Radiocarbon dates from the inter-tidal peat bed at

0 downloads 0 Views 279KB Size Report
Sep 30, 2011 - The publisher does not give any warranty express or implied or make ... The publisher shall not be liable for any loss, actions, claims, proceedings, ... Prior to the 1960s peat was visible both on the beach and in cliff exposures at .... the Bann estuary В8 km to the southwest of Portrush (Figure 1, Hamilton and.
This article was downloaded by: [University of Ulster at Coleraine] On: 09 May 2012, At: 01:07 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Irish Geography Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rigy20

Radiocarbon dates from the inter-tidal peat bed at Portrush, County Antrim a

a

a

Peter Wilson , Kieran Westley , Ruth Plets & Michael Dempster

a

a

Environmental Sciences Research Institute, School of Environmental Sciences, University of Ulster, Coleraine, Northern Ireland Available online: 30 Sep 2011

To cite this article: Peter Wilson, Kieran Westley, Ruth Plets & Michael Dempster (2011): Radiocarbon dates from the inter-tidal peat bed at Portrush, County Antrim, Irish Geography, 44:2-3, 323-329 To link to this article: http://dx.doi.org/10.1080/00750778.2011.620415

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-andconditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Irish Geography Vol. 44, Nos. 23, JulyNovember 2011, 323329

Radiocarbon dates from the inter-tidal peat bed at Portrush, County Antrim Peter Wilson*, Kieran Westley, Ruth Plets and Michael Dempster

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

Environmental Sciences Research Institute, School of Environmental Sciences, University of Ulster, Coleraine, Northern Ireland Two new 14C dates from a bed of compact inter-tidal peat at Mill Strand, Portrush, are reported and discussed in relation to two previously obtained ages from peat at Dhu Varren, a short distance inland. The date from the top of the Mill Strand peat is statistically inseparable from that of the peat top at Dhu Varren. Together these dates indicate the cessation of peat growth occurred 6950-6550 cal. BP as a result of sand deposition associated with sea-level rise. However, dated samples from deeper within the respective stratigraphies are not compatible. Their respective ages in relation to their elevations pose a problem of interpretation. Keywords: inter-tidal peat; Ireland

14

C dating; relative sea-level; Portrush; Northern

Introduction At a large number of sites around the coast of Ireland there are inter-tidal and/or sub-tidal peat beds (e.g. Prior et al. 1981, Carter et al. 1989, Shaw and Carter 1994). At many of these sites valuable palaeoenvironmental data pertaining to vegetation history, human activity, and relative sea-level fluctuations have been obtained from the peat, and utilisation of 14C dating has provided a timeframe for these events. Such peat accumulations have facilitated the reconstruction of local and regional patterns of coastal evolution. The existence of a bed of compact peat at Mill Strand (aka West Strand), Portrush, County Antrim (C 854401) has been known about for over 100 years (Gray 1879). Prior to the 1960s peat was visible both on the beach and in cliff exposures at the head of the beach, but sea-defence works have since obscured the cliff. Peat can now only be seen in the inter-tidal zone, usually in late winter when storms have removed sand from the beach (Figures 1 and 2). Archaeological finds, primarily flints belonging to the Mesolithic cultural period (10,0006,000 cal. BP) have been made within peat layers in the Portrush area (most notably at the Springhill locality within Portrush town (Simpson 1889)). However, with the exception of a dense, apparently in situ, cluster of flint flakes and cores discovered by Patterson (1896), reported finds from the Mill Strand peats are sparse. For instance, Simpson (1889) could not find any flints in the exposed peat while Hewson (1934) claimed that worked flints were rare in the peat but considerably more abundant in the overlying sand. *Corresponding author. Email: [email protected] ISSN 0075-0778 print/ISSN 1939-4055 online # 2011 Geographical Society of Ireland http://dx.doi.org/10.1080/00750778.2011.620415 http://www.tandfonline.com

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

324 P. Wilson et al.

Figure 1. Top: Location of Mill Strand, Portrush. Bottom: Mill Strand, Portrush showing Jessen’s (1949) section, location of the late 1980s temporary exposure at Dhu Varren, and boreholes and core sites drilled in 2009. (Aerial photograph courtesy of the Ordnance Survey of Northern Ireland.)

Patterson’s (1896) finds were described as ‘much like those found in the Larne gravels’: a statement later verified by Woodman (1978) and implying a Late Mesolithic (8,5006,000 cal. BP) date for the artefacts and enclosing peat. The first palaeoecological investigation to be conducted (Erdtman 1928) indicated that Betula, Corylus and Pinus dominated the oldest part of the peat, with Quercus and Ulmus also represented. This part of the stratigraphy was ascribed to the Boreal climatic phase (10,5007,800 cal. BP). In the upper part of the peat Alnus was dominant, followed by Corylus, Betula, Ulmus, Quercus and Pinus. This assemblage was taken to indicate an age equivalent with the early Atlantic climatic phase (7,8006,800 cal. BP) and that the peak of sea-level rise followed that period. Much more detailed stratigraphical and palaeoecological work was reported by Jessen (1949). The section that he logged and described in detail extended for

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

Irish Geography 325

Figure 2. Exposures of peat on Mill Strand 2005.

600 m along the foreshore (Figure 1). It was demonstrated that the peat had developed on top of stoneless, shell-free sand that formed low ridges at two places. The sand was considered to be blown sand because its grain size was considerably finer than that of both the beach sand and the blown sand that covered the peat. The sand ridges were interpreted as old dunes. From both pollen and plant macro-fossil identifications Jessen (1949) concurred with the view of Erdtman (1928) that the peat was of late Boreal  early Atlantic age and had developed before the maximum of sea-level rise. He envisaged that peat growth had taken place in the depressions between and on the flanks of sand dunes as a result of drainage impedance due to sea-level rise. As sea level rose and peaked the peat was inundated and buried beneath 23.5 m of wave-lain beach sand. Later, sea-level fell and aeolian dunes developed on top of the beach sand. In the late 1980s, bulk samples of peat from a temporary exposure at Dhu Varren, 300 m inland from the Mill Strand outcrops (Figure 1), were 14C dated (Figure 3 and Table 1). The top of the peat (Beta-36943), at 4.15 m OD, returned an age of 5920980 BP; a sample from 1.4 m below the top (Beta-36944) yielded an age of 73109100 BP (Wilson and Carter 1990). The base of the peat was not seen. These ages are broadly consistent with the late-Boreal  early Atlantic age for the peat proposed by Erdtman (1928) and Jessen (1949), and the Late Mesolithic attribution of Patterson’s (1896) finds.

Further dates In 2009, in connection with a trans-Atlantic cable landing station, three boreholes (BH01-BH03) were drilled at Mill Strand. BH01 (ground surface elevation 9 m OD) passed through peat at a depth of between 2.6 m and 1.3 m OD. BH02 and BH03 did not encounter peat but in the area between them a percussion core was taken from a peat outcrop (Figure 1). The peat surface was at 1.4 m OD, peat thickness was 1.1 m, and it was underlain by sand. A bulk sample from the top of the peat (UBA-14416) and an unidentified plant macrofossil from the base of the peat (UBA-14417) were 14C dated to 5839930 BP and 6482930 BP respectively (Figure 3 and Table 1).

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

326 P. Wilson et al.

Figure 3. Stratigraphic logs and 14C dates for Dhu Varren and Mill Strand, and borehole logs for BH01-BH03.

Discussion We have no reason to reject any of the four available dates for the Portrush peat and therefore they are treated as accurate age estimates. All ages have been calibrated using CALIB REV 6.0.0 (Reimer et al. 2009) and the 2s range for each is given in Table 1.

Irish Geography 327 Table 1. Details of

14

C dates from Dhu Varren and Mill Strand.

Sample ID

Laboratory Code

Material

DV01 DV02 PRH01 PRH02

Beta-36943 Beta-36944 UBA-14416 UBA-14417

Peat Peat Peat Plant macrofossil

14

C age

5920980 73109100 5839930 6482930

Calibrated age: yr BP d13C˜ (2s range) 25.0* 25.0* 27.8 34.1

69506540 83407960 67376563 74407320

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

Note: *Estimated value.

The ages from the top of the peat at each site are statistically inseparable. This may be considered surprising given that the peat at Mill Strand is susceptible to wave erosion in late winter when beach sand is usually stripped away and the peat exposed. It is likely that peat growth at both sites was terminated at the same time as a result of sand deposition associated with sea-level rise. Therefore, if any significant erosion of the peat at Mill Strand had occurred since it became exposed on the beach face, the 14C age would have been older than that obtained from Dhu Varren, because the dated sample would have represented a position from deeper within the stratigraphy rather than the true top of the peat. That the ages are indistinguishable strongly suggests that any erosion of the Mill Strand peat has been extremely limited. The Mill Strand age thus corroborates the Dhu Varren age and together they indicate that the peat was overwhelmed by beach sand 69506550 cal. BP. It is inferred that this event indicates the timing of the maximum of Holocene sea-level rise and is consistent with 14C ages and their interpretation from peats and organic-rich muds at the Bann estuary 8 km to the southwest of Portrush (Figure 1, Hamilton and Carter 1983, Battarbee et al. 1985, Wilson and Plunkett 2010). The base of the peat at Mill Strand is significantly younger than the peat from 1.4 m below the surface at Dhu Varren (Table 1). Acceptance of both ages implies peat initiation occurred at different times in different places, as noted for peat beds exposed in sand cliffs along the Lough Foyle shore of Magilligan Foreland, 20 km west of Portrush (Wilson and Bateman 1986). However, given that the base of the Mill Strand peat is at a lower elevation than that at Dhu Varren (Figure 3), this argument is difficult to accept. The expectation would be that peat initiation at the lower elevation site would have commenced somewhat earlier than at the higher elevation site. The converse situation could only have arisen if the lower site had been regularly flooded and organic sediment prevented from accumulating. In turn this would imply a relatively high sea-level across the site until 7400 cal. BP, followed by a relative fall to allow peat accumulation, and then a relative rise at 69506550 cal BP to terminate peat growth. Such a scenario has not previously been proposed. An alternative scenario is that the recently dated Mill Strand outcrop is not in situ but was originally located within the sand cliff that bordered the beach prior to the construction of the modern sea wall. At some point, it slumped or fell down from the cliff face, possibly as a result of undercutting by waves, and was deposited on the beach. It is worth noting that Jessen (1949) mapped exposed peat deposits within the sand cliff as high as 5 m OD. Therefore, the outcrop may have formed at a similar level to that of the Dhu Varren peat. In addition to explaining the apparent

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

328 P. Wilson et al. discrepancy between the dates from the base of each sequence, if the erosive event occurred relatively recently (e.g. prior to sea wall construction but within, say, the previous century), it would also explain the limited erosion of the Mill Strand peat surface (see above). However, this hypothesis requires the peat block to have maintained its original orientation when falling, without toppling or overturning as is common in these situations, in order to produce the observed pattern of 14C dates. It would then have to survive subsequent wave erosion while lying exposed on the beach. Moreover, the sand deposit underlying the peat that was recovered in the percussion core contained traces of organic fragments perhaps indicating that the peat had in fact developed in situ. At present we are unable to discount any of the 14C ages and choose between the alternative explanations for the apparent anomaly. Irrespective of this problem the age from Dhu Varren remains the earliest age (83507950 cal. BP) yet available for peat initiation at Portrush. Submerged and inter-tidal peat beds and organic-rich muds have been recorded and 14C dated at several locations at the Bann estuary (Figure 1, Hamilton and Carter 1983, Battarbee et al. 1985, Wilson and McKenna 1996 , Wilson and Plunkett 2010) and their relationship to sea-level changes discussed. Some of these materials pre-date the maximum of Holocene sea-level rise, others post-date it, but together they assist in constraining the curve of sea-level change along the north coast of Northern Ireland.

Conclusions One of the 14C dates obtained from inter-tidal peat at Mill Strand corroborates an earlier obtained age for the cessation of peat growth due to burial by sand. The other age, for peat initiation, is somewhat problematic in being both younger than and at a lower elevation than nearby peat that is significantly older. Although the ages add to a growing body of evidence demonstrating marked environmental changes during the Holocene along the north coast of Northern Ireland, additional 14C dates are required to resolve outstanding issues. Acknowledgements We thank the Northern Ireland Environment Agency for providing funding for the two most recently acquired 14C dates and Metoc Plc/Glover Site Investigations for access to the core and the borehole data. Thanks are also due to Robin Edwards for assistance with core sampling. Michael Dempster was a member of the Northern Ireland Environment Agency at the time this work was undertaken.

References Battarbee, R.W., Scaife, R.G., and Phethean, S.J., 1985. Palaeoecological evidence for sea-level change in the Bann estuary in the Early Mesolithic period. In: P.C. Woodman, ed. Excavations at Mount Sandel 197377. Belfast: HMSO, Northern Ireland Archaeological Monographs 2, 111120. Carter, R.W.G., Devoy, R.J.N., and Shaw, J., 1989. Late Holocene sea levels in Ireland. Journal of Quaternary Science, 4, 724. Erdtman, G., 1928. Studies in the postarctic history of the forests of north-western Europe. Geologiska Fo ¨ reningens Fo ¨ rhandlingar, 50, 123192.

Downloaded by [University of Ulster at Coleraine] at 01:07 09 May 2012

Irish Geography 329 Gray, W., 1879. The character of the rudely-worked flints of the north of Ireland, chiefly in Antrim and Down. Journal of the Royal Historical and Archaeological Association of Ireland 5 (4th series), 109143. Hamilton, A.C. and Carter, R.W.G., 1983. A mid-Holocene moss bed from eolian dune sands near Articlave, Co. Londonderry. Irish Naturalists’ Journal, 21, 7375. Hewson, L.M., 1934. Notes on the Irish sandhills. Journal of the Royal Society of Antiquaries of Ireland. 4 (7th series), 231244. Jessen, K., 1949. Studies in Late Quaternary deposits and flora-history of Ireland. Proceedings of the Royal Irish Academy, 52B, 85290. Patterson, W.H., 1896. On a find of worked flints in submerged peat at Portrush, Co. Antrim. Journal of the Royal Society of Antiquaries of Ireland, 26, 383384. Prior, D.B., Holland, S.M., and Cruickshank, M.M., 1981. A preliminary report on late Devensian and Early Flandrian deposits on the coast at Carnlough, County Antrim. Irish Geography, 14, 7584. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J., Turney, C.S.M., van der Plicht, J., and Weyhenmeyer, C., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 050,000 years cal BP. Radiocarbon, 51, 11111150. Shaw, J. and Carter, R.W.G., 1994. Coastal peats from northwest Ireland: Implications for late-Holocene relative sea-level change and shoreline evolution. Boreas, 23, 7491. Simpson, W.J., 1889. Notes on worked flints found on a raised beach at Portrush in August 1886 . Proceedings of the Royal Irish Academy, 1, 7677. Wilson, P. and Bateman, R.M., 1986. Nature and palaeoenvironmental significance of a buried soil sequence from Magilligan Foreland, Northern Ireland. Boreas, 15, 137153. Wilson, P. and Carter, R.W.G., 1990. Portrush  Mill Strand. In: P. Wilson, ed. North Antrim and Londonderry. Field Guide 13. Dublin: Irish Association for Quaternary Studies, 3539. Wilson, P. and McKenna, J., 1996. Holocene evolution of the River Bann estuary and adjacent coast, Northern Ireland. Proceedings of the Geologists’ Association, 107, 241252. Wilson, P. and Plunkett, G., 2010. Age and palaeoenvironmental significance of an inter-tidal peat bed at Ballywoolen, Bann estuary, Co. Londonderry. Irish Geography, 43, 265275. Woodman, P.C., 1978. The Mesolithic in Ireland. Oxford: Archaeopress.