FLUXMANCHE radiotracers measurements: A contribution to the dynamics of the English Channel and North Sea

International audienceFrom 1986 to 1992, 125Sb released by the nuclear fuel reprocessing plant at La Hague has been used to study the transfer of waters from mid-Channel areas towards the Straits of Dover. Since 1940, these investigations have formed part of the MAST 53 FLUXMANCHE programme and have involved repeated measurements on a Straits of Dover transect. A discussion of the results is presented here which takes into account new information provided by a hydrodynamic model developed in the framework of FLUXMANCHE; it combines knowledge of the monthly fluxes of water trough the Straits of Dover and week-by-week simulation of the 125Sb activities of waters flowing in this aera. It is proposed that soluble radionuclides introduced into the central Channel are transferred towards the Straits of Dover over a time which varies from four to seven months depending on the route taken

The distribution of artificial radionuclides in the English Channel, southern North Sea, Skagerrak and Kattegat, 1990–1993

International audienceThis paper presents the initial results concerning the distribution of artificial radionuclides from research cruises conducted in collaboration by three European institutes as part of an EEC MAST research project. Ten cruises were undertaken covering the English Channel, southern and eastern North Sea, Skagerrak and Kattegat over a period of 2.5 yr. A large number of analyses of four artificial radionuclides (13'Cs, "Tc, '*%b, !%r), which behave conservatively in seawater, provided information about the general distribution of water masses and circulation patterns as well as about single transport events in the study area. Controlled liquid releases from the La Hague nuclear fuel reprocessing plant are transported eastwards, forming a characteristic distribution pattern in the Channel and the southern North Sea. This includes a near-coastal "plume" and a distinct boundary between waters contaminated predominantly by La Hague and by Sellafield. Spatial and temporal distributions of radionuclide ratios were used, for the first time, to calculate transit times from the English Channel to the coast of Jutland. The data published herein provide an essential input to the calibration of numerical models simulating water transport processes. The results demonstrate the continuing usefulness of artificial radionuclides as oceanographic tracers, even at the very low concentrations observed at present in northwest European Shelf waters

Spatial and Temporal Distribution (1987-91) of 125Sb Used to Trace Pathways and Transit Times of Waters Entering the North Sea from the English Channel

International audienceThe labelling of marine waters off north-western Europe by artificial radio-nuclides discharged by the nuclear fuel reprocessing plants at Sellafield and La Hague provides a potentially useful tool for the study of hydrodynamic processes. In this context, the present investigation introduces a tracer, 125 Sb, which is conservative within the watermass and which is characteristic of releases from La Hague. Analysis of the data collected between 1987 and 1991 shows that a large part of the Channel waters entering the North Sea follow a route along the Belgian, Dutch, German and Danish coasts, while undergoing a low degree of dilution with other marine waters circulating in the central North Sea. The westward extension of the Channel plume is variable with time, but the general distribution is in good agreement with the ICES box model. Estimates are given for the transit times from Cap de la Hague to various sectors of the North Sea; it takes 15-17 months for labelled waters to reach the Norwegian Channel

Genetic, phenotypic, and environmental correlations in black medic,Medicago lupulina L., grown in three different environments

International audienceWe have investigated the relationship between phenotypic and genetic correlations among a large number of quantitative traits (36) in three different environments in order to determine their degree of disparity and whether phenotypic correlations could be substituted for their genetic counterparts whatever the environment. We also studied the influence of the environment on genetic and phenotypic correlations. Twenty accessions (full-sib families) ofMedicago luPulina were grown in three environments. In two of these two levels of environmental stress were generated by harvesting plants at flowering and by growing plants in competition with barley, respectively. A third environment, with no treatment, was used as a control with no stress. Average values of pod and shoot weight indicate that competition induces the highest level of stress. The genetic and phenotypic correlations among the 36 traits were compared. Significant phenotypic correlations were obtained easily, while there was no genetic variation for 1 or the 2 characters being correlated. The large positive correlation between the genetic and phenotypic correlation matrices indicated a good proportionality between genetic and phenotypic correlations matrices but not their similarity. In a given environment, when only those traits with a significant genetic variance were taken into account, there were still differences between genetic and phenotypic correlations, even when levels of significance for phenotypic correlations were lowered. Consequently, it is dangerous to substitute phenotypic correlations for genetic correlations. The number of traits that showed genetic variability increased with increasing environmental stress, consequently the number of significant genetic correlations also increased with increasing environmental stress. In contrast, the number of significant phenotypic correlations was not influnced by the environment. The structures of both phenotypic and genetic matrices, however, depended on the environment, and not in the same way for both matrices

Percentage contribution of inputs from the Atlantic, Irish Sea, English Channel and Baltic into the North Sea during 1988: A tracer-based evaluation using artificial radionuclides

International audienceA sampling cruise carried out in the North Sea in July 1988 has enabled the construction of general maps showing the distribution of salinity and temperature as well as the activities of the radionuclides 125 Sb, 137 Cs and 134 Cs. An inspection of the raw data and the distribution patterns so obtained shows the important role of meteorological fluctuations in the movement of near-surface waters throughout the North Sea. A simple mixing model is used to interpret the North Sea dataset in terms of four contributory water-masses (the Atlantic, Irish Sea, English Channel and Baltic), each of which can be identified by its specific characteristics. Maps are drawn up which indicate the contribution factor in percentage of each of these water masses at all points in the North Sea to a precision better than 15%. Pt is also possible to extract the contribution of direct fallout resulting from the Chernobyl accident; comparison with previously obtained results allows an evaluation of the rate of renewal of surface waters in the North Sea over a period of two years. In all areas, this value appears to be better than 75%

Effects of Coulomb interactions on the electronic structure of the Si(001) surface

International audienceThe electronic structure of the Si(001) surface is obtained with a tight-binding method developed in the slab geometry. The effects of local Coulomb interactions on the stability of antiferromagnetic solutions and on the width of the gap in the surface states at the Si(001) surface reconstructed with symmetric dimers are first visualized with a two-orbital model Hamiltonian. It allows us to study the effect of the coupling between the dangling and the bonding orbitals. The calculation performed with the tight-binding method supports the results of the model and provides an explanation of the semiconducting character of the reconstructed Si(001) surface with symmetric dimers. For realistic parameters which include the atomic values of the interorbital exchange, an antiferromagnetic order of (2×2) symmetry is stable. It is argued that even if the long-range antiferromagnetic order is destabilized by quantum fluctuations in quasi-two-dimensional surface states, the short-range antiferromagnetic order induced in the dangling orbitals by Coulomb interactions is sufficient to explain the semiconducting behavior of symmetric dimers

LA SÉDIMENTATION LAGUNAIRE DES DERNIERS SIÈCLES EN LANGUEDOC-ROUSSILLON. DONNÉES SÉDIMENTOLOGIQUES, ISOTOPIQUES ET PALYNOLOGIQUES

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Processing of cartographies from steam generator tubes using Eddy current testing with an absolute coil

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