Morpho-sedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula,SW France |
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| Institution: | 1. DHI, Agern Allé 5, 2970 Hørsholm, Denmark;2. Technical University of Denmark, Department of Mechanical Engineering, Nils Koppels Allé, Bygn. 403, 2800 Kgs. Lyngby, Denmark |
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| Abstract: | The Medoc Peninsula is a triangular area of land between the Atlantic Ocean on the west and the Gironde estuary on the east. The Gironde, orientated SE-NW, is the largest estuary in France. On the Medoc Peninsula Holocene sediments cover a substratum of Tertiary limestones and Plio-Pleistocene fluvial terraces. The Gironde originated as an incised valley during the Weichselian glacio-eustatic fall (100 000–18 000 B.P.) and has acted as a sink for fine sediment throughout the Holocene sea level rise (SLR) (18 000 B.P. onwards). Conversely, the Atlantic littoral zone, characterised by sandy beaches and dunes, has been subjected to erosion throughout the Holocene transgression.The incised valley of the present Gironde was inundated by the sea approximately 10 000 B.P. At this time, the rate of SLR exceeded that of sediment supply, producing a large accommodation space in which transgressive tidal-estuarine muds and sands were deposited. As the rate of SLR decreased around 6000-4000 B.P., sedimentation became more pronounced and the available accommodation space began to decrease significantly. Landward-derived fluvial sediments began to prograde over the tidal muds and sands, and a first generation of salt marshes formed in the lateral valleys. Around 2575-1420 B.P., a sandy chenier ridge formed at the edge of a first generation of salt marshes and wholly or partly separated them from the Gironde. On the eastern side of the ridge a second generation of marshes began to form after 1200 B.P. Reclamation of the first and second generations of marshes occurred during the seventeenth and eighteenth centuries, respectively. On the estuarine side of the eighteenth century dyke the modern intertidal flats and salt marshes began to form after the eighteenth century. Sedimentation within the estuary decreased the accommodation space and led to the increased transport of sediment to the shelf after 2 000 B.P. This process was also aided by climatic and anthropogenic factors. Future evolution of the Gironde estuary is likely to consist of further marsh growth and chenier development. However, future increases in the rate of SLR, and the degree of storminess, may cause a shift to an erosional regime in parts of the lower estuary.According to previous work 25, 66], three different dune fields migrated landwards across the Atlantic littoral zone over the Holocene period. A field of isolated barchan dunes moved landwards over the Plio-Pleistocene fluvial terraces before 5100 B.P. From some time after this, until around 3000 B.P., a field of parabolic dunes was active. Finally, a barchan dune field was active from around 3000-2000 B.P. to the end of the eighteenth century/beginning of the nineteenth century, when the dunes were stabilised by pine plantations. It is tentatively suggested, using these dates and dune morphology, that dune formation was controlled by sand supply governed by the rate of SLR. However, the role of climatic changes such as aridity, storminess, windiness and the associated effects of vegetation cover, is also likely to be important. In the future the Atlantic coast is likely to continue to erode, although the extensive plantation of the aeolian dunes with pine forests is likely to prevent large-scale transgressive activity. |
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