Dawson, Alastair

Dawson, Sue

Jordan, Jason

English

Lithostratigraphical and biostratigraphical investigation of marshes adjacent to coastal dune sequences in the Scottish Outer Hebrides show inland-tapering sand units enclosed within organic sediments. The sand sheets are considered to have been deposited by past windstorm activity, while radiometric dating appears to indicate deposition during the last 2000 years except for the well-known period of Medieval warmth that is here considered to have occurred between c AD600 and 1400. It is argued that the episodes of sand-blow indicated by the deposits may reflect periods of increased cyclogenesis in the North Atlantic associated with increased sea-ice cover, an increase in the thermal gradient associated with the polar atmospheric and oceanic fronts as well as colder air temperatures. It is also noted that the diminished North Atlantic winter storminess during Medieval times was broadly coincident with the expansion of Viking culture

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North Atlantic climate change and Late Holocene windstorm activity in the Outer Hebrides, ScotlandSue Dawson Geography, School of Social and Environmental Sciences,  University of Dundee, Perth Road, Dundee, DD1 4HN  Alastair G Dawson Aberdeen Institute of Coastal Science and Management,  School of Geosciences, St Mary’s, University of Aberdeen,  Aberdeen, AB24 3UF  Jason T Jordan Department of Geography, Environment and Disaster  Management, George Eliot Building, Coventry University,  Priory Street, Coventry, CV1 FB         Scottish Archaeological Internet Report 48, 2011 www.sair.org.uk27CONTENTSList of Illustrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .281 Abstract  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .292 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303 Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314 Chronology and Climate Dynamics  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .325 Implications for Archaeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .346 Conclusion   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .357 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3628LIST OF ILLUSTRATIONS 1 Field site locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 Concentrations of Na+ (ppb) at GISP2 drill site, central Greenland, ad­987–1987;  and 2-sigma calibrated radiocarbon dates showing the start of windstorm sedimentation  at study sites (after Meeker & Mayewski 2002)  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 323 Graph showing the number of weeks with ice around Iceland, for 20-year intervals  from ad­860–193 (after Koch 1937) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3329Lithostratigraphical and biostratigraphical investi-gation of marshes adjacent to coastal dune sequences in the Scottish Outer Hebrides show inland-tapering sand units enclosed within organic sediments. The sand sheets are considered to have been deposited by past windstorm activity, while radiometric dating appears to indicate deposition during the last 2000 years except for the well-known period of Medieval warmth that is here considered to have occurred between c ad­600 and 1400. It is argued that the episodes of sand-blow indicated by the deposits may reflect periods of increased cyclogenesis in the North Atlantic associated with increased sea-ice cover, an increase in the thermal gradient associ-ated with the polar atmospheric and oceanic fronts as well as colder air temperatures. It is also noted that the diminished North Atlantic winter stormi-ness during Medieval times was broadly coincident with the expansion of Viking culture.1 ABSTRACT30In recent years reconstructions of Late Holocene climate variability of the North Atlantic region have been enhanced considerably as a result of Greenland ice core research. In particular, our knowledge of changes in North Atlantic storminess has been transformed as a result of the published chronology of Na+ (sea salt) concentration changes from the GISP2 drill site (Meeker & Mayewski 2002). Annual changes in sea-salt concentration at GISP2 calcu-lated for the last 1,400 years have been interpreted as a regional signal of North Atlantic winter stormi-ness change. The sea-salt concentration time series is resolved to an annual scale and thus provides a high-precision long-term record of storminess change that can be compared with other records of change (e.g. meteorological time-series). In this paper, we explore in more detail the results of Dawson et al (2004) that described Late Holocene coastal windstorm deposits from the Scottish Outer Hebrides. Radiocarbon dating of peat deposits enclosing the sand units were used to construct a local chronology of Late Holocene windstorm events that were, in turn, used to test the Meeker and Mayewski (2002) hypothesis that an increase in winter storminess took place after c ad­1400–20 during the so-called Little Ice Age (LIA), this having been preceded by a period of negligible storm activity during the Medieval Warm Period (MWP).2 INTRODUCTIONIllus 1   Field site locations, Scottish Outer Hebrides 31The Outer Hebrides island chain (illus 1) extends from broadly north to south across a latitudi-nal range of 6ºN–8ºN, thus occupying an area presently influenced by the North Atlantic storm track. The islands are characterised by Lewisian gneiss bedrock that rises to heights locally in excess of 00m, but along the Atlantic margin the bedrock is characterised by sub-horizontal rock surfaces located a few metres above sea level. These are mantled by largely stabilised dune systems consist-ing of coastal dunes to seaward and a gently sloping surface to landward, where the system ends within peat areas, rocky outcrops and many small lochs. The sand-dominated landscape is locally known as machair, a Gaelic word that describes a low-lying, grass-dominated coastal plain composed of calcar-eous sand, with a  calcium carbonate content up to c 80% (Ritchie & Whittington 1994). Notwithstand-ing the apparent uniformity of the machair surface, the occurrence of organic horizons within dune sediment exposures implies a complex evolutionary history. Studies of sediment exposures and of foreshore peat deposits have been undertaken by several researchers, including Ritchie (1979, 198), Ritchie and Whittington (1994) and Gilbertson et al (1996; 1999) that, in turn, have led to the development of a model of machair evolution. The model is based on the concept that during the Holocene rise in sea level, glacially-derived sands from offshore were mobilised and mixed with comminuted shell debris. This material was reworked into coastal ridges and moved onshore over pre-existing lacustrine and organic deposits (Ritchie 198). The complex nature of machair evolution was demonstrated by research undertaken in the Uists by Ritchie and Whittington (1994), who  showed that locally variable stratigraphic sequences had resulted from non-synchronous sand-blow (windstorm) events. Ritchie and Whittington (1994) demonstrated that the earliest sands represent ‘overlapping landform suites’ which, through long-term erosion, outcrop today in the present intertidal zone. They argued that the onset of sand movement was non-synchronous across the Outer Hebrides and had been strongly influenced by local coastal configuration. They maintained that the culmination of the Holocene rise in sea level resulted in a reduction in offshore sediment supply to the coast. Since relative sea level reached its approximation position c 2000 years ago, it was argued that coastal sediment transport during recent millennia has been principally associated with a recycling of pre-existing coastal sediments and a possible degradation of pre-existing coastal dunes and machair.3 STUDY AREA 32Dawson et al (2004) described dated storm events for seven sites across the Outer Hebrides that exhibit distinctive age clustering. Only three of the dated storm horizons exhibit standard errors that place the sediments within parts of the MWP time interval. By contrast, the majority of the uppermost dated horizons appear to have been deposited after c ad­ 1300. The stratigraphical evidence indicates that a sustained period of machair and dune devel-opment in which shelly sands encroached inland ended  between the end of the MWP and perhaps as recently as 200 years ago.It is tempting to develop a hypothesis in which the frequency of storm events is related to past episodes of climate change, normally described in terms of former air temperature characteristics, with greater storm frequencies during periods of cold (e.g. the LIA), and periods of reduced storminess during episodes of warmth (e.g. the MWP). Such explan-ations, however, are fraught with difficulty since the westerly winds associated with severe North Atlantic cyclones (storms) are normally associated with higher than average winter temperatures. Equally, it can be argued that the coldest episodes of the LIA may have been associated with signifi-cantly reduced storminess owing to the expansion of the polar anticyclone and a southward movement of the polar atmospheric and oceanic fronts across the North Atlantic (Lamb 199).Meeker and Mayewski (2002) provided the most 4 CHRONOLOGY AND CLIMATE DYNAMICS Illus 2   Top: concentrations of Na+ (ppb) at GISP2 drill site, central Greenland, AD 987–1987 shown as departures from the long-term average (after Meeker & Mayewski 2002). Bottom: 2-sigma calibrated radiocarbon dates showing the start of windstorm sedimentation at study sites (see also illus 1).33comprehensive record of North Atlantic storminess for the last c 1,400 years based on measurements of sea-salt (Na+) concentrations in the GISP2 Greenland ice core (illus 2). Their measurements show that reduced storminess was a feature of the MWP that lasted between c ad­600 and 1400. They detected a marked increase in storminess frequency (increased Na+ concentration) that commenced at this time and observed that this ‘state’ of increased storminess has lasted until the present.These observations should also be considered within the context of the classic research of Lamb (1979), who argued that the cold conditions of the LIA were associated with an increased meridional (north–south) thermal gradient across the North Atlantic that, in turn, led both to a southward dis-placement of the North Atlantic storm track (the polar atmospheric front) and of polar waters (the southern limit of the polar oceanographic front). Considered together, the results of Lamb (1979) and Meeker and Mayewski (2002) point to an increase in winter storminess across the British Isles after c ad­ 1400–20 and preceded by diminished winter storminess during the MWP. It is argued here that such increases in the thermal gradient in the North Atlantic atmosphere provide a simple explanation for the greater frequency of windstorm sediment deposition and machair formation post-ad­1400 (and possibly also for the time interval studied here prior to the MWP). The dated sand units are plotted on the same timescale to illustrate the clustering of the dates during the LIA time period. This pattern of stormi-ness increase also coincides with an increase in Icelandic sea ice cover that commenced during the late 13th century (Koch 1937) (illus 3).Stratigraphical studies from sites along the Atlantic margin of Scotland and Ireland, as well as from the North Sea region, provide support for the chronology of sand mobilisation described here. Gilbertson et al (1999) also describe late Holocene phases of sand movement in the Outer Hebrides c 200–600 years ago (c ad­1400–1800), c 1,300–1,700 years ago (c ad­300–700) and c 3,300–3,800 years ago (c 1800–1300bc) based on luminescence dating of calcareous machair sands. Orford et al (2000) concluded that the majority of coastal dune systems in eastern England were produced during the LIA. Similarly, Delaney & Devoy (199) noted that in western Ireland remobilisation of sand across back barrier lagoonal sites took place over the last c 400 years.It could be argued that some of the dated windstorm events in the Outer Hebrides may be related to anthropogenic activity related to over-population,  animal grazing pressures and the harvesting of marram grass for domestic purposes during the 18th and 19th centuries (cf Angus 1994, 1997). Although such processes were undoubt-edly important in machair destabilisation and aeolian sediment transport, we maintain here that the increase in North Atlantic storminess that commenced at c ad­1400–20 and has continued until the present was the key factor accounting for the ages of the windstorm deposits described from the sites examined within this paper.Illus 3   Graph showing the number of weeks with ice around Iceland, for 20-year intervals from AD 860–1935 (based on Koch 1937).34A key implication for archaeological research is that North Atlantic winter climate may have changed dramatically between  the MWP and LIA. For example, during the MWP a diminished thermal gradient in the North Atlantic may have resulted in a substantial decrease in cyclogenesis that most probably also coincided with a marked decrease in sea ice extent and a significant decrease in winter rainfall. By contrast, a marked increase in winter storminess coupled with an increase in rainfall took place during the LIA. Considered together, these factors imply that it is misleading simply to consider the principal difference between the MWP and the LIA as a marked change in temperature. The real difference between the two time intervals is a radical difference in the atmospheric circula-tion of the northern hemisphere. During the time interval between c ad­600 and 1400, North Atlantic winters were rarely characterised by extreme storminess and high rainfall, while sea ice cover was never as extensive as it was during parts of the LIA. The ice core data point to the most benign conditions of the MWP as having occurred during the 9th and 10th centuries ad, and it may not be a coincidence that this time interval coincided with the first European settlements of Greenland and Iceland. Similarly, the disappearance of Viking civi-lisation in Greenland during the first decades of the 1th century ad coincides not only with a dramatic increase in winter storminess (a phenomenon not witnessed by earlier Viking generations), but also by a dramatic increase in the extent of sea ice that may have greatly hindered navigation. Whatever the ages of specific coastal sediment units at Viking sites in the Outer Isles, the Vikings themselves are likely to have developed styles of land use that were unsuited to the stormy and wet winters that accom-panied the onset of the LIA.In respect of coastal change, it may be useful to envisage many coastal Viking settlements as having been buried as a result of LIA windstorm activity and sand drift. Thus, it may have been the case that many Viking settlements in the Outer Hebrides may have evolved in landscapes where coastal dunes were much rarer features than they are at present. It is recognised, however, that many areas of machair in the Outer Hebrides may already have been in existence prior to the MWP, having owed their origins to windstorm activity during earlier intervals of stormy conditions during the Holocene.  IMPLICATIONS FOR ARCHAEOLOGY3This paper has considered the age and origin of sand sediments enclosed within organic deposits at sites within the Scottish Outer Hebrides and has also discussed patterns of North Atlantic climate change for the last c 2,000 years. It is suggested that a chronology of past storminess is recorded in the coastal sand dune sequences along the Atlantic coastline of the Outer Hebrides. The morphological and stratigraphical evidence indicates a concentra-tion of windstorm activity during the LIA with a major reduction in sand drift during Medieval times between c ad­ 600 and 1400. The geological data indicate that windstorm deposition may have taken place during periods of climate deterioration and sea-ice expansion across the northern North Atlantic, associated with an increased thermal gradient favourable for increased production of winter storms. Dated inferred sand-drift episodes across the Atlantic seaboard of north-west Europe show a broad synchronicity with increased sand mobilisa-tion in eastern England, south-west Ireland and the Outer Hebrides, implying a regional response to a change in northern hemisphere atmospheric circu-lation (Lamb 199; Gilbertson et al 1999; Wintle et al 1998; Wilson et al 2001).6 CONCLUSION36Angus, S 1994 ‘The conservation importance of machair systems of the Scottish Islands, with particular reference to the Outer Hebrides’, in J M Baxter & M B Usher (eds) The Islands of Scotland: A Living Marine Heritage 95–120. HMSO, Edinburgh.Angus, S 1997 The Outer Hebrides: The Shaping of the Islands. The White Horse Press, Harris.Dawson, S; Smith, D E; Jordan, J & Dawson AG 2004 ‘Late Holocene coastal sand movements in the Outer Hebrides, NW Scotland’, Marine Geology 210, 281–306.Delaney, C & Devoy, R 199 ‘Evidence from sites in western Ireland of late Holocene changes in coastal environments’, Marine Geology 124, 273–87.Gilbertson, D D; Kemp, R A & Grattan, J (eds) 1996 The Outer Hebrides: The Last 14,000 Years. Sheffield Environmental and Archaeological Research Campaign in the Hebrides (SEARCH) Vol. 2, 27. Sheffield Academic Press, Sheffield.Gilbertson, D D; Schwenninger, J L; Kemp, R A & Rhodes, E J 1999 ‘Sand-drift and soil formation along an exposed North Atlantic coastline: 14,000 years of diverse geomorphological, climatic and human impacts’, Journal of Archaeological Science 26, 439–69.Hammer, C; Mayewski, P A; Peel, D & Stuiver, M (eds) 1997 ‘Greenland summit ice cores: Greenland ice sheet project 2 and Greenland ice core project’, Journal of Geophysical Research 102, 26, 315–886.Koch, L 1937 ‘The East Greenland ice’, Meddelelser om Grønland, Ud givne af Kommissionen for Videnskabelige Undersøgelser I Grønland, Bd 130, Nr 3. Lamb, H H 1971 Climate, History and the Modern World. Methuen Press, London.Lamb, H H 1979 ‘Climatic variations and changes in the wind and ocean circulation: the Little Ice Age in the North-east Atlantic’, Quaternary Research 11, 1–20.Lamb, H H 199 Climate, History and the Modern World. 2nd Edition, Routledge, London. Meeker, L D & Mayewski, P A 2002 ‘A 1,400-year high resolution record of atmospheric circulation over the North Atlantic and Asia’, The Holocene 12, 27–66.Orford, J D; Wilson, P; Wintle, A G; Knight, J & Braley, S 2000 ‘Holocene coastal dune initia-tion in Northumberland and Norfolk, eastern UK: climate and sea-level changes as possible forcing agents for dune initiation’, in I. Shennan & J. Andrews (eds) Holocene Land-Ocean Inter-action and Environmental Change around the North Sea, 197–217. Geological Society Special Publications 166, London.Ritchie, W 1979 ‘Machair chronology and devel-opment in the Uists and adjacent islands’, Proceedings of the Royal Society of Edinburgh 77, 107–122.Ritchie, W 198 ‘Inter-tidal and sub-tidal organic deposits and sea level changes in the Uists, Outer Hebrides’, Scottish Journal of Geology 21, 161–76.Ritchie, W & Whittington, G 1994 ‘Non-synchro-nous aeolian sand movements in the Uists: the evidence of the intertidal organic and sand deposits at Cladach Mor, North Uist’, Scottish Geographical Magazine 110, 40–46.Wilson, P; Orford, J D; Knight, J; Bradley, S M & Wintle, A G 2001 ‘Late Holocene (post-4,000 years bp) coastal development in Northumber-land, north-east England’, The Holocene 11, 215–29.Wintle, A G; Clarke, M L; Musson, F M; Orford, J D & Devoy, R J N 1998 ‘Luminescence dating of recent dune formation on Inch Spit, Dingle Bay, south-west Ireland’, The Holocene 8, 331–39.7 REFERENCES

North Atlantic climate change and Late Holocene windstorm activity in the Outer Hebrides, Scotland

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North Atlantic climate change and Late Holocene windstorm activity in the Outer Hebrides, Scotland

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