Holocene coastal changes and infilling of the La Perroche marsh (French Atlantic coast)

Abstract

The La Perroche marsh is a small Holocene marsh on the southwestern coast of the Oleron island (Bay of Biscay, France) presently occupied by a freshwater swamp and separated from the ocean by a continuous sand dune ridge. The study of its infill shows first basal conglomeratic deposits with calcareous pebbles on a Cenomanian rough substrate. The Holocene infill since 6000 years BP begins with a very fast sedimentation of sands. Then, the sedimentation becomes finer, with lagoonal to freshwater influences recorded by ostracods. The distribution of these deposits inside the marsh is asymmetric, with a thick sandy-gravely layer in the eastern part and a succession of sands, clays and organic layers in the western part. Communication with the sea is episodic, and the sedimentation is controlled by an active channel, as shown by the presence of displaced coastal-marine faunas at some sites. Around 5000 years BP, a coastal marsh began to develop with high organic matter contents. These organic deposits presently outcrop on the beach, suggesting a more seaward location of the shoreline at that time. Generally, these sediments are azoic. When fauna is present, it indicates brackish to freshwater environments showing a progressive isolation of the system. A second phase of terrigenous sedimentation occurred after a transgressive event around 2800-2500 years BP, but the communication with the sea was then very reduced, as indicated by mainly freshwater faunas. This transgressive event appears to be synchronous with a positive sea-level tendency recorded, throughout northwestern European coasts, between 3000 and 2000 years BP. The definitive closing of the marsh occurred around 2100 years BP, after a second phase of freshwater marsh. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Résumé − Changements côtiers datant de l'Holocène et comblements du marais de La Perroche (côte atlantique française). Le marais holocène de La Perroche, localisé sur la côte sud-ouest de l'île d'Oléron (golfe de Gascogne, France), est actuellement un marais d'eau douce séparé du domaine océanique par un cordon dunaire continu. L'étude de son comblement au cours des 6 000 dernières années montre d'abord une invasion marine sur un substrat très accidenté d'âge cénomanien. Ensuite, se déposent probablement rapidement des sédiments sableux, témoins à la fois de l'établissement et du démantèlement de cordons dunaires et de l'érosion de l'arrière pays. Puis, la sédimentation devient plus fine et de type laguno-lacustre ainsi que le montrent les faunes d'ostracodes souvent associées aux sédiments. La distribution des dépôts dans le marais apparaît dés le début dissymétrique avec la présence d'épais niveaux sablo-graveleux à l'est et des successions de sables, d'argiles et de dépôts organiques à l'ouest. Les communications avec la mer sont épisodiques et le dépôt des sédiments est commandé par une chenalisation très active. Celle-ci est soulignée par la présence à proximité immédiate du chenal de faunes de type marin, le plus sou...

Holocene salt‐marsh sedimentary infilling and relative sea‐level changes in West Brittany (France) using foraminifera‐based transfer functions

“…a BP at an elevation of −3.80 m NGF (Goslin et al 2013 (Van Geel et al 1996;Barber et al 2003Barber et al , 2004Magny 2004;Dark 2006;Gandouin et al 2009;Charman 2010;Swindles et al 2013;Tisdall et al 2013), characterized by colder and wetter climatic conditions, with increased rainfall and largely enhanced storminess (Van Geel et al 1996;Clarke & Rendell 2009). Major disruptions of coastal sedimentary environments were also identified all along the European coasts, either in the English Channel region (Long & Hughes 1995;Billeaud et al 2009;Sorrel et al 2009;Lespez et al 2010;Tessier et al 2012) or along the Atlantic coasts Clavé et al 2001;Moura et al 2007;Sorrel et al 2009). Archaeological remains suggest less dense settlement patterns at the end of the Late Bronze Age (Coquillas 2001;Pailler et al 2011; and several abandonment of settlements, notably due to an important eolian event within the coastal areas of NW Europe (Guilcher & Hallégouët 1991;Tisdall et al 2013).…”

Late Holocene evolution of the southwestern Doñana National Park (Guadalquivir Estuary, SW Spain): a multivariate approach

“…5), each one constituted by species with analogous ecology (Carbonel, 1980 ;Clave ¤ et al, 2001) : (a) fresh to low brackish water assemblage : Cyclocypris laevis, Cypridopsis vidua, Cypris bispinosa, Herpetocypris chevreuxi, Ilyocypris gibba and Cyprinotus salinus ; (b) high brackish water assemblage : Cyprideis torosa (smooth ornamentation), Leptocythere castanea and Loxoconcha elliptica ; and (c) coastal, marine assemblage : Carinocythereis whitei, Neocytherideis subulata, Palmoconcha guttata, Palmoconcha laevata, Pontocythere elongata, Semicytherura sp. and Urocythereis oblonga.…”

Holocene sedimentary infilling of a tide-dominated estuarine mouth. The example of the macrotidal Seine estuary (NW France)

“…On the other hand, the period 3500-2500 yr B.P. is known all along the coasts of northern Europe as a period of climatic deterioration with severe coastal retreats and barrier destructions [Long et al, 1996;Régnault et al, 1996;Spencer et al, 1998;Clavé et al, 2001;Baeteman et al, 2002;Meurisse et al, 2005;Billeaud et al, 2010]. This hypothesis of a climatic crisis to explain the major environmental change recorded in the sedimentary infilling of the Seine mouth is preferred here, since the impact of this crisis on coastal areas has been widely documented, including already in the outer Seine Estuary [Sorrel et al, 2009].…”