The spatial distributions of (1) surface sediment characteristics (D0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ13C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.

On the Bay of Biscay continental shelf, there are several mid-shelf mud patches including La Grande Vasie`re to the north, the West Gironde Mud Patch (WGMP) off the Gironde estuary and the Basque Mud Patch close to the Spanish border. In general, these deposits are several meters thick and cover coarser substrate. Questions remain about their storage capability for fine particles and carbon. This work investigates the sedimentation of the WGMP in order to develop a first estimate of organic carbon (OC) burial. Interface sediment cores were collected at nine stations along two cross-shelf transects in October-November 2016. X-radiograph imaging and grain-size analyses were used to characterize sedimentary structures. Pb-210(xs) depth profiles were established to calculate sediment (SAR) and mass (MAR) accumulation rates. Sedimentary structures indicate episodic sandy inputs overlying older deposits at proximal sites, and relatively continuous sedimentation at seaward locations. On the outer-central portion of the northern transect, a maximum SAR (0.47 cm yr(-1)) was observed, suggesting a depocenter. On the southern transect, excluding two stations where sedimentary inputs appear massive but sporadic, the SARs are lower (<0.3 cm yr(-1)). Quantitative estimates of OC burial rates increase seaward with a maximum of 45 gC m(-2) yr(-1). To evaluate carbon loading independent of grain-size variability, OC values were normalized to surface area of sediments (SA). Interestingly, a qualitative comparison of OC burial efficiencies using the OC/SA ratio highlights three groups of sites (low, medium and relatively high OC burial efficiency) which are likely related both to different sedimentary environments and variable deposition conditions linked to local environmental conditions and depth. This work highlights the likely control of hydrodynamic intensity and sedimentary inputs on the amount of OC stored in the WGMP sediments.

High coastal erosion rates along the beaches of the Médoc peninsula urge for the study of the Pleistocene and Holocene deposits buried under the modern dune. Those deposits are known to be exceptionally rich in archaeological remains (from the Mesolithic at least). The aggressive predation of these fossil deposits by the sea explains the urgency of exploring their content and their sedimentological history. Within these formations, the “Lède du Gurp” archaeological site located on the shore, on the administrative boundary of Soulac-sur-Mer and Grayan-et-l’Hôpital, is of tremendous value for palaeogeographical and palaeoenvironmental reconstructions of the Médoc peninsula. This site has preserved remains of dense anthropic activity but also evidence of the history of the estuary’s migration before our contemporary era (i.e. recent centuries). Here we present results of new sedimentological investigations conducted as part of the LITAQ project on and around the site. Continuous and undisturbed sections several metres thick (up to 3.5 m) have been obtained by sampling (sediment slides) and coring operations in the field and studied for their lithostratigraphy (using classic and X-ray imaging coupled with spectrocolorimetry, X-ray fluorescence and grain-size analyses together with radiocarbon dating) along with simultaneous archaeological investigations. Our study refines the palaeoenvironmental evolution of the site over at least the Neolithic period and supports previous interpretations of the origin of these atypical lithofacies, i.e. the deposition of a thick Holocene sequence of clays and peats in a depression associated with karstification processes. Our new datings and observations suggest the possible occurrence of a thermokarst system during the coldest phases of the last deglaciation.

The cordgrass Spartina anglica is an introduced species that tends to invade sheltered sand and mudflats, at the upper low marsh level. In Arcachon Bay, a lagoon in South West of France, the cordgrass can also replace Zostera noltii beds. The consequence of cordgrass presence on macrobenthic fauna was estimated and compared to adjacent habitats (bare sands, Z. noltii sea grass beds) during one year. The communities of the three habitats were characterised by low species richness, low abundance and biomass (when Hydrobia ulvae, 90% of abundance, is not considered) and high seasonal stability. The infaunal assemblages were particularly homogeneous between habitats without any characteristic species.Cordgrass eradication experiments were performed and zoobenthic recolonisation was observed the following year. Modifications in benthic fauna were observed on epifauna only.These results highlight the limited structuring effect of habitat heterogeneity at high tidal levels and in soft-bottom sediments where desiccation becomes the dominant factor determining infauna community structure. (c) 2006 Elsevier Ltd. All rights reserved.