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TL;DR: In this article, surface heat-flow data from the Eastern Gulf of Aden indicate a thermal anomaly that has persisted after continental break-up, which may have been caused by small-scale convection that occurred during and after rifting.
Abstract: During the early stages in the formation of divergent margins, the lithosphere experiences large changes in temperature that can determine its strength and influence magma generation. Heat-flow data from the Eastern Gulf of Aden indicate a thermal anomaly that has persisted after continental break-up. This anomaly may have been caused by small-scale convection that occurred during and after rifting. During the early stages in the formation of divergent margins, the lithosphere experiences large changes in temperature that can determine its strength and influence magma generation1,2. This, in turn, may play a key role in continental rifting, break-up, and subsequent subsidence. Here we present surface heat-flow data from the Eastern Gulf of Aden, which is a recently formed divergent margin between Africa and Arabia3. In the deeper parts of the margin the heat flow is high and constant, but it decreases abruptly near the shelf-slope. Our numerical models, in conjunction with geophysical and geological constraints, suggest that the data are best explained by a thermal anomaly in the upper mantle that has persisted after continental break-up. We suggest that this anomaly is related to small-scale convection that occurred during and after rifting. Similar anomalies could have characterized other divergent margins: for example, the presence of shallow-water sediments deposited after the opening of the Atlantic Ocean4,5,6 hints at lower subsidence than would have occurred in the absence of persistent thermal anomalies.
63 citations
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TL;DR: In this article, a tomographic reconstruction of electron density perturbations in the acoustic frequency band is presented, with a timing consistent with an infrasonic wave generated by the path of seismic surface waves.
Abstract: SUMMARY
The coupling between the solid earth and its atmosphere is responsible for vertically propagating infrasonic waves generated by seismic surface waves. These pressure waves are amplified as they propagate upward, and produce perturbations of ionospheric electron density. The electron density perturbations above California, due to the seismic surface waves generated by the Denali earthquake on 2002 November 3, have been imaged from GPS data by a tomographic method. The integrated electron content along GPS ray paths presents a noise level that is lower in the acoustic wave frequency band than in the gravity wave frequency band. Therefore, the filtered GPS data from Californian networks are inverted for a tomographic reconstruction of electron density perturbations in the acoustic frequency band. The inversion is properly resolved only in a small number of areas due to the geometry of GPS ray paths. In these areas, a wave propagating upward at 1.2 ± 0.3 km s−1 and horizontally at 4 ± 1 km s−1 is observed, with a timing consistent with an infrasonic wave generated by the path of seismic surface waves. The discrepancies between the observed electron density perturbation structure and the expected infrasonic wave can be explained by the poor resolution of the inverse problem or by a simple model of interactions between the neutral wave and the plasma. Future development of dense GPS networks and the advent of the Galileo system will overcome the resolution problems, and allow us to relate ionospheric perturbations to the seismic signal. Such a relation can be used to constrain the source and propagation of seismic waves as well as upper atmosphere characteristics.
63 citations
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02 May 2000-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, Ramaman et al. showed that glass modifications under β-irradiation can be correlated with the migration of sodium outside of the Si and B glass network formers.
Abstract: Raman, 11B Nuclear Magnetic Resonance (NMR) and X-ray Photoelectron (XPS) spectroscopy investigations have been made on a series of β-irradiated aluminoborosilicate glasses. This work shows the following changes of the glass structure under β-irradiation: (i) an increase of glass polymerization, (ii) an increase of boron in a trigonal environment and (iii) no modification in the aluminum environment during irradiation. These glass modifications under β-irradiation can be correlated with the migration of sodium outside of the Si and B glass network formers. Moreover, XPS experiments show a strong decrease of the sodium concentration at the surface of these irradiated glasses. This study presents therefore strong evidences of migration and segregation processes of sodium during a β-irradiation in the bulk for these simplified aluminoborosilicate glass compositions.
63 citations
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TL;DR: In this article, the authors describe the organic speciation of Fe in the Kerguelen Archipelago and identify the mechanisms of natural iron fertilization in the Southern Ocean.
Abstract: During the Ke rguelen O cean and P lateau compared S tudy (KEOPS; January–February 2005) cruise, the area southeast of the Kerguelen Archipelago in the Indian sector of the Southern Ocean was investigated to identify the mechanisms of natural iron fertilization of the Kerguelen Plateau. In this study, the organic speciation of Fe is described. Samples were determined immediately on board using competing ligand-adsorptive cathodic stripping voltammetry (CL-AdCSV). The dissolved organic ligands were always in excess of the dissolved Fe concentration, increasing the residence time in the water column and the potential availability for phytoplankton. The concentration of the dissolved organic ligands ranged from 0.44 to 1.61 n Eq of M Fe (=complexation site for Fe), with an average concentration of 0.91 n Eq of M Fe (S.D.=0.28, n =113) and a mean logarithm of conditional stability constant (log K′) of 21.7 (S.D.=0.28, n =113). A second weaker dissolved organic ligand group was detected in 32% of the samples, with Fe-binding characteristics at the edge of the detection window of the applied method. The occurrence of the highest concentrations of dissolved organic ligands in the wind-mixed surface layer and near the sediment at the bottom of the water column indicated that both phytoplankton and the sediment act as sources. Both sources are in concert with the general conclusions from the KEOPS research on the sources of Fe, where Fe was regenerated, organic Fe-binding ligands were formed in the upper layers, and both Fe and ligands were supplied by the sediment.
62 citations
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TL;DR: In the early Cretaceous, successive tectonic phases and several sea level falls resulted in the emersion of the main part of western Europe and the development of thick "lateritic" weathering as discussed by the authors.
Abstract: During the early Cretaceous, successive tectonic phases and several sea level falls resulted in the emersion of the main part of western Europe and the development of thick "lateritic" weathering. This long period of continental evolution ended with the Upper Cretaceous transgressions. During this period, the exposed lands displayed a mosaic of diverse morphologies and weathered landscapes.
Bauxites are the most spectacular paleoweathering features, known for long in southern France. Recently, new residual outcrops have been identified, trapped in the karstic depressions of the Grands Causses. Other bauxitic formations, containing gibbsite, have also been recognised, occurring with the Clay-with-Jurassic-cherts in the southeastern border of the Paris Basin. These bauxitic formations overlay Jurassic limestone and are buried beneath Upper Cretaceous marine deposits. The recognition of bauxites up north into the southern Paris Basin significantly widens the extension of the Lower Cretaceous bauxitic paleolandscapes.
On the Hercynian basements thick kaolinitic weathering mantles occur. They have been classically ascribed to the Tertiary. The first datings of these in situ paleosoils, by means of paleomagnetism and/or radiogenic isotopes, record especially early Cretaceous ages. This is the case for the "Siderolithic" formations on the edges of the French Massif Central, but also for the kaolinitic profiles in the Belgian Ardennes. In the Flanders, the Brabant basement is deeply kaolinised beneath the Upper Cretaceous cover. These paleosoils show polygenetic evolutions. The relief of these basement paleolandscapes may have been significant. There where probably high scarps (often of tectonic origin) reaching 200 m in elevation or beyond, as well as wide surfaces with inselbergs, as in the present day landscapes of tropical Africa and South America.
On the Jurassic limestone platforms occur diverse kaolinitic and ferruginous weathering products. Around the Paris Basin they show various facies, ranging from kaolinitic saprolites to ferricretes. Due to the lack of sedimentary cover, the age of these ferruginous and kaolinitic weathering products has been debated for long, most often allocated to the Siderolithic sensu lato (Eocene-Oligocene). Recent datings by paleomagnetism have enabled to date them (Borne de Fer in eastern Paris Basin) back also to the early Cretaceous (130 ± 10 Ma). These wide limestone plateaus show karstified paleolandforms, such as vast closed and flat depressions broken by conical buttes, but also deep sinkholes in the higher areas of the plateaus and piedmonts. The depth of the karst hollows may be indicative of the range of relative paleoelevations. Dissolution holes display seldom contemporaneous karst fillings, thus implying that the karstland had not a thick weathering cover or that this cover had been stripped off before or by the late Cretaceous transgression. Nevertheless, some areas, especially above chert-bearing Jurassic limestone or marl, show weathering products trapped in the karst features or as a thick weathering mantle.
In the Paris Basin, the Wealden gutter looked like a wide floodplain in which fluvio-deltaic sands and clays were deposited and on which paleosoils developed during times of non-deposition. The edges of the gutter were shaped as piedmonts linked up with the upstream basement areas. The rivers flowing down to the plain deposited lobes of coarse fluvial sands and conglomerates. The intensity of the weathering, the thickness of the profiles and their maturation are directly dependent on the duration of the emersion and the topographic location relative to the gutter. Near the axis of the gutter, where emersion was of limited duration, the paleoweathering features are restricted to rubefaction and argillization of the Lower Cretaceous marine formations. On the other hand, on the borders of the basin and on the Hercynian basement, where emersion was of longer duration, the weathering profiles are thicker and more intensively developed.
The inventory of the Lower Cretaceous paleoweathering features shows the complexity of the continental history of this period. Moreover, the preserved weathering products are only a part of this long lasting period, all the aspects relative to erosion phases are still more difficult to prove and to quantify. In this domain, apatite fission tracks thermochronology (AFTT) can be helpful to estimate the order of magnitude of denudation. Residual testimonies and subsequent transgressions may enable to estimate relative elevations, but in return, we presently have no reliable tool to estimate absolute paleoelevations. In the work presented here, the inventory enabled to draw a continental paleogeographic map showing the nature of the weathering mantles and the paleolandscape features, just as paleoenvironments and paleobathymetry presently appear on marine paleogeographic maps. For the future, the challenge is to make progress in dating the paleoweathering profiles and especially in the resolution of these datings, in order to correlate precisely the continental records with the different events which trigger them (eustatism, climate, regional and global geodynamics). The final goal will be to build up a stratigraphic scale of the "continental geodynamic and climatic events" in parallel with "sequential stratigraphy" in the marine realm.
62 citations
Authors
Showing all 903 results
Name | H-index | Papers | Citations |
---|---|---|---|
Claude J. Allègre | 106 | 327 | 35092 |
Paul Tapponnier | 99 | 294 | 42855 |
Francesco Mauri | 85 | 352 | 69332 |
Barbara Romanowicz | 67 | 284 | 14950 |
Geoffrey C. P. King | 64 | 157 | 17177 |
Yi-Gang Xu | 64 | 271 | 14292 |
Jérôme Gaillardet | 63 | 199 | 14878 |
François Guyot | 61 | 292 | 12444 |
Georges Calas | 60 | 266 | 10901 |
Ari P. Seitsonen | 59 | 212 | 45684 |
Michele Lazzeri | 58 | 140 | 57079 |
Bernard Bourdon | 58 | 118 | 9962 |
Gianreto Manatschal | 56 | 200 | 10063 |
Nikolai M. Shapiro | 56 | 154 | 15508 |
Guillaume Morin | 55 | 156 | 7218 |