Palaeo-Environmental Study Area P17 the Atlantic Coast, North Médoc, France

Palaeo-environmental Study Area P17 The Atlantic Coast, North Médoc, France PALAEO-ENVIRONMENTAL STUDY AREA P17 THE ATLANTIC COAST, NORTH MÉDOC, FRANCE Plate P17 Palaeo-environmental investigations on the shore near Soulac-sur-Mer, Atlantic Coast, France LOCATION Geographical description (Figure P17.1) The Aquitaine coast (Plate P17) is a 240 Km long low coast, bordering the Bay of Biscay in southwestern France. Between the mouth of the Gironde Estuary, in the north, and the mouth of the Adour river, in the south, this represents 5% (and the least protected part) of the total length of the French coastline. The coast is bordered inland by a forested complex of Holocene dune systems, isolating several coastal lakes. This long beach-dune sandy barrier is only interrupted by the inlet of the Arcachon lagoon and, towards the south, by the mouths of minor channels draining coastal lakes and know locally as "courants". The area of land containing the dunes is known as ‘’Les Landes de Gascogne’’. The distribution of dunes differs along the coast. In the north crescentric or barchan dunes are dominant, whilst parabolics occur in the centre and increase in importance towards the south. All the dunes formed under a predominant wind direction of 280 N. Historical records show that the rates of erosion vary along the Atlantic coast, ranging from 1,0- 2,0 meters per years in the northern part of the area (5-10 meters per years close to the Gironde mouth), to 0,2-1,0 meters per years in the southern section. 1.2 Coastal description The northern part of the Aquitaine Coast is the a continuous sandy coast, bordered by linear foredune which is maintained artificially by the Forestry Service (O.N.F-Office National des Forêts). 1 Palaeo-environmental Study Area P17 The Atlantic Coast, North Médoc, France The Gironde coast is formed by two linear sedimentary cells, of differents orientations. Between the Cape of Pointe de La Négade and the Pointe de Grave, the orientation of the coast is N55°; between Pointe de La Négade and the Cap Ferret spit, the coast is orientated approximately N- S (N7°). We focussed our studies on the north of the Atlantic Coast between Pointe de Graves and the Hourtin lake (Figure P17.1). 1.3 Regional Authority Conseil Régional d' Aquitaine, Conseil General de Gironde. Towncouncil of: Le Verdon-sur- Mer, Soulac-sur-Mer, Grayan-et-L'Hopital, Vensac, Vendays-Montalivet, Naujac-sur-Mer and Hourtin. 1.4 Designation (protected areas) IGN map, 25 000e, 1433 OUEST and 1434 OUEST. 2. MODERN HUMAN GEOGRAPHY The french Aquitain coast is one of the Europe's less populated regions (348,000 people). However, as a tourist attraction there is an average increase of the population of more than 150% during July and August : some of the smaller town, such as Soulac-sur-Mer, can see their population multiplied by 10. The littoral zone is strewn with isolated bathing stations most often seetled on the foredune; consequently, these stations are especially sensitive to changes in the shoreline position. The cities surrounding this study area are seaside resorts. Their winter population are respectively: Le Verdon-sur-Mer, 1 274; Soulac-sur-Mer, 2 720; Grayan-et-L'Hopital, 728; Vensac, 694; Montalivet, 1 827; Naujac-sur-Mer, 631; Hourtin, 2 323. 3. CONTEMPORARY COASTAL PROBLEMS: (DUNE MOVEMENT, EROSION, ACCRETION; I.E. COASTAL CONTEXT) (Appendix P17.2) The macrotidal (2-5 m range) Gironde coast, situated to the south of the Gironde estuary, is dominated by N-W swell. The predominent cape Pointe de la Négade separated two linear sections of this coastline, with different orientations and different dynamic settings: a northern section, dominated by a maximum resultant northerly longshore drift (4000,000m3 /yr); and the southern section by a 630,000 m3 /yr southerly drift. Presently, sediment movements along the Gironde coast are considered to be the results of the combined action of: 1. longshore drift removing sediment and transporting it alongshore 2. aeolian transport 3. onshore-offshore exchange, between the shore and the nearshore zone and connected with the sight increase of sea level. A synthesis of numerous studies allows erosion over (at least) the past 37 years to be estimated; for certain sections for the past 200 years. The average rate of erosion decreases on both side of the Pointe de La Négade, where it reached the maximum between 1957-1994 (7.3 m/yr). The northern section, protected by dykes and sea defences between the estuary mounth at Pointe de La Négade and Soulac, has been stabilised. Today, only the limited area of the Huttes is not protected and is subjected to erosion of 9.6 m/yr; it is an area of great concern. To the south, the average rate of erosion decrease rapidly-it reached 0.4 m/yr in Montalivet-and then was relatively constant around 1 m/yr as far as the Grang-Crohot. The 2 Palaeo-environmental Study Area P17 The Atlantic Coast, North Médoc, France extremity of the Cap Ferret spit has been generally accreting to seawards; it also has extended to the south, for the past 200 yeras. Since 1970, the tip of the spit has undergone erosion. Erosion of the Gironde coast results from a sedimentary budget in deficit by some 350,000 m3/yr escarping northward (the difference between the northely longshore drift (400,000 m3/yr) and 50,000 m3/yr retained in the system by the process of erosion/accretion of the Huttes beach and St Nicolas sandbank) and 630,000 m3/yr transported toward the south. Aeolian loss appears minimal and offshore loss, due to relative sea-level rise, is estimated to be between 600,000 and 900,000 m3/yr. The system receives no fluvial supply of sand and the primary source of sediment originates from coastal erosion, mobilised by longshore drift. Total sediment loss for the Gironde coast ranges between 1,580,000 and 1,880,000 m3/yr; this explains the average erosion rate, of between 1 and 2 m/yr. Whilst the longshore transport is quite well known, the onshore/offshore movements, especially those caused by the present-day sea-level rise and/or aeolian action, are still poorly understood and require further investigation. 4. KNOWLEDGE, HISTORY, ARCHAEOLOGICAL AND PALAEO-ENVIRONMENTAL SETTING OF THE COASTAL COMMUNITY 4.1 Sedimentological and Palaeo-environmental data concerning the study area Along the Aquitaine coast aeolian sand dunes cover an area from 0.2 to 10 km wide, and enclose several large coastal lakes (Figure P17.1). The only previous stratigraphic and cartographic work on the sandy dune system in this region was carried out by the B.R.G.M. (Bureau de Recherches Géologiques et Minières - Marionnaud, 1972; Dubreuilh and Marionnaud, 1973). They divided the dune complex into four systems on the basis of morphology. These generations are orientated in a linear pattern along the coast with the oldest located furthest inland and the youngest near the present littoral zone (Figures P17.2 and P17.3). Assuming that the dunes stretched beneath each other to the coast where they were exposed as overlying sand units separated by paleosols, in which only three dates were available: 5100, 3000 and 2300 B.P.. Dubreuilh (1971) suggested the age of the dunes from east to west as follow: (i) Meso- to Neolithic dune system (Boreal to Atlantic) constituted by isolated barchans or barchanoid ridges. This dune system disappears just to the north of Hourtin lake and is absent to the south of the Medoc peninsula. The system was considered to be older than 5100 B.P. based on a date obtained from a peat, outcropping on the present day coast beneath the foredune, considering it as a "lateral equivalent" of the base of the dune system (Dubreuilh, 1971). (ii) Proto-historic dune system from Copper Age to Bronze Age (Subboreal to the early Sub- Atlantic) constituted by parabolic dunes and interpreted as moving between 5100 and 3000 B.P.. (iii) Historic dune system, constituted by barchanoid dunes, found only in localised areas in north Medoc near Soulac and to the south seaward of the lake of Hourtin. This system was interpreted as a sand sheet formed between 3000 and 2300 B.P., and covered by dunes moving from this time until the 19th century. (iv) Present day littoral dunes, forming an artificial barrier several hundred metres wide along the coast. These were established in the middle of the last century to protect the recently planted pine forests further inland. The morpho-chronology of Dubreuilh (1971), Marionnaud (1972), and Dubreuilh and Marionnaud (1973) although apparently rigorous, shows a number of problems when examined more closely. Firstly, the authors do not explain the east-west succession of parabolic-barchan- parabolic-foredune forms found to the west of Hourtin lake. Secondly, to the north of Hourtin Lake, similar barchanoid dunes are interpreted as belonging to two different generations: Meso- Neolithic and Historic. Finally, the cartography and morphological description of Marionnaud (1972) and Dubreuilh and Marionnaud (1973) does not correspond with old maps and historical 3 Palaeo-environmental Study Area P17 The Atlantic Coast, North Médoc, France records (Masse, 1707; Belleyne, 1762; Cassini, 1798; Buffault, 1942; Bressolier et al., 1990; Froidefond and Prud'homme, 1991; Barrere, 1992). These records show that dunes in many areas in north Medoc were classified as being active before 5100 B.P. or from 5100 to 2300 B.P. (Marionnaud, 1972; Dubreuilh and Marionnaud, 1973), but were in fact mobile during the last century. Additionally, the historical evidence shows the existence of a first generation of parabolic dunes stabilised before the last century.

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