Quaternary Science Reviews 21 (2002) 89–101

New data for the in Great Britain and D.Q. Bowena,*, F.M. Phillipsb, A.M. McCabec, P.C. Knutza, G.A. Sykesa a Department of Earth Sciences, University of Cardiff, Cardiff, CF10 3YE, UK b Department of Earth and Environmental Sciences, NewMexico Tech, Socorro, NM 87801, USA c School of Environmental Sciences, University of Ulster, Coleraine BT52 1SA, Received 15 March 2001; accepted 28 August 2001

Abstract

Understanding the history of the British and Irish Ice Sheet (BIIS) at the Last Glacial Maximum (LGM) has been advanced by new approaches, in particular, by cosmogenic nuclide surface-exposure dating, aminostratigraphy of ‘shelly’ glacial deposits, AMS radiocarbon dating, and the evidence from continental margin marine cores, all of which supersede the previously weak geochronologic control. It was formerly believed that Great Britain and Ireland was largely ice free between the last (oxygen isotope sub-stage 5e) and the Late Devensian, when the LGMoccurred. As such the BIIS was effectively out of phase with Laurentide and Scandinavian ice sheets, as well as inferences of ice volume from oxygen isotope stratigraphy. The BIIS during the Late Devensian maximum was also perceived as having been relatively stationary. New evidence shows that the LGMwas an important event during the evolution of an earlier BIIS when the extent of ice was greater. Repeated rafting events over the past 50 ka are shown by marine cores, while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36Cl ages on glaciated surfaces and glacial boulders, that are indicative of deglacial events between 40 and 12 ka. These appear to be associated with Heinrich events, the earliest being inferred as Heinrich 4 at about 40 ka. During this advance, the BIIS and Scandinavian Ice Sheet (SIS) were in contact and all of Ireland was glaciated. The ice sheet appears to have fluctuated several times between 40 and 25 ka, although evidence for this is poorly preserved. But the 36Cl and 14C evidence is clear that the BIIS reached its LGMmaximum size about 22 ka, soon after Heinrich Event 2, when the BIIS and SIS were not in contact. One cluster of 36Cl and 14C ages, at 21.471.3 ka, records an initial pulse of that was followed by extensive deglaciation about 17.470.4 ka just before Heinrich Event 1, when the ice sheet readvanced. Contrary to previous views, the BIIS probably existed throughout much of Devensian time as a mobile and sensitive ice sheet, during which the LGMadvance was but one important event. In places, glacial deposits of the earlier Devensian glaciation have previously been incorrectly identified as products of the later LGMglaciation. r 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction the precise timing of the maximum extent of ice during the Last Glacial Maximum (LGM). The British and Irish Ice Sheet (BIIS) consisted of The definition of the LGMused here is that lowland and upland centres of ice dispersal that were the recommended by the EPILOG project (Environmental main driving forces of ice sheet configuration and flow Processes of the : Land, Oceans, ) at on to the continental shelf. Thickened wedges, ‘Chronozone Level 1’, between 19 and 23 cal ka (Mix ice-contact morainic ridges and terminal outwash et al., 2001). Following the recommendation of that spreads show that deglaciation was characterized by project, radiocarbon ages are calibrated to calendar ages shrinking ice sheet margins that contracted back to most using the CALIB-4 calibration scheme. of the original centres of ice dispersion. But until The standard view of the Devensian (Wisconsinan/ recently little sedimentologic, paleoclimatic and geo- Weichselian) was that the maximum extent of LGMice chronologic evidence has been available to understand largely coincided with the limit of ‘fresh’ relatively the environments of deposition, the climate forcing or unmodified glacial and fluvioglacial landforms. This stemmed from Wright’s (1914, 2nd edition 1937) distinction between ‘older’ and ‘newer drifts’, followed by Charlesworth (1928) in Ireland with the delineation *Corresponding author. Tel.: +44-29-2087-4830; fax: +44-29-2087- 4326. of the ‘South Irish End ’ that marked the LGM E-mail address: bowendq@cardiff.ac.uk (D.Q. Bowen). limit. It was then correlated with other ‘newer drift’

0277-3791/02/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0277-3791(01)00102-0 90 D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101 landforms in and Wales (Charlesworth, 1929). 2. Cosmogenic exposure ages Mitchell et al. (1973) defined the Late Devensian, when the LGMoccurred, as between 26,000 and 10,000 In many cases, pre-existing geochronologic limita- radiocarbon years ago. But they believed that all of tions have been overcome through the application of previous Devensian time, from the ‘last interglacial’ to cosmogenic nuclide surface-exposure dating. Cosmo- 26 14C ka BP, was ice free. This implied that the BIIS genic nuclides (3He, 10Be, 21Ne, 26Al, 36Cl) are produced was out of phase with both Laurentide and Scandina- in rocks at the surface of the earth by the action of vian ice sheets, as well as with ice volume changes cosmic radiation (Cerling and Craig, 1994). Most of the inferred from oxygen isotope stratigraphy (Shackleton cosmic ray flux is attenuated within the top one to two and Opdyke, 1973). meters of the surface of the solid earth and thus The CLIMAP reconstruction of the LGM (CLIMAP, processes such as glaciation that excavate rock 1981) adopted views then current on the location from greater depths and deposit it on the surface can of the ice margin in England (Boulton et al., 1977), be dated by measuring the accumulation of cosmogenic Wales (Bowen, 1973), and southern Ireland (Synge, nuclides. Gosse and Phillips (2001) have recently 1970). But in western Ireland it depicted ice that reviewed the principles and methods of cosmogenic extended offshore. It also showed the BIIS and nuclide dating. Scandinavian Ice Sheet (SIS) to be in contact in the For this study, glacial erratic boulders and glacially North Sea (Andersen, 1981). The IGCP-24 project smoothed bedrock were sampled around the former ice (Glaciations in the Northern Hemisphere) showed an margins. Criteria described in Gosse and Phillips (2001) LGMice margin corresponding to those shown in Fig. 1 were used in selecting sample sites. The possibility of for Ireland and Scotland. But in England and Wales it coverage by peat was a primary consideration but it is corresponded with the ‘earlier Devensian ice maximum’ believed that the sample sites presented here had, at shown on Fig. 1. This project also concluded that the most, a thin peat blanket. Close attention was given to BIIS and SIS were not in contact during the LGM weathering and surface degradation, attempting to (Bowen et al., 1986). sample whenever possible surfaces exhibiting glacial The timing of the ‘LGM’ was adduced from only five smoothing or striations. The samples were processed for ‘maximum’ radiocarbon ages from four localities. These 36Cl analysis by dissolution in hydrofluoric acid. An were 18,2407250 14C yr BP (21.97 cal ka) and isotopically enriched, stable 35Cl spike was added to the 18,5007499 14C yr BP (21.7 cal ka) at Dimlington, east- samples during processing so that the 36Cl and stable Cl ern England (Penny et al., 1969); 30,500 14CyrBP at concentrations could be determined by isotope dilution Four Ashes, West Midlands of England, (Shotton, mass spectrometry during analysis by accelerator mass 1967b); 18,000 14C yr BP (21 cal ka) in northeast Wales spectrometry at Purdue Rare Isotope Measurement (Rowlands, 1971); and 30,500 14C yr BP in northwest Laboratory (PRIME Lab) (Elmore et al., 1979). Sur- Ireland (Colhoun et al., 1972). Ubiquitous radiocarbon face-exposure ages were calculated using the data ages for deglaciation throughout the British Isles were reduction program CHLOE (Phillips and Plummer, available from peat samples that infilled holes, 1996) according to the methods and production although these mainly provided Blling/Allerd ages. constants given in Phillips et al. (2001) and Gosse and The dearth of reliable ages for establishing the age of the Phillips (2001), and are given in Tables 1 and 2. Surface- LGMcontrasts greatly with North America where the exposure ages are affected by rock-surface rates, advection of moist air from the Gulf of Mexico and thus ages are listed for possible erosion rates promoted tree growth close to the southern Wisconsinan ranging from 0 to 3 mm/kyr. Sample-specific erosion ice margin. Except for ‘interstadial’ tree growth during rates were estimated based on field observations. The oxygen isotope stage 5 (Jones and Keen, 1993) there is ages corresponding to the best-estimate erosion rates are no evidence in Great Britain of tree growth during the in bold type in Table 1. Devensian other than during the Blling–Allerd. Valuable information on extent of ice has been Europe may have been isolated from warm air masses obtained in Scotland based on cosmogenic dating of by west-to-east topographic barriers, and advection of trim lines around nunataks, from which ice profiles can warm moist air masses occurred only from the west be extrapolated (Stone et al., 1998). In Ireland and during times of relatively brief interstadial warmth and Gower, however, sampling was carried out close to the strong thermohaline circulation. Thus, without appro- limits of glacial stratigraphic units that define the former priate organic samples for radiocarbon ages, most ice sheet margins. The distribution of ages resulting sectors of the Devensian ice sheet lacked adequate from this sampling approach have yielded coherent geochronologic control. This remained so until the patterns, both geographically, in relation to previously advent of cosmogenic rock exposure ages, amino acid mapped glacial stratigraphic boundaries, and tempo- geochronology and AMS 14C dating of monospecific rally, in relation to ice-discharge events inferred from marine microfaunas. the marine record. D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101 91

Fig. 1. Ice margins and localities referred to in the text. 92 D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101

Table 1 36 Cl ages, locations, elevation, topographic scaling (ST), and effective fast neutron attenuation length, as a function of surface orientation, (Lf;e) for Ireland samplesa

36 Group No. Location Cl ages given assumed erosion rate Latitutude Longitude Elevation ST Lf;e (1N) (1W) (m) (unitless) (g/cm2) Zero Erosion 1 mm/kyr 3 mm/kyr (ka) (ka) (ka)

5 23 Lough Accorymore 12.770.7 12.470.6 11.870.6 53.9817 10.17 200 0.980 170 22 Accorymore House 13.470.6 13.570.6 13.670.6 53.975 10.165 190 0.996 164

4 21 Kilkee 15.871.1 15.371.0 14.570.9 52.675 9.67 66 0.985 155 20 Lough Bray 15.971.1 15.871.1 15.771.1 53.175 6.25 420 1.000 170

3 19 Leckan More 18.371.2 17.671.1 16.771.0 54.125 6.175 354 0.993 161 18 Lough Nakeeroge 16.471.9 16.671.9 17.072.1 53.9967 10.1367 5 0.980 161 17 Lough Nahanagan 17.971.0 17.570.9 17.170.9 53.04 6.38 450 0.971 150 16 Windy Gap 17.070.6 17.170.6 17.370.6 54.05 6.25 243 0.984 161 15 Rasharkin 17.972.7 17.772.6 17.572.6 54.992 5.43 186 0.990 170 14 Lough Nakeeroge 17.573.7 17.773.8 18.074.0 53.9967 10.1367 5 0.980 164

2 13 Loop Head 21.570.9 20.371.8 19.172.6 52.63 9.75 120 0.990 169 12 Cornarone 22.373.1 20.972.7 19.172.3 53.23 9.45 30 0.990 170 11 Sheep’s Head Pen. 21.171.3 21.371.3 21.871.4 51.62 9.58 152 0.995 170 10 Carrigacapeen 22.873.7 22.073.4 20.973.1 51.817 9.504 30 0.998 165 9 Lough Accorymore 21.575.2 21.675.3 22.175.6 53.975 10.165 190 1.000 170 8 Mottee Stone 23.172.2 22.372.0 21.471.9 52.88 6.2 250 0.993 161 7 Hatton Farm 25.473.3 23.672.8 21.672.5 52.43 7.12 245 0.980 168

1 6 Malin Head 25.171.1 25.171.1 25.171.2 55.38 7.373 55 0.869 124 5 Bloody Foreland 30716 31717 32719 55 8 70 0.990 170 4 Knocknagravure 32.875.1 32.975.1 33.475.5 54.092 10.082 20 1.000 170 3 Bray 41717 36713 32711 53.16 6.25 213 0.988 157 2 Rathpatrick Cross 40.274.3 36.573.6 33.173.2 52.283 7.067 75 0.990 169 1 St. Molan’s Cave 37.571.5 33.971.3 30.571.1 52.867 7.117 244 0.990 169 Kerry Head 228726 168716 181729 51.82 9.5 36 0.980 168

a Ages corresponding to best estimate of erosion rates are indicated by bold type.

Fig. 2 shows five groups of 36Cl exposure ages that chromatographs for analytical reliability is described indicate deglaciation. Group 1 indicates deglaciation in Bowen (2000). Aminostratigraphic units are defined ages of about 3771 and 3271.3 ka. Group 2 36Cl as stratigraphic units defined or characterized on the average ages are 2271.1 ka, but with 14C ages included basis of the racemization ratios of particular amino this becomes 21.471.3 ka. Group 3 36Cl ages have a acids in their contained fossils, while aminozones are mean age of 17.370.04 ka, but with 14C ages included regional aminostratigraphic units. Amino acid geochro- this becomes 17.470.4 ka. Groups 4 and 5 36Cl ages nology is a geochronologic framework of aminozones correspond with the transition to the Blling and with with numerical ages provided by independent geochro- the , respectively. nologic dating methods (Bowen, 2000). These concepts allow the subdivision of the glacial stratigraphic record and, with independent dating calibration, provide 3. Aminostratigraphy and amino acid (d-Aile/l-Ile) geochronologic ages for glacial events. geochronology Three genetic stratigraphic units are described. (1) Glaciomarine muds containing in situ marine faunas. Amino acid geochronology was applied to bivalves These have tight clusters of d-Aile/l-Ile ratios, where and gastropods from marine, glaciomarine, and glacial one standard deviation is characteristically less than deposits containing marine shells. The time-dependent 10% of the mean. (2) Glaciomarine deposits containing epimerization of l-isoleucine to d-alloisoleucine in the derived marine shells that exhibit a wider range of d- marine fossils was measured using High Pressure Liquid Aile/l-Ile shell ratios, but with a distinct cluster of the Chromatography. Analytical procedures that utilize the youngest ratios. (3) Shelly glacial , deposited Wehmiller international standard are described in by a terrestrial ice sheet that crossed a sea area, that Bowen et al. (1998), and a system of screening exhibit a wide range of d-Aile/l-Ile shell ratios but also Table 2 Chlorine-36 ratio and chemical composition of Ireland samples

36 Group No. Location Cl/ Cl CO2 Na2O MgO Al2O3 SiO2 P2O4 K2O CaO TiO2 MnO2 Fe2O3 B Gd U Th (10 15Cl) (ppm) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (ppm) (ppm) (ppm) (ppm) ..Bwne l utraySineRe Science Quaternary / al. et Bowen D.Q. 5 22 Lough Accorymore 10575 24.1 0.34 0.42 0.16 3.05 92.50 0.01 1.32 0.02 0.27 0.006 1.25 6.5 1.0 0.0 0.0 21 Accorymore House 431720 12.3 1.11 1.69 0.55 10.0 80.05 0.01 3.71 0.94 0.28 0.030 1.83 14 2.0 1.2 1.8

4 21 Kilkee 8275 30.9 1.19 1.01 0.46 6.26 86.51 0.04 0.95 0.02 0.57 0.010 2.04 22 4.0 1.8 6.3 20 Lough Bray 262718 45.9 0.59 3.25 0.19 13.7 75.15 0.05 5.26 0.38 0.08 0.011 0.42 12 3.0 1.5 0.5

3 19 Leckan More 13178 157 0.52 2.24 4.00 10.9 62.70 0.05 0.83 11.77 0.76 0.096 5.55 16 5.5 2.4 9.3 18 Lough Nakeeroge 9117103 10.1 3.40 0.56 1.94 16.5 61.80 0.02 7.27 0.52 0.66 0.060 6.66 7.0 3.5 1.3 2.9 17 Lough Nahanagan 202711 48.13 1.08 3.41 0.44 13.54 75.69 0.05 3.16 0.64 0.14 0.04 1.10 12.50 1.50 1.50 0.50 16 Windy Gap 562720 20.4 2.58 1.47 3.47 26.8 46.58 0.02 0.09 16.46 0.23 0.051 2.92 14 2.5 0.5 0.5 15 Rasharkin 181726 40.5 2.02 2.55 8.14 16.0 45.37 0.14 0.16 8.93 1.90 0.207 14.2 11 4.0 0.5 0.5 14 Lough Nakeeroge 6357130 9.54 1.37 1.95 0.89 12.0 75.10 0.01 4.41 0.25 0.27 0.033 2.86 8.0 3.0 2.1 4.6

2 13 Loop Head 11978 29.1 4.81 2.82 1.45 16.7 63.98 0.12 1.22 0.06 0.97 0.054 6.36 43 6.0 3.1 6.3 7 12 Cornarone 119 14 59.4 0.56 4.64 0.37 10.5 80.14 0.03 1.61 0.21 0.18 0.024 1.17 11 2.5 1.6 5.7 v 11 Sheep’s Head Pen. 490730 11.1 2.23 0.88 1.69 11.6 73.16 0.02 2.71 0.01 0.81 0.049 6.00 38 5.0 3.0 9.4 89–101 (2002) 21 iews 10 Carrigacapeen 126718 36.0 1.62 1.40 1.26 8.34 81.54 0.03 1.48 0.06 0.55 0.037 2.89 20 4.5 2.1 4.6 9 Lough Accorymore 7957185 7.46 0.93 0.96 0.52 7.56 82.72 0.01 2.82 0.14 0.52 0.042 2.75 9.0 2.0 1.7 5.9 8 Mottee Stone 178716 60.3 0.61 4.17 0.29 14.4 74.31 0.06 2.68 1.48 2.68 0.041 0.80 12 3.0 1.4 1.0 7 Hatton Farm 146715 44.3 1.25 0.05 0.26 3.91 91.00 0.01 1.56 0.01 0.20 0.005 1.90 24 1.0 1.5 3.1

1 6 Malin Head 320713 20.2 0.64 0.39 0.20 5.75 88.28 0.01 2.97 0.11 0.34 0.005 1.13 7.5 2.0 3.0 9.4 5 Bloody Foreland 235071180 3.70 0.19 4.40 0.04 12.2 78.27 0.01 3.40 0.53 0.04 0.007 0.42 6.5 1.5 7.0 16 4 Knocknagravure 583783 21.6 0.34 4.04 0.11 12.8 77.09 0.02 4.25 0.36 0.08 0.045 0.44 8.5 4.0 4.4 4.7 3 Bray 215782 19.9 0.34 0.00 0.08 1.38 96.47 0.02 0.41 0.01 0.17 0.002 0.32 13 2.0 1.4 1.0 2 Rathpatrick Cross 211721 44.3 1.25 0.05 0.26 3.91 91.00 0.01 1.56 0.01 0.20 0.005 1.90 24 1.0 1.5 3.1 1 St. Molan’s Cave 23579 44.3 1.25 0.05 0.26 3.91 91.00 0.01 1.56 0.01 0.20 0.005 1.90 24 1.0 1.5 3.1 Kerry Head 768764 52.0 0.95 0.56 0.61 4.44 90.02 0.02 1.38 0.03 0.22 0.008 1.51 19 3.5 1.0 1.3 93 94 D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101

Fig. 2. 36Cl cosmogenic rock exposure ages (triangles) and radiocarbon (cal) ages (boxes) plotted against latitude. Roman numerals indicate Irish 36Cl exposure localities (Fig. 1 and Table 1). Large standard deviations for 2, 3 and 5 (Table 1) are not plotted. Arabic numerals indicate 36Cl localities in Wales: i, Cwm Idwal col; ii, Gower, South Wales; iii & iv, Cwm Idwal (Phillips et al., 1994). 14C (cal) ages: a, Dimlington, England; b, Belderg, Ireland; c, Killard Point, Ireland; d, Kilkeel, Ireland; e,Cranfield Point; f, Rough Island, Ireland; g, Glanllynnau, Wales. Geochronologic groups (see text). For localities see Fig. 1. have a distinct cluster of the youngest ratios. In (2) and immediately after the LGM. It is converted to a (3), the d-Aile/l-Ile age of the youngest faunal cluster geochronology by radiocarbon ages at two sites. First, defines a maximum age for the stratigraphic unit. For all 14C ages of 18,2407250 yr BP (21.96 cal ka) and three units, independent calibration converts the d-Aile/ 18,5007499 yr BP (21.7 cal ka) on arctic silts from the l-Ile of the youngest faunal element into a geochronol- Dimlington Bed, eastern England. This lies in hollows ogy that indicates a maximum geochronologic age for its on the surface of the Bridlington Member (‘Basement host sediment. ’) from which derived Macoma shells provide LGM Two principal aminozones have been recognized and aminozone ages (Eyles et al., 1994). Second, 14C ages of calibrated by U-series and 14C. The earlier aminozone 16,9407120 yr BP (19.60 cal ka) and 17,3007100 yr BP corresponds with d-Aile/l-Ile ratios characteristic of (20.01 cal ka) on in situ Macoma shells from a Gilbert- oxygen isotope sub-stage 5e that is calibrated by U- type delta, part of the Belderg Formation, Donegal Bay, series ages (Bowen, 2000). On first inspection it might Western Ireland (McCabe et al., 1986). appear that two separate aminozones may be identified The 5e aminozone was originally defined from in this 5e aminozone (Fig. 3), but its integrity is fossiliferous marine deposits in the Netherlands confirmed by the range of d-Aile/l-Ile values from two (above) and was based on the moderate-rate racemizing unequivocal sub-stage 5e sites: the Severn Estuary in genera Arctica, Macoma, Hiatella and Mya. Its Wales and Castricum (Eemian parastratotype) in The definitive d-Aile/l-Ile ratio was 0.1870.02 (Miller and Netherlands. Marine shells of this aminozone have 14C Mangerud, 1985). New evidence from the Severn ages>37 14C ka BP at Castle Hill in Scotland, and Estuary in southeast Wales, however, expands that Derryogue, Ireland, while an age of 34.74 14C ka BP was definition. In littoral deposits at Goldcliff, near New- obtained on ‘bulk shell fragments’ at Moel Tryfan in port, the Macoma d-Aile/l-Ile ratio is 0.1370.03 Wales (Foster, 1970). (Fig. 3), with corresponding ratios for the slower-rate The younger aminozone corresponds in time with racemizing Nucella of 0.0970.01and Littorina sp.of marine transgression immediately before, during, and 0.08470.012 (both standardized to L. Littorea) (Allen, D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101 95

Fig. 3. Mean amino acid ratio (d-Aile/l-Ile) ratios plotted against Mean Annual Temperature. The sloping line is the approximate boundary between the 5e and LGMaminozones and represents decreasing rates of epimerization with lower temperatures. Circles: marine deposits in situ. Triangles: shelly glacial deposits. Localities: 1, Severn Estuary, Wales; 2, Castricum (The Netherlands); 3, Hunstanton, England; 4, Derryogue, Ireland; 5, Caithness, Scotland; 6, Isles, Scotland; 7, Malin Beg, Ireland; 8, Ballyrisk, Ireland; 9, Clava (‘raft’), Scotland; 10, Castle Hill, Scotland; 11, Moel Tryfan, Wales; 12, Ballycotton Bay, Ireland; 13, Traeth-y-Mwnt, Wales; 14, Abermawr, Wales; 15, Belderg, Ireland; 16, Isle of Man; 17, Dimlington, England; 18, Skerries, Ireland; 19, northwest Isle of Man; 20, Shanganagh, Ireland; 21, Sistrakeel, Ireland; 22, Sourlie, Scotland. For localities see Fig. 1.

2000). These unexpectedly lower ratios may be consis- sheet, in which the LGMwas but one important event at tent with a longer rather than a shorter interglacial about 22 ka. Here, we discuss the sequence of Devensian (Winograd et al., 1997) when sea level was close to BIIS events as inferred from new geochronologic data. present for longer than previously thought, and it may resolve an apparent double sea level during 5e (Bowen 4.1. Pre-LGM et al., 1985). Miller and Mangerud (1985) showed that aminozone boundaries in Europe sloped from south to Group 1 36Cl exposure ages (Table 1) around the north as a function of the integrated temperature at margins of Ireland indicate deglaciation of an ice sheet different localities. The practice of plotting the current that formerly extended offshore. These range between mean annual temperature (MAT) at a site against the 37.5 and 25.1 ka and show that glaciated terrain outside mean d-Aile/l-Ile ratio as surrogate for the effect of the LGM, formerly thought to be Munsterian (pre-5e) integrated temperatures on racemization (Wehmiller in age (Synge, 1970), was glaciated during oxygen and Miller, 2000) is shown on Fig. 3 for Great Britain isotope stage 3. The 36Cl exposure ages of Group 1 are and Ireland. It demonstrates that Bowen and Sykes probably snapshots of the deglacial history of a mobile (1988) were mistaken in their assumption that amino- ice sheet that may have fluctuated considerably. But, as zones were not affected by any significant south to north inferred from d-Aile/l-Ile and 36Cl data, its final retreat temperature gradient because of more or less equable from the continental shelf, at about 25 ka (Malin Head), maritime temperatures. The small d-Aile/l-Ile separa- probably led to the first extensive marine transgression tion between the 5e and ‘LGM’ aminozones is attributed of British and Irish seaways since that of oxygen isotope to limited racemization of the 5e fauna, which was sub-stage 5e. driven by relatively brief intervals of interstadial warmth The glaciation of Caithness and Orkney, northeast during Dansgaard–Oeschger interstadials. Scotland, was affected by a Scottish ice sheet that was deflected onshore in a northwesterly direction by the presence of Scandinavian ice in the North Sea. Before 4. The sequence of events deflection, the ice sheet had crossed the floor of the Moray Firth where it picked up marine shells, subse- Unlike previous views, new data have shown that the quently incorporated into glacial deposits in Caithness BIIS was a long-lived feature that probably evolved for and Orkney. On the south side of the Moray Firth, the much of Devensian time as a mobile and sensitive ice ice similarly crossed the coastal strip to deposit shelly 96 D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101

Fig. 4. Total amino acid ratios v Free amino acid ratios (d-Aile/l-Ile) for Macoma, Arctica, Hiatella and Mya from shelly glacial deposits in Caithness, The Orkney Isles, and Castle Hill (14C>37 ka) (northeast Scotland), and the Severn Estuary (aminozone 5e), South Wales. This should be compared with Fig. 3. Comparison of the data with the standard oxygen isotope sub-stage 5e d-Aile/l-Ile ratios shows that the youngest faunas represented in the diagram are time-equivalent to oxygen isotope sub-stage 5e. glacial sediments at Castle Hill (Peacock and Merritt, land, 1842) provide further evidence for the extent of 1997) and adjacent King Edward (Miller et al., 1987). glaciation. The marine fauna was derived from the floor The age of the youngest derived shells corresponds with of the by an ice sheet that ascended a ‘gentle aminozone 5e (Figs. 3 and 4). Thus the glaciation of incline’ (Wright’s, 1914, 2nd edition 1937), but with a these areas occurred before the LGMwhen the BIIS and vertical ascent among the greatest known (Flint, 1957). SIS were in contact in the North Sea. The approximate It contains shells of Macoma with 5e Aminozone d-Aile/ extent of the BIIS (Fig. 1) may be marked by offshore l-Ile ratios that confirm the major extent of the pre- morainic structures offshore from northeast (Hall and LGMglaciation. The major coeval Welsh ice advance is Bent, 1990) and northwest Scotland (Stoker et al., 1994). confirmed by a 36Cl deglaciation age of 38.575.4 ka In eastern England, ice extended to as was (recalculated from Phillips et al., 1994) from a col above suggested by Straw (1979), who named it an ‘Early the headwall of Cwm Idwal (Darwin, 1842). The Devensian’ glaciation. This pre-LGMglaciation is extent of this glaciation in Wales is uncertain, but confirmed by derived shells of Macoma in the Hun- indications of its southwestern margins may be repre- stanton (glacial) Formation with aminozone 5e d-Aile/ sented by the relatively high-level (B180 m) fluvioglacial l-Ile ratios. In English West Midlands and the Welsh landforms between Traeth-y-Mwnt and Abermawr Borderlands, Irish Sea and Welsh ice was coeval (Wills, (Fig. 1). These invite comparison with the similar 1937, 1952). The Irish Sea ice penetrated the Cheshire– features in Norfolk (Sparks and West, 1964), also Shropshire–Staffordshire region as far south as the attributed to the pre-LGMglaciation (Straw, 1979). ‘Wolverhampton Line’ where, at Four Ashes, a 14C age of 30 ka BP was previously used to provide a maximum 4.2. The LGM age for the LGMglaciation (Shotton, 1967b). Unlike the glacial topography north of the Whitchurch The LGMadvance followed ice retreat when, as Moraine, however, that to the south is notably subdued, shown by the extensive LGMaminozone marine fauna and aminozone 5e d-Aile/l-Ile ratios from its shelly incorporated into its deposits, marine transgression gravels suggest that such a major geomorphic contrast flooded the seaways of Great Britain and Ireland reflects an age difference (Boulton and Worsley, 1965; (Fig. 3). At Dimlington, eastern England, the Bridling- Worseley, 1970). Such recognition of a pre-LGM‘Main ton Member (‘Basement Till’)Folder than 21.97 and Irish Sea’ glaciation to the ‘Wolverhampton Line’ 21.70 cal ka (Penny et al., 1969)Fcontains a 5e amino- (Fig. 1) reverts to the previous views of Wills (1937, zone Macoma fauna (Eyles et al., 1994), unlike the 1952) and Shotton (1967a). glacial deposits of Norfolk (above). The LGMbound- The shelly sands and gravels at over 400 m on Moel ary, therefore, lies between Dimlington and Norfolk, Tryfan, and at similar heights on adjacent north-facing and Straw (1979) placed it at the Killinghome– mountains in northwest Wales (Trimmer, 1831; Buck- Hogsthorpe in Lincolnshire (Fig. 1). In the D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101 97

Cheshire–Shropshire lowland, LGMice advanced to the 5. Land-sea correlation Whitchurch Moraine (Boulton and Worsley, 1965) that was coeval with Welsh ice from the west (Wills, 1937, A 30-m core from the Barra Fan (MD95-2006) off the 1952). Hebridean margin (location on Fig. 1) shows a detailed In northeast Scotland, LGMice advanced into the record of from the British ice sheet back to Moray Firth, but not as far as Castle Hill or southern about 45 cal ka (Fig. 5) (Knutz, 2000). Knutz et al. Caithness (Sutherland, 1984) (Fig. 1). At Clava (Fig. 1), (2001) provide a detailed analysis of the data (shown on its deposits incorporate a raft of earlier, pre-LGM Fig. 5). Ice-rafted debris (IRD) input from the ice sheet glacial sediments that has an exclusively 5e aminozone is inferred from glaciomarine deposits that contain a fauna and 14C ages of 43.8 ka BP, >39.4 ka BP, series of coarse-grained turbidites. Between 10 and >43 ka BP, and >41.2 ka BP (Merritt, 1992). The 45 cal ka, IRD deposition increased every 2 to 3 ka, absence of raised shorelines in the Orkney Isles is similar to the timescale of Dansgaard–Oeschger events. further evidence that the area was not glaciated during Glaciomarine sediments are interbedded with silty, the LGM(Smith, 1997, 2000). In Ireland, the LGMice CaCO3 rich deposits, shown as shaded bands on margin corresponds with that largely established by Fig. 5, that indicate times of enhanced bottom current Charlesworth (1928) as refined by Synge (1970). In West activity and high surface water productivity in the Wales (Fig. 1) LGMaminozone faunas date the Rockall Trough. These are correlated with GISP2 glaciomarine sediments at Traeth-y-Mwnt and Aber- interstadials (Knutz et al., 2001). mawr (Fig. 3), while in South Wales a 36Cl age (below) Some provisional correlations may be made between from the giant erratic boulder of Arthur’s Stone in land and sea records over this period. The land-sea Gower defines the LGMextent on its ice margin. correlation model is based on evidence from northeast Overall, the LGMlimit of the BIIS is similar to the Ireland, where 14C and 36Cl geochronology shows that IGCP-24 reconstruction (Bowen et al., 1986), although the response of the ice sheet to forcing associated with less extensive in eastern and western England (Fig. 1). Heinrich Events was one of deglaciation (from 36Cl age Compared with the CLIMAP (1981) reconstruction it is clusters), readvance and ice surging, followed by much less extensive, and the BIIS and SIS were not in regional deglaciation (from 14C ages) (McCabe and contact. Clark, 1998). The cluster of 36Cl ages between 37.5 and The timing of the LGMis largely established on its 36 ka (1, 2, and i on Fig. 2) may correspond with southern and western margins from 36Cl exposure Heinrich Event 4 and the IRD peaks BF 13.1 and 13.2 in deglaciation ages on glaciated surfaces and glacial the Barra Fan core. They indicate deglaciation of boulders close to the former ice margin. In Ireland, the southeast Ireland and also Eryri (Snowdonia) in north- average 36Cl age for an early pulse of deglaciation is west Wales, areas not subsequently covered by the BIIS 21.871 ka. These data also show that all ice centres during the LGM, except for the Cork-Kerry in were in phase (Fig. 2). If calibrated 14Cl ages for southern Ireland. A subsequent group of 36Cl ages deglaciation from glaciomarine deposits that indicate between 33 and 31 ka (4 and 5 on Fig. 2) may similarly early deglaciation within the ‘Chronozone Level 1’ time- be related to ice retreat from northwestern Ireland band are included, the average age is 21.271.4 ka. In following Heinrich Event 3. Finally, samples 7 and G eastern England, at Dimlington, early deglaciation is indicate that retreat may be associated with Heinrich also indicated by calibrated 14C ages of 21.97 cal ka and Event 2. Although the data are sparse, they can be 21.70 cal ka. The oldest 36Cl ages from sites near or on interpreted as evidence that the BIIS reached its the ice margin are 22.372 ka on the Mottee Stone in maximum size before 37 ka. It pulsed in response to Ireland, and 23.272 ka at Gower South Wales (above) Heinrich and/or Dansgaard–Oeschger events, and it (Figs. 1 and 2). The average age of all 36Cl and attained successively smaller maximum limits with each calibrated 14C ages for the BIIS LGMis cycle, including Heinrich Event 1. This pattern is 21.471.3 cal ka. consistent with the trend of oxygen isotope values from Further deglaciation of LGMice is indicated by the the Greenland ice cores (Fig. 5). cluster of Group 3 36Cl ages of 17.370.5 ka, or with the The greater preservation of evidence from the later inclusion of calibrated 14C ages, 17.470.4 ka. This cycles, and hence the larger number of samples pattern of pre-LGMdeglaciation, accompanied by available, allows a more detailed reconstruction of marine transgression, LGMadvance, and LGMretreat events subsequent to the LGM. The 36Cl and 14C ages accompanied by marine transgression, was repeated fall into two distinct groups (Groups 2 and 3 in Table 1) with the subsequent Heinrich Event 1 advance, in with mean ages of 21.471.3 and 17.470.4 cal ka. These northeast (McCabe and Clark, 1998) and northwest can be related to the marine record of the Barra Fan Ireland, where its limit coincides with that of the LGM (core MD95-2006) and the continental slope south of (McCabe, in preparation). Elsewhere, the extent of Ireland on the northern side of the Bay of Biscay (core Heinrich 1 ice is indicated by McCabe et al. (1998). MD95-2002). Interpretation of physical and paleonto- 98 ..Bwne l utraySineRe Science Quaternary / al. et Bowen D.Q. v es2 20)89–101 (2002) 21 iews

Fig. 5. Land-sea correlation: Barra Fan core MD95-2006, 36Cl exposure ages, GISP2 (Grootes and Stuiver, 1997) and VM23-81 (Bond and Lotti, 1995). D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101 99 logic evidence from core MD95-2002 by Zaragosi et al. * These clusters appear to be associated with Heinrich (2001) indicates that following Heinrich 4, there was a events. The most extensive Devensian glacial advance gradual amelioration, leading to an incursion of warm was between isotope stage 5e and B37.5 ka, when all North Atlantic Drift (NAD) between 20.7 to 19.5 cal ka. of Ireland and Scotland were ice covered. It is The group 2 36Cl ages, which are all close to the LGM probable that the BIIS and SIS were confluent in margin, fall in the range 23.6–20.3 cal ka, as do the 14C the North Sea at this time, a conclusion also reached ages at Dimlington. Immediate subsequent retreat from for sometime before 26 cal ka by Serjup et al. (1996). the maximum or near-maximum position of these Its most likely age is B40 cal ka and it may have been sample localities is demonstrated by calibrated 14C ages associated with Heinrich event 4. of 19.8 cal ka at Belderg and 19.4 cal ka (16,7607130 * The BIIS reached its LGMmaximum size at 14C yr BP) at Kilkeel, that are associated, respectively, B22 cal ka, immediately following Heinrich event 2. with retreat of the Donegal Bay and Irish Sea ice The BIIS and SIS were not in contact at the LGM, streams. and northeast Scotland and Norfolk were ice free. The MD95-2002 results show a cold reversal between In parts of Ireland and Wales the LGMice margin 19.5 and 18.5 cal ka cal marked by IRD peaks BF5 and may coincide with that of the Heinrich 2 Event, BF4 in MD95-2006. There are no 36Cl ages that but in general its margin corresponds with previous correspond to this interval, and their absence indicates views. a time of ice advance. At 18.5 cal ka warm NAD * The LGMat B22 cal ka is a stage in the evolution of returned to the region and culminated in rapid the Devensian ice sheet. It followed deglaciation and sedimentation and fresh water indicators at MD95- marine transgression across a depressed crust because 2002 that Zaragosi et al. (2001) interpret as the result of marine shells dating from that transgression (LGM rapid deglaciation of the BIIS. The timing of this aminozone) characterize its glacial and glaciomarine interval, between 17.7 and 16.9 cal ka, corresponds with deposits. the range of group 3 ages between 18.0 and 16.7 cal ka, * One cluster of 36Cl and 14C ages at 21.471.3 cal ka and confirms major retreat of the BIIS. This was records an initial pulse of deglaciation. The fresh reversed between 16.9 and 15.8 cal ka, the peak of appearance of LGMfluvioglacial landforms may be Heinrich Event 1. During this time the BIIS readvanced, due to release of meltwater and glacial debris that as shown in northwest (McCabe and Clark, 1998) and built up thickened sediment wedges on land and on northeast Ireland (McCabe, in preparation), and in the continental shelf at this time. England and Scotland (McCabe et al., 1998). Its extent * This was followed by extensive deglaciation at in most regions, however, fell short of that reached at 17.470.4 cal ka. During this phase, subglacial im- the LGM. No 36Cl ages are available for the period of prints and ice marginal patterns record massive advance, but 36Cl samples 20 and 21 (Fig. 2) at B15.5 ka deglaciation from the continental shelf towards record its deglaciation. The H1 ice advance to tidewater terrestrial centres of ice dispersion. This was accom- areas produced the large lithic peak BF2 in MD95-2006 panied by a change from wet to dry basal ice regimes (Fig. 5) and a similar one in MD95-2002. in lowland areas, prior to ice build up to the H1 event (McCabe and Clark, 1998). * During the Heinrich 1 Event the ice sheet surged 6. Conclusions forwards, although in most places not to the LGM limits (McCabe et al., 1998). H1 drumlinisation * The pre-existing dearth of 14C ages for estimating the was followed by ice sheet collapse across northern timing and extent of the LGMhas been overcome by Britain. cosmogenic 36Cl surface-exposure dating, amino acid geochronology of marine shells in glacial deposits, AMS radiocarbon dating and evidence from con- tinental shelf margin marine cores. Acknowledgements * In contrast to previous views, the last BIIS can now be seen as a long-lived feature that probably existed D.Q. Bowen acknowledges NERC Research Grants through most of the Devensian as a highly mobile for the aminostratigraphic investigations, the 36Cl and sensitive ice sheet of which the LGMadvance exposure ages and the cost of field work. F.M. Phillips was but one important evolutionary event. thanks Cecilia McCord for the 36Cl sample preparation * The marine cores indicate repeated iceberg rafting and Lois Phillips for her perseverance in locating glacial events on millennial timescales over at least the past erratics. We thank Killian McDaid (Coleraine) and 50 ka. The inference of numerous glacial advances is Alun Rogers (Cardiff) for cartographic work, and Bill supported by several clusters of 36Cl ages that McCoy and an anonymous reviewer for their comments indicate deglaciation events between 37.5 and 12 ka. on the manuscript. 100 D.Q. Bowen et al. / Quaternary Science Reviews 21 (2002) 89–101

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