Leverhulme Trust

The Leverhulme Trust Research Fellowship:

North Atlantic Oscillation impacts on sand drift 


Abstract
Variations in the state and intensity of the North Atlantic Oscillation (NAO) control the location and strength of westerly storm tracks moving across the Atlantic and into Europe. In historic times, persistent and intense storminess affecting western European coasts caused periods of sand invasion resulting in village abandonment and problems for agriculture. This research seeks to apply absolute dating methods to coastal dunes in Portugal and France to resolve past episodes of sand invasion in order to establish a record of changes in the state of the North Atlantic Oscillation over the last 10,000 years.


Research Outline
Dunefields are conspicuous features of many locations along the Atlantic coasts of western Europe. Within these Atlantic dune systems there is sedimentary evidence for episodic sand mobilisation spanning the Holocene period (the last 10,000 years or ‘10ka’), alternating with periods of stability characterised by intercalated peat or soil development and forest growth (Clarke et al., 2002; Wilson et al., 2005). Historical records deriving from the 17th and 18th centuries from coastal areas across Europe, describe persistent storminess driving significant sand invasion causing problems for human settlement and agriculture (Lamb & Frydendahl, 1991). In Denmark, France and England, archival accounts testify to villages, churches and estate buildings being buried by dunes and abandoned (Lamb, 1995).

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Within the last decade, the importance of the North Atlantic Oscillation (NAO) in driving the location of westerly storm tracks within the Atlantic Basin (Figure 1), and its dominant role in controlling the nature of European climate, has been increasingly recognized. Sand invasion across Europe is known to have occurred during the cooler period of the Little Ice Age. Despite our increased understanding of the role of the NAO in driving changes in storm track locations and intensities (Dawson et al., 2002), the spatial coherence of NAO impacts within the European sedimentary record has only recently been investigated (Clarke & Rendell, 2006). Research by the author (Clarke & Rendell, 2006) has shown that sand invasion during historical times in dunefields of southern France and western Portugal was asynchronous, apparently controlled by the status of the NAO. This fellowship proposal seeks to build on these new findings through identifying whether sand invasion throughout the pre-historical period of the Holocene is also asynchronous in these French and Portuguese dunefields, driven by changes in the status of the NAO. In particular, this work seeks to evaluate whether the abrupt, millennial-scale shifts to a cooler and more disturbed climate found in ice core and ocean records at 1.4ka, 2.8ka, 4.2ka, 5.9ka (Bond et al., 1997), impacted synchronously upon dune building on Atlantic coasts or whether an advance in sea-ice strengthened the persistence of one state of the NAO, with consequent asynchronous sand invasion.

NAO

Figure 1. North Atlantic Oscillation: (a) NAO+ enhanced westerlies bring rain (dashed shading) to the Britain and Scandanavia, while enhaced tradewinds drive Saharan dust plumes across the Atlantic to the Caribbean; (b) NAO- reduced strength westeries and tradewinds, rain (dashed shading) over the Iberian Peninsula and western Mediterranean.[from Clarke & Rendell, 2006]


Historical records of the NAO derive from archive sources, instrumental data (Luterbacher et al., 2002) and tree-rings (Glueck & Stockton, 2001) and span the period from the present back to 1429 years AD. At present, little is known about the state of the NAO prior to this time period. Given the sensitivity of coastal dunes to changes in location of westerly storm tracks, these dunes provide an aeolian sedimentary record of NAO status. Beyond the historical period, dunes in France were emplaced 3.3-3.6ka (Clarke et al., 2003) and Portugal 1.4ka, 2.2ka, 8.2 and 9.1ka (Clarke & Rendell, 2006). Given the apparent sensitivity of coastal dunefields to Atlantic storminess, a systematic and detailed sampling and dating programme can provide unique insights into the state and dominance of the NAO during the Holocene period. This research aims to use optically-stimulated luminescence (OSL), in comparison with radiocarbon (14C) techniques, where applicable, to determine (a) the timing of Holocene sand drift and dune building episodes and (b) periods of land surface stability associated with soil development, in order to provide a regional chronology for Atlantic storminess and dune building. Comparison of chronologies between sites will allow the persistence and intensity of the NAO to be evaluated for this period of Holocene. OSL techniques are applicable to aeolian sands and provide a date for the timing of sand grain burial within a dune or sand sheet (Aitken, 1998) whereas 14C can be applied to carbon extracted from peat or buried soils. The coastal fringes of both Portugal and Aquitaine contain undated mid-early Holocene sands and buried soils (Clarke et al., 2002; Clarke & Rendell, 2006). This project seeks to exploit this sedimentary archive to better understand Atlantic climate changes throughout the Holocene period.

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Photographic diaries of fieldwork undertaken for this project are linked here: Portugal in July 2006 and France in January 2007.

References
Aitken, M.J. (1998) An Introduction to Optical Dating. Oxford University Press

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. (1997) A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278, 1257–66.

Clarke, M.L. & Rendell, H.M. (2006) The effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal. The Holocene, 16 (3), 341-355.

Clarke, M.L., Rendell, H.M., Tastet, J-P., Clavé, B. & Massé, L. (2002) Late Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France, The Holocene, 12, 231-238.

Dawson, A.G., Hickey, T., Holt, T., Elliot, L., Dawson, S., Foster, I.D.L., Wadhams, P., Jonsdottir, I., Wilkinson, J., McKenna, J., Davis, N.R., Smith, D.E. (2002) Complex North Atlantic Oscillation (NAO) Index signal of historic North Atlantic storm-track changes. The Holocene, 12, 363-369.

Glueck, M.F. & Stockton, G.W. (2001) Reconstruction of the North Atlantic Oscillation, 1429-1983. International Journal of Climatology, 21, 1453-1465.

Lamb, H.H. (1995) Climate, History and the Modern World. 2nd edition, Routledge, London.

Lamb, H.H. & Frydendahl, K. (1991) Historic Storms of the North Sea, British Isles and northwest Europe, Cambridge University Press.

Luterbacher, J., Xoplaki, E., Dietrich, D., Jones, P.D., Davies, T.D., Portis, D., Gonzalez-Rouco, J.F., von Storch, H., Gyalistras, D., Casty, C. and Wanner, H. (2002) Extending North Atlantic Oscillation reconstructions back to 1500. Atmospheric Science Letters 2, 114-124.

Wilson, P., McGourty, J & Bateman, M.D. (2005) Mid- to late-Holocene coastal dune event stratigraphy for the northeast coast of Northern Ireland. The Holocene, 14, 406-416.