Armorican Shelf Research Articles

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 Impact of Tides on the Bay of Biscay Dynamics

The impact of tides on the Bay of Biscay dynamics is investigated by means of an ocean model twin-experiment, consisted of two simulations with and without tidal forcing. The study is based on a high-resolution (1/36∘) regional configuration of NEMO (Nucleus for European Modelling of the Ocean) performing one-year simulations. The results highlight the imprint of tides on the thermohaline properties and circulation patterns in three distinct dynamical areas in the model domain: the abyssal plain, the Armorican shelf and the English Channel. When tides are activated, the bottom stress is increased in the shelf areas by about two orders of magnitude with respect to the open ocean, subsequently enhancing vertical mixing and weakening stratification in the bottom boundary layer. The most prominent feature reproduced only when tides are modelled, is the Ushant front near the entrance of the English Channel. Tides appear also to constrain the freshwater transport of rivers from the continental shelf to the open ocean. The spectral analysis revealed that the tidal forcing contributes to the SSH variance at high frequencies near the semidiurnal band and to the open ocean mesoscale and small-scale features in the presence of summer stratification pattern.

Controls on near-bed oxygen concentration on the Northwest European Continental Shelf under a potential future climate scenario

Dissolved oxygen concentrations in the ocean are declining on a global scale. However, the impact of climate change on oxygen in shelf seas is not well understood. We investigate potential future changes in oxygen on the northwest European continental shelf under a business as usual greenhouse gas emissions scenario (Representative Concentration Pathway RCP8.5). Regions of the European shelf are thermally stratified from spring to autumn, which can cause oxygen depletion in sub-pycnocline waters. A transient climate-forced model simulation is used to study how the temperature, salinity and concentration of near bed dissolved oxygen change over the 21st century. In warming and freshening water, the oxygen concentration declines in all shelf regions. The climate change signal emerges first in salinity, then in temperature and finally in near bed oxygen. Regions that currently experience oxygen depletion (the eastern North Sea, Celtic Sea and Armorican shelf) become larger in the future scenario and oxygen depletion lasts longer. Solubility changes, caused by changes in temperature and salinity, are the dominant cause of reducing near bed oxygen concentrations in many regions. Until about 2040 the impact of solubility dominates over the effects of the evolving ecosystem. However, in the eastern North Sea by 2100, the effect of ecosystem change is generally larger than that of solubility. In the Armorican Shelf and Celtic Sea the ecosystem changes partially mitigate the oxygen decline caused by solubility changes. Over the 21st century the mean near bed oxygen concentration on the European shelf is projected to decrease by 6.3%, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. For monthly minimum oxygen the decline is 7.7% with the solubility component being 50% of the total.

Multiscale seismic reflectivity of shallow thermoclines

AbstractSeismic oceanography is coming of age as an established technique of observation of the thermohaline structure of the ocean. The present paper deals with the seismic reflectivity of the Armorican Shelf seasonal thermocline, west of France, based on two seismic experiments performed with a sparker source. The peak frequency was 500 Hz for the ASPEX experiment, where the thermocline was located at 27 m water depth, and reduced to 400 Hz associated to a higher source level for the IFOSISMO experiment, where the thermocline was 12 m deeper. Despite these settings, only the first experiment could clearly highlight the thermocline reflector, providing the first seismic observation of a shallow oceanic structure. To better understand the limitation of high‐resolution seismic devices in detecting weak oceanic features, we develop a wavelet‐based seismic analysis and consider, as a first approximation, a simple thermocline modeled by a Gauss error function, allowing an analytical expression for the associated seismic reflectivity. We show that the acoustic impedance profile of the thermocline is mainly controlled by a sound velocity proportional to the temperature. We show that the seismic reflectivity is controlled by the reflection coefficient of the large‐scale structure of the thermocline and by an attenuation factor which depends on the ratio between the seismic wavelength and the characteristic size of the thermocline. Depending on this ratio, the strength of the thermocline‐related reflection may be too weak to be detected by seismic measurement.

Surface layer circulation derived from Lagrangian drifters in the Bay of Biscay

Based on Lagrangian drifter trajectories collected from 1992 to 2009, a seasonal climatology of the circulation in the Bay of Biscay has been computed. The buoys deployed during several cruises are fitted with drogues at depth between ~15m and ~80m to follow the current in surface layers. After a pre-processing of the dataset, 281 trajectories corresponding to a cumulated time series about 71years long have been obtained to describe the residual seasonal circulation. Main features have been identified: an general northwesterly circulation over the deep ocean, a cyclonic along-slope circulation with a poleward slope current along the Aquitaine, Armorican and Celtic shelves (3.5 to 7cms−1), a marked winter eastward flow along the North Spanish slope (larger than 5cms−1 in average), and weak currents on the continental shelf from April to September (lower than 2.5cms−1 in summer). Further circulation patterns have been identified and quantified as the westward current in spring and summer along the North Spanish slope and shelf with speeds reaching 13.5cms−1. A cyclonic circulation cell has also been confirmed on the Armorican shelf from October to March. Intense poleward currents (about 10–15cms−1) have been observed from October to March on the shelf.The density of trajectories from 2004 to 2009 also allowed exploring the interannual variability of the circulation during two main half-year periods from October to March and April to September. Through this analysis, we described the variability of the Iberian Poleward Current intrusions in the Bay of Biscay during recent years as well as the interannual fluctuations in the development of a poleward autumn current on the Armorican shelf.

Buried fluvial incisions as a record of Middle–Late Miocene eustasy fall on the Armorican Shelf (Bay of Biscay, France)

High-resolution seismic data have been acquired in June 2008 on the Armorican Shelf (AS), in the northern Bay of Biscay, in order to reassess its stratigraphic architecture in detail and to study the impact of eustasy, tectonic and sediment delivery on the margin sedimentary record. Several profiles show fluvial-type incisions of several tens of meters (up to 54 m) associated to a widespread erosion surface. Several hypotheses are proposed for the stratigraphic position of this surface and incisions. We suggest Middle to Late Miocene age. Considering the relatively quiescent tectonic activity of the margin, we infer that the relative sea-level fall responsible of aerial incision on the Miocene shelf is eustasy-related. We propose the attested Serravallian–Tortonian eustatic lowstand (c. 11.6 Ma) as the key event responsible of such erosion and incision. This event marks the early beginning of the high-amplitude sea-level fluctuations that culminated during the Pleistocene and significantly controlled the present day AS morphology. The variability of vertical incision observed along single reaches can be explained by the confluence of several tributaries, the sinuosity of the channel and can be amplified by the unconsolidated nature of the Miocene substratum. The main pathways of the fluvial network corresponding to these buried valleys have been reconstructed and connections to other existing networks are proposed.

Circulation on the Armorican shelf (Bay of Biscay) in autumn

Three drifters drogued at 65 m were launched on a transect on the Armorican shelf of the Bay of Biscay for 4 years. The experiments were conducted in autumn. They revealed a north-westward, poleward current over the 100 m isobath and a very weak eastward current over depths comprised between 120 and 150 m. A model was used to assess the role of residual tidal currents and wind-induced circulation. The results show that the former are quite weak and the latter do not explain the average velocity of over 10 cm s − 1 . It is thought that this current is mainly driven by the density gradient induced by the breakdown of stratification. Hydrological data and satellite images from the period are discussed, in the light of this hypothesis.

Sardine spawning off the European Atlantic coast: Characterization of and spatio-temporal variability in spawning habitat

Data on the occurrence of sardine ( Sardina pilchardus) eggs from 42 national ichthyoplankton surveys along the European Atlantic coast were collated in order to describe the spawning habitat and spawning distribution of sardine in recent decades (1985–2005). A modification of existing spawning habitat characterisation techniques and a newly developed method to compare the probability of egg presence across surveys carried out with different sampling gears were used. Results showed that sardine spawning off the Atlantic European coast is mainly restricted to the shelf area, with the main geographical range being between the Strait of Gibraltar (the southern limit of data available for this analysis) and the middle part of the Armorican shelf (latitude around 47.5° North), and along a temperature range of 12–17 °C. Spawning grounds within these limits show a nearly continuous geographical distribution, covering a large proportion of the shelf of the Iberian peninsula and adjacent waters, except for: (1) a persistent gap at the north west corner of the Iberian peninsula, (2) a small secondary break at the Spanish–French border in the inner part of the Bay of Biscay and (3) at the south west corner of the peninsula where there is a narrowing of the shelf width. These discontinuities were used to separate spawning into four nuclei and to describe the changes in spawning distribution in the time series. The relative importance of each nucleus and the degree of separation between adjacent nuclei varies between years, with the exception of the permanent gap at the northwest corner of the Iberian peninsula, which is persistent throughout the time series. Year to year changes in the proportion of the potential spawning habitat in which spawning actually occurred, changing from around 60% before the mid 1990s to around 40% thereafter, and did not show any relationship with spawning stock biomass. Evolution of potential habitat occupation over the Armorican shelf shows larger variability than that observed in the Iberian peninsula, with percentages of occupation ranging from around 30% up to nearly 80% of the shelf in recent years (within the limitations of the relatively sparse data for this region).

Seasonal mean circulation on the Irish shelf—a model-generated climatology

Subtidal circulation over the shelf sea west of Ireland is computed diagnostically. The 3D circulation is driven by observed/analyzed density fields and standard mean seasonal wind fields; subject to spatially-variable vertical mixing fields from a detailed tidal model; and resolved on a bimonthly scale. The calculation extends from the Armorican Shelf to the Hebrides, in order to include large-scale effects. Three bimonthly “seasons” are presented: late winter (March–April) when commercially important marine fish are spawning; spring (May–June) when anadromous fish are in transit along the shelf northward; and summer (July–August) when harmful algae are observed in the region.

Detection and monitoring of algal blooms using SeaWiFS imagery

SeaWiFS (Sea viewing Wide Field of view Sensor) imagery is used to monitor the birth, development and termination of phytoplankton blooms in the north-east Atlantic. Extensive patches of water giving high reflectance of visible light were observed from the beginning of February to the end of July 1998 in the Celtic and Armorican Shelf regions between 45° and 60° N and interpreted as phytoplankton. Eighteen relatively cloud-free SeaWiFS images have been analysed in order to detect the spatial, temporal and spectral development of the blooms within the two study areas. Consistency of the spectral signature and the very high broadband reflectance leads to the conclusion that these are blooms of the widespread coccolithophore Emiliana huxley, although no in situ data during the study period are available to validate this hypothesis. The utility of SeaWiFS data for studying the life history of phytoplankton blooms is confirmed.

The deep-sea Armorican depositional system (Bay of Biscay), a multiple source, ramp model

The deep-sea Armorican depositional system, located in the central part of the Bay of Biscay, is a medium-sized turbidite system with a surface area of more than 30,000 km2. The whole system is a mud/sand-rich submarine ramp on a passive margin. The medial ramp is characterised by the presence of six distinct tributary channels which form three systems: the Guilcher, Crozon and Cornouaille systems. The distal ramp corresponds to divergent braided secondary channels and associated lobes. Variations in hydrodynamic conditions on the outer Armorican Shelf during the last climatic cycle are the major factor controlling facies shifts and system growth. Thus, the Armorican depositional system is a delta-fed submarine ramp during low sea-level glacial conditions, and an outer shelf-fed submarine ramp at times of sea-level rise.

Nitrogen Uptake by Phytoplankton in a Shallow Water Tidal Front

Nitrogen uptake by phytoplankton was studied in a shallow water tidal front and its adjacent stratified and well-mixed sectors on the Armorican shelf (north-west Europe) in July 1992. The front was characterized by relatively high phytoplankton biomass (>3·4μgl−1) and high ammonium and nitrate uptake rates (1·05 and 0·63mmolm−2h−1respectively) compared with the stratified (0·71 and 0·21mmolm−2h−1) and well-mixed (0·49 and 0·25mmolm−2h−1) waters. Ammonium regeneration rates at the front were of the same order as ammonium uptake rates indicating intense recycling of nitrogen by microheterotrophs. The high total nitrogen uptake, in terms of both absolute and chlorophyll-specific rates, in the front suggests that in situ growth, rather than physical processes, is the main cause of phytoplankton abundance in the front. Despite high nitrate uptake rates, the f -ratios (ρNO−3/(ρNO−3+ρNH+4)) were lower than in the deep water tidal fronts, because of low nitrate concentrations below the thermocline.

A new bathyphotometer for bioluminescence measurements on the Armorican continental shelf (northeastern Atlantic)

A new bathyphotometer has been developed by the Oceanographic Laboratory of the French Naval Academy which has performed a series of in situ bioluminescence measurements since 1994. It is used as an additional probe to a Seabird ® CTD. It measures planktonic bioluminescence. It is associated with a SeaTech ® fluorometer. The Armorican shelf has particularly been investigated since July 1994. Results obtained in January and April 1995 off Brittany (France), at the entrance to the Bay of Douarnenez and in the northern Bay of Biscay are presented as examples. A method to separate the bioluminescence background from sharp peaks is presented. The strength of bioluminescence signals was related to seasonal planktonic biomass fluctuations, low in winter, higher in spring.

Late Autumn Distribution and Seasonality of Chlorophyll-a at the Shelf-Break/Slope Region of the Armorican and Celtic Shelf

Simultaneous satellite and ship measurements have characterized the structure of temperature and nitrate concentration of the Shelf-Break Front of the Armorican and Celtic Sea in late autumn (November) 1985. A chlorophyll-a distribution associated with this feature is analysed in relation to the shelf-break frontal genesis and properties, and in relation to the seasonality of in situ concentrations of chlorophyll-a at the shelf-break/slope region, adjacent shelf and adjacent ocean near ~ 47·5°N.

Apparition soudaine de Protolloydolithus (Trilobita, Trinucleidae) dans l'Ordovicien (Llandeilo) du Massif Armoricain: Un essai d'interprétation

Protolloydolithus, known from the Ordovician of Britain and Portugal, also occurs in the Armorican Massif, in Llandeilo siltstones from the Andouillé Formation (Laval synclinorium). The sudden appearance of the genus, linked to a transgressive event, can be considered as the result of an immigration from southern Britain into the deep water parts of the Armorican shelf.

Celtic and Armorican slope and shelf residual currents

Current meter data collected over the last 20 years are presented and used to describe the residual currents on the Celtic and Armorican slope and shelf regions. On the slopes, a poleward current of about 6cm s −1 exists at the 500m depth contour. At mid depths, these currents are directed onslope, whereas near the bottom the flow in markedly downslope, reaching mean speeds of about 15cm s −1 near 6°40′W. The downslope currents are thought to be largely tidally induced and balanced by Stokes transports. The total slope transport near 48°N is about 4Sv. On the upper slopes (<1000m depth) the transport increases poleward. On the outer Celtic shelf, a weak (2 cm s −1) counter-current flowing southeastwards was observed. On the Armorican shelf, the residual flow is again nothwestwards and this coastal flow appears to continue northwards across the mouth of the English Chanel and past the Isles of Scilly with typical mean upper layer speeds of about 2cm s −1. Southwest of Ireland the flow is again northwesterly. Numerical model simulations show that the eastern slope boundary current of the NE Atlantic can be driven by realistic distributions of seawater density. The simulations also show only a small wind driven barotropic response on the Celtic and Armorican shelf region and that a component of the residual shelf flows, like the slope current, may be driven by pressure distributions arising from regional differences in the distribution of seawater density, or from non local wind stress.

Deep structure of the Aquitaine shelf: constraints from expanding spread profiles on the ECORS Bay of Biscay transect

This paper describes the analysis and interpretation of six Expanding Spread Profiles (ESP) which were shot approximately perpendicular to a 300 km long vertical reflection profile along the eastern continental margin of the Bay of Biscay (Aquitaine shelf) by the French ECORS program in association with Hispanoil. This transect crosses various tectonic features of different ages: the Armorican shelf, the Parentis basin and the Cantabria shelf. Velocity—depth models have been derived from the ESPs by the combination of two complementary methods using time-distance and intercept-slowness domains. They provide important constraints for the analysis of the vertical reflection data. The velocities allow definition of crustal layering with a 5.8-6.2 km s −1 upper crust and a 6.5–7.1 km s −1 lower crust. This layering matches the change of reflectivity observed on CDP data with a relatively transparent upper crust and upper mantle in opposition to a highly layered lower crust. Important variations of the thickness of these two layers are revealed by this study. The most important one occurs beneath the Parentis basin with a 15 km shallowing of the upper mantle, the velocity distribution suggesting that major crustal thinning has taken place at the cost of a large part of the lower crust.

A numerical model for the generation of tidally forced nonlinear internal waves over topography

A three layer numerical model is developed to investigate tidally forced nonlinear internal wave generation over large scale one-dimensional bottom topography. Advective effects due to the barotropic tide are retained. Numerical experiments are presented which demonstrate how internal wave propagation can be arrested by the imposed tide. The model is applied to Knight Inlet on the British Columbia coast and to the edge of the Armorican Shelf in the Celtic Sea. Observations over the Celtic Sea continental slope reveal and the model confirms that the internal tidal velocity can exceed the maximum strength of the barotropic M 2 tide. The presence of these velocity surges plays an important role in bottom sediment transport. The velocity field in each layer predicted by the model for the Celtic Sea slope is in agreement with the observations. When applied to a sill in Knight Inlet, the model predicts that internal waves are generated over the sill top and subsequently propagate in both directions in the form of internal bores. On either side of the sill a residual current is produced in each layer.

Deep seismic reflection and refraction profiling along the Aquitaine shelf (Bay of Biscay)

Summary. The 300 km ECORS - Bay of Biscay profile was carried out along the Aquitaine shelf and comprised a complete set of experiments including zero-offset and 7.5 km constant offset vertical seismic reflection and six expanding spread profiles. Large offset recordings were fundamental for the definition of the layered lower crust and the Moho, while ESPs provided decisive complementary information for the geological interpretation. These data show a strong variation in crustal thickness from about 20 km beneath the rifted Parentis basin, a failed arm of the oceanic Bay of Biscay, up to 35 km to the north below the Armorican shelf, in the Hercynian domain, and to the south below the Cantabria shelf, in the vicinity of the Pyrenean deformation front. The results have important implications for the behaviour of the crust during the formation of rifted sedimentary basins and during continental collision.

Solitary internal waves in the Celtic Sea

An examination is made of tidally generated high frequency internal wave trains propagating from the shelf-break of the Celtic Sea and Armorican shelf. The waves propagate on-shelf after maximum off-shelf tidal streaming at speeds of about 70 cm s −1 giving a wavelength between successive tidal disturbances of 30 km. As the waves travel on-shelf they are advected by the barotropic tide and can take on a solitary wave form.