Géochimie d'une toposéquence de sols tropicaux du Tchad : utilisation des méthodes statistiques

In der tropischen Bodenkette von Mindera (Tschad) konnten verschiedene pedologische und geochemische Faziestypen festgestellt werden. Von den höher gelegenen ausgewaschenen Typen zu den angereicherten Typen der tieferen Lagen findet man Kaolinit, konkretionierte Eisen-und Manganhydroxyde, Montmorillonit und schliesslich Kalziumkarbonat. Mit Hilfe statistischer Methoden ist es möglich die Proben in Gruppen zusammenzufassen, die Beziehungen zwischen Hauptund Spurenelementen festzustellen sowie deren Lokalisation in verschiedenen mineralogischen und geochemischen Faziestypen zu erkennen. So erscheint die landschaftliche Verteilung der Elemente sehr nuanciert : die Zusammensetzung der Bestandteile eines Faziestyps ist nicht konstant längs der Bodenkette, die relative Mobilität der Elemente hängt von dieser Fazies und den vorhandenen geochemischen Schranken ab.Геохимия последовательного ряда тропических почв чада. применение статистических методов. В ряде тропических почв Миндеры, территория Чад, различные педологические и геохимические фации располагаются, во времени и пространстве, в определенном порядке. Они характеризуются, от вышележащей выщелоченной до нижележащей замкнутой среды, следующим последовательним рядом : каолинит, конкреционные гидроокиси железа и потом марганца, монтмориллонит и карбонат кальция. Статистические методы (кистевой анализ, факторный анализ) позволяют выделить группы образцов, связь между главными элементами и микроэлементами, также как и их локализацию в различных минералогических и геохимических определенных фациях. Таким образом, хроматографическая схема элементов в ландшафтах представляется нюансированной : состав компонентов одной фации не постоянен в последовательном ряду ; порядок относительной подвижности элементов меняется в зависимости от фаций и природы геохимических барьеров.In the tropical soil catena of Mindera, Chad, several pedological faciès are distributed in space and time. From the leached environments of the upper part of the catena to the confined environments of the lower part, the succession is the following : kaolinite, iron then manganese hydroxide concretions, montmorillonite and calcium carbonate. The statistic methods (factor analysis and cluster analysis) permit the autours to establish groups of samples, relations between major elements and trace elements, as well as their distribution in the different mineralogical and geochemical facies defined. The scheme of the chromatography of elements in the landscapes is thus discriminative : the composition of the constituents of a facies is not constant along the catena ; the order of relative mobility of elements varies as a function of facies and of the nature of geochemical barriers.Dans la chaîne de sols tropicaux de Mindera, au Tchad, différents faciès pédologiques et géochimiques s'ordonnent dans l'espace et dans le temps. Ils sont caractérisés, depuis les milieux lessivés de l'amont aux milieux confinés de l'aval, par la succession suivante : kaolinite, hydroxydes concrétionnés de fer puis de manganèse, montmorillonite et carbonate de calcium. Les méthodes statistiques (analyse de grappe et analyse factorielle) permettent d'établir des groupes d'échantillons, les relations entre éléments majeurs et traces, ainsi que leur localisation dans les différents faciès minéralogiques et géochimiques définis. Le schéma de la Chromatographie des éléments dans les paysages est donc nuancé : la composition des constituants d'un faciès n'est pas constante le long de la toposéquence ; l'ordre de mobilité relative des éléments varie en fonction des faciès et de la nature des barrières géochimiques.Karpoff Anne-Marie, Bocquier Gérard, Isnard Pierre, Tardy Yves. Géochimie d'une toposéquence de sols tropicaux du Tchad. Utilisation des méthodes statistiques. In: Sciences Géologiques. Bulletin, tome 26, n°4, 1973. Sédimentologie et géochimie de la surface. pp. 315-338

Active oceanic spreading in the northern north Fiji basin : results of the NOFI cruise of R/V l'Atalante (Newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August-September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along an off-axis from 170°40'E to 178°E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. (Résumé d'auteur

Clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower Miocene carbonate sediments of the Bahamas platform: Isotopic and thermodynamic evidence.

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Metalliferous sediments within lava sequences of the Sumail ophiolite (Oman): Mineralogical and geochemical characterization, origin and evolution

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Fluid-Sediment Interactions in the Nankai Subduction Zone (NanTroSEIZE): Pore Fluid and Mineralogical Records

International audienceThe Nankai Trough Seismogenic Zone Experiment (IODP), SW Japan, aimed at unraveling hydrologic processes at subduction megathrusts and in situ rock and fluid properties to understand the fault zone behavior during earthquake nucleation and rupture propagation. At different sites, characterization of the sediments at high resolution allows tracing fluid-rock reactions that are either associated to early transformations (hydration or dehydration of clay minerals) during the accretion phase or to mineral recrystallization processes linked to deformation phases in gouge zones. In the sedimentary strata at the front of the accretionary prism changes in clay composition are inferred from chemical trends observed in pore fluids, which further indicate the influence of volcanic ash alteration (zeolite formation). Along the megasplay fault, recrystallization processes (e.g. pyrite) are observed in foliated gouge zones at the contact with non-foliated silty clay. The data highlight the importance of small-scale observations for process and reactive phases such as clays in such variable fluid-sediment reactions

The Chenaillet Ophiolite in the French/Italian Alps: an ancient analogue for an Oceanic Core Complex?

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Anatomically modern human in Southeast Asia (Laos) by 46 ka.

International audienceUncertainties surround the timing of modern human emergence and occupation in East and Southeast Asia. Although genetic and archeological data indicate a rapid migration out of Africa and into Southeast Asia by at least 60 ka, mainland Southeast Asia is notable for its absence of fossil evidence for early modern human occupation. Here we report on a modern human cranium from Tam Pa Ling, Laos, which was recovered from a secure stratigraphic context. Radiocarbon and luminescence dating of the surrounding sediments provide a minimum age of 51-46 ka, and direct U-dating of the bone indicates a maximum age of similar to 63 ka. The cranium has a derived modern human morphology in features of the frontal, occipital, maxillae, and dentition. It is also differentiated from western Eurasian archaic humans in aspects of its temporal, occipital, and dental morphology. In the context of an increasingly documented archaic-modern morphological mosaic among the earliest modern humans in western Eurasia, Tam Pa Ling establishes a definitively modern population in Southeast Asia at similar to 50 ka cal BP. As such, it provides the earliest skeletal evidence for fully modern humans in mainland Southeast Asia

Anatomically modern human in Southeast Asia (Laos)

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