JOURNAL OF QUATERNARY SCTENCE (2001) 16(5) 497_506Copyright @ 2001 John Wiley & Sons, Lrd.DOI: 10.1002/jqs.63B

Sea-level change in the lrish Sea since the LastGlacial Maximum: constraints from isostaticmodell ingKURT TAMBECK+ and ANTHONY P. PURCEILResearch School of Earth Sciences, Australian National University, Canberra 0200, Australia

Lambeck, K. and Purcell, A' P' 2001. SeaJevel change in the lrish Sea since the Last Claciat Maximum: constraints from isostatic modelling. J, euaternary Sci.,Vol .16 pp. 497-506. ISSN 0267-8179.Received 4 September 2000; Revised 7 March2OOl: Accepted 22 March2OOl

ABSTRACT: Models of glacio-hydroisostatic sea-level change have been published for the BritishIsles that are broadly consistent with the observational evidence, as well as with glaciologicalconstraints. lt has been argued, however, that the models fail to represent sea-level change alongthe lrish Sea margins and in southern Ireland for the post-deglaciation period. The argumãnt restson the interpretation of the depositional environment of the elevated 'lrish Sea Drift' on both sidesof the lrish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the Brit ish lslesare consistent with the former interpretation in that sea-levels on either side of the lrish Sea, southof about the lsle of Man, are not predicted to have risen above present sea-level at any time sincethe deglaciation of the lrish Sea. This implies that ice over both the lrish Sea and lreland wasrelatively thin (ca. 600-700 m over lreland). lf the glaciomarine interpretation of the elevated lrishSea Drift is correct, then the maximum ice thickness over central and southern lreland would haveto reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea-level change to beconsistent with the Holocene evidence, this thick ice sheet could not have extended to theèasternside of the lrish Sea. Nor could it have been very thick at its northern and western l imits. lf suchan ice model is extreme and incompatible with glaciological observations then the alternative is toaccept t lre inl-erpretatit¡n of the Irish Sea Drift as terrestrial in origin. Copyright O 200] John Wiley& Sons, Ltd.

KEYWORDS: lrish sea; lrish sea Drifç sea-level; glacial isostasy; British lsles ice sheet.

Journal of Quaternãry Science

Introduction

Bendigeidfran and the host... sailed towards Ireland, and in thosedays the deep water was not wide. He went by wading. Therewere but two riveß, the Lli and the Archan were they called, butthereafter the deep water grew wider when the deep overflowedthe kingdoms.

The Mabinogionl

That sea-levels have fluctuated in the lrish Sea has been knownfor a long t ime and perhaps the earl iest surviving l i teraryreference to it is in the Mabinogion.l lrrespective of the originalsource or meaning of this tale, the issue of whether it has beenpossible to wade across the waters between lreland and Britainat some t ime in the past is a signif icant factor in any study ofthe similari t ies and dif ferences in fauna and f lora of the two

* Correspondence to: Professor K. Lambeck, Research School of Earth Sciences,Australian National University, Canberra 0200, Australia.E-mail: Kurt.Lambeck@anu.edu.aurTranslated by G. Jones and T. Jones, Everyman, London. The oldest preservedmanuscript collections of the Mabinogion stories date to about AD I 300-1 325as the Llyfr Cwyn Rhydderch (the White Book of Rhydderch). Existing fragmentsof the stories go back a further i 00 years.

islands, of the origins of sediments found in the Irish Sea andits adjacent shores, or of the ice-cover over the region duringthe last glaciation. That sea-levels have been lower in the pastis based on more quantitative evidence than the Mabinogiontale, and is attested to by observations, for example, of thesubmerged Holocene forest of Borth. However, when trackingthe change back in time the unambiguous evidence quicklypeters out and the pre-Holocene records are more debatable.

The sea-level signal recorded in geological strata andmorphology is the change in level with respect to the land,which itself may be vertically displaced. lt is, therefore, arelative displacement and includes any change in oceanvolume and any deformation of the land surface beneathand adjacent to the oceans. Usually these two contributionsare closely related; the factors that change ocean volumesalso cause deformation of the land surface. On the time-scale of thousands of years the primary cause of relativesea-level change in the lrish Sea is the effect of the icesheets sweeping back and forth across the continental landmasses-including the movement of ice across the Brit ishlsles-and the concomitant changes in global ocean volume.The Brit ish Isles region is not known for high levels of tectonicactivity and if any tectonically driven vertical displacements

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occurred they are l ikely to have been small and local, and notto have made major contributions to sea-level change. Anysediments deposited on the floor of the lrish Sea during thelast glacial cycle may have caused some subsidence but thisis also unlikely to be a dominant factor in regional and recentsea-level change: a cursory examination of the distributionof recent sediments in the lrish Sea indicating that they arerelatively thin and occur in pockets of l imited areal extent.

Along the shores of the Irish Sea, sea-level changes duringthe recent glacial cycle are therefore primarily a consequenceof the growth and decay of the regional and global ice sheets,with contributions from:

1. changes in ocean volume because of the melting of thelarge Late Pleistocene ice sheets;

2. crustal rebound from the disappearance of the local icesheet over the Brit ish lsles, including the associated changesin the regional gravity f ield or geoid;

3. crustal rebound and gravity changes associated with thechanging ice sheets of Scandinavia, North America and, toa lesser extent, Antarctica.

Together, these contributions result in complex spatial andtemporal changes in sea-level even for relatively restrictedareas such as the lrish Sea.

Evidence for sea-level change along the shores of the lrishSea is l imited and insufficient to completely describe thisregional spatial and temporal variation and mathematicalmodels are required to interpolate between the fragmentsof evidence available. Such models for glacio-hydroisostaticsea-level change have been presented previously for the Britishfsles, including lreland and the Irish Sea (Lambeck, 1993a,b,1995, 1996). The reason for addressing this region once againis that it has been said that these models are deficient for thelrish Sea (McCabe, 1997). One can have confidence in modelsonly if they have widespread applicabil ity so that any claim tothe contrary does merit examination.

The theory for the Earth's response to surface loading formsone of the foundations of computational geophysics and hasbeen tested over a wide range of similar problems (e.g.O'Connell, 1971; Peltier, 1974, 1998; Cathles, 1975; Wu andPeltier, 1983; Sabadini et al., 199i; Tushingham and Peltier,1991 , 1992; Wolf, 1996; Fjeldskaar 1997; Vermeersen andSabadini, 1997; Lambeck and Johnston, 1998; Lambeck ef a/.,' l 998; Mitrovica and Milne, 1998; Kaufmann and Lambeck,2000). The earth-model parameters entering into these modelsalso are reasonably well understood and although differentsolutions may lead to somewhat different values for theresponse function of the planet, these differences are smallwhen compared with the magnitude of the discrepanciesbetween predictions and observations that are said to occuralong the Irish Sea margins (McCabe, 1997). The issue is eitherone of an interpretation of the sea-level evidence or of themagnitude of the glaciation over the Brit ish Isles.

The previously published sea-level models for Ireland andthe lrish Sea were constrained primarily by Holocene data, theobservations consisting of relatively wel l-constrai ned sea-levelindicators that have a well understood relationshio to meansea-levels at the time of formation as well as being radiocar-bon dated (Carter, 1982; Carter et a|.,1989). Sedimentologicalarguments for high sea-levels during deglaciation of the lrishSea were ignored in these models because the interpretationof the field data was not universally accepted and because thisevidence was at odds with the better founded Holocene evi-dence. The observational debate centres on the interpretationof the ' lr ish Sea Drift ' , bordering bofh sides of the lrish Sea.Two contrasting hypotheses have been presented for the origin

of these sediments: the conventional view that the sedimentswere deposited by a terrestrial glacier prior to the rise in sea-level and inundation of the lrish Sea (Harris, 1991; Austinand McCarroll, 1992; McCarroll and Harris, 1992), and thealternative view that the sediments were deposited in shallow-water, glaciomarine environments around retreating tidewaterice margins (Eyles and McCabe, 1989; McCabe, 1995, 1997).ln the first interpretation, sea-levels, once the area becameice-free, would have been lower than today, whereas in thesecond interpretation the levels exceeded presentvalues by asmuch as 150 m: on the Welsh side these levels are reportedas having attained .|40 m or more near Banc-y-Warren andB0 m on the Llin Peninsula, and on the lrish side levels up to150 m above present have been suggested for localities nearDrogheda (McCabe, 1987; Eyles and McCabe, 1989; Warren,1991). We are not in a position to make a judgement onthe sedimentological arguments for either model but we donote that:

1. there are more recent papers in which various argumentshave been presented for a terrestrial origin of thesesediments on the Welsh side of the lrish Sea (scourse andAustin, 1994; McCarroll, 1995; Hart, I995; Harris ef a/.,1997; Thomas et al., 1998);

2. that the issues raised therein have not been addressed inthe recent paper by McCabe (1997);

3. thatthe postulated sea-levels along the Irish coast have beenreduced from the high levels previously cited to about 20 mat Dundalk and about 'l 0 m between Cork and Wexford.

Clacial rebound models, constrained by Holocene sea-levelevidence only, do not lead to predictions of well-elevatedshorelines for late-glacial t imes (i.e. the interval between theend of maximum glaciation and the disappearance of the LateDevensian ice over the Brit ish Isles). Instead, they lead to anlrish Sea that could indeed have been waded across soon afterthe ice retreated from the basin, with the width and timingof the land bridge being rather crit ically dependent on thethickness of the earlier ice (cf. Lambeck, '1995, "| '996). Thusthese model predictions are consistent with the conventionalwisdom on the origin of the Late Devensian lrish Sea Driftand do not support models of glaciomarine deposition andsubseouent rebound. Because the models are based on soundphysics and work well elsewhere, any defect in the modelsmust l ie in the input parameters. If the parameters requiredto predict well-elevated shorelines are clearly at odds withother l ines of geophysical or glaciological evidence, then thesedimentological model has to be reconsidered. This is whatthis paper sets out to do: to examine under what conditionsglacial rebound models can predict relative sea-level changeacross the lrish Sea that is consistent with the Holoceneevidence and that can also predict high sea-levels duringdeglaciation of both sides of the lrish Sea.

Model Predictions for Sea-level Change in thelrish Sea

The glacio-hydroisostatic model is based on a formulation ofplanetary deformation that is widely used in glacial reboundstudies and found to be adequate to describe the observedphenomena associated with glacial cycles of surface loading,including the sea-level response and global rotational andgravitational responses (see the references quoted above).The application of this model to the Brit ish lsles rebound

SEA-LEVELS IN THE IRISH SEA SINCE THE LAST CLACIAL MAXIMUM

and sea-level change has been discussed in some detail byLambeck (1993a,b, 1995) and has been applied to partsof Britain by Shennan et al. (2OOOa,b). The model for theice sheet over the Brit ish Isles is given in Lambeck (1995).The maximum ice thickness was constrained by the few LastClacial Maximum (LGM) trimline observations then available,but the adopted values for Scotland are consistent with themore recent results by Ballantyne et al. (1998). The assumedmaximum LCM ice thickness over Ireland was of the order of600-700 m, chosen to be consistent with geomorphologicalevidence that the highest peaks in lreland stood out asNunataks during the LCM (Watts, 1977). lce over the lrishSea was kept relatively thin because of the suggestion thatthe lsle of Man may not have been wholly ice covered at thetime of peak glaciation (Thomas, 1977). Recent inversions forice thickness indicate that the broad features of this ,standard,ice model are consistent with the sea-level data (Johnston andLambeck, 2000), but detailed regional studies also indicatethat more local modifications are appropriate, such as in northand northwest Scotland (Shennan et al.,2OOOa).

The observational data base used in these inversions rarelyextends beyond Holocene time and the accuracy of estimatesof ice volumes for the time of maximum glaciation are reducedsubstantially if there is no independent information on the icethickness for the early periods. The introduction of observedtrimline constraints in the inversions does, however, lead tomuch improved solutions for the earlier ice-sheet dimensionsflohnston and Lambeck, 2000). One other outcome of thissolution is that late-glacial melting appears to be delayed byabout I000 yr from that assumed previously, or equivalently,if the ice-retreat isochrons are unmodified, that the coupling ofthe ice to the basal rock and sediments was stronger during theearly period than implied by the adopted ice-height profi les.

Since the publication of these rebound models severalnew data sets have become available that serve as a testof the model. Particularly important are the isolation basinresults by Shennan etal. (2000a) from localit ies where theearlier evidence was based on ambiguous geomorphologicalindicators such as the rock platform elevations. (Subsequentdating by Stone ef a/. (1996) has supported the younger Dryasage for the last major reshaping of the Main Rock platform.)Figure 1a i l lustrates the comparison of model predictions andobservations for the Kentra Moss and Arisaig region of westernScotland. Agreement between observations and predictionshere is excellent. Further north, in Applecross and Coigach,the two begin to diverge (Shennan et al., 2OO0a) and thisbecomes more marked for Caithness (Fig. l b) and Orkney(Fig. Ic), where the recent data by Dawson and Smith (.|997)and De La Vega et al. (2000), respectively, indicate that theobserved sea-levels lie well above the predicted levels. Thusthe Caithness-Orkney region is unlikely to have been ice-free as assumed, following Sutherland (1984), in the standardmodel. (Experiments with models in which the ice-sheetthickness was increased north of the Creat Clen did not modifvthe solutions for the earth-model parameters (mantle viscosityand lithospheric thickness), indicating that the solution for thelatter is effectively independent of details of the ice model.)New results for the Humber Estuary (Shennan et al., 2OOOï)indicate good agreement between observed and predictedvalues (F ig. 1d) .

These and other comparisons indicate that the reboundmodel of Lambeck (1 993b, 1 995) provides a reasonableoverallrepresentation of the relative sea-level change around theBritish lsles and that any discrepancies between observed andpredicted sea-levels can be attributed to:

f . inadequacies in the adopted standard ice model-additio-nal ice over northernmost Scotland, including an extensionof the ice over the Orkney lslands, and possibly a later thanassumed melting over Scotland;

2. a refinement of the earth-model parameters including agreater degree of viscosity depth stratification (Lambecket a1.,"1996).

Nowhere over Creat Brita¡n do the comparisons of obser-vations with predictions suggest that the basic tenets of themodel are invalid. Nor are the resulting model parametersgrossly inconsistent with independent estimates of either themantle response function or of the ice sheet over the region.Thus, if the model does not reproduce the postulated elevatedshorelines along the lrish Sea coast (McCabe, 1997) then it isthe ice sheet over lreland and the lrish Sea that needs to bereconsidered. The question then is, what modifications to theice model are required to produce well-elevated pre-Holoceneshorelines without leading to substantial disagreements withthe sea-level evidence from the Holocene period.

Figure 2a illustrates the contributions to sea-level changearising from the combined northern European, North Amer-ican and Antarctic ice sheets at three epochs, the LCM (at22OOO yr BP), the Younger Dryas (at 10000 yr ago) and themid-Holocene (at 6000 yr ago). (All ages are in uncalibratedradiocarbon years before present.) The spatial variability overthe lrish Sea generally is small, about 10% of the signal,but not negligible when compared with the observationalaccuracy of the better sea-level indicators. The shape of thecontours i l lustrates the three main contributions: the Scandi-navian glacio-isostatic effect in the northeast, the Laurentidecontribution in the northwest, and the hydroisostatic effec!with Ireland rebounding upwards with respect to the adjacentocean. The corresponding ice-volume equivalent sea-level risehas been inferred from LGM and late-glacial observations ofsea-level at localit ies far from the former ice sheets (Lambeckand Chappell , 2OO1). Figure 2b gives the relative sea-level,for the same epochs, resulting from the Brit ish Isles ice only.This i l lustrates the broad zone of uplift, init ially somewhatlarger than the area covered by the ice and then graduallyshrinking through the combined effect of stress-relaxation inthe mantle and ice retreat. For there to be raised palaeoshore-lines at any locality this regional uplift must exceed the globalcontribution. Figure 2c i l lustrates the total predicted sea-levelfor the three epochs. Only over Scotland, northwest Englandand northeastern lreland are palaeoshorelines predicted tohave occurred above present level, but mostly at times whenthe area was still ice-covered. Shorelines south of about thelsle of Man are predicted to lie below the present sea sur-face throughout the LCM and the subsequent deglaciation.Only for the late Holocene are low-elevation raised shore-lines predicted, but then sti l l only north of about Angleseyand Drogheda. Figure I includes comparisons of predictedand observed sea-levels at coastal s¡tes in northwest England,Wales and lreland and agreement between the two generallyis satisfactory. Time-series predictions for three lrish localities,Donaghadee, Dundalk and Wicklow, and one Welsh locality,Borth, are i l lustrated in Fig. 3. Only for the northernmost siteof Donaghadee are elevated palaeoshorelines predicted oncethe site became ice free.

There are two options for increasing the rebound overthe lrish Sea: modify the earth-model parameters or modifythe local ice load. Because the model acts essentially asan interpolator between observations, with the interpolationparameters specified as mantle-response functions and an ice-height scale factor, some trade-off between the two classes ofearth- and ice-parameters can occur. This has been illustrated

500 JOURNAL OF QUATERNARY SCIENCE

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in Lambeck et al. (1996, Fig. 4) and Lambeck (1996, Fig. 15)for the Great Britain and Ireland solutions. For example, bothsolutions indicate that different combinations of values forl ithospheric thickness and upper mantle viscosity lead to

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predictions that remain in reasonable agreement with theobserved sea-levels, provided that the thickness of the ice isadjusted, although one particular combination of parametersgives a better fit to the observations than all the others. In

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time (x1000 laC years) before present

Figure 1 Comparison of model predictions and observed values for relative sea-level change for some selected sites. The observations are shownby either open circles with error bars or by the solid lines representing upper and lower limits. The upper and lower Iimits of the model predictionsare shown by the broken lines. (a-d) Comparisons for some more recent observational evidence that was not included in the original reboundsolutions of Lambeck (1 993b, 1995). The observational data are from (a) Shennan et al. (20O01>), (b) De La Vega ef a/. (2000), (c) Dawson andSmith (1997) and (d) Shennan et a/. (2000a). (e-f) Comparisons for some of the previously discussed observational evidence (Lambeck, 1993b,1996). The original field data are from (e) Heyworth and Kidson (1 982), (f) Carter (1 982), (g) Carter et al. (1989), (h) Carter ef a/. (1989) andSti l lman (1968).

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SEA-LEVELS IN THE IRISH SEA SINCE THE LAST CLACIAL MAXIMUM

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Figure 2 Predicted sea-level change for the lrish Sea and adjacent regions from (a) the melting of the far_field ice sheets of North America,Northern Europe and Antarctica, (b) the melting of the ice sheet over tñe British lsles and (c) thã combined melting of all ice sheets

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Figure 3 Predicted components of sea-level at four sites: Dundalk, Wicklow, Donaghadee and Borth. (i) Contribution to sea-ievel change tromthe distant ice sheets, (ii) contribution from the British ice sheet, and (iii) the total preãi.t"d sea-level change

JOURNAL OF QUATERNARY SCIENCE

particular/ increasing the l ithospheric thickness and the uppermantle viscosiÇ requires a concomitant increase in modelice thickness and, if one is prepared to lower the criteria foracceptance of a solution, an overall increase in ice thicknesscould be contemplated. Within the framework provided bythe style of the assumed ice sheet over the British lsles andthe requirement that the predictions are in broad agreementwith observations, however, modifying the earth responsefunction does not lead to a substantially different conclusionregarding the elevations of the late-glacial shorelines. (Whatit does lead to is a significantly less satisfactory comparisonover Scotland, with the rebound pattern being much broaderthan observed and it is the wavelength information containedwithin these Scottish observations that constrains the modelparameters at least as much as the amplitude information.)An alternative means of increasing the rebound over lreland,without increasing it over Scotland, is to consider variationin lateral upper mantle structure between the two regions.The inversion of the lrish Holocene data alone results inearth-model parameters that are consistent with, althoughless accurate than, those obtained from the Creat Britaindata (Lambeck, 1996). Thus there is no compelling reasonto introduce models with major lateral variation in mantlestructure, always within the framework provided by the ice-sheet model and observational data set. (The rheologicalparameters for these two regions are also consistent withthose found for Scandinavia (Lambeck et al., 1998).)

A further possibil i ty for increasing shoreline elevations alongthe lrish Sea coast is to increase the ice load over thatregion without modifying the load over the remainder ofthe ice sheet so as to largely preserve the agreement betweenmodel predictions and observations for Scotland and northernEngland and to remain consistent with the trimline evidencefor ice-surface elevations. (Because a basic assumption ofthe model is that the lithosphere supports shear streises, theresponse to a surface load is regional and the consequenceof modifying the load locally extends beyond the zone ofchange.) To raise the predicted shoreline elevation at Dundalkand Wicklow to, say 10-20 m above present sea level at'I 4 ka, requires that the crustal rebound from the melting ofthe British ice sheet must be increased by between 25 and45 m for Dundalk Bay and between 55 and 75 m at Wicklow(Fig. 3). (We adopt the lower range of 'observed' estimates hererather than the values of up to 1 50 m suggested previously byMcCabe (1987) and Eyles and McCabe (1989). lf the lattervalue is adopted then all the following scaling factors mustbe increased correspondingly.) The predicted crustal reboundfrom the Brit ish Isles alone for this epoch is 55 m and 25 mat these two sites respectively, and to bring the shorelines to10-20 m above sea-level requires a one and a half to tr¡¡ofoldincrease in local rebound at Dundalk and a three to fourfoldincrease for Wicklow. This would have to be done withoutsignificantly modifying (i) the Holocene rebound in the region,and (i i) the pre-Holocene and Holocene rebound for Scotlandand northern England. If such a scaling is applied to the Brit ishice sheet as a whole, the outcome is wholly inconsistent withany observational evidence for Scotland and northern England.For example, a global upwards scaling by only 1.5 yields bestfitt ing model parameters that are not implausible, but thevariance of misfit between observations and predictions isalmost doubled (cf. Fig. 5c of Lambeck et a|.,1996). Clearly, ifthe ice sheet is to be modified without an impact on the maincentre of rebound, the ice thickness has to be increased onlyover Ireland and the lrish Sea.

Two numerical experiments have been conducted to explorefurther what ice increases are implied by elevated post-deglaciation shorelines along the Irish Sea coast. In the first,

the lrish Sea ice is defined as the area between longitudes-3' and -7' and south of 55" north. Thus it includes Walesand the eastern margin of lreland. The 'standard' ice modelfrom Lambeck (1995) serves as starting point for the upwardsscaling of the ice over the Irish Sea. That is, the north-southice stream will be increased in thickness by a uniform scalingparameter and wil l be allowed to flow over land on both sidesof the lrish Sea. The contribution of this ice load to the sea-level change is i l lustrated in Fig. 4, along with the contributionfrom the remainder of the Brit ish ice sheet as well as thedistant ice sheets. The predicted rebound ̂fi-s at 14 ka for thestandard model does not exceed 20 m at both Dundalk andWicklow. This represents about 4Oo/" and 757o respectively ofthe contribution A,f6-¡ to the sea-levels at these two localitiesfrom the total standard British Isles ice sheet. (Southern Irelandis on the periphery of the 'Brit ish ice' and the rebound isthe sum of two opposing effects; the local rebound from theformer region and the subsidence from the peripheral zone ofthe latter.) For comparison, the contribution from the far field(Scandinavia, North America and Antarctica) Àf¡-¡ is stronglynegative at this time, about -70 m. To produce shorelines atan elevation Afo the local component has to be increased bya factor B1 of approximately

Ér : lÄfo - {(afu_,- ^l_,) * afn}]/À(i_.

For 10-m elevated shorelines at Dundalk Bay during 15-14 ka,fu : 2; for 30-m shorelines at the same locality, & : 3.

Figure 4 i l lustrates the predicted sea levels for models, basedon a more rigorous calculation than the above, in which theIrish Sea ice has been scaled upwards by factors of two andthree, whereas the rest of the Brit ish ice sheet has remainedunchanged. At Dundalk, elevated shorelines of about 30 melevation are predicted at the time that the ice margin stoodat the Main Drumlin Moraine (ca. 17000 yr ago) only if thethickness of the lrish Sea ice is increased about twofold. AtWicklow, a three-fold increase is required to produce well-elevated shorelines. Further south, at Wexford, the requisiteincrease is even greater. At Donaghadee, representative ofthe northeastern corner of Ireland, raised shorelines arepredicted for late-glacial times but the lowstands reportedfor Counties Antrim and Down at the end of the Pleistoceneand earliest Holocene (Carter, 1982) (Fig. i) are not nowreproduced. In the west, at Donegal, for example, models withincreased ice thickness lead to elevated Holocene highstandsthat are inconsistent with the observational evidence (Carteret al., 1989). Likewise, the predicted Holocene sea-levelsfor the Welsh coast, such as at Borth, now lie above theobserved values, in conf lict with the submerged forest evidence(Heyworth and Kidson, 1982).

In the second experiment the margin of the enhancedice sheet is shifted westward and the arèa of increased icethickness is defined as lying west of -4' and south of 56'north. Thus Wales is mostly excluded from any enhancementof the ice thickness over the lrish Sea. Figure 5 i l lustrates theresults for models in which the ice sheet over this lr ish sectorhas been scaled up by factors oî Br-2 and 3, leaving theremainder of the Brit ish ice sheet unchanged. The results aresimilar to those for the first test.

1. For þ2:2, shorel ines at Dundalk are predicted to occurat elevations of about 50 m for the time that the ice frontstood at the Main Drumlin Moraine; at Wicklow highervalues (B2 : 3) are requlred to produce such elevations; atWexford higher values again (þ2 > 4) are required to bringlate-glacial levels above present-day sea level.

SEA-LEVELS IN THE IRISH SEA SINCE THE LAST CLACIAL MAXIMUM

40

-601 8

-801 8

time (x 1000 l4C years) before present

Figure 4 Relative sea-level predictions (solid lines) for the models in which the Irish Sea ice has been scaled by þ, : 1,2 and 3 (curves 1,2 and 3respectively)' The curve with Br : 1 corresponds to the same model predictions as used in Figs 1 and 2. The dasired curves 4, 5 and 6 representthe contribution to sea-level change predicted from only the lrish Sea ice, scaled by the same p, values

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2. The lowstands observed in Down and Antrim for earliestHolocene time are not reproduced with B2 > 3.

3. At Donegal on the northwest coast, the predicted Holocenehighstand approaches 15 m for þz:3, compared withan observed position that lies below present-day sea-level(Carter et al., 1989\.

4. At Borth, however, the predicted values are more consistentwith the observed Holocene evidence than for the firstexperiment.

The two experiments indicate that, if the sedimentologicalevidence for glaciomarine sequences d¡ctates that raisedshorelines occurred along the eastern and southern Irish coast,any increase in ice over that of the standard model has tobe restricted to central and southern lreland, with the totalice thickness increasing from south to north. Also, for both

modifications, raised late-glacial shorelines (e.g. the postulated140 m Banc-y-Warren level or the 80 m Llin level (Eyles andMcCabe, 1989)) and a r¡sing sea-level in late Holocene t¡me(e.g. the Borth results) are incompatible with the Holoceneobservations for the coastal sites of Cardigan Bay (Figs 4and 5) and, if the better founded Holocene result is preferred,this indicates that any increase in ice thickness over lrelandis limited to the western parts of the lrish Sea. This doesnot lead to levels at 140 m above present sea-level for theperiod immediately after deglaciation, however, as suggestedby Eyles and McCabe (1989). (ln fact, the two observaiións ofrising Holocene sea-levels yet a highstand up to 140 m abovepresent only a few thousand years earlier, are incompatiblewith any plausible rebound model in which the l ithosphereacts as a continuous elastic or high-viscosity layer.) Finally,the increased thickness of the ice could not have extended

40

JOURNAL OF QUATERNARY SCIENCE

-60

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time (x 1000 14C years) before present

Figure 5 Same as Fig. 4 but for the sca led ice models over lreland and the lrish Sea

far to the west without resulting in raised shorelines along theAtlantic coast that have not been observed (e.g. at Donegal,and for Bantry Bay in Fig. 1, Carter et al., 1989).

One possible way of increasing the rebound atWicklow andfurther south is to argue that (i) the LCM ice margin extendedbeyond the Southern lrish Moraine l imit that was assumedin the standard ice-sheet model (consistent with Eyles andMcCabe, 1989), or (ii) that the retreat of ice over the Irish Seawas less rapid than assumed. (The small, possibly independentice-cap over Munster was so small as to have a negligible effecton sea-level change.) Ice also may have extended furthersouthwards over the Celtic Sea (Scourse et al., 1990, 199.1 )than assumed in the 'standard' ice model, but this ice flow

was probably relatively thin and mobile as well as restrictedprimarily to the sea-floor. Hence, contributions to isostaticrebound from such an extens¡on are small, particularly at thelrish Sea margins. The retreat of ice across the lrish Sea in thestandard model was constrained by an age of about 1700OyrBP for the Main Drumlin Moraine (McCabe, 1987) and byice-free conditions in northern Wales before about 18000 yrBP (Shot ten and Wi l l iams,1971) and on the Is le of Man by18000 yr ago (Stephens and McCabe, 1977). A rapid retreatof the ice across the southern lrish Sea (and southern Ireland)between the time of maximum glaciation about 22 000 yrago for the Brit ish lsles and ice-free conditions in northernWales, and possibly at the lsle of Man, by 18 000 yr ago when

15-80 L

1 81 8

SEA-LEVELS IN THE IRISH SEA SINCE THE LAST CLACIAL MAXIMUM s05

sea-level globally was quite constant (Yokoyama et al.,2OOO),is not consistent with the ice sheet being very thick and centredover the southern region.

of Scandinavia and a reduction in ice volume increases thepalaeowater depths.)

The predicted land bridge occurs at about 51" north lat¡tudeand 7' west longitude and its location is determined largelyby the global changes rather than by the dimensions of the iceover the Irish Sea. Models with different thicknesses of ice overthe lrish Sea differ mainly in the locations of the shorelinesin Cardigan Bay and in the width of the coastal plain onthe Irish side. Thus sediment cores from these localit ies maycontain ¡mportant information for constraining the ice models.For all models considered, the predicted land bridge is onlya few metres above coeval sea-level and is likely to havebeen flooded by the melt waters from the northern ice. Thus,although it could indeed have been crossed by wading byBendigeidfran 'with all that there was of string minstrelsy onhis own back', it may not have provided an effective or friendlyroute for other faunal and floral migrations from Britain (andEurope) to Ireland.

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Conclusion

Models of glacial rebound for a particular region must beconsistent with observations of relative sea-level change,with field evidence for the location of ice margins andice thickness, and with the geophysical theory of rebound.For the lrish Sea and its margins the available evidence isinconsistent with the interpretation of the elevated glacialsediments ¡n eastern lreland having been deposited in ashallow-water, glaciomar¡ne environment. The maximum icethickness required over southern lreland to produce suchshorelines is three to four times greater than that inferredfrom the Holocene evidence, or at least 2000 m thick, muchgreater than over Scotland and northern England. To avoidinconsistencies with the Holocene sea-levels from the Welshcoasq which indicates that sea levels were rising throughoutthis interval (Heyworth and Kidson, 1 982), such ice thicknesseswould have to be restricted to the western side of the lrish Sea.but as noted above, such raised levels are inconsistent withthe Holocene evidence from nearby localit ies. The apparentabsence of raised shorelines along the west coast of lrelandmeans that this additional ice could not have extended tothe western margin of the ice sheet. lf the increase in icethickness is extended into northern Ireland, then the predictedsea-levels for Antrim and Down remain above the present levelthroughout late-glacial and Holocene time, inconsistent withthe relative lowstands that have been reported there (Carter,1982). Furthermore, the ice would have had to vanish quicklyin order that southern lreland and the lrish Sea south of theMain Drumlin Moraine were ice-free before .l2000 yr ago. lfsuch ice models appear to be extreme and incompatible withglaciological evidence and concepts, and we are not aware ofany evidence or models that support such a radical departurefrom 'standard' ice models, then the alternative is to adopt theconventional interpretation of the ,lrish Sea Drift, as havingbeen deposíted by a terrestrial glacier prior to the global risein sea level. Such an interpretation is consistent with sea-levelmodels of relatively thin lrish Sea ice that is largely fed by iceover Scotland and northern England.

A critical test of the rebound model can come from estimatesof the water depths in the Irish Sea during the ice retreat stagesas well as from evidence of whether or not a land bridgeexisted between Britain and lreland (see e.g. Devoy, 1985).Previous model results showed that the location, t iming andelevation of such a land bridge was rather crit ically dependenton the ice thickness over lreland (Lambeck, , l996) and it alsowill be dependent on the ice thickness over the lrish Sea: anincrease in ice thickness over the lrish Sea increases the waterdepths in late-glacial t ime and suppresses any land bridges thatmay otherwise form. A decrease in ice thickness over lrelandhas a similar effect because the lrish Sea lies in the peripheralbulge zone of the ice load. For the thin-ice model a tenuousland bridge is predicted to occur ar about 1 6 000 yr ago. Thesmall difference between this and the earlier reconstructionof Lambeck (l 996) is mainly the consequence of the reducedamount of ice in the Scandinavian ice sheet used in the presentcalculations and in some minor rebound-model refinements(Lambeck et al., 1998). (Analogous to the above argument, thispart of the Brit ish lsles l ies within the peripheral bulge zone

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