Comportement de l'uranium dans les systemes volcaniques. Exemple du Vico (Latium, Italie)

SIGLEINIST T 77641 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Répartition de l'uranium dans les roches volcaniques

Uranium distribution is investigated in a range of volcanic rocks using the fission track method. Samples from the Chaîne des Puys (Massif Central, France), Vico, Vulsini and Cimino (Latium, Italy) volcanic series have been studied. Even for different degrees of differentiation and variable uranium content in the rocks, the systematic concentration of uranium in glasses and groundmasses reflects its stability in magmatic liquids. The crystallinity and texture of the groundmass are shown to play a key rôle in uranium distribution. Uranium-mineral associations differing from those existing at the magmatic stage, and the modification of uranium distribution in groundmasses argue for mobilisation and redistribution taking place in late and post-magmatic stages. Uranium distribution between groundmass and mineral phases suggests that U-silicate and/or U-phosphate complexes can stabilize uranium in magmatic liquid and control its relative enrichment in accessory minerals.La méthode des traces de fission de l'uranium est appliquée à l'étude de sa répartition dans les roches volcaniques. Les exemples étudiés proviennent des séries de la Chaîne des Puys (Massif Central, France), du Vico, du Vulsini et du Cimino (Latium, Italie centrale). — L'enrichissement systématique de l'uranium dans les mésostases et les verres par rapport aux phases cristallines, quels que soient le degré de différenciation et la teneur en uranium des laves, traduit la stabilité préférentielle de l'uranium dans les liquides magmatiques. — Le degré de cristallinité des mésostases et leurs textures déterminent le caractère homogène ou non de la répartition de l'uranium dans celles-ci. — Des phénomènes de mobilisation et de redistribution de l'uranium à des stades tardifs, voire postmagmatiques, sont mis en évidence par des associations uranium-minéraux qui diffèrent de celles du stade magmatique et par des modifications de répartition dans les mésostases. La distribution de l'uranium entre mésostase et phases minérales que l'on observe conduit à proposer l'existence de formes complexées de type urano-silicatées ou urano-phosphatées stabilisant cet élément dans les liquides magmatiques et préfigurant son enrichissement sélectif dans les minéraux accessoires les plus fréquents des roches volcaniques.Flehoc Christine, Toulhoat Nelly, Villemant Benoît. Répartition de l'uranium dans les roches volcaniques. In: Bulletin de Minéralogie, volume 110, 2-3, 1987. Les mécanismes de concentration de l'uranium dans les environnements géologiques

Un conifère fossile d’âge médiéval (1160‑1 260 Cal. AD) au bord du Lac Laramon, altitude 2 359 m (Vallée de la Clarée, Briançonnais, Hautes-Alpes)

La souche partiellement ennoyée et le tronc couché d’un grand arbre ont été trouvés à une altitude de 2 359 m, sur le talus d’érosion d’une pelouse ceinturant le lac glaciaire de Laramon (vallée de la Clarée, est du Briançonnais). Une datation 14C de la partie externe de la souche a donné un âge de 830 ± 30 BP (1 160 – 1 260 cal. AD), situant la vie de l’arbre vers la fin de l’époque médiévale, période au climat plutôt clément dans les Alpes. Le bois est parfaitement conservé. Sa structure, très simple, est celle d’un résineux ; des canaux résinifères suggèrent qu’il pourrait s’agir d’un pin ou d’un mélèze, espèces dont de vieux individus de taille et morphologie comparables s’observent dans la région jusqu’à une altitude de 2 000 m. Il est probable qu’à l’époque médiévale, des conditions climatiques plutôt clémentes permettaient à de tels arbres de vivre plus haut. La souche se trouve actuellement les « pieds dans l’eau » indiquant un creusement du lac après l’époque médiévale. Nous suggérons que ce creusement se serait produit lors du Petit Age Glaciaire, époque à laquelle se seraient mis en place les derniers dépôts glaciaires.The partially immersed stump and collapsed trunk of a big tree have been found at an altitude of 2359 m, on the erosional bank of short grassland surrounding the glacial Laramon lake (Clarée valley, eastern Briançonnais massif). The external part of the stump has given a 14C date of 830 ± 30 BP (1 160 – 1 260 cal. AD) setting the life of the tree during the Medieval period. The wood is perfectly preserved, with a very simple structure typical of conifers. The occurrence of resin channels suggests that the tree could be a pine or a larch; size- and shape-comparable specimens of these conifers are today present in the region at altitudes up to 2 000 m. It is suggested that mild climatic conditions during the Medieval period enabled such trees to live higher. The presence of permanent water at the foot of the stump indicates an expansion of the lake since the Medieval period. It is suggested that digging of the lake occurred during the Little Ice Age concurrently with the emplacement of the last glacial deposits

Marine mollusk shells record the seasonal variations of temperature during the Mid Eocene Climatic Optimum in the Paris Basin

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Geochronological arguments for a close relationship between surficial formation profiles and environmental crisis (c. 3000–2000 BP) in Gabon (Central Africa)

International audienceWe present new 14C data on charcoal fragments recovered from the lower (coarse-grained Stone Line) and upper (fine-grained Cover Horizon) portions of surficial formation profiles in Gabon. These data and others compiled from the literature enable a reconstruction of the Upper Holocene geological regional history of Gabon. The connection between the geological events recorded in the surficial formations and the upper Holocene environmental crisis is discussed and a scenario connecting geological events with climatic and environmental changes is proposed. Such a scenario suggests that following the climatic crisis, the reconstitution of soils by aeolian sedimentation could have been an important factor of Bantu expansion

Marine mollusk shells record the seasonal variations of temperature during the Mid Eocene Climatic Optimum in the Paris Basin

International audienceDuring the Middle-Late Eocene, the Earth transitioned from a greenhouse to icehouse period. Within this period, a warming phase of 500 kyr called MECO (Middle Eocene Climatic Optimum) took place at the beginning of the Bartonian (from 40.5 Ma to 40 Ma - C18n). This event is characterized by a negative shift in the δ18O profile of benthic foraminifera associated with an increase of 4 to 6 °C in surface and deep ocean waters. The peak of the MECO is also characterized by a short δ13C negative excursion at 40.0 Myr during an overall increasing trend of δ13C. This positive trend of the δ13C curve appears to be related to an atmospheric increase in the pCO2, but the causes remains unclear.Unlike the oceanic domain, few datas exist for the characterization of the MECO in coastal areas. Additionally, important component of the climatic context, such as the seasonal gradient of temperature, remain unknown. To unravel these uncertainties, this work focuses on the nearshore Eocene sedimentary records of the Paris Basin, which presents an important and remarkably well preserved paleobiodiversity of marine mollusk shells. Previous studies have confirmed that the MECO event is well recorded in Bartonian sediments, but due to several uncertainties, its stratigraphic position remains to be specified. Here we present a composite section that spans a stratigraphic interval covering the middle Lutetian (falunière de Grignon outcrop) and the Bartonian (Horizon de Mont-Saint Martin Formation, le Guépelle section and the Sables de Cresnes Formation). Thus, we combine different proxies provided by 18O, 13C and ∆47 analyses of marine mollusk shells sampled in these sections in order to clarify the stratigraphic position of the MECO in the sedimentary succession of the Paris Basin and to constrain the climatic expression of this hyperthermal event in shallow marine environment. Isotopic analyses were performed on the shells of 3 Bartonian mollusks species: 2 bivalves represented by Bicorbula gallica and Crassostrea cucullaris and 1 gastropod represented by Torquesia sulcifera. Stable isotope (δ18O and δ13C) results both show a characteristic negative excursion at the end of the Sables du Guépelle formation, in the lower part of the Bartonian. Clumped isotope analyses were performed on some specimens of B. gallica and T. sulcifera in order to better constrain the composition of δ18Ow throughout the stratigraphic interval studied. These results indicate significant decreases in local δ18Ow over the lifetime of most individuals, interpreted as large infra-annual variations in salinity. Paleotemperatures calculated from the previously constrained δ18Ocarbonate increase by 4 to 10 °C during the MECO event , while the seasonal temperature variation decreases from 11-13 °C to 8 °C during the negative isotopic excursion of the end of the Sables du Guépelle formation.Based on these new results, we propose that the MECO is recorded in the top of the Sables du Guépelle formation marked by a warming period and a lower seasonal temperature gradient. These results lead to a better chimio-chronostratigraphic calibration of the Bartonian deposits of the Paris Basin

Marine mollusk shells record the seasonal variations of temperature during the Mid Eocene Climatic Optimum in the Paris Basin

International audienceDuring the Middle-Late Eocene, the Earth transitioned from a greenhouse to icehouse period. Within this period, a warming phase of 500 kyr called MECO (Middle Eocene Climatic Optimum) took place at the beginning of the Bartonian (from 40.5 Ma to 40 Ma - C18n). This event is characterized by a negative shift in the δ18O profile of benthic foraminifera associated with an increase of 4 to 6 °C in surface and deep ocean waters. The peak of the MECO is also characterized by a short δ13C negative excursion at 40.0 Myr during an overall increasing trend of δ13C. This positive trend of the δ13C curve appears to be related to an atmospheric increase in the pCO2, but the causes remains unclear.Unlike the oceanic domain, few datas exist for the characterization of the MECO in coastal areas. Additionally, important component of the climatic context, such as the seasonal gradient of temperature, remain unknown. To unravel these uncertainties, this work focuses on the nearshore Eocene sedimentary records of the Paris Basin, which presents an important and remarkably well preserved paleobiodiversity of marine mollusk shells. Previous studies have confirmed that the MECO event is well recorded in Bartonian sediments, but due to several uncertainties, its stratigraphic position remains to be specified. Here we present a composite section that spans a stratigraphic interval covering the middle Lutetian (falunière de Grignon outcrop) and the Bartonian (Horizon de Mont-Saint Martin Formation, le Guépelle section and the Sables de Cresnes Formation). Thus, we combine different proxies provided by 18O, 13C and ∆47 analyses of marine mollusk shells sampled in these sections in order to clarify the stratigraphic position of the MECO in the sedimentary succession of the Paris Basin and to constrain the climatic expression of this hyperthermal event in shallow marine environment. Isotopic analyses were performed on the shells of 3 Bartonian mollusks species: 2 bivalves represented by Bicorbula gallica and Crassostrea cucullaris and 1 gastropod represented by Torquesia sulcifera. Stable isotope (δ18O and δ13C) results both show a characteristic negative excursion at the end of the Sables du Guépelle formation, in the lower part of the Bartonian. Clumped isotope analyses were performed on some specimens of B. gallica and T. sulcifera in order to better constrain the composition of δ18Ow throughout the stratigraphic interval studied. These results indicate significant decreases in local δ18Ow over the lifetime of most individuals, interpreted as large infra-annual variations in salinity. Paleotemperatures calculated from the previously constrained δ18Ocarbonate increase by 4 to 10 °C during the MECO event , while the seasonal temperature variation decreases from 11-13 °C to 8 °C during the negative isotopic excursion of the end of the Sables du Guépelle formation.Based on these new results, we propose that the MECO is recorded in the top of the Sables du Guépelle formation marked by a warming period and a lower seasonal temperature gradient. These results lead to a better chimio-chronostratigraphic calibration of the Bartonian deposits of the Paris Basin

Corrélations stratigraphiques des dépôts Bartonien du Bassin de Parisgrâce à l’outil géochimique (d18O - d13C)

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