NW France Research Articles

Influence of small- and large-scale ecological factors on the centipede (Chilopoda) assemblages of Armorican forests (NW France)

Despite their high functional role as abundant predators, the drivers of centipede assemblages remain poorly known in forests, especially in France. Using replicated depletion quadrats, we sampled five forests distributed over the Armorican Massif. A total of 5209 individuals belonging to 20 species was collected between 2015 and 2017. Assemblage composition was surprisingly constant across parcels, with only different dominant species in the oldest parcel (Bercé, Pays-de-Loire) and in the forest of Cerisy (Normandy). The latter hosted two exclusive species, likely because of both biogeographic and abiotic conditions, Geophilus truncorum and Lithobius aeruginosus. Species richness was overall close among sites and parcels, and therefore little influenced by tree age or dominant species. Individual densities varied between site pair-matched parcels, probably because of differences in temporal forest continuity rather than differences in tree ages. In quadrats, the presence of numerous ants strongly affected the number of centipedes, with global densities divided by an average factor of 7. Also at small spatial scale, we did not find a strong vertical stratification of centipede species in terms of presence-absence, with only 2 species exclusive of soil and none for litter, but with important differences in the relative abundance of dominant species. This study suggests that centipedes are important components of the forest soil and litter fauna, should be more frequently studied in French forests and have a potential of bio-indicators of local conditions in these habitats, as well as of temporal forest continuity.

Environmental controls on the distribution of brGDGTs and brGMGTs across the Seine River basin (NW France): implications for bacterial tetraethers as a proxy for riverine runoff

Abstract. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial lipids that have been widely used as environmental proxies in continental paleorecords. Another group of related lipids, branched glycerol monoalkyl glycerol tetraethers (brGMGTs), has recently been proposed as a potential paleotemperature proxy. Nevertheless, the sources and environmental dependencies of both brGDGTs and brGMGTs along the river–sea continuum are still poorly understood, complicating their application as paleoenvironmental proxies in some aquatic settings. In this study, the sources of brGDGTs and brGMGTs and the potential factors controlling their distributions are explored across the Seine River basin (NW France), which encompasses the freshwater-to-seawater continuum. BrGDGTs and brGMGTs were analyzed in soils, suspended particulate matter (SPM), and sediments (n=237) collected along the land–sea continuum of the Seine basin. Both types of compounds (i.e., brGDGTs and brGMGTs) are shown to be produced in situ, in freshwater and saltwater, based on their high concentrations and distinct distributions in aquatic settings (SPM and sediments) vs. soils. Redundancy analysis further shows that both salinity and nitrogen dominantly control the brGDGT distributions. Furthermore, the relative abundance of 6-methyl vs. that of 5-methyl brGDGTs (the IR6Me ratio), the total nitrogen (TN), the δ15N, and the chlorophyll a concentration co-vary in a specific geographical zone with low salinity, suggesting that 6-methyl brGDGTs are preferentially produced under low-salinity and high-productivity conditions. In contrast to brGDGTs, the brGMGT distribution appears to be primarily regulated by salinity, with a distinct influence on the individual homologues. Salinity is positively correlated with homologues H1020a and H1020b and negatively correlated with compounds H1020c and H1034b in SPM. This suggests that bacteria living in freshwater preferentially produce compounds H1020c and H1034b, whereas bacteria that primarily grow in saltwater appear to be predominantly responsible for the production of homologues H1020a and H1020b. Based on the abundance ratio of the freshwater-derived compounds (H1020c and H1034b) vs. their saltwater-derived homologues (H1020a and H1020b), a novel proxy, the Riverine IndeX (RIX), is proposed to trace riverine organic matter inputs, with high values (>0.5) indicating a higher riverine contribution. We successfully applied RIX to the Godavari River basin (India) and a paleorecord across the upper Paleocene and lower Eocene from the Arctic Coring Expedition at Lomonosov Ridge, showing its potential applicability to both modern samples and paleorecords.

Trace elements in coralline algae as a new proxy for seawater chemistry and metal pollution

The abundances of some macronutrients, and trace elements (K, Al, P, V, Mn, Co, Ni, Cu, Zn, Rb, Sr, Mo, Cs, Ba, Pb, Th, U, Y and REE) were determined in a series of coralline algae (Lithothamnion corallioides) samples (n = 101) collected alive in the Bay of Brest and the Iroise Sea (Western Brittany, NW France), in order to assess the potential of these algae as archives of seawater chemistry and potential metal pollution. REE and Y (REY) patterns are similar in shape to those of local seawater, exhibiting similar La, Ce and Y anomalies, but with abundances ranging between 4 and 5 orders of magnitude higher than seawater values. Variations in La anomalies (La/La* = 1.29–2.08), Y anomalies (Y/Ho = 38.6–55.8), and heavy rare earth enrichments (Prsn/Ersn = 0.22–0.52) are consistent with mixing of seawater with rivers flowing into the Bay of Brest. The behavior of other trace elements, such as Al and Cs, also reflects this mixing. Other parameters and processes can control the abundances of the other elements measured. For example, the presence of organic matter in studied samples controls the abundances of K and Rb. The abundances of base metals (e.g., Co, Ni, Cu) are highly sensitive to the various pollutants present in the Bay of Brest. In particular, the Pb content of coralline algae clearly reflects the pollution caused by mining of a nearby Pb deposit from the 18th to the early 20th century. Our results demonstrate the potential of coralline algae not only for tracing water masses, but also for studying metal pollution.

The Lower Cambrian deposits of the Le Rozel Formation (Normandy, NW France): Insight into a newly described ichnofossil assemblage

The rise of morphological complexity and taxonomic diversity of trace fossils during the Cambrian Period has been a prominent focus to track the evolution and diversification of early animals. Here, we present a newly described trace fossil assemblage from the lower Cambrian Le Rozel Formation in Normandy (Northwestern France) composed of the following ichnogenera: Archaeonassa, Bergaueria, Helminthoidichnites, Helminthopsis, Nereites, Psammichnites, and Treptichnus. Trace fossils are exquisitely preserved on top of strata (i.e., epirelief), and are distributed through the entire thickness of the siliciclastic succession of about 200 m. This thick sedimentary succession shows sedimentary environments with ripples generated by unidirectional current processes or by oscillatory processes, such as hummocky cross-stratification (HCS) storm facies, tidal facies with flaser stratification, and unidirectional current ripples. In addition, syneresis cracks and microbially induced sedimentary structures (MISS) suggest that the Le Rozel Formation was deposited in a shallow marine shelf environment. New U-Pb dating of detrital zircon grains suggests a late Ediacaran maximum deposition age of 549 ± 3 Ma. This dating is supplemented by the substantial preservation of the three-dimensional burrow systems (Treptichnus pedum) and the large sediment bulldozers (Nereites, Psammichnites). Collectively, our findings suggest that the trace fossils assemblage was deposited during the Terreneuvian Epoch, beneath the first archaeocyaths and the first trilobite-bearing facies from both other lower Cambrian Carteret and Saint-Jean-de-la-Rivière Formation. The ichnofauna described from the Le Rozel Formation complements the worldwide trace fossil record around this critical time of life history.

Île Dumet (Armorican Massif, France) and its glaucophane eclogites: the little sister of Île de Groix

Abstract. Blueschist-facies rocks are scarce within the Variscan orogen. Two main occurrences are known in the Armorican Massif (NW France), at Île de Groix and Bois-de-Céné. Another glaucophane occurrence was discovered in 1988 but went unnoticed; it is located on Île Dumet, an uninhabited island off the coast of southern Brittany, in the estuary of the river Vilaine. Orthogneiss occurs on the SW half of the island; the original granitoid magma had intruded mica schists on the NE half, where numerous 1 to 10 m long boudins of mafic rocks occur. These lenses are typically retrogressed into plagioclase-bearing amphibolite, but a few contain remnants of glaucophane-bearing eclogite, which also occurs as numerous loose blocks along the NE coast of the island, suggesting that the best-preserved eclogites lie in situ offshore in that direction. The glaucophane eclogites contain garnet, omphacite, quartz, amphibole, clinozoisite/epidote, minor phengite, paragonite, rutile and rare apatite. Prograde metamorphic evolution is indicated by garnet crystals zoned from Mn-rich cores to Mg-richer rims (typically (core→rim): Alm44→58 Prp1→12 Grs33→29 Sps22→1) and amphibole grains with glaucophane nuclei and Ca–Na-amphibole overgrowths that show sharp transitions, supporting evolution through a solvus, as predicted by the thermodynamic modelling. Modelling of the P–T conditions using the P–T pseudosection technique indicates a peak of metamorphism at about 620 ∘C and 16 kbar. The retrograde evolution of the metabasites is evidenced by the late formation of albite, titanite and ferro-actinolite. The surrounding mica schists, composed of quartz, garnet, phengite, paragonite and chlorite, were also largely retrogressed during exhumation. The orthogneiss of the SW part of the island does not show clear evidence of high-pressure metamorphism, since the magmatic feldspars are still preserved, similarly to the orthogneiss of Les Sables Rouges on the island of Groix. Île Dumet and the western part of the Vilaine estuary represent a blueschist-facies equivalent to Île de Groix (Brittany) and Bois-de-Céné (Vendée) on the mainland. All three occurrences occupy the centres of wide synforms whose concentric units are, from rims to core (i.e. from base to top), (a) a high-T migmatitic basement; (b) Cambro-Ordovician metasediments and acid metavolcanites (“porphyroids”); and (c) blueschist-facies mica schists and metabasites, serpentinites, and minor orthogneisses derived from a pre-Variscan oceanic accretionary prism. There are about 10 similar occurrences within the Ibero-Armorican Arc, forming a discontinuous high-pressure belt, but most of them have remained unnoticed due to a high degree of retrogression.

Reproduction of Marphysa sanguinea Annelida, Polychaeta (Eunicidae), at Mount Edgecombe, Plymouth, near the type locality in Southwest England

Abstract The reproductive cycle of Marphysa sanguinea is described for a population at Mount Edgecombe, Plymouth, near the type location in Southwest England, using a data set obtained previously (October 1999 to September 2000). The species is iteroparous without schizogamy, spawning prior to October 1999 and during a short breeding season in 2000 from end August through September. The sexes are separate with a sex ratio of 1:1. Mature oocytes and spawned eggs are 215 μm in diameter and spermatozoa of the ectaqua sperm type. Mature gametes of both sexes are discharged through paired coelomoducts, and the diploid chromosome number is 28. Proliferation of new coelomic gametes from paired gonads began within a month of spawning and continued for 8–9 months but ovulation was suppressed in June and July. Attempts to undertake fertilisation using spawned oocytes and active spermatozoa were unsuccessful. The size of discharged oocytes suggests a short pelagic larval duration of a few days. This is the first publication about the reproduction of this species, and our results suggest that M. sanguinea is restricted to intertidal areas in SW England, NW France and southern North Sea. The highly synchronised pattern of reproduction observed is not compatible with a quasi-cosmopolitan species range indicating that this species has been mistakenly reported from around the world. Future studies of the genus should combine rigorous taxonomy with observations of reproduction to facilitate comparison among Marphysa spp.

Ordovician sedimentation and basin development in the North Armorican Massif, NW France: Field evidence from the northern Cotentin Peninsula

AbstractField evidence from the northern Cotentin Peninsula and regional data are used to construct a tectono-stratigraphic model for the Ordovician which characterizes basin development in the North Armorican Massif. In La Hague, 15 m of transgressive marine sandstone belongs to the Dapingian age Grès Armoricain Formation which onlaps lower Cambrian, rift-fill deposits via an unconformity. Approximately 450 m of overlying Darriwilian strata are dominated by shallow marine sandstone showing hummocky cross-stratification with subordinate shales containing trace and body fossils. Together, these facies support an interpretation of offshore shallow marine strata overlying a break-up unconformity. Regional analysis indicates the time gap at the unconformity is 20–40 Ma and formed from crustal upwarping, which was greatest in the north of the Armorican Massif. Dapingian strata (Grès Armoricain) thins irregularly to the north (0–94 m), interpreted to reflect passive onlap onto residual relief associated with the uplift and the initiation of thermal subsidence on the margin. The succeeding Darriwilian strata (Schistes de Beaumont to Grès de May) conversely display a steady thickening (161–623 m) to the north, the stratal patterns suggesting that from the Darriwilian onwards, the ocean basin to the north was firmly established as the main locus of subsidence on the continental margin.

Elliptical body fossils from the Fortunian(Early Cambrian) of Normandy (NW France)

Body fossils have been discovered in the Fortunian deposits of the Rozel Cape, in Normandy (NW France). The material consists of about 80 specimens preserved on a shale surface, recently observed at the base of a cliff at the Cap Rozel, in the Cotentin region. The fossils, centimetric in size, have an elliptical outline, with a peripheral bulge, generally without other conspicuous ornamentation, but showing sometimes concentric or radial lines possibly of taphonomic origins. In addition, these body fossils are preserved parallel to the bedding plane, locally rich in horizontal trace fossils (e.g. Archaeonassa Fenton & Fenton, 1937, Helminthoidichnites Fitch, 1850, Helminthopsis Heer, 1877) and also complex treptichinids burrows (e.g. Treptichnus pedum (Seilacher, 1955)) sometimes associated with microbial mats. The sedimentological characteristics of these deposits (ripple marks, syneresis cracks) correspond to a shallow marine shelf environment, with a variable hydrodynamism in the intertidal zone, low for surfaces showing elliptic fossils and syneresis cracks, higher for surfaces with ripple marks. These new discoveries unravel the potential of the Fortunian strata from Normandy and provide new information about the early Cambrian biocenoses.

New CNN-based tool to discriminate anthropogenic from natural low magnitude seismic events

SUMMARY With the deployment of high quality and dense permanent seismic networks over the last 15 yr comes a dramatic increase of data to process. In order to lower the threshold value of magnitudes in a catalogue as much as possible, the issue of discrimination between natural and anthropogenic events is becoming increasingly important. To achieve this discrimination, we propose the use of a convolutional neural network (CNN) trained from spectrograms. We built a database of labelled events detected in metropolitan France between 2020 and 2021 and trained a CNN with three-component 60 s spectrograms ranging frequencies from 1 to 50 Hz. By applying our trained model on independent French data, we reach an accuracy of 98.2 per cent. In order to show the versatility of the approach, this trained model is also applied on different geographical areas, a post-seismic campaign from NW France and data from Utah, and reaches an accuracy of 100.0 and 96.7 per cent, respectively. These tests tend to hypothesize that some features due to explosions compared to earthquakes are widely shared in different geographical places. In a first approach, we propose that it can be due to a contrast in the energy balance between natural and anthopogenic events. Earthquake seismic energies seem to be more continuous as a function of frequency (vertical bands features in a spectrogram) and conversely for explosions (horizontal strips).

Supplemental Data 1. The oldest articulated ranid from Europe: A Pelophylax specimen from the earliest Oligocene of Chartres-de -Bretagne (NW France)

Supplemental Data 1. The oldest articulated ranid from Europe: A Pelophylax specimen from the earliest Oligocene of Chartres-de -Bretagne (NW France)

Sedimentological and palynological records since 10 ka BP along a proximal-distal gradient on the Armorican shelf (NW France)

New findings acquired in Armorican shelf (core MD08-3204 CQ: Bay of Quiberon and core VK03-58bis: South Glénan islands) depict Holocene paleoenvironmental changes since 10 ka BP through a multi-proxy dataset including sedimentological and palynological data. First, grain-size analyses and AMS-14C dates show a common sedimentary history for both study cores. The slowdown of the relative sea level (RSL) rise was accompanied by a drop of the sedimentation rates between ∼8.3 and 5.7 ka BP; the rates had been relatively higher at the onset of the Holocene. This interval led to the establishment of a shell-condensed level, identified in core VK03-58bis by the “Turritella layer” and interpreted as a marker for the Maximum Flooding Surface. Palynological data (pollen and dinoflagellate cyst assemblages) acquired in core MD08-3204 CQ argue for an amplification of the fluvial influence since 5.7 ka BP; the establishment of the Highstand System Tract (i.e., mixed marine and fluviatile influences on the platform) then accompanied the slowdown of the RSL rise rates. On the shelf, the amplification of Anthropogenic Pollen Indicators (API) has been better detected since 4.2 ka BP, due not only to the increase of human impact but also due to a stronger fluvial influence on the shelf during the Late Holocene. Palynological data, recorded on the 8.5–8.3 ka BP interval along an inshore-offshore gradient, also demonstrate the complexity of the palynological signals such as i) the fluvial influence that promotes some pollen taxa (Alnus and Corylus) from proximal areas and ii) the macro-regionalization of palynomorph sources in distal cores. In addition, the comparison of palynological tracers, including API, over the last 7 kyrs BP, with South Brittany coastal and mid-shelf sites subject to northern vs. southern Loire catchment areas, shows a major hydro-climatic effect on the reconstructed palynological signals. Strengthened subpolar gyre (SPG) dynamics, combined with recurrent positive North Atlantic Oscillation (NAO) configurations, appear responsible for increased winter precipitation and fluvial discharge over northern Europe, as in Brittany. Conversely, weakened SPG intervals, associated with recurrent negative NAO-like modes, are characterized by intensified winter fluvial discharge over southern Europe. Interestingly, at an infra-orbital timescale, we record major peaks of API during periods of strengthened (/weakened) SPG dynamics in sites subject to detritic-organic matter inputs from Brittany (/Loire) watersheds.

The Bay of Mont Saint Michel. Sedimentary facies, morphodynamics and Holocene evolution of a hypertidal coastal system

The aim of this 3-day field trip in the Bay of Mont-Saint-Michel, located between Normandy and Brittany (NW France), is to examine the different sedimentary environments, which compose this hypertidal coastal system (a tide-dominated, wave-influenced embayment, a tide-dominated estuary, a wave-dominated sandy shoreline). Hydrodynamics, sedimentary facies, and sequences, Holocene infill, and evolution will be discussed through field observations, sediment cores, very high resolution seismic, and ground-penetrating radar data. The field trip includes a sightseeing tour of Mont-Saint-Michel and an overview of Norman gastronomy.

The Early Cambrian Deposits of the Le Rozel Formation (Normandy, Nw France): Insight into a New Ichnofossil Assemblage

The Early Cambrian Deposits of the Le Rozel Formation (Normandy, Nw France): Insight into a New Ichnofossil Assemblage

Distribution of modern dinocysts in surface sediments of southern Brittany (NW France) in relation to environmental parameters: Implications for paleoreconstructions

Dinoflagellate cyst assemblages from 15 modern surface sediment samples of the Bay of Quiberon (Southern Brittany shelf) have been examined to assess their potential as marine bio-indicators for paleoenvironmental reconstructions in a shallow coastal environment. Some discrepancies are noted in the distribution of dinocyst taxa in the study area, and particularly regarding dinocyst concentration and diversity (26 different taxa identified in total) as well as heterotrophic taxa percentages. We suggest that the proportion of heterotrophic taxa is, in an embayment of 15 m deep in average, mainly attributed to bottom water oxygenation and sediment granulometry, both acting on species-selective degradation after dinocyst deposition. More precisely, higher heterotrophic abundances are found under lower oxic conditions and in fine grain-size sediment samples, leading to caution about their use as productivity indicators in coastal environments when these parameters are not fully addressed. The comparison of the Bay of Quiberon data with surface sediment samples and top cores from previously published data makes it possible to establish a transect of the modern dinocyst distribution from inshore to offshore areas in the northern Bay of Biscay, allowing to identify different ecological groups according to the hydrological and bathymetric contexts: i) an estuarine assemblage strongly dominated by Lingulodinium machaerophorum, ii) a proximal coastal assemblage dominated by L. machaerophorum and, to a lesser extent, Spiniferites bentorii, iii) a neritic assemblage dominated by L. machaerophorum, Spiniferites ramosus and cysts of Pentapharsodinium dalei, and iv) an oceanic group dominated by Spiniferites mirabilis and Operculodinium centrocarpum.

Energy Characteristics of Acoustic Emission at the Volume-Expansion Point of a Rock Bridge: A New Insight into the Evolutionary Mechanism of Coastal Cliff Collapse

The recession of a coast can destabilize coastal cliffs. The stability of a cliff is controlled by a rock bridge. Identifying the volume-expansion point of rock bridges is crucial to assess cliff stability, but currently there are few identifying methods. Using a numerical analytical tool, we investigate the acoustic emission characteristics during shear tests on rock bridges. Acoustic emission events with a high energy level, i.e., characteristic events which occur at the volume-expansion point of rock bridges, can indicate this point. The characteristic events, the mainshock (the maximum event corresponding to rock-bridge rupture), and the smaller events between them constitute a special activity pattern, as the micro-seismicity during the evolutionary process of a coastal cliff collapse in Mesnil-Val, NW France showed. This pattern arises in rock bridges with different mechanical properties and geometry, or under different loading conditions. Although the energy level of characteristic events and mainshocks changes with the variation of the conditions, the difference of their energy level is approximately constant. The spatial distribution of characteristic events and mainshocks can indicate the location of rock bridges. These findings help to better understand the evolutionary mechanism of collapses and provide guidelines for monitoring the stability of coastal cliffs.

Enigma variations: characteristics and likely origin of the problematic surface texture Arumberia, as recognized from an exceptional bedding plane exposure and the global record

AbstractArumberia is an enigmatic sedimentary surface texture that consists of parallel, sub-parallel or radiating ridges and grooves, most commonly reported from upper Neoproterozoic – lower Palaeozoic strata. It has variably been interpreted as the impression of a small metazoan, a ‘vendobiont’, a physical sedimentary structure formed on a substrate with or without a microbial mat covering, or a non-actualistic microbial community. In this paper we contribute new insights into the origin of Arumberia, resulting from the discovery of the largest contiguous bedding plane occurrence of the texture reported to date: a 300 m2 surface in the lower Cambrian Port Lazo Formation of Brittany, NW France. We compare the characteristic features of Arumberia at this locality with 38 other global records, revealing four defining characteristics: (1) the three-dimensional (3D) morphology of exposed Arumberia lines (either positive relief ‘ridges’ or negative relief ‘grooves’) records fully preserved cords within clay laminae; (2) lines may transition laterally into reticulated patterns; (3) characteristic parallel and sub-parallel Arumberia lines can become modified by desiccation on emergent substrates prior to interment; and (4) Arumberia are streamlined with palaeoflow in successions showing evidence of unidirectional currents, but are organized parallel to ripple crests where strata were sculpted by oscillatory flows. These characteristics indicate that Arumberia records a 3D entity, distinct in material properties from its host sediment, which occurred in very shallow water settings where it was prone to passive reorganization in moving water, and desiccation when water drained. A literature survey of all known Arumberia occurrences reveals that the most reliable examples of the form are stratigraphically restricted to a 40 Ma interval straddling the Ediacaran–Cambrian boundary (560–520 Ma). Together these characteristics suggest that Arumberia records the remains of extinct, sessile filamentous organisms (microbial or algal?) that occupied very shallow water and emergent environments across the globe at the dawn of the Phanerozoic Eon.

Potential Impact of Photoinhibition on Microphytobenthic Primary Production on a Large Intertidal Mudflat

AbstractMicrophytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into the photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high‐resolution tri‐dimensional hydrodynamic model coupled to an MPB model with or without photoinhibition. Photoinhibition leads to a 20% (−0.79 × t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (−29%) than on the lower shore (−5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (−22% and −23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter‐annual time scale.

Evolution of a Holocene banner bank controlled by morphodynamics and structural setting of a macrotidal coast: Saint-Brieuc Bay (NW-Europe)

The morphology and internal structure of the Horaine Bank (Bay of Saint-Brieuc, NW France) are described based on multibeam echosounder and high-resolution seismic datasets coupled with vibro-core data. The Horaine Bank shows large-scale bedforms in the lee of a submerged rocky shoal, which allowed defining it as a Banner Bank. The internal structure of the sandbank reveals four seismic units (U1–U4) on a Cambrian basement (U0). The basal unit U1 is interpreted as reworked lowstand fluvial sediments those infilled micro incised valleys during a rise in sea level. This unit is overlain by paleo-coastal barrier sand-spit (U2) whose development was controlled by swell in the context of a rapid rise in sea level. The successive prograding unit (U3) is interpreted as flooding deposits in continuity with unit U2. The unit U4 is characterized by oblique reflectors oriented in two opposite directions. This last unit, dated post 3500 yr BP, corresponds to migrating dunes superimposed on the bank and observable in the high-resolution bathymetric data. The strong correlation between tidal currents and the apparent clockwise migration of dune crests suggests the presence of a tidal gyre controlling the present-day dynamics of most of the Horaine bank dunes. This study proposes a new model for the construction of banner banks characterized by the gradual transition of a sand spit to a banner bank during marine transgression and ensuing hydrodynamic variability.

New insights into the early evolution of horizontal spiral trace fossils and the age of the Brioverian series (Ediacaran–Cambrian) in Brittany, NW France

AbstractIn northwestern France, the Brioverian series is a thick siliciclastic succession deposited during the Cadomian cycle (c. 750–540 Ma). In the uppermost Brioverian beds, previous studies unravelled an assemblage dominated by simple horizontal trace fossils associated with microbially stabilized surfaces. Here, we report Spirodesmos trace fossils – one-way, irregular and regular horizontal spirals – from Crozon (Finistère, Brittany), Montfort-sur-Meu and St-Gonlay (Ille-et-Vilaine, Brittany). After reviewing the literature on horizontal spiral trace fossils, an Ediacaran–Fortunian Spirodesmos pool is identified from marginal-marine to shelf settings, while an Ordovician–Recent trend formed in the deep-marine realm. These results suggest that an onshore–offshore migration in Spirodesmos took place during Ediacaran–Fortunian to Ordovician time, similar to what happened in graphoglyptids. In addition, the age of the uppermost Brioverian beds (Ediacaran or early Cambrian) is still a pending question. Here, we report two new U–Pb detrital zircon datings from sandstone samples in St-Gonlay, giving maximum deposition ages of 551 ± 7 Ma and 540 ± 5 Ma. Although these results do not discard an Ediacaran age for the uppermost Brioverian beds, a Fortunian age is envisioned because the new dating corroborates previous dating from Brittany, Mayenne and Normandy. However, the intervals of error of the radiometric dating, and the dominance of non-penetrative trace fossils associated with matgrounds (an ecology more typical of the Ediacaran Period), do not allow definitive conclusions on the age of the uppermost Brioverian beds.

Coastal chalk cliff retreat rates during the Holocene, inferred from submarine platform morphology and cosmogenic exposure along the Normandy coast (NW France)

Submerged marine terraces potentially provide crucial information on past sea-level variations and paleo-coastline locations that may be used to estimate long-term coastal erosion rates. The Normandy coastline has recently been surveyed using a shallow water high-resolution mapping system. We identified a new continuous submarine platform, called the inner platform, limited by a shore parallel edge located between -9 m and -10 m (NGF) along the Normandy chalk coastline. A lower rock platform, called the outer platform, ranging from about -14 m to -17 m (NGF) appears locally. This corresponds to inherited preserved submarine terraces created during a past sea level highstand. The high cosmogenic 10Be concentration measured at the end of Mesnil-Val inner shore platform (including intertidal and subtidal shore platforms) is attributed to the last glacial cliff location at 6.5 ky ± 1 ky. From the spatial edge location of the inner platform in Normandy, we estimated cliff retreat rates since 6.5 ky ± 1 ky ranging from 0.051 ± 0.008 m/y to 0.090 ± 0.014 m/y from place to place. Comparisons with the current coastal chalk cliffs indicate a mean retreat rate estimated over the contemporary period suggesting such long-term retreat rates are 33% to 57% lower than the contemporary ones (0.10 m/y to 0.18 m/y). This confirms a contemporary acceleration of chalk cliff system retreat rates.