Coastal Change in Tropical Overseas and Temperate Metropolitan France Inferred from a National Monitoring Network: A Summary from the Current Special Issue

International audienceThis paper provides an overview of the papers published in this Special Issue « Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France » of Journal of Coastal Research. This special issue reflects the large diversity of mainland and tropical French coast and highlights the complex, site-and timescale-specific, combination of factors driving coastal evolution. Many contributions reveal a strong climate control on storm wave activity and, in turn, coastal response. In some contributions, the inherited geology and anthropogenic factors clearly appear affect coastal change. This special issue emphasises the need to monitor the coast combining different means to improve our understanding and predicting capacities of the natural variability of coastal response in a changing climate

Incised-valley morphologies and sedimentary-fills within the inner shelf of the northern Bay of Biscay

This study is a first synthesis focused on incised-valleys located within the inner shelf of the Bay of Biscay. It is based on previously published results obtained during recent seismic surveys and coring campaigns. The morphology of the valleys appears to be strongly controlled by tectonics and lithology. The Pleistocene sedimentary cover of the shelf is very thin and discontinuous with a maximum thickness ranging between 30 and 40 m in incised-valley fills. Thus the incised bedrock morphology plays a key-role by controlling hydrodynamics and related sediment transport and deposition that explains some variations of those incised-valley fills with respect to the previously published general models

Archaeal Methane Cycling Communities Associated with Gassy Subsurface Sediments of Marennes-Oléron Bay (France)

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2009/publication-6165.pdfInternational audienceIn Marennes-Oleacuteron Bay, a macro-tidal bay located on the French Atlantic coast, kilometer-scale acoustic turbidity reveals an accumulation of free gas in the sediment. Large concentrations of organic matter and rapid sedimentation rates provide ideal settings for biogenic methane cycling. We integrate seismic, sedimentologic, biogeochemical and molecular genetic approaches to determine whether microbial methane cycling is involved in this process. Here we show that the acoustic turbidity upper boundary matched with X-ray facies displaying fissures with the highest methane concentrations, demonstrating the existence of methane bubbles in the sediment. 16S rRNA and mcrA gene clone libraries were dominated by sequences affiliated to the three known ANME lineages and to putative methanogens. Sequences related to the marine benthic group B (MBG-B) and miscellaneous crenarchaeotal group (MCG) were also detected. However, the highest methane concentration facies was the only section where active Archaea were detected, using reverse-transcribed rRNA, indicating that these communities were involved either directly or indirectly in the methane cycling process. Moreover, three metabolically active novel uncultivated lineages, related to putative methane cycling Archaea, could be specifically associated to these methane bearing sediments. As methane cycling Archaea are commonly retrieved from deep subseafloor and methane seep sediment, the study of coastal gassy sediments, could therefore help to define the biogeochemical habitats of deep biosphere communities

Enregistrements des changements environnementaux dans les sédiments littoraux (cas des pertuis charentais et du bassin d'Arcachon)

Cette thèse avait pour objectif d évaluer le potentiel des sédiments littoraux à enregistrer les changements environnementaux. Trois sites d études ont été choisis sur la côte atlantique française: la flèche sableuse de la Pointe d Arçay pour l enregistrement du climat de la houle ; la Baie de Marennes-Oléron et le Bassin d Arcachon pour la discrimination des nombreux forçages contrôlant le comblement des estuaires. La Pointe d Arçay est une flèche sableuse à crochons qui présente un taux d allongement moyen de 26 m/an. Les approches géomorphologiques et de modélisation de la houle depuis 1979 ont mis en évidence que les houles les plus énergétiques favoriseraient un fort allongement de la flèche alors que les houles moins énergétiques favoriseraient un faible gain sédimentaire et un recourbement de l extrémité de la flèche. Ce mécanisme semble expliquer le développement successif des crochons le long de la flèche depuis la deuxième moitié du 20ème siècle. La Baie de Marennes-Oléron et du Bassin d Arcachon ont été analysés séparément, puis comparés, grâce à deux approches complémentaires, la stratigraphie (sismique THR et carottages) et les évolutions morphologiques séculaires. Le comblement de la Baie de Marennes-Oléron est rapide depuis le 19ème siècle et est complexe depuis 8 000 ans BP en raison des multiples forçages qui le contrôlent. Le comblement du Bassin d Arcachon a un budget équilibré depuis le 19ème siècle et semble principalement contrôlé par sa fermeture partielle depuis 2500 ans BP. Les nombreuses différences observées entre ces deux environnements estuariens montrent les importances relatives des forçages locaux comme la morphologie du substratum, la marée, la houle, les apports sédimentaires et l impact de l homme.The purpose of this PhD thesis was to evaluate the potential of coastal sediment in recording environmental changes. With this aim in view, three coastal environments have been chosen along the French Atlantic coast: the Arçay sandspit for past wave climate record; the Marennes-Oléron Bay and Arcachon Lagoon to decipher among the multiple forcing parameters controlling estuaries sediment-fill. The Arçay Spit is a compound spit displaying an elongation mean rate of 26 m.yr-1. Morphological and wave modeling approaches since 1979, show that energetic swells seem to cause massive sand accumulation and spit elongation, whereas less energetic swells appear to be responsible for small sand accumulation and spit curvature. Such mechanism seems to be responsible of the successive hook ridges accretion observed along the Arçay Spit since the second half of the 20th century. The Marennes-Oléron Bay and Arcachon Lagoon have been successively analyzed and compared following a combination of VHR seismic, sediment cores and historical and present-day bathymetries. The sediment-fill of the Marennes-Oléron Bay is rapid since the 19 century and complex since 8000 yrs BP, due to multiple forcing parameters. The sediment-fill of the Arachon Lagoon displays a balanced budget since the 19th century and seems to be mainly controlled by the partial closure of this lagoon since 2500 yrs BP. Numerous differences observed in those two estuarine environments give insight on relative influence of local forcing parameters including bedrock morphology, tide, waves, sediment supply and human impact.LA ROCHELLE-BU (173002101) / SudocSudocFranceF

Wave control on the rhythmic development of a wide estuary mouth sandbank: A process-based modelling study

International audienceThis study presents the numerical hindcast of the morphological changes and stratigraphy of a wide estuary mouth sandbank located along the French Atlantic coast (Marennes-Oléron Bay), and primarily investigates the respective contribution of tide and waves to its pluri-decadal evolution. Firstly, the morphodynamic hindcast appears strongly improved when tide and waves are considered in the forcing rather than only tide. Secondly, the modelling results suggest that the strong seasonal variation of the wave climate in this area explains the rhythmic lateral accretion of the bank (i.e. normal to the currents main direction) observed in seismic reflection profiles. The grain size variation in the area of sediment accretion is also characterized by a seasonal cycle. Finally, repetitive bathymetric surveys and seismic profiles revealed that the lateral accretion of the bank is associated with about one seismic reflector per year, which is in agreement with our modelling results when suggesting that these seismic reflectors are due to the seasonal variation of the grain size. These new results may be considered for the morphological evolution and the stratigraphy of other tidal sandbanks potentially affected by waves

Stratigraphic records and variability of incised valleys and estuaries along French coasts

International audienceSome of the striking results of the papers published in the special publication "French Incised valleys, estuaries and lagoons" of the Bulletin de la Société géologique de France are presented and compared. The selection of papers within this volume focuses exclusively on the recent progress made on modern French incised valleys, estuaries and lagoons around the coasts of France. Those papers together with abundant papers recently published on modern French incised valleys provide new insights for the knowledge on these sedimentary systems. The large amount of new results obtained is indebted to an extensive exploration within a large variety of estuaries, lagoons and coastal areas, from macrotidal tide-dominated, to microtidal wave-dominated, with also meso- to macrotidal mixed tide- and wave-dominated estuaries. These data allow comparing incised valleys within the same setting of tectonically stable and sediment starved margins, but showing contrasted conditions of hydrodynamics, sediment supply and bedrock control. At a stratigraphic level, sea-level variation is the main parameter controlling incised valley formation and sediment fill. The first-order controlling factor explaining the observed variations in valley fills is hydrodynamics. Three valley-fill categories are highlighted: tide-dominated, mixed tide-and-wave and wave-dominated, that match the classification based on hydrodynamics and morphology of present-day estuaries or lagoons. The second-order controlling factor explaining the observed variations in valley fills is the antecedent morphology of the bedrock, which in turn controls hydrodynamics and sediment supply. Finally, a promising result is the demonstration of the potential of incised valley fills to record high frequency environmental changes related to climate events and human activities

Hypothetical Influence of Bacterial Communities on the Transfer of 14C-Depleted Carbon to Infaunal Foraminifera: Implications for Radiocarbon Dating in Coastal Environments

(IF 1.53 [2018]; Q1)International audienceLittle is known about the potential complications that may arise from the use of coastal foraminifera for radiocarbon (14C) dating. The aim of this study is to report the fortuitous finding of 14C-dated Haynesina germanica individuals picked from two sediment cores (Pertuis Charentais, France), which appeared 2500–2000 years older than their expected age of deposition. Stratigraphical and micropaleontogical evidence have ruled out the possible effect of reworking of calcareous tests from previous strata. Similar anomalous 14C ages were obtained on abundant lignocellulose debris recovered from the cores, which are supplied by rivers flowing into the study area. Given that H. germanica is an infaunal species, we hypothesize that in-situ living individuals acquired the 14C-depleted isotopic signature of lignocellulose debris within the sediment prior to definitive burial, following an unexplored pathway of carbon transfer between the two compartments. Based on the literature, we propose a plausible explanation, which involves bacterial communities living in the study area. This putative role of bacteria may have considerable importance for past and future studies of Holocene environmental changes in coastal environments. Further work is now needed to explore this hypothesis with more robust, direct evidence based on comprehensive geochemical, geochronological and microbiological studies