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Low-lying coasts and small islands, such as in the Lesser Antilles, are particularly vulnerable to hurricane-induced marine floods. In September 2017, Category 5 Hurricane Irma, with winds up to 360 km/h, hit the northern Caribbean islands and caused the destruction of 95 % of the structures on Barbuda Island. We investigated the geomorphological impacts and the sedimentological record of this hurricane in Barbuda’s Codrington Lagoon. Following Hurricane Irma, two wide inlets developed across the Codrington sandy barrier. One of these inlets enlarged and was still opened four years later. From available data, it seems that this barrier remained continuous for the last 250 years before Hurricane Irma. At a longer time scale, very high-resolution seismic exploration combined with sediment cores sampled in Codrington Lagoon were used to investigate Irma deposit and environmental changes for the last 3700 years. The evolution from a low-energy small and shallow lagoon to the modern wide and high-energy lagoon recorded by the lagoon sediment-fill was related to both long-term sea-level rise and subsidence. The top of the lagoon fill consists of a thick and extensive sand sheet recording an abrupt increase in energy. Given its location at the top of the cores, its very recent age supported by short-lived radionuclides data, together with large inlets opening and barrier erosion after Irma implying a large sand supply to the lagoon, this sand sheet was attributed to Hurricane Irma. From our cores, it appears that this deposit is unique over more than 3700 years. Both the opening of a new inlet and the thick upper sand sheet support the exceptional character of Irma at the scale of centuries to millennia. Our study reinforces the idea that Hurricane Irma was exceptional in terms of intensity and may be associated with global warming.

International audience; Extensive bathymetric and two-dimensional seismic surveys have been carried out and cores collected in Pago Pago Bay (Tutuila, American Samoa) in order to describe and gain a better understanding of the sediment fill of the bay, which was affected by the 2009 South Pacific Tsunami. Eight sedimentary units were identified over the volcanic bedrock. The basal transgressive unit displays retrograding onlaps towards the shore, whereas the overlying seven aggradational layers alternate between four draping units and three pinching out seaward units. 'Core to seismic' correlation reveals that draping units are composed of homogeneous silts, while pinching out units are dominated by very coarse coral fragments showing fresh cuts, mixed with Halimeda plates. The basal unit is attributed to transgressive sedimentation in response to flooding of the bay after the last glacial maximum, followed by the upper aggrada-tional units corresponding to highstand sedimentation. The changeovers in these upper units indicate an alternation between low-energy silt units and high-energy coral debris units interpreted as tsunami-induced deposits. The 14 C dating reveals that high-energy sedimentation units can last up to approximately 2000 years while low-energy sedimentation units can last up to approximately 1000 years. This alternation, deposited during the last high-stand, may be explained by cycles of tectonic activity and quiescence of the Tonga Trench subduction, which is the main source of tsunamigenic earthquakes impacting the Samoan archipelago. In the uppermost silt unit, only the geochemical signature of the terrestrial input of the 2009 SPT backwash deposits was detected between 7 cm and 9 cm depth. Hence, Pago Pago Bay offers a unique sediment record of Holocene bay-fill under the impact of past tsunamis intermittently during the last 7000 years.

Throughout the last 20 ka, tropical Africa was the site of significant hydrological changes related to variations in the intensity of the African monsoon. Recent studies conducted in lake and deltaic sedimentary records suggest that gradual long-term monsoon humid oscillations (African Humid Period ~14 – ~6 ka) were punctuated by centennial-scale episodes of hyperaridity (HS1, YD, 8.2 ka, 4.2 ka). These abrupt hydrological changes could have caused drastic transformations in landscape patterns, providing favourable ecosystems, but also produced arid environments that may have restricted human exploitation of the landscape.The Abhe Lake basin in the Central Afar region (Ethiopia & Djibouti) is the endorheic receptacle of freshwater and terrigenous inputs from the Ethiopian Highlands, and is a hydro-sedimentary system sensitive to hydroclimatic change. Since 2014, in the context of the VAPOR-Afar and PSPCA programs (in Ethiopia and Djibouti respectively), we have intensified scientific investigations over this basin with the objective of exploring the evolution of Holocene hydroclimatic change, its impact on landscapes and on prehistoric human behaviour during one of the most forcible transitions in human history, that from a dominantly hunter-gathering way of life to the food production societies one.We present the first results of this research that combines paleoclimatological, geomorphological and geoarchaeological studies based on a new set of 14C datations on two lacustrine cores, on several morpho-sedimentary outcrops and on 8 new archaeological sites spanning the Early to Late Holocene. Our results allow us to: a) refine the temporal occurrence and the modalities of the African Humid Period and of the 8.2 and 4.2 ka hyperarid episodes, coupling paleolake-level reconstitution and lacustrine sediment analysis (paleolimnology, geochemistry); b) evaluate the impact of these hydroclimatic oscillations on perilacustrine sedimentary formation processes, especially for pedogenic formations; c) and better understand the relationship between environmental change and the development of one of the most important economic and cultural innovations, that of domestication.

Throughout the last 14 ka, tropical Africa experienced significant hydrological changes that were mainly driven by the orbital precession cycle, which controls the intensity of the African monsoon. Recent studies conducted in lake and deltaic sedimentary records suggest that long-term monsoon humid oscillations (African Humid Period / AHP ~14 – ~6 ka) were punctuated by centennial-scale episodes of hyperaridity. However, the abrupt or gradual aridification modalities since the end of the AHP and the modalities of the centennial-scale episodes, as well as their impacts on past and current environments are still debated.The Lake Abhe basin in the Central Afar region (Ethiopia & Djibouti) is the endorheic receptacle of freshwater originating in the Ethiopian Highlands, and represents a hydro-sedimentary system sensitive to hydro-climatic changes in East Africa. Today it is characterized by residual lakes (Gamari and Afambo lakes) and a hyper-arid climate, while during the AHP, the Abhe basin was occupied by a Mega-lake and by humid environmental conditions. Holocene climatic disruptions drastically changed the landscapes and ways of life along this basin. The aim of this study is to describe, interpret and estimate the impact of hydro-climatic oscillations on the evolution of Lake Abhe’s littoral lacustrine environments and palaeolandscapes since the AHP from different viewpoints.Indeed, this research combines paleoclimatological and geomorphological studies based on a new set of 14C ages on two lacustrine cores and on several morpho-sedimentary outcrops spanning the Early to Late Holocene. Our results allow us to: a) refine the temporal occurrence and the hydrological modalities of the AHP including short-term arid episodes linked to Younger Dryas and 8.2 ka North Atlantic events; b) recognise some paleo-shoreline geomorphic features linked to lake level fluctuations, as well as the development littoral pedological horizons and the activation/shutdown of the perilacustrine fluvial network during humid and arid events; c) track these changes until the present day, and discuss their evolution scenario in the near future.Comparing with other regional climatic records, we show how Lake Abhe basin was highly reactive to East African monsoonal regimes, and how current hydrological changes could impact its environments.

Following recent tsunamis, most studies have focused on the onshore deposits, while the offshore backwash deposits, crucial for a better understanding of the hydrodynamic processes during such events and offering an opportunity for sedimentary archives of past tsunamis, have mostly been omitted. Here, we present a unique sedimentary record of the backwash from two historical tsunamis sampled in a sheltered bay in American Samoa, namely the 2009 South Pacific Tsunami and the 1960 Great Chilean Earthquake Tsunami. Although not always concomitant with a marked grain size change, backwash deposits are identified by terrestrial geochemical and mineralogical signatures, associated with basal soft sediment micro-deformations. These micro-deformations, including asymmetric flame structures, are described for the first time in historic shallow marine backwash deposits and lead us to propose an improved depositional mechanism for tsunami backflow based on hyperpycnal currents. Moreover, this study brings a potential new criterion to the proxy toolkit for identifying tsunami backwash deposits, namely the basal soft sediment micro-deformations. We suggest that further studies focus on these micro-deformations in order to test the representability of this criterion for tsunami backwash deposits. Sheltered shallow marine environments in areas repeatedly impacted by tsunamis have a higher potential for the reconstruction of paleo-tsunami catalogs and should be preferentially investigated for coastal risk assessment.

Little 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.

International audience; The Lesser Antilles are a densely populated region where local populations and industrial facilities are concentrated at the coastlines, and are therefore exposed to many rapid-onset hazards such as hurricanes and tsunamis. However, the historical catalog of these events is too short to allow risk assessment and return period estimations, and it needs to be completed with long-term records of washover deposits in coastal sedimentary environments such as lagoons. In this study, two sediment cores were taken in March 2018 in a small coastal lagoon on Scrub Island (northeastern Caribbean). Sedimentological, geochemical, microfaunal and chronological analyses enabled us to identify 25 sandy layers resulting from high-energy-marine floods. Two of these layers were interpreted as tsunami deposits based on sedimentological (rip-up clast of the underlying cohesive substrate and internal mud laminae), and geochemical evidence. The most recent deposit is associated with the transatlantic tsunami triggered by the 1755 CE Lisbon earthquake. The older one is the thickest sandy layer recorded in the lagoon, with an age range of 1364-1469 cal. CE, as determined using 14 C dating. This event was recorded in sedimentary archives of both the northern and the southern part of the Caribbean, with its large spatial extent, supporting a distant tsunamigenic origin. The 23 remaining sandy layers were interpreted as storm deposits, based on sedimentological and chronological data, with the three most recent layers being correlated with historical hurricanes. This new 1600 year-long record displays similarities with that of the Bahamas, with the periods of intense hurricane activity being in antiphase with those of the northeastern US coast. This regional comparison may provide evidence for a latitudinal forcing of hurricane tracks through time in relation to climate fluctuations.

International audience; Marine and estuarine sediment records reporting impacts of historical land use changes exist worldwide, but they are rarely supported by direct quantified evidence of changes in denudation rates on the related catchments. Here we implement a spatially-resolved RUSLE soil erosion model on the 10 000 km2 Charente catchment (France), supplied with realistic scenarios of land-cover and climate changes since 1500, and compare the results to a 14C-dated estuarine sediment record. Despite approximations, the model correctly predicts present-day Charente river sediment load. Back-cast modelling suggests that the Charente catchment is an interesting case where the sediment supply did not change despite increase in soil erosion resulting from 18th-century deforestation because it was mitigated by drier climate during the same period. Silt-sand alternations evidenced in the sediment record were correlated with sub-decadal rainfall variability.

A dataset of 15 geo-referenced orthomosaics photos was generated to address long-term shoreline change along approximately 270 km of high-energy sandy coast in SW France between 1950 and 2014. The coast consists of sandy beaches backed by coastal dunes, which are only disrupted by two wide tidal inlets (Arcachon and Maumusson), a wide estuary mouth (Gironde) and a few small wave-dominated inlets and coastal towns. A time and spatially averaged erosion trend of 1.12 m/year is found over 1950–2014, with a local maximum of approximately 11 m/year and a maximum local accretion of approximately 6 m/year, respectively. Maximum shoreline evolutions are observed along coasts adjacent to the inlets and to the estuary mouth, with erosion and accretion alternating over time on the timescale of decades. The two inlet-sandspit systems of Arcachon and Maumusson show a quasi-synchronous behaviour with the two updrift coasts accreting until the 1970s and subsequently eroding since then, which suggests that shoreline change at these locations is controlled by allocyclic mechanisms. Despite sea level rise and the well-established increase in winter wave height over the last decades, there is no capture of significant increase in mean erosion rate. This is hypothesized to be partly the result of relevant coastal dune management works from the 1960s to the 1980s after a long period of coastal dune disrepair during and after the Second World War. This study suggests that long-term shoreline change of high-energy sandy coasts disrupted by inlets and/or estuaries is complex and needs to consider a wide range of parameters including, non-extensively, waves, tides, inlet dynamics, sea level rise, coastal dune management and coastal defences, which challenges the development of reliable long-term coastal evolution numerical models.

Understanding wet avalanche intensity and the role of past environmental changes on wet avalanche occurrence is a main concern especially in the context of a warming climate and accelerated environmental mutations. Avalanches are closely related to fast cryosphere changes and may cause major threats to human society. Here, we used the sedimentary archive of the Alpine Lake Lauvitel (Lac du Lauvitel; western French Alps) to establish the first long-term avalanche record in this Alpine region. For this purpose, we used a novel CT-scan methodology that allows the precise identification of coarse material – from sand to pebble – transported to the lake and embedded within the finer continuous sedimentation. We identified a total of 166 deposits over the last 3300 yr cal. BP. In parallel, a detailed pollen analysis gave an independent record of environmental changes. Based on modern observation, lake monitoring, seismic investigations and sedimentological evidences, coarse material deposits were attributed to wet avalanche events. Our results highlight the effect of vegetation cover on the avalanche hazard while a period of strong frequency increase occurred after 780 yr cal. BP. In Lake Lauvitel, this period corresponds to a major forest clearance induced by the rise of human land use. Climate forcing on the avalanche hazard was investigated before and after the vegetation shift. On a multicentennial scale, wet avalanches preferably occur during periods of larger glacier extent, in which higher winter precipitation probably generates a sufficiently thick snow cover. On a sub-centennial scale, avalanches are more frequent during periods of relative warming, resulting in a destabilization of the same snow cover in spring season. Our results highlight as well the role of forest cover in mitigating wet snow avalanches' occurrence. In the context of predicted warmer temperatures, this study raises the question of whether a wet avalanche hazard increase may be expected in the near future especially at higher altitudes.

Washover deposits are among the most remarkable sedimentary signatures of dune breaching and coastal inundation. Overwash of the barrier leading to washover development can be related either to storms or tsunamis but the distinction between these two processes based on the depositional record is still a matter of debate. Hence, studying recent well-documented washover deposits, combining sedimentological and hydrodynamic investigations is a key approach to identify the processes governing the washover deposition and to analyze their sediment record within the washover. During the winter of 2013–2014, the coasts of the Bay of Biscay experienced an exceptional wave climate, leading to dune breaching and washover deposition at many locations, including the Gatseau Spit (South of Oleron Island, France). Field observations, aerial photographs, topographic measurements, GPR investigations, trenches and cores, grain size analysis and magnetic fabric measurements were carried out and complemented with hydrodynamic modeling of tides, storm surges, short waves and infra gravity waves in order to compute maximum runup during the winter of 2013–2014. Such an interdisciplinary approach led to propose that: (1) large overwash events occurring during the winter of 2013–2014 were mainly driven by infra gravity (IG) waves combined with high tides; (2) the washover elongated outer shape was strongly controlled by the antecedent morphology of the back-barrier area; (3) one overwash flow driven by one IG wave is recorded by one normally graded lamina composed of quartz sand at base and heavy minerals enriched sand at top; (4) dominant settling of the particles from a suspension during sedimentation occurred in the end of each overwash flow in response to flow velocity decrease; (5) laminae can be grouped in laminasets generally composed of thinning upward laminae, related to overwash flow depth decrease during falling tide and to erosion of basal laminae by highest overwash flows occurring more likely around the high tide; (6) the number of laminasets in a proximal position is in the same order of magnitude as the number of modeled highest runup events, offering an almost complete record of overwash events that occurred during the winter 2013–2014.

International audience; Sediment records of two contrasting backshore coastal marshes, extremely vulnerable to recent and historical marine flooding events, located on the SW coast of France, have been investigated using a multiproxy approach. The studied marshes are 30 km apart and have been flooded by similar storm events (7 marine floods in the last 250 years). One is located in a wave-exposed coast but isolated from the sea by a sediment barrier, whereas the other is located in a sheltered estuarine environment and isolated from the sea by a dike. One core was collected in each marsh and information on grain-size, foraminifera, shell contents and stable carbon isotopes was obtained along with an age model using 210Pb, 137Cs and 14C. Core data combined with historical maps give evidence of a typical estuarine backfilling, part of the Holocene regressive parasequence, including an intertidal mudflat at the base and a backshore environment at the top. Despite the absence of grain size anomalies, marine flood-related sedimentation in the backshore area of both marshes is identified by a mixture of marine and terrestrial features, including marine fauna, vegetation debris and variation in the δ13C signature of the organic fraction. Very low sedimentation rates between flood events and/or bioturbation prevents the identification of individual episodic marine floods in the sediment succession. Comparison of the two sedimentary successions shows that the foraminifera deposited by marine submersions are of two different types. Foraminifera are monospecific and originate from the upper tidal mudflat in the sheltered marsh; whereas in the backshore marsh located in a wave-exposed environment, they show higher diversity and originate from both shallow and deeper water marine environments. Thisstudy shows that wave exposure can control the faunal content of marine flood sediment records in coastal marshes.

There is a growing interest for marine flooding related to recent catastrophic events and their unintended consequences in terms of casualties and damages, and to the increasing population and issues along the coasts in a context of changing climate. Consequently, the knowledge on marine flooding has progressed significantly for the last years and this review, focused on storm-induced marine submersions, responds to the need for a synthesis. Three main components are presented in the review: (1) a state-of-the-art on marine submersions from the viewpoint of several scientific disciplines; (2) a selection of examples demonstrating the added value of interdisciplinary approaches to improve our knowledge of marine submersions; (3) a selection of examples showing how the management of future crises or the planning efforts to adapt to marine submersions can be supported by new results or techniques from the research community. From a disciplinary perspective, recent progress were achieved with respect to physical processes, numerical modeling, the knowledge of past marine floods and vulnerability assessment. At a global scale, the most vulnerable coastal areas to marine flooding with high population density are deltas and estuaries. Recent and well-documented floods allow analyzing the vulnerability parameters of different coastal zones. While storm surges can nowadays be reproduced accurately, the modeling of coastal flooding is more challenging, particularly when barrier breaches and wave overtopping have to be accounted for. The chronology of past marine floods can be reconstructed combining historical archives and sediment records. Sediment records of past marine floods localized in back barrier depressions are more adequate to reconstruct past flooding chronology. For the two last centuries, quantitative and descriptive historical data can be used to characterize past marine floods. Beyond providing a chronology of events, sediment records combined with geochronology, statistic analysis and climatology, can be used to reconstruct millennial-scale climate variability and enable a better understanding of the possible regional and local long-term trends in storm activity. Sediment records can also reveal forgotten flooding of exceptional intensity, much more intense than those of the last few decades. Sedimentological and historical archives, combined with highresolution topographic data or numerical hindcast of storms can provide quantitative information and explanations for marine flooding processes. From these approaches, extreme past sea levels height can be determined and are very useful to complete time series provided by the instrumental measurements on shorter time scales. In particular, historical data can improve the determination of the return periods associated with extreme water levels, which are often inaccurate when computed based on instrumental data, due to the presence of gaps and too short time-series. Longterm numerical hindcast of tides and surges can also be used to provide the required time series for statistical analysis. Worst-case scenarios, used to define coastal management plans and strategies, can be obtained from realistic atmospheric settings with different tidal ranges and by shifting the trajectory of storms. Management of future crises and planning efforts to adapt to marine submersions are optimized by predictions of water levels from hydrodynamic models. Such predictions combined with in situ measurements and analysis of human stakes can be used to define a vulnerability index. Then, the efficiency of adaptation measures can be evaluated with respect to the number of lives that could be potentially saved. Numerical experiments also showed that the realignment of coastal defenses could result in water level reduction up to 1 m in the case where large marshes are flooded. Such managed realignment of coastal defenses may constitute a promising adaptation to storm-induced flooding and future sea level rise. From a legal perspective, only a few texts pay specific attention to the risk of marine flooding whether nationally or globally. Recent catastrophic events and their unintended consequences in terms of death and damages have triggered political decisions, like in USA after hurricane Katrina, and in France after catastrophic floods that occurred in 2010. 151-184pp Volume 165 DHA/NEC Earth-Science Reviews

International audience; Storm-induced coastal flooding is among the most destructive natural disasters, as seen recently in the Bay of Bengal, the Gulf of Mexico and the Philippines. This study presents a high resolution hindcast of the flooding associated with Xynthia, a mid-latitude storm that severely hit the central part of the Bay of Biscay in February 2010. A 2DH fully coupled modeling system is applied to the NorthEast Atlantic Ocean, with a resolution locally reaching a few meters along the coastline of the study area. Such a fine resolution was required to adequately represent the dikes and the barriers that usually prevent the area from flooding, but results in a N1,700,000 element unstructured grid. The comparison with the available data reveals that waves and water levels are reproduced with normalized errors of the order of 10% and 5%, respectively. The extension of the flooding is also well reproduced, although with some underestimations along the coastline and overestimation in the inner part of large marshes. These limitations are explained by a lack of spatial resolution locally and the absence of several processes in the model such as infragravity waves and wave runup. The comparison between our baseline simulation and a simulation where the flooding is disabled by increasing the dike height reveals differences in maximum water levels locally reaching 1.0 m. This result is of key importance for coastal management strategies and also questions classical engineering approaches relying on one-way nesting.

Chaumillon, E.; Cange, V.; Gaudefroy, J.; Mercle, T.; Bertin, X., and Pignon, C., 2019. Controls on shoreline changes at pluri-annual to secular timescale in mixed-energy rocky and sedimen...

International audience; This 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.

(IF 2.38; Q1); International audience; Abundant sedimentological and geochronological data gathered on European sandy coasts highlight major phases of increased high-energy sedimentation in the North Atlantic Ocean during the late Holocene. Owing to an inconsistent use of the terminology, it is often difficult to determine whether studies have described storm-built or wave-built deposits. Both deposits can be identified by overall similar coarse-grained sedimentary facies, but may provide contradictory paleoenvironmental interpretations. The aim of this study is to address this issue, by analysing a set of published 14C ages recovered from wave-built sediment bodies of the Pertuis Charentais (France). Integration of 14C data highlights seven coarse-grained sedimentation pulses (CSPs) that reflect a synchronous increase in wave-induced sediment supply occurring around 2650, 2420, 1240, 970, 800, 650 and 450 cal. yr B.P. CSPs can be matched with geochronological data published on other exposed sandy coasts of the western Europe. In first order, CSPs were preserved during phases of decreased storm activity and thus limited coastal erosion in the North Atlantic Ocean from 2700 to 2000 and from 1200 to 600 cal. yr B.P. In second order, CSPs occurred during positive NAO and/or negative EA/WR peaks, which are known from previous studies to induce higher waves and to enhance wave-induced sediment transport in the Pertuis Charentais. Chronological boundaries of CSPs could be used as a diagnostic tool to differentiate storm deposits indicative of sudden, episodic very high-energy sedimentation from wave deposits resulting from the sustained action of above-average westerly winds blowing across the North Atlantic Ocean.

International audience; Present-day coastal barriers represent around 15% of the world's oceanic shorelines, and play an important role as early warning indicators of environmental change. Among them, wave-dominated barriers are dynamic landforms that tend to migrate landward in response to storms and sea-level change. High rates of sediment supply can locally offset the global retrogradation trend, providing valuable records of past environmental change occurring on transgressive coasts. However, geochronological control limits the temporal resolution of such records to millennial or centennial timescales, and the decadal or even faster response of wave-built barriers to historical climate changes is therefore poorly understood. In this study, we show that shoreline dynamics of sand spits reconstructed from old cartographic documents has been synchronous on both margins of the North Atlantic Ocean since about 1800 CE. Spit growth accelerated drastically during three periods lasting about 15 years, characterised by positive North Atlantic Oscillation (NAO) and negative East Atlantic–West Russia (EA–WR) atmospheric circulation patterns. These changes are in phase with periods of increased volcanic activity. We use a high-resolution wave hindcast (1948–2014 CE) in a reference area to confirm the association between NAO and EA–WR as a proxy for offshore and nearshore wave height and for associated longshore sediment transport (LST) involved in spit growth. A 24-month lagged correlation between sediment transport and volcanic aerosol optical thickness (concentration of ashes in the atmosphere) is observed, suggesting that spit shoreline dynamics at the decadal timescale is partially forced by external climate drivers via cascading effects on atmospheric circulation patterns and wave climate. Our results imply that NAO variability alone is not sufficient to understand the evolution of wave-built coastal environments. The associated sediment record can be used to reconstruct multi-decadal variability of other climate patterns.

International audience; This study shows the short-term sediment dynamics (2010) and the long-term morphological evolution (1905 to 2008) of a tidal sand bar emplaced in the Gironde Estuary, the Plassac Tidal Bar. This tidal sand bar was selected because it corresponds to a morphological category (lobate shape) that was not previously studied in the setting of estuarine bay-head deltas. A recent high-resolution bathymetric survey (2010) evi- dences the sediment transport pattern, inferred from the lee face orientation of subaqueous dunes. This sediment transport pattern, together with results from previous studies, explains the five main mechanisms for the tidal bar evolution identified from 29 bathymetric maps since 1905: namely, flood ramp infill, partial ebb shield breaching, lateral accretion of the ebb spits and ebb shield lengthening, generated by the merging of mini-flood lobes on the outer sides of the ebb spits. Lateral accretion seems to be a key-process of sediment accretion for lobate tidal sand bars. Most of the evolutions are explained by tidal processes, but a fluvial influence is evidenced by correlating the presence of mini-flood lobes (migrating seaward from the upper reaches of the bay-head delta) and the lengthening of the tidal bar with periods of high fluvial discharge. A ca. 25-year periodicity in both the fluvial discharge and the tidal bar width, length and volume variations is evidenced and suggests a climate control on the tidal bar evolution. A classification of lobate tidal sand bars is proposed in order to distinguish tide-dominated and wave-influenced sand bars emplaced in large estuary mouth or tidal inlets, from tide-dominated and fluvial-influenced sand bars emplaced in bay-head deltas.

International audience; – This synthesis of the morphological and stratigraphical evolutions of the Marennes-Oléron Bay (west coast of France) combines bathymetric data (1824 and 2003) and very high-resolution seismic profiling groundtruthed by vibracore samples. The Marennes-Oléron Bay is characterised by a very high sedimentation rate and appears to be an ideal place to investigate the sedimentary record of the major environmental changes that occurred since the last several millennia. The sediment budget of the Marennes-Oléron Bay, between 1824 and 2003, is clearly positive. The flood-dominated northern Marennes-Oléron Bay displays sediment gain in both intertidal and subtidal areas whereas the ebb-dominated southern Marennes-Oléron Bay displays sediment gain restricted to the intertidal area and deepening of subtidal channels. In addition, human influences such as oyster farming may play a role in the sediment gain of the bay. The sediment-fill of the northern Marennes-Oléron Bay consists of five main phases: (1) lenticular units and flooded intertidal flats recording lower sea level periods before 7500 yr B.P.; (2) tidal channel-fills recording changes in tidal drainage pattern from 7500 to 5000 yr B.P.; (3) a subtidal unit which constitutes the main phase of sediment fill in the northern part of the bay from 5000 to 1500 yr B.P.; (4) a major channelized erosional surface related to huge coastline changes from 1500 to 1000 yr B.P.; and (5) a mud drape emplaced during the last millennia and potentially recording historical human impact (deforestation and land reclamation). The sediment fill of the southern Marennes-Oléron consists of sandbanks, mixed sand-and-mud flats and tidal channels, mainly emplaced under wave-and-tide processes since the last centuries. Despite its relatively thin (20 m at the maximum), recent and rapid sediment fill, the stratigraphic organization and morphological evolution of the Marennes-Oléron Bay is very complex and spatially variable. Like in many other estuaries , sediment fill of the Marennes-Oléron Bay was successively controlled by relative sea level changes, and then by sediment supply driven by hydrodynamic changes related to huge coastline migrations, and finally by human activities. Moreover, this kind of " rocky coast " estuary, where the sediment-fill is very thin and discontinuous, is characterised by a bedrock control at each phases of the sediment fill both in terms of preservation in topographic lows and in terms of control on hydrodynamics and related sediment input. – Cette synthèse sur les évolutions morphologiques et stratigraphiques de la baie de Marennes-Oléron (côte ouest de la France) repose sur des données bathymétriques (1824 et 2003), des enregistrements sismiques à très haute résolution et des carottages. La baie de Marennes-Oléron qui se caractérise par de très forts taux de sédimentation repré-sente un site idéal pour étudier l'enregistrement sédimentaire des changements environnementaux qui se sont produits lors des derniers millénaires. Le budget sédimentaire de la baie de Marennes-Oléron, entre 1824 et 2003 est clairement positif. Dans la partie nord de la baie, dominée par le flot, le gain sédimentaire se situe à la fois dans les zones inter– et subtidales, alors que dans la partie sud de la baie, dominée par le jusant, le gain sédimentaire se limite aux zones intertidales avec un creuse-ment des chenaux subtidaux. L'activité ostréicole semble jouer un rôle dans le gain sédimentaire au niveau des zones in-tertidales. Le comblement sédimentaire de la partie nord de la baie de Marennes-Oléron comprend cinq phases principales, de la base vers le sommet : (1) des unités lenticulaires et des estrans enregistrant des périodes de plus bas niveau marin avant 7 500 B.P. ; (2) des comblements de chenaux témoignant de réorientations dans le drainage de la baie entre 7 500 et 5 500 B.P. ; (3) une unité subtidale mise en place entre 5 500 et 1 500 B.P. et qui constitue la plus grande partie du comblement ; (4) une surface d'érosion majeure liée à une mobilité drastique du trait de côte entre 1 500 et 1 000 B.P. ; (5) un drapage vaseux déposé depuis le dernier millénaire et enregistrant probablement l'impact anthropique de la défo-restation et de la poldérisation. Dans la partie sud de la baie de Marennes-Oléron des bancs de sable, des estrans mixtes, tel : 33 (0) 5 46 45 72 31 ; fax : 33 (0) 5 46 45 82 74. eric.chaumillon@univ-lr.fr Manuscrit déposé le 15 octobre 2008 ; accepté après révision le 15 janvier 2009. sablo-vaseux et des chenaux tidaux constituent un comblement rapide depuis les derniers siècles principalement. L'ensemble des données montre que malgré un comblement sédimentaire peu épais (20 m au maximum), récent et rapide, les évolutions morphologiques et l'organisation stratigraphique de la baie de Marennes-Oléron sont très complexes et variables dans l'espace. Comme dans la plupart des estuaires, le comblement sédimentaire de cette baie a suc-cessivement été contrôlé par les changements relatifs du niveau de la mer, puis par les apports sédimentaires gouvernés par des changements hydrodynamiques en relation avec des migrations importantes du trait de côte et enfin par les acti-vités humaines. Une originalité de ce type d'estuaire de côte rocheuse, caractérisé par une couverture sédimentaire très fine et discontinue, est le contrôle par la morphologie du socle rocheux à toutes les étapes du comblement, tant du point de vue de la préservation des sédiments dans les creux topographiques, que du point de vue de la modification des para-mètres hydrodynamiques et des apports sédimentaires qui leurs sont associés.

International audience; During the Late Holocene, the rate of sea level rise decreased and climate changes, hydrodynamic processes or anthropogenic impacts became predominant parameters governing the sedimentary infill of estuarine environments. The aim of this study is to describe the response of past benthic mollusc communities to these forcing factors. Mollusc skeletal remains were sampled from three 8000, 5500 and 2600 year-long sedimentary records in the Marennes–Oléron Bay (Atlantic Coast, France), where environmental changes have previously been identified thanks to a combination of very high resolution seismic profiles and sedimentological data. Molluscan assemblages successfully record environmental changes, even at the smallest temporal scale. They provide relevant information regarding bathymetry, salinity, hydrodynamics, oxygen content and sediment supply. In the 8000 year-long record, the transition from an upper to a lower intertidal mudflat community provides evidence of a transgressive phase that occurred between 8000 and 7600 years BP. In the 5500 year-long record, an oyster bank developed on the slopes of a tidal channel has been recovered. The composition of the fossil community and the careful examination of skeletal remains suggest that the channel was hydraulically inefficient, in response to the sediment infilling of its drainage area. In the 2600 year-long record, a soft-bottom, shallow-water, subtidal mollusc community, similar to that found nowadays in the Marennes–Oléron Bay, has been recovered from both lower mixed sand-mud and upper muddy units separated by a major regional unconformity dated to about 1000 years BP. This sharp transition is recorded by a few rare species, and was responsible for the local extinction of the bivalve Lepton squamosum. However, the abundance of the most common species and the quantity of fragmented shells remain stable, which may indicate that mud drape deposition was related to an increase in mud supply rather than a decrease in water energy. The level of detail of the palaeoenvironmental reconstructions varies, depending on the ecological requirements of the species. Stenotypic indicator species provide accurate, quantitative information, whereas eurytypic species add undesired variability to the assemblages. Since fossil molluscs are easy to collect and to identify and provide long-term, smoothed records of environment variations, this study suggests that they can be used prior to any other biological proxy, as a first step into reconstructions of Holocene coastal environments.

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2009/publication-6165.pdf; International audience; In 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.

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.

International audience; Both sedimentological and mollusc death assemblage analyses from a 14C-dated core, coupled with seismic stratigraphicalanalysis within the Marennes-Oléron Bay (French Atlantic coast), provide a detailed record of environmental changes during theend of the Holocene rapid sea-level rise. Lithofacies and mollusc fauna changes evidence the transition from a former intertidal baremudflat to a subtidal environment. The newly shown former sea-level record, computed from radiocarbon-dated Scrobiculariaplana shell, matches previously published local sea-level curves. This drowned mudflat belongs to an incised valley-fill thatprovides new insights into valley-fill successions along the French Atlantic coast.; Une étude combinée de stratigraphie sismique, d’une part, et d’analyses sédimentologiques et de thanatocénoses de mollusques réaliséessur un carottage, d’autre part, a été effectuée en baie de Marennes-Oléron. Elle révèle l’ennoiement progressif d’une paléovasièreintertidale mise en place durant la remontée rapide du niveau marin de l’Holocène et son remplacement par des habitats marins depetits fonds vaseux de mode abrité. La datation de Scrobicularia plana indique que ce nouvel enregistrement d’un paléoniveaumarin est conforme aux courbes du niveau marin local antérieurement publiées. Le remplissage sédimentaire de la vallée incisée, oùse situe cette paléovasière intertidale, apporte des informations complémentaires sur le remplissage sédimentaire holocène desvallées incisées de la côte atlantique française.

International audience; This 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.

Resume De nouvelles donnees de sismique tres haute resolution et de vibro-carottages acquises dans le pertuis Breton (littoral vendeo-charentais) permettent de mettre en evidence une organisation originale de remplissage de vallee incisee. Le comblement consiste principalement en cinq prismes progradants superposes, constitues par des sables marins. Ces prismes montrent des depocentres retrogradants et sont caracterises a leur toit par une discontinuite plane qui se prolonge par une plate-forme sous-marine. Ce dispositif est tres similaire aux prismes retrogradants transgressifs des marges continentales et suggere que l'essentiel du remplissage du pertuis Breton soit transgressif et probablement controle par la derniere remontee eustatique. Pour citer cet article : N. Weber et al., C. R. Geoscience 336 (2004).

New, very high-resolution (25 cm) seismic profiles have revealed the internal architecture of the infilling of a macrotidal bay, the Marennes-Oléron Bay, France. Within this geometry, a major seismic unconformity has been correlated with core data. This correlation provides evidence that the seismic unconformity corresponds to a sharp grain-size decrease. Both seismic and core data indicate that this change of grain size can be interpreted as a record of a recent (around 1,000 years b.p.) decrease in hydrodynamical energy with time and/or a larger supply of fine-grained material. This recent environmental change can be related to natural infilling of the Marennes Oléron Bay, and to tidal prism decrease, increasing human activities (e.g. land reclamation, deforestation, agricultural land use) and climate fluctuations during the late Holocene, such as the transition between the cold period of the Dark Ages (1,550–1,050 years b.p.) and the Medieval warm period (1,050–550 years b.p.).

International audience; This study analyzes historical archives to produce a database of storm-induced coastal flooding in the French cen- tral part of the Bay of Biscay since 1500 AD. From this new database, 46 coastal floods have been reported for the last 500 years (1 event every 11 yr on average), which demonstrates the high vulnerability of this region to coast- al flooding. The limitations of historical archives prevent concluding to a change in storminess over the period. The six largest coastal floods that occurred since 1900 are further investigated because numerous informations are available in terms of meteorological conditions as well as maximum water levels reached. Associated storm surges are also modeled using a simplified methodology, relying on a fully coupled wave and circulation modeling system directly forced by the available meteorological data. The analysis of modeling results and histor- ical archives reveals firstly that several meteo-oceanic settings can lead to extensive coastal flooding in the stud- ied area. The analyzed events allow distinguishing three main settings: (1) a small storm surge associated with the highest astronomical tidal levels, (2) a large storm surge, induced either by extreme winds or a particular sea-state and associated with high astronomical tidal levels and, (3) a large storm surge enhanced by extreme wave conditions. Second, maximum water levels reached during these 6 coastal floods were shown to be higher than the 100 years return period water level estimated in several recently published studies. This inconstancy can be explained mainly by the lack of extreme water level records in tide gauge datasets used in these previous studies. To a slightest degree, this study questions the validity of classical statistical approaches when analyzing extreme water levels data resulting from contrasting meteo-oceanic settings. It is concluded that historical ar- chives combined with storm surge modeling could be a valuable approach to better estimate the return period of extreme sea levels and improve the understanding on the vulnerability of mixed-energy coastal environments such as the study area.

Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north-south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean-Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area. ©2001 Elsevier Science B.V. All rights reserved.

International audience; We studied sediment cores from Lake Vens (2,327 m asl), in the Tine'e Valley of the SW Alps, to test the paleoseismic archive potential of the lake sediments in this particularly earthquake-sensitive area. The historical earthquake catalogue shows that moderate to strong earthquakes, with intensities of IX-X, have impacted the Southern Alps during the last millennium. Sedimentological (X-ray images, grain size distribution) and geochemical (major elements and organic matter) analyses show that Lake Vens sediments consist of a terrigenous, silty material (minerals and organic matter) sourced from the watershed and diatom frustules. A combination of X-ray images, grain-size distribution, major elements and magnetic properties shows the presence of six homogenite-type deposits interbedded in the sedimentary background. These sedimentological features are ascribed to sediment reworking and grain sorting caused by earthquake-generated seiches. The presence of microfaults that cross-cut the sediment supports the hypothesis of seismic deposits in this system. A preliminary sediment chronology is provided by 210Pb measurement and AMS 14C ages. According to the chronology, the most recent homogenite events are attributable to damaging historic earthquakes in AD 1887 (Ligure) and 1564 (Roquebillie're). Hence, the Lake Vens sediment recorded large-magnitude earthquakes in the region and permits a preliminary estimate of recurrence time for such events of *400 years.

Resume : La tempete Xynthia a severement touche les littoraux situes dans la partie centrale du Golfe de Gascogne dans la nuit du 27 au 28 fevrier 2010. Cette tempete a genere une surcote de plus d’1.50 m en phase avec une maree haute de vives-eaux, ce qui a entraine la submersion de nombreuses zones basses du littoral. Afin de mieux comprendre cette catastrophe, nous avons developpe un systeme de modelisation numerique realisant le couplage complet entre les vagues et la circulation hydrodynamique, et qui permet de reproduire de facon precise les vagues, la maree et la surcote pendant Xynthia. L’analyse des resultats numeriques et des mesures disponibles montre que la surcote exceptionnelle pendant Xynthia resulte d’un transport d’Ekman fortement amplifie par la presence de vagues jeunes et cambrees. L’extension de notre grille de calcul vers les zones potentiellement inondables nous permet de reproduire de facon realiste la submersion marine associee a Xynthia, alors que les breches dans les digues ne sont pas representees. Ce resultat suggere que, pour la majorite des zones inondees, les volumes de surverse sont largement dominants par rapport aux volumes d’eau s’ecoulant par les breches. La comparaison entre notre simulation de reference et une simulation ou la submersion a ete rendue impossible en relevant les digues montre de facon surprenante des hauteurs d’eau le long du trait de cote localement 1.0 m plus hautes lorsque la submersion est rendue impossible. Ce resultat, qui montre que les submersions marines peuvent impacter de facon significative les hauteurs d’eau en mer, a de lourdes implications en termes de gestion des zones cotieres. Il remet egalement en question les approches classiques d’ingenierie reposant sur l’emboitement de plusieurs grilles sans retroaction des grilles entre elles. Mots-cles : Xynthia, Surcote, Submersion marine, Modelisation numerique.

International audience; This study presents new 14C AMS ages obtained from the sedimentary sequence of Lake Allos (elevation: 2,230 m, Verdon valley, Southern Alps, France). These ages provide new evidence on the last deglaciation over the Southern Alps, enabling us to revise the chronological framework previously established. Sedimentation in the lake began between 12,300 and 13,500 cal. yr BP (2 sigma interval). The two glacial stages identified in a cirque downstream of Lake Allos (at 1,900 m a.s.l.) and on the glacial threshold (at 2,300 m a.s.l.) are hence older. Our results are in good agreement with previous conclusions based on palynological evidence from high-elevations peat bogs and lakes that suggested an earlier deglaciation, at the onset of the Lateglacial.; Cet article présente les datations 14C AMS obtenues dans la séquence sédimentaire du lac d’Allos (alt. 2 230 m, vallée du Verdon, Alpes du Sud, France). Ces données contribuent à la connaissance de la chronologie de la déglaciation à l’échelle des Alpes du Sud et permettent de revoir le cadre chronologique proposé antérieurement sur ce site. La sédimentation lacustre démarre entre 12 300 et 13 500 ans cal. BP (2 sigmas). Les deux stades glaciaires identifiés en aval du lac à 1 900 m et sur le verrou glaciaire à 2 300 m sont par conséquent plus anciens. Nous rejoignons ici les conclusions des travaux antérieurs sur les tourbières et lacs d’altitude qui, sur la base des données polliniques, suggéraient une déglaciation très précoce remontant au début du Tardiglaciaire.

International audience; Estuarine tidal bars emplaced in estuaries are complex sediment bodies composed of sand and mud provided by rivers. This study focuses on the Plassac Tidal Bar, which is the unique lobate-shape tidal bar of the Gironde Estuary, located at the very upstream extremity of the bay-head delta, because no detailed sedimentological studies have been made on such geomorphological category of tidal bar. Its stratigraphy was investigated using a dense grid of 65 very high resolution seismic profiles, ground-truthed by 6 to 9 meter-long cores and time-controlled by historical bathymetric surveys. The base of the tidal bar consists of a tidal erosional surface. This surface has eroded muddy tidalites, corresponding to the maximum flooding surface of the Gironde Estuary. The bar is composed of two vertical sequences of facies, composed of thickening-up cross stratified fine-to medium-grained sand beds at the top and decimeter-thick mud-layers at the base. Those mud layers constitute the master bedding of the bar and are dipping in a channelward direction, perpendicularly to the tidal flows and to the sediment transport directions. One phase of lateral accretion of the Plassac Tidal Bar is correlated with variations in fluvial discharge of the last decades and suggests a climate control on the internal architecture of this sandbar. Low water periods lead to the upstream migration of the turbidity maximum and to the deposition and consolidation of the muddy master bedding planes. During the subsequent flood periods, the muddy master bedding planes are partially eroded and then buried by the fluvially-sourced sandy mini-flood lobes that merge with the tidal bar. The internal architecture of the Plassac Tidal Bar is controlled by a lateral accretion process, its master bedding surfaces which record this process are dipping perpendicularly to the axis of the tidal flows. All the other tidal bars of the bay-head delta (around a dozen) exhibit an elongated morphology and prograde longitudinally by seaward frontal accretion occurring in the axis of the tidal flows. The master bedding surfaces which record this process are seaward dipping along the axis of the tidal flows. Those variations in internal accretion surfaces orientation of tidal bars are of considerable importance and must be taken into account for studies of ancient tidal deposits, paleoenvironment reconstructions and for heterogeneities characterization and quantification of reservoir properties in these types of sandstones.

Multichannel seismic data were acquired across several areas of the Mediterranean Ridge (M.R.) in the eastern Mediterranean Sea during the MCS Prismed survey (March 1993). The M.R. is a wide, high-standing and elongated swell, emplaced between the southern Aegean and Africa as a consequence of the long-term convergence and the ongoing collision between Europe and Africa. Sedimentary piling and associated deformation have built a thick deformed sediment wedge, the internal geological structures of which remain poorly understood. Processing of two MCS lines (PM 02 and PM 03) across the western branch of the Mediterranean Ridge reveals that Messinian evaporite-bearing formations, which were deposited within the deep Ionian oceanic basin 5 my ago, play an important part in the present structural deformation of the M.R. accretionary wedge. Within the outer M.R., the Messinian “lower evaporites” are believed to act as one of the main decollement levels, while the top of these formations may operate as a disharmonic layer below thin Plio-Quaternary sediments. The present-day M.R. toe appears to be affected by ductile-flow deformation. However, most of the accreted outer wedge is believed to have resulted from the progressive piling up and associated thrusting of Messinian and overlying Plio-Quaternary sediments. Toward the northeast (i.e., toward the central M.R. domain), a major backthrust zone indicates a structural boundary between the outer, wide and convex M.R. and its inner, rather flat, domain, which is inferred to act as a backstop.

National audience

International audience; The structure and function of estuarine food webs change in response to both natural and anthropogenic stresses. The construction of quantitative food webs and their analysis by means of Ecological Network Analysis provides outputs that have been used in many studies to assess system development, stress, robustness, resilience and maturity. Here we attempt to relate to the physical characteristics of the environment, ecosystem indices derived from Ecological Network Analysis. Ten models of food webs were gathered, across a selection of soft-bottom estuaries representative of a large morphological and hydrodynamic diversity, from wave-dominated to mixed energy tide-dominated systems. The selection allowed the comparison of their derived Ecological Network Analysis indices, because of similarities of accuracy in the representation of detritus and bacteria, and because models took into account all trophic levels up to top-predators. In order to obtain comparable physical characteristics, global models were used for a homogeneous description of tide and tidal prisms. Spearman correlations, hierarchical ascendant clustering and Redundancy Analysis were applied to examine the relationship between Ecological Network Analysis indices and physical characteristics. The set of four physical variables selected (catchment area, tidal range at neap tide, index of tide-wave domination and latitude in absolute value) explained 67% of the structure of the Ecological Network Analysis indices. This implies that the physical forcing related to climate, hydrodynamics and morphology is essential for determining the ecological emergent properties of the food web. In the European policy context of determining the ‘good ecological status’ of coastal ecosystems, it implies that the use of Ecological Network Analysis indices for basing the determination of operational indicators should be done, taking into account this context of a strong influence of physical factors.

Les immenses surfaces inondees lors de la tempete Xynthia et le drame humain associe ont rappele l’extreme sensibilite des marais et plaines cotieres de Charente-Maritime et Vendee a la submersion marine. Des mesures acquises pendant et apres cette tempete permettent de realiser des simulations qui visent a mieux comprendre et mieux predire les phenomenes de submersion marine. Une modelisation oceanographique complete de la surcote et des inondations a terre necessite des calculateurs tres puissants et de longues heures de calcul. Nous avons teste une methode alternative qui consiste a utiliser un leve topographique a haute resolution (laser aeroporte, Litto3D IGN) couple a un modele oceanographique de surcote, dont l’espace de calcul est limite au domaine oceanique, afin de realiser des submersions statiques et des calculs de surverse. Les resultats obtenus sont realistes sur des marais de petites dimensions. En revanche, cette methode, ne donne pas des resultats satisfaisants sur des marais de grande taille, en particulier si ceux-ci ne sont pas draines par un estuaire ou un chenal de maree en connexion avec l’espace marin. En effet le parametre cle est la distance entre le trait de cote et l’inondation maximale. Dans le cas ou cette distance est grande, la dynamique de propagation de l’onde d’inondation devient predominante et une modelisation de l’ecoulement est incontournable pour des simulations realistes.

Resume : La tempete Xynthia a severement touche les littoraux situes dans la partie centrale du Golfe de Gascogne dans la nuit du 27 au 28 fevrier 2010, en generant une surcote de plus d’1.50 m au maregraphe de La Pallice (La Rochelle). Cette surcote etait en phase avec une maree haute de vives-eaux, ce qui a entraine la submersion de nombreux cordons dunaires et inonde les zones basses du littoral. Notre etude vise a etudier les contributions des vagues dans la surcote associee a Xynthia, en combinant modelisation numerique et mesures in situ. Un nouveau systeme de modelisation numerique, base sur le code de circulation oceanique SELFE et le modele spectral de vagues WWMII, a ete mis en place pour l’Atlantique nord-est. Le couplage entre les vagues et les courants est complet en 2DH et en 3D et les deux modeles partagent la meme grille de calcul non structuree et la meme decomposition de domaine. Une fois calibre, ce systeme de modelisation numerique permet de reproduire les niveaux d’eau pendant Xynthia avec une erreur de l’ordre de 0.10 m et les hauteurs et periodes des vagues avec des erreurs de l’ordre de 15%. L’analyse des resultats numeriques montre d’abord que la surcote a ete amplifiee par la presence de vagues tres jeunes et cambrees, qui augmentent la rugosite de l’ocean. Cet etat de mer est lie a la trajectoire atypique de la tempete Xynthia, qui a traverse le Golfe de Gascogne du SO vers le NE, reduisant la zone de generation des vagues a quelques centaines de kilometres. A la cote, les gradients de tension de radiation des vagues contribuent a la surcote totale de quelques centimetres dans les ports a plus de 0.40 m au niveau des plages directement exposees aux houles oceaniques. Cette etude montre que, pendant la tempete Xynthia, les vagues ont autant contribue a la surcote que le vent et les gradients de pression atmospheriques. Mots-cles : Surcote – Xynthia – Vagues – Pertuis charentais

Investigation of Lake Allos sediments revealed ~ 160 graded layers, interpreted as flood deposits, over the last 1400 yr. Comparisons with records of historic floods support the interpretation of flood deposits and suggest that most recorded flood events are the result of intense meso-scale precipitation events. As there is no evidence for any major changes in erosion processes in the catchment since the Medieval Warm Period (MWP), we interpret the Allos record in terms of repeated intense precipitation events over the last millennium, with a low flood frequency during the MWP and more frequent and more intense events during the Little Ice Age. This interpretation is consistent with the pattern of increasingly humid conditions in the northwestern Mediterranean region. This long-term trend is superimposed on high frequency oscillations that correlate with solar activity and autumnal North Atlantic Oscillation (NAO). Finally, a comparison of flood records across the northwestern Mediterranean region showed that intense precipitation events in Allos (east of the Rhône Valley) were out of phase with events in the Cévennes (west of the Rhône) but in phase with events in eastern Spain. Supported by meteorological analyses, this suggests an oscillation in atmospheric circulation patterns over the northwestern Mediterranean.

Mollusc death assemblages were recovered in 98 subtidal sampling stations on the seafloor of the shallow Pertuis Charentais Sea (Atlantic coast of France). Taxonomic composition and spatial distribution of death assemblages were investigated, as well as their response to sediment grain size (field data), bottom shear stress (coupled tide and wave hydrodynamic modelling), and sediment budget (bathymetric difference map) Results showed that molluscs are likely to be reliable paleoenvironmental indicators since death assemblages were able to acquire ecological changes within years (decadal-scale taphonomic inertia), and live-dead agreement inferred from existing data on living benthic communities was high, except close to river mouths and intertidal mudflats that provide terrestrial and Intertidal species to subtidal death assemblages, respectively. Taxonomic composition of these within-habitat death assemblages strongly depended on sediment grain size and bottom shear stress, similarly to living subtidal communities. Post-mortem dispersal of shells, owing to relatively low bottom shear stress in the area, was only of a few 10s to 100s of meters, which shows that death assemblages preserved environmental gradients even at a fine spatial scale Sediment budget had also a significant influence on death assemblages. Thick-shelled epifaunal species were correlated with erosion areas on one side, and thin-shelled infaunal species with deposition on the other, showing that mollusc fossil assemblages could be used as indicators of paleo-sedimentation rate This new proxy was successfully tested on a previously published Holocene mollusc fossil record from the same area It was possible to refine the paleoenvironmental interpretation already proposed, in accordance with existing strangraphic and sedimentological data. (C) 2010 Elsevier Ltd. All rights reserved