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Populus nigra L. establishment and fluvial landform construction: biogeomorphic dynamics within a channelized river 
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下载免费PDF全文 Dov Corenblit Johannes Steiger Gaspard Charrier José Darrozes Virginia Garófano‐Gómez Alexandre Garreau Eduardo González Angela M. Gurnell Borbála Hortobágyi Frédéric Julien Luc Lambs Sébastien Larrue Thierry Otto Erwan Roussel Franck Vautier Olivier Voldoire 《地球表面变化过程与地形》2016,41(9):1276-1292
Populations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants. We propose a conceptual model of biogeomorphic unit construction for channelized, lateral stable rivers. We consider the resultant biogeomorphic units as functional from an ecological point of view because P. nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity as a result of successful colonization, especially downstream of mature poplar stands. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Claudine Malfilatre Philippe Boulvais Marie-Pierre Dabard Sylvie Bourquin Erwan Hallot Didier Pallix Denis Gapais 《Comptes Rendus Geoscience》2012,344(1):14-24
The Comblanchien limestone is a well-known building stone that deserves a quantitative characterization in order to be able to certify its origin. We compare petrographical and geochemical characteristics of limestones commercialized under the name of Comblanchien to those of one of their competitors (VATC, a Portuguese coated grain-bearing limestone). From macroscopic and microscopic observations, the distinction is somehow difficult but the coated grain-bearing Comblanchien samples have δ13C values (from +1.75 to +2.25‰) different from those of the VATC sample (negative δ13C value). As an example of application of these results, three samples from a building site near Paris are also studied in order to verify their provenance. We conclude that they do not come from quarries from the Comblanchien area, but likely from Portugal. The study shows that a detailed analysis of building limestones may provide a robust tool for discriminating between currently mined stones of different provenances, and thus open the possibility to attach to a given limestone a fingerprint, an identity card, and if required a commercial label. 相似文献
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The current morphology of the martian lithospheric magnetic field results from magnetization and demagnetization processes, both of which shaped the planet. The largest martian impact craters, Hellas, Argyre, Isidis and Utopia, are not associated with intense magnetic fields at spacecraft altitude. This is usually interpreted as locally non- or de-magnetized areas, as large impactors may have reset the magnetization of the pre-impact material. We study the effects of impacts on the magnetic field. First, a careful analysis is performed to compute the impact demagnetization effects. We assume that the pre-impact lithosphere acquired its magnetization while cooling in the presence of a global, centered and mainly dipolar magnetic field, and that the subsequent demagnetization is restricted to the excavation area created by large craters, between 50- and 500-km diameter. Depth-to-diameter ratio of the transient craters is set to 0.1, consistent with observed telluric bodies. Associated magnetic field is computed between 100- and 500-km altitude. For a single-impact event, the maximum magnetic field anomaly associated with a crater located over the magnetic pole is maximum above the crater. A 200-km diameter crater presents a close-to-1-nT magnetic field anomaly at 400-km altitude, while a 100-km diameter crater has a similar signature at 200-km altitude. Second, we statistically study the 400-km altitude Mars Global Surveyor magnetic measurements modelled locally over the visible impact craters. This approach offers a local estimate of the confidence to which the magnetic field can be computed from real measurements. We conclude that currently craters down to a diameter of 200 km can be characterized. There is a slight anti-correlation of −0.23 between magnetic field intensity and impact crater diameters, although we show that this result may be fortuitous. A complete low-altitude magnetic field mapping is needed. New data will allow predicted weak anomalies above craters to be better characterized, and will bring new constraints on the timing of the martian dynamo and on Mars’ evolution. 相似文献
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Erwan Le Guerroué 《Comptes Rendus Geoscience》2010,342(3):204-214
Carbonate δ13C values provide a useful monitor of changes in the global carbon cycle because they can record the burial ratio of organic to carbonate carbon. The most pronounced isotope excursions in the geologic record occur during the Neoproterozoic and have assumed a central role in the interpretation of biogeochemical events preceding the Ediacaran and Cambrian radiations. The most profound negative carbon isotope excursion is best recorded in the Ediacaran-aged Shuram Formation of Oman and has potential equivalents worldwide including the Wonoka Formation of South Australia and other sections in China, India, Siberia, Canada, Scandinavia and Brazil. All these excursions are less well understood than those in the Phanerozoic because of their unusual magnitude, long duration (> 1 Ma) and the difficulty in correlating Neoproterozoic basins to confirm independently that they do indeed record global change in the mixed ocean reservoir. Alternatively, these δ13C anomalies could reflect diachronous diagenetic processes. Currently none of these excursion are firmly time constrained and critical to their interpretation is a coherent reproducibility and synchroneity at the global ocean scale. Here we use available strontium isotope record as an independent chronometer to test the timing and synchroneity of the Shuram δ13C and its potential equivalents. The use of the 86Sr/87Sr ratio allows the reconstruction of a coherent, global δ13C record calibrated independently against time. The calibrated δ13C curve indicates that the Shuram negative anomaly spans several tens of millions of years and reaches values below −10‰. This carbon isotopic anomaly therefore represents a meaningful oceanographic event that fundamentally challenges our understanding of the carbon cycle as defined in the Phanerozoic. 相似文献
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Franck Bourdelle Karim Benzerara Olivier Beyssac Julie Cosmidis Daniel R. Neuville Gordon E. Brown Jr. Erwan Paineau 《Contributions to Mineralogy and Petrology》2013,166(2):423-434
Estimation of Fe3+/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe3+/ΣFe ratio in silicates using Fe L2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe3+/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe3+ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L3-edge. This method allows mapping of Fe3+/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L2 maximum for Fe3+, which is compared to the total integral intensity of the Fe L2-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe3+/ΣFe ratios at the submicrometre scale in geological samples. 相似文献
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This research investigates the dynamics of the axial tidal flow and residual circulation at the lower Guadiana Estuary, south Portugal, a narrow mesotidal estuary with low freshwater inputs. Current data were collected near the deepest part of the channel for 21 months and across the channel during two (spring and neap) tidal cycles. Results indicate that at the deep channel, depth-averaged currents are stronger and longer during the ebb at spring and during the flood at neap, resulting in opposite water transport directions at a fortnightly time scale. The net water transport across the entire channel is up-estuary at spring and down-estuary at neap, i.e., opposite to the one at the deep channel. At spring tide, when the estuary is considered to be well mixed, the observed pattern of circulation (outflow in the deep channel, inflow over the shoals) results from the combination of the Stokes transport and compensating return flow, which varies laterally with the bathymetry. At neap tide (in particular for those of lowest amplitude each month), inflows at the deep channel are consistently associated with the development of gravitational circulation. Comparisons with previous studies suggest that the baroclinic pressure gradient (rather than internal tidal asymmetries) is the main driver of the residual water transport. Our observations also indicate that the flushing out of the water accumulated up-estuary (at spring) may also produce strong unidirectional barotropic outflow across the entire channel around neap tide. 相似文献
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The western Mediterranean late Oligocene–Miocene basins (Alboran, Valencia and Provençal basins) are a coherent system of interrelated troughs. In all basins normal faults and thermal subsidence migrated toward the east progressively moving to the Miocene-to-Pleistocene Algerian and Tyrrhenian basins. All those troughs appear elements of the back-arc opening related to the eastward roll-back of the W-directed Apennines–Maghrebides subduction zone, similarly to western Pacific back-arc settings.
These late Oligocene–early Miocene basins nucleated both within the Betic cordillera (e.g. Alboran sea) and in its foreland (Valencia and Provençal troughs). The N40–70° direction of grabens is oblique to the coexisting N60–80°-trending orogen and shows its structural independence from the orogenic roots. Thus, as the extension cross-cuts the orogen and developed also well outside the thrust belt front, the westernmost basins of the Mediterranean had to develop independently from the Alps-Betics orogen. Therefore, the Alboran extension, considered a classic example of a basin generated by the collapse of an orogen, cannot be ascribed to the detachment or annihilation of the lithospheric root. In contrast with the eastward migrating extensional basins, the Betic-Balearic thrust front was migrating westward producing interference or inversion structures. 相似文献
These late Oligocene–early Miocene basins nucleated both within the Betic cordillera (e.g. Alboran sea) and in its foreland (Valencia and Provençal troughs). The N40–70° direction of grabens is oblique to the coexisting N60–80°-trending orogen and shows its structural independence from the orogenic roots. Thus, as the extension cross-cuts the orogen and developed also well outside the thrust belt front, the westernmost basins of the Mediterranean had to develop independently from the Alps-Betics orogen. Therefore, the Alboran extension, considered a classic example of a basin generated by the collapse of an orogen, cannot be ascribed to the detachment or annihilation of the lithospheric root. In contrast with the eastward migrating extensional basins, the Betic-Balearic thrust front was migrating westward producing interference or inversion structures. 相似文献
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Neoproterozoic carbonates are known to show exceptional variations in their carbon isotopic ratios, and in the absence of biostratigraphy and a firm geochronological framework, these variations are used as a correlation tool. However, it is controversial whether the carbon isotope record reveals a primary oceanographic signal or secondary effects such as diagenesis. The Shuram Formation of the Nafun Group of Oman allows a stratigraphic test of this problem. The Nafun Group (Huqf Supergroup, Oman) in the Huqf area of east-central Oman consists of inner carbonate ramp facies of the Khufai Formation overlain by marine, storm-generated, red and brown siltstones of the Shuram Formation. Towards its top, the Shuram Formation is composed of distinctive shallowing-upward, 4–17-m-thick parasequences cropping out continuously over 35 km, which show recessive swaley cross-stratified siltstones capped by ledges comprising wave-rippled, intraclast-rich ooidal carbonate. These storm-dominated facies show a regional deepening in palaeobathymetry towards the south. The carbonates of the Shuram Formation are marked by an extreme depletion in 13 C in bulk rock. δ13 C values quickly reach a nadir of −12‰ just above the Khufai-Shuram boundary and steadily return to positive values in the overlying mainly dolomitic Buah Formation. The Shuram excursion is thought to be ca . 50 Myr in duration and extends over 600 m of stratigraphy. Carbon isotopic values show a systematic variation in the parasequence stack, with values varying both vertically through the stratigraphy (∼2‰ per 45 m) and laterally in the progradation distance (∼1‰ over 35 km). This supports a primary, oceanographic origin for these extremely negative carbon isotopic values and independently argues strongly against diagenetic resetting. 相似文献