Richard Maire

Nuclear microprobe analysis of uranium-rich speleothems: Methodological aspects

Abstract The study of the chemical composition of speleothems, calcium carbonate concretions, can reveal past climatic and environmental changes. We investigated the micro-chemical distribution of trace elements in speleothems formed of an unusual mixture of calcite and aragonite, originating from the Aranzadi gallery, ‘Gouffre de La Pierre-Saint-Martin’, Pyrenees, France. PIXE and RBS were performed simultaneously with a 3.0 MeV proton beam, focused down to 5.0 μm diameter. Quantitative element mapping revealed that U and Sr concentrations were the highest ever observed in speleothems of meteoric origin. Moreover, within the same samples, Sr and U were more concentrated by a factor 10 in aragonite mineral phase than in calcite. This can be explained by the crystalline orthorhombic structure of aragonite that presents a site of coordination wider than in calcite allowing to trap large cations such as Sr 2+ and (UO 2 ) 2+ . These results helped to raise new hypothesis on the very specific growth conditions of some speleothems from the Aranzadi gallery.

Multicellular Consortia Preserved in Biogenic Ductile-Plastic Nodules of Okondja Basin (Gabon) by 2.1 Ga

We highlight complex ductile nodules found in the FB2 formation of the Okondja francevillian Basin (Gabon) dated to 2.1 Ga, during the Great Oxygenation Event (GOE). More than 500 specimens were collected from unmetamorphosed pelites in an excellent state of preservation. These nodules are divided into two groups: globular and elongated forms, one to three cm in diameter and over six cm long along the axis of elongation. They are characterized by two hemispheres separated by a central zone. They have a remarkable radial structure in spheroidal forms, and often have a polyphased structure in elongated forms. Chemical and microscopic analyses indicate that these nodules are formed over 80% of a fabric of micro-quartz, fossilizing calcite grains (rounded grains and remnants of biofilms) and clay channels. These slightly pyritized nodules also contain iron in the form of hematite and goethite. There are several types of micro-organisms, several biomorphic iron particles (generally less than 300 μm in size) among which are preserved some multicellular clusters measuring between 50 and 250 microns, and some larger biomorphs. Their organization is very complex with fibro-radial and polyphased internal fabric and a discrete external peripheral system. The very low values of δ13C carb in calcite (-17 and -26‰) suggest precipitation of calcite from decomposition of organic matter in anoxic photosynthesis conditions. The morphological, petrographic, geochemical and isotopic characteristics of these nodules indicate a biological origin such as microbial/algal consortia, associated with eukaryotic organisms. They lived on the seafloor, buried just below the surface of the sediment, in calm, shallow and oxido-reducing environment.

An Akouemma hemisphaeria Organic Macrofossils Colony HostingBiodiversity Assemblage on the Seafloor of Okondja Basin (Gabon) datedat 2.2 Ga

A colony of silico-carbonate Akouemma nodules found in Akou River sedimentary formations of the Palaeoproterozoic Okondja Basin consists of two groups, spheroidal (ovoid) nodules and elongated nodules. These nodules, which consist of two hemispheres separated by a median disc, are composed essentially of micro-quartz associated with calcite of extra-polymeric substance (EPS) type, clay minerals, organic carbon and oxides and sulphides of iron. They contain tubular microfossils, pluricellular clusters, microorganisms and vesicles, and have undergone considerable deformation by mutual lateral compression in tabular beds. They were interpreted as biogenic nodules hosting microorganisms. We provide the following additional supporting evidence: Akouemma nodules exhibit internal fibro-radial fabrics initially composed of fibres and carbon particles; the initially well-organized structures are decaying and are in particles and fragments that are dispersed in the undeformed siliceous mass. These internal fabrics are strongly highlighted by Al- K- (Ti)-rich clay minerals that are often in close association with fibres and carbon particles. They had likely a vegetative reproduction by duplication. We infer that the Akouemma nodules are macrofossils of sessile soft-plastic body organisms. These macrofossils, recently dated at 2.2 Ga, are designated “Akouemma hemisphaeria” and bring a new vision to the “large colonial organisms” found in the Franceville Basin.

Study of Subterranean Floods in OceanicSubpolar Karst of Madre de DiosArchipelago (Patagonia, Chile)

Madre de Dios archipelago is located, at 50°30′ S, on the Pacific front. The karst areas in the reef limestone of Upper Paleozoic of Chilean Patagonia have long remained unexplored because of their remoteness, difficult access and very inhospitable cold, wet and windy climate. The annual rainfall is 7–8 m/year and the average wind speed reaches 70 km/h almost unidirectional (W to NW), and involve strong floods at a high rate. To study the flood dynamics, several underground sites have been instrumented in 2008 and recovered in 2010.

Les grandes cavités alpines. Répartition et contexte hydrogéologique

Les Alpes occidentales possedent une quarantaine de cavites connues depassant 500 m de profondeur. Les grands systemes karstiques se situent principalement dans les chaines subalpines de la zone externe et secondairement dans la zone interne. Ils se developpent dans des dispositifs structuraux varies : synclinaux perches, gouttieres et voutes anticlinales plongeantes, structures chevauchantes complexes. La zone de transfert vertical et oblique est caracterisee par une circulation vadose surtout cascadante (puits-meandres, collecteurs obliques) tandis que la zone de base noyee a une importance varia¬ ble selon les reseaux.