International audienc

International audienceIn order to reconstruct the former sea-levels and to better characterize the history of Holocene salt-marsh sedimentary infillings in West Brittany (western France), local foraminifera-based transfer functions were developed using Weighted-Average-Partial-Least-Squares (WA-PLS) regression, based on a modern dataset of 26 and 51 surface samples obtained from salt-marshes in both the bay of Tressseny and the bay of Brest. Fifty cores were retrieved from Tresseny, Porzguen, Troaon and Arun salt-marshes, which were litho- and biostratigraphicaly analyzed in order to reconstruct palaeoenvironmental changes. A total of 26 AMS 14C age determinations were performed within the sediment successions. The Holocene evolution of salt-marsh environments can be subdivided into four stages: (1) a development of brackish to freshwater marshes (from c. 6400 to 4500 cal. a BP); (2) salt-marsh formation behind gravel barriers in the bay of Brest (from 4500 to 2900 cal. a BP); (3) salt-marsh erosion and rapid changes of infilling dynamics due to the destruction of coastal barriers by storm events (c. 2900-2700 cal. a BP); (4) renewed salt-marsh deposition and small environmental changes (from 2700 cal. a BP to present). From the application of transfer functions to fossil assemblages, 14 new sea-level index points were obtained indicating a mean relative sea-level rise around 0.90±0.12 mm a-1 since 6300 cal. a BP

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This study focuses on the recurring climate conditions required for the largest storms occurring in NW France (Brittany). It is based on the analysed records of storm events along Western Brittany coast (see Part I). In this manuscript (Part II), storm recurrence is explored along with forcing mechanisms. Periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms (North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) modes) triggering the most destructive storms. Then, palaeostorm events are discussed at the Holocene scale, from 6000 yr BP to present, to verify the forcing mechanisms. Most recorded events appear to be linked with cooling episodes, mostly in winter, a transition to or from a negative winter NAO mode, a positive AMO mode. Extreme storms occur immediately prior to the 'Medieval Warm Period' (MWP). Maximum effects are reached prior to the onset of the MWP and during the Maunder and Dalton solar minima. Low storm activity occurred during the Spörer Minimum linked to an acceleration of the Atlantic Meridional Overturning Circulation (AMOC). Main storm triggers seem to correspond to a positive AMO mode with an unstable jetstream configuration driving a negative NAO. In this study, four specific weather configurations were defined to explain each type of recorded storminess. The strongest storms correspond to low AMO and decennial-negative NAO modes (e.g. 'Little Ice Age'), or high AMO in association with dominant low NAO modes, as during the early Middle Age and present-day period. Fresh or warm oceans in association with a positive NAO mode are stormy but with very low sting storms frequency. Although in agreement with the orbital forcing and the Holocene glacial history, increasing storm frequency and intensity is most probably partly biased by continuous sea-level rise and resulting erosion.

Our study aims to understand the recurring climatic conditions prevailing during the largest storms reaching NW France (Brittany). These storms are responsible for the breaching of coastal barriers and major flooding of lowlands. In a first part of our work, we examine the morphological impact and stratigraphic record of storm events along Western Brittany rocky coasts, with a special focus on the southern coast of the Bay of Audierne, the most exposed coast of the region. In a second paper ('Middle-to Late-Holocene Storminess in Brittany (NW France): Part II'), we shall focus on the chronology of storm events and their climate forcing conditions. Drilling transects and stratigraphic analyses were first undertaken to constrain chronology, strength and wind direction during the main Holocene storm events. New dates, observations and a relative sea-level (RSL) curve were then used to inform discussion of the necessary climatic and morphologic conditions leading to destructive storm events. Most recorded events appear to be linked with cooling episodes of the Holocene and a RSL close to present. Some storms are clearly responsible for breaching and dune building or remobilisation. We demonstrate that storm frequency and intensity appear to rise in a stepwise manner during the late Holocene. Maximum efficiency is reached during the 'Little Ice Age' with clustered events probably lasting several days, but major storms also occurred immediately prior to the 'Medieval Warm Period'. We suggest that recent coastal dune building from c. ad 1100 until now, despite a sea level close to present and continuously rising, may be a direct consequence of the restoration of beaches after periods of recurrent storminess. This building activity often occurred during dry negative North Atlantic Oscillation (NAO) events, in connection with the available sedimentary supply.

Résumé L'érosion des côtes à falaises est souvent exprimée par le biais des vitesses de recul du rivage. Ces taux de retrait donnent une bonne appréciation de leur érosion, mais ils masquent une partie du problème, c'est-à-dire les processus par lesquels les falaises sont érodées. Une étude diachronique des évolutions d'une partie de la côte crayeuse du Pays de Caux (Normandie, France) basée sur la comparaison des plans cadastraux et de photographies aériennes, ainsi que sur une approche naturaliste associée à des mesures de terrain, a permis de mesurer les vitesses de retrait de ces falaises et d'identifier chacun des processus d'érosion qui les affectent. Les modalités des évolutions de ce rivage à falaises peuvent alors être proposées en y incluant les impacts récents des activités anthropiques. Les processus marins interviennent essentiellement dans la mobilisation des sédiments littoraux et dans l'évacuation des accumulations des produits de l'érosion subaérienne ; l'érosion des falaises est principalement dominée par les processus subaériens qui déterminent les mouvements de masse volumineux dont les débris recouvrent la plate-forme littorale au pied des falaises. Comme ces accumulations massives persistent durant plusieurs décennies, elles réduisent l'alimentation des accumulations littorales de galets en aval de la dérive et elles favorisent l'attaque du pied des falaises adjacentes par les vagues. L'intensification des activités humaines sur le rivage par le biais de la construction de jetées et d'épis perpendiculaires au trait de côte ainsi que les extractions des galets de silex qui ont réduit l'alimentation naturelle des cordons de galets ont renforcé la fragilisation des falaises. De ce fait, les vitesses de retrait des falaises et le nombre moyen de mouvements de masse annuels ont été doublés au cours de la seconde moitié du siècle dernier.

par photo-interprétation. Elle utilise comme références les lignes des pleines mers et du pied de dune. Les variations observées de la position du pied de dune entre 1954 et 1980 témoignent d'un littoral en érosion à un rythme moyen de-0,83 m/an. L'étude de la cinématique de la ligne des pleines mers entre 1980 et 2005 indique des évolutions importantes consécutives à l'aménagement de la jetée du port de Nouakchott en 1986. Le trait de côte a progressé de plus de 800 m en amont-dérive alors qu'au sud de l'épi, en aval-dérive, la côte a reculé de plus de 500 m. Les principales limites de l'approche méthodologique de cartographie de la mobilité du trait de côte résident dans les erreurs d'interprétation des lignes de référence ; celles-ci sont plus spécifiquement liées ici à la qualité des images aériennes utilisées et aux variations à court terme de la position de la limite des pleines mers. Ce sont là autant de sources d'erreur qu'il faut analyser et estimer avant de tirer des conclusions sur la cinématique littorale. MOTS CLÉ : trait de côte-ligne des pleines mers-pied de dune-Nouakchott-photographies aériennes-érosion-accrétion

Impacts from natural and anthropogenic coastal hazards are substantial and increasing significantly with climate change. Coasts and coastal communities are increasingly at risk. In addition to short-term events, long-term changes, including rising sea levels, increasing storm intensity, and consequent severe compound flooding events are degrading coastal ecosystems and threatening coastal dwellers. Consequently, people living near the coast require environmental intelligence in the form of reliable shortterm and long-term predictions in order to anticipate, prepare for, adapt to, resist, and recover from hazards. Risk-informed decision making is crucial, but for the resulting information to be actionable, it must be effectively and promptly communicated to planners, decision makers and emergency managers in readily understood terms and formats. The information, critical to forecasts of extreme weather and flooding, as well as long-term projections of future risks, must involve synergistic interplay between observations and models. In addition to serving data for assimilation into models, the observations are also essential for objective validation of models via hind casts. Linked observing and modeling programs that involve stakeholder input and integrate engineering, environmental, and community vulnerability are needed to evaluate conditions prior to and following severe storm events, to update baselines, and to plan for future changes over the long term. In contrast to most deep-sea phenomena, coastal vulnerabilities are locally and regionally specific and prioritization of the most important observational data and model predictions must rely heavily on input from local and regional communities and decision makers. Innovative technologies and nature-based solutions are already helping to reduce vulnerability from coastal hazards in some localities but more focus on local circumstances, as opposed to global solutions, is needed. Agile and spatially distributed response capabilities will assist operational organizations in predicting, preparing for and mitigating potential community-wide disasters. This white paper outlines the rationale, synthesizes recent literature and summarizes some data-driven approaches to coastal resilience.