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101.
Lúcia T. da Rosa-Costa Jean-Michel Lafon Alain Cocherie Claude Delor 《Journal of South American Earth Sciences》2008,26(4):445-462
The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones. 相似文献
102.
New palaeomagnetic analyses have been carried out in the Kerguelen Islands on 32 lava flows of well-established age (20–22 Ma). Combined with previous studies, they yield a reliable pole for the Lower Miocene for the Antarctic plate: N = 59 flows. 349.9°E, 83.5°S, A95 = 6.1°. A reversal sequence reversed-normal has been identified in the Port Jeanne d'Arc section. 相似文献
103.
Alessandro Morbidelli Claude Froeschlé 《Celestial Mechanics and Dynamical Astronomy》1995,63(2):227-239
On the basis of the general theory of Hamiltonian systems, we consider the relationship between Lyapunov times and macroscopic diffusion times. We find out that there are two regimes: the Nekhoroshev regime and the resonant overlapping regime. In the first case the diffusion time is exponentially long with respect to Lyapunov times. In the second case, the relationship is polynomial although we do not find any theoretical reason for the existence of a universal power law. We show numerical evidences which confirm our theoretical considerations. 相似文献
104.
Bacteria from lake mud grown under anaerobic conditions with powdered barite present significantly increase the amount of barium that dissolves in the culture medium. Innocula of lake mud were added to a growth medium specified for sulfate-reducing bacteria. Powdered barite was added to each culture flask and flasks were purged with nitrogen, sealed and maintained at 20°C for 17 days. When the cultures were opened for analysis, hydrogen sulfide and iron sulfide had formed in all growing cultures. In cultures containing barite as the only sulfate, the barium content of the culture solutions had risen to an average of 3.3 ppm (maximum = 3.8 ppm, minimum = 1.8 ppm). Cultures containing ferrous sulfate and barite showed less than 0.1 ppm barium in solution. Cultures that had been sterilized by poisoning or autoclaving showed approximately 0.5 ppm barium in solution. Sulfate-reducing bacteria may be responsible for the high barium concentrations recorded in streams draining barite mining areas. 相似文献
105.
106.
Claude Oudot 《Marine Chemistry》1982,11(6):573-588
The distribution of molecular nitrogen in the equatorial area of the eastern Atlantic is shown for a warm season (April) and a cold season (August) and interpreted in the light of the thermal field.The measured nitrogen always exceeds the solubility. The maximum supersaturations of nitrogen are always located in the layer with vertical nitrogen gradient, at the top of the thermocline. The extreme supersaturations occur more often in the warm season than in the cold season. They express either an eddy mixing (boundaries of the Equatorial undercurrent), or the advective transport of a water mass (salinity maximum of the South Atlantic subtropical water).
Résumé
La distribution de l'azote moléculaire en zone équatoriale de l'Atlantique Est est présentée en saison chaude (avril) et en saison froide (août) et interprétée à la lumière du champ thermique.L'azote mesuré est toujours en excès sur la solubilité. Les sursaturations maximales d'azote sont toujours repérées dans la couche de gradient vertical d'azote, au sommet de la thermocline. Les sursaturations extrêmes paraissent être plus nombreuses en saison chaude qu'en saison froide. Elles traduisent soit un mélange turbulent (frontières du Sous Courant Equatorial) soit le transport advectif d'une eau type (maximum de salinité de l'Eau Subtropicale de l'Atlantique Sud). Mar. Chem., 11: 573–588. 相似文献107.
A new hypothesis about the surface circulation in the eastern basin of the mediterranean sea 总被引:3,自引:1,他引:3
In time, the circulation of the Atlantic Water (AW) in the eastern basin of the Mediterranean Sea has been described differently, according to two major representations. The historical one, which began with the scheme from Nielsen in 1912 and has been refined up to the 1980s, favours a counterclockwise circulation in the whole basin, with AW flowing in its southern part as a broad flow off Libya and Egypt (from the Ionian to the Levantine subbasins), then continuing along Middle-East and Turkey before flowing back westwards. The more recent one, issued in the 1990s, favours a clockwise circulation in the northern part of the Ionian continuing offshore across the basin from the Cretan to the central part of the Levantine as the so-called “Mid-Mediterranean Jet”. This jet is depicted then as splitting both clockwise in the southeastern part of the basin and counterclockwise off Turkey (where this representation agrees with the former). Because the recent representation cannot be considered as a refinement of the historical ones, we have been interested in understanding why a given data set available to everybody is interpreted in such different ways.In the Algerian subbasin, the combined use of satellite infrared images and a significant amount of in situ data sets (hydrology and both Eulerian and Lagrangian current measurements) allowed us to solve a similar controversy. Therefore, we examined the circulation features in the eastern basin, undertaking the detailed analysis of 1000 daily and weekly composite images spanning the period 1996–2000, and of monthly composite images available since 1985. Whenever in situ observations were available, we have confronted them with the satellite thermal signatures and have shown that both are consistent. This paper focuses on the overall (basin scale) results while the detailed ones are published in an other paper. The new scheme we propose is basically a refined version of the historical ones: the circulation of AW is counterclockwise in the whole eastern basin but it is more constrained alongslope than previously thought, and the broadening historically schematised appears to be due to intense mesoscale eddies mainly generated by the instability of this circulation. 相似文献
108.
The Petit Rhône Canyon (Gulf of Lions) is incised by a narrow meandering thalweg. Two current meters were moored near the bottom, one in the channel and the other on the levee. None of the measured currents is associated with the sediment transport peculiar to deep-sea fan building. This transport is mainly influenced by relatively high speeds (1 h means up to 48 cm/s; 1 day means > 30 cm/s) measured in diverse directions. 相似文献
109.
Changes in the northern Gulf of St. Lawrence ecosystem estimated by inverse modelling: Evidence of a fishery-induced regime shift? 总被引:1,自引:0,他引:1
Claude Savenkoff Martin Castonguay Denis Chabot Mike O. Hammill Hugo Bourdages Lyne Morissette 《Estuarine, Coastal and Shelf Science》2007,73(3-4):711-724
Mass-balance models have been constructed using inverse methodology for the northern Gulf of St. Lawrence for the mid-1980s, the mid-1990s, and the early 2000s to describe ecosystem structure, trophic group interactions, and the effects of fishing and predation on the ecosystem for each time period. Our analyses indicate that the ecosystem structure shifted dramatically from one previously dominated by demersal (cod, redfish) and small-bodied forage (e.g., capelin, mackerel, herring, shrimp) species to one now dominated by small-bodied forage species. Overfishing removed a functional group in the late 1980s, large piscivorous fish (primarily cod and redfish), which has not recovered 14 years after the cessation of heavy fishing. This has left only marine mammals as top predators during the mid-1990s, and marine mammals and small Greenland halibut during the early 2000s. Predation by marine mammals on fish increased from the mid-1980s to the early 2000s while predation by large fish on fish decreased. Capelin and shrimp, the main prey in each period, showed an increase in biomass over the three periods. A switch in the main predators of capelin from cod to marine mammals occurred, while Greenland halibut progressively replaced cod as shrimp predators. Overfishing influenced community structure directly through preferential removal of larger-bodied fishes and indirectly through predation release because larger-bodied fishes exerted top-down control upon other community species or competed with other species for the same prey. Our modelling estimates showed that a change in predation structure or flows at the top of the trophic system led to changes in predation at all lower trophic levels in the northern Gulf of St. Lawrence. These changes represent a case of fishery-induced regime shift. 相似文献
110.
The anatomy, fine structure, diffusion distances and respiratory surface areas of the gills of two species of the polychaete family Alvinellidae have been investigated. Each gill consists of a stem on which are inserted two opposite rows of respiratory elements: flat sickle-shaped lamellae in Alvinella pompejana and cylindrical filaments in Paralvinella grasslei. Both lamellae and filaments have a ciliated mucous epidermis, a central layer of supporting and muscle cells and are devoid of coelomic cavity. Each respiratory element possesses one afferent and one efferent marginal vessel, united to each other distally, and connected proximally to separate longitudinal vessels running in the stem. Superficial parallel blood spaces connect the marginal vessels across the lamella or filament. Deeper, between the basal laminae of the epidermis and that of the central cell layer, a blood sinus is also present. The marginal vessels and the superficial blood spaces actually are intraepidermal extensions of this deep blood sinus. The diffusion distances are very small owing to the intraepidermal position of the respiratory blood spaces. The specific gill surface areas in A. pompejana and P. grasslei are the largest known today in polychaetes, respectively 12 and 47 cm2 per g wet weight. The distinctive features of the gills are possibly related to a low oxygen content of the ambient seawater. Numerous crystalline granules scattered in the gill epidermis suggest that this epithelium has a detoxifying function. 相似文献