The South America VLF NETwork (SAVNET)     

Jean-Pierre Raulin  Pedro Correia de Matos David  Rubens Hadano  Antonio C. V. Saraiva  Emilia Correia  Pierre Kaufmann 《Earth, Moon, and Planets》2009,104(1-4):247-261

In this paper we present the South America VLF NETwork (SAVNET), a new observing facility at very low frequencies. It has been recently installed at different locations spread over Latin America, in Brazil, Peru and Argentina. It consists of a network of seven Very Low Frequency (VLF) receivers with the main scientific objective of monitoring the solar activity on short (minutes to hours) and long (years) time scales. Other objectives include a better understanding of the spatial structure of the South Atlantic Magnetic Anomaly, the study of atmospheric phenomena and the search for genuine seismic-electromagnetic effects. After discussing the scientific goals, the details of the installation are presented as well as the first results recently obtained.  相似文献   

Conflicting structural and geochronological data from the Ibituruna quartz-syenite (SE Brazil): Effect of protracted “hot” orogeny and slow cooling rate?     

Sylvain Petitgirard  Alain Vauchez  Marcos Egydio-Silva  Olivier Bruguier  Pierre Camps  Patrick Moni  Marly Babinski  Mathieu Mondou 《Tectonophysics》2009,477(3-4):174

The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Araçuaí Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (~ 750 °C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530–535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at ~ 570–580 Ma, i.e., an HT deformation > 35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from ⁴⁰Ar–³⁹Ar thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, ⁴⁰Ar–³⁹Ar mineral ages suggest a very low cooling rate: < 3 °C/My between 570 and ~ 500 Ma and ~ 5 °C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling “hot orogens” that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric.  相似文献   

Evidence for a warmer period during the 12th and 13th centuries AD from chironomid assemblages in Southampton Island, Nunavut, Canada     

Nicolas Rolland  Isabelle Larocque  Pierre Francus  Reinhard Pienitz 《Quaternary Research》2009,72(1):27-37

This study presents the Late-Holocene evolution of a northern Southampton Island (Nunavut, Canada) lake, using fossil chironomids supported by sedimentological evidences (XRF, grain size and CNS). All proxies revealed a relatively stable environment during the last millennium with short-lived events driving changes in the entire lake ecosystem. The chironomid-based paleotemperatures revealed variations of significant amplitude coincident with changes in the sediment density and chemical composition of the core. Higher temperature intervals were generally correlated to lower sediment density with higher chironomid concentration and diversity. Higher temperatures were recorded from cal yr AD 1160 to AD 1360, which may correspond to the Medieval Warm Period. Between cal yr AD 1360 and AD 1700, lower temperatures were probably related to a Little Ice Age event. This study presents new information on the timing of known climatic events which will refine our knowledge of the paleoclimate and climatic models of the Foxe Basin region. It also provides a new framework for the evolution of such freshwater ecosystems under the “Anthropocene” and underlines the importance of including sedimentological proxies when interpreting chironomid remains as this combined approach provides an extended overview of the past hydrological and geochemical changes and their impacts on lake biota.  相似文献   

Decoupling between the middle and upper crust during transpression-related lateral flow: Variscan evolution of the Aston gneiss dome (Pyrenees, France)     

Yoann Denle  Philippe Olivier  Grard Gleizes  Pierre Barbey 《Tectonophysics》2009,477(3-4):244

We present a structural, AMS, microstructural and kinematic study of the Aston gneiss dome (French Pyrenees), which consists of a core made up of orthogneiss and paragneiss intruded by numerous sills of Carboniferous peraluminous granite. The orthogneiss corresponds to a former Ordovician granitic laccolith. Four Variscan events have been evidenced in this gneiss dome: (i) D1 deformation observed only as relics in the orthogneisses and their country-rocks located above the sillimanite isograd, and characterized by a NS to NE–SW non coaxial stretch associated to top to the south motions (NS convergence); (ii) D2-a deformation observed in the orthogneisses and their country-rocks, mainly migmatitic paragneisses, located below the sillimanite isograd and in the peraluminous granites whatever their structural level, and characterized by an EW to N120°E stretch associated to a top to the east flat shearing (lateral flow in the hot middle crust in a transpressive regime); (iii) D2-b deformation characterized by EW-trending megafolds corresponding to the domes in the middle crust and by EW-trending tight folds with subvertical axial planes in the metasedimentary upper crust; (iv) subvertical medium-temperature mylonitic bands developed by the end of the transpression.The Aston massif is a good example of decoupling between a cold upper crust and a hotter middle crust overheated by a thermal event originated in the upper mantle. This decoupling allowed the lateral flow of the migmatitic middle crust along a direction at high angle with respect to the more or less NS-trending direction of convergence. We suggest that the HT-LP metamorphism developed before the formation of the domes during D2-a, coevally with the emplacement of numerous sills of peraluminous granite, whereas the emplacement of the large calc-alkaline plutons in the upper crust occurred by the end of D2-b. Our data invalidate the previous geodynamical models based on either early or late extensional regime to explain the development of the HT-LP metamorphism. This new interpretation of the dynamics of the Variscan crust of the Pyrenees is consistent with recent studies conducted in Archaean and Palaeoproterozoic hot continental crusts having undergone oblique convergence, and characterized by a competition between vertical thickening and lateral flow induced by the important rheological contrast between two thermally different levels.  相似文献   

Identification of a sulfide derivative with a bicyclic hydrocarbon skeleton related to squalene. Part II: synthesis of a S-spiro-like dithiolane triterpenoid     

Pierre Adam  Philippe Schaeffer  Sylvestre Gug  Estelle Motsch  Pierre Albrecht 《Organic Geochemistry》2009,40(8):885-894

A sulfide with a bicyclic squalene hydrocarbon skeleton present in sediments from various depositional settings and of different geological age has been identified by synthesis of a reference compound. In contrast to previous expectations [cf. Schouten, S., Sinninghe Damsté, J.S., de Leeuw, J.W., 1995. A novel triterpenoid carbon skeleton in immature sulphur-rich sediments. Geochimica et Cosmochimica Acta 59, 953–958; Gug, S., Schaeffer, P., Adam, P., Klein, S., Motsch, E., Albrecht, P., 2009. Identification of a sulfide derivative with a bicyclic hydrocarbon skeleton related to squalene. Part I: Synthesis of a dithiane triterpenoid. Organic Geochemistry 40, 876–884.], this sulfide bears two “S-spiro type“ thiolane moities, but not the two thiochromane moieties originally envisaged. The synthesis resulted in the formation of a complex isomeric mixture of compounds, all co-eluting in gas chromatography (GC) in the form of one broad peak, as is the case with the geochemical compound. The spiro-type sulfide was formed by intramolecular sulfurization of a polyunsaturated bicyclic squalene derivative of unknown biological origin. The presence of this sulfide in a number of Triassic to Miocene marine sediments from various and contrasting depositional settings (upwelling environments, shallow continental platforms, lagoonal sub-basins, hypersaline environments), all characterized by strongly anoxic conditions, but with or without photic zone anoxia, indicates that the microorganisms biosynthesizing the precursor bicyclic squalene occupy diverse ecological niches.  相似文献   

Rapid and cyclic aeolian deposition during the Last Glacial in European loess: a high-resolution record from Nussloch, Germany   总被引：1，自引：0，他引：1  

Pierre Antoine  Denis-Didier Rousseau  Olivier Moine  Stphane Kunesch  Christine Hatt  Andreas Lang  Hlne Tissoux  Ludwig Zller 《Quaternary Science Reviews》2009,28(25-26):2955-2973

This paper reports the results of an investigation of the Weichselian Upper Pleniglacial loess sequences of Nussloch (Rhine Valley, Germany) based on stratigraphy, palaeopedology, sedimentology, palynology, malacology and geochemistry (δ¹³C), supported by radiocarbon, TL and OSL dating. Grain-size and magnetic susceptibility records are taken at 5 cm intervals from the Upper Pleniglacial (UPG) loess. The data indicate cyclic variations in loess deposition between ca 34 and 17 ka, when the sedimentation rate is especially high (1.0–1.2 m per ka for more than 10 m). The grain-size index (GSI: ratio of coarse silt versus fine silt and clay) shows variations, which are assumed to be an indirect measurement of wind intensity. The sedimentation rate, interpreted from the profiles, indicates high values in loess (Loess events LE-1 to LE-7) and low or negligible values in tundra gley horizons G1 to G8. OSL ages from the loess and ¹⁴C dates from organic matter in the loess show that loess deposition was rapid but was interrupted by shorter periods of reduced aeolian sedimentation. Comparison between the data from Nussloch and other European sequences demonstrates a progressive coarsening of the loess deposits between ca 30 and 22 ka. This coarsening trend ends with a short but major grain-size decrease and is followed by an increase to a new maximum at 20 ± 2 ka (“W” shape). Correlation between the loess GSI and the Greenland ice-core dust records, suggests a global connection between North Atlantic and Western European global atmospheric circulation and wind regimes. In addition, the typical Upper Pleniglacial loess deposition begins at ca 30–31 ka, close to Heinrich event (HE) 3, and the main period of loess sedimentation at about 25 ± 2 ka is coeval to HE 2. Correlation of magnetic susceptibility and grain-size records shows that the periods, characterised by high GSI, coincide with an increase in the amount of ferromagnetic minerals reworked from the Rhine alluvial plain. They suggest enhancement in the frequency of the storms from N–NW. These results are integrated within a palaeogeographical model of dust transport and deposition in Western Europe for the Weichselian Upper Pleniglacial (or Late Pleniglacial).  相似文献   

Chondritic micrometeorites from the Transantarctic Mountains     

Matthias Van GINNEKEN  Luigi FOLCO  Carole CORDIER  Pierre ROCHETTE 《Meteoritics & planetary science》2012,47(2):228-247

Abstract– On the basis of morphological and petrographic characteristics, eight “giant” unmelted micrometeorites in the 300–1100 μm size range were selected from the Transantarctic Mountain micrometeorite collection, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of scanning electron microscopy, electron probe microanalyses, and synchrotron X‐ray diffraction allow their classification as chondritic micrometeorites. The large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions, therefore allowing a better definition of their parent material. A large variety of material is observed: five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite, one to CV chondrite, one to CM chondrite, and one to CI chondrite parent materials. Besides reporting the first occurrence of a CV‐like micrometeorite, our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin.  相似文献   

PULSE: a finite element code for solving adiabatic nonradial pulsation equations     

Pierre Brassard  Stéphane Charpinet 《Astrophysics and Space Science》2008,316(1-4):107-112

We briefly present the nonradial adiabatic pulsation code PULSE first developped for white dwarf asteroseismology and now used to compute adiabatic oscillation properties for various types of stellar objects. Numerical tests show that the code is able to provide the accuracy (for a given stellar model) required to deal with the precision in frequency expected from the COROT long runs. While the ultimate objective is to compare the output of various pulsation codes (see these proceedings), we already emphasize problems that need to be addressed concerning, in particular, the mesh resolution of the input stellar models and its impact on the accuracy at which frequencies can be computed.  相似文献   

Snow cover monitoring in the Northern Patagonia Icefield using MODIS satellite images (2000–2006)     

Paulina Lopez  Pascal Sirguey  Yves Arnaud  Bernard Pouyaud  Pierre Chevallier 《Global and Planetary Change》2008,61(3-4):103-116

The snow cover of the Northern Patagonia Icefield (NPI) was monitored after applying the Normalized Difference Snow Index (NDSI) and the Red/NIR band ratio to 134 Moderate Resolution Imaging Spectroradiometer (MODIS) images captured between 2000 and 2006. The final results show that the snow cover extent of the NPI fluctuates a lot in winter, in addition to its seasonal behaviour. The minimum snow cover extent of the period (3600 km²) was observed in March 2000 and the maximum (11,623 km²) in August 2001. We found that temperature accounts for approximately 76% of the variation of the snow cover extent over the entire icefield. We also show two different regimes of winter snow cover fluctuations corresponding to the eastern and the western sides of the icefield. The seasonality of the snow cover on the western side was determined by temperature rather than precipitation, while on the east side the seasonality of the snow cover was influenced by the seasonal behaviour of both temperature and precipitation. This difference can be explained by the two distinct climates: coastal and continental. The fluctuations in the winter snow cover extent were more pronounced and less controlled by temperature on the western side than on the eastern side of the icefield. Snow cover extent was correlated with temperature R² = 0.75 and R² = 0.74 for the western and eastern sides, respectively. Since limited meteorological data are available in this region, our investigation confirmed that the change in snow cover is an interesting climatic indicator over the NPI providing important insights in mass balance comprehension. Since snow and ice were distinguished snow cover fluctuations can be associated to fluctuations in the snow accumulation area of the NPI. In addition, days with minimum snow covers of summer season can be associated to the period in which Equilibrium Line Altitude (ELA) is the highest.  相似文献   

Interplay of S and As in Mekong Delta sediments during redox oscillations   总被引：1，自引：1，他引：0  

Van T.H.Phan  Fabrizio Bardelli  Pierre Le Pape  Raoul-Marie Couture  Alejandro Fernandez-Martinez  Delphine Tisserand  Rizlan Bernier-Latmani  Laurent Charlet 《地学前缘(英文版)》2019,10(5):1715-1729

The cumulative effects of periodic redox cycling on the mobility of As,Fe,and S from alluvial sediment to groundwater were investigated in bioreactor experiments.Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated:Matrix A(14 m deep)had a higher pyrite concentration than matrix B(7 m deep)sediments.Gypsum was present in matrix B but absent in matrix A.In the reactors,the sediment suspensions were supplemented with As(Ⅲ)and SO_4~(2-),and were subjected to three full-redox cycles entailing phases of nitrogen/CO_2,compressed air sparging,and cellobiose addition.Major differences in As concentration and speciation were observed upon redox cycling.Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles.Indeed,a high pyrite content associated with a low SO_4~(2-)content resulted in an increase in dissolved As concentrations,mainly in the form of As(Ⅲ),after anoxic half-cycles;whereas a decrease in As concentrations mainly in the form of As(Ⅴ),was instead observed after oxic half-cycles.In addition,oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation,increasing the As pool available for mobilization.The same processes seem to occur in sediment with the presence of gypsum,but,in this case,dissolved As were sequestered by biotic or abiotic redox reactions occurring in the Fe—S system,and by specific physico-chemical condition(e.g.pH).The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work,and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta(Vietnam).  相似文献