Littoral lakes along the Atlantic shorelines of Southern Brittany, France: Archives of sea level variations and storm events during Holocene times
Résumé
Littoral lakes are characteristic of the Atlantic shorelines of Brittany, western France. These lakes are generally small and shallow and are separated from the Atlantic Ocean by dune systems. In certain areas of the littoral, these lakes alternate with small rias that are known to have been open toward the sea for at least the last centuries. In Morbihan, southern Brittany, a legend indicates that during the 5th century, a princess of Wales known as Princess Ninok escaped with her vessel and escort to a major storm by mooring along the shoreline of the "Lannénec Pond", that is today a small freshwater littoral lake with no link with the sea. Cores have been obtained from this lake as well as from a nearby ria know as the "Loc'h". A 6 m long core (LA-1) was obtained from the Lannénec Pond, covering, while a 5 m long core (LO-1) was obtained from the Loc'h, both covering the last 6000 years. Sedimentological and micropaleontological studies have been conducted on these two cores, with peculiar attention concerning marine and freshwater microfauna and microflora. Foraminifer and dinoflagellate associations found in the LA-1 core clearly demonstrate that the Lannénec Pond was connected with the open ocean several times during the last 6000 years, alternating with periods of closed, freshwater lake separated from the ocean by a dune system. The chronology obtained on the LA-1 core shows that open sea conditions existed during the 5th century, thus permitting Princess Ninnok's vessel to moor along the Lannénec shoreline. On the contrary, micropaleontological data obtained for the LO-1 core show that marine conditions prevailed all along the last 6000 years, evolving from almost dry sea-swamp environment to open sea. No lacustrine conditions have been identified on this core. Alternating phases of lacustrine and open sea environments in the Lannénec Pond may relate to sea level fluctuations during the last 6000 years. Nevertheless, such sea level changes may also have affected the nearby Loc'h, thus resulting in establishing temporary lacustrine conditions in this area. Considering the present-day hydrodynamics along this part of the Morbihan shoreline, major storm events may induce huge sediment transport along the shoreline, resulting in possible damming of estuaries. Such conditions may have prevailed in the Lannénec area, while the Loc'h area appears to be preserved from such sediment transport. Storm frequency along the Morbihan coastline can be established from high resolution studies of littoral lakes.