on palaeonunataks. An alternative interpretation of trimlines as englacial thermal boundaries between a lower zone of erosive ice at pressure melting point and a higher zone of cold-based ‘protective’ ice has recently gained favour. We have tested these competing hypotheses through cosmogenic 10 Be exposure dating of 14 high-level glacially deposited erratic boulders sampled from above-trimline blockfields on five plateaus in Wester Ross, NW Scotland. Nine samples yielded post-LGM exposure ages ranging from 14.0
0.7 ka to 16.5
0.9 ka or 14.9
0.9 ka to 17.6
1.1 ka, depending on the locally derived 10Be production rate employed in the age calculation. The remaining five samples produced widely differing apparent exposure ages of w23 ka to w177 ka that are inferred to reflect complex exposure histories. The nine postLGM blockfield ages demonstrate (1) deposition of erratics on summit blockfields (and well above trimlines) by the last ice

sheet, implying that the last BritisheIrish ice sheet overtopped the sampling sites; and (2) preservation of blockfields on summits over-ridden by glacier ice, indicating that the overlying ice cover remained cold-based throughout the growth, expansion and downwastage of the last ice sheet. Our results conclusively refute the palaeonunatak hypothesis of trimline formation for our study sites. 2. As altitude-consistent trimlines can be traced across mountains throughout much of NW Scotland and the Hebrides, we infer that all high-level trimlines in these areas represent englacial thermal boundaries between a lower zone of wet-based ice and an upper zone of cold-based ice within a thick ice sheet that overtopped all summits. We suggest that a similar interpretation applies to high-level trimlines identified on mountains in Ireland, Wales and the English Lake District. 3. Our evidence that plateau blockfields apparently escaped modification by the last ice sheet is consistent with the view that blockfields may have evolved throughout much or all of the Quaternary on plateaus subject to slow subaerial lowering, with preservation of blockfields under successive Pleistocene ice sheets. 4. The averaged post-LGM exposure ages of plateau-top erratics at our study sites indicate that higher summits in NW Scotland emerged from the ice between w16 ka and w15 ka. Summit emergence at this time implies substantial downwastage of the last ice sheet prior to the rapid warming that terminated the Dimlington Stade (zGS-2), and before the Wester Ross Readvance, which culminated near the mouths of nearby fjords. However, comparison of our exposure ages with the retrodictions of proxy climate-driven thermo-mechanical coupled models (Hubbard et al., 2009) suggest that ice-sheet downwastage may have isolated thin cold-based ice caps on summit plateaus, and that our exposure ages relate to disappearance of such ice caps rather than the downwastage of the regional ice surface. More generally, although lithologically distinct erratics are fairly rare on higher British and Irish mountain summits, glacially deposited ‘perched’ boulders are common, and should allow replication of our approach to provide a definitive test of trimline genesis elsewhere. Our results also suggest that it should be possible to employ surface exposure dating using cosmogenic isotopes to establish the timing of ice sheet downwastage from summits to low ground through sampling of glacially emplaced boulders over a range of altitudes at key sites. Finally, we note that confirmation that the high-level trimlines in NW Scotland represent thermal boundaries within the last ice sheet opens up the possibility of using trimline altitudes as a constraint in the future development of thermo-mechanically coupled ice sheet models. Acknowledgements This research was supported by a grant from the UK Natural Environment Research Council (NE/H010831/1). We thank Ole Humlum for assistance in the field, Maria Miguens-Rodriguez for assistance with sample preparation, Henry Patton for providing icesurface altitude maps generated by the Hubbard et al. (2009) icesheet model, Phil Hughes and Brad Goodfellow for discerning reviews of the original paper, and Graeme Sandeman (University of St Andrews) for preparation of figures. References André, M.-F., 2002. Rates of postglacial rock weathering on glacially-scoured outcrops (Abisko-Riksgränsen area, 68 ). Geografiska Annaler 84A, 139e150.

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