Electron microprobe U–Th–Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapá Block, southeastern Guiana Shield, Northern Brazil

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.     

New palaeomagnetic results from the Kerguelen Islands

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.     

On the relationship between Lyapunov times and macroscopic instability times

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.     

Microbial mobilization of barite

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.     

Le mécanisme de la précipitation du Co3 Ca et la Détermination de L'épaisseur du Dépôt annuel de la vase dans le lac de Neuchâtel

Sans résumé     

La distribution de l'azote moleculaire N2 dans l'Atlantique equatorial oriental

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.     

A new hypothesis about the surface circulation in the eastern basin of the mediterranean sea

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.     

Deep strong currents and sediment transport in the Northwestern Mediterranean Sea

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.     

36.5万年以来西边界底流冰期-间冰期不稳定性与ODP 646站位(拉布拉多海)黏土级沉积碎屑Sm/Nd比率的关系

1区域概况随着气候—海洋系统模拟工作力度的加大,相关古气候改变北大西洋深层水(NADW)生产力速率的论著十分亟需。大多数作者认为整体的NADW生产力在冰期降低,他们的推断范围不完全一致。冰期更大量的南大洋水体涌入北大西洋,而且北大西洋深部对流现在已被更浅的和更南部的对流所取代。有关更新世末次冰期—间冰期旋回NADW演变,主要研究成果如下:(1)沿冰岛—苏格兰溢流水团路径从冰岛盆地采集柱状样,对其进行磁各向异性测试。变化结果同过去20万a水体可变强度相关联。冰期氧同位素2期、4期、6期较之间冰期1期、3期、5期有30%的强度衰…