AMS 14C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet

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AMS

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C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet A.M. McCabea,, P.U. Clarkb, J. Clarka

a

School of Biological and Environmental Science, University of Ulster, Coleraine BT52, Northern Ireland b Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA Received 4 January 2004; accepted 1 June 2004

Abstract Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred X18.3 kyr 14C BP along the northwestern Irish coast, in agreement with a deglacial age of 22 36Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m 16.7 14C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL 15.0 14C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr 14C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 14C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction Throughout much of the 20th century, the general interpretation of Late Devensian deglaciation of the British Isles was of monotonic ice retreat towards centres of ice dispersion, possibly interrupted by stillstands or minor readvances (Mitchell et al., 1973). More recently, work by D.Q. Bowen and others has reassessed this perspective by reconstructing distinct events within the deglacial stratigraphy, relating these to large-scale Corresponding author. Tel.: +44 2870 324653; fax: +44 2890 324911. E-mail address: [email protected] (A.M. McCabe).

0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.06.019

depositional systems, and dating each event (McCabe et al., 1998; McCabe and Clark, 1998, 2003; Bowen, 1999; Bowen et al., 2002). The resulting recognition that the BIIS was sensitive to the abrupt climate changes that originated in the North Atlantic Ocean led to a new perspective of a dynamic BIIS characterized by large and rapid fluctuations. Proxy marine data of circumNorth Atlantic ice sheets, particularly ice-rafted debris (Bond and Lotti, 1995; Bond et al., 1999; Grousset et al., 2000; Scourse et al., 2000), similarly indicate millennialscale variability in ice sheet activity, although it remains difficult to interpret the specific ice sheet response of advance or retreat from these records (McCabe and Clark, 1998; Clark and Pisias, 2000).

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That large changes in the BIIS occurred in response to the abrupt climate changes that characterized the North Atlantic region during the last deglaciation may be anticipated because of its relatively small size and its location immediately adjacent to the North Atlantic Ocean (Fig. 1). Testing ice sheet-climate linkages on millennial timescales from terrestrial sequences is generally hampered by the relatively low preservation potential of dateable glacial deposits. In the case of the BIIS, however, extensive glacial sequences recording deglaciation are often preserved because of the rapidity and scale of initial deglaciation 17–18 14C kyr BP and the fact that subsequent ice-margin readvances were less extensive, resulting in sediment offlap towards inland centres of ice dispersion. Furthermore isostatic depression around the margins of the retreating BIIS in northern Britain resulted in deposition of fossiliferous marine sediments below, within and above ice-marginal successions, thus providing suitable material for radiocarbon dating. For these reasons, well-dated stratigraphic records of ice sheet variability and relative sea level change exist in northern Britain (McCabe and Clark, 1998, 2003).

Current evidence from the BIIS now demonstrates that North Atlantic climate change contributed to early deglaciation as well as to subsequent ice sheet reorganization with associated readvances characterized by widespread changes in ice flow lines and centres of ice dispersion (McCabe and Clark, 1998). We note that Synge (1968) first inferred large shifts in centres of ice dispersion from patterns of cross-cutting moraines and striae in western Ireland, but he did not have radiometric control to identify the millennial timescale of these events. Here we summarize and update AMS 14C constraints on the deglacial chronology of northern Britain (Table 1). Deglacial stratigraphies associated with the BIIS margin represent either ice-contact sediments or marine muds that in several cases were overrun by readvancing ice (Figs. 2–5). Because these stratigraphies represent separate sectors of the BIIS, our AMS 14C control allows us to evaluate whether regional ice margin fluctuations are in phase as well as their relation to possible climate forcings. In addition, 14C constraints on RSL changes reveal the timing of isostatic and eustatic changes that contributed to the RSL record.

Fig. 1. Deglacial records in the north Irish Sea Basin and patterns of ice sheet flow during the Killard Point Stadial. Location and stratigraphy of critical sections with dated marine mud beds are shown in relation to ice sheet limits. Insets show location of study areas in the British Isles.

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Table 1 AMS 14C ages Locality/description/comments

Laboratory number

Radiocarbon age (yr BP71s)

Source

Rough Island, County Down: samples from marine mud draping drumlins. Dates marine transgression after ice sheet collapse. Killard Point, County Down: samples from marine mud interbedded in terminal outwash from the Killard Point moraine. Dates ice sheet readvance. Rathcor Bay, County Louth: samples from deformed marine mud below deposits of the Rathcor moraine. Constrains readvance of Dundalk Bay ice lobe. Linns, County Louth: samples from deformed mud below ice pushed morainic sediments. Constrains ice readvance on south side of Dundalk Bay ice lobe. Cranfield Point, County Down: samples from marine mud below outwash from the Carlingford Bay ice lobe. Dates ice-free interval prior to readvance. Cooley Point, County Louth: samples from marine mud below an intertidal boulder pavement and outwash from the Dundalk Bay ice lobe. Records ice free interval prior to ice sheet readvance. Corvish, County Donegal: samples from marine muds. Dates final deglaciation of Inishowen Peninsula, dates ice readvance into Trawbreaga Bay and constrains early deglaciation of the continental shelf.

AA21822

12,740795K

McCabe and Clark (1998)

AA22820 AA22821

13,7857115K 13,9957105K

McCabe and Clark (1998) McCabe and Clark (1998)

AA56701

14,2507130K

This paper

AA56700

14,157769K

This paper

AA56700 AA21819

14,7057130K 15,6057140K

McCabe and Clark (1998) McCabe and Clark (1998)

AA17693 AA17694 AA17695

15,0207110K 15,3907110K 15,4007140K

McCabe and Haynes(1996) McCabe and Haynes(1996) McCabe and Haynes(1996)

AA32315 AA45968 AA45967 AA45966 AA33831 AA33832 SSR-2714

14,0457100K 15,7207160K 15,1907150K 16,0607430K 15,025795K 17,1407110E 16,9707100

McCabe McCabe McCabe McCabe McCabe McCabe McCabe

AA56707 AA56706 AA56705 AA56704 AA53589 SSR-2713 AA56703 CAMS-89688 CAMS-89687 CAMS-89686 AA22351 AA22352

15,928767+ 15,989774+ 18,275799+ 16,4307130K 16,5807120+ 16,5407120 16,227783K 16,9707190K 16,540770K 16,640770K 16,7607130K 16,7507160K

This paper This paper This paper This paper This paper McCabe et al. (1986) This paper Clark et al. (submitted) Clark et al. (submitted) Clark et al. (submitted) McCabe and Clark (1998) McCabe and Clark (1998)

Fiddauntawnanoneen, County Mayo: samples from glaciomarine mud on south side of Donegal Bay. Postdates early deglaciation of continental shelf and records possible global meltwater pulse 19 cal kyr BP. Belderg Pier, County Mayo: samples from glaciomarine iceberg zone mud. Postdates early deglaciation of the continental shelf. Records possible global meltwater event 19-kyr BP (cal.) or destablisation of ice margin in Donegal Bay.

Kilkeel Steps, County Down: samples from marine mud contained within channels (sea level lowstand) cut into deglacial diamict. Dates global meltwater pulse at 19-kyr BP (cal.).

and Clark (2003) and Clark (2003) and Clark (2003) and Clark (2003) and Clark (2003) and Clark (2003) et al. (1986)

AMS 14C ages from monospecific samples of Elphidium clavatumK, Macoma calcarea+ and Quinqueloculina seminulumE. Samples SSR-2714 and SSR-2713 from bulk samples of M. calcarea. Samples corrected for a typical 400 yr reservoir age and an older reservoir age which probably occurred during cold deglaciation phases when NADW formation rates slowed.

2. Marine microfaunas for AMS

14

C

Marine microfaunas found within muddy sediments exposed around the Irish Sea basin and elsewhere in northern Britain may originate by several processes. Some faunas that are mixed components of different ages represent reworking of older sediments either directly by ice (Austin and McCarroll, 1992) or by post-mortem resuspension in water derived from subglacial channels (Haynes et al., 1995). These conditions undoubtedly varied spatially and temporally around the decaying BIIS, which displayed both subglacial deposi-

tion and glaciomarine deposition at its margin (Eyles and McCabe, 1988). In either case, however, the fauna represent mixed ages and thus cannot be used for radiocarbon dating. Marine muds may also contain microfaunas that represent in situ biocoenoses. These faunas are dominated by the foraminifera Elphidium clavatum (85–95%) and the ostracod Roundstonia globulifera (5–10%) that shows intact instars. Shell tests are characterized by pristine glossy preservation together with a range of sizes and an absence of temperate forms. Some muds also contain variable percentages of other Arctic

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Fig. 2. Cartoon illustrating the facies geometry, facies types, sites sampled for AMS 14C dating and the main deglacial events recorded at Kilkeel Steps, County Down (Clark et al., 2004). This channel was cut into glaciomarine diamict during a fall in relative sea level. Four similar fluvial channels cut into glaciomarine diamict are exposed along cliff sections between Kilkeel and Cranfield Point (McCabe, 1986). Marine muds infilling these channels record a rise in relative sea level at 19 cal. kyr BP. The dates (Table 1) are from monospecific samples of E. clavatum in pristine condition including delicate ornamentation and preservation of the last aperture. Original samples were 490% E. clavatum with most size fractions being present and an absence of any warm species. In addition the delicate valves of the open shelf ostracod Polycope sp. were still joined by short hinges. The absence of reworked species and the pristine nature of the full assemblage strongly suggests an in situ marine microfauna.

microfaunas, including fragile polymorphinids, lagena and miliolids (McCabe and Clark, 2003). Similar opportunistic biocoenoses have been recorded from contemporary arctic–subarctic areas recently vacated by tidewater glaciers (Hald et al., 1994). These in situ faunas, which are compositionally distinct from other mixed assemblages that are commonly redeposited at ice sheet margins by reworking (Haynes et al., 1995), provide an outstanding opportunity for developing a 14C chronology of glacial and sea level fluctuations in northern Britain (McCabe and Clark, 1998, 2003). The AMS 14C dates presented in this paper (Table 1) are based mainly on monospecific samples of the foraminifera E. clavatum obtained by boiling and sieving 10–20 kg of marine mud.

3. Three new dated sites We describe new 14C ages that constrain the timing of deglacial events at three sites in Ireland. We then place these events into the regional context of the last deglaciation of the BIIS based on previous 14C dating.

3.1. Kilkeel Steps The Mourne Plain occurs between the Mourne Mountains and the Irish Sea (Fig. 1). During initial deglaciation of the northern Irish Sea Basin, decaying ice masses retreated west into Carlingford Lough, northeastwards along the eastern flanks of the mountains, and northwards into the mountains, creating a depositional basin in the area now occupied by the Mourne Plain (Fig. 1). McCabe (1986) argued that the deposits beneath the Mourne Plain are glaciomarine because of their position below the marine limit (30 m asl), their sediment geometry, the presence of thick (o10 m) muds with dropstones, interbedded facies sequences with dropstones, and the overlying raisedbeach facies. The general vertical facies succession, from stratified diamict (Fig. 6a and b) (debris flows) to interbedded sand and gravel (Fig. 7a and b) (subaqueous outwash) to beach sand/gravel (Fig. 8a) (upper shoreface), is characteristic of an emergent deglacial facies sequence. In addition, Stephens and McCabe (1977) noted that the late-glacial raised beach facies ended at specific moraines, such as the Cranfield moraine that marks a readvance ice limit at the mouth

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Fig. 3. Stratigraphic log and radiocarbon dates from Belderg Pier, County Mayo. See Fig. 4 for location and regional setting.

of Carlingford Lough (Figs. 1 and 8b). These field relations confirm that a high RSL existed at the same time as sediment delivery to depocentres (e.g. Sandpiper Pit) on the Mourne Plain from adjacent ice margins (Fig. 1). The sediment geometry of the coastal exposures cut into the Mourne Plain show that following deglaciation of the Mourne Plain, southward flowing rivers from the mountains eroded broad channels into the diamict that was deposited as the ice margins retreated across the site of the Mourne Plain (Figs. 1 and 2). These channels are graded to a sea level similar to or lower than at present, indicating that by the time they formed, the region had emerged by isostatic uplift since formation of the marine limit 30 m asl. Marine mud now filling the river channels indicates that a falling RSL due to isostatic emergence was subsequently reversed by a rise in global sea level (Fig. 9). We sampled the channel fill at Kilkeel Steps from four levels containing a well-preserved marine microfauna dominated (490%) by E. clavatum and also containing the open shelf species of Polycope sp. (Didie et al., 2002).

Five AMS 14C ages indicate rapid sedimentation of the muds (o10 m asl), with radiocarbon ages at the base being statistically the same as the radiocarbon age at the top (Fig. 2) (Table 1) (Clark et al., 2004). The five ages have the same weighted mean 14C age (16,640740 14C yr BP) as the weighted mean of three 14C ages (16,3607220 14C yr BP) on a single sample of Barbados coral that has a corresponding weighted mean U/Th age of 19,000770 yr BP. Accordingly, Clark et al. (2004) interpreted these new dates and their relation to the channel-fill facies as indicating that the rise in eustatic sea level at Kilkeel Steps is the same event (19 cal. kyr BP) that was previously inferred from sediment facies and microfossil assemblages from Bonaparte Gulf (Yokoyama et al., 2000). 3.2. Belderg Pier Hinch (1913) proposed that the clay exposed at Belderg Pier (o20 m asl), northwest Ireland, was deposited from floating ice. Synge (1968), Colhoun (1973), and Davies and Stephens (1978) subsequently

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Fig. 4. Deglacial records from northwestern Ireland. Stratigraphic position of critical facies successions and dated glaciomarine/marine mud beds are summarized in logs. Note the inferred western limit of Killard Point Stadial ice and regional ice flow patterns inferred from subglacial bedforms. The fairly continuous zone of subglacial transverse moraines located in the upper Erne, Shannon and Omagh basins may have formed beneath the lowland ice masses which remained after early deglaciation (McCabe and Clark, 2003, McCabe et al., 1992).

interpreted the clay as a basal till created by onshore ice movement. McCabe et al. (1986) described the sedimentary succession and faunas and concluded that the deposits were in situ glaciomarine sediments, undisturbed by glaciotectonics (Figs. 3 and 4). The schistose bedrock around the pier was moulded and striated by northwesterly ice flow onto the continental shelf (Fig. 4). This surface is overlain directly by laminated mud and sand containing occasional pebbles, massive mud, lens-shaped bodies of cobble gravel, and a massive tabular unit of diamictic mud which can be traced discontinuously for about 11 km along this coastal sector. We interpret the basal part of the succession as ice proximal because of rapid grain size and facies changes. Rain-out, gravelly debris flows and density underflows are all typical processes which operate when ice vacates a site, with textural variability reflecting continuously changing transport mechanisms (Powell, 1984). The thick and uniform aspect of the diamictic mud forming the upper part of the section is best interpreted as near-continuous rain-

out from sediment plumes together with iceberg rafting and suppression of bottom current activity. McCabe et al. (1986) originally dated this site by conventional 14C dating of paired valves of Macoma calcarea (16,5407120 14C yr BP; SSR-2713). We have obtained six new AMS 14C dates from Belderg Pier from micro- and macrofaunas in order to test validity of the original conventional dates and to evaluate whether there are significant age differences in the vertical profile (Fig. 3). Six of the seven dates support the original conventional 14C age and suggest that glaciomarine sedimentation occurred between 16,5807120 yr 14C BP and 15,928770 yr 14C BP. One older date (18,275+100 14 C yr BP) on Quinqueloculina seminulum, however, differs markedly from the other dates measured on both macro- and microfaunas. We suggest that this outlier, measured on a different species, may be derived from older sediments. The sediments at Belderg Pier postdate the last major ice sheet advance onto the continental shelf and record subsequent glaciomarine conditions along an isostati-

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Fig. 5. Inferred ice sheet limits, stratigraphic logs and dated marine mud beds around Dundalk Bay. Ridge moraine patterns are shown from the northern and southern margins of the ice sheet lobe which readvanced during the Killard Point Stadial. Muds dated to 14.2 14C kyr BP at Linns and Rathcor are disturbed and overlain by deposits formed during the ice sheet readvance across Dundalk Bay from north central Ireland. Older marine muds (15.4 14C kyr BP) overlain by an intertidal boulder pavement occur outside this ice sheet limit at Cooley Point (McCabe and Haynes, 1996). However at Dunany Point on the south side of the bay the intertidal boulder pavement has been overrun by the ice lobe (see Fig. 10b). Note that the raised late glacial beaches (horizontal shading) only occur immediately outside the inferred ice sheet limits between Dunany Point and Rathcor.

cally depressed southern margin of Donegal Bay (Figs. 3 and 4). Early deglaciation of the continental shelf off County Donegal, northernmost Ireland, occurred before 17,1407110 14C yr BP (McCabe and Clark, 2003). The oldest 14C dates from Belderg Pier (16,4307130 14C yr BP, 16,5807120 14C yr BP, 16,5407120 14C yr BP) suggest that deglaciation of Donegal Bay may have lagged this early deglaciation to the north by several hundred years. We also consider the possibility that the base of the Belderg section is incomplete and deglaciation of this region occurred earlier, as suggested by the age of 16,9707100 14C yr BP on M. calcarea from marine mud exposed at Fiddauntawnanoneen one kilometre to the east (McCabe et al., 1986) (Table 1). 3.3. Dundalk Bay McCabe et al. (1987) used late Pleistocene moraines and other landforms to reconstruct glaciomarine environments at the margins of an ice lobe that

grounded in Dundalk Bay (Fig. 5). Ice-contact ridges, delta-fan and apron sequences were divided into two major facies associations. Facies on the southern margin of the ice lobe consist of muds and diamictic muds with minor amounts of sand/gravel (10%) forming welldefined morainal banks (Figs. 5 and 10a). The Dunany ridge (o30 m high, 7 km in length) is the largest of these. The ridge at Linns is more subdued (10–15 m high) and consists of a core of marine mud overlain by listric shear surfaces developed in sand, gravel and mud (Fig. 5). These structures rise southwards, recording compression and stratigraphic duplication, and are a result of ice movement from the north. The overall pattern of muddy morainic ridges and tectonic thrusting within the ridges indicates proglacial thrusting and stacking which translated the ice lobe laterally. The steep slope on the northern face of the Dunany ridge is a good example of an ice contact face with the sediments in front (to the south) only tilted rather than extensively deformed.

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Fig. 6. (a) Stacked beds of texturally variable diamict from coastal exposures, Ballymartin, County Down. Beds are massive or show weak coarse tail grading, and contain laminations continuous for 4–5 m (see Fig. 6b). Clast lithologies show variability on a metre scale recording debris flow inputs from different geological provenances when ice vacated the Mourne Plain. These glaciomarine facies were deposited by debris flows, underflows and ice rafting as ice masses vacated the Mourne Plain during early deglaciation (McCabe, 1986). Figure is 1.7 m. (b) Undeformed and graded laminations deposited in conformity with subjacent beds of massive diamict, Ballymartin, County Down. Laminae were deposited from sandy underflows. The juxtaposition of laminae and massive beds confirm rapid changes in depositional processes as the sediment pile (13 m) accumulated in an ice contact, glaciomarine environment. Rule is 0.5 m long.

Fig. 7. (a) Large lonestone (dropstone) within laminated sand, Sandpiper Pit, south County Down. This deposit is a flat, subaqueous spread located 0.5 km east (ice distal) of the Cranfield Point moraine at the mouth of Carlingford Lough. Spade is 0.9 m long. (b) Detail from poorly sorted and weakly stratified silty sand beds, Sandpiper Pit, County Down. Amalgamated beds represent rapid deposition from high-density sandy underflows. Small clasts (o2 cm) are therefore aligned parallel to bedding planes while the ab planes of the larger clasts have steep plunges suggesting an ice rafted origin. The subaqueous outwash was deposited from the margin of the Carlingford Bay ice lobe directly into the sea. Spade is 1.9 m long.

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Fig. 8. (a) Laminated marine muds dated to 19 cal. kyr BP contained within channels (see channel margin on left of photo) at Kilkeel, south County Down. Channels were cut into glaciomarine diamict during a sea level lowstand due to isostatic uplift following early deglaciation. These deposits are truncated by late glacial raised beach sand and gravel (top of the photo) formed during the later Killard Point Stadial when an ice lobe in Carlingford Bay readvanced as far south as Cranfield Point and Ballagan Point. Section is 10 m high. (b) Marine mud (dated to 14.7 and 15.6 14C kyr BP) at Cranfield Point overlain by diamict deposited during the ice sheet readvance in Carlingford Bay which reached the Cranfield Point Moraine. Section is 5 m high.

The second facies association on the northeastern margin of the ice lobe was deposited in a narrow arm of the sea and consists of a series of coalescing, iceproximal, coarse clastic deltas interbedded distally with subaqueous sand and diamicton. At Rathcor, these deposits have been ice pushed and overlie deformed fossiliferous marine muds (Fig. 5). Two northwest/ southeast trending ridges mark this ice limit and a younger ice marginal ridge that parallel the present shoreline (Fig. 5). Both ridges stratigraphically overlie marine muds along the northern margin of the bay, and at one location (Giles Quay) the outwash is interbedded with thick (o1 m) beds of marine mud, demonstrating contemporaneity (McCabe et al., 1987). Advance of this ice lobe into Dundalk Bay was part of a major southeasterly ice sheet readvance from the lowlands of north-central Ireland to the coastal parts of Counties Down and Louth (Figs. 1 and 11). The ice sheet limit is marked by a narrow (1–3 km) terminal

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Fig. 9. Laminated marine deposits showing strong rhythmic bedding of sand and mud couplets, Kilkeel Steps, south County Down. These are contained within channels and record a global meltwater pulse at 19-cal. kyr BP. Trowel is 30 cm long.

zone of ice contact landforms and terminal outwash stretching from Dunany Point northeastwards to Killard Point, a distance of about 130 km (Fig. 1) (McCabe and Clark, 1998). Ice lobe configuration in Carlingford Lough and Dundalk Bay are controlled by the presence of topographic highs. Synchroneity between the icesheet limits (Fig. 1) is based on the presence of lateglacial raised beaches currently at 19–20 m asl that only occur outside of these ice limits. Additional evidence for synchroneity is indicated by a regionally coherent set of ice sheet flow lines that lead directly to the moraines (Fig. 1). The moraines were formed by the subglacial flux of sediment to tidewater ice sheet margins (Figs. 11 and 12). The new AMS 14C dates from Linns (14,157769 14C yr BP) and Rathcor Bay (14,2507130 14C yr BP) confirm the synchroneity of ice sheet readvance recorded by linear moraines on the northern and southern sides of Dundalk Bay (Fig. 5). More specifically, these dates represent ice advance into marine muds that already existed along bay margins and therefore the ice sheet

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from both lowland and upland centres of ice dispersion to its maximum limit (Bowen et al., 2002). The published limit of ice extent onto the continental shelf is poorly constrained in time and space, and will continue to be revised as new information is gathered (e.g., McCabe and Clark, 2003). The classical definition of the maximum extent of terrestrial ice is that it coincides with the limit of ‘fresh’, unmodified glacial and glaciofluvial landforms. Bowen et al. (2002) reported 36 Cl ages on glaciated surfaces and glacial boulders close to the former postulated ice sheet limit suggesting that deglaciation of the southern BIIS margin in Ireland began shortly after 22 36Cl kyr BP. Here we summarize six subsequent events from within the deglacial stratigraphy based on AMS 14C dating of microfaunas from marine and glaciomarine muds. 4.1. 18.0– 16.7

Fig. 10. (a) Coastal section in a morainal bank at Dunany Point, County Louth (McCabe et al., 1987). Facies sequence consists of stacked beds of muddy diamict which are texturally very variable. Beds are planar or contained within shallow channels and record a wide range of debris flow activity. Minor stratified interbeds emphasize the dip of beds southwards away from the ice-contact slope of the ridge which may indicate some ice pushing. Section is 22 m in height. (b) Plan view of a striated and bevelled boulder pavement beneath the diamict sequence on the south side of Dundalk Bay at the base of the Dunany Point exposure (Fig. 10a). Note that the pavement is pressed into the finer grained sediments below. The feature is similar to the intertidal boulder pavement exposed at Cooley Point on the north side of Dundalk Bay (see Fig. 16b). Pen is 12 cm long.

advance slightly postdates them (Fig. 12). The Dundalk Bay dates are in excellent agreement with dates from Killard Point (13,7857115 14C yr BP, 13,9957105 14C yr BP) (Table 1) at the northeastern limit (Co. Down) of the moraine system because these are from muds interbedded in terminal outwash (Figs. 13a and b) (McCabe and Clark, 1998). Accordingly, the Dundalk Bay dates further constrain the age of maximum ice sheet readvance during the Killard Point Stadial at 14.0 14 C kyr BP.

4. Synthesis of major deglacial events During the last glacial maximum in the British Isles, regional ice-flow indicators indicate that ice advanced

14

C kyr BP-initial deglaciation

Deglaciation of the continental shelf is not well constrained, but four sites provide detailed information. McCabe and Clark (2003) discuss associated terrestrial evidence. Although the age of 18,275+100 14C kyr BP from Belderg Pier (Table 1) reflects a reworked sample of Quinqueloculina seminulum (Fig. 3), we suggest that the monospecific sample came from pre-existing sediment that records deglaciation at that time. Further work is needed to test this hypothesis, but if this date is valid, then it identifies a previously unrecognized phase of early deglaciation along the northwestern BIIS margin that is contemporaneous with deglaciation of the southern BIIS margin at 22.0+1.1 36Cl kyr BP (Bowen et al., 2002). In the northern Irish Sea Basin, the deposits at Kilkeel Steps, County Down, record the deglaciation of the Mourne Plain on the southern flanks of the Mourne Mountains (McCabe, 1986) (Figs. 1 and 2). These stacked sequences of stratified and laminated diamictons (Fig. 6a and b) record glacigenic input from three discrete sources as well as from ice rafting. These sediments have not yet been dated directly, but 36Cl ages indicate that the BIIS started to retreat from its southern limit 22.0+1.1 36Cl kyr BP (Bowen et al., 2002), and the entire sequence predates marine mud with ages from 16,540770 14C yr BP to 16,9707190 14C yr BP (Table 1). Moreover, as summarized previously, this entire sequence represents deglaciation followed by a fluctuation in RSL (fall followed by rise) associated with isostatic emergence and subsequent eustatic rise that flooded the coast and deposited the marine mud. Accordingly, this sequence of events clearly requires some time between deglaciation (o22 36Cl kyr BP) and marine deposition (19 cal. kyr BP). What caused deglaciation of the Irish Sea Basin and possibly the northwestern Irish coast at 22 cal. kyr BP?

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Fig. 11. Depositional model showing relationships between fast ice flow, drumlinization and subglacial debris flux to tidewater ice margins where large morainal banks accumulate (after Eyles and McCabe, 1989).

Fig. 12. Cartoon of the glacigenic system operating during the Killard Point Stadial in the north Irish Sea Basin showing the relationships between subglacial bedforms, dated marine muds, stratigraphy, terminal outwash and relative sea level (after McCabe et al., 1998).

In particular, we note that deglaciation occurred during the interval of relative sea level and climate stability formally defined as the Last Glacial Maximum (19,000–23,000 yr BP) (Mix et al., 2001). In the absence of a clear climatic forcing, we attribute deglaciation to relative sea level rise associated with isostatic subsidence (Eyles and McCabe, 1988).

At Corvish, north County Donegal, marine muds at the head of Trawbreaga Bay indicate that deglaciation of northernmost Ireland occurred before 17,1407110 14 C yr BP (Fig. 4) (McCabe and Clark, 2003). The fully marine microfauna and the paucity of ice-rafted detritus indicate that the ice sheet margin had retreated well inland by this time. As discussed previously, 14C ages

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Fig. 13. (a) Panoramic view of a large channel (200 m across and 20 m in depth) which conducted meltwater from a major glacial efflux at Killard Point, County Down. It is infilled with disorganized beds of boulder gravel. Bedding planes vary from sharp to amalgamated. Sediments record deposition from high-energy concentrated suspensions. The multistoreyed channels infilled with coarse gravels comprise the subaqueous sediment apron deposited as terminal outwash at Killard Point. (b) Laminated red mud and sand deposited from suspension at the base of an abandoned subaqueous channel at Killard Point. These muds are interbedded with gravelly outwash and diamict (debris flows) and record low-energy deposition following channel avulsion. Muds are dated to 14 14C kyr BP (McCabe and Clark, 1998). Spade is 0.9 m long.

from Belderg Pier and Fiddauntawnanoneen also identify this deglacial event on the southern margin of Donegal Bay. What remains unclear is whether a significant ice-margin readvance onto the continental shelf occurred between initial deglaciation 18.3 14C kyr BP and this subsequent deglacial event at 17 14C kyr BP, or whether the ice margin remained near the coast throughout this interval. We note the similarity of ages from Corvish (17,1407110 14C yr BP) and southern Donegal Bay (16,9707100 14C yr BP, 16,5807120 14C yr BP) that constrain deglaciation of the northwestern Irish coast and ages from Kilkeel Steps (16,540770 14C yr BP to 16,9707190 14C yr BP) (Table 1) that constrain an abrupt rise of global sea level. We suggest that the two events may be related, whereby an abrupt sea level rise following the last glacial maximum lowstand (Yokoyama et al., 2000) flooded the recently deglaciated Irish Sea Basin coastline while also destabilizing the marinebased western BIIS margin that extended onto the

Fig. 14. Satellite image of subglacial bedforms around the head of Donegal Bay and the Lower Lough Erne Basin. Ice sheet centres were located astride the Donegal highlands in the north and the Omagh Basin to the east. Superimposed, cross-cut and striated bedforms record two major ice flow events in the area surrounding the head of the bay (Knight and McCabe, 1997). Image is 45 km across from east to west.

continental shelf, initiating deglaciation of northwestern Ireland (Fig. 14). The widely spaced sites recording deglaciation at this time indicate a decrease in ice sheet size by about two thirds (McCabe et al., 1998). The precise area occupied by ice following withdrawal inland is difficult to evaluate but the almost continuous zone of pristine subglacial, transverse ridges (Rogen moraines) across the north Irish lowlands between Lurgan and Strokestown (County Roscommon) (150 km) suggests that the ice may have reorganized as domes across the lowlands accompanied by basal conditions which favoured formation of the transverse moraines (Fig. 15a and b). Placing subglacial moraine formation at this time is supported by the observation that these transverse ridges are overprinted only once during the subsequent ice sheet advance 14 14C kyr BP. (Killard Point Stadial) (McCabe et al., 1998). 4.2. 18.0– 16.7

14

C kyr BP-fall in relative sea level

The stratified and laminated diamicts deposited on the Mourne Plain (County Down) represent deposition

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Fig. 15. (a) Subglacial transverse ridges from the Upper Lough Erne Basin, north-central Ireland. Ridge patterns are highlighted by the outlines of intervening lakes. These ridges have been cross-cut, overprinted and moulded by ice flow from the north during the Killard Point Stadial. Image is 35 km across. (b) Subglacial transverse moraines from the Upper Shannon Basin showing partial crosscutting though the ridges are mostly in tact. Overprinting occurred during ice sheet reorganization towards the end of the Killard Point Stadial. Image is 35 km across.

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by rain-out, ice rafting, and resedimentation by debris flows some distance seaward from successive ice-grounding zones as ice vacated this sector of the Irish Sea Basin (Fig. 6a and b) (McCabe, 1986). This early deglaciation is also recorded by a marine limit of 30 m asl between Ballymartin and Annalong (Stephens and McCabe, 1977). Sediment geometry for about 6 km along this coast shows that the diamicts were deposited originally as a continuous sediment apron. Subsequent isostatic uplift occurred led to incision of channels into the sediment apron by rivers originating in the Mourne Mountains (Fig. 2). Because the marine limit at 30 m asl occurred sometime between 22 36Cl kyr BP (Bowen et al., 2002) and 19 cal. kyr BP (time of deposition of mud in channel), when global sea level was 130–140 m lower than present (Yokoyama et al., 2000), these data also suggest a net isostatic uplift of 160–170 m of the Mourne Plain. 4.3. 16.7

14

C kyr BP—eustatic sea level rise

The thick (11 m) marine muds that infill the erosional channels at Kilkeel Steps (Figs. 2 and 9) record a rapid rise in sea level 19 cal. kyr BP (Clark et al., 2004). We hypothesize that the similarity in 14C ages from deglacial sediments at Corvish and Belderg Pier indicate that the same 19 cal. kyr BP sea level rise triggered deglaciation of remaining marine-based margins off the northwestern Irish coast. Although deposited at the same time, facies at Belderg Pier differ from those at Kilkeel because they were deposited from sediment plumes and ice rafting in association with a nearby ice margin along the outer southern margin of Donegal Bay (Fig. 4). These iceberg zone muds may be related to the destabilization of the BIIS margin triggered by the 19 cal. kyr sea-level rise, whereby the ice margin remained within the inner bay following the initial deglaciation. That such a tidewater ice margin existed as this time is clearly possible because of its proximity to major centres of ice dispersion in the Omagh, Shannon and Erne basins (Fig. 4). 4.4. 15.0 present

14

C kyr BP—relative sea level similar to

A bed (205 m long) of closely spaced single boulders (Fig. 16a and b) is exposed within 1 m of present sea level at Cooley Point on the outer northern margin of Dundalk Bay. The boulders form an undulating pavement that continues below the eroded cliffline (Fig. 16b). Boulders are mainly flat lying and are pressed into fossiliferous marine mud and are overlain by sand and gravel deposited as glacial outwash. Upper clast surfaces are bevelled and striated with multiple crosscuts. At some points, the boulder bed is draped by

Fig. 16. (a) Intertidal boulder bed one clast thick pressed into marine mud, Cooley Point, County Louth. Note ice bevelled upper surfaces of the clasts and the intervening mud pillars. Section is 25 m long and 1.3 m high. (b) Mosaic-like packing of ice bevelled boulders into mud and detail of undulations in pavement surface, Cooley Point. Spade is 0.9 m long.

laminated sand and mud. Modern analogues suggest that the mosaic-like pattern of this boulder bed formed as an intertidal pavement in a subarctic setting, with seasonal sea ice playing an important role in organizing the pavement through sorting, stranding, pushing, pressing, dragging, and rocking of individual clasts (Rosen, 1979; Hansom, 1983, 1986). Nearshore intertidal boulder pavements typically develop within a tidal range of 2–4 m (Tanner, 1939), suggesting that RSL was similar to present when the Cooley Point pavement formed. 14C ages on foraminifera from marine muds beneath the pavement suggest that the pavement formed less than 15,0207110 14C yr BP (McCabe and Haynes, 1996) (Table 1). The overlying outwash was deposited in association with the subsequent ice readvance to Rathcor 14,2507130 14C yr BP (Table 1). The late glacial terrace which buries this pavement is contemporaneous with the ice limit at Rathcor and records a rise in RSL of 20 m. (Fig. 5). Fifteen kilometres away on the south side of Dundalk Bay, exposures at the base of the Dunany ridge moraine (Figs. 5 and 10b) show similar boulder pavements as

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that exposed at Cooley Point (Fig. 16a and b), suggesting that intertidal conditions were present around bay margins. 4.5. 14.0

14

C kyr BP—the Killard Point Stadial

A significant ice sheet readvance, the Killard Point Stadial, occurred in northern Britain 14.0–14.5 14C kyr BP (McCabe and Clark, 1998; McCabe et al., 1998), and represents the only glacial fluctuation known to occur between the maximum of the last glaciation and the Loch Lomond Stadial in northern Britain. At its type area at Killard Point, this readvance is marked by terminal outwash with 14C ages of 13,7857115 14C yr BP and 13,9957105 14C yr BP on E. clavatum from interbedded marine sediments (Fig. 1, Table 1). This readvance is also dated at Corvish, where it is constrained to have occurred sometime between 15,025795 14C yr BP and 14,0457100 14C yr BP (McCabe and Clark, 2003), and Dundalk Bay, where it occurred after 14,150769 14C yr BP (Table 1). These data indicate that the readvance occurred in response to cooling triggered by Heinrich event 1, which started at 14.5 14C yr BP (Bond et al., 1999), and ended just before 14,0457100 14C yr BP at Corvish and shortly after 13,7857115 14C yr BP at Killard Point (but see below). The elements of the glacial system comprising the readvance are known in some detail (Figs. 1, 11 and 12). Evidence for marked readvances during this stadial occur around the margins of the northern Irish Sea Basin, in the Inishowen Peninsula of County Donegal, and along the large marine embayments of western Ireland (Figs. 4, 14 and 15a). McCabe et al. (1998) reconstructed the ice flowlines associated with this ice sheet readvance from the last subglacial imprints recorded in the north Irish lowlands (Figs. 4 and 14). Erosion of preexisting bedforms was associated with subglacial sediment fluxes towards tidewater ice margins, resulting in continuous and thick sediment wedges at ice sheet margins, such as the extensive sediment spread at Askillaun on the southern margin of Clew Bay (Fig. 17a and b). The main subglacial imprints of this readvance show that regional drumlinization occurred along a series of curved flowlines which in the northwest were strongly influenced by topographic controls within compartmented mountain blocks or outliers of Carboniferous strata (Fig. 4). Some reconstructed flowlines extend inland into and across a prominent zone of earlier formed, transverse (Rogen) moraine (McCabe et al., 1999; Knight et al., 1999). Along the paths of former ice streams, bedforms show a morphological continuum down ice from unmodified transverse ridges to remoulded/overprinted, crosscut or streamlined forms. Recent mapping confirms that subglacial transverse

Fig. 17. (a) Stacked beds of texturally variable diamict dipping westwards, Askillaun, Clew Bay, western Ireland. Lenses of gravel within shallow channels and lines of clasts emphasize the stratification in the exposure. Facies and sediment transitions are common. Occasional large (o2 m across) dropstones are embedded at all levels in the sequence. The feature is part of an extensive apron (later eroded by meltwater into a ridge) of ice contact glaciomarine sediment deposited by sediment gravity flows in front of ice retreating east towards the head of the bay. Figure at base of cliff is 1.8 m high. (b) Large boulder (3 m across) resting on glaciomarine diamict and overlain by delicate draped laminations (right of photo), near Old Head, Clew Bay western Ireland. Boulder top surface is strongly bevelled and striated by ice flow to the northwest. The large boulder is a dropstone trapped in massive diamict and bevelled as ice oscillated across its surface and along the ice proximal sediment apron. This boulder is at the same level as a single line of smaller ice-bevelled clasts set in massive diamict. The draped laminations confirms that after the basal ice had lifted off the clast bed, subaqueous deposition from density underflows commenced. This interpretation of basal ice marginal oscillations across poorly sorted sediments at tidewater margins has been described by Eyles (1988) from the Yatataga Formation, Alaska.

ridges extended as a southwest to northeast linear zone across the north-central lowlands (Fig. 4). Pristine and partly modified transverse ridges are best preserved in the large basins of the Upper Shannon, Upper Erne and Omagh (Fig. 15a and b), corresponding to the areas where the main ice masses remained following the early deglaciation of the BIIS. Overprinted bedform patterns show that these ice sheet centres

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became reorganized following maximum glaciation, resulting in changed flow lines (Knight and McCabe, 1997). Probably the most significant of these are the northerly ice flows that began in Joyce Country and extended north into Clew Bay and as far north as Crossmolina (Fig. 4). Nevertheless, the presence of pristine or weakly remoulded bedforms (Fig. 15b) suggests that some areas of the BIIS represent former ice domes where subglacial flow was sluggish.

the Killard Point readvance at Corvish, northernmost Ireland (Table 1) (McCabe and Clark, 2003). If this is the case, however, it is difficult to reconcile rapid ice recession following the Killard Point maximum shortly after 13.8 14C kyr BP with maintenance of a high RSL at Rough Island some 1000 years later, when isostatic emergence is expected to have caused a fall in RSL. Further work is required to understand the significance of the Rough Island RSL indicator.

4.6. 13.7 kyr BP—stagnation zone retreat There are no marked deglacial events yet identified on the north Irish lowlands following the Killard Point Readvance. Esker systems across the Plains of Mayo and fluvioglacial outwash surfaces in many topographically controlled settings testify to rapid deglaciation associated with stagnation zone retreat (SZR). North of the Killard Point ice limit, there are no fluvioglacial spreads until the core areas of ice dispersion are reached around the Lough Neagh Basin. Thus far, the only radiocarbon constraints on the end of the Killard Point Stadial and the start of SZR in the Irish Sea Basin are the age of 13,7857115 14C yr BP from Killard Point outwash and an age of 12,740795 14 C yr BP from marine mud at Rough Island (Strangford Lough), about 15 km north of Killard Point (McCabe and Clark, 1998). Based on the rapid rates of sedimentation associated with the outwash facies at Killard Point from which the sample dated 13.8 14C kyr BP is derived, we believe that deglaciation occurred shortly thereafter, which is consistent with the age of 14,0457100 14C yr BP for deglaciation following

5. Conclusions We use 29 radiocarbon dates to constrain the last deglaciation of the western sectors of the last BIIS. Several deglacial events are identified that reflect significant fluctuations of the ice sheet margin and of RSL. Not surprisingly, we find that standard views of ice sheet history (e.g., Boulton et al., 1977) require updating, because most of these models (conceptual and numerical) were static, based on an amalgam of generalized LGM ice flowlines of mixed ages, and assumed monotonic ice-margin retreat (e.g., Lambeck and Purcel, 2001). New radiometric data now identify a dynamic BIIS that was characterized by large fluctuations in its margin and centres of mass (McCabe and Clark, 1998, 2003; McCabe et al., 1998; Bowen et al., 2002). These data also identify substantial changes in RSL that reflect a combination of isostatic emergence and eustatic rise (Fig. 18). Nevertheless, an overview of facies and facies geometries formed during deglaciation suggest that

Fig. 18. Summary chart of changes in relative sea level points, County Down, eastern Ireland. These data show that marine and glaciomarine conditions existed for much of the deglaciation period (17–14 14C kyr BP) in the north Irish Sea basin. Dated sea level trends and field evidence do not validate the monotonic deglaciation depicted by Lambeck and Purcel’s (2001) model simulations of RSL in the Irish Sea basin.

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C. Bryant (NERC), D. Harkness (NERC), and T. Guilderson (LLNL) for radiocarbon dates. This work is support by grants from the University of Ulster and the US National Science Foundation.

References

Fig. 19. Wave-influenced, rhythmically bedded couplets of sand and mud showing well-developed sandy ripple trains capped by laminated mud drapes, Portballintrae, County Antrim (McCabe et al., 1994). Each couplet represents a storm event and interactions between wave base and bottom sediments. Occasional lonestones represent carriage by shore ice. These facies have a low preservation potential and occur above glaciomarine diamict and below the emergent clastic raised beach gravel. Exposure is 4 m high.

much work remains to be done to fully understand the significance of stratigraphically complex sequences currently exposed around the coastline of the British Isles. For example, the discovery of intertidal boulder pavements, wave-influenced rhythmically bedded sediments (Fig. 19), and marine muds within glacigenic successions show that detailed sedimentological and palaeontological work can help environmental reconstruction. This is especially important because deglaciation was a time of rapid environmental change and subsidiary facies often have a low preservation potential.

Acknowledgements We thank K. McDaid and L. Rogers for cartographic design, N. McDowell for photographic design, and

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