Coastal peats from northwest Ireland: implications for late-Holocene relative sea-level change and shoreline evolution

Coastal peats from northwest Ireland: implications for late-Holocene relative sea-level change and shoreline evolution JOHN SHAW AND RICHARD W. G. CARTER?

Shaw, J. & Carter, R. W. G. 1994 (March): Coastal p a t s from northwest Ireland: implications for late- Holocene relative sea-level change and shoreline evolution. Boreas, Vol. 23, pp. 74-91. Oslo. ISSN 0300-9483.

Peat and organic rich sediments at coastal sites in extreme northwest Ireland have accumulated in a wide variety of environments, often strongly influenced by late Holocene changes in relative sea level and by geomorphic processes. A deep peat sequence on the coast of Aranmore Island accumulated initially in a lake and subsequently in a freshwater marsh environment. The long pollen record serves as a template for regional events. It extends over much of the Holocene and shows relatively high levels of Pinus pollen up to just before the disappearance of this taxon at c. 3600BP. Coastal peat occurrences elsewhere are much thinner and have accumulated over shorter periods; they contain further evidence to show that coastal areas were well-wooded compared with today, and that Pinus was an important woodland component prior to c. 4000 BP. At sites in Gweebarra Bay intertidal peats record the closure of small estuaries by geomorphological events during the past 5000 years. Coastal sites at Ballyness, Clonmass, and Trawenagh display regressive stratigra- phies ~ minerogenic marine sediments are overlain by silty peats capped by highly organic freshwater peats. Basal radiocarbon dates range from 4500 to 3300BP. The silty peats are interpreted as having formed in salt-marsh environments and contain distinctive pollen spectra, marked by high levels of Pinus and Compositae Liguliflorae pollen. The data suggest that relative sea level attained levels close to that of today by the mid-Holocene in this region. The pattern of relative sea-level change agrees well with that predicted by geophysical modelling. John Shaw, Geological Survey of Canada, Atlantic Geoscience Centre, Bedford Institute of Oceanography, Box 1006, Dartmouth, Nova Scotiu, B2Y 4A2, Canada; Richard W. G. Curter, formerly of the Department of Environmental Studies, University of Ulster, Coleraine BT52 ISA, Co. Londonderry, Northern Ireland; 2nd February, 1993 (revised 8th November, 1993).

B o r n

Northwest Ireland (Fig, I) is an area of particular interest to students of sea-level variations as it lies within an area especially sensitive to both short- (10' to lo2 years) and long-term ( lo3 to lo4 years) environmental changes. In the short-term, the exposed coast is influenced by both oceanic and mesoscale atmo- spheric fluctuations, while over the longer period the area straddles the divide between the European isostatic and eustatic provinces (Carter 1990a). The latter are reflected in the course of land-sea changes during the Holocene (Pirazzoli 199 1) which have generated raised beaches in northern Britain and Scandinavia and submerged coasts in the south of the British Isles, France and northern Spain. On a more regional scale, northwest Ireland was marginal to a major late Pleis- tocene ice mass. Boulton et al. (1977) present a model of the British Isles ice sheet maximum indicating coalescing ice-domes, one of which was centred in north west Scotland and the other in east central Ireland. Ice from these domes spread out, merging along an east to west line approximating to southern and Northern Ireland. Their reconstruction suggests very rapid thinning of ice across most of County Donegal and an ice-free in the extreme northwest around Malin Head. A later model (Boulton et al.

t Deceased

1985) indicates an isolated late-stage ice mass in northwest Ireland. The history of ice dome development and decay is reflected in the patterns of deglacia- tion and subsequent sea-level history.

Previous studies of sea-level in Ireland (Carter 1982; Devoy 1983; Carter et al. 1989) have emphasised the continuing imbalance that has been maintained over the last 15,000 years as both local and regional isostatic recovery has interacted with global changes in ocean level to impart distinctive rising and falling signatures to the late-Plestocene and Holocene sea level curves. Carter ( 1982), synthesising relative sea- level data from northeast Ireland, identified an early to mid-Holocene transgression that rose to several metres above present mean sea level, before falling back to a lower level.

The aim of this paper is to consider the evidence for late-Holocene sea-level change in northwest Ireland. As Pirazzoli (1977) has identified at a global scale and Carter et al. ( 1989) have recognised in Ireland, there is a relative paucity of sea-level information relating to the last few thousand years. While attempts to link Holocene with recent (historical) sea-level change have been made for the North Sea (Shennan & Wood- worth 1991), and further afield from eastern North America, relatively little is known about the Atlantic seaboard of Europe. This paper suggests that exami- nation of organogenic sequences from climatically-

BOREAS 23 (1994) Holocene sea -level change, Ireland 75

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Fig. I. Map of the study area showing locations mentioned in the text.

and oceanographically-sensitive zones, such as northwest Ireland, can help interpret coastal evolution and assist in predicting future sea level impacts.

Godwin (1943) reiterated the views of earlier work- ers when he stated that the occurrence of peat and other organic remains in the intertidal zone is not, of necessity, proof of a change in sea-level. However, he also observed that '. . . in a great majority of instances the submerged peats truly reflect marine transgression' (p. 199). In County Donegal a number of occurrences of intertidal peat and other organic remains have been previously reported. These chiefly comprise a Quercus log at + 1.3 m overlain by gravel on a beach at Drumskellan (Fig. 1) and dated at 6995 k 100 BP (UB-206) (Colhoun et al. 1973), freshwater peat at Termon (Fig. l), dated at 3840 f 120 BP (Gif-5484) by Gaulin et al. (1983), and peat at Rossbeg Harbour (Fig. 1) dated at 6995 f 105 (UB-2095) (Pearson 1979).

These isolated studies involving single I4C datings merely serve to highlight the potential of the area for sea-level studies. Furthermore, they represent only a small proportion of the occurrences of organic remains in the intertidal zone in the region. In this paper we describe a number of newly investigated sites, discuss the environments in which the organic materi-

als accumulated, and present conclusions regarding the course of relative sea level and coastal evolution during the past five thousand years.

Background

Physical setting

County Donegal forms a distinct region of high ter- rain with an irregular, deeply-embayed coastline (Stephens 1970). The highlands are dissected by numerous glacially deepened valleys and surrounded by a narrow fringe of coastal lowlands. The coastline is bedrock-dominated, with several major sea loughs including fjards (Lough Foyle, Sheephaven and Mul- roy Bay) and fiords (Lough Swilly), numerous lesser inlets, and a scattering of islands. The non-bedrock coastline (King 1966) includes both small coastal dune systems backing dissipative beaches in coastal com- partments, and large sand-dominated coastal systems in the larger embayments. The latter comprise shallow, sand-choked estuaries containing well-developed flood and ebb structures, sheltered at their seaward margins by dissipative sandy beaches backed by high coastal dunes (Shaw 1985; Wilson 1990; Carter 1990;

76 John Shaw and Richard W. G. Carter BOREAS 23 (1994)

Quigley 1991; Carter &Wilson 1992). Salt marshes are common in the estuaries, but are generally small in extent, partly, in several instances, as a result of land reclamation in the last century.

Outside sheltered embayments the wave climate is dominated by high-energy Atlantic swell and storm waves. Modal values of significant wave period and deepwater wave height are 10.5 s and 2 m respectively (Shaw 1985); the 50 year extremes of period and height are 16 s and 30 m. Most waves approach from the west, and the most common approach direction recorded off Malin Head is 280" (Mollison 1982). The mean spring range of the semi-diurnal tides increases from 2.3 m in Lough Foyle to 3.5 m at the entrance to Lough Swilly, and is between 3.0 m and 3.5 m elsewhere. The climate is wet and windy, with prevailing winds from the southwest. The coastal areas and uplands are largely devoid of woodland, except for scattered softwood forestry plantations dating from the late nineteenth century.

Late Quaternary changes in relative sea level Late Quaternary relative sea-level changes in this region have been summarised by Devoy (1983) and Carter et al. (1989). The morphological evidence indicates that in northern Donegal a series of raised late-glacial ( 17,000-20,000 years BP) strandlines reach heights of +22 m (Stephens & Synge 1965). To the west these beaches become lower, so that south and west of Bloody Foreland (Fig. 1) no late-glacial raised beaches have been identified. To the north and east the Irish raised beaches have been correlated with Scottish equivalents (Synge & Stephens 1966) together indicating a major zone of isostatic recovery centred on the western Highlands (e.g. Dawson 1984). From a local late-glacial high stand, sea level fell, so that by 11,500- 12,000 BP the shoreline must have been some distance seaward of the present coast. Carter (1982) suggested a sea-level minimum of - 30 m was likely, a depth which would have allowed establishment of a landbridge between northeast Ireland and western Scotland (Devoy 1985). More recently Peacock et al. (1992) and Peacock (1992) have indicated water depths on the shelf off western Scotland may have been lowered by 90 to 100 m in the late-Pleistocene and early Holocene. Whatever the minimum sea-level position, it is apparent that the sea then rose rapidly flooding the northwest Irish estuaries between 7500 and 9000 BP (Carter 1982, 1990a; Carter er al. 1989) and reaching a peak sometime between 6500 and 4500 years ago. (The timing and height of this peak will be discussed later, as will the regional implications of the evidence from northwest Ireland.)

The present coastal scenery of northwest Ireland has been inherited in part from the late-glacial and in part from the mid-Holocene. It is likely that many of the spectacular coastal cliffs, rising to over 350m at

Slieve League in west County Donegal, were ice- eroded forms subsequently cosmetically-trimmed by marine processes. Rate of change on these late-glacial cliffs is exceedingly slow (Davies & Stephens 1978) although some show evidence of sporadic landslips and rockfalls. Of equal importance was the glacial debris deposited near the coast or on the shelf. This has been scoured and transported, accumulating as coastal beaches, dunes and marshes. Evidence from I4C dates on coastal dune deposits suggests strongly (Carter & Wilson 1992) that formation of the present Irish dune systems did not occur until after the peak mid-Holocene sea-level, when, presumably, a regressive sea-level provided conditions conductive to onshore transport from the shelf, This middle Holocene phase may be associated, therefore, with the construc- tion of the beach/dune complexes, which in places mask the earlier, more erosional landforms.

Holocene environmental change The pattern of vegetation changes during the Holocene in this region parallel those elsewhere in Ireland (Jessen 1949; Mitchell 1956; Smith & Pilcher 1973). The spread of Corylus was followed by that of Quercus, Pinus and Ulmus. The expansion of Alnus (Smith & Pilcher 1975; Smith 1984), is dated at between 5.1 and 7.7 ka BP and was later in uplands than in lowlands and coastal areas. The Ulmus decline, an event of some controversy (Goransson 1980; Hirons & Edwards 1986; Perry & Moore 1987) is a near-synchronous horizon in Irish pollen diagrams, with dates ranging between 5.1 and 5.3 ka BP (Smith & Pilcher 1973).

Pinus stumps are common in Donegal, at the base of blanket bog, and testify to well-wooded conditions which prevailed there until historic times. At Glen- veagh, 12 km inland from the north coast, pollen of this taxon drops to less than 5% of all pollen by 2550 k 100 BP (Telford 1978) and Pinus stumps below blanket bog date to 3850 ? 100 BP and 3880 f 100 BP. These data accord well with what is known of the behaviour of Pinus in Ireland. While during the early Holocene Pinus pollen accounted for a high percentage of arboreal pollen, particularly in northern and western Ireland (Bennet 1984), levels subsequently declined and the species disappeared in the late-Holocene. The final decline has been placed 300- 400 years either side of 4000 BP (Smith & Pilcher 1973), and has been attributed to a variety of causes (Bennet 1984; Bradshaw & Browne 1987).

Much vegetation change was the consequence of human occupation of the north of Ireland, which dates from the Mesolithic (around 8000 years ago) but assumes greater importance from the Neolithic on- wards, when much of the natural vegetation was destroyed (Mallory & McNeill 1991). Many coastal sites were occupied, and it is probable that Neolithic disturbance, in particular, led to significant instability

BOREAS 23 (1994) Holocene sea -level change, Ireland I1

in the coastal dune systems. Certainly in County Donegal evidence of human activity is extensive (Lacy 1983).

Methodology Sites were examined between 1981 and 1988 - their locations are shown on Fig. 1. Peat was sampled using Hiller and Russian corers and minerogenic sediments using a vibracorer system patterned on that described by Finklestein & Prins (1981). Sixteen samples were radiocarbon dated (Table l), a majority of which have been adjusted by normalisation to -25%, "C. Unad- justed dates are noted in the text and in figures.

Pollen was prepared by standard techniques (Faegri & Iversen 1975) including treatment by sodium hy- droxide and hydrofluoric acid, and acetolysis. Pollen from Ballyness was prepared using the plastic micro- sphere method (Ogden 1986) to provide absolute concentrations. In the relative pollen diagrams pollen sum is either arboreal pollen, arboreal plus Coryloid pollen, or, because of low arboreal amounts, all pollen and spores. Identification was according to Moore & Webb ( 1978), with subdivision of the Plantaginaceae according to Faegri & Iversen (1975). Pollen grains of Corjlus and Myrica were not differentiated from one another, and are described as Coryloid. However, it is assumed that in most cases, most of the Coryloid pollen is Corylus.

Eleveations are relative to Belfast Datum, which is 2.7 m above Ordnance Datum Dublin. Mean sea level (or mean tide level: M.T.L.) is assumed to be Om. EIevations at Clonmass and Trawenagh were tied to Ordnance datum by surveying. At a majority of sites bench marks could not be located, and elevations were determined by surveying to sea level and using tide tables, assuming that M.T.L. was at Om O.D. The tidal ranges are those given in the Admiralty Tide Tables, Vol. 1 (1984).

Results Given the length of the County Donegal coastline (over 1000 km) and the constraints of time, only a selection of sites was chosen for detailed study. Initial reconnaissance revealed numerous potential sites, many of which remain univestigated. The sites discussed in this paper are therefore not an exhaustive set, but we would argue that they are representative of coastal conditions in northwest Ireland.

Aranmore Island On the pocket beach on the south coast of Aranmore Island, 4.00 m of freshwater peat overlies 1.35 m of gytja, the base of which is at -2.35 m, and 0.20 m of grey clay. The clay contains high percentages of Rumex and Betula pollen, and the local pollen assemblage zone is equivalent to Zone IV, Pre-Boreal (Jessen 1949). As shown on Figure 2, the five local

Table 1. Radiocarbon dates referred to in the text. All were collected by the first author except where indicated. Dates where the I3C/'*C ratio is omitted are unadjusted. The dates were performed at the Scottish Universities Research and Reactor Centre (SRR), The Radiocarbon Dating Research Unit of the Queen's University of Belfast (UB), and at Beta Analytic Ltd., Coral Gables, Fla. (Beta).

'3C/'ZC Height Location I4C yr BP Lab. No. %, (m O.D.) Material dated

Clonniass I670 k 50 SRR-2395 - 20.6 -0.2 Wood (Taxus) Clonniass 3990 k 70 SRR-2396 -25.0 + 1.5 Wood (Alnus) Clonmass 4540 f 100 SRR-2397 -28.3 +2.33 to +2.63 Wood (Alnus) Clonmass 3600 & 50 SRR-2398 - 30.1 +3.31 to +3.41 Organic-rich sand Clonmass 800 & 40 SRR-2399 -29.0 +3.93 to +3.96 Organic-rich sand Trawenagh 2890 f 60 SRR-2604 -27.9 +1.29 to +1.30 Peat Trawenagh 3990 f 90 SRR-2605 -25.0 +0.9 to +0.92 Silty peat Trawenagh 4200 80 SRR-2606 -25.9 -0.63 to -0.58 Organic-rich silt Roshin Pt. 4950 k 80 UB-2676 - 27.7 f0.9 to +0.92 Peat Helgoland 2095 f 100 UB-2677 -22.9 +1.45 to +1.55 Peat Aranmore 3250 k 150 UB-2678 -27.8 +0.95 to +1.20 Peat Clonmass 3300 k 95 UB-2679 -27.7 t1 .20 to t1 .28 Silty peat Naran 815 f 95 UB-2680 -28.4 t1 .70 to + I 3 3 Peat Ballyness 4410 * 100 Beta-34314 -27.7 +0.17 to +0.19 Silty Peat

Sheskinmore I040 k 70 Beta-22238 - +1.40 to +1.45 Peat Portstewart 8960 f 110 Beta-34315 -29.8 - 6.0 Peat

Ballyness 3720 k 90 Beta-42528 ~ +0.47 to +0.49 Peat

Rossbeg Hr. 6995 f 105' UB-2095 ~ +0.7 Peat Termon 3840 k 1202 Gif-5484 ~ -0.75 Peat Drumskellan 6995 f 100' UB-206 ~ + 1.3 Wood New Bridge 5810 f 140" Gif-5483 - -0.2 Peat Maghera 3415 k 135' UB-2 I77 - + 2.6 Charcoal

' Pearson 1979 (collected by F. G. Mitchell); Gaulin el al. 1983; Colhoun et a/. 1973; 4Gaulin, pers. comm. 1982; Carter 1982 (collected by P. J. Newbould).


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BOREAS 23 (1994) Holocene sea-level change, Ireland 19

pollen assemblage zones above -2.35m also equate with those of Jessen. In zone 1, -2.35 to -2.0 m, the proportion of organic sediment increases upwards. A lake environment is indicated, with Nymphaea replac- ing Myriophyllum as the dominant aquatic taxon. The expansion of Crylus and decline of Betula suggest that this phase is the equivalent of Zone V, Boreal.

Zone 2 extends from -2.0 m to - 1.0 m. Below - 1.6 m the organic proportion of the sediment increases upwards; above this level percent loss on igni- tion is constant at about 80%. High arboreal pollen amounts suggests a closed woodland around the pond. Corylus is dominant but Pinus, Quercus and Ulmus increase through time. This zone is equivalent to Zone VI, Boreal.

Zone 3, from - 1 .O m to -0.15 m, is equivalent to Zone VIIa, Atlantic, defined by Jessen (1949) by the rise of Anus above its empirical limit. Based on evidence from Rossbeg harbour (see below), this event occurred c. 7000BP at Aranmore. The advent of high Alnus levels is coincident with the end of the lake phase, marked by the transition from detritus to herbaceous peat, and the disappearance of Nymp haea .

Zone 4, -0.15 m to + 1.40 m, is equivalent to Zone VII b (Sub-Boreal). The lower zone boundary is marked by a decline of Ulmus, the upper boundary by the disappearance of Pinus, just after 320 k 150 BP. Zone 5 extends from + 1.40 m to the uppermost level counted in the section, at +2.6 m. It begins with the Pinus disappearance and is assumed to be equivalent to part of Zone VIII, Sub-Atlantic. High amounts of Ericaceae pollen point to the development of ombrogenous bog.

This site contains an exceptionally long and contin- uous pollen record which shows that coastal areas of Donegal experienced a sequence of vegetational changes similar to those that have been documented for other parts of Ireland. It exhibits the high Pinus levels recorded in pollen diagrams from coastal re- gions of western Ireland (Huntley & Birks 1984; Ben- net 1984). Moreover, It shows the final disappearance of Pinus just after 3250 BP, following a sharp decline which, based on the level in the section of the Ulmus decline of c. 5200BP, probably commenced c. 4000 BP.

All of the peat and gyttja accumulated in freshwater settings. However, a marine influence is indicated by a zone of disseminated, fine sand, believed to be aeolian, at the level of the Ulmus decline in the stratigraphy (at the present M.T.L.). With a sea level only slightly lower than today a low isthmus would have extended from the study site to the mainland. A dune system on the seaward side of the isthmus probably supplied blown sand to the site. When the isthmus was submerged by rising sea level after 5 ka, the dunes migrated to their present position on the island, northeast of the study site, and the supply diminished.

Submerged forest sites

The Aranmore site just described is exceptional; most intertidal peats in Donegal accumulated over briefer intervals of time. Evidence from the majority of such instances reinforces the view that coastal areas were more heavily wooded in the past. For example, intertidal peat at Termon dated at 3840 * 120 BP (Gaulin et al. 1983) contains high levels of Pinus pollen and numerous horizontal, compacted Quercus trunks, suggesting a wooded environment on what is presently a bare, exposed headland.

Many intertidal peats contain macrofossil remains of Pinus. Four hundred metres north of the Aranmore site, Pinus stools are embedded within early Holocene freshwater peat in the intertidal zone. An impressive and well preserved array of large Pinus stools rests on granite bedrock at Annagary Strand, embedded in 1 m thick Phragmites peat. The bases of the stools are at approximately + 1.6 m, the M.H.W.S. level.

At New Bridge Bay Piizus stools and horizontal trunks, and a few Betula trunks, are contained in 0.1 m of highly compacted peat in the intertidal zone. The base of the peat is 0.10 m above M.T.L., which is at -0.2m O.D. in this area. The peat contains very little pollen, but the presence of Pinus stools suggests that it predates c. 4000 BP. This is confirmed by Gaulin (per. comm. 1982) who stated that the base of the peat on the beach is dated at 5810 f 140 BP (Gif-5483). The peat is overlain in places by indurated sand and conglomerate, cemented by pyrite which was almost certainly deposited in a marine environment (Berner et al. 1979). Compacted Phragmites peat exposed at Rossbeg Harbour, the site dated at 6995 BP by G.F. Mitchell (Pearson 1979), has a pollen assemblage with 5% Ahus (based on a sum of arboreal plus Coryloid pollen), suggesting that this taxon was above its empirical limit by this time.

Lower Clonmass estuary Evidence of a strong maritime influence upon peat accumulation is present at the mouth of the Clonmass Estuary, where recent marine erosion of coastal dunes has exposed sandy sediments containing organic-rich horizons. These contain twigs and roots of Ahus dated at 4540 f 100 BP (SRR-2397) and 3990 * 70 BP (SRR-2396) (Table 1). Above these remains is a thin ( < 0.2 m) lens of compacted peat containing disseminated sand, Taxus seeds, and twigs of Taxus and Alnus. A 0.1 m-thick sample spanning the full thick- ness of the peat is dated at 3600 * 50 BP (SRR-2398). The pollen diagram (Shaw 1985) shows high amounts of arboreal pollen. Ahus and Coryloid pollen are dominant, with relatively high Salix amounts. Ulmus values are low and only one Pinus grain was noted. Hedera values are conspicuously high.

The peat formed in a wooded swamp environment in a depression in coastal dunes, with Alnus, Salix and

80 John Shaw and Richard W. G. Carter

Taxus growing on the site. Pinus pollen appears to be absent at 3600 BP, yet, at an upper estuary site (see below), Pinus levels remained above 50% after 3300 BP. This apparent contradiction is discussed later.

Above the peat is grey brown sand containing several thin, organic-rich horizons, the highest of which is dated at 800 f 40 BP (SRR-2399). These represent temporary phases of stability during the renewed aeolian activity which overwhelmed the swamp. Wood from the outer several centimetres of one of three Taxus stools lying on the adjacent beach at M.T.L. is dated at 1670 f 50 BP (SRR-2395). The presence of Taxus in a coastal marsh was also recorded in a similar environment at Magilligan, 100 km to the east, where seeds of this taxon were found by McMorris (1979). The most recent aeolian activity at the site is represented by the uppermost unit, from +3.3 to 8.3 m, comprising shelly sand. Carbonate is absent from all the underlying sediments, probably due to decalcification caused by soil leaching. Again a parallel may be drawn with the Magilligan site (Wilson & Bateman 1986).

Roshin Point (Gweebarra Bay) At Roshin Point, site A (Fig. 1; see also Fig. 3), 0.40 m of compacted black peat is exposed in the intertidal

BOREAS 23 (1994)

Table 2. Detailed lithostratigraphy at Roshin Point, site A.

Elevation (m) 1.30 to 0.90

0.90 to 0.48

0.48 to 0.03

0.03 to -0.35

-0.35 to -1.22

Black, highly compacted, fissile, well-humified herbaceous peat with disseminated sand, scattered wood fragements, and Phragmites rhizomes. Dark grey brown micaceous silty find sand, with dark brown wavy laminae up to 2 mm. Numerous dark brown blebs and streaks. Wood fragment at 15 cm. Grey micaceous silty fine sand with dark brown laminae 0.5- 1.0 mm thick. Scattered vertical monocot. roots 0.5-1.0 cm long. Wood fragment at 0.4 m. Dark grey brown micaceous silty fine sand with numerous wavy laminae, some lenticular and up to 4 mm thick, increasing in frequency with depth. Massive light brown grey medium sand with shell fragments up to 1 mm.

zone of a small, sandy tombolo. The peat overlies 1.25 m of laminated silty sand on top of shelly medium sand (Table 2). A basal peat sample (+0.92 m to +0.90 m) is dated at 4950 & 80 BP (UB-2676). The pollen diagram (Fig. 4) shows relatively low Ulmus values and an upwards decline in Pinus beginning at 1.16 m. Coryloid and Gramineae amounts increase greatly above this level. The sharp contact between the peat and the underlying sand, and the lact of evidence for a salt-marsh interval in the pollen stratigraphy,

Fig. 3. Air photograph of Gwee- barra Bay showing Roshin Point in the foreground, the extensive coastal dunes at the entrance to the Gweebarra Estuary, and the narrow entrance to Trawenagh Bay in the distance.

BOREAS 23 (1994) Holocene sea -level change, Ireland 8 1

Herbaceous peat

suggests the sudden isolation of a small estuary from the ocean, probably by blown sand (as happened at Helgoland - see below). The peat was compacted by barrier roll-over, and is now exposed on the seaward face of the contemporary barrier.

At a second site about 0.5 km from the first ( B on Fig. 1, Table 3), on intertidal mud flats, 0.45 m of herbaceous peat lies under 0.34m of fine sand, and over shelly medium sand. The peat unit has wood fragments in the upper part and Phragmites rhizomes at the base. In the pollen diagram (Fig. 5) high Pinus values occur at the base of the peat. Chenopodiaceae amounts are relatively high and foraminifera test lin- ings were noted during the count (58% of the pollen sum). Above this level is a series of aquatic pollen types: Typha, Alisma, Nymphaeu, Potamogeton and Hydrocotyle. The Gramineae peak at 0.5 m may be due to the presence of Phrugmites. Cyperaceae amounts rise above above this level. The pollen spectra show a marine influence in the lowest several centimetres, succeeded by a series of freshwater aquatic environments, reedswamp and finally Cyperaceae-dominated mire. No radiocarbon date is available but the high Pinus levels suggest that the basal peat is older than c. 4000 BP.

Helgoland and Naran Small dune-dominated barriers at Helgoland and Naran (Fig. 1) protect low wetlands which are infilled tidal inlets. Behind Helgoland Strand 0.60 m of sandy, dark brown, herbaceous peat, with Phragmites rhi-

Fig. 4. Pollen diagram from peat exposed on the foreshore of a small tombola at Roshin Point, Gweebarra Bay (site A). The absence of any marine influence suggests a sudden regressive event.

Table 3. Detailed lithostratigraphy at Roshin Point, site B.

Elevation (m) 1.10-1.00

1.00-0.80 0.80-0.76 0.76-0.70

0.70-0.50 0.50-0.31

0.31 -0.10

Dark grey fine sand with complete bivalves and shell fragments, strongly reducing. Grey fine sand with shell fragments. Dark grey organic-rich grey fine sand. Dark brown herbaceous peat with wood fragments, including Betula. Very dark brown herbaceous peat. Very dark brown herbaceous peat with Phragmites rhizomes and rootlets. Grey brown foraminiferous medium sand with shell detritus and Phrugmites rootlets which penetrate down from the overlying unit.

zomes in the lowest 0.05 m, overlies dark grey shelly fine sand with vertical monocot. roots. A sample from + 1.55 m to + 1.45 m is dated 2095 _+ 100 BP (UB- 2677). The peat has low levels of arboreal pollen (2-6%), shrub pollen (10- lSO/,), and no Pinus pollen, indicating an environment denuded of trees, similar to that of today. As at Roshin Point (site A) there is a sharp transition between peat and the underlying minerogenic sediment, showing that the closure of the inlet was a sudden event, probably due both to sediment infilling and aeolian activity.

A similar sequence of events behind the Naran barrier. East of a small freshwater lake (the remnant of a tidal inlet), 0.21 m of sandy herbaceous peat is sandwiched between aeolian sand and underlying shelly sand. The base of the peat, from + 1.70 m to + 1.83 m, dates to 815 f 95 BP (UB-2680).


r I I I I

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7

0.6 -

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Fig. 5. Pollen diagram from a location close to Roshin Point (site B), Gweebarra Bay, showing a gradual transition from marine to freshwater conditions

Loughros More

Loughros More is an embayment on the west coast of Donegal, conspicuous for the extent of coastal dunes at its mouth and the presence of a massive sand cliff formed by migrating tidal channels (Shaw 1985). She- skinmore Lough is a shallow lake, fringed by Phrag- mites beds, located between the dunes and the surrounding hills. On the floor of a side valley, herbaceous peat, 1.82 m thick and containing Phragmites rhizomes, overlies micaceous fine sand containing foraminifera and marine diatoms (Shaw 1985). A 5 cm thick basal peat sample, from +1.40m to + 1.45 m, is dated at 1040 f 70 BP (Beta-22238) (unadjusted). In the pollen diagram of the lowest 0.20 m of peat (Shaw 1985), Pinus pollen is absent, and arboreal pollen levels are extremely low. The evidence suggests a sharp transition away from marine influence when an inlet became isolated from the sea, possibly as a result of instability in the Loughros More dunes just before c. 1OOOBP. Sheskinmore Lough, like the lake behind the Naran dunes, is thus a freshwater remnant of a former estuary.

Upper Clonmass Estuary

In the upper Clonmass estuary (B on Fig. 1) a relatively narrow intertidal mudflat is fringed by high saltmarsh. Scattered mounds of freshwater peat on top of the saltmarsh are remnants of a formerly more extensive cover, removed by cutting. Vibracoring here showed 0.46m of herbaceous peat above 0.32m of silty peat with upwards-decreasing inorganic content. This unit overlies 1.80 m of micaceous silty sand, with shelly fine sand at the base of the section (TabIe 4). A thin (0.24 m) interval of organic-rich silty fine sand was present at M.T.L.

The silty sands had no carbonate and contained foraminifera tests, typically Trochammina inJata, a high salt-marsh type (Scott et al. 1981). A sample from the base of the silty peat is dated at 3300 If: 95 BP (UB-2679). In the pollen diagram (Fig. 6) two local pollen assemblage zones occur, with their boundary located at the top of the silty peat at + 1.52 m. In the lower zone Betula amounts are high and Pinus decreases upwards. Compositae Liguliflorae amounts increase upwards and Plantago maritima

BOREAS 23 (1994) Holocene sea -level change, Ireland 83

Table 4. Lithostratigraphy, Clonmass Estuary site B (upper estuary).

Elevation (m) 1.98 to 1.52 1.52 to 1.20

Dark brown herbaceous peat Grey brown silty peat, increasingly laminated with depth, downward increasing inorganic content Grey brown micaceous silty sand, monocot. roots, dark laminae, 2-5 mm thick, upwards fining Dark grey brown organic-rich silty fine sand, disseminated mica, roots, organic lumps up to 3 mm; upper boundary convoluted, lower sharp over 1 mm Dark grey massive silty fine sand with roots Grey shelly fine sand

1.20 to 0.03

0.03 to -0.21

-0.21 to -0.60 -0.60 to -0.85

falls. This zone probably represents a salt-marsh environment, in which the marine influence decreased with time. The upper zone is dominated by Coryloid and grass pollen, with Ericaceae increasing upwards to very high percentages. It indicates a freshwater environment. At the base of the upper zone, and overlap- ping with the lower zone, Compositae Liguliflorae values are very high. A similar, large Compositae Liguliflorae peak occurs at two other sites: Ballyness and Trawenagh. At all three locations the peak is at the interface between salt-marsh and freshwater peats.

In the upper Clonmass Estuary Pinus pollen levels remained high at 3300 BP, and subsequently declined, yet at Aranmore the decline in Pinus began c. 4000 BP. Moreover, peat from a vegetated hollow in coastal dunes at the mouth of the Clonmass estuary, dated at 3600 BP contained no Pinus pollen (see above). At Glenveagh, 12 km inland (Telford 1978), the Pinus curve finally fades out at 2550BP. TWO factors might explain the contradiction: (i) at the estuary mouth (site A) Pinus was underrepresented because of a dense local woodland cover; and (ii) Pinus pollen was being transported from distant areas to the estuary in streams, and was over-represented at the upper site (B), until, with the waning of marine influence, freshwater peat began to accumulate with- out this pollen influx.

The lithostratigraphy and pollen data show that a shallow marine setting was superseded by salt- marsh and freshwater marsh environments. Assuming that the basal salt-marsh peat accumulated somewhere between the former M.H.W.N. (mean high water neap) and M.H.W.S. (mean high water springs) tidal levels, and that the range was the same as today (M.H.W.S. = 1.7m; M.H.W.N.=0.7m; M.T.L. = 0.0 m), then mean sea level was between +0.5 m and -0.5 m at 3300 f 95 BP.

Ballyness Ballyness spit (Rutherford 1979; Shaw 1985; Carter 1990b) is located in an exposed setting at the extreme

Table 5. Detailed lithostratigraphy at Ballyness.

Elevation (m) 1.47- I .02 1.02-0.47 0.47-0.17

0.17 +

Black, crumbly, well humified herbaceous peat Dark brown, moderately humified, herbaceous peat Faintly laminated, dark brown silty peat, inorganic content increasing downwards Dark olive grey micaceous silty sand, rootlets.

northwest corner of Donegal and consists of high coastal dunes on a foundation of gravel beach ridges with crests at - 3 m. It encloses a shallow, sand-choked estuary with eroding peat faces along its shore in several places. Behind the narrow proximal end of the spit 1 .O m of dark herbaceous peat overlies 0.30 m of silty peat and upwards-decreasing organic content, on top of micaceous silty sand containing foraminifer tests. (Table 5). The base of the silty peat, from +0.19 m to +0.17 m, is dated at 4410 f 100 BP (Beta-34314). An unadjusted radiocarbon date on peat from the interval + 0.49 m to + 0.47 m is 3730 f 90 BP (Beta-42528).

The pollen diagram (Fig. 7) has four local assemblage zones. Zone 1 has high levels of Pinus and Plantago maritima, and small but significant amounts of Chenopodiaceae. It coincides with the silty peat, and is interpreted as a salt marsh deposit. Zone 2 has lower levels of Pinus pollen, high amounts of Plantago maritima and Gramineae, and marked peak in Composite Liguliflorae, high amounts of indeterminable palynomorphs, and reduced concentrations. Zone 3 contains no Pinus pollen, but this taxon returns at the top of Zone 4.

These data show regressive overlap, with successive changes from intertidal flat, through intertidal salt- marsh, to a supratidal, freshwater environment. As- suming again that the basal salt-marsh peat formed between the former M.H.W.N. and M.H.W.S. levels, and the tidal range has not altered (M.H.W.S. = 1.7 m; M.H.W.N. =0.7m; M.T.L. =O.Om), then mean sea level was in the range - 0 S m to -1.4m at 4410+ 100 BP. Furthermore, if it is assumed that the highest salt-marsh deposit accumulated between the former M.H.W.S. and H.A.T. (highest astronomical tide) levels (H.A.T. = 2.2 m), then mean sea level at 3730 & 90 BP (unadjusted age) stood somewhere in the - 1.2 m to - 1.7 m range.

Trawenagh Bay Trawenagh Bay is a bedrock-bounded inlet connected to the ocean by a narrow channel (see Fig. 3). Cliffs of peat several metres high are exposed by erosion along the shore. Monolith and vibracorer section showed 1.5 m of black herbaceous peat with scattered Myrica rootlets, overlying 0.55m of silty peat with upwards decreasing inorganic content. This was on top of micaceous silty sand containing a single, organic-rich layer (see Table 6).

00

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BOREAS 23 (1994) Holocene sea-level change, Iretand 85

Fig. 7. Pollen diagram from peat exposed on the shore of the Ballyness estuary. Pinus amounts are relatively high in the lower (salt-marsh) zone, and this taxon is still present after 3730 BP.

Table 6. Detailed lithostratigraphy. Trawenagh Bay.

Elevation (m) 2.85 to 2.75

2.75 to 1.45

Black fibrous peat, disseminated fine sand and shell fragments Black well humified herbaceous peat, numerous roots of Myricu, especially at 2.05, 1.75 and 1.65 m Dark brown silty peat, organic content decreasing downwards, from 80% to 2%, sharp lower boundary Dark grey brown micaceous silty fine sand, roots. Horizontal Phrugmites rhizomes at the top of the unit. Increasingly frequent dark laminae below 0 m Dark brown organic rich sand silt (organic content lo'%"). convoluted upper boundary, lower boundary sharp Massive grey brown micaceous silty sand

1.45 to 0.90

0.90 to -0.58

-0.58 to -0.63

-0.63 to - 1.29

Three radiocarbon dates from the site are: 2890 60 BP on peat at 1.29 m to 1.30 m (SRR- 2604); 3990 k 90 BP on silty peat at 0.90 m to 0.92 m (SRR-2605); and 4200 k 80 on organic-rich silty sand at -0.63 m to -0.58 m (SRR-2606).

In the pollen diagram for the organic sediments (Fig. 8) two pollen assemblage zones are suggested. In the lower zone, +0.90 to + 1.25 m, arboreal pollen types decline upwards. Pinus shows a particularly sharp fall, and wedges out at the upper zone boundary. Plantago maritima and Compositae Tubu- liflorae values decline from relatively high levels at Compositae Liguliflorae increases towards the top of the zone. This zone lies within the unit of peat with upwards - increasing organic content. The upper zone is dominated by Gramineae, Coryloid-type and Eri- caceae, with Composite Liguliflorae values falling away above the lower zone boundary. Some of the Coryloid pollen is probably Myrica, since Myrica macrofossils were present.

The stratigraphic sequence is similar to those at Ballyness and Clonmass, particularly the latter, where a lower, organic-rich unit was also present. The silty peat formed with an external input of inorganic material, and represents a salt-marsh environment in which the marine influence declined. Using the assumptions made for Clonmass and Ballyness (see above) then mean sea level at 3990f90BP was in the range

86 John Shaw and Richard W. G. Curter BOREAS 23 (1994)

- . .

. .

2

- . . - . - . ’ -289060BP

(SRR-2604) ._ _ . .

K L.O.I.

0 50 1

-\7 f 1 i

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1 //

1 i

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./*

Pollen sum I All pollen and spores

u 10%

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0 Sand

Fig. 8. Pollen diagram from a site on the shore of Trawenagh Bay showing a lower salt-marsh environment, and an upper freshwater bog environment.

+0.2m to -0.8m; the estimate for the range at 2890 60 BP is -0.4m to -0.9 m. If the lower, organic-rich silt accumulated in a salt-marsh environment, then mean sea level at 4200k80BP is constrained to the range - 1.3 to -2.3 m.

Discussion

Palynology of salt-marsh deposits

Two aspects of the pollen and spore assemblages of salt-marsh deposits at Clonmass, Ballyness, and Trawenagh are worthy of discussion. The first is the unusually high, and problematic, levels of Composite

Liguliflorae pollen. Similarly high levels have been reported from archaeological sites in arid areas and seasonally dessicated lake sediments (Bottema 1975), from coastal peats in southwest Ireland (Carter et al. 1989), and in oxidised horizons within a Holocene peat sequence in New England (Shaw & van de Plass- che 1991). This type is highly resistant to decay (Havinga 1984), and the high levels at Donegal sites may result from poor preservation conditions at the transition between salt marsh and freshwater hydros- eres. This hypothesis is supported by the presence of high amounts of indeterminables and, at Ballyness at least, low absolute concentrations of palynomorphs at these levels - Hall (1981) argued that relatively high amounts of ‘deteriorated or indeterminable’ pollen


and low concentrations pointed to altered assemblages, ie. assemblages which do not reflect the pollen rain at a site.

Compositae Liguliflorae pollen is also considered an indicator of man-managed habitats (see Moore et al. 1991: 190). The Liguliflorae pollen from the Ballyness site is predominantly the Cichorium intybus type of Moore et al. 1991, and a highly speculative interpretation is that the Liguliflorae pollen is derived from Taraxacum sp. growing on high salt marshes which were being used for grazing purposes during the Ne- olithic and Bronze Age periods.

The second interesting aspect of the salt marsh deposits is the high Pinus levels, which Shaw (1985) argued reflected over-representation; the upward declines indicated a waning marine influence, and less recruitment of pollen from distant sources in estuarine waters. However, at Trawenagh and Clonmass the decline is accentuated since it coincides with the regional decline of Pinus, whereas in the somewhat older peat at Ballyness, a high Pinus presence continues after the end of marine influence at 3730 BP, although at reduced levels.

Holocene relative sen-level changes

The relative sea-level curve for the region is shown in Fig. 9. There is little evidence with which to define precisely the curve in the early Holocene; its position is constrained by a date of 8960 f 110 BP (Beta- 34315) on freshwater peat with an early Holocene pollen assemblage, collected from a depth of 6 m in the estuary of the River Bann, Northern Ireland. However, unlike the areas further east [where the early-Holocene transgression rose aboue present sea level (see Carter 1982)], in Co. Donegal the peak sea level appears never to have reached present datum. The lack of evidence for marine incursion at the relatively exposed Aranmore Island site (apart from a subsidiary presence of possible aeolian sand) strongly suggests sea level peaked below present. The most obvious explanation is that sites to the west were further from the isostatic lift centre, a pattern similar to that described in western Scotland (qu. Ritchie 1966, 1985; Dawson 1984). Going east from Co. Donegal evidence of a raised Holocene shoreline first appears in Lough Foyle, where marine deposits at Drumskellan were dated to around 6900 BP (Colhoun et al. (1973).

The curve after 5000BP is fixed by radiocarbon dates from salt-marsh peats at Clonmass, Ballyness, and Trawenagh which, because they formed within a narrow vertical range, are reliable indicators of palaeo-sea levels. However, the curve is a simplifica- tion - the plotted range estimates for mean sea level based on salt-marsh deposits are inconsistent by up to 0.5 m. This can be attributed to a number of factors: ( 1) errors in estimating elevation; (2) the potential

(0

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m" O . -

a 9:

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7 -

0 0 1 3 8 8 8 8 - Radiocarbon years BP

Fig. 9. Plot of relative sea-level index points discussed in the text and a simplified mid- to late-Holocene relative sea-level curve for western Donegal. The data may reflect a slight inter-regional variation in sea levels.

existence of a east to west decline in Holocene shoreline elevations; (3) the probability that tidal ranges within the estuaries may differ markedly from those outside. Tidal range may alter through time as a result of natural infilling of an estuary, progressive con- straint of tidal flow by barrier growth, or anthropo- genic activity. If the range is reduced at some sites, the effect is to push estimates of former sea level higher; (4) the possibility that relative sea level rise was stepped rather than smooth - Van de Plassche (1991) has argued convincingly for the existence of alternat- ing periods of slow and rapid sea-level rise during the past 2000 years on the coast of Connecticut, USA. It is likely that all four factors combine to reduce cer- tainty.

Data from intertidal freshwater organic deposits are also used to constrain the post-5000 BP portion of the sea-level curve. At Roshin Point basal peat at f0 .9 m overlies minerogenic sediments. If the latter were deposited below the M.H.W.S. level (+ 1.7 m), then mean sea level was 0.8 m below the present level prior to 4950 BP. At Helgoland, immediately prior to the estuary closure at c. 2095 BP, mean sea level was about 0.3 m below the level of today. At Naran, basal peat at + 1.7 m dates to 815 f 95 BP (UB- 2680), suggesting mean sea level was very close to the present level at that time. The numerous occurrences of freshwater peat and rooted trees in the intertidal zone can be used to infer minimum depressions of palaeo-relative sea levels by assuming that they accumulated above the M.H.W.S. level, and that the lowest level of tree growth was between M.H.W.S. and the H.A.T. level. Thus, mean sea level at New Bridge Bay, where peat is at -0.2 m, was depressed by at least 1.8m at 5810BP. The Taxus trunk at Clon- mass, at -0.2m and dated at 1670BP, indicates mean sea level depressed by at least 2.2m at that time. This is in conflict with other data, and it is likely, as Shaw (1985) conjectured, that the stump is not in situ.


Impact of the transgression at the coast One consequence of the early Holocene transgression on this exposed oceanic coast would have been a transfer of clastic debris from the shelf towards the land. Long-period waves and frequent storms acting over a hydrodynamically-shallow shelf would have been more than sufficient to remobilise and rework any unconsolidated material, which would have been driven onshore. Larger particles (gravels/cobble size) would have been particularly susceptible to landward transport under the influence of low frequency, non- linear wave action. As sea level rose, so the coastal deposits would have transplanted landward, amalga- mating with material from terrestrial sources such as cliffs or rivers. Offshore surveys (e.g. Caston 1976; Evans et al. 1980) indicate a largely current-swept shelf with only limited thicknesses or veneers of recent sedimentary material. (The work of Peacock et al. (1992) off western Scotland indicates a marked outer depth limit of about 100 to 120 m to the rock-scoured zone of the shelf, which may correspond to the outer wave base of early Holocene sea. However, Peacock ( 1992) believes this evidence needs further investigation as it conflicts with the model predictions of Lambeck ( 199 1 a)).

As the transgression waned after 5000 BP, gravel shorelines formed. This phase is dated to pre- 4410 BP at Ballyness (see above). The beaches were supplied by sediment eroded from glacigenic deposits both offshore and onshore. For example, sediment was supplied to the Ballyness barrier by the erosion of the coastal bluffs which extent west to Bloody Foreland. The early barrier was composed of gravel, with sets of gravel beach-ridge recurves in the wide distal section, and with a thin sporadic cover of beach aeolian sand since formed into calcrete. The development of gravel barriers at or near the time of peak sea-level would have established a shoreline reaching well above mean tide height. Gravel beaches in County Donegal may reach from 5 to 14metres above M.W.L. depending upon orientation and exposure.

Accumulation of coarse material at the shoreline provided loci for the development of other deposits, notably sand beaches and dunes, but also quiet-water deposits such as tidal flats and marshes. The establishment of a gravel framework would encourage further textural partition of sediment, allowing sand to accumulate in the nearshore/upper shoreface and silts and clays in back barrier environments. The most striking aspect of the region today, the dominance of sand in littoral systems, commenced with a slight fall in sea- level after the transgression had reached its peak; this allowed the release of large volumes of sub-aqueously stored material (mainly sands) to be moved ashore, as wave base fell. This process, which probably continued for several hundred if not a thousand years, achieved an unequalled phase (at least in the Holocene) of

sediment abundance, and saw the rapid development of beach and dune complexes throughout northwest Ireland (Carter & Wilson 1992). County Donegal was particularly favoured, as the likely dominance of onshore winds would have accentuated the depositional process. This dynamic coastal environment is the con- text within which many coastal peat occurrences must be interpreted.

The newly formed barriers created sheltered settings in otherwise exposed locations. An instructive example is again found at Ballyness, where the formation of the barrier prior to 4410 BP permitted the development of salt marshes fringing the estuary. Subse- quently, organic deposits intimately associated with coastal barriers formed for a variety of reasons. One of the most common causes has been closure of tidal inlets due to sediment infilling or aeolian action (cf. Murphy 1980), which has allowed freshwater peat to accumulate in backbarrier settings - this, as noted above, has occurred at Naran and Helgoland. At Clonmass an interdune depression was the site of peat accumulation.

While the sandy barriers of Donegal are, for the most part, stationary (Shaw 1985) migration has occurred in some cases, exposing former backbarrier organic deposits. The best illustration is at Roshin Point, where the barrier moved across the back-barrier freshwater marsh, exposing fissile, compacted peat on the beach face. However, the most pervasive type of change on barriers is that caused by ebb-channel migration. At Clonmass, a migrating channel destroyed the old coastal dune system during this century, exposing the complex suite of interdune deposits described briefly above. Away from sandy barriers systems, most intertidal exposures for organic deposits in result from general coastal retreat which exposes freshwater peat and rooted forests in the intertidal zone.

Regional sign$cance

The evidence presented in this paper suggests that relative sea levels in northwest Ireland have varied due to both local (sediment budget) and more regional factors (glacial unloading, climatic change). Of these glacial unloading has had the most profound influence, most likely associated with the decay and disso- lution of the ice sheets in northern Britain (Sissons 1983) although there may be a small Irish component, which Devoy (1991) suggests may underlie the somewhat anomalous south to north variation in Irish sea-level curves.

The County Donegal sea-level data fit the general model of rheological behaviour moving away from an ice centre (Clark et al. 1978; Quinlan & Beaumont 1981). In western Scotland the late- to post-glacial sea level hardly fell below present sea level (Peacock et al. 1978), but further afield the Holocene curve is largely

BOREAS 23 (1994)

80 -

60 -

40 -

Holocene sea -level change, Ireland 89

submerged, save for the transgression peak. The elevation and timing of this peak appears to vary from east to west. Jardine (1975, 1982) and Griffiths (1988) describe cliff notches and estuarine sediments at + 5 to +6 metres 0 . D on the Solway coast of Scotland falling between 7900 and 6400BP. In the Firth of Clyde Dickson et al. (1978) bracketed the peak between 6900 and 5400BP, almost exactly the same as that recorded in north County Antrim at heights of 2 to 3 metres by Carter (1982). The peak does not reach present sea-level in County Donegal and the pollen analytical evidence suggests it may been after 4000 BP. In County Down, about 150 km south of the County Antrim sites discussed by Carter, Singh & Smith ( 1973) recorded a transgression peak at 3 100 BP. However the general lack of good sea-level index points renders this information as ‘suggestive’ rather than definitive and further investigations are clearly needed. A somewhat similar but perhaps more extreme situation existed in the Outer Hebrides, where evidence points to a stationary sea-level of -3 to - 5 m 0.D from 9000 to 5000 BP before rising further (Ritchie 1979). There is also a problem in reconciling these onshore data and interpretations of Peacock (1992) and Peacock et al. (1992).

Building on earlier modeling by Peltier & Andrews (1976), Lambeck (1991a, 1991b) has applied a high- resolution model of isostatic crustal rebound to the British Isles. By and large this model both demon- strates the coherency of sea-level interpretations from one investigations to another and the systematic re- sponse of the area to unloading of the last ice sheet.

Lambeck (1991a, Fig. 7) presents a series of predicted sea-level curves for Ireland, including four for northwest and northeast Ireland (Fig. 10). There is a very good agreement between the data from the present study which should, and do, fall near the lower bound of the envelope. The earlier curve of Carter (1982) based on a range of index points mainly from northeast Ireland also fits the model, although there appears to be a substantial deviation from field evidence for the higher, earlier shorelines, but this argument falls outside the scope of this paper.

Conclusions

1. The coastal record of County Donegal is consistent with that of an area evolving under a high energy wave and wind regime, with extensive organisation of both fine and coarse clastic sediments.

2. The Holocene relative sea-level curve appears to vary alongshore, with sites on the west coast showing no evidence of the mid-Holocene peak above present mean sea level which is common further east.

3. The relative sea-level history matches that predicted by recent isostatic rebound models.

Acknowledgemenrs. - We acknowledge the various contributions to this research made be Alan Hamilton, William Magowan, Arthur Ward, Brendan OKane, Nigel McDowell, Roger Flower, and Ken Hirons. Charles Schafer and Bob Mott reviewed early drafts of the manuscript. The research was funded by NERC. This is Geological Survey of Canada Contribution Number 43992.

I I

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8

8

0’

0 - _ _ _ _ _ - - - - - _ _ * - * - - - - - _ _ _ _ _ ’ ,!15ka BP “ 0 Q) > ce .- CI -

This study -. Fig. 10. Diagram showing the envelope of Lambeck’s (1991a) predicted relative sea-level curves for

a“ -20- /

Northwest Ireland northeast and northwest Ireland (dashed lines), together with the curve for the study area, and the curve for northeast Ireland from Carter (1982).

BOREAS 23 (1994) 90 John Shaw and Richard W. G. Carter

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Documents

Coastal peats from northwest Ireland: implications for late-Holocene relative sea-level change and shoreline evolution