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71.
Variability of atmospheric 14C content often complicates radiocarbon-based chronologies; however, specific features such as periods of constant 14C age or steep changes in radiocarbon ages can be useful stratigraphic markers. The Younger Dryas event in the Northern Hemisphere is one of those periods, showing conspicuous 14C wiggles. Although the origin of those variations is not fully understood, we can make practical use of them and determine: (i) whether the Younger Dryas was global in extent; if so, (ii) were the initial cooling and the final warming synchronous worldwide; and (iii) what are the implications of these similarities/differences? Here we report high-resolution AMS 14C chronologies from the mid-latitudes of South America that pinpoint a cool episode between 11,400 and 10,20014C yr B.P. The onset of the final cool episode of the Late Glacial in the southern mid-latitudes, i.e., the Huelmo/Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by ∼550 calendar years. Both events ended during a radiocarbon-age plateau at ∼10,20014C yr B.P. Thus, the Huelmo/Mascardi Cold Reversal encompasses the Younger Dryas, as well as a couple of short-term cool/warm oscillations that immediately preceded its onset in the North Atlantic region. 相似文献
72.
Anne Le Friant Georges Boudon Jean-Christophe Komorowski Christine Deplus 《Comptes Rendus Geoscience》2002,334(4):235-243
Results from recent fieldwork and the Aguadomar marine survey in the Lesser Antilles clearly indicate that the volcanic field of southern Dominica has experienced three major edifice collapse events. This led to formation of the most voluminous debris avalanches known in the Caribbean Arc. Submarine hummocky morphology with plurikilometric megablocks is characteristic of debris avalanche deposits. We propose that steep slopes on the western Caribbean side of the island and intense hydrothermal alteration lead to recurrent large-scale edifice collapses. Therefore islands in the Lesser Antilles face a non-negligible risk from generation of tsunamis associated with potential future edifice collapse. To cite this article: A. Le Friant et al., C. R. Geoscience 334 (2002) 235–243. 相似文献
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74.
The Lower Palaeozoic low-grade metamorphic rocks of the Brabant Massif are largely buried below a thick cover of post-Palaeozoic strata. Along the top of the subcrop, they comprise remnants of Cretaceous to Tertiary weathering profiles that represent the lower part of thick saprolites. The alteration of the chlorite- and muscovite-dominated Palaeozoic rocks was characterized by the destruction of chlorite, accompanied by the formation of kaolinite and iron oxides and/or iron hydroxides. The first product of chlorite weathering seems to have been regularly interstratified chlorite-vermiculite or chlorite-smectite, which is now represented by interstratified chlorite-muscovite with regular ordering. Outside the thin transitional zones in which this mineral occurs, the rubefied intervals show only little variation in composition, which is due to the replacement of chlorite by kaolinite over short vertical distances and the stability of muscovite throughout the preserved parts of the saprolite. The rubefied rocks do have a somewhat different composition along the top of some profiles, which is related to an interaction with groundwater after burial, resulting in smectite formation, feldspar weathering and iron dissolution. Groundwater interaction is also responsible for the occurrence of weathering without rubefaction, outside the areas with saprolite remnants, which resulted in vermiculite, smectite and kaolinite formation. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
75.
76.
Francis Sondag Jean Loup Guyot Jean Sbastien Moquet Alain Laraque Georges Adele Grard Cochonneau Jean Claude Doudou Christelle Lagane Philippe Vauchel 《水文研究》2010,24(11):1433-1445
This study presents the data collected within the framework of an Observatory of Research in Environment on the Amazonian Basin—the ORE HYBAM. It relates to the dissolved and solid loads of the two main rivers of French Guiana, the Maroni and Oyapock rivers, running on the Guiana Shield and draining respectively 64 230 and 24 630 km2. The low coefficient of variation of the average annual flows of the two rivers indicates an inter‐annual hydrological stability probably related to the immediate vicinity of the Atlantic Ocean. The sedimentary load is mainly composed of quartz and kaolinite. The total suspended solid (TSS) concentrations are among the world's lowest values; they range from 1 to 130 mg l−1 during the hydrological cycle, with averages of 22 and 12 mg l−1, for the Maroni and Oyapock rivers, respectively. The seasonal variability of these values is significantly higher than that of hydrologic flows, but without simple relationship with the discharge. Water chemical composition of the two rivers indicates a very weak mineralization, very similar to that found in the Amazonian rivers running on the Brazilian and Guianese shields, and in the Congo River and its tributaries in the Central African Shield. Seasonal variations are observed in both basins; they correspond to higher concentrations during low water stage (from October to February) and to more diluted water during the flood, from April to July. A signature enriched in Cl− is present at the Saut Maripa station on the Oyapock River indicating a more marked influence of the trade winds in this basin. The computation of atmospheric contributions to ions budget indicated a weak contribution for Ca2+ and Mg2+, which originates mainly from water‐rock interactions in both stations, while more than half of Na+ is derived from atmospheric inputs. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
77.
Linear sandbanks appear in the lee of coastal headlands where the hydrodynamics are dominated by strong tidal currents and the seabed is characterized by an abundance of sands. They may develop as symmetrical sandbanks on either sides of the headland or as an unique banner bank. The present study numerically investigates the combined effects of waves and tide on the initial development of headland-associated sandbanks. A morphological model based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) is applied to an idealized Gaussian shaped headland for waves conditions varying in heights and directions at the offshore boundary. The coupling considers the effects of the interactions between the wave and current bottom boundary layers, namely the enhanced levels of turbulence near the bottom and the increase of the total bottom shear stress. Waves substantially modify the initial development of sandbanks formed by suspension narrowing their width and reorienting them along the side of the headland. They weakly impact the morphogenesis of sandbanks by bedload favoring on a short-time scale the growth of symmetric circular-shaped features and a central depositional spit prolonging the headland tip. Waves of transverse directions toward the tip of the headland contribute to the initiation by suspension of a well-developed feature in the headland side of low energy limiting the seabed evolution in the exposed area. 相似文献
78.
Claudia Hohmann Georges Ona-Nguema Gordon E. Brown Jr. Andreas Kappler 《Geochimica et cosmochimica acta》2011,75(17):4699-4712
Sorption of contaminants such as arsenic (As) to natural Fe(III) (oxyhydr)oxides is very common and has been demonstrated to occur during abiotic and biotic Fe(II) oxidation. The molecular mechanism of adsorption- and co-precipitation of As has been studied extensively for synthetic Fe(III) (oxyhydr)oxide minerals but is less documented for biogenic ones. In the present study, we used Fe and As K-edge X-ray Absorption Near Edge Structure (XANES), extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Mössbauer spectroscopy, XRD, and TEM in order to investigate the interactions of As(V) and As(III) with biogenic Fe(III) (oxyhydr)oxide minerals formed by the nitrate-reducing Fe(II)-oxidizing bacterium Acidovorax sp. strain BoFeN1. The present results show the As immobilization potential of strain BoFeN1 as well as the influence of As(III) and As(V) on biogenic Fe(III) (oxyhydr)oxide formation. In the absence of As, and at low As loading (As:Fe ≤ 0.008 mol/mol), goethite (Gt) formed exclusively. In contrast, at higher As/Fe ratios (As:Fe = 0.020-0.067), a ferrihydrite (Fh) phase also formed, and its relative amount systematically increased with increasing As:Fe ratio, this effect being stronger for As(V) than for As(III). Therefore, we conclude that the presence of As influences the type of biogenic Fe(III) (oxyhydr)oxide minerals formed during microbial Fe(II) oxidation. Arsenic-K-edge EXAFS analysis of biogenic As-Fe-mineral co-precipitates indicates that both As(V) and As(III) form inner-sphere surface complexes at the surface of the biogenic Fe(III) (oxyhydr)oxides. Differences observed between As-surface complexes in BoFeN1-produced Fe(III) (oxyhydr)oxide samples and in abiotic model compounds suggest that associated organic exopolymers in our biogenic samples may compete with As oxoanions for sorption on Fe(III) (oxyhydr)oxides surfaces. In addition HRTEM-EDXS analysis suggests that As(V) preferentially binds to poorly crystalline phases, such as ferrihydrite, while As(III) did not show any preferential association regarding Fh or Gt. 相似文献
79.
Amélie Bordage Etienne Balan Johan P. R. de Villiers Robert Cromarty Amélie Juhin Claire Carvallo Georges Calas P. V. Sunder Raju Pieter Glatzel 《Physics and Chemistry of Minerals》2011,38(6):449-458
The oxidation state of vanadium in natural and synthetic Fe–Ti oxides is determined using high-energy resolution fluorescence-detected
X-ray absorption spectroscopy (HERFD-XAS). Eleven natural magnetite-bearing samples from a borehole of the Main Magnetite
Layer of the Bushveld Complex (South Africa), five synthetic Fe oxide samples, and three natural hematite-bearing samples
from Dharwar supergroup (India) are investigated. V K edge spectra were recorded on the ID26 beamline at the European Synchrotron Radiation Facility (Grenoble, France), and the
pre-edge features were used to determine the local environment and oxidation state of vanadium. In the case of the magnetite
samples (natural and synthetic), we show that vanadium is incorporated in the octahedral site of the spinel structure under
two oxidation states: +III and +IV. The variations of the pre-edge area are interpreted as various proportions in V3+ and V4+ (between 9.5 and 16.3% of V4+), V3+ being the main oxidation state. In particular, the variations of the V4+/V3+ ratio along the profile of the Main Magnetite Layer seem to follow the crystallization sequence of the layer. In the case
of the hematite samples from India, the pre-edge features indicate that vanadium is substituted to Fe and mainly incorporated
as V4+ (between 40 and 72% of V4+). We also demonstrate the potentiality of HERFD-XAS for mineralogical studies, since it can filter out the unwanted fluorescence
and give better resolved spectra than conventional XAS. 相似文献
80.
Karim Benzerara Jennyfer Miot Guillaume Morin Georges Ona-Nguema Feriel Skouri-Panet Céline Férard 《Comptes Rendus Geoscience》2011,343(2-3):160-167
Microorganisms can mediate the formation of minerals by a process called biomineralization. This process offers an efficient way to sequester inorganic pollutants within relatively stable solid phases. Here we review some of the main mechanisms involved in the mediation of mineral precipitation by microorganisms. This includes supersaturation caused by metabolic activity, the triggering of nucleation by production of more or less specific organic molecules, and the impact of mineral growth. While these processes have been widely studied in the laboratory, assessment of their importance in the environment is more difficult. We illustrate this difficulty using a case study on an As-contaminated acid mine drainage located in the South of France (Carnoulès, Gard). In particular, we explore the potential relationships that might exist between microbial diversity and mineral precipitation. The present review, far from being exhaustive, highlights some recent advances in the field of biomineralogy and provides non-specialists an introduction to some of the main approaches and some questions that remain unanswered. 相似文献