35 research outputs found
Observations sur les microfaciès des roches sédimentaires prélevées sur la marge armoricaine
L'étude des dragages de roches et de sédiments effectués sur la marge armoricaine entre 47 et 48, de latitude Nord permet d'en préciser l'évolution paléogéographique et tectonique et de l'intégrer à celle du golfe de Gascogne. L'analyse de mi crofaciès des échantillons rocheux dont l'âge varie du Jurassique supérieur au Néogène met en évidence - d'une part, l'appartenance de la bordure nord de la marge armoricaine au domaine mésogéen au moins jusqu'au Crétacé moyen; - d'autre part, la permanence d'un régime de plate-forme carbonatée qui paraît fonctionner du Tithonique au Tertiaire inférieur, sur laquelle ont pu s'installer des édifices récifaux notamment au Crétacé moyen. Pendant toute cette période la sédimentation a compensé la subsidence; - enfin, l'accentuation de la subsidence à partir de l'Éocène (accompagnée d'une diminution de la production de calcium $) qui est vraisemblablement en relation avec les phénomènes tectoniques majeurs qui ont affecté à ce moment la bordure sud de la plaque européenne
Evidence de Messinien à l'Ouest de Gibraltar. Caractéristisation faunistique et isotopique
Seismic profiles studies have shown in the province west of Gibraltar Straits the existence of an acoustically opaque layer interpreted as a massive allochthonous structure put in place during the Tortonian. Several sedimentary basins developped on the top of this structure. Core Alb. KS09 comes from one of these basins. In this core two sedimentological and faunistic units can be distinguished :
a) 0 cm to 11 cm, Pleistocene sediments (N22-N23) with subtropical atlantic foraminiferal assemblages.
b) 11 cm to 147 cm, lower Messinian sediments (N17) with Globorotalia conomiozea, G. mediterranea associated with temperate species.
Therefore, at 11 cm there exists a hiatus of about 5 m.y. Oxygen and carbon isotopic compositions of planktonic and benthonic foraminifera show relatively large variations (exceeding 1% ?), The ratios tend to be higher in the more recent levels and there is an increase in the surface-to-bottom gradient of these values. Up to 50 cm, oxygen isotope ratios of benthonic foraminifera are lower than those of foraminifera of the same age (N17) in the Atlantic. The deep waters of this basin might have been warmer than those of the Atlantic. However, between levels 65 cm and 125 cm the sudden increase in the surface-to-bottom δ 18O and δ 13C gradients as well as the replacement of G. mediterranea, G. saphoae and G. conomiozea by Globorotalids belonging to the scitula and menardii groups might be explained by an important admixture with Atlantic waters resulting from a higher sea level or from a deepening of the basin. This saline surface waters might have induced a stagnation and a decrease in oxygen content of the deeper water (pyritic, black coloured sediments), as well as a decrease of the 13C/12C ratios of benthonic foraminifera (increase in residence time of deep waters). This particular episod may illustrate a more general pattern of sea water exchange in the peri-mediterranean basins, during the late Miocene.Le secteur situé à l'Ouest du détroit de Gibraltar est caractérisé par la présence d'une unité allochtone mise en place au Tortonien ; sur cette unité se sont développés un ensemble de petits bassins sédimentaires. Les sédiments prélevés dans l'un d'entre eux contiennent une microfaune de Foraminifères planctoniques d'âge Messinien inférieur (biozone à Globorotalia conomiozea, sous-zone à Globorotalia mediterranea) ayant des affinités méditerranéennes. Les compositions isotopiques (18O/16O) des tests aussi bien des espèces benthiques que planctoniques sont voisines de celles mesurées sur des espèces provenant de Méditerranée. Compte tenu de l'évolution structurale de ce secteur à la fin du Miocène on peut envisager comme hypothèse l'appartenance de ce bassin au domaine méditerranéen.Vergnaud-Grazzini Colette, Pastouret L. Evidence de Messinien à l'Ouest de Gibraltar. Caractéristisation faunistique et isotopique . In: Géologie Méditerranéenne. Tome 7, numéro 1, 1980. Aspects dynamiques du passage Miocène – Pliocène en Méditerranée. pp. 135-145
(Table 2) Results of oxygen and carbon isotopic analysis in the tests of benthic and planktic foraminifera, sediment core CH22KW31
Late Quaternary deep sea sedimentation off the Niger delta
The oxygen isotopes ratios of benthic foraminifera and detailed radiocarbon ages of the organic matter of an over 15 m long sediment core from the outer Niger delta allow us to date the oxygen isotope stage boundaries 1/2 to 11500 (+/- 650) years BP, 2/3 to approximately 23000 (+/- 2000) years BP. The composition of the predominantly terrigenous clays and accessory pelagic fossils reflects the evolution of the climate over the southwestern Sahel zone and the response of the Eastern Tropical Atlantic to these climatic fluctuations during the Late Quaternary.
The dilution of the pelagic fossil concentrations by the terrigenous material and the oxygen isotopes ratios of planktonic foraminifera indicate large fluctuations in the freshwater discharge from the Niger, with high precipitations over the drainage area of this river from 4500 (+/- 300) to 11500 (+/- 650) years BP and from 11800 (+(- 600) to 13000 (+/- 600) years BP while the time intervals in between were as dry as today. Relative increase of kaolinite during wet phases and the association of smectite, chlorite and attapulgite during dry ones characterize the response of the weathering in the Niger drainage basins to the climatic fluctuations. The occurrence of 10-14 A mixed-layers prior to 26000 years BP is correlated with moderate alteration of the crystalline substratum outcrops from the middle-lower part of the Niger Basin. High quartz concentrations are particularly typical for the transition between oxygen isotope stages 1 and 2 at the inception of heavy precipitations in the southern Sahel zone. Sedimentation rates were quite constant, 30-35 cm/1000 years; they became unusually large at the beginning of the Holocene from 10900 (+/- 650) to 11500 (+/- 650) years BP where they reached more than 600 cm/1000 years.
Bottom waters around 1100 m depth in the Gulf of Guinea responded to changes in paleo-oceanography of the entire Atlantic Ocean as well as to local influences. Abnormal carbon isotopes ratios and the drastic changes from a highly diversified fauna (during stages 2 and 3. and during the last part of stage 1 after approx. 7000 years BP) to a poorly diversified fauna in the intervenin time span point to the development of a local benthic environment which cannot easily be compared with the corresponding continental and slope environments of the entire Atlantic Ocean
Late quaternary climatic changes in western tropical africa deduced from deep-sea sedimentation off the Niger delta
The oxygen isotopes ratios of benthic foraminifera and detailed radiocarbon ages of the organic matter of an over 15 m long sediment core from the outer Niger delta allow us to date the oxygen isotope stage boundaries 1,2 to 11500 (+ 650) years BP. 2/3 to approximately 23000 (+ 2000) years BP. The composition of the predominantly terrigenous clays and accessory pelagic fossils reflects the evolution of the climate over the southwestern Sahel zone and the response of the Eastern Tropical Atlantic to these climatic fluctuations during the Late Quaternary. [NOT CONTROLLED OCR]Nous avons suivi sur 30 000 ans environ l'alternance phase sèche-phase humide caractéristique du Pléistocène des basses latitudes, ainsi que son empreinte dans l'histoire sédimentaire, à travers l'étude d'une carotte prélevée dans la partie externe du delta du Niger. La courbe des variations des teneurs en isotopes de l'oxygène des tests de foraminifères benthiques a été calibrée par datations au (14)O : les zones de transition entre les stades isotopiques 1.2 et 2.3 ont pu être datées respectivement de 11 500 (+ 650) ans BP et de 23000 (+2000) ans BP environ. [OCR NON CONTRÔLE
Annotated record of the detailed examination of Mn deposits from R/V "Le Suroit" GEOMANCHE75/1 stations
We report here preliminary results from the detailed sampling of granitic basement at a depth of 4.000 m at the lower edge of the Armorican continental margin near 48°N and 12°W. We consider that we have sampled here the extreme lateral limit of the continental crust where local exposures emerge from under a cover of Cainozoic,Mesozoic and probable Palaeozoic sediments
Annotated record of the detailed examination of Mn deposits from R/V "Jean Charcot" GEOMANCHE 76/2 stations
This report presents the preliminary results of the study of rocks and sediments obtained by dredging during four cruises of the R/V Jean Charcot and R/V Le Suroit during a cooperative program between CNEXO and CEPM (CH 58: April 1975; SU 01: December 1975; CH 66: February 1976; and CH 67: March 1976). Several dredges on the continental slope of the Goban Spur recovered "granitic" rocks on two morphological structures, Granite Cliff and "Menez Bihan" (Pautot et al., 1976), in water depths ranging from 3200 to 4200 meters. According to the radiometric age and petrology, the granodiorite appears to have a close affinity with similar igneous facies which have been described in Iberia (Capdevilla et al., 1973) and attributed to the Variscan intrusive episode which is also found in the nearby continental area (southwest Great Britain and Brittany, France). They often are coated by a centimeter-thick layer of ferromanganiferous deposits (Shaaf et al., 1977)
Efficient photo-thermal activation of gold nanoparticle-doped polymer plasmonic switches
International audienceWe report on the photo-thermal activation of dielectric loaded plasmonic switches comprised of gold nanoparticle-doped polymer deposited onto a gold film. The plasmonic switches rely on a multi-mode interferometer design and are fabricated by electron beam lithography applied to a positive resin doped with gold nanoparticles at a volume ratio of 0.52%. A cross-bar switching is obtained at telecom wavelengths by pumping the devices with a visible beam having a frequency within the localized surface plasmon resonance band of the embedded nanoparticles. By comparing the switching performances of doped and undoped devices, we show that for the modest doping level we consider, the power needed to activate the doped switches is reduced by a factor 2.5 compared to undoped devices. The minimization of activation power is attributed to enhanced light-heat conversion and optimized spatial heat generation for doped devices and not to a change of the thermo-optic coefficient of the doped polymer