A depth profile of
239Pu and
240Pu specific activities and isotope ratios was determined in an inter-tidal sediment core from the Esk Estuary in the northeastern Irish Sea. The study site has been ...impacted with plutonium through routine radionuclide discharges from the Sellafield nuclear reprocessing plant in Cumbria, NW England. A pronounced sub-surface maximum of ~
10
kBq
kg
-1 was observed for
239
+
240
Pu, corresponding to the peak in Pu discharge from Sellafield in 1973, with a decreasing trend with depth down to ~
0.04
kBq
kg
-1 in the deeper layers. The depth profile of
239
+
240
Pu specific activities together with results from gamma-ray spectrometry for
137Cs and
241Am was compared with reported releases from the Sellafield plant in order to estimate a reliable sediment chronology. The upper layers (1992 onwards) showed higher
239
+
240
Pu specific activities than would be expected from the direct input of annual Sellafield discharges, indicating that the main input of Pu is from the time-integrated contaminated mud patch of the northeastern Irish Sea. The
240Pu/
239Pu atom ratios ranged from ~
0.03 in the deepest layers to >
0.20 in the sub-surface layers with an activity-weighted average of 0.181. The decreasing
240Pu/
239Pu atom ratio with depth reflects the changing nature of operations at the Sellafield plant from weapons-grade Pu production to reprocessing spent nuclear fuel with higher burn-up times in the late 1950s. In addition, recent annual
240Pu/
239Pu atom ratios in winkles collected during 2003–2008 from three stations along the Cumbrian coastline showed no significant spatial or temporal differences with an overall average of 0.204, which supports the hypothesis of diluted Pu input from the contaminated mud patch.
► Depth profiles of
239Pu and
240Pu determined in a northeastern Irish Sea sediment. ► Dominant Pu source is the Irish Sea mud patch contaminated by Sellafield discharges. ► Historical Pu isotopic ratios reflect the change of Sellafield operations. ► Contemporary Pu signal characterised by
240Pu/
239Pu atom ratios >
0.20.
This article presents a synthesis of recent works performed on two macrotidal tide-dominated estuaries located along the southern coast of the English Channel, the Seine estuary and the ...Mont-Saint-Michel Bay (NW France). On the basis of very high resolution seismic data combined with sediment cores and AMS dating, these works allow reconstructing the Holocene infill of such singular estuaries, yet rarely documented in the literature. Rate in sea-level rise, bedrock morphology, sediment supply are the main controlling factors of the general infill pattern. Both infills are composed of two units, the transgressive systems tract (TST) and highstand systems tract (HST). The maximum flooding surface (MFS) is dated around 6500cal BP when the Holocene sea-level rise slowed down, and corresponds to the main architectural change. The TST is poorly preserved compared with the HST that constitutes the main depositional unit, making this incised valley fill deviating from the classical models. The HST is composed above the axis of the incised valley of a highly wandering estuarine channel belt characterized by deep tidal reworking throughout its building. On the margin of this high-energy estuarine system, the HST is made of wave-dominated barriers, tidal flats and offshore tidal banks.. They constitute dominantly aggradational systems, the aggradation rate of which is in accordance with the rate of sea-level rise from 6500cal BP onwards. Consequently, the impacts of the rapid climate changesthat characterized mid- to late Holocene times can be preserved in these marginal successions. Various sedimentary and geometrical signatures indicate that periodic enhanced storminess episodes, probably related to the Bond Cold Events (~1500years periodicity), are responsible for the most drastic environmental changes in these macrotidal tide-dominated settings. The role of long term tidal cycles as well as the impact of human activities are also considered.
Studies on the consequences of dredging on estuarine morphology and its sedimentary dynamics are common, but the impacts of dumping dredge spoil in coastal open settings are rarely found in ...scientific literature. An experimental study was conducted over the period 2012-2013 to monitor the physical impacts of dredged material dumped at two adjacent sites (one million cubic metres at each) on the inner shelf of the Bay of Seine in France (eastern part of the English Channel, La Manche). As recently reinforced in the EU Marine Strategy Framework Directive (MSFD), knowledge on the location and intensity of human impacts (e.g. on marine ecosystems) is critical for effective marine management and conservation. So, two methods of disposition were tested to evaluate the impacts of dumping on the environment and thus propose recommendations for future dumping. The strategy is based on a Before-After-Control-Impact (BACI) approach, in which the spatio-temporal variability was studied by analysing the morphological and sedimentological characteristics over a period of 28 months, from November 2011 to April 2014, also including recovery of the seafloor after cessation of the dumping activities. The first experimental dumping operation (MASED) was carried out regularly for 8 months at a single point and generating a conical deposit of 5m in height, while the second dumping (MABIO) lasted for 12months involving four steps in the dumping process. In the second case, a wider area was covered, leading to the formation of a smaller deposit of 2 m in height. The dumped deposits consisted of muddy fine sand, whereas the inner shelf seafloor in this area is covered with fine to medium sand. As a result, muddy fine sand accumulated at or near the two dumping sites, with a maximum mud (i.e. particles<63 μm or>4 Φ) content of 50% compared to<5% before dumping operations. Videos obtained from a LVB200 Seabotix ROV, highlighted the heterogeneity of the sea floor around the dumping areas. Due to hydrodynamic forcing (wave climate and tidal currents), about 50% (MABIO) and 75% (MASED) of the volume of dredged material remained at the end of the dumping periods. After dumping ceased, a further 5% of material for MABIO and 20% for MASED, was transported out of the study area. For the latter, a spreading of fine particles was observed extending from the conical deposit towards the south west. To favour long-term exploitation, a more dispersive dumping over a wider surface area is recommended (e.g. MABIO)
•We monitor physical impacts of dredged material dumped over the period 2012-2013.•We analyse the morphological and sedimentological characteristics over a period of 28 months.•The dumped deposits consisted of muddy fine sand, whereas the inner shelf seafloor is an area covered with fine to medium sand.•Due to hydrodynamic forcing, 50% to 75% of the volume of dredged material remained at the end of the dumping periods.
The aim of this study is to understand the evolution of the superficial sedimentation, over the last 150 years on the mouth of the Seine estuary. An indicator is created, calculating the percentage ...area corresponding to the different facies for each set of available data (mid-19th century to 2009). The shift between the 1970s (16% of surface area of mud and muddy sand), the 1990s (about 50%) and 2009 (5%) appears clearly. The decrease in the muddy area is balanced with an increase of sandy mud and muddy sand surface. This evolution could be explained by river flow rate activity. The Seine estuary is a naturally tide-dominated estuary; however, engineering activities have increased the energy of the channelized river. Considering the fine-grained fraction in the superficial sediments and its variations, the Seine estuary has shifted to a river-dominated system.
In estuarine ecosystems, metallic and organic contaminants are mainly associated with fine grain sediments which settle on mudflats. Over time, the layers of sediment accumulate and are then ...transformed by diagenetic processes mainly controlled by microbial activity, recording the history of the estuary’s chemical contamination. In an environment of this specific type, we investigated the evolution of the chemical contamination and the structure of both total and active microbial communities, based on PhyloChip analysis of a 4.6-m core corresponding to a 40-year sedimentary record. While the archaeal abundance remained constant along the core, a decrease by one order of magnitude in the bacterial abundance was observed with depth. Both total and active microbial communities were dominated by
Proteobacteria
,
Actinobacteria
, and
Firmicutes
in all sediment samples. Among
Proteobacteria
, alpha-
Proteobacteria
dominated both total (from 37 to 60 %) and metabolically active (from 19.7 to 34.6 %) communities, including the
Rhizobiales
,
Rhodobacter
,
Caulobacterales
, and
Sphingomonadales
orders. Co-inertia analysis revealed a relationship between polycyclic aromatic hydrocarbons, zinc and some polychlorobiphenyls concentrations, and the structure of total and active microbial communities in the oldest and most contaminated sediments (from 1970 to 1975), suggesting that long-term exposure to chemicals shaped the structure of the microbial community.
Ground-based solar observations are strongly affected by optical turbulence. The concept of a new instrument which allows one to measure both spatial and temporal parameters of atmospheric turbulence ...has been proposed in the late 1990s. The instrument MISOLFA (Moniteur d'Images Solaire Franco-Algérien) is based on this concept and has been developed over the past 10 years in the framework of a ground-based solar astrometry programme and in parallel to the development of several night time turbulence monitors at Calern Observatory, south of France. In this paper, we first describe its instrumental concept, the technical choices that were made to meet the specifications and discuss the difficulties encountered. Using numerical simulations, we present and test the methods that can be used in order to estimate the turbulence parameters from both MISOLFA image and pupil planes. The effect of finite outer scale on Fried parameter estimation from a simple estimate of the angle-of-arrival variance is clearly shown. Finally, we present the first results obtained with the instrument fully operating in its two observing planes. We obtained a mean value of angle-of-arrival coherence time of 5.3 ms, and good agreement is found between spatial parameters obtained with image and pupil planes. First estimates of the atmospheric structure constant
$C_{n}^2(h)$
and outer scale L
0(h) profiles are also presented which illustrates the profiling capacities of the new instrument.
In estuarine ecosystems, trace metals are mainly associated with fine grain sediments which settle on mudflats. Over time, the layers of sediments accumulate and are then transformed by diagenetic ...processes, recording the history of the estuary’s chemical contamination. In such a specific environment, we investigated to what extent a chronic exposure to contaminants could affect metal-resistant sedimentary bacteria in subsurface sediments. The occurrence and diversity of cadmium resistance genes (cadA, czcA) was investigated in 5- and 33-year-old sediments from a highly contaminated estuary (Seine France). Primers were designed to detect a 252-bp fragment of the czcA gene, specifically targeting a transmembrane helice domain (TMH IV) involved in the proton substrate antiport of this efflux pump. Although the cadA gene was not detected, the highest diversity of the sequence of the czcA gene was observed in the 5-year-old sediment. According to the percentage of identity at the amino acid level, the closest CzcA relatives were identified among Proteobacteria (α, β, γ, and δ), Verrucomicrobia, Nitrospirae, and Bacteroidetes. The most abundant sequences were affiliated with Stenotrophomonas. In contrast, in the 33-year-old sediment, CzcA sequences were mainly related to Rhodanobacter thiooxydans and Stenotrophomonas, suggesting a shaping of the metal-resistant microbial communities over time by both diagenetic processes and trace metal contamination.
Context. For the past thirty years, modern ground-based time-series of the solar radius have shown different apparent variations according to different instruments. The origins of these variations ...may result from the observer, the instrument, the atmosphere, or the Sun. Solar radius measurements have been made for a very long time and in different ways. Yet we see inconsistencies in the measurements. Numerous studies of solar radius variation appear in the literature, but with conflicting results. These measurement differences are certainly related to instrumental effects or atmospheric effects. Use of different methods (determination of the solar radius), instruments, and effects of Earth’s atmosphere could explain the lack of consistency on the past measurements. A survey of the solar radius has been initiated in 1975 by Francis Laclare, at the Calern site of the Observatoire de la Côte d’Azur (OCA). Several efforts are currently made from space missions to obtain accurate solar astrometric measurements, for example, to probe the long-term variations of solar radius, their link with solar irradiance variations, and their influence on the Earth climate. Aims. The Picard program includes a ground-based observatory consisting of different instruments based at the Calern site (OCA, France). This set of instruments has been named “Picard Sol” and consists of a Ritchey-Chrétien telescope providing full-disk images of the Sun in five narrow-wavelength bandpasses (centered on 393.37, 535.7, 607.1, 782.2, and 1025.0 nm), a Sun-photometer that measures the properties of atmospheric aerosol, a pyranometer for estimating a global sky-quality index, a wide-field camera that detects the location of clouds, and a generalized daytime seeing monitor allowing us to measure the spatio-temporal parameters of the local turbulence. Picard Sol is meant to perpetuate valuable historical series of the solar radius and to initiate new time-series, in particular during solar cycle 24. Methods. We defined the solar radius by the inflection-point position of the solar-limb profiles taken at different angular positions of the image. Our results were corrected for the effects of refraction and turbulence by numerical methods. Results. From a dataset of more than 20 000 observations carried out between 2011 and 2013, we find a solar radius of 959.78 ± 0.19 arcsec (696 113 ± 138 km) at 535.7 nm after making all necessary corrections. For the other wavelengths in the solar continuum, we derive very similar results. The solar radius observed with the Solar Diameter Imager and Surface Mapper II during the period 2011–2013 shows variations shorter than 50 milli-arcsec that are out of phase with solar activity.
Sediment cores were collected at the outlet of the highly anthropogenized catchment of the Seine River at two contrasting sites: a flood plain of the lower Seine River and a quasi-permanently ...submerged harbour basin (or wet dock) in the upper tidal estuary.
Analyses of artificial radionuclides (137Cs and plutonium isotopes), coupled with hydrological and bathymetric data, lead to a precise dating of the sediment cores collected at the two sites.
137Cs signals originating from global fallout (early 1960s) and from the Chernobyl accident (1986) are identified, but at different levels due to the incomplete nature or variable continuity of the records. Anomalous 238Pu concentrations found at both sites (1–2 Bq kg−1) are attributed to unknown industrial releases originating from upstream. Interpolating 137Cs sediment activities under the assumption of a constant sediment rate, those releases were dated back to 1975 ±1, thus providing a local but reliable time-marker.
Age models have highlighted a very contrasting sediment filling dynamics in these two sites. This study presents the first sediment record of alpha- and gamma-emitting artificial radionuclides obtained at the outlet of the huge catchment area of the River Seine, over a period covering the last 50 years.
•The sediment dynamics is studied at two contrasting sites in the Seine River.•137Cs, 238Pu an 239,240Pu profiles are provided and used as time markers.•Radionuclide and hydrodynamics data are combined to build up accurate age models.•It is the first step for a reconstruction of the Seine River contamination history.
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