Journal of the Geological Society, London, Vol. 153, 1996, pp. 853-872, 15 figs, 2 tables. Printed in Northern Ireland
Glaciation and sea-level change for Ireland and the Irish Sea since Late Devensian/Midlandian time
KURT LAMBECK Research School of Earth Sciences, The Australian National University,Canberra, ACT 0200, Australia
Abstract: The sea-level change around the coast of Ireland and the Irish Sea for the past 20 000 years is primarily the combined result of the glacio-isostatic adjustment of the crust to the removal of ice over the British Isles and the total eustatic change from the global ice sheets. However, the isostatic effects due to theremoval of ice from northernEurope and NorthAmerica and the addition of meltwater into the oceans also make a significant contribution. Predictions of sea-level change, based on glacio-hydro-isostatic models arecompared with observations to constrainthe ice volume over Ireland at the time of the last glacial maximum and the maximum ice height appears to have been of the order of 600 m. The models predict well the spatial variability in sea-level change observed across theregion for Holocene and Lateglacial time, with levels abovepresent being predicted only for northeastern Ireland and north of about Morecambe Bay. The models do not support suggestions that Lateglacial levels along the east coast of Ireland or the coast of Wales were 50-150 m above their present levels. Consistent models that would produce such large Lateglacial highstands are incom- patible with all other sea level and glacial evidence for the British Isles. Palaeobathymetry and palaeoshoreline reconstructions for the Irish Sea indicates that a tenuous landbridge between Britain and Ireland developed only across the Celtic Sea, between about 18000 and 14000 years BP.
Keywords: Holocene, Ireland, Irish Sea, glacial rebound, changes in level.
The evidence for Holocene and Late Midlandian sea level instances this has to be inferred from the sea-level data around the Irish coast is abundant yet qualitative and few itself. In some instances, where a good temporal and spatial detailedobservational records are available. At best the distribution exists, this separation of earth-modeland record is based ondata brought together from different ice-modelparameters can be achieved, as is the case for localities, collected by different investigators and correspond Great Britain. If it can then be assumed thatthe Irish to indicators with different responses to changes in sea level. crustalresponse does not differ greatly from that of its Despiteshortcomings, a considerable spatial variability in eastern counterpart, it may become possible to resolve some sea-level changehas been recognised around the coastline of the questionsabout the ice sheetand palaeo-shoreline andqualitative regional patterns have beenestablished indicators over the Irish region. This is what this paper sets (Carter et al. 1989). This pattern is broadly consistent with out to do. the predictions of past sea-level positions based on models Following the introduction the first section outlines very of the glacio-hydro-isostatic adjustments of the crust and sea briefly the glacial rebound model and presents some model level in response to the melting of both the large and distant resultsfor simple configurations in orderto illustrate the ice sheets and the smaller local ice sheet over the British importance or otherwise of the various componentsthat Isles (Lambeck 1991~). However, the observational contributeto the sea-level change. The second section constraints on the ice sheet over Ireland, as well as on the discusses some more realistic predictions based on the local sea-level fluctuations, appearto be inadequatefor ice model previously developed (Lambeck 1993b). The evaluating precisely the Earth’sresponse function tothe third section briefly reviews the sea-level data and compares changing crustal load. This is in contrast to the situation for these with the predictions and draws some conclusions about Great Britain where the spatial and temporal distribution of the ice sheet over Ireland and shoreline migration since the relevantobservational materials is more complete and time of the last glacial maximum. where it has been possible to invert the observational data for both the Earth structure andcertain ice sheet parameters (Lambeck 1991b, 1993a, 6; Lambeck et al. 1996). The glacial rebound model and predictions for Some of the limitations of present knowledge of the Irish simple configurations ice sheet include: the limits of the ice at the time of the last glacial maximum, particularly in the Irish Sea; the rates of The rebound model ice retreat over southern Irelandand the Irish Sea: the thickness of the ice at the time of, and subsequent to, the Detailedaccounts of the glacial rebound model for Great last glacial maximum. Thisknowledge can beimproved if Britain have been given elsewhere (Lambeck 1993a, b) and Late- and Postglacial observations are available of sea-level a simple heuristic description of the model is adopted here. change around and within the margins of the ice sheet. For The problem is one of estimating sea-level change in such observations to be useful, however, theEarth’s response to thegrowth or decay of continental-based ice response to loadingmust also be known and in most sheets. The zeroapproximation solution is theeustatic 853
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sea-levelmodel inwhich sea level changes in a spatially Axially symmetric model predictions uniform manner. But because of the gravitational attraction While it isnow recognized that sea-level change exhibits between the ice and melt-water, and because of the Earth’s both spatial and temporal variability, the magnitude of the deformation under the time-dependent ice and water loads, departures from eustasy do not always appearto be well this gives a poorrepresentation of theactual change. understood in discussions of observational evidence. Some Instead, in response to the melting of the Late Pleistocene simple quantitative models will therefore be examined first ice sheets, sea level haschanged in a complex spatial before considering more realistic models. Such models that manner since the time of the last glacial maximum at about are illustrative of the various contributions are axially 20 000-18 000 years ago. symmetric ice sheetsthat ignore the consequences of the This change at location cp; and time t can be expressed in complex geometry of the coastlines. Consider a model-earth the following schematic way that is covered by ocean except for a continental polar cap of radius R,, . This cap is covered by an ice sheet of the Al(cpJt) = AlAt) + Alr(cp, f) + Al,(cp?t) + Aldcp, t) (1) same radius with a radial height Z(r) dependence given by where Al(cp, t) is the relative change in sea level at time t (Paterson 1971)
with respect to the present. Ale([) is the eustatic sea-level r I \I51(14 change defined as Z(r, t)= Z,,(t) L 1 - (&l- 1 change in ocean volume (2) and A‘~(f) = area of ocean surface Z,,( l) = a (5) and is a function of time only. The second term AL,(cp, t) in where ZJt) is the maximum ice height at time t. R is the (1) represents the change in sea level that would occur if the radius of the ice load at time t, and a is a constant. This ice Earth were rigid such that the only spatial variation is the sheet is assumed to have been in place for a very long time result of the changing gravitational attraction of the ice and such thatat the onset of melting a state of mechanical water. Near the margin of a growing ice sheet, for example, equilibriumhas been reached betweenthe load and the the gravitational attraction of the ice sheet will be such as to stress state of the lithosphere. Three different ice loads will raise sea level while further away it will fall. This will be be considered,representing respectively theLaurentian, superimposed on the associated eustaticchange and the Scandinavian, and British ice sheets (Table 1). In all three total sea-level change will be spatially variable. The third the ice load is initially assumed to have been removed term is the glacio-isostatic term. It allows for the sea-level instantaneously at 12 000 years ago but in some later models change in response to both the modification of the Earth’s a more realistic melting profile is assumed. surface and of the gravitationalpotential as the planet The earth-model is defined by an elastic lithosphere of deforms under the changing ice load. The last term in (1) is thickness H,, by an upper mantle of uniform viscosity v,,,,, the hydro-isostatic term that describes the sea-level change and by a lower mantle of uniform viscosity The division resulting fromthe changingwater load. As the ice sheet v/,,?. between the upper and lower mantle is assumed to coincide decays and the melt-water is distributed non-uniformly into with the 670 km seismic discontinuity. Analyses of glacial the oceansbecause of the gravitational and isostatic rebound from numbera of different localities have deformation effects, theEarth’s crust deforms spatially suggested that a strong contrast in average viscosity occurs under the changing load. The resulting modification of sea between the upper and lower mantle (Nakada & Lambeck level is described by this term. 1989; Lambeck 19938). Table 2 summarizes the nominal The description the rigid term requires a knowledge of of parameters adopted, parameters that yield a very satisfac- the distribution of the ice at the time for which sea level is tory agreement between observations and predictions of sea calculated. The glacio-isostatic corrections, in contrast, level throughout the British region (Lambeck 19936, 1995). require a full history of the ice sheets because of the viscous Figure 1 illustrates the deflection of the Earth’s crust in nature of the Earth’s response to stress and the calculation response tothe ice load atthe time of the maximum of this term requires a knowledge of the Earth’s rheology. glaciation for each of thethree ice models. For the two The hydro-isostatic term is also a function of this rheology, larger ice sheets, where the radius of the load much exceeds as well as of the change in ocean shapeand volume in the effective lithospheric thickness, the maximum deflection response to the crustal displacement and eustatic rise. The at the centre of the load approaches the local isostatic limit, relative sea-level equation (1) is thereforean integral similar to solutions for loading of a thin plate over a fluid equation that is solved iteratively (Clark et al. 1978; Nakada medium (Lambeck & Nakiboglu 1980). For the small load, & Lambeck 1987; Mitrovica & Peltier 1991; Johnston 1993). characteristic of the British ice sheet dimensions, the Solutions of the equations for realistic ice and earth models are computationally intensive and are usually obtained by expanding the surface loads in surface spherical harmonics Table 1. Parameters of the three axisyrnrnetric ice models and out to very high degrees (typically greater than degree 200). their equivalent eustatic sea-level (esl) rise Once a solution is found, the location of past shorelines and Model Radius Maximum Equivalent the palaeo-topography or bathymetry h(cp, t) attime t degrees ice height esl (m) follows from the present topography h(cp, r,,) as (km) (m)
c) = h(cp, L)- Al(9, t). (3) GB 2.5 (280) 1500 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVELCHANGE SEAFORIRISH THE 855 Table 2. Summary of effective parameters defining the three-layered whereas for the British ice sheet it is less than 10 m for the mantle model same earth model with the amplitude being both a function of the ice height and the load radius. The narrow crustal Lithospheric thickness 65 km moat that formsbetween the ice margin and the crustal Depth of lower-upper mantle boundary 670 km bulge is generally small, its width never exceeding about Average upper mantle viscosity 4 X 10~"Pa S 2" beyond the ice margin and its amplitude generally less Average lower mantle viscosity 10~'Pa S than about 100 m below the maximum level of the uplifted Density and elastic moduli Dziewonski & Anderson arch. (1981) Figure 2 illustrates the evolution of the deflection of the crust through time after the instantaneous removal of the ice for thethree models. This deflection is expressed in this maximum deflection is much reduced, because the maximum ice thickness is less and the load is partly supported by the instance with respect tothe position of the crust atthe present time, some 12 000 years after the removal of the ice. elastic stresses within the lithosphere. Figure 1 also illustrates the small moat and arch structure of the Earth's That is, it represents the uplift or subsidence of the crust that has occurred during the interval between the specified surface that forms on the periphery of the ice load at the epoch andthe present. The relaxation 12000 years after time of the ice loading. This would representthe radial unloading will generally not be complete and the amount of displacement of the surface of the crust and would be remaining rebound beneath the centre of the former ice load superimposed on the glacially unperturbed topography. The is about 100 m for the two larger ice sheets and about 20 m position of the maximum uplift of the crustal arch in each for the smallest ice model. After the initial elastic deflection case occurs about 2" (about 220 km) beyond the ice margin (the difference between the results for the epochs 12' and for this particular earth model and extendsout to a 12- at infinitely small intervals before (12') and after (W) considerable distance beyond the load radius, out to about melting at 12 000 years the rebound of the crust beneath 10" for the British-sized ice load and out to about 30" for the BP) the ice load is initially rapid and then slows down. Thus the Laurentide-sized load. Of importance is the dependence of pattern aat fixed site is one of an approximately the magnitude of the uplift on the load height and radius, exponentially decaying curve with time (Fig. 3a). The being an order of magnitude less for the smallest load than peripheralmoat and arch also decrease in amplitude as a for the largest load model: for the Laurentide, the maximum consequence of the viscous stress relaxation (Fig. 3b & c). peripheral bulge deflection of the crust is about 100 m Any horizontal displacements of these features are small, if not zero, particularly for the larger loads (e.g. Fig. 2f). The change in the deflection of the crust, or the shift in the radial distance of the surface from the Earth's centre of mass, can be measured using certain geodetic methods of E ."M positioning but the requisitemeasurement accuracy has f been available only for the past decade and no significant .I8 results have yet been obtained. Also, recent measurements of the tail-end of the relaxation process do not provide strongconstraints onthe rebound models whose charac- -200 teristic time scales are of the order 103-104 years. Thus the more usual and useful observable is the change in the position of sea level relative to its present location, as reflected in raised or submerged shorelines. Figure 4 illustratesthe components that contribute to this total sea-level change for the same three models. The rigid term (curve 1, Fig. 4) is non-zero only while there is ice and, in the instantaneous unloading models considered here, it vanishes immediately after 12000 years ago. For the small ice sheet the rigid component is generally unimportant (Fig. 4a) but that of the larger ice models can be quite substantial (Fig. 4c). For all models the contribution is positive out to a distance of about 60", beyond which it becomes negative. The glacio-isostatic term comprises two parts; the I deformation of the crustal surface with respect to which the 0 5 10 15 20 25 30 relative sea-level change is measured (curve 3, Fig. 4), and distance (in degrees)from centre of ice load the change in gravitational potential associated with this deformation (curve 2,Fig. 4). Generallythe first part will Fig. 1. Crustal deflection for the three zonal ice models, be the major one although the second contribution remains corresponding approximately to the dimensions of the British Isles (GB), Scandinavia (SCAN), and Laurentia (LAUR) ice sheets, for significant, particularly for the larger ice models (Fig. 4~). the state of regional isostatic compensation. The crustal deflections The total glacio-isostatic sea-level curve, including the are for the time of maximum glaciation. The effect of the water load gravitational attraction of the ice, is dominated in all three is ignored. The arrows in the lower figure denote the location of the cases by the surface deformation. Thus the prediction of the edge of the ice sheet for each of the three models. The horizontal latter alone gives a reasonable qualitative and quantitative scale is in degrees of colatitude (1" = 110 km) from the centre of the understanding of the sea-level response in thearea load. previously covered by ice where the rebounding crust is seen Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 856 K.LAMBECK -50 .l0 I 0 h E - 50 *E 100 0 8 ‘5 150 200 8U 250 GB 300 1 0 2 4 6 8 l( a b -100 I 1 t ,*--.12 - h 0 vE Y 200 V2 C .41Y 400 V 6 ‘ 600 I 800 J 100 5 10 15 25 20 30 8 12 10 1418 16 20 C d -100 ,--- : ‘, .I 3 LAUR LAUR 50 0 10 20 30 40 2515 20 30 e distance (in degrees) from centre of iceload f distance (in degrees) from centre of iceload Fig. 2. Evolution of the crustal deflection, relative to the present position of the surfaces for the three zonal ice models in which the ice load has been removed instantaneously at T = 12 OOO years BP. All epochs are in units of 1000 years. The curve 12+ corresponds to the deformation at times greater than 12 000 years ago, and the curve 12- corresponds to a time immediately after the ice has been removed. The difference between these two curves defines the elastic response to the unloading. (a, b) The deflection under the GB load with (b) corresponding to an expanded scale across the peripheral moat and arch. (c, d) The deflection under the SCAN load. The arrow in (d) marks the ice margin of this ice model. (e, r) The deflection under the LAUR load. Note the different distance and deflection scales used. as a relative fall in sea level. Beyond the immediate ice limit rise in sea level (Fig. 3c). In this moatand arch zone, all the narrow peripheral crustal moat is also an area where the terms are important if an accurate quantitative description is non-eustatic sea-level change is one of a small relative fall sought, particularly for the larger ice sheets. In large part whereas the arch area is a region of a relative (non-eustatic) this is a result of the different wavelengths of the rigid and Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SE A -LE VE L CHANGE FOR THESEA-LEVEL FORCHANGE IRISH SEA 857 h WE f 200 a> 1 100 P ’3 3 0 0 15 10 5 0 0 5 10 a a 100 1 1 SCAN 80 h 60 WE .g 40 20 B4 $0 ’------.-- -20 t2 1 40 b 15 10 5 0 0 5 10 15 20 20 t LAUR I I -200 15 10 5 0 0’ 15 5 10 20 25 30 C time (x1ooO years BP) C distancedegrees)(in from centre of sheetice Fig. 3. Predicted crustal rebound corresponding to Fig. 2 for the Fig. 4. The contributions to the total sea-level change, relative to three load models at radial distance (a) r = 0, (b) r = R, (radius of the present, at a time before the instantaneous unloading at 12 000 the ice load), and (c) r = R, (distance out to the centre of the arch). years BP for the three zonal ice models. (1) the gravitational attraction of the ice on a rigid earth, (2) the change in gravitational attraction associated with the deformation of the earth, (3) the glacio-isostatic componentsand the neglect of theformer displacement of the Earth’s surface in response to the loading, (4) will generallylead to a poor representation of sea level the sum of the contributions 1 to 3. across the moat and arch zone, particularly for the larger ice sheets. The total ice load terms are also illustrated in Fig. 4 for (Fig. 5). In all three cases the primary evolution of sea-level thethree ice sheetsat the time of maximum glaciation change through time is one of a decreasing amplitude of the (curve 4) and at several epochs after the removal of the ice departure from eustasy with time, with little or no migration Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 858 K. LAMBECK Isles, the global eustatic change will dominate the sea-level change (see Fig. 11, below). At least fourfactors will perturbthe results of these 200 h simple models and limit their generalization to real E GB v i situations, as well as demand more detailed models in order 2 for any direct comparisons with observations to be relevant. -8 First, melting of the ice sheetsdoes not occur instan- 2 100 taneously and both the radius of the ice sheet and its height - change over a period of typically about loo00 years. The 1 resulting interplay of the two time-dependent processes, the .g ice retreatand the viscous relaxation process, further modifies the sea-level response in the ice margin zone. zo Figure6 illustrates the schematic evolution of the cross 5 section of theGB ice sheet models together with the -50 I 1 correspondingpredicted sea-level change duetothe 0 2 4 6 8 10 combined gravitational attraction and glacio-isostatic terms. a In this model the onset of melting starts initially at 22 OOO years ago and ended 12000 years ago. The change beyond 600 the immediatearea of ice loading remains small but the structure of the curve changes with time, with the crustal arch (the zone of negative sea-level change inFig. 6c) h migrating towards the load centre as the ice retreats, but vE 400 always by a relatively small amount. Second, in the above 2 model the ice occurs on a continental base that rises above 8 sea level such that the shoreline remains fixed in time. But ti if the ice sheet is grounded on sea floor, the moat structure - 200 is correspondingly broadened by the retreating ice and P expandingocean basin. This is illustrated inFig. 7 for an .g idealized situation where the sea floor, in the absence of the 2 ice load, is flat. Irish Sea localities would correspond 0 approximately to distances greaterthan about 1" and the differential deflection of the sea floor from north to south -100 (increasing distance) would be quite substantial. However, 0 5 10 15 20 because the overalleustatic sea-level change is also large, b raised shorelines are predicted to occur only in areas within 600 the originally ice-loaded zone. Third, to these glacio-isostatic correctiveterms must be addedthe water 500 I-...12- LAUR 4 load terms to give a complete picture of the sea-level change h for the region. Generally these terms are small when .S400 compared with the glacial rebound terms near the centre of %b the ice load but they are nevertheless important, particularly B 300 8 for epochs after the completion of melting, and add to the g 200 complexity of the sea-level function in the zone immediately P beyond the ice limit (Lambeck 1993a; Johnston 1993). P 100 Fourth,the assumption of regional isostatic equilibrium having beenattained before the onset of melting will 0 generally not be valid. In reality the build up of the ice 2 sheets has been quite rapid and the duration of the interval -100 of maximum glaciation is comparatively short so that this -200 equilibrium state will not have been achieved, with the effect 0 5 10 l5 20 25 30 thatthe magnitude of the crustal deflection and sea-level C distance (in degrees) from centre of ice sheet change is overestimated (Lambeck 1993~). All four limitations are relaxed in the detailed models discussed Fig. 5. Total glacio-isostatic contribution, including the gravita- below. tional attraction, to relative sea-level change at epochs immediately These simple resultsdescribe well the response of the after the instantaneous ice unloading at 12 000 years BP (12-) and at crust to rapidly changing ice loads. They also illustrate that 11 000 (ll),9000 (9), 7000 (7), S000 (S) and 3000 (3) years before some of the heuristic models forthe magnitude and present (a) for the representation of the zonal ice model of the evolution of the crustal peripheral arch or bulge are quite British Isles, (b) Scandinavia. and (c) Laurentia. unrealistic. For example, Eyles & McCabe (1989),in discussing sea-level change within the Irish Sea Basin, argue thatthe glacio-isostatic depression of the sea floor, the of the locations at which the maximum departures occur. crustal moat structure in Fig. 5, occurs many hundreds of Superimposed upon the isostatic terms must be the eustatic kilometresdistant from the ice loads, creating deep sea-level rise from the totality of the ice sheets and near the peripheral basins in which thick successions of glacial and margins of small ice sheets, such as the one over the British marine sediments are deposited and which are subsequently Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVELCHANGE SEAIRISHFOR THE 8.59 a 0 1 2 3 4 a 300, l l500 16 OOO yun BP 1000 C 8 500 E 0 -500 1 2 3 4 5 b -50 012345678 b 20 l 0 1 2 3 4 5 C 10 Is'., 750 10 OOO years BP 500 5 250 -10 1 I 3 4 5 6 7 a -250 I 1 C distance(in degrees)from centre of sheetice 0 1 2 3 4 5 Fig. 6. The predicted relative sea level for the zonal GB model in d radius (in degrees)from centre of ice load which melting starts at 22 000 years BP and ends soon after 12 OOO Fig. 7. Schematic evolution of the GB ice sheet and crustal years BP. (a) Ice profiles at selected epochs, (b) predicted sea-level deflection for a shallow-water marine-based ice sheet at four change due to the rigid and glacio-isostatic components (the arrow epochs: (a) the glacial maximum: (b) at 16 OOO years BP; (c) at denotes the limit of the ice at 22 000 years BP), (c) same as (b) but 13 OOO years ago when the ice load is restricted to Scotland; (d) at for the moat-arch zone beyond the initial ice sheet limit. 10 OOO years ago. The ice profile through time corresponds to that illustrated in Fig. 6a. If the eustatic rise of the major ice sheets is uplifted. This is not based on quantitative modelling but on included, then at 22 000 years BP the crustal deflection outside the analogiesdrawn from glacial rebound calculations for the ice load is less than the eustatic change of about 130m and no Laurentide ice sheetand from foreland basin models raised shorelines of last glacial maximum age are predicted beyond developed for understandingsedimentary basin evolution. the load radius. At 13 000 years BP, for example, the global eustatic Neither analogy isvalid when applied tothe British ice sea level is about 9.5 m and raised shorelines are predicted only out sheet's influence on the Irish Sea Basin. The first analogy is to a distance of about 0.8" ( Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 860 K. LAMBECK as if fluid. Thus the deformation is solely determined by the Devensian time to 500 years in Lateglacial time. (See also rheology of the lithosphere, the effective thickness of which Lambeck 1995, fig. 3) In this model the ice over Ireland is is considerably less at these long time scales than at the 104 assumed to have formed largely by the outwards growth of year time scale because of stress-relaxation within the layer. local ice domes that coalesced to cover much of the island. In contrast, for the shorter time scales of the glacial cycles The ice sheet over the Irish Sea is assumed to have been the the load is partly supported by stresses within the mantle result of the movement of ice from Scotland across the and the deformations will be less, particularly for the small NorthChannel of the Irish Sea where itwas deflected ice loads. Clearly, the application of models developed for southwards to form the Irish Sea glacier. The maximum other purposes can lead to misleading interpretations if a extent of the ice sheet is assumed to have occurred along the proper scaling for time and length scales is not applied. South of IrelandMoraine atabout 22000 years BP (e.g. For a region such as the British Isles, the total sea-level Eyles & McCabe 1989) and to have extended over the Irish change will be a function of the isostatic contributions from Sea as far as the southern entrance to St Georges Channel the local ice sheet as well asfrom the larger andmore (e.g. Garrard & Dobson 1974; Eyles & McCabe 1989), distant ice sheets of Laurentia and Fennoscandia, located at although relatively thin ice cover may have extended further distances of about 40-45" and 15-20" away respectively. At southwards (e.g. Scourse 1991; Scourse et al. 1990). A small these distances the rigid contributions from these two zonal ice cap is also assumed to have existed briefly over the high ice sheets over the British Isles is positive and ranges from ground of County Kerry and County Cork in southwestern about 7 to 10 m for the former and from 11 to 16 m for the Ireland. The southern limit of the ice sheet over Wales and latter ice sheet (Fig. 4). The glacio-isostatic contributions southern England appears to be reasonably well constrained from the Laurentia-sized ice model range from about 1 to 5 and the limit proposed by Bowen (1981) andAndersen m overthe British Isles region,whereas the contribution (1981) has been largely adopted. The ice over Wales, from the smaller but nearer Fennoscandian ice model ranges dominated by a local dome over the high ground, coalesced from about -11 to +l m over the same region. Clearly the to the west and north with ice that advanced down the Irish contributions from these, and any other, distant ice sheets Sea basin from Scotland (see also papers in Ehlers et al. need to be considered if accurate predictions for the glacial 1991). In western Ireland the ice model is assumed not to rebound are sought. have extended much beyond the present shoreline. To the Despite theirlimitations, several semi-quantitative norththe ice extendedinto the Malin Sea (Davies et al. conclusions can be drawn from these simple model results. 1984) but over a smaller area than proposed by Boulton et Foremost,they illustrate the complex spatial patterns of al. (1977) andAndersen (1981). (Fig. 10 below, illustrates sea-level change that can develop at the periphery of the ice the ice limits.) sheets asa result of the combined,sometimes competing, Estimates of the maximum ice thickness over Ireland effects of the time-dependentgravitational attraction rangefrom 1500 m (Denton & Hughes 1981) to 750 m between the ice and water andthe glacio- and (Boulton et al. 1985) based on theoretical ice model hydro-isostatic factors. Not only the local ice-sheet but also considerations, andto only 400-500 m based on themore distant, and in this case larger, ice sheets of geomorphological evidence (Watts 1977). Insofar as the Laurentia and Fennoscandia must be considered. Moat and present model is largely constrained by observational arch structures do develop around the margins of the ice evidence and inferences this last value, 500 m, was adopted sheet, which can give rise to the rapidly changing gradients in Lambeck(19938) although in a second model, the in the sea-level surface when expressed relative to present maximum ice thickness over the region had been increased sea level. But the magnitudes are generally small for the to 600m. This latter model is used here. Ice heights over Britishcase, particularly when compared with thetotal northern Scotland were also increased in that model to eustatic sea-level changes thatare associated with the match betterthe observations there, but this has minimal quasi-simultaneous melting of the large ice sheets. influence on the predictions forIreland. The evolution of the ice heights with time are based on the assumption that More realistic models profiles of the form (4) remain valid for any section through the ice sheet with the constant (Y scaled from the starting profile for that section at t = 22000 (Lambeck 19938). The Ice models ice over the Irish Sea is assumed to have been relatively For effective glacial rebound modelling in the Irish Sea and thin, consistent with the suggestion that the Isle of Man may surrounding region, a high resolution spatial and temporal not have been completely ice covered during the peak of description of the evolution of the ice load is required since glaciation, although the proposed ice thickness here is at leastLast Interglacial time about120000 years BP greater than proposed by Thomas (1977). The maximum ice (Lambeck 1993a). The ice models adopted for the major ice thickness over Wales is assumed to have been 400 m. sheets over Fennoscandia, Laurentia, and Antarctica are the If the Isle of Man was largely ice free by about 18000 same as those previously used in the discussions of sea-level years ago as suggested by Stephens & McCabe (1977) then change overGreat Britain. The model is a global one in the ice retreat across the Irish Sea andsouthern Ireland which all four contributions to the sea-level changes in (1) must have been rapid initially. This is consistent withice are evaluated for each of the ice sheets but inwhich the free conditions in northern Wales implied by the spatialdetail of thecomponent ice sheets decreases with radiocarbon date Birm-146 (Shotten & Williams 1971). distancefrom the British Isles. The British ice model for Across Ireland the ice front at this time is assumed to have Devensian time has been discussed in Lambeck (19936) and stood north of the Galtrim Moraine which is believed to be will be used here asastarting model forthe rebound olderthan 18000 years. The Main Drumlin Moraine is calculations. It is defined on a 25 km by 25 km grid and at assumed to have an age of 17 000 years (McCabe 1987), and time steps ranging from a few thousand years in prelate the ice is assumed to have retreated to this location by that Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVELCHANGE FOR THE IRISH SEA 861 time. This moraine has also been traced on to the adjacent .” sea floor and this has been used to define the northwards ice movements in the Irish Sea. Retreat along thenorthern margin of Ireland appears to have been less rapid initially 20 i\2. and thenorthern limits of the ‘Drumlinevent’ moraines have been taken to mark the ice limit at 17000 years ago 0 (Bowen et al. 1986; McCabe et al. 1986). By 15 years BP 000 2v 4 the ice sheet is restricted to the northern and northwestern 0 parts of Ireland and theconnection with the Scottish ice -2 -20 m sheet is tenuous. Ireland is assumed to have been ice free 2 by about 14000 years BP (Carter 1982; McCabe 1987). .-2 -40 The ice movementsbefore 22000 years BP have been Lm reconstructed from palaeotemperature records for Scotland, 2 England and Ireland, onthe basis that when pre-Late -60 Midlandian or Devensian temperatures are similar to those at a Lateglacial period then the ice sheets were also similar. -80 This part of the ice model is less critical forpredicting Lateglacial sea-level changes than is the ice model for the past 20000 years and this simplification appearsadequate forpredicting Lateglacial and postglacial sea levels 15 10 5 (Lambeck 1993~). a Some preliminary model predictionsof sea-level change 2o Additional to the requirement of a realistic description of 1 the ice sheet is a model of the rheological response of the Earth to changing surface loads. The model adopted here is the simple three-layermodel introduced in the earlier section (Table 2) as it was found to provide a very adequate representation of the rebound of Great Britain, describing well all the principal characteristics of the sea-level change observed throughout that region (Lambeck 1993b). Further analyses with more complex earth models (Lambeck et al. 1996) indicate thatthe three-layer representation is adequate for all present purposes. * ‘H/ 5. Broad Haven Figure 8 illustrates the predicted mean sea levels for sites aroundthe coast of Ireland(see Fig. 9 forlocations). In thesepredictions all melting of the distant ice sheets has ceased by6000 years BP. The resultscorrespond to mean sea level and shoreline features formed at a higher tide level 20 I5 10 5 0 will lie above these predictions by the corresponding tidal b amplitude. Thenortheast coastalsites of Ballycastle and Donaghadee show a sea-level pattern that is characterized by an initially falling sea level, a stationary level at about 11 000-12 000 years BP, followed by a rise in sea level up to about 6000 years BP after which the level gradually falls to its present value (Fig. 8a). Studies for other regions have indicated that the ocean volumes may have increased over the last 6000 years sufficiently to raise sea level by about 2 m ,-.a (Lambeck1993b; Lambeck & Nakada 1990) such that the WE e, predictedmid-Holocene highstands in Fig. 8 may be too -5 -20 high by this amount. Withthisadjustment, the m mid-Holocene highstand of mean sea level is predicted to 2 occur between about Dundalk to Fanad Head,with- the .- -40 maximum highstand occurringbetween Belfast Loch and m 3 Ballycastle. Earlierhighstands are predicted to occur over -60 a shorter section of coastline between aboutDonaghadee -80 Fig. 8. Predicted sea-level change for sites around Ireland and the Irish Sea (see Fig. 9 for locations of sites) (a) for northeastern and -100 southern sites in Ireland, (b) for western localities, and (c) for sites 20 15 10 5 0 in northwestern England and Wales. C time (x1OOO years BP) Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 862 K.LAMBECK 56 MALM SEA 54 Fig. 9 Location map for sites mentioned in text and in Fig. 8. Sites numbered la to 8a refer to the sites 1 to 8 in Fig. 8. Sites numbered lb to 9b and lc to 8c 52 refer to the locations in Figs 8b and 8c respectively. Other sites are: A, Portrush (Londonderry): B, Belfast Loch; C, Fanad Head on Lough Swilly (Donegal): D, Mell; E. Dublin; F, Cardigan and CELTIC Banc-y-Warren (Wales): G, Bryncir and SEA Hendre (north Wales). The Irish mo- raines are denoted by SIM (the Southern Irish Moraine), GM (the Galtrim Moraine), MDM (the Main Drumlin Moraine and its extension to the Isle of 50 Man), and DEM (the northern Drumlin -10 -8 -6 -4 -2 event Moraine). and Ballycastle, with the youngest features predating about 10 with a spatial variability of about 1.5 m. At 16000 years 14000years BP. Thus, provided that ice freeconditions BP this latter contribution ranges from about 0 to 20 m over existed in this area by this time,pre-Holocene raised the same region. shorelines only older than about 14 000 years BP can be Along the west coast of Ireland the predicted pattern of expected to have formed with this model. In the adopted sea-level change is more complex because of the deeply ice model Ballycastle was ice free by about 1.5 000 years BP penetrating Lochs. Compare forexample, the predictions and any raised shorelines, now less than lOm abovesea for Slyne Head and Galway, or LoopHead and Limerick level, would have had to form in only a brief interval of (Fig. 8b).Observations from such sites would provide about 1000 years before the sea level fell below its present significant constraints on both the rheology and the ice position. The southern coast of County Down was ice free parameters.Nowhere along the west coast are highstands after about 16000 years BP but at this time the predicted predicted since the onset of melting and the change for the mean sea levels were below their present level and no raised past 12 000 years has been one of rising levels throughout, shorelines of Lateglacial age are predicted to have occurred albeitat spatially and temporally variable rates. This either here or further south. Minimum sea levels at depths difference between the east and west coast response reflects of up to about -30 m are predicted to occur prior to the the fact that it is the ice over northern England and start of theHolocene in northeastern Ireland. With Scotland, rather than the ice over Ireland, that determines increasing distance from the ice sheet these minima increase the dominantcharacter of the spatial variation in the in magnitude and occur earlier in time until the sea-level sea-level. Alongthe eastern side of the Irish Sea,the curves begin to resemble those for sites well beyond the ice predictions are similar to those for northeastern and eastern sheet limits (Fig. 8). Note that because of the rapid build up Ireland, reflecting the increasing distance from the centre of of the ice sheet immediatelybefore the onset of melting, rebound of the sites from north to south. The highstand in maximum depthsat the moredistant sites such asBantry mid-Holocene time is predicted to occur only north of Bay occursometime after theonset of deglaciation. Note Morecambe Bay and the Lateglacial lowstand becomes also that the maximum depth reached is less thanthe progressively deeper and earlier from north to south (Fig. correspondingeustatic change at the last glacial maximum 8c). Later glacial highstands are predicted to occur at Isle of by substantialamounts. At 22000 years BP, forexample, Man beforeand up to about 18000 years BP but will not the combined glacio-hydro-isostatic contributionfrom the have formed because the region wasice covered until this major ice sheets averages about S0 m over the region of Fig. time. Likewise, conditions forshoreline formation would Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 S EA -LEV EL CHANGE FOR THE IRISH THE SEA-LEVEL FORCHANGE SEA 863 56 54 52 50 56 54 52 50 - 1'0 -S -6 -4 -2 Fig. 10. The predicted isobases of sea level relative to present levels (continuous black curves), the palaeo-shorelines (the limits of the shaded regions). and the ice limits and ice heights at selected epochs (continuous grey lines). The ice contour interval is 100 m. The minimum land elevation of the landbridge (the sill height) at 18000 years BP is S m, that at 15 OOO years BP is 3 m. not have occurred in this model forthe coast of the eustaticchange in Late Holocenetime has been applied. Cumberland,despite sea levels having been higher thanThese results can be compared with Fig. 3 of Lambeck presentbefore about 15 000 years BP. (1995) which are based on thesame earth model andan ice Figure 10 illustrates the spatial pattern of sea-level model identical in all respectsexcept thatthe maximum ice changeacross the region forseveral epochs. They are based thickness over Ireland is 500m ascompared with 600m in on the same model as before except that the correction to the present model. Illustrated in Fig. 10 are (i) the ice limits Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 864 K. LAMBECK 56 54 52 50 -10 -8 -6 -4 -2 -10 -8 -6 -4 -2 Fig. 10. (Continued.) and the ice heights, (ii) the contours of sea-level change with about 12 000 years BP the magnitudes of the two rates are respect topresent sea level, and (iii) the approximate about equal and the local sea level remains constant relative positions of the shorelinepredicted by eqn 3. As already to the land until such a time as the eustatic term dominates indicated by the results of Fig. 8 and by the simple and relative sea levels begin to rise. At 6000 years BP the axisymmetric models, the scope for development of eustatic rise has become very small as the melting of the Lateglacial raised shorelines outside of the area of glaciation major ice sheets has terminated,and the residual crustal is limited throughout the region in question and the model rebound dominates to produce the small amplitude predicts highstands only for northeastern Ireland, primarily mid-Holocene highstand (Fig. lla). This highstand in Late Holocene time with sea levels having beena few is significant here because it indicates that mantle relaxat- metresabove their present level. The principal difference ion is still important several thousand years afterthe betweenthese andthe earlierresults in Lambeck (1995) last ice vanished over Britain and this helps constrain concerns the predicted locations of the palaeo-shorelines. In the timeconstants (or mantle viscosity) of the relaxation particular therather broad, lowlying butpersistent process. landbridge between Ireland and Englandpredicted by the Furthersouth, in Dundalk Bay andsouthwards for earlier model from about 19 000 years to 13 000 years BP is example, the crustal rebound still occurs, in part in response now much more tenuous asa result of the increased ice load to the removal of the ice load over Great Britain and in part over Ireland. With an increased local and small ice load the in response to the ice load over Ireland, but its amplitude is sea floor in this region is suppressed by a greater amount small andthe eustaticcontribution dominates throughout whilst the eustatic sea-level change remains essentially Lateglacial time (Fig. llb) although small amplitude unchanged. Another difference is thatthe earlier results mid-Holocene highstands may still develop. The major were based on a less accurate and lower spatial resolution contribution to the crustal rebound remains the removal of model of the bathymetry (the ETOPOJ model, NOAA ice over Great Britain because the length-scale of the Irish 1985) whereas the presentresults are based onthe ice cap is much smaller andthe load is supportedto a 2.5' X 2.5' bathymetry data base compiled by GETECH greaterextent by the lithosphere. Farther south again, in (1995). Bantry Bay for example (Fig. llc), the rebound from the Thenature of these sea-level variations aroundthe removal of the British Isles ice is much reduced and does region can be explained in a qualitative way as the sum of notexceed about 1Om at the time of the Lastglacial the local crustal rebound term and the global eustatic term maximum. Also, because the ice retreat occurs early in this (Fig. ll), ignoring forthe moment the glacio- and area, relatively little rebound occurs after about 10 OOO years hydro-isostatic terms from the distant ice sheets. In BP (a residual of < 1.5 m for the model considered here). northeasternIreland the crustal rebound is substantial The glacio-hydro-isostatic contributions of these distant ice (curve 1, Fig. lla), primarily in response to the removal of sheets has not been included in these qualitative examples the ice over Scotlandand northern England. Initially this although at the level of precision sought these contributions rebound exceeds in magnitude the eustatic rise (curve 3) but become important. Particularly at the southern sites such as is of opposite sign such that the initial sea-level response is Bantry Bay, these terms are larger than the isostatic terms one of an apparent falling level or of relative land uplift. By associated with the British Isles ice sheet. Figure lld Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVELCHANGE FOR THE IRISH SEA 865 150 Dundalk Bay Louth 100. W -150 20 15 10 5 0 20 15 10 5 0 a b 30 -5 . . .-.- S. -10 20 15 10 5 0 20 15 10 5 0 C time (X 1000 years BP) d time (X 1 OOO years BP) Fig. 11. (a) to (c) Schematic construction of the sea-level curves at three localities. Curve 1 represents the rebound due to the deglaciation of Great Britain, curve 2 represents the eustatic change from the global ice sheets, and curve 3 represents the sum of these two contributions. (a) A site in northeastern Ireland (e.g. Ballycastle or Donaghadee) or northwestern England (Whitehaven). (b) A site towards the edge of the ice sheet such as Dundalk Bay in Ireland or Morecambe in northwestern England. (c) A site further away from the centre of the ice load such as Bantry Bay. (d) The total glacio-hydro-isostatic contribution from the distant ice sheets at two of the sites (note the different scale). illustrates these far-field effects for two of the sites: results wavelength of the rebound are dependent on this parameter. thatare representative of the spatial variability and A reduction in upper mantle viscosity increases the rebound magnitude of these isostatic contributions. allin areas (Fig. 12c& d). In observationally well- As indicated, the amplitudes of the sea levels can be constrained problems such as for Great Britain, a separation modified either by changing the earth-model parameters or of these two parameters as well as the ice thickness can be by modifying the ice load. The earth-model dependence of effected but it remains to be demonstrated that this is so for the response to the removal of the ice over the British Isles the Ireland and Irish Sea data. This is discussed further in only is illustrated in Fig. 12. Because the load is of small the next section. spatial dimension, this response is sensitive to primarily the lithospheric thickness and to the upper mantle viscosity and Observations and comparisons with predictions insensitive tothe choice of lower mantle viscosity. A decrease in lithospheric thickness increases the magnitude of the rebound in areas near the centre of the ice load, such as Evidence for Late Midlandian and Holocene sea-level for Ballycastle (Fig. 12a) but the behaviour is the opposite change in Ireland for sites near the former margin of the ice sheet such as the The observational evidence for the Donegal coast between Cardigan Bay site (Fig. 12b) because both the amplitude and Gweebarra and Lough Swilly (Fanad Head) (Carter et al. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 866 K. LAMBECK Ballycastle Ulster OI -20I l5 10 I -a m 5 0 m 15 10 5 0 a b m 25 Ballycastle Cardigan .... Ulster ..... WdeS 150. m --.~~=2x10~~as **..cm= 2x10~Pas h 'Ev a, 0. -50 -5 20 15 10 5 0 m 15 10 5 C time (x1OOO years BP) d time (~1000years BP) Fig. 12. Earth-model dependence of the crustal rebound due to the ice removal over the British Isles, at Ballycastle (a, c) and Cardigan (b, d). (a, b) Lithospheric thickness dependence for an upper mantle viscosity of 4 X 102"Pa S and (c, d) upper (above 670 km depth) mantle viscosity dependence for a lithospheric thickness of 65 km. All models assume an average lower mantle viscosity of 1022 PaS. Observations are generally available only after about 10000 years BP and such data from sites near the centre of the load (e.g. Ballycastle cannot clearly distinguish models with low H/ and v,,, values from models with high H, and v,,, values. At Cardigan Bay this separation of H/ and v,,,, is easier to effect and illustrates the importance of observational data from different localities 1989, fig. 12) represents a typical example of the difficulty of been reduced to mean sea level (MSL) and errors of *l m constructinga precise local sea-level curve. First, the have beenadopted here. Observationsbefore about 8000 observations are from sites separated in distance by up to 70 years BP are fragmentary for this location. km and in close proximity tothe ice sheet limits. Thus Forsouthwest Ireland Carter et al. (1989) have considerable spatial variability in sea-level response can be established 5 depth-age points for mean high water (MHW) anticipatedasillustrated by the examples in Fig. 8b. levels fromthree relatively closely spaced sites in County Compare, for example, the predictions for Donegal, Malin Kerry. The evidence is based on the transition from More, and Inishowen in Fig. 8. Second, the nature of the freshwater to brackish-water peats, and thedata base available dataadds greatly tothe uncertainty of any appearsto be reasonably homogeneous. All heights have depth-agerelation because a variety of different sea-level been reduced to mean sea level using the present estimate of indicators, reflecting formation at different levels within the the tidal range. The observations for this locality suggest a tidal range, are included in this compilation. Even if only gradually rising sea level over the past 5000 years without a the age-depth relations of peats are included and the data is highstand having developed (Fig. 13b). In addition, Stillman restricted to localities within theGweebarra Bay area,no (1968) has established a terrestrial-marine transition at accurate result for the sea-level change emerges. Figure 13a -60 m within Bantry Bay dated at about 11 500 years BP. illustrates the upper and lower limits of the sea-level curve Similar results from County Cork (Carter et al. 1989, fig. 10) for the Donegal region as inferred by Carter et al. (1989) but extend the trend of rising sea levels from about 8000 years the caveat on its reliability cannot be over emphasized. The BP to the present (Fig. 13c). principal result is that the present level was apparently first The recordextends further back along the coast of reached at about 6000 years BP, that a small amplitude and northern Ireland (Carter 1982) although thedata is again short-duration highstand may have occurred at this time, varied,inhomogeneous and unreliable. Figure 13d and that sea level may have dropped below its present level illustrates theupper and lower limits to the MSL curves after about 4000 years BP. The estimate by Carter et al. has proposed by Carter (1982) for the east and north coasts of Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVEL CHANGE FOR THE IRISH SEA 867 1 0. / - M -I IE -2 - -3 - 4- -5 - Kerry southwestIreland 8 6 4 2 0 4 -5 4 -3 -2 -1 0 a b m C. Londonderry 8~Antrim -10 . Cork southern Ireland I -10 -8 -6 4 -2 0 m 15 10 5 0 C time (x1ooO years BP) d time (x1OOO years BP) Fig. 13. Observational evidence for Ireland. (a) Donegal, (b) County Kerry, (c) County Cork, (d) Ulster, County Down, and Antrim, Londonderry. In (a) and (d) only the upper and lower limits to the observed estimates of the sea-level change are illustrated. In (b) and (d) these limits are superimposed on the individual observations. Ulster.Along the eastcoast, County Down, Lateglacial eastern Ulster (Co. Down) places age limits of about 17 000 shorelines have been reported at heights of 10 to 16 m OD years BP forthe oldestand highest elevatedshoreline at but, based on the observation from Roddans Port (a few about 19 m just outside the Main Drumlin Moraine and kilometres south of Donaghadee) of terrestrial deposits over about 16 000-15 000 years BP for locations to the north. marine sediments at 12 110 f 150 years BP,levels must have Evidence in support of higher Lateglacial sea levels been near or below their present values before about 12 000 along the Irish coast have been reported and much debated. years BP. Other marine-terrestrial alternations indicate that For example, the Me11 Formation north of Dublin and just sea level continued to fall before again rising by about 9000 outside the Main Drumlin Moraine, occurs at 30-40 m OD years BP, although the minimum height reachedat about and is reported to contain fauna that are indicative of water 10 000 years BP is unknown. Along the coast of Antrim and depths of 90-110 m. Thus sea levels at the time of sediment Londonderry of northern Ulster, Lateglacial ridges and deposition have sometimesbeen inferred to have been platforms have been identified at elevations up to 20-22 m 120-150 m above the present level (e.g. Eyles & McCabe OD, with levels falling tothe present value before about 1989; Warren 1991). The age estimates of these deposits 12000 years BP (Fig. 13d). Thedepth of the subsequent range, however, from the penultimate cold stage to the Late minimum lowstand is again undefined. Within the Midlandian (Hoare 1991). Also questions have been raised observational uncertainties, the sea-level curves for the two as to whether the sediments are in situ or have been localities are, with the possible exception of theLate transported to theirpresent position (e.g. Warren 1991; Holocene part of the record, comparable. Hoare 1991). Eyles & McCabe (1989) propose that these The raised beaches and shorelines noted along the Ulster high levels along theeastern coast of Ireland are of coast lie within the limits of the ice sheet at its maximum Lateglacial age but the evidence appears sketchy and extent and they are assumed to postdate the ice retreat over inadequate for constraining the models of glacial rebound. the region. Then, in orderto be consistent with the ice model discussed above, the oldest shorelines along the north Sea-level change along the eastern margin of the Irish coast of Ireland cannot exceed about 17 000 years BP for the highest features west of Inishowen (Devoy 1983) and about Sea 16000 years BP for the lower shorelineseast of Portrush. A primary source of information on Holocene sea levels Thisestablishes the maximum ages forthese features along theeastern margin of the Irish Sea comes from adopted inFig. 13d. Likewise, the model ice retreat over northwest England where Tooley (1978) has described the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 868 K. LAMBECK transgression from about 9000 years BP to the present for a earth-model (Table 2) determined from the British data set region extendingfrom aboutthe Duddon Estuary in the (Lambeck 19938; Lambeck er al. 1996). A major northto Liverpool Bay in the South.As previously discrepancy between observations and predictions occurs for demonstrated,the spatial variability of the predicted the Donegal data (Fig. 14a) andthe comparison suggests sea-level variation for this region can be important because that a greater volume of ice may be required over Ireland of the glacial rebound (Fig. 8c). Thus the sea-level than assumed in the model. But inview of the uncertainty information from this region has been grouped to form four of the data from this locality it is probably inappropriate to regional curves: theDuddon Estuary,Morecambe, the attach much significance to this discrepancy. The agreement Ribble Estuary, and Formby. The Duddon Estuary data is betweenobservations for the two southern sites in Kerry the least satisfactory but it does point to sea levels in Late and Cork is more satisfactory. In particular, themodel Holocene time having beenabove the present level. The prediction is consistent with the observation by Stillman Morecambe data forms a sea-level curve that lies marginally (1968) of the position of sea level at about 11 300 years BP above the Ribble Estuary and Formby data and below the (Fig. 14b). Agreement for the two northeastern localities is Duddon Estuary result, consistent with the predicted trends also reasonable although the discrepancies, similar for both (Fig. 8c). These regional curves, reduced to MSL have been sites (Fig. 14d, e), may be significant. In both instances the discussed further in Lambeck (19938). predicted mid-Holocene highstand lies above the observed Several lines of evidence suggest that earlier sea levels values, suggesting that the ice load may be excessive in this along the northern Cumberland coast have been higher than area. In contrast, the predicted lowstand atabout 12000 the present level. These include sea caves, benches,and years BP precedes the observed (inferred) timing of this water-erodedor wave-cut notches at Whitbarrow (c30 m event and this points to a need to increase the ice load over OD) and Humphrey Head (c35 m OD) and other sea-level the region in early Lateglacial time. But this is inconsistent indicatorsbetween 15 and 30 melevation (Tooley 1985, with the observationsbefore about 16000 years BP where 1987). Eyles & McCabe (1989) have suggested even higher the discrepancy implies an excessive amount of ice. Lateglacial sea levels, reaching 150 m. Quantitative data for Alternatively, the formation of the highstand along the the ages of formation of these features is absent, except that Ulster coast occurred 1000-2000 years earlierthan the if they formed in Lateglacial time and are not remnants of limiting ages inferred above. Again, in the absence of more earlier cycles of glaciation, they postdatethe ice retreat accurate observational data, it is probably unwise to draw across the region andare therefore youngerthan about more than indicative conclusions from these comparisons. 16000 years BP so as to be consistent with the adopted ice Certainly the observational data is inadequate, both in its model (Fig. 10). Like their Irish counterpartsand similar chronological control and geographic distribution,for features identified on the Isle of Man,these undated inferringimproved ice model constraints. Figure 14 also elevated featurescannot be used in any quantitative illustrates the comparisons between the observations and rebound analysis. predictions for some of the northwest England and Wales Observations of sea-level change along the coast of localities. Agreement between the two here is much Wales have been reviewed by Heyworth & Kidson (1982). improved, reflecting in large part the better quality of the The more important evidence for present purposes is from observational data, andthat these data form part, albeit Cardigan Bay and consists primarily of thedepth of small, of the overall data set used in Lambeck (19938) to freshwater tomarine transitions(or the reverse)deduced infer the optimum earth-model parameters. Certainly these from peats and in situ tree stumps and roots of submerged observations do not point to a need for major modifications forests. Thelatter data are based onthe assumption that to the ice sheet over Great Britain. thetrees havebeen killed by the rising groundwater One possible explanation for some of the discrepancies associated with a rise of sea level and, despite reservations could be thatthe choice of earth-modelparameters is expressed about theseobservations (Tooley 1979), these inappropriate.Consider the observationequation (Lam- observations are assumed to be indicative of an upper limit beck 19938) tothe sea-levelcurve. Result forthe DoveyEstuary and Borth region of central Wales are discussed further in A<;(% t)+ .,(V> t>= AlXcp, t) Lambeck (19938). Other localities along the coast are = Al;(t) + PAl,B(cp, t)+ Al:-Tcp, t) (6) represented only by a few isolated points. Together, these data suggest that sea levels along this section of the coast where A[;’ is the observed sea-level data point j (j= l...J) were about 25-30 m below their present level at about 9000 at location cp and time r, E, is the correction to this years BP. Evidence for sea levels before this time is limited. observation, Alf((p, r) is the model prediction corresponding Eyles & McCabe (1989) have suggested that Lateglacial sea to the jthobservation, A[; is the eustatic sea level as defined levels may have been up to 140 m above their present level before, A[: is the glacio-hydro-isostatic correction (Alp + near Banc-y-Warren, at 50 m near Cardigan, and near 80 m AlY + ACE) forthe British ice sheet and Alf-’ is the nearHendre and Bryncir onthe Llyn Peninsula but this glacio-hydro-isostatic contribution from the more distant ice interpretation is questionable because thesedeposits are sheets of Fennoscandia,Laurentia, and Antarctica. P is a glaci-lacustrine deltas andcannot be used as evidencefor single scaling parameter for the height of the ice over the high relative sea-level. (Austin & McCarroll 1992; Harris British Isles. The equation assumes thereforethat the ice 1991; McCarroll & Harris 1992). limits and ice retreatare known and thatthe only inadequate knowledge of the ice model is its height and that any such inadequacy can be remedied by a single factor, Comparison of observations with predictions uniform in spaceand through time. The unknowns in this The comparisons of the Irish observations with predictions observationequation arethe scale parameterand the are illustrated inFig. 14 forthe optimum three-layered earth-modelparameters that define the hiBand the Al”. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVELCHANGE FOR THE IRISH SEA 869 I I l0 I------ Donegal Bantry Bay Cork I/ northwestern Ireland Kerry southern Ireland l086420 121086420 l08642 a b C 30 i southeastern Ulster coast Down Antrim and Londonderry 20 Morecambe Bay northwest England ~~ 20 15 10 5 0 8642 d €l0 0 -10 -20 Ribble Estuary Formby -30 Dovey Estuary I ‘I northwest England I northwest England Cardigan Bay l08642 8642 1 8642 g time (x1OOO yean, BP) time (x1OOO ycan BP) i tim (x1ooO ycan BP) Fig. 14. Comparison between observations (with error bars or, in (d) and (e) by dashed lines as upper and lower limits) with predictions (continuous line) for Ireland (a-e), northwest England (f-h) and Wales (i). Thelatter parameters include the lithospheric effective sheet are relatively small, the predicted crustal rebound and elastic thickness H, and a set of values v, describingthe sea-level change is most sensitive to the uppermantle viscosity layering of the mantle beneaththe lithosphere, viscosity v,,,,,and the lithospheric thickness H, and solutions although only three layer models, as previously defined, are of equation 6 are sought only for p, H, and vi,,,, with the considered here. A measure of goodness of fit of a solution lower mantle (below 670 km depth) viscosity set to with a particular set of earth-model parameters is given by = 10’’ Pa S (Lambeck et al. 1996). The solution is found by searching through the parameter space of p , H, and 1’ for the minimum estimate of the variance function defined r2= -C [UJ’(cp,t) - U?(cp, t)1’/4 (7) J,=, by equation 7. This function is illustrated inFig. 15 as contours of constant variance X’ and is based on all the data where U, is the a priori standard deviation of each fromIreland, except for the unreliableDonegal data, and observation. the observational data from the eastern side of the Irish Sea. Because the spatialdimensions of the British Isles ice Thissolution points to v,,,,,--- (3-4)10*” Pa S, H, = 80- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 870 K. LAMBECK discrepancies with observations thatdo occur can be attributed in partto inadequacies in this data and to limitations of the details of the ice model. Nowhere does the model predict the large highstands on both sides of the Irish Sea that have been suggested by Eyles & McCabe (1989). At Dundalk Bay, forexample, the maximum predicted sea levels even for a time when the region was still ice covered, lie close to or below present sea level (Fig. 8) because the predictedcrustal rebound is insufficient to overcome the eustatic change. To produce Lateglacial levels I at 120-150 m as suggested by Eyles & McCabe (1989), after the region became ice free, would requirea two- to three-foldincrease in the crustal rebound. This would require a corresponding increase in ice over the region but, because the crustal response is both more regional than the ice cover and effective over a considerable period of time after the ice is removed, this is inconsistent with the more reliable sea-level evidence for the British Isles as a whole. Likewise, the model predictions cannotsupport the inference of high Lateglacial sea levels in Cardigan Bay such as the140m suggested by Eyles & McCabe (1989) at Banc-y-Warren. The predicted crustal rebound here is only a few tens of metres because of the relatively large distance from the main centre of loading to the north and because the relatively small horizontal dimensions of the ice over 8 Wales ensures that this local load's contribution to rebound isvery small. To producethe requisite crustal rebound of more than 200 m would require a ten-fold increase in the ice load. Modifications of the earth-modelparameters cannot rlum(x1020W S) produce such highstands either. First, the more reliable data suggests thatthe parameters describing therebound of Fig. 15. Contours of equal variance Zz (defined by eqn. 7) as Great Britain are also appropriate for the Irish Sea region. function of lithospheric thickness and upper mantle viscosity (thick Second, no choice of parameters, other than a lithosphere lines) and the corresponding p estimates (thinner lines with values and mantle of zero strength, could producerebound that in italics), based on the Irish data from all sites (except Donegal) approaches the requisite amplitudes. and the Irish Sea sites in northwestern England and Wales. Conclusion Sea-level change around the Irish coast since the time of the 120 km, p = 1.3. A similar solution, based on the four Irish last (Midlandian) glacial maximum is not well constrained sites only, yields v,,,,,i= 3 X 10*", H, = 60-80 km and a similar observationally but the observed pattern is consistent with value for p . These solutions compare with q,,,,,=(4- glacio-hydro isostatic models derived from . thebetter 5)102"Pa S, H, i= 60-70 km and p = 1.0 for the more reliable constrained model for Great Britain. The sea-level pattern and much larger data set for Great Britain which included is primarily the result of the rebound in response to the the eastern Irish Sea data. In the light of some of the deglaciation of the former ice sheet over the British Isles observationaluncertainties of the Irish data these and the eustatic change from the distant ice sheets, although differences are probablynot significant and conclusions the isostatic contributions from the latter, primarily the drawn about lateral variation in mantle rheology would be Laurentian and Fennoscandian ice loads, are not insig- premature. Solutions based on all of the British Isles data nificant. The model predictions are in broad agreement with yield thesame result as theGreat Britain only solutions, the observations for Holoceneand Lateglacial time but reflecting the dominant role played by the better constrained some discrepancies occur which may point to inadequacies observations for Britain. Thus the best interim solution for in the details of the ice model. The currently available Ireland is probably one in which the earth-model parameters observational data, however, are inadequate for estimating are set equal to those derived from the overall solution and improved ice models. Better chronological constraint on the theparameter is estimated from the Irish data only. This pre-Holocene and Holocene sea-level indicators is desirable, leads to p = 0.93-0.96 depending on whether the Donegal as is new data from other localities, particularly from the data is excluded or included, indicating that the assumed ice west coast. Offshore observations of the timing of the thickness estimates over Ireland are consistent with the sea-level transgression in Lateglacial time would be rebound data and models. particularly valuable for constraining the rebound model. In general, the glacio-hydro-isostatic reboundmodel Despite the limitations of the observational sea-level provides a good description of the observed sea-level around constraints on the glacio-hydro-isostatic model, some Ireland and the Irish Sea for Holocene and Lateglacial time. conclusions appear robust. One is that the rebound models There is no strong argumentfor lateral variations in the place a global constraint on the ice volume over Ireland and mantle parameters on the basis of this analysis andthe the models indicate that the maximum ice thickness at the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021 SEA-LEVEL CHANGE FOR THE IRISH SEA 87 1 time of the last glacial maximum probably did not exceed suggested (e.g. Savage 1966; Devoy 1985) because the about 600-800 m, depending on the choice of earth-model predicted magnitude of the crustal deflection under the ice parameters: earth models consistent with the evidence for load in the region exceeds the eustatic lowering of sea level Great Britain, point to the lower limit while the less-reliable throughout Lateglacial time. The absence of such a models based onthe Irish data only, point tothe upper northern landbridge is consistent with seismic-stratigraphic limit. Some of the differences between the observed and evidence (Wingfield 1995) as well as withfossil evidence predicted sea-levels do point to a need for an improved ice (Preece et al. 1986). The height of the Celtic Sea sill formed model over Ireland in which the ice limits, the ice retreat, by the emerged sea floor is only a few metres, reaching 5 m and the ice thickness, is better constrained. Questions about at 18 000 years BP and only 1 m at 14 000 years BP. Thus the the southern limit of the ice sheet in the Celtic Sea, whether actual land connection may be quite tenuous because, until it extended as farsouth as the Scilly Isles, forexample, such a time as the ice barrier between northern Ireland and cannotbe resolved with the present sea-level data. Better Scotland is broken, it will damthe meltwater up tothe information on the earlier loading cycle of the Irish ice sheet height of the sill, or lower if it is locally broken by overflow is also desirable. channels. Thus the periglacial lake level limits could have The models do not supportthe existence of the high been up to 5 m greater for sites north of the sill than the Lateglacial sea levels along both sides of the Irish Sea that limits indicated in, fcr example, Fig.10c. The tidal have been suggested by Eyles & McCabe (1989). Models amplitude in the Celtic Sea may also have been a few meters thatare consistent with the Holocene data,or with the so that the exposedsea floor is likely to have provided a observationalevidence over Britain as a whole, predict hostile environmentfor any flora or faunaattempting a Lateglacial highstands only in northeastern Ireland and with crossing. This is generally consistent with the fossil record amplitudes of only a few tens of metres, generally consistent of rather limited fauna1 contact between Ireland and Britain with the observedshorelines in this region (Carter et al. in Late Pleistocene and Holocene time (e.g. Savage 1966; 1989), but no reasonablecombination of earth-and Preece et al. 1986). Both this andthe earlier model ice-model parameters predicts the highstands up to150m (Lambeck 1995) place thelandbridge earlier in time than above present sea level that have been inferred for Cardigan the sometimes supposed date of about 11 500-10500 years Bay or Dundalk Bay. An interpretation other than in terms (e.g. Preece et al. 1986). of Late Midlandian glacio-isostatic rebound is required to While many questions remain unresolved, the model explain these deposits. predictions do point to areas where improved observational Once the relative sea levels can be modelled with some constraints are most desirable if a full understanding of the confidence the palaeowater depthsand shorelines can be ice-sea-earth interactions is soughtand if the full predicted using the relation (3). Such results are illustrated implications of these interactions on palaeogeographic and in Fig. 10 based on the earth model that best describes the palaeo-environmental reconstructions are to be understood. totalobservational data base for the British Isles. These The needfor improved sea-level and ice distribution predictions differ from the earlier results of Lambeck (1995) informationhasalready been identified. Oneother in that the land connection in the latter was predicted to be requirement is a model forpalaeo ocean tide corrections. more extensive. One reason for this was that in the original The palaeo sea-level indicators do not usually correspond to paper the 5' X 5' ETOPO-5 data base (NOAA 1985) was meansea level and tidal corrections are required. In the used which tends to smooth the bathymetric data, whereas present model the assumption made is that tidal ranges have in the present paper the more detailed 2.5' X 2.5' data base not changed with time, an assumption that is unlikely to be prepared by GETECH (1995) hasbeen adopted.Another valid forearly Holocene and earlier times because of the reasonfor the difference is thatthe predictions of the substantialchanges in coastal geometryand water depths positions of palaeoshorelines in shallow and gentle sloping that have occurred (see Fig. 10). Improved sedimentological sea floor environments can bequite sensitive to certain evidence for changes in sea floor topography since the time model parameters, in this case the ice height. The greater of the last glacial maximum would also make a valuable ice volume over Ireland in the present model compared with contribution to improved reconstructions of palaeoshorel- the earlier model reported in Lambeck (19936, 1995) leads ines and water depths. to a somewhat greater deflection of the sea floor and hence to a reduced exposure of the sea floor in Lateglacial time. Observations of ages and locations of shorelines within the Author's e-mail address: [email protected]. present Irish Sea, or depth-ageinformation on sediments identifying the marine transgression, would be of great value References in further constraining the models. Because the maximum ice limits in this model persist for ANDERSEN. G.B. 1981. Late Weichselian ice sheets in Eurasia and Greenland. In: DENTON,G. 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Quaternary deposits and glacial stratigraphy in Ireland. London, Special Publications. %,209-242. Received 13 October 1995; revised typescript accepted 5 July 1996. Scientific editing by Graham Shimmidel. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/6/853/4889371/gsjgs.153.6.0853.pdf by guest on 02 October 2021