Mid- to late-Holocene environmental evolution of the Loire estuary as observed from sedimentary and microfaunal characteristics

We used sedimentological and foraminiferal characteristics of four sedimentary cores, supported by paleogeographical and historical data, to reconstruct the depositional history of the inner Loire estuary (Near Saint-Nazaire, France) and the response of benthic foraminifera to the mid- to late-Holocene marine flooding of the incised valley. These were further used to evaluate the consequent changes in estuarine morphological and hydro-sedimentary patterns during this time period. Our results described significant changes in hydro-sedimentary dynamics over the past ~ 6 kyrs BP. At our location, these changes expressed the combined influence of marine (e.g., tide, storm waves) and fluvial dynamics (e.g., floods), which are linked, on a broader scale, to sea-level variations and the regional climate regime. Three main periods stand out: (1) from ~ 6.0 to ~ 2.5 kyrs BP, when the sea-level rise slowed down, a large brackish bay extended over and around the study area. The fine-grained tidal rythmites recorded north of the Bilho bank (the main tidal bar located in our study area) indicated a calm depositional environment, protected from the main riverine influence. The presence of thick flood deposits from ~ 5.4 to ~ 4.0 kyrs BP near the Bilho bank indicates further the dominance of humid conditions. (2) From ~ 2.5 kyrs BP to ~ 1850 CE (pre-industrial state), sea-level stabilized at its present value, and the pre-existing bay was progressively infilled. North of the Bilho bank, near a major mudflat (Méan), the generally homogenous sedimentation composed of silty muds rich in organic matter indicated a sheltered environment; the main water flow channel being located south of the Bilho bank. Within this overall homogenous sedimentation, foraminiferal assemblages described rather accurately the progressive infilling of the valley (indicated by a decrease in the proportions of outer estuarine species), accompanied with the channelization of the main entering marine currents (tide, storm waves) (indicated by an increase in the proportions of transported species from the adjacent upper continental shelf), and finally the buildup of the Méan mudflat and the stabilization of the environment to its present day configuration (indicated by the dominance of autochthonous inner estuarine species). (3) Since 1850 CE, the human impact progressively modified the general landscape of our study area with the construction of the Saint-Nazaire shipyard, the digging of the northern navigation channel and the polderization of the northern Bay. The southern channel was progressively abandoned by the main water flow in favor of the newly dug northern channel, causing the southern migration of the Bilho sandbank and the progressive filling of the southern channel

The Miguasha Fossil-Fish-Lagerstätte: a consequence of the Devonian land-sea interactions

International audienceThe evolution of vertebrate assemblages in terms of fluctuating environments has rarely been investigated for the Devonian period. Variation of biodiversity (richness, abundance and species composition) in the diverse Devonian fish assemblage of the Escuminac Formation (Quebec, Canada) is analysed in response to changes in lithofacies, depositional environment and taphonomy through time. Five sequences within an inner wave-dominated estuary show shifts in continentalisation. Although a ubiquitous fish assemblage is identified throughout the formation, species are more diversified and species composition is better structured during relative sea-level rise than during still-stand and relative sea-level fall. Konservat and Konzentrat Fossil- Lagerstätte horizons occur in the transgressive phase of the sequences

L'estuaire externe de la Vilaine : Paléoenvironnement et dynamique actuelle. Imagerie acoustique

Rapport mission Bingolain

Écoulements uniformes et graduellement variés en lit composé versus couches de mélanges libres

36th IAHR World Congress, La Haye, NLD, 28-/06/2015 - 03/07/2015International audienceMixing layers associated with uniform and gradually varied flows (GVF) in compound channel are experimentally investigated in two flumes, featuring a rectangular or a trapezoidal main channel (MC). These shear layers are compared with free mixing layers. Starting with uniform flow, the GVF are generated by an imbalance in the upstream discharge distribution between the floodplain (FP) and the MC. The GVF are longitudinally evolving under the influence of four external forcings: (i) a two-stage geometry, (ii) a varying vertical flow confinement (quantified by the relative flow depth, Dr = hf / hm, where hf and hm are the flow depths in the FP and MC); (iii) a variable lateral depth-averaged mean flow; (iv) a variable velocity ratio, l = Us/(2Uc), where Us = Ud2-Ud1 is the velocity difference, Uc = 0.5(Ud1+Ud2) is the mean velocity across the mixing layer, Ud1 and Ud2 are the depth-averaged velocities outside the mixing layer in the FP and MC, respectively. In the case of weakly or moderately sheared flows (l < 0.3 - 0.35), the peak values of scaled depth averaged Reynolds-stress, denoted Max (gd), are independent of l, but increase with a decrease in flow confinement, to reach the maximum values observed for free mixing layers. In the case of highly sheared flows (l > 0.3 - 0.35), the values of Max (gd) become independent of the flow confinement as l increases, reaching values that can be greater than the ones observed for free mixing layers. Despite the flow confinement, the high values of l trigger the development of 2D large coherent structures without interaction with the 3D bed-induced turbulence. Lastly, with nearly constant values of l and Dr, it was found that the scaled shear-layer turbulence was mainly dictated by the lateral flow. For both uniform flows and test cases with a lateral flow to the FP, the Rayleigh's inflection point criterion is fulfilled. This gives rise to large 2D structures and high values of Max (gd). By contrast, for test cases with a significant lateral flow to the MC, the convex velocity profiles without inflection-point are associated with low levels of Max (gd). Lastly, the trapezoidal MC was found to enhance the turbulent exchange compared with the rectangular one

The Holocene transgression as recorded by incised-valley infilling in a rocky coast context with low sediment supply (southern Brittany, western France)

International audienceA combination of morphobathymetric studies, very high-resolution seismics, core sampling and radiocarbon age data is used to investigate the latest stage of the sedimentary infilling of incised valleys in southern Brittany, related to the Holocene transgression. Owing to the bedrock morphology of this highly irregular rocky coast, two main types of valleys are defined by topographic rocky highs parallel to the coastline: 1) wide and rather shallow incised valleys offshore from a topographic sill, 2) narrow and relatively deep valleys between the sill and the coast (ria-type valley). The sedimentary infilling in both types of valleys becomes highly differentiated as the transgression advances onto the coastal area. In the wide valley seaward of the topographic sill, the infilling consists mainly of offshore heterolithic facies while, in the ria-type valley, most of the infill is composed of brackish mudflat deposits and estuarine tidal muddy sands. As the transgression proceeds, the rocky highs are flooded and the whole area is finally covered by the offshore facies. Radiocarbon dating indicates that: 1) the marine ravinement surface is highly diachronous (a few thousand years cross-shore); 2) the top of the offshore facies, coarser and very shelly, represents an episode of condensed sedimentation from about 3000 to 4000 years ago, amalgamating the maximum flooding surface (MFS) and the highstand systems tract (HST). However, we observe a muddy drape, strongly bioturbated in places, in the most proximal areas, overlying the offshore facies. It is thought to represent the modern and most recent stage of sedimentary infilling. This mud cover is made of fine-grained sediments of fluvial and biological origin, and is interpreted as a prograding HST. It reflects an increased influx, partly due to human activities. Finally, the main features of incised valley sedimentary infilling in a rocky coast context with low sediment supply can be characterized by (i) the very strong control of bedrock morphology, (ii) the diachronous character of the transgression, (iii) the late position of the MFS, and (iv), the highly reduced volume of the HST

Le projet Alert : une analyse de la vulnérabilité du patrimoine culturel côtier dans l'Ouest de la France.

International audienc

Coastal Changes and Cultural Heritage (1) : Assessment of the Vulnerability of the Coastal Heritage in Western France

International audiencePresent climatic change and anthropogenic pressure increasingly affect the coastal zone. The Channel and Atlantic seaboards of France are no exception, with hundreds of archaeological sites currently threatened by the accelerated relative rise in sea level, erosion, and various anthropogenic modifications to the environment. In this article we propose a multidisciplinary approach to assess the vulnerability of archaeological heritage in the West of France. We also aim to establish an original methodology (and specific tools) for the observation of risks and the definition of impacts they may have on heritage sites. This research opens up a theoretical perspective for building a vulnerability model for archaeological heritage that takes into account the evolution of the environment and the specificity of each site over the course of the next 20 to 30 years

Basement control on shaping and infilling of valleys incised at the southern coast of Brittany.

International audienceThe shape and infilling of the submerged parts of valleys incised along the southern coast of Brittany (France) have been investigated using very high-resolution seismics and a small number of piston cores. The valley location and morphology are found to be controlled mostly by submarine topography, which is marked by a well-developed fault zone that lies between the modern coast and a prominent basement-cored island and shoal complex located 5--15 km offshore. The faults controlled the shape of the valley networks and the amount of incision along the valley profile. They were probably active until the end of incision, because the valley thalwegs show scarps up to 10 meters high where they are crossed by these faults. The valleys were incised during the Quaternary lowstands of sea level, and most of the fill was emplaced during the last postglacial sea-level rise. The valley fills form a transgressive succession, consisting mainly of fluvial deposits at the base (possibly amalgamated from older sequences) overlain by tide-dominated estuarine deposits and capped by offshore muds. The most prominent internal surfaces are the tidal- and wave-ravinement surfaces. The valley-fill architecture is strongly dependent on the valley morphology (depth of incision, width of the valleys, and extent of estuarine intertidal areas). Estuarine deposits inside narrow and linear valleys are mostly aggrading muds, whereas those inside large and dendritic valleys dominantly comprise sandier, tidal-channel and bar deposits