A 200-km2 area in the Bay of Concarneau on the South Brittany coast was surveyed acoustically using different sidescan sonars (a 100-kHz EdgeTech DF1000, and a 240-kHz Reson SeaBat 8101). The area corresponds to a sector of the REBENT network. It was selected for its physical and biological characteristics, reflecting the sedimentary heterogeneity and biological diversity of Brittany's coastal seafloors. The work presented here illustrates the methodology for mapping subtidal seabed habitats in the context of the network. Backscatter mosaics were produced covering 100% of the survey area. Extensive ground-truthing was carried out involving 93 Shipek grab samples and 25 drop-down video profiles. From interpretation of the acoustic facies, 40 biological soft-bottom stations were sampled using a Hamon grab to characterize macrobenthic communities (>2 mm). The results indicated considerable variation in backscatter responses in relation to high densities of macrobenthic species (Lithothamnion, Asterias, Haploops, Maldane, Ophiocomina), and a wide variety of substratum types present within a relatively small area. Dense biocenoses of maerl were accurately surveyed from 20-m to <5-m depth (Lower Astronomical Tide; LAT). Boundaries of Haploops communities are associated with dense small pockmarks in the centre of the bay. The relationships between sediment sometimes colonized by macrobenthic species and backscatter responses are discussed.

This study details the sedimentary infilling of an original tidal-dominated estuary system during the final stage of the last marine transgression. The Bay of Brest is confined and connects the rivers Elorn and Aulne, to the sea of Iroise by a narrow strait encasing a well preserved paleo-channel. The compilation of high-and very-high-resolution bathymetric and seismic data, constrained by sediments datations, allows us to classify the paleo-morphology of the bay into three stepped domains: the paleo-valley floor surrounded by fluvial terraces, the central plateau, and the shallow embayments. Taking into account the main factors controlling the infilling, including sea-level rise, substratum morphology, and hydrodynamics, the stratigraphic scheme of the bay has been reconstructed. The lowstand system track (LST) is assumed to correspond to relict Pleistocene continental deposits associated with the last low sea level around 21,000 cal yr B.P. The transgressive phase, represented by the transgressive system track (TST), is divided into two parts. The lower part (TST1) constitutes the first stage of transgressive deposition in the bay. Starting around 9000 cal yr B.P. and currently conserved in the shallowest parts, it is characterized by tidal flats associated with deposits in the inner estuary. This stage ends at about 7000 cal yr B.P. and is separated from the upper part (TST2) by a tidal ravinement surface that occurs around 7700 cal yr B.P. at the foot slope. TST2 formed sand bodies in the central part of the bay between 6800 and 3000 cal yr B.P. These deposits, interpreted as tidal banks, are associated with the outer estuarine environment. The maximum flooding surface (MFS), dated around 3000 and 2000 cal yr B.P., marked the installation of a highstand system track (HST) under the combined influence of tidal currents, storms events, and anthropogenic activity. The geometry deposition of each system track is controlled at the first order by the combination of the irregular Holocene sea level rise and the inherited rocky substratum morphology. The retreat of the shoreline, and estuarine environments, is function of the successive flooding of stepped domains. The direct evolution of the volume of the assumed accumulated sea water in the bay favoured the gradual installation of the present-day hydrodynamic circulation. Highlights ► Sedimentary records retrace the landward retreat of tidal processes at the Holocene. ► The stepped bedrock allows unique preservation of depositional environments. ► The bed-rock morphology has a key-role on the conservation of sediment deposition.

Dutertre, M., Hamon D., Chevalier C., and Ehrhold, A. 2013. The use of the relationships between environmental factors and benthic macrofaunal distribution in the establishment of a baseline for coastal management – ICES Journal of Marine Science, 70: 294–308. Relationships between benthic macrofauna and natural abiotic factors were studied along the coastal fringe of South Brittany, situated north of the Gulf of Biscay on the French Atlantic continental shelf. Within the framework of the REBENT network, sediment characteristics, depth, and macrofaunal abundance were determined for 95 stations spread over five subtidal sectors, using a combination of seabed acoustic remote sensing systems and grab sampling. The physico-chemical properties of the water column and the hydrodynamic conditions were generated by validated three-dimensional environmental models which take into account variations over shorter temporal scales. Multivariate analyses ranked 16 natural abiotic variables according to the significance of their influence on the macrofauna. Together these variables explained 51% of spatial variation in the macrofauna, with morpho-sedimentological and hydrological factors contributing 22% and 26%, respectively. The outputs from validated three-dimensional environmental models appear to be useful interpretational tools for benthic ecology studies, especially in estuarine and coastal ecosystems with high environmental variability due to regular freshwater inputs. Ten major species assemblages were identified using biological and physical characteristics. The results provide important baseline knowledge for future ecosystem and resource management.

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International audienceAn exceptional structural picture of the immerged Variscan basement, offshore the Leon metamorphic domain, is supplied by high-resolution LiDAR and echosounder data recorded in the Molène archipelago, western Brittany (France). Various types of fabrics are identified and, from in situ rock sample analyses further combined with field structural data, are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures. Our onshore/offshore study leads us to propose a two-phase kinematic model that emphasizes the role of a strike-slip duplex in an EW-trending relay zone linking the North Armorican and Pierres Noires ductile shear zones (NASZ, PNSZ). Dextral shearing occurred within a transtensional setting, synchronously with magmatic intrusions (St-Renan granite and an offshore gabbro-diorite complex) dated at 314-320 Ma by new U-Th/Pb ages. It post-dated an early regional foliation related to top-to-the-NE ductile transpressional shearing. Our study emphasizes the key role of strike-slip tectonics in the NW part of the Armorica Variscan belt