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251.
Wetlands are valued for their high biodiversity and for their ecosystem services. However, we still do have a poor understanding of their role in the redox transformation of contaminants such as mercury, particularly in fluvial settings. Seasonal and spatial variations in dissolved gaseous mercury (DGM) distribution and production were studied in the Bay St. François, a wetland in the St. Pierre Lake, a fluvial lake of the St. Lawrence River, in Quebec, Canada. A high spatial resolution for DGM, with samples taken every 10-cm depth, was used in field measurements. Through a series of parallel field and incubation experiments, we assessed the main factors determining Hg(0) transformations as a function of depth, seasons, and presence/absence of macrophyte beds. Besides light penetration in the water column and water temperature, iron and dissolved organic carbon likely stimulated Hg(II) reduction. Inversely, chloride favored Hg(0) oxidation. Macrophytes and associated epiphytes appeared to be important sites of adsorption/absorption of Hg(II) and likely of DGM. It seems however that the effects of macrophytes were restricted to immediately adjacent waters. Near the bottom, under anoxic conditions, the reduction of Hg(II) was highly promoted. In addition, sediments and decomposing macrophytes seemed to release DGM and/or reducible Hg to bottom waters. Overall, differences in DGM between surface and bottom waters tended to be more accentuated than observed differences in DGM between macrophyte beds and sites devoid of plant. 相似文献
252.
Gweltaz Maho Xavier Fayoux Stphane Guillot Eduardo Garzanti Paul Capiez Georges Mascle 《Journal of Asian Earth Sciences》2006,26(6):695-707
In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time. 相似文献
253.
Xavier Emery 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(1-2):53-65
In the geostatistical analysis of regionalized data, the practitioner may not be interested in mapping the unsampled values
of the variable that has been monitored, but in assessing the risk that these values exceed or fall short of a regulatory
threshold. This kind of concern is part of the more general problem of estimating a transfer function of the variable under
study. In this paper, we focus on the multigaussian model, for which the regionalized variable can be represented (up to a
nonlinear transformation) by a Gaussian random field. Two cases are analyzed, depending on whether the mean of this Gaussian
field is considered known or not, which lead to the simple and ordinary multigaussian kriging estimators respectively. Although
both of these estimators are theoretically unbiased, the latter may be preferred to the former for practical applications
since it is robust to a misspecification of the mean value over the domain of interest and also to local fluctuations around
this mean value. An advantage of multigaussian kriging over other nonlinear geostatistical methods such as indicator and disjunctive
kriging is that it makes use of the multivariate distribution of the available data and does not produce order relation violations.
The use of expansions into Hermite polynomials provides three additional results: first, an expression of the multigaussian
kriging estimators in terms of series that can be calculated without numerical integration; second, an expression of the associated
estimation variances; third, the derivation of a disjunctive-type estimator that minimizes the variance of the error when
the mean is unknown. 相似文献
254.
Xavier Emery 《Mathematical Geology》2006,38(7):801-819
Multigaussian kriging aims at estimating the local distributions of regionalized variables and functions of these variables
(transfer or recovery functions) at unsampled locations. In this paper, we focus on the evaluation of the recoverable reserves
in an ore deposit accounting for a change of support and information effect caused by ore/waste misclassifications. Two approaches
are proposed: the multigaussian model with Monte Carlo integration and the discrete Gaussian model. The latter is simpler
to use but requires stronger hypotheses than the former. In each model, ordinary multigaussian kriging gives unbiased estimates
of the recoverable reserves that do not utilize the mean value of the normal score data.
The concepts are illustrated through a case study on a copper deposit which shows that local estimates of the metal content
based on ordinary multigaussian kriging are close to the optimal conditional expectation when the data are abundant and are
not dominated by the global mean when the data are scarce. The two proposed approaches (Monte Carlo integration and discrete
Gaussian model) lead to similar results when compared to two other geostatistical methods: service variables and ordinary
indicator kriging, which show strong deviations from conditional expectation. 相似文献
255.
256.
257.
Xavier Emery 《Mathematical Geology》2007,39(2):205-223
This work deals with a family of geostatistical models used in application fields concerned with a change of support, such
as mineral resources evaluation and polluted soil management. Three models are examined: the discrete Gaussian, Hermitian
and Laguerrian models, which rely on a transformation of the variable of interest (mineral grade or pollutant concentration)
defined on point and block supports into variables with Gaussian or gamma univariate distributions and isofactorial bivariate
distributions. The focus is given to the relationships between the transformed variables at both supports, and to the conditions
that these relationships imply on the model parameters. Additionally, guidelines are given for improving the variogram analysis
of the transformed variables and for validating the change-of-support model. 相似文献
258.
259.
Gian-Maria Zuppi Jean-Paul Novel Martial Dray Xavier Darmendrail Serge Fudral Claude Jusserand Gérard Nicoud 《Comptes Rendus Geoscience》2004,336(15):1371-1378
The presence in the French–Italian Alps of several springs characterized by a high sodium–chloride content, depleted in oxygen-18, tritium free and with low carbon-14 activity, is well known. The geological, hydrological, and geochemical studies in progress for the new high-speed railway tunnel crossing the French–Italian Alps, show, in the drilled boreholes at a mean elevation of 700 masl, the presence of brines. Brines appear to be the basis of the geological, structural, and hydrodynamic systems, and fill the largest faults and the main tectonic contacts. The brines squeezed by tectonic activity, are mobilised at depth by freshwaters from the surface. To cite this article: G.-M. Zuppi et al., C. R. Geoscience 336 (2004). 相似文献
260.
Xavier Guillot Muzahim Al-Mukhtar Faiza Bergaya Jean-Marie Fleureau 《Comptes Rendus Geoscience》2002,334(2):105-109
Porosity measurements on a clay material, carried out through various microscopic techniques, show a significant difference with the total porosity macroscopically estimated. Analyses have allowed to assign this difference, whose importance is linked to the hydromechanical stresses sustained by the material, to microporous spaces lower than 2 nm and to closed pore volumes, not accessible by the used techniques. Besides, the role of the drying methods of samples, which do not allow the removal of a part of the water, is highlighted. Its elimination would lead to irreversible microtextural modifications, prejudicial to the understanding of the macroscopic behaviour of the material. To cite this article: X. Guillot et al., C. R. Geoscience 334 (2002) 105–109. 相似文献