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Late-Holocene and Younger Dryas glaciers in the northern Cairngorm Mountains, Scotland

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We present 17 cosmogenic Be-10 ages of glacial deposits in Coire an Lochain (Cairngorm Mountains), which demonstrate that glacial and nival deposits cover a longer timescale than previously recognised. Five ages provide the first evidence of a late-Holocene glacier in the British Isles. A previously unidentified moraine ridge was deposited after c. 2.8 kyr and defines a small slab-like glacier with an equilibrium line altitude (ELA) at c. 1047 m. The late-Holocene glacier was characterised by rapid firnification and a dominance of sliding, enabling the glacier to construct moraine ridges in a relatively short period. Isotopic inheritance means that the glacier may have existed as recently as the Little Ice Age' (LIA) of the 17th or 18th century ad, a view supported by glacier-climate modelling. Nine Be-10 ages confirm a Younger Dryas Stadial (YDS) age for a cirque-floor boulder till, and date the glacier maximum to c. 12.3 kyr when the ELA was at c. 963 m altitude. Both glaciers existed because of enhanced accumulation from wind-blown snow, but the difference in ELA of only c. 84 m belies the YDS-LIA temperature difference of c. 7 degrees C and emphasises the glacioclimatic contrast between the two periods. Three Be-10 ages from till boulders originally deposited in the YDS yield ages <5.5 kyr and indicate snow-avalanche disturbance of older debris since the mid-Holocene, as climate deteriorated towards marginal glaciation.
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The Holocene
2014, Vol. 24(2) 141 –148
© The Author(s) 2013
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DOI: 10.1177/0959683613516171
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Introduction
Scotland’s last glaciers are generally accepted to have vanished at
the close of the Younger Dryas Stadial (YDS), around 11.5 kyr
(Golledge et al., 2008). Geomorphological evidence of Holocene
glaciers has proved elusive, though a favourable climate for
cirque glaciation in the ‘Little Ice Age’ (LIA, c. ad 1300–1850,
Matthews and Briffa, 2005) is implied by historical accounts of
year-round snow cover and lake ice in the 17th and 18th centuries,
and by the small temperature depression calculated to be suffi-
cient to allow net annual snow accumulation in high-altitude
cirques (Harrison et al., forthcoming; Kington, 2010; Lamb,
1995; Manley, 1949; Sugden, 1977). Attention has focused on
attempts to show that moraines ascribed to the YDS (locally
termed the Loch Lomond Stadial or LLS) were actually late Holo-
cene in age (Sugden, 1977), but 14C dating of cirque lake sedi-
ments (Batterbee et al., 2001; Rapson, 1985) does not support a
LIA age of cirque moraines in the northern Cairngorm Mountains.
This paper reports cosmogenic 10Be ages and glaciological recon-
structions of cirque moraines in Coire an Lochain (Figure 1) to
examine the evidence for both YDS and subsequent glacial
advances.
Geomorphological evidence
Several glacial and nival landforms are identified within the
cirque (Figure 1). The cirque-floor boulder sheet was deposited as
supraglacial melt-out till during retreat from the YDS maximum
extent, and was ascribed a YDS age by Sissons (1979). The
maximum glacier extent is clearly defined by its distal edge,
although lack of a distinct terminal moraine ridge suggests that
the maximal extent was short-lived. A lateral moraine can be
traced up the eastern glacier margin, constraining reconstruction
of the glacier ablation zone.
The cirque backwall comprises a debris slope leading up to the
Great Slab, a stepped, plucked, low-angled (c. 25°–30°) granite
slab which merges upslope with the vertical 100-m-high headwall
(Figures 2 and 3a). The junction of slab and cliff is free of rockfall
talus, in contrast to ledges elsewhere on the cirque walls. The rock
scenery of the upper cirque is ‘clean’ and has been interpreted as
the source of a pre-YDS rock slope failure by Ballantyne (2013).
Bordering most of the east edge of the slab, and the lower west
edge, are small but prominent steep-sided debris ridges, of which
the eastern ridge extends for >100 m obliquely downslope (Fig-
ures 1 and 3b). The western ridge terminates abruptly upslope at
a rock wall. The two ridges do not meet at the base of the Great
Late-Holocene and Younger Dryas glaciers
in the northern Cairngorm Mountains,
Scotland
Martin Kirkbride,1 Jez Everest,2 Doug Benn,3 Delia Gheorghiu4 and
Alastair Dawson5
Abstract
We present 17 cosmogenic 10Be ages of glacial deposits in Coire an Lochain (Cairngorm Mountains), which demonstrate that glacial and nival deposits
cover a longer timescale than previously recognised. Five ages provide the first evidence of a late-Holocene glacier in the British Isles. A previously
unidentified moraine ridge was deposited after c. 2.8 kyr and defines a small slab-like glacier with an equilibrium line altitude (ELA) at c. 1047 m. The
late-Holocene glacier was characterised by rapid firnification and a dominance of sliding, enabling the glacier to construct moraine ridges in a relatively
short period. Isotopic inheritance means that the glacier may have existed as recently as the ‘Little Ice Age’ (LIA) of the 17th or 18th century ad, a view
supported by glacier-climate modelling. Nine 10Be ages confirm a Younger Dryas Stadial (YDS) age for a cirque-floor boulder till, and date the glacier
maximum to c. 12.3 kyr when the ELA was at c. 963 m altitude. Both glaciers existed because of enhanced accumulation from wind-blown snow, but
the difference in ELA of only c. 84 m belies the YDS–LIA temperature difference of c. 7°C and emphasises the glacioclimatic contrast between the two
periods. Three 10Be ages from till boulders originally deposited in the YDS yield ages <5.5 kyr and indicate snow-avalanche disturbance of older debris
since the mid-Holocene, as climate deteriorated towards marginal glaciation.
Keywords
beryllium 10, Cairngorm Mountains, cosmogenic dating, glacier reconstruction, ‘Little Ice Age’, Younger Dryas
Received 4 September 2013; revised manuscript accepted 15 November 2013
1University of Dundee, UK
2British Geological Survey, UK
3UNIS, Norway
4SUERC, UK
5University of Aberdeen, UK
Corresponding author:
Martin Kirkbride, University of Dundee, Nethergate, Dundee, DD1
4HN, UK.
Email: m.p.kirkbride@dundee.ac.uk
516171HOL24210.1177/0959683613516171The HoloceneKirkbride et al.
research-article2013
Fast-track report
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142 The Holocene 24(2)
Slab, but form an opposing pair, and are interpreted as ice-
marginal moraines.
The Great Slab (Figure 3b) is a source of full-depth snow ava-
lanches in heavy-snow years, which are continuing to trim the lower
ends and proximal slopes of the moraine ridges to deposit elongated
splays of avalanche-reworked debris downslope (Figure 1). The
avalanche debris apron extends to the lake (Figure 2), whose small
eastern bay is interpreted as an avalanche-impact pit. Snow ava-
lanches from the south-western cirque headwall onto lake ice have
been observed to generate floods downstream through the till sheet
(Bullivant, personal communication, 2011).
A glacial origin of the ridges is interpreted on the basis of their
form and location. They contain matrix-supported angular granite
clasts, up to boulder size, in a matrix of coarse sand and grit (Figure
3c). Boulder surfaces are pink in colour and markedly less edge-
rounded compared with the grey weathered boulders on nearby
slopes and till sheets (Figure 3a). Moraine boulders are smaller than
in the cirque-floor till sheets, and soil cover is thin with no horizon
development. The ridges are oriented at a highly oblique angle to
the headwall above, do not extend up to it and extend too far
downslope for gravitational transport of debris across a firn field to
have been the main debris source (cf. Ballantyne and Benn, 1994).
For these reasons, a protalus origin for the ridges is discounted. No
clast shape analysis has been conducted because a significant con-
tribution to construction of the ridges may have come from rework-
ing of pre-existing debris, which could have incorporated material
with different transport histories. Thus, clast shape data are unlikely
to provide diagnostic evidence of genesis.
Field and laboratory methods
The location and relatively unweathered clasts of the Great Slab
moraine indicates a significantly younger age than the cirque-floor
till. Exposure-age dating was used (1) to test the presumed YDS
Figure 1. Geomorphology of Coire an Lochain showing the locations, sample numbers and exposure ages of 10Be samples and the former
glacier extents in the cirque. Key: GS = Great Slab; 1 = moraine ridge crest; 2 = boulder sheet; 3 = margin of YDS till sheet; 4 = avalanche-
impact pit; 5 = rock faces; 6 = rock slab; 7 = rock outcrops; 8 = frost-shattered mountain-top debris; 9 = scree slopes; 10 = polygenetic debris
cones and avalanche aprons; 11 = lake and 12 = permanent stream. Contour lines are in metres above sea level. Easting and northing reference
points refer to UK National Grid.
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Kirkbride et al. 143
age for the latter, which was initially sampled with a view to inves-
tigating whether the whole deposit was of a single age, and (2) to
investigate whether the Great Slab moraine represents evidence of
a small Holocene glacier in the upper cirque. Rock samples were
collected from large embedded boulders protruding from the
moraine (Figure 3c), to avoid more recent debris deposited on the
ridge surface by rockfall or snow avalanches. Samples CLU#1 to
CLU#3 were from the longer right-lateral moraine, and CLU#4 and
CLU#5 from the left-lateral moraine. Six samples were collected
from each of the inner and outer parts of the cirque-floor boulder
till, labelled LCI#1 to LCI#6 and LCO#1 to LCO#6, respectively.
All were from the upper surfaces of boulders large enough to pro-
trude above the surrounding boulder layer to minimise snow
shielding. Sample locations are shown in Figure 1.
Samples were processed at the Cosmogenic Isotope Analysis
Facility (CIAF) in the Scottish Universities Environmental
Research Centre (SUERC). Rock samples were crushed and
ground, and quartz was separated from other minerals by mechan-
ical (magnetic) and chemical (hexafluorosilicic acid treatment
and hydrofluoric acid leaching) procedures. 10Be extraction and
target preparation followed procedures modified from Kohl and
Nishiizumi (1992) and Child et al. (2000). The 10Be/9Be ratios
were measured using the 5 MV accelerator mass spectrometer
(AMS) at SUERC (Xu et al., 2010). The Be ratios were converted
to nuclide concentration in quartz. Details of sample locations and
analytical data are given in Table 1. We provide two sets of expo-
sure ages (Table 2), both calculated using the Cronus-Earth online
calculator v. 2.2 (http://hess.ess.washington.edu/). However, the
first set is calculated using a global production rate (4.39 ± 0.37
atoms/g/yr; Lm scaling) obtained from age-constrained calibration
measurements (Balco et al., 2008). The use of these widespread
calibrations points leads to an increase of the scaling
uncertainties.
New local calibration sites indicate that the global production
rate used in the Cronus-Earth calculator is too high (e.g. Balco
et al., 2009; Fenton et al., 2011). The second set of exposure ages
was calibrated using a locally derived 10Be production rate (NWH
LPR12.2; Ballantyne and Stone, 2012; Fabel et al., 2012), which
is lower than the global production rate and has lower uncertain-
ties related to scaling (sea-level high-latitude production rate 3.99
± 0.13 atoms g/yr; Lm scaling). Thus, the 10Be ages calculated
using the LPR (Table 2) are older and more precise than 10Be ages
calculated with the global production rate. Although the NWH
LPR12.2 is not independently constrained, it corresponds closely
to the Loch Lomond production rate (LLPR) of 3.92 ± 0.18
atoms/g/yr (Lm scaling; Fabel et al., 2012). The LLPR is based on
the 10Be concentration from boulders on the Loch Lomond termi-
nal moraine and is independently constrained by radiocarbon dat-
ing (MacLeod et al., 2011).
Results
Chronology
The whole cirque-floor till sheet yields ages consistent with depo-
sition in the YDS (Table 2). For the inner area of till (samples
LCI#1 to LCI#6), the LPR calibration gives three ages of between
11.5 ± 0.5 and 13.1 ± 0.6 kyr (mean = 12.3 kyr). Three samples
yield mid-Holocene ages of between 2.7 and 5.1 kyr. These boul-
ders are interpreted to have been overturned after deposition
either by snow avalanching from the western slopes of the cirque,
or by avalanche-generated flood waves from the lake (freshly
avalanche-disturbed boulders were observed nearby after the
2009–2010 winter). Four of the six samples from the outer till
range in age from 12.1 to 12.8 kyr; the other two (LCO#1: 14.4
kyr and LCO#6: 13.7 kyr) may be inherited ages from older fore-
land debris incorporated into the YDS till close to the maximum
of the advance. The mean age of samples LCI3, LCI4, LCI6, and
LCO2 to LCO5 is 12.3 kyr – several centuries older than the dated
Figure 3. Details of the late-Holocene glacial landforms in the upper part of Coire an Lochain. (a) Looking south-west to the prominent right-
lateral moraine on the near side of the Great Slab. Note the absence of debris accumulation between the slab and the headwall in (b) view
from the left to the right-lateral moraine, across the plucked steps of the Great Slab. The right-lateral moraine is the steep grassy boulder ridge
at the far side of the slab – (c) a boulder, embedded in the late-Holocene right-lateral moraine, sampled for 10Be dating.
Figure 2. View from the north into Coire an Lochain. Moraine
crests in the upper cirque are indicated by white dotted lines. Below
the Great Slab, an apron of snow-avalanche and debris-flow debris
extends downslope to the main cirque lake (just visible to the right).
The bouldery till sheet of YDS is indicated.
GS: Great Slab.
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144 The Holocene 24(2)
YDS maximum at the type site in the south-western Scottish
Highlands (MacLeod et al., 2011).
Five 10Be ages from the Great Slab moraine (Table 2) range
from 0.9 ± 0.3 years (CLU#4) to 5.5 ± 0.5 years (CLU#3) using the
LPR calibration: four are younger than 2.8 ± 0.5 kyr. The wide
spread of ages is interpreted as an effect of isotopic inheritance.
Low rockfall rates in the Holocene (Ballantyne and Harris, 1994)
would logically be associated with variable concentrations of
cosmogenic beryllium in cliff faces, whose exposed surfaces will
comprise a mosaic of small facets of very different exposure ages.
Rock falls can therefore be expected to contain varying concen-
trations of cosmogenic beryllium which are inherited by the
moraine ridges following a short glacial transport path. It follows
that 10Be ages should be interpreted as maximum ages for the
moraines, and that the youngest boulder sampled provides the
closest chronological control on moraine age. By this reasoning,
Table 1. Sample locations and analytical details for the late-Holocene moraine (CLU) and the YDS till sheet (LCI and LCO).
Sample AMS ID Latitude (°N) Longitude (°W) Altitude (m) Thickness (cm) Density (g/cm) Shielding
(factor)a
10Be ± σ (atoms/g
quartz)b
CLU#01 b6524 57.1043 −3.678 1040 2 (0.9833) 2.7 0.9205 27,772 ± 3353
CLU#02 b6525 57.1044 −3.678 1052 3 (0.9751) 2.7 0.9205 16,513 ± 1723
CLU#03 b6526 57.1044 −3.678 1043 4 (0.967) 2.7 0.9205 54,292 ± 3858
CLU#04 b6527 57.1036 −3.6796 1043 5 (0.959) 2.7 0.8646 7945 ± 1273
CLU#05 b6531 57.1037 −3.6794 1036 4 (0.967) 2.7 0.8646 22,140 ± 2491
LCI#1 b2447 57.1 −3.68 896 5 (0.959) 2.7 0.9888 27,628 ± 1139
LCI#2 b2448 57.1 −3.68 922 1 (0.9916) 2.7 0.9853 54,096 ± 1588
LCI#3 b2353 57.1 −3.68 914 1 (0.9916) 2.7 0.9867 129,395 ± 3869
LCI#4 b2354 57.1 −3.68 903 1 (0.9916) 2.7 0.9854 120,034 ± 3299
LCI#5 b2355 57.1 −3.68 896 1 (0.9916) 2.7 0.9826 46,081 ± 1720
LCI#6 b2356 57.1 −3.68 899 1 (0.9916) 2.7 0.987 135,747 ± 4359
LCO#1 b2359 57.1 −3.68 915 2.5 (0.9792) 2.7 0.9824 149,051 ± 3450
LCO#2 b2349 57.1 −3.68 917 2 (0.9833) 2.7 0.9824 125,882 ± 3803
LCO#3 b2360 57.1 −3.68 924 2 (0.9833) 2.7 0.9798 127,762 ± 3805
LCO#4 b2350 57.1 −3.68 916 5 (0.959) 2.7 0.98 129,841 ± 7664
LCO#5 b2361 57.1 −3.68 915 3 (0.9751) 2.7 0.9824 127,517 ± 3837
LCO#6 b2362 57.1 −3.68 909 2 (0.9833) 2.7 0.985 141,717 ± 4074
aShielding by distant objects after Dunne etal. (1999).
b10Be/9Be blank-corrected ratios and 10Be concentrations are referenced to NIST SRM 4325 (2.79 × 1011; Nishiizumi etal., 2007). The processed blank
10Be/Be ratios were generally between 2% and 18% (two ratios of 25% and 41%) of the sample 10Be/Be ratios and were subtracted from the measured
ratios. Uncertainties (±1σ) include all known sources of analytical error.
Table 2. Exposure ages and associated analytical uncertainties based on global and local production rates.
Sample Global PR NWH 12.2 LPR
Exposure age (kyr) Internal σ (kyr) External σ (kyr) Exposure age (kyr) Internal σ (kyr) External σ (kyr)
CLU#1 2475 403 457 2779 452 462
CLU#2 1467 264 293 1647 296 302
CLU#3 4919 430 608 5524 483 518
CLU#4 768 267 275 862 299 301
CLU#5 2139 339 386 2400 380 389
LCI#1 2425 100 234 2722 113 145
LCI#2 4519 133 416 5076 150 228
LCI#3 10944 331 1017 12301 373 562
LCI#4 10253 285 945 11523 321 507
LCI#5 3944 148 374 4429 166 224
LCI#6 11633 378 1091 13077 426 617
LCO#1 12833 301 1170 14428 339 599
LCO#2 10752 329 1000 12084 370 554
LCO#3 10877 328 1011 12226 369 557
LCO#4 11411 682 1213 12826 768 884
LCO#5 11003 335 1024 12367 377 566
LCO#6 12173 355 1129 13685 399 615
PR: production rate.
Sample thickness correction using a rock density of 2.7 g/cm and attenuation length of 160 g/cm.
Exposure ages calculated using the Lm scaling schemes in the Cronus-Earth (http://hess.ess.washington.edu/). Global PR ages was assumed to be 4.39 ±
0.37 atoms/g/yr. NWH LPR12.2 exposure ages are calculated using a production rate of 3.99 ± 0.13 atoms/g/yr based on a deglaciation age of 12.2 kyr
in Scotland (Ballantyne and Stone, 2012; Fabel etal., 2012).
The calculated age uncertainties are expressed as 1σ. The external uncertainties include the internal (analytical) and the total (including calibration and
scaling) uncertainties.
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Kirkbride et al. 145
the maximum moraine age lies between c. ad 850 and ad 1450
based on CLU#4 and deposition during the LIA is possible. A
more cautious approach is to regard CLU#3 as an outlier, in which
case, the earliest possible age for the moraine is <2.8 kyr, during
or after the climatic deterioration of the Sub-Atlantic period, but
as discussed below, this does not discount an LIA age for the
glacier.
Glacier reconstruction and climatic implications
Glacier morphology was reconstructed at a scale of 1:6250 and
contoured at a 10 m interval, from which surface slopes and hyp-
sometric curves were calculated and former equilibrium lines
estimated using an accumulation area ratio of 0.6 (Figure 4). Iso-
pach maps were constructed by subtraction of present-day relief
from the former glacier surface topography by grid sampling of
the reconstructed glacier surfaces. Glaciological reconstructions
followed the method of Carr et al. (2010), based on the best-fit
regressions of empirical relations from contemporary glaciers.
An independent estimate of summer (June–August) temperature
was used to calculate total accumulation and winter balance at
the equilibrium line altitude (ELA) of each glacier. Accumula-
tion is used to estimate ablation gradients, which are in turn
applied to the hypsometric curves to estimate mass loss per
altitudinal band of each ablation zone (mean annual ablation
multiplied by the area between contour lines). In equilibrium
mass balance, total mass loss from an ablation zone equates to
the balance discharge through the ELA, which divided by cross-
sectional area gives the balance velocity. The creep velocity
equation of Paterson (1994) is then applied to calculate the aver-
age rate of ice flow due to deformation, using flow law parame-
ters of n = 3 (exponent) and 5.3 × 10−15/s/kPan (constant). The
latter value is for temperate ice so a lower value may be appropri-
ate for the YDS glacier, in which case the present results will
overestimate creep velocity. Average sliding velocities through
the ELA are estimated as the difference between the balance and
creep velocities.
The mean summer ELA temperature for the late-Holocene gla-
cier was taken as the present (1981–2000) temperature minus
1.5°C (see Harrison et al., forthcoming). Present-day temperature
was based on Aviemore weather data recorded at 228 m altitude,
adjusted to the ELA using an empirical lapse rate of 0.007°C/m
calculated from the temperature difference between Aviemore and
Cairngorm summit (1245 m, 2.5 km to the east-northeast). The
mean summer temperature for the YDS glacier ELA was derived
from a contemporary chironimid-based mean July temperature of
6.8°C (from Brooks et al., 2012) from Abernethy Forest, 230 m
a.s.l. and 15 km to the north. The mean July temperature was
Figure 4. Reconstructions of the Younger Dryas (top) and late-Holocene (bottom) glaciers in Coire an Lochain, to the same scale. Surface
topography is shown on the left, and ice thickness on the right, each with a 10 m contour interval.
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146 The Holocene 24(2)
adjusted to a June–August average using their present-day ratio,
and then to the ELA using the same lapse rate. The YDS July tem-
perature at 230 m altitude was 7.3°C cooler than the present day.
The results of the glaciological and climatic reconstructions
are shown in Table 3. The startling statistic is the three-fold
decrease in precipitation and accumulation values in the YDS,
associated with ablation-season temperatures close to freezing
and very low turnover. In contrast, the late-Holocene glacier
required high winter accumulation to sustain the glacier through
summers averaging 4.1°C across the ablation season, and 6.9°C
for July. Similar average shear stresses between the two glaciers
mask the contrast between a thin, steep and more dynamic late-
Holocene glacier in which 85% of motion was through basal slid-
ing on a smooth, impermeable rock slab, and a thicker, colder and
less dynamic YDS glacier, which existed in a cold continental
environment (Table 3). One commonality between the glaciers is
that both depended strongly on enhanced accumulation by blown
snow from the surrounding plateau (Harrison et al., forthcoming;
Sissons, 1979). This offset the aridity of the YDS and the warmth
of the late Holocene to allow glaciers to form and survive in each
period.
In Carr et al.’s (2010) method, sliding velocity is pragmati-
cally a residual value after other parameters have been calculated,
and is ultimately dependent on the best-fit summer temperature-
accumulation regression. Ohmura et al.’s (1992) relation of June–
August temperature and approximate annual precipitation (winter
accumulation plus summer precipitation at ELA) includes the
standard deviations around the best-fit curve, allowing a range of
possible precipitation scenarios to be modelled. Taking the June–
August ELA temperature of 6.0°C for the late-Holocene glacier
(Table 3), Ohmura et al.’s relation also yields an annual precipita-
tion value of c. 2.70 m/yr but with a ±1σ range of 2.25–3.25 m/yr.
At the upper end of this range, mass turnover in the Coire an
Lochain glacier could therefore have been 20% greater, with a
higher sliding velocity to maintain equilibrium.
Reconstruction of the late-Holocene glacier gives results that
are climatically and glaciologically credible. However, the ques-
tion remains as to whether a small accumulation zone of 27,500
m2 combined with a high accumulation rate was able to hold firn
for a sufficiently long period to form glacier ice above the ELA.
This question is addressed by comparing an estimated mean resi-
dence time of snow and ice in the accumulation zone with the
time taken for attainment of the firn–ice transition (a density of
0.85 g/cm3: Paterson, 1994). For a calculated accumulation-zone
volume of 413,000 m3 and a balance discharge through the ELA
of 4150 m3/yr, the mean residence time would have been c. 100
years. In temperate glacial environments where saturation of the
snowpack occurs, the firn–ice transition can be reached in as little
as 5–7 years (Paterson, 1994). Glacier ice therefore would have
formed quickly above the ELA.
However, a residence time of c. 100 years also requires the
glacier to have survived for longer than this in order to construct
moraines at lower altitude, and this may pose a problem given the
marginal conditions for glaciation in the late Holocene. The pos-
sibility exists that the glacier may not be accurately represented
by Carr et al.’s model due to its thin, steep form and smooth
impermeable bed: conditions of high shear stress combined with
low normal stress promote effective basal pressure variations and
bed separation (Benn and Evans, 2010). A significantly higher
mass turnover may have been possible if sliding velocity was
faster, sustained by greater local wind-blown enhancement of
accumulation than the above calculations have estimated. By
back-calculating from balance velocity to the mean specific net
accumulation required to maintain equilibrium, a balance velocity
of 5 m/yr would require net accumulation of only 0.67 m/yr with
a 27 years accumulation-zone residence time. Corresponding val-
ues for a balance velocity of 10 m/yr are 1.34 m/yr and 14 years.
These scenarios seem more plausible, and would have allowed
the glacier to construct moraines within a lifespan of much less
than 100 years, although ablation would also have to be greater
than calculated by the Carr et al. method.
Discussion: A LIA glaciation in the
Cairngorm Mountains?
The orbitally driven decline in Holocene temperatures since the
Climatic Optimum (Mayewski et al., 2004; Wanner et al., 2008,
2011) led to the earliest neoglacial glacier advances in high-
latitude and high-altitude mountains in Europe and Scandinavia,
where the terrain was high enough for the glaciation threshold to
be crossed at earlier stages of the mid-Holocene cooling. Subse-
quent cooling led to a later onset of neoglacial advances in lower
ranges. The Scottish Highlands were only approaching the glacia-
tion threshold late in the Holocene. Our results demonstrate that a
small glacier existed in the upper cirque of Coire an Lochain since
c. 2.8 kyr, after snow-avalanche disturbance of YDS boulders
since the mid-Holocene. This finding is the first firm evidence of
Holocene glacier formation in the British Isles. The glaciocli-
matic reconstruction shows that the ELA lay closer, in altitudinal
terms, to the YDS glacier than a simple temperature driver would
allow. This emphasises both the importance of enhanced precipi-
tation for allowing glacier ice to form (Harrison et al., forthcom-
ing), and the aridity of YDS winters in the Cairngorm
Mountains.
A question of great interest is whether the glacier existed dur-
ing the period of LIA glacierisation (sensu Matthews and Briffa,
2005) after ad 1300. Local low and high-altitude proxy tempera-
ture records (Barber et al., 1999; Dalton et al., 2005) show a gen-
eral cooling throughout the second half of the Holocene, so that
glacier formation prior to 2.8 kyr (if samples CLU#1 to CLU#5 are
accepted as maximum ages) is highly unlikely. However, these
records have insufficient chronological control to judge when the
greatest sustained cooling occurred within the last 2.8 millennia.
While there is a general view that the post-Mediaeval cooling
exceeded earlier cool periods (Wanner et al., 2011), palaeoeco-
logical records from the Netherlands (Van Geel et al., 1996) and
Switzerland (Hieri et al., 2003; Larocque-Tobler et al., 2010) indi-
cate cooler temperatures in the Sub-Atlantic period than during
the last 500 years. Thus, the Holocene proxy temperature record
does not appear to provide a clear cold ‘spike’ with which to cor-
relate the late-Holocene glacier. From historical climatology, the
Table 3. Reconstructed climatic and glaciological parameters for
the Coire an Lochain glaciers, following the method of Carr etal.
(2010).
Variable Late-Holocene
glacier
YDS glacier Units
Glacier area 36,200 310,500 m2
Glacier volume 456,000 9,625,000 m3
ELA 1047 963 m
Mean July ELA temperature 6.9 1.7 °C
Mean June–August ELA
temperature
6.0 0.8 °C
Mean May–October ELA
temperature
4.1 −1.1 °C
Winter balance at ELA 2.3 0.6 m/yr
Total accumulation at ELA 2.7 0.9 m/yr
Ablation gradient −0.0076 −0.0034 m/m/yr
Balance velocity 1.4 0.7 m/yr
%Basal sliding 85 58 %
Average τ61 58 kPa
YDS: Younger Dryas Stadial; ELA: equilibrium line altitude.
by guest on November 2, 2016hol.sagepub.comDownloaded from
Kirkbride et al. 147
decade ad 1690–1700 is generally regarded as having had the
lowest air temperatures throughout the period ad 1350–1700
across northern Europe (Lamb, 1995). The period was associated
with a marked decrease in sea-ice cover across the northern North
Atlantic and Greenland Sea that had the effect of displacing the
polar oceanic and atmospheric fronts to the south (Lamb, 1995).
There is also limited evidence that the exceptional cold and
increased winter snowfall that characterised this period was asso-
ciated with a predominance of easterly and northeasterly winds
during winter (Dawson, 2009). Similar, although less extreme
conditions, prevailed across Scotland for much of the 18th cen-
tury, culminating in the extreme cold of the 1780s.
A significant question is whether the long, snowy winters of
the late 17th and 18th centuries occurred frequently enough to
generate and sustain glacier ice for a sufficient period to form
moraine ridges. Several summaries of historical climate (Dawson,
2009; Kington, 2010; Lamb, 1995) concur that the late 17th and
18th centuries experienced the most sustained ‘glacier-friendly’
climates, and several periods when climate approached glaciation
are noted by Lamb (1995) and Kington (2010). The central Eng-
land temperature series (Lamb, 1995) places the greatest post-
Mediaeval cool temperatures within this period. We suggest here
that the period ad 1650–1790 was the most conducive time for
glacier formation in Scotland during the last c. 2.8 kyr, when the
North Atlantic Oscillation was dominantly in negative mode
(Luterbacher et al., 2002). The glacier described here probably
constructed its moraines within this interval, and may have sur-
vived until the mid-19th century. Peat-based proxies of wetness
from northern Britain, including sites in the Cairngorm Moun-
tains, indicate high water tables and greater humidity around ad
1640, between ad 1715 and 1850, and between ad 1790 and 1830
(Barber et al., 1999; Blundell and Barber, 2005; Mauquoy et al.,
2008). These periods would have been associated with greater
snow accumulation on the Cairngorm Plateau.
The glaciological reconstruction of the late-Holocene glacier
creates a picture of a thin, steep, sliding glacier existing close to
the limit of climatic viability, facilitated by enhanced snow accu-
mulation, rapid transformation to glacier ice, and short ice resi-
dence time. Thus, construction of small ice-marginal moraines
would have been aided by a rapid supply of debris to the ablation
zone margin. The location of the Holocene glacier is unusual to
the extent that we suspect there would have been very few, if any,
similar topoclimatic niches in the British mountains where con-
temporary late-Holocene glaciers could have formed (cf. Harri-
son et al., forthcoming). The glacier existed partly because the
Great Slab supported an accumulation zone on a relatively low-
angle surface above 1050 m altitude. The upper cirque is nar-
rowly enclosed by vertical faces, providing effective shading for
such a small glacier, and accumulation enhancement by the
extensive plateau above is very effective, even today. Only such
a combination of geomorphological factors could allow marginal
glaciation in recent millennia of a warm Atlantic Ocean and
dominantly westerly cyclonic airflow. Conversely, altitudes of
the main cirque floors in the Cairngorm Mountains appear to
have been just too low to allow ice to survive for long enough to
construct moraine ridges, and reconnaissance of several cirques
has produced evidence of pronival deposition rather than Holo-
cene moraines.
In conclusion, this study has demonstrated the former pres-
ence of a glacier ice in the British Isles in the late Holocene. It
seems probable that a glacier existed in Coire an Lochain in the
LIA. However, few other sites in the Cairngorm Mountains are
likely to have correlative moraines due to stringent topoclimatic
criteria that would have been needed to be met. Glacioclimatic
reconstructions highlight the extremes of glaciological environ-
ment represented by the Younger Dryas cirque glacier and the
late-Holocene niche glacier. The irony is that the elusive landform
evidence for recent glacial activity was discovered in one of the
most visible locations in the massif.
Acknowledgements
Cosmogenic nuclide dating was supported by the Natural Envi-
ronmental Research Council CIAF SUERC allocations 9095.1010
and 9011.0405. The support of the British Geological Survey is
acknowledged. Field assistance was provided by Iain Forteath and
Scott Tipple. We thank Stephan Harrison (University of Exeter)
and Nic Bullivant (Cairngorm Mountain) for helpful discussions
on different aspects of this research, and Simon Carr (QMUL)
and an anonymous referee for their constructive comments on an
earlier version of this paper.
Funding
The research received no specific grant from any funding agency
in the public, commercial, or not-for-profit sector.
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Cairngorm Club Journal, Vol.18 (97), 188-201
Article
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It is widely believed that the last glaciers in the British Isles disappeared at the end of the Younger Dryas stadial (12.9-11.7 cal. kyr BP). Here, we use a glacier-climate model driven by data from local weather stations to show for the first time that glaciers developed during the Little Ice Age (LIA) in the Cairngorm Mountains. Our model is forced from contemporary conditions by a realistic difference in mean annual air temperature of -1.5 degrees C and an increase in annual precipitation of 10%, and confirmed by sensitivity analyses. These results are supported by the presence of small boulder moraines well within Younger Dryas ice limits, and by a dating programme on a moraine in one cirque. As a result, we argue that the last glaciers in the Cairngorm Mountains (and perhaps elsewhere in upland Britain) existed in the LIA within the last few hundred years, rather than during the Younger Dryas.
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Lateglacial and early-Holocene mean July air temperatures have been reconstructed, using a chironomid-based inference model, from lake-sediment sequences from Abernethy Forest, in the eastern Highlands of Scotland, and Loch Ashik, on the Isle of Skye in north-west Scotland. Chronology for Abernethy Forest was derived from radiocarbon dates of terrestrial plant macrofossils deposited in the lake sediments. Chronology for Loch Ashik was derived from tephra layers of known ages, the first age-depth model of this kind. Chironomid-inferred temperatures peak early in the Lateglacial Interstadial and then gradually decline by about 1 °C to the beginning of the Younger Dryas (YD). At Abernethy Forest, the Lateglacial Interstadial is punctuated by three centennial-scale cold oscillations which appear to be synchronous with the Greenland Interstadial events GI-1d, when temperatures at Abernethy fell by 5.9 °C, GI-1c, when temperatures fell by 2.3 °C, and GI-1b, when temperatures fell by 2.8 °C. At Loch Ashik only the oscillation correlated with GI-1d is clearly defined, when temperatures fell by 3.8 °C. The start of the YD is clearly marked at both sites when temperatures fell by 5.5 °C at Abernethy Forest and 2.8 °C at Loch Ashik. A warming trend is apparent during the late-YD at Abernethy Forest but at Loch Ashik late-YD temperatures became very cold, possibly influenced by its close proximity to the Skye ice-field. The rapidly rising temperatures at the YD - Holocene transition occur about 300 years earlier at both sites than changes in sediment lithology and loss-on-ignition. The temperature trends at both sites are broadly similar, although between-site differences may result from the influence of local factors. Similar climate trends are found at other sites in the northern British Isles. However, the British summer temperature records differ in detail from trends in the oxygen-isotope records from the Greenland ice-cores and from other chironomid-inferred temperature records available from Scandinavia, north-west Europe and central Europe, which suggest important differences in the influence of climatic forcing at regional scales.
Article
Glaciers and Glaciation is the classic textbook for all students of glaciation. Stimulating and accessible, it has established a reputation as a comprehensive and essential resource. In this new edition, the text, references, and illustrations have been thoroughly updated to give today's reader an up-to-the minute overview of the nature, origin, and behavior of glaciers and the geological and geomorphological evidence for their past history on earth. The first part of the book investigates the processes involved in forming glacier ice, the nature of glacier/climate relationships, the mechanisms of glacier flow, and the interactions of glaciers with other natural systems such as rivers, lakes, and oceans. In the second part, the emphasis moves to landforms and sediment, the interpretation of the earth's glacial legacy, and the reconstruction of glacial depositional environments and palaeoglaciology.
Article
Presents current knowledge on the effects of periglaciation on the British landscape. The book is in 4 parts, the first of which provides an introduction to periglaciation and the Quaternary environment and chronology. The second part deals with the periglaciation of lowland Britain (below 400m asl), including chapters on ice wedge casts, pingos, mass wasting and landscape modification. The third part looks at the periglaciation of upland Britain, which generally is underlain by more resistant rocks, and includes topics on frost weathering, patterned ground, solifluction, talus slopes and related landforms. The final part of the book examines the periglacial enviroments of three contrasting periods: the Dimlington Stadial; the Loch Lomond Stadial and the present day environment in upland Britian. An extensive reference list is provided. -R.Gower
Article
Recent dating of catastrophic rock-slope failures (RSFs) in the Scottish Highlands has confirmed that many occurred in the millennium following deglaciation. This implies that numerous Lateglacial RSFs occurred in the interval between ice-sheet deglaciation (˜15–14 ka) and the final retreat of glacier ice (12.5–11.5 ka) at the end of the Loch Lomond Stade (LLS), but have not been recorded because RSF runout debris was removed by LLS glaciers. The morphology of postglacial RSFs is used as a guide to identification of the failure sites of such Lateglacial RSFs. Key elements are a steep (>55°) headscarp separated by a pronounced break of slope from a subjacent quasi-rectilinear or stepped failure plane inclined at 35–45°; flank scarps, headscarp tension cracks and detached blocks may also be present. Inferred Lateglacial RSF sites occur on trough walls, spur ends and within cirques. Many small valley-side scarps lacking runout debris are inferred to represent sites of minor Lateglacial RSFs. The implications of these findings for trough development, mountain summit morphology and cirque evolution are discussed. Future research questions relating to RSF sites that predate the Last Glacial Maximum and the contribution of Lateglacial RSFs to the sediment budget of LLS glaciers are outlined.
Article
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 wet-based ice at pressure melting point from cold-based ice on summit plateaus. We test these competing hypotheses through Be-10 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 Be-10 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 similar to 15-16 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 similar to 14.7 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. (c) 2012 Elsevier Ltd. All rights reserved.