96 research outputs found
New indirect evidence of Permian bi-modal volcanism from sediment petrology in the Orobic Basin (Central Southern Alps, Italy)
The Lower Permian Cabianca Volcanite (lower Collio Fm. Auct.), representing the filling of a large caldera, is dominantly acidic, as most coeval volcanics in the Southern Alps: intermediate rocks are rare and nearly irrelevant by volume (only two small andesite breccia pipes outcrop on the flanks of Mt. Cabianca). Subsurface data document the occurrence of an andesite body in the Novazza mine and a diatreme 2 km to the east (crossed by a prospection well), on the left Serio River valley. Out of the moat, Lower Permian pyroclastics outcrop along tens of kilometres, but intermediate rocks are unreported, except for the Val Pradini \u201cporfirite amigdalare\u201d in the west (Ornica). Permian volcanics and Variscan metamorphics are covered by Lower Permian sediments deposited in a transtensional basin, bound by steep faults and fringed by alluvial fans. The alluvial fan conglomerate of Val Sanguigno, north of Novazza, is dominated by andesitic clasts (cobbles-boulders), which reasonably derived from lavas that were outcropping beyond the southern boundary fault of the basin.
Investigations of the petrographic composition of the alluvial fan conglomerates carried out east and west of Mt. Cabianca, allowed comparing the composition of the volcanic rocks preserved in the depocentre (Cabianca Caldera) with the volcanic rocks cropping out outside of it, recorded in the clasts of the alluvial fans at the borders of the Orobic Basin. In the eastern (Lake Barbellino) and central (Val Sanguigno) parts of the basin, conglomerates at the southern border of the basin contain abundant intermediate pebbles, and an increasing proportion of basement clasts to the west. Conglomerates from the northern margin of the basin (Ponteranica Conglomerate, Pizzo dei Tre Signori massif) mainly contain pebbles and cobbles of metamorphic basement, acidic welded tuffs and, subordinately, of grey-green volcanics. In both cases, microscope analysis of associated litharenites revealed significant quantities of intermediate to basic volcanic lithic fragments. Visual estimates suggest that basic to intermediate clasts represent 15 to 20% of the bulk volume.
Petrographic composition of Lower Permian conglomerates confirms that Permian streams eroded both volcanic rocks and the Variscan basement, but documents that in the source area of the siliciclastics (i.e. highs bordering the basin) intermediate-basic volcanics were more abundant than in the Cabianca Volcanite. The relatively scarcer acidic pebbles in the conglomerate can be partly ascribed to the fact that they were still incoherent and reluctant to produce pebbles transportable over significant distance. Nevertheless, the high amount of basic-intermediate rocks documents that outside of the caldera these rocks were volumetrically abundant. This fact may be explained suggesting the existence of fractures and faults able to drive and trap basic-intermediate lavas documenting a clear bi-modal distribution of extrusives at the surface
Lithostratigraphy and facies architecture of a Lower Permian continental succession in Central Southern Alps (Orobic Basin, Italy)
The Lower Permian Pizzo del Diavolo Fm. of the Orobic Alps (Laghi Gemelli Gr., upper Collio Fm. Auct.) records, along an E-W area more than 50 km long, events that occurred in a complex continental depositional system, during semi-arid climatic condiitons and in a transtensional tectonic setting. This formation postdates intense volcanic activity and overlays parts of a large caldera and its vast surrounding areas, covered by dominating pyroclastic deposits interbedded with rare braid-plain and lacustrine sediments (Cabianca Volcanite). Post-volcanic sediment distribution reflects the existence of connected half-grabens, characterised by transverse sedimentary input (coarse-grained alluvial fans) evolving into fine-grained heterolitic deposits in the depocentre, hosting ephemeral playa-lakes.
Field mapping of two marginal sectors of the Orobic Basin (Pizzo dei Tre Signori massif and Lake Barbellino area), coupled with facies analysis of the Pizzo del Diavolo Fm., led to the identification of significantly different sedimentary evolutions. At the eastern and western ends of the basin, the Pizzo del Diavolo Fm. consists of alluvial fan and floodplain facies associations that differ from the previously studied stratigraphic architecture described in the central part of the basin. The southern borders of the studied areas are characterised by coarse-grained, fining-upward alluvial fan deposits (Val Sanguigno Conglomerate) at their base. The petrographic composition of the conglomerates from the northern border of the basin (Ponteranica Conglomerate) indicates differences in the exposed and eroded rocks from the northern and southern highs, with changes along the basin borders, where conglomeratic units with dominating basement clasts (Mt. Aga Conglomerate) occur. Floodplain facies are similar all along the basin, even if with thickness changes (up to 700 metres in the western part).
The stratigraphic architecture observed in the eastern and western sectors markedly differs from that described in the central part of the basin (where a well-organised succession of two fining-upward cycles is described), preventing a detailed correlation of the events across the basin. Also the relative abundance of facies in these three sectors of the basin is different: fine-grained sediments dominate in the central part, whereas coarser deposits occur to the west (Pizzo dei Tre Signori massif) and to the east (Lake Barbellino). The complex architecture of the basin, the difficulty in recognising events or trends that can be traced all across the basin and the different petrographic composition of conglomerates along the northern and southern margins of the basin suggest not only that tectonics controlled facies distribution and depositional environments but also the existence of sub-basins characterised by different evolutions within the greater Orobic Basin, further supporting the envisaged role of strike-slip tectonics
Atomic scale investigation of silicon nanowires and nanoclusters
In this study, we have performed nanoscale characterization of Si-clusters and Si-nanowires with a laser-assisted tomographic atom probe. Intrinsic and p-type silicon nanowires (SiNWs) are elaborated by chemical vapor deposition method using gold as catalyst, silane as silicon precursor, and diborane as dopant reactant. The concentration and distribution of impurity (gold) and dopant (boron) in SiNW are investigated and discussed. Silicon nanoclusters are produced by thermal annealing of silicon-rich silicon oxide and silica multilayers. In this process, atom probe tomography (APT) provides accurate information on the silicon nanoparticles and the chemistry of the nanolayers
Lack of Phylogeographic Structure in the Freshwater Cyanobacterium Microcystis aeruginosa Suggests Global Dispersal
Background : Free-living microorganisms have long been assumed to have ubiquitous distributions with little biogeographic signature because they typically exhibit high dispersal potential and large population sizes. However, molecular data provide contrasting results and it is far from clear to what extent dispersal limitation determines geographic structuring of microbial populations. We aimed to determine biogeographical patterns of the bloom-forming freshwater cyanobacterium Microcystis aeruginosa. Being widely distributed on a global scale but patchily on a regional scale, this prokaryote is an ideal model organism to study microbial dispersal and biogeography.
Methodology/Principal Findings : The phylogeography of M. aeruginosa was studied based on a dataset of 311 rDNA internal transcribed spacer (ITS) sequences sampled from six continents. Richness of ITS sequences was high (239 ITS types were detected). Genetic divergence among ITS types averaged 4% (maximum pairwise divergence was 13%). Preliminary analyses revealed nearly completely unresolved phylogenetic relationships and a lack of genetic structure among all sequences due to extensive homoplasy at multiple hypervariable sites. After correcting for this, still no clear phylogeographic structure was detected, and no pattern of isolation by distance was found on a global scale. Concomitantly, genetic differentiation among continents was marginal, whereas variation within continents was high and was mostly shared with all other continents. Similarly, no genetic structure across climate zones was detected.
Conclusions/Significance : The high overall diversity and wide global distribution of common ITS types in combination with the lack of phylogeographic structure suggest that intercontinental dispersal of M. aeruginosa ITS types is not rare, and that this species might have a truly cosmopolitan distribution
Biodiversity inventories in high gear: DNA barcoding facilitates a rapid biotic survey of a temperate nature reserve
Comprehensive biotic surveys, or âall taxon biodiversity inventoriesâ (ATBI), have traditionally been limited in scale or scope due to the complications surrounding specimen sorting and species identification. To circumvent these issues, several ATBI projects have successfully integrated DNA barcoding into their identification procedures and witnessed acceleration in their surveys and subsequent increase in project scope and scale. The Biodiversity Institute of Ontario partnered with the rare Charitable Research Reserve and delegates of the 6th International Barcode of Life Conference to complete its own rapid, barcode-assisted ATBI of an established land trust in Cambridge, Ontario, Canada
The spatial resolution of 3D atom probe in the investigation of single-phase materials
International audienceThe resolution of three-dimensional atom probe (3DAP) is known to be mainly controlled by the aberrations of the ion trajectories near the surface of the specimen. A model has been developed to compute the ion trajectories in 3D near a sharp hemispherical electrode defined at the atomic scale. Simulations were applied on one-phase binary alloys. The influence of the evaporation fields of chemical species is studied. Simulated desorption images are consistent with experiments in both ordered alloys and random solid solution. An extra loss in the lateral resolution is observed in disordered alloys as compared to pure metals. The predicted order of evaporation provided by this model is in excellent agreement with experiments. The stacking sequence of atomic planes reconstructed from simulated data is shown to be disturbed in a similar way as observed in real experiments with 3DAP. Copyright (C) 2000 Elsevier Science B.V
The spatial resolution of 3D atom probe in the investigation of single-phase materials
International audienceThe resolution of three-dimensional atom probe (3DAP) is known to be mainly controlled by the aberrations of the ion trajectories near the surface of the specimen. A model has been developed to compute the ion trajectories in 3D near a sharp hemispherical electrode defined at the atomic scale. Simulations were applied on one-phase binary alloys. The influence of the evaporation fields of chemical species is studied. Simulated desorption images are consistent with experiments in both ordered alloys and random solid solution. An extra loss in the lateral resolution is observed in disordered alloys as compared to pure metals. The predicted order of evaporation provided by this model is in excellent agreement with experiments. The stacking sequence of atomic planes reconstructed from simulated data is shown to be disturbed in a similar way as observed in real experiments with 3DAP. Copyright (C) 2000 Elsevier Science B.V
Apports de la tomographie atomique dans l'observation et l'analyse des interfaces
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Modeling artifacts in the analysis of test semiconductor structures in atom probe tomography
International audienceIn this paper, the investigation of boron delta layers by atom probe tomography is used to demonstrate that a sub nanometer resolution (0.9 nm full-width at half-maximum, FWHM) can be achieved. This resolution is surprisingly lower than the intrinsic resolution observed in silicon (0.2 nm). Reconstruction artifacts are suggested. In this paper, the extent of reconstruction artifacts is evaluated using a model that reproduces the field evaporation of the sample and the image reconstruction. It is shown that reconstruction artifacts can only account for half of the resolution degradation, suggesting an actual physical depth of delta doped B layer of about 0.5 nm
Modeling artifacts in the analysis of test semiconductor structures in atom probe tomography
International audienceIn this paper, the investigation of boron delta layers by atom probe tomography is used to demonstrate that a sub nanometer resolution (0.9 nm full-width at half-maximum, FWHM) can be achieved. This resolution is surprisingly lower than the intrinsic resolution observed in silicon (0.2 nm). Reconstruction artifacts are suggested. In this paper, the extent of reconstruction artifacts is evaluated using a model that reproduces the field evaporation of the sample and the image reconstruction. It is shown that reconstruction artifacts can only account for half of the resolution degradation, suggesting an actual physical depth of delta doped B layer of about 0.5 nm
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