Atlantic Shelf Research Articles

Setting the record straight on drivers of changing ecosystem states

AbstractThis short communication is a response to the critique by Greene (2012), who puts forward the argument that the dynamics of northwest Atlantic continental shelf ecosystems are strongly influenced by changes in Arctic climate, as indexed by surface salinity. The argument, in its essence, discounts any indirect effects from over‐exploitation of top predators on lower trophic levels in northwest Atlantic ecosystems. Frank et al. (Science 308, 2005, 1621; Nature 477, 2011, 86) reported the existence of cascading trophic interactions with particular emphasis on the eastern Scotian Shelf. Greene argues that the events occurring in the Gulf of Maine/Georges Bank region are representative of all Northwest Atlantic shelf systems, despite previous research (Frank et al. Trends Ecol. Evol. 22, 2007, 236; Petrie et al. Fish. Oceanogr. 18, 2009, 83) that has shown a differential pattern of forcing ranging from top‐down in species‐poor, cold water systems to bottom‐up in warmer, more species‐rich systems, including Georges Bank.

Towards a more balanced view of marine ecosystems

AbstractFrank et al. (Nature, 477, 2011, 86) hypothesize that the slow recovery of the Scotian Shelf ecosystem to its structure prior to the early 1990s is a result of the oscillatory, runaway consumption dynamics of the forage fish complex. Their hypothesis is based on the underlying assumptions that this ecosystem's dynamics are governed by top‐down forcing, predator–prey role reversals, and a trophic cascade extending from demersal predatory fish, especially cod, down to the level of nutrients (Science, 308, 2005, 1621; Nature, 477, 2011, 86). Greene and Pershing (Science, 315, 2007, 1084) called into question the underlying assumption of an extended trophic cascade. They argued that while top‐down forcing can be important at higher trophic levels in many Northwest Atlantic Shelf ecosystems, its impacts on zooplankton, phytoplankton, and nutrients are minor or nonexistent. Instead, they argued that lower trophic‐level dynamics in these ecosystems are governed by climate‐associated, bottom‐up forcing. Observations over the past three decades (Greene et al. Ecology, 89, (Suppl 11), 2008, S24; MERCINA Oceanography, 25, 2012, 208.) are consistent with predictions from the climate‐forcing hypothesis proposed by Greene and Pershing (Science, 315, 2007, 1084), while those from the past decade (MERCINA, Oceanography, 25, 2012, 208) demonstrate that the trophic cascade hypothesis as proposed by Frank et al. (Science, 308, 2005, 1621) should be rejected. A closer examination of these observations also calls into question the hypothesis of ecosystem recovery in the Northwest Atlantic being governed by the oscillatory, runaway consumption dynamics of the forage fish complex.

Recent Arctic Climate Change and Its Remote Forcing of Northwest Atlantic Shelf Ecosystems

During recent decades, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. Additionally, shifts in the Arctic's atmospheric pressure field have altered surface winds, ocean circulation, and freshwater storage in the Beaufort Gyre. These processes have resulted in variable patterns of freshwater export from the Arctic Ocean, including the emergence of great salinity anomalies propagating throughout the North Atlantic. Here, we link these variable patterns of freshwater export from the Arctic Ocean to the regime shifts observed in Northwest Atlantic shelf ecosystems. Specifically, we hypothesize that the corresponding salinity anomalies, both negative and positive, alter the timing and extent of water-column stratification, thereby impacting the production and seasonal cycles of phytoplankton, zooplankton, and higher-trophic-level consumers. Should this hypothesis hold up to critical evaluation, it has the potential to fundamentally alter our current understanding of the processes forcing the dynamics of Northwest Atlantic shelf ecosystems.

Temperature characteristics of bacterial sulfate reduction in continental shelf and slope sediments

Abstract. The temperature responses of sulfate-reducing microbial communities were used as community temperature characteristics for their in situ temperature adaptation, their origin, and dispersal in the deep sea. Sediments were collected from a suite of coastal, continental shelf, and slope sediments from the southwest and southeast Atlantic and permanently cold Arctic fjords from water depths ranging from the intertidal zone to 4327 m. In situ temperatures ranged from 8 °C on the shelf to −1 °C in the Arctic. Temperature characteristics of the active sulfate-reducing community were determined in short-term incubations with 35S-sulfate in a temperature gradient block spanning a temperature range from 0 to 40 °C. An optimum temperature (Topt) between 27 °C and 30 °C for the South Atlantic shelf sediments and for the intertidal flat sediment from Svalbard was indicative of a psychrotolerant/mesophilic sulfate-reducing community, whereas Topt ≤20 °C in South Atlantic slope and Arctic shelf sediments suggested a predominantly psychrophilic community. High sulfate reduction rates (20–50%) at in situ temperatures compared to those at Topt further support this interpretation and point to the importance of the ambient temperature regime for regulating the short-term temperature response of sulfate-reducing communities. A number of cold (<4 °C) continental slope sediments showed broad temperature optima reaching as high as 30 °C, suggesting the additional presence of apparently mesophilic sulfate-reducing bacteria. Since the temperature characteristics of these mesophiles do not fit with the permanently cold deep-sea environment, we suggest that these mesophilic microorganisms are of allochthonous origin and transported to this site. It is likely that they were deposited along with the mass-flow movement of warmer shelf-derived sediment. These data therefore suggest that temperature response profiles of bacterial carbon mineralization processes can be used as community temperature characteristics, and that mixing of bacterial communities originating from diverse locations carrying different temperature characteristics needs to be taken into account to explain temperature response profiles of bacterial carbon mineralization processes in sediments.

Directly measured net denitrification rates in offshore New England sediments

Continental shelf sediments are important locations for denitrification in the marine environment. Here, we report a robust, directly measured set of sediment net denitrification rates from five sites off the coast of southern Rhode Island (USA). Using the N2/Ar technique, we sampled three of these sites at an almost bimonthly rate from October 2009 to January 2011, and two additional sites during July 2010. Net denitrification ranged from approximately 20 to 75 N2–N μmolm−2h−1 with a mean of 44±4 N2–N μmolm−2h−1. These values are similar to other studies that report direct-measurements of sediment net N2 fluxes located along the East coast of the United States. Net sediment denitrification rates were not significantly different between sites, nor did the rates change over the sampled temperature range (5–17°C). Net denitrification rates were weakly correlated, but exhibited significant relationships with sediment oxygen demand. Positive and negative fluxes of nitrate along the sediment–water interface suggest that both direct and coupled denitrification are occurring. Sediment molar C:N ratios were consistent temporally and spatially, indicating that sediment organic matter quality may be responsible for the similar rates of net denitrification at our sites. If we apply our rates to the 360,000km2 North East United States shelf area, we calculate that 0.14Tmol of N per year are removed, accounting for just over 5% of the total N entering the North Atlantic shelf region. Additionally, we predict a historical denitrification rate based on primary production estimates, which indicate that denitrification rates may have been three times higher in the past than our current measurements. Thus, as seen in a nearby coastal system, the nitrogen cycle in offshore New England regions could have changed during the last century.

Benthic infaunal community variability on the northern Svalbard shelf

Marine benthic communities are effective indicators of environmental change. Yet in the Arctic, there are few empirical tests of how sustained climatic change may influence community structure. Northern Svalbard is influenced by both warm Atlantic and cold Arctic water masses, providing an opportunity to assess potential effects of long-term environmental changes by examining spatial variation in community structure. We examined benthic macroinfaunal communities and sediment pigments under Atlantic and Arctic water masses on the northern shelf and fjords of Svalbard. We report on infaunal biomass, abundance, species composition, and diversity at 10 stations spanning 79°–81°N and ranging in depth from 200 to 500 m. Benthic biomass averaged 128 g WW m−2 (48–253 g WW m−2), mean density was 3,635 ind. m−2 (780–7,660 ind. m−2), and species richness varied from 45 to 136 taxa stn.−1. Abundance-based community structure clustered stations in groups related to water mass characteristics, with Atlantic and Arctic shelf stations being well distinguished from each other. Dominant taxa were different in Atlantic- and Arctic-influenced locations. Faunal biomass was highest in the Atlantic-influenced fjords, followed by Arctic fjords and Arctic shelf stations, with Atlantic shelf stations having the lowest biomass. Species richness and diversity were inversely related to biomass. Patterns in faunal biomass were strongly correlated with sedimentary pigments (R 2 = 0.74 for chl a and R 2 = 0.77 for phaeopigments), with large differences in sedimentary pigment concentration among stations. These relationships suggest that benthic fauna on the northern Svalbard shelf are food limited and dependent on predictable, albeit episodic, delivery of organic matter from the water column.

Deep-water coral reefs from the Uruguayan outer shelf and slope

We report the finding of monospecific scleractinian (i.e. Lophelia pertusa) reefs from the Uruguayan outer shelf and slope during an exploratory joint research cruise onboard the R/V 'Miguel Oliver' during January-February 2010. Acoustic mapping of the seafloor allowed the detailed analysis of 8,944 km(2), where some 17 structures identified as mounds were detected. Isolated cusps or groups of small mounds were the two main morphologies observed. Mound summit depths ranged from 167 to 326 m. The average height of the mounds was 35 m, reaching a maximum of 67 m. In all sampled mounds, the presence of live coral and/or coral rubble was detected, while absent from surrounding soft sediment bottoms. Some mounds were associated with fluid seepages. This is the first report of deep-sea coral reefs on the Uruguayan continental shelf and slope, and represents the southernmost Western Atlantic shelf and slope record of L. pertusa to date.

Trophodynamics and functional feeding groups of North Sea fauna: a combined stable isotope and fatty acid approach

The trophodynamics of pelagic and benthic animals of the North Sea, North Atlantic shelf, were assessed using stable isotope analysis (SIA) of natural abundance carbon and nitrogen isotopes, lipid fingerprinting and compound-specific SIA (CSIA) of phospholipid-derived fatty acids (PLFAs). Zooplankton (z), epi- and supra-benthic macrofauna were collected in the Southern Bight, at the Oyster Grounds and at North Dogger, 111 km north of the Dogger Bank. The study included 22 taxonomic groups with particular reference to Mollusca (Bivalvia and Gastropoda) and Crustacea. Primary consumers (Bivalvia) were overall most 15N enriched in the southern North Sea (6.1‰) and more depleted in the Oyster Grounds (5.5‰) and at North Dogger (2.8‰) demonstrating differences in isotopic baselines for bivalve fauna between the study sites. Higher trophic levels also followed this trend. Over an annual cycle, consumers tended to exhibit 15N depletion during spring followed by 15N enriched signatures in autumn and winter. The observed seasonal changes of ? 15N were more pronounced for suspension feeders and deposit feeders (dfs) than for filter feeders (ffs). The position of animals in plots of ? 13C and ? 15N largely concurred with the expected position according to literature-based functional feeding groups. PLFA fingerprints of groups such as z were distinct from benthic groups, e.g. benthic ffs and dfs, and predatory macrobenthos. ? 13CPLFA signatures indicated similarities in 13C moiety sources that constituted ? 13CPLFA. Although functional groups of pelagic zooplankton and (supra-) benthic animals represented phylogenetically distinct consumer groups, ? 13CPLFA demonstrated that both groups were supported by pelagic primary production and relied on the same macronutrients such as PLFAs. Errors related to the static categorization of small invertebrates into fixed trophic positions defined by phylogenetic groupings rather than by functional feeding groups, and information on seasonal trophodynamic variability, may have implications for the reliability of numerical marine ecosystem models.

Quaternary primary productivity in Porcupine Seabight, NE North Atlantic

Biogenic opal and calcium carbonate contents in the Quaternary fine-grained sediments in the deep sea coral mound area (Hole U1317E and Hole U1318B drilled by IODP Expedition 307) in Porcupine Seabight, southwest off Ireland, were measured to estimate the primary productivity (Pp) of the surface seawater in NE Atlantic. The results from the two holes commonly show relatively high Pp values of 10–30 g cm−2 ky−1, having stratigraphic cyclicity reversely correlated with the δ13C profile. The high Pp could be attributed to the oceanic setting controlled by the Eastern North Atlantic (Central) Water and Shelf Edge Current, which have been strongly influenced by the obliquity forcing of the Milankovitch cycle. A positive covariance was observed between high Pp and the mound development in U1317E, implying that the organic matter was the principal food source for the mound coral community. It is proposed that the pelagic marl matrix of the mound sediments provides information of the surface water productivity that is useful for reconstructing the paleoceanography and the paleoclimate in the NW Atlantic.

Modelling the timing and duration of dormancy in populations of Calanus finmarchicus from the Northwest Atlantic shelf

Calanus finmarchicus relies on dormancy to thrive in the seasonal environment of the boreal Atlantic. The lipid accumulation window (LAW) hypothesis proposes that a seasonal window of environmental conditions allows developing individuals to store enough lipids for dormancy to be safely initiated. Successful dormancy requires a sufficient amount of lipids to fulfil the reduced metabolic demand of the dormant individual and to sustain the final maturation process. We used a pattern-oriented modelling approach that implements the LAW hypothesis and employs a genetic algorithm for parameter estimation, in order to reproduce the observed phenology and demography of C. finmarchicus populations from the two contrasting regions, the Gulf of St. Lawrence (GSL) and the Gulf of Maine (GoM) in the northwest Atlantic shelf. In the GSL, the model reproduced the timing of dormancy, the abundance and individual condition of late copepodid stages. In the GoM, the model produced a semi-annual dormancy pattern, as no locally produced individual could last the 6–8 months of dormancy inferred from the available observations. Further testing requires extending demographic time series, including lipid condition of late copepodid stages in the GoM, and the implementation of a 3-D modelling framework that would explicitly address the complex interactions between circulation and population dynamics of C. finmarchicus over the entire northwest Atlantic shelf.

Regional Objective Analysis for Merging High-Resolution MERIS, MODIS/Aqua, and SeaWiFS Chlorophyll- a Data From 1998 to 2008 on the European Atlantic Shelf

In this paper, we define the method that is used to merge high-resolution multisensor chlorophyll-a (chl-a) data on the Ireland-Biscay-Iberia Regional Ocean Observing System area from 1998 to the present at a resolution of 1.1 km. The method is based on geostatistics and is known as kriging. The merged variable is the daily anomaly of chl-a, with the anomaly being defined as the difference between the daily image and the mean historical field for the considered day. For each day, the continuous anomaly image is generated using the kriging method, and the mean historical field is then added to obtain the cloudless field of chl- a. The initial satellite chl-a data set used in the merging procedure is derived from the daily level-2 water leaving radiances of three ocean color sensors: the Sea-Viewing Wide Field of View Sensor on the Orbview platform, the Moderate Resolution Imaging Spectroradiometer on the Aqua platform, and the Medium Resolution Imaging Spectrometer Instrument on the ENVISAT platform. The chl-a concentration is obtained using a specific algorithm developed by Ifremer, known as OC5 product. Before merging, each satellite-derived chl-a product has been compared to in situ data and has been validated using a matchup data set. After this validation against in situ data, intercomparisons between the satellite data sets have been performed. As the chl-a anomaly variability depends, in this region, on the season and the distance from the shore, local space-time semivariograms have been calculated to estimate the spatiotemporal dependence or covariance of the chlorophyll anomalies. The semivariograms, used in the estimation of the kriged anomaly, are defined by their nuggets (noise), their spatial and temporal range (maximum distance for a nonnull covariance between the anomalies), and their sill (maximum variance). The spatial range of the semivariogram has been approximated locally on a regular grid. The nuggets and the sills have been deduced from the square of the mean of the chl-a concentration (the climatological reference) as we have observed a classical proportionality effect between the square of the chl-a mean, the variance of the distribution, and the parameters of the semivariograms. Compared to each original product, the analysis shows a complete coverage and differences with the in situ data that are statistically equivalent to those observed with the initial satellite data set. The merged product offers a number of applications for environmental monitoring such as the monitoring of the eutrophication risk required by the Water Framework Directive of the European Union.

The influence of the Brazil and Malvinas Currents on the Southwestern Atlantic Shelf circulation

Abstract. The oceanic circulation over the southwestern Atlantic shelf is influenced by large tidal amplitudes, substantial freshwater discharges, high wind speeds and – most importantly – by its proximity to two of the largest western boundary currents of the world ocean: the Brazil and Malvinas currents. This review article aims to discriminate the dynamical processes controlling the interaction between this extensive shelf region and the deep-ocean. The discussion is focused on two broad regions: the South Brazil Bight to the north, and Patagonia to the south. The exchanges between the Brazil Current and the South Brazil Bight are characterized by the intermittent development of eddies and meanders of the Brazil Current at the shelfbreak. However, it is argued that this is not the only – nor the most important – influence of the Brazil Current on the shelf. Numerical simulations show that the thermohaline structure of the South Brazil Bight can be entirely ascribed to steady state, bottom boundary layer interactions between the shelf and the Brazil Current. The Malvinas Current does not show the development of eddies and meanders, but its influence on the Patagonian shelf is not less important. Models and observations indicate that the Malvinas Current not only controls the shelfbreak dynamics and cross-shelf exchanges but also influences the circulation in the shelf's interior.

Remote sensing with SPOT-4 for mapping kelp forests in turbid waters on the south European Atlantic shelf

Remote sensing has become an increasingly used technique for the thematic mapping of large marine areas. In recent years, many researchers have successfully applied these techniques in different places for benthic mapping in clear waters; however, areas with turbid waters present important limitations that are gradually being solved by recent technological advances. In this context, the main objective of the present study is to develop and validate a methodology for mapping intertidal and subtidal kelp forests in the Galician coast (NW Spain), based on images from SPOT-4 (Satellite Pour l’Observation de la Terre). Three analysis methods have been applied: visual analysis and interpretation, unsupervised classification (cluster) and supervised classification (angular classification and maximum likelihood classification). Classification percentages higher than 70% in all substrates were obtained both using visual analysis and interpretation and maximum likelihood classification.

Internal waves on continental and island shelves of the open ocean: A comparative analysis for observations on the New York and Hawaii shelves

Based on the results of processing experimental data obtained from measurements of current velocities and water temperatures on the United States Atlantic Shelf and near the Hawaii Islands (the island of Oahu and Mamala Bay), we perform a comparative analysis of the characteristics of internal waves in these representative areas of continental and island shelves of the ocean. These investigations indicate that the internal-wave fields in these areas are very different from one another in both the low- and high-frequency ranges. On the Atlantic Shelf, we have regularly observed tandems of powerful internal solitons clearly seen on space imagery of the oceanic surface. On the island shelves, soliton-type internal waves were less seldom seen as very specific oscillations. The absence of surface manifestations of even powerful solitons in Mamala Bay is explained both by the large pycnocline depth and by the fact that the vertical structure of these solitons is controlled by the second rather than the first mode, as it is on the Atlantic Shelf.

Tide model comparison over the Southwestern Atlantic Shelf

Sea surface height (SSH) as measured by satellites has become a powerful tool for oceanographic and climate related studies. Whereas in the open ocean good accuracy has been achieved, more energetic dynamics and a number of calibration problems have limited applications over continental shelves and near the coast. Tidal ranges in the Southwestern Atlantic (SWA) continental shelf are among the highest in the world ocean, reaching up to 12 m at specific locations. This fact highlights the relevance of the accuracy of the tidal correction that must be applied to the satellite data to be useful in the region. In this work, amplitudes and phases of tidal constituents are extracted from five global tide models and three regional models and compared to the corresponding harmonics estimated from coastal tide gauges (TGs) and satellite altimetry data. The Root Sum Square (RSS) of the misfit of the common set of the five tidal constituents solved by the models ( M 2, N 2, S 2, K 1 and O 1) is higher than 18 cm close to the coast for two of the regional models and higher than 24.5 cm for the rest of the models considered. Both values are too high to provide an accurate estimation of geostrophic non-tidal currents from satellite altimetry in the coastal region. On the other hand, the global model with the highest spatial resolution has a RSS lower than 4.5 cm over the continental shelf even when the non-linear M 4 overtide is considered. Comparison with in-situ current measurements suggests that this model can be used to de-tide altimetry data to compute large-scale patterns of SSH and associated geostrophic velocities. It is suggested that a local tide model with very high resolution that assimilates in-situ and satellite data should meet the precision needed to estimate geostrophic velocities at a higher resolution both close to the coast and over the Patagonian shelf.

Sand bodies and incised valleys within the Late Quaternary Sanaga–Nyong delta complex on the middle continental shelf of Cameroon

Seismic data and sediment cores collected offshore from the Sanaga River and Nyong River mouths were used to analyse a loose mantle of yellow to reddish sandy gravel with a range of fluvial and deltaic characteristics cropping out in the middle part (25–65 m water depth) of the continental shelf of Cameroon. Contrary to most of the Atlantic shelves, where the lowstand systems tract was destroyed by erosion, we found 80–120 ms (60–90 m) of sediment mainly in the middle part of the shelf, which correspond credibly to MIS 2–4. Fluvial paleodrainage systems are preserved beneath the shelf and individual filled channels with planar infillings were mapped that cross the shelf along two surfaces of erosion. These incisions suggested westerly and northwesterly drainage shifts during presumed lowered base level. The presence of closely spaced channel fills suggests repeated avulsion of a single stream during a long-lasting sub-aerial erosion period. The seismic facies of these Pleistocene deposits distinguish themselves clearly from well-stratified older strata showing deformation (Pliocene) or intense folding (Miocene). The orientation of the paleovalleys appears strongly controlled by the N60°E trending cross faults within Mesozoic–Cenozoic strata. On the basis of approximately 80,000 years of accumulation, an estimate of the yearly mass of sediment transported was calculated for the Sanaga and Nyong rivers and compared to the present sediment load of the two rivers. It is estimated that as much as 50% of the sediment discharge must have bypassed the upper slope. We show that deposition took place near the terrestrial break in slope even though the coastline had already regressed across the shelf. In contrast, the gradient of the lower course of the nearby Niger River remains below 0.05% through several hundreds of kilometres and no lowstand deposits have been preserved. We conclude that the break in slope, even terrestrial, is the determining factor regarding preservation of shelf deposits during relative sea level rise.

Pattern and scale of variability among Northwest Atlantic Shelf plankton communities

Continuous plankton recorders (CPRs) have been used in the Northwest Atlantic for almost 50 years. While data collected by these surveys have provided valuable information on long-term variability in plankton populations, all previous analyses have been limited to only a portion of the geographic range of the available data. Here we present an analysis of the CPR data from the Mid Atlantic Bight to the Labrador Sea. Across this wide geographic range, we found many common associations among the taxa. In particular, the changes in most regions were strongly size structured, with small and medium copepods varying together and often positively related to indicators of phytoplankton abundance. The time series from nearby regions were strongly correlated; however, after 1990, the spatial pattern became more complex. During this period, several of the copepod taxa, noticeably Calanus finmarchicus and Centropages typicus, experienced a series of anomalies that appeared to propagate from northeast to southwest. Although the direction of propagation was consistent with the shelf circulation, the anomalies propagated at a rate much slower than typical current speeds. The timing of the copepod anomalies and their phase speed were similar in character to observed changes in salinity and the position of the Shelf Slope Front. The correspondence between the changes in the plankton community and changes in the physical environmental suggests that physical conditions are a strong driver of interannual variability in Northwest Atlantic Shelf ecosystems.

Effect of random hydrodynamic inhomogeneities on low frequency sound propagation loss in shallow water

Low frequency (100–500 Hz) sound propagation loss on the US Atlantic continental shelf and in the Barents Sea in the presence of stochastic surface waves, and for the US Atlantic shelf also in the presence of internal waves, is studied for the range of up to 150 km by means of numerical simulations. Qualitative difference between sound propagation loss behavior on the US Atlantic shelf and in the Barents Sea is demonstrated for summertime conditions even without random inhomogeneities. It is shown that whereas internal waves have a weak effect on propagation loss, surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m/s.

Long-term stability of δ13C with respect to biological age in the aragonite shell of mature specimens of the bivalve mollusk Arctica islandica

The stable carbon isotope ratio in bivalve shells ( δ 13C S) is an enigmatic geochemical archive whose interpretation is often frustrated by the intrusion of variable and unpredictable vital effects which can influence the mix of metabolic and dissolved inorganic carbon (DIC) in the shell material. The impacts of vital effects and rapid changes in calcification rates on the variability and value of δ 13C S have been described in a number of studies and in many bivalve species with typical lifespans between a few years and a few decades, δ 13C S has been observed to change (usually decreasing) with biological age. Very long-lived animals, by contrast, spend most of their lives in the mature, slow-growing phase, and it might be expected that in these instances the effect of changes in calcification rates on δ 13C S would be less marked, or even absent. Analysis of δ 13C S in mature (> 40 years old) Arctica islandica, reported here, indicates that this is the case. δ 13C S in shell samples with biological ages between 42 and 391 years from four distinct sites in the North Atlantic shelf seas (Gulf of Maine, north Icelandic shelf, Irish Sea and North Sea) shows no age-related trend. This suggests that metabolic vital effects in mature A. islandica may be reasonably stable at the population level. If the drivers of isotopic disequilibrium between shell and ambient environment can be identified and quantified, it may be feasible to adjust for them and use δ 13C S in mature A. islandica to investigate long-term changes in nutrient sources and as a robust proxy for δ 13C of environmental DIC.

Sedimentary architecture of the Loire River drowned valleys of the French Atlantic shelf

AbstractThe analysis of a dense grid of high resolution seismic profiles collected offshore the present day Loire River estuary indicates the presence of a thick and complex Pleistocene coastal wedge between the coast and 50 meters water depth. Most of this coastal wedge is preserved in a fossil valley network starting 10 km off the coast on the “Precontinent Breton” topographies and wedging out progressively 50 km in the offshore where the shelf flattens. This system is comprised of three main valleys 30 km long, 40–60 m deep and 0.7 to 4 km wide each, in average, with a northern valley incompletely filled by sediment. These valleys are incised into Eocene (Ypresian-Bartonian) sedimentary rocks lying unconformably on the metamorphic and magmatic rocks of the South Armorican Massif basement. The coastal wedge is comprised of six seismic units. From the base of the valleys to the seafloor, these units are successively interpreted as (1) colluvial (U1) and braided river deposits (U2), overlain by restricted marine to estuarine sediments (U3), and (2) straight to meandering fluvial deposits (U4) giving rise vertically to floodout marine sediments (U5). The whole sediment pile is capped by open marine bioturbated mudstones (U6).This succession of seismic units is organised in two depositional sequences bounded by an unconformity of regional extent, which corresponds to a drastic change in the paleovalley fill architecture. The lower sequence fills up the southern and central valleys when the upper sequence fills up the northern valley network. Both sequences are sharply truncated by a ravinement surface at the base of the offshore shales formed during the Holocene marine transgression. The correlation of the observed depth of the incisions and transgression surfaces with the global sea-level curve provides an indirect estimate of the ages of the depositional sequences. The lower sequence is probably Saalian (130–200 Ka, MIS 6) and the upper one Weischelian (110–12 Ka, MIS 2–4 and 5a–d). The ages and the infill of these valleys are very close to the ones observed in the largest paleovalleys of the inner French Atlantic shelf (Gironde, English Channel) and consistent with the sedimentary record of the continental Loire River (stepped terraces). In the later case, the Saalian period corresponds to a sharp increase on the incision of the river that shaped the morphology and determined the location of the present-day Loire valley.