Long term monitoring of meiofauna densities after the Amoco Cadiz oil spill

Long term temporal variations in nematode and copepod densities were monitored in the upper and lower part of the Bay of Morlaix (West Channel), which was heavily contaminated by the 1978 Amoco Cadiz oil spill. Deseasonalization of abundance data allowed estimates to be made of the different components, namely the annual and seasonal trends and the residue. Reduced nematode abundance was long lasting in both types of sediment investigated: it appeared to be a gradual decrease in sublittoral sand but there was a dramatic reduction in estuarine mud 2 years after disturbance. Copepod variability, however, was related to seasonal factors.     

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank,North Atlantic ocean

Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast.We applied the⁴⁰Ar/³⁹Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma).Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events.The basement rocks of Gorringe Bank reveal distrubed⁴⁰Ar/³⁹Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.     

Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3

A detailed topographic and geophysical survey of the Daiichi-Kashima Seamount area in the southern Japan Trench, northwestern Pacific margin, clearly defines a high-angle normal fault which splits the seamount into two halves. A fan-shaped zone was investigated along 2–4 km spaced, 100 km long subparallel tracks using narrow multi-beam (Seabeam) echo-sounder with simultaneous measurements of gravity, magnetic total field and single-channel seismic reflection records. Vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported. The northern and southern extensions of the normal fault beyond the flank of the seamount were delineated. Materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount. Canyons observed in the upper landward slope terminate at the mid-slope terrace which has been uplifted since start of subduction of the seamount. Most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern Japan Trench wall. This survey was conducted on R/V “Jean Charcot” as a part of the Kaiko I cruise, Leg 3, in July–August 1984 under the auspices of the French-Japanese scientific cooperative program.     

Nouvelles synthéses hydrothermales du zircon: substitutions isomorphiques; relation morphologie-milieu de croissance

Zircon has been synthesized from a large number of mineral salts and also from one organic compound of zirconium. Its crystallization requires an acid reactive environment.The obtained crystals have been studied morphologically and the crystallographic forms obtained are linked to the presence of specific ions.The study of substitutions in the zircon network shows that hydro-zircon αZr(SiO₄)_1?x(OH)_4x will form only when fluorine is present. Other varieties of the α and β phases ZrSiO₄ have been obtained, substituting Hf, U, for Zr, and Ge for Si and an immiscible α phase ThSiO₄.Germanium enters the zircon network only at low temperature. Uranium and Hf, but not Th, can substitute for Zr in zircon.     

Dynamical aspects of katabatic wind evolution in the Antarctic coastal zone

The spatial evolution of katabatic winds along idealized slopes representative of Antarctic terrain is examined using a hydrostatic, two-dimensional primitive equation model with high resolution. A downslope momentum-forces analysis is made of simulations in which katabatic flow reaches steady state, with emphasis on physical mechanisms in the coastal zone. The importance of the reversal of the pressure gradient force in the coastal zone, causing the sudden decay of katabatic winds, is discussed.     

Seismic evidence of a crustal overthrust in the Western Alps

Seismic data provided by the ALP75 experiment in the northwestern part of the Alpine Arc has been re-evaluated using ray-tracing techniques in a laterally inhomogeneous medium, thus improving a previous interpretation. The structure of the crust definitely appears to be different in the Northern Subalpine Chains where it is layered with an ESE mean dip of 30° and in the inner part of the Alpine Are where no clear stratification can be derived from the available data.In the Northern Subalpine Chains the boundary between the upper and lower crust is found to be extremely uneven whereas the Moho discontinuity underneath as well as a boundary in the lower crust shows no evidence of unevenness. This pattern can be ascribed to the rheological properties of the crust which has a birttle behaviour at shallow depths and a more plastic behaviour underneath. A velocity reversal is found under the inner part of the Alpine Arc at a depth of 11 to 23 km.An attempt is made to synthesize deep seismic sounding data which have been accumulated since 1956 in southeastern France. The crust is stratified to the north of a line Cévennes-Aiguilles Rouges and amorphous to the south. This line, which is the former boundary between the European plate and the Provence subplate, is thought to have acted as a weak zone during the Alpine orogeny. A crustal overthrust could have occurred in this region which would have been furthered by the higher plasticity of the low-velocity zone. Finally we speculate on the physical significance of intracrustal velocity reversals and suggest they are associated with active tectonic areas where dislocation densities are high.A Lithospheric Seismic Profile along the Axis of the Alps; Paper No. V.     

Remplacements isomorphiques par synthèses hydrothermales dans les silicates fluores: Topaze et zunyite

The substituted topaz Al₂GeO₄(OH,F)₂, Al₂(Si_xGe_1-x)O₄(OH,F)₂ and Ga₂GeO₄(OH,F)₂, as well as the substituted zunyite Al₁₃Ge₅O₂₀(OH,F)₁₆F₂Cl have been synthetised. The unit-cell parameters of these artificial minerals, isomorphs of natural compounds, have been determined; the corresponding lists of d_hkl are given. The study by I.R. spectroscopy of these compounds with a total or a partial replacement of Si by Ge in the same way as Ga-Ge topaz suggests the assignment of certain absorption bands to characteristic structural groups. The thermal decomposition of AlSi and AlGe topaz into mullite has been studied.