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261.
Élisa Morichon Thierry Allard Daniel Beaufort David Quirt 《Physics and Chemistry of Minerals》2010,37(3):145-152
Natural radiation-induced defects were identified in specimens of sudoite (Al–Mg di-trioctahedral chlorite) related to unconformity-type
uranium deposits at the base of the Athabasca Group (Saskatchewan, Canada), using electron paramagnetic resonance (EPR) spectroscopy
at X- and Q-band frequencies. X-band spectra indicate the presence of a main native defect, named the As-center, whose EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g
// = 2,051 and g
⊥ = 2,005, and a secondary defect with apparent component g = 2,025. The study of oriented specimens shows that the main defect has its g
// component perpendicular to the (ab) plane of sudoite. The As-center corresponds to an electron hole located on oxygen atoms of the structure and is likely associated with Si, according
to the lack of hyperfine structure. The As-center in sudoite has EPR parameters similar to the A-center in kaolinite and dickite, and the Ai-center in illite. The saturation behavior of EPR spectra as a function of power demonstrates that native defects of sudoite
are different from those known in other clays, such as kaolinite, dickite or smectite, but are similar to those of illite.
The isochronal annealing data suggest that the main defect in sudoite is stable to more than 300°C. The corresponding defects
characterized in sudoite may have the potential for tracing past radionuclide migration around unconformity-type uranium deposits. 相似文献
262.
Volker Kahlenberg Daniela Girtler Erik Arroyabe Reinhard Kaindl Daniel M. Többens 《Mineralogy and Petrology》2010,100(1-2):1-9
Single crystals of devitrite (Na2Ca3Si6O16) were synthesized as pale-yellow transparent needle shaped crystals using a Na2MoO4-flux. Experiments aiming to prepare the K-equivalent of devitrite from the corresponding K2MoO4-flux were unsuccessful. The crystal structure of devitrite was determined from single-crystal X-ray diffraction data (Mo-Kα radiation, 2θmax.?=?25.34°, Rint?=?2.66%) and refined in space group P $ \bar{1} $ (no. 2) to R(|F|)?=?3.08% using 2,513 observed reflections with I?>?2σ(I). Unit-cell parameters are: a?=?7.2291(8), b?=?10.1728(12), c?=?10.6727(12) Å, α?=?95.669(9), β?=?109.792(10), γ?=?99.156(9)°, V?=?719.19(14) Å3, Z?=?2. The structure belongs to the group of multiple chain silicates consisting of dreier quadruple chains, i.e. the crystallochemical formula can be written as $ {\hbox{N}}{{\hbox{a}}_2}{\hbox{C}}{{\hbox{a}}_3}\left\{ {{\mathbf{uB}}{,4}_\infty^1} \right\}\left[ {^3{\hbox{S}}{{\hbox{i}}_6}} \right.\left. {{{\hbox{O}}_{16}}} \right\} $ . Linkage between the bands running along [100] is provided by double chains of edge sharing CaO6-octahedra as well as additional more irregularly coordinated Na- and Ca-cations located in the tunnel-like cavities of the mixed tetrahedral-octahedral framework. Structural investigations were completed by Raman and infrared spectroscopical studies. The allocation of the bands to certain vibrational species was aided by density functional theory (DFT) calculations. 相似文献
263.
Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote,eastern Canary Islands,Spain 总被引:1,自引:0,他引:1
Daniel R. Muhs James Budahn Gary Skipp Joseph M. Prospero DeAnna Patterson E. Arthur Bettis III 《地学学报》2010,22(6):399-410
Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island’s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace‐element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major‐element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region. 相似文献
264.
The southeast area of the Argentine Pampas is characterized by the presence of an unconfined aquifer in a wide plain. A methodology is proposed that deals with the aquifer vulnerability where the homogeneity of the hydrogeological variables used by traditional methods (in this case, DRASTIC-P) causes vulnerability maps to show more than 80% of the territory under the same class. This absence of discrimination renders vulnerability maps of little use to decision-makers. In addition, the proposed methodology avoids the traditional vague classification (high, low, and moderate vulnerability) which is highly dependent on subjectivity in its association of each class with hydrogeological considerations. That traditional vulnerability assessment methodology was adapted using a geographic information system to reclassify classes, based on the Natural Breaks (Jenks) method. The pixel-to-pixel comparison between the result obtained by the DRASTIC-P and the reclassified classes generates the so-called operational vulnerability index (OVI), which shows four classes, associating each with different hydrogeological requirements to make decisions. 相似文献
265.
266.
Medium- and Long-term Recovery of Estuarine and Coastal Ecosystems: Patterns,Rates and Restoration Effectiveness 总被引:1,自引:0,他引:1
Ángel Borja Daniel M. Dauer Michael Elliott Charles A. Simenstad 《Estuaries and Coasts》2010,33(6):1249-1260
Many estuarine and coastal marine ecosystems have increasingly experienced degradation caused by multiple stressors. Anthropogenic
pressures alter natural ecosystems and the ecosystems are not considered to have recovered unless secondary succession has
returned the ecosystem to the pre-existing condition or state. However, depending upon the scales of time, space and intensity
of anthropogenic disturbance, return along the historic trajectory of the ecosystem may: (1) follow natural restoration though
secondary succession; (2) be re-directed through ecological restoration, or (3) be unattainable. In order to address the gaps
in knowledge about restoration and recovery of estuarine and coastal ecosystems, this special feature includes the present
overview and other contributions to provide a synthesis of our knowledge about recovery patterns, rates and restoration effectiveness.
From the 51 examples collated in this contribution, we refine the recovery from the list of stressors into six recovery mechanisms:
(1) recovery from sediment modification, which includes all aspects of dredging and disposal; (2) recovery by complete removal
of stressors limiting natural ecosystem processes, which includes tidal marsh and inundation restoration; (3) recovery by
speed of organic degradation, which includes oil discharge, fish farm wastes, sewage disposal, and paper mill waste; (4) recovery
from persistent pollutants, which includes chemical discharges, such as TBT; (5) recovery from excessive biological removal,
related to fisheries and (6) recovery from hydrological and morphological modification. Drawing upon experience both from
these many examples and from an example of one comprehensive study, we show that although in some cases recovery can take
<5 years, especially for the short-lived and high-turnover biological components, full recovery of coastal marine and estuarine
ecosystems from over a century of degradation can take a minimum of 15–25 years for attainment of the original biotic composition
and diversity may lag far beyond that period. 相似文献
267.
The paper deals with the approximation and optimal interpolation of functions defined on the bisphere
\mathbb S2×\mathbb S2\mathbb {S}^{2}\times \mathbb {S}^{2} from scattered data. We demonstrate how the least square approximation to the function can be computed in a stable and efficient
manner. The analysis of this problem is based on Marcinkiewicz–Zygmund inequalities for scattered data which we present here
for the bisphere. The complementary problem of optimal interpolation is also solved by using well-localized kernels for our
setting. Finally, we discuss the application of the developed methods to problems of texture analysis in material science. 相似文献
268.
Daniel Mikeš 《Central European Journal of Geosciences》2010,2(3):420-432
Lack of age dates in the terrigenous Cenozoic sediments of the Duero and the Ebro sedimentary basins has complicated tecto-stratic correlation across the two basins. We tentatively synthesize a range of existing studies and new data to construct a rough general paleogeography throughout Upper Cenozoic times. The more extensive erosion of the Ebro has been previously attributed to the earlier moment of opening. We tentatively analyse lithostratic data to conclude that the lower knick-point and different lithologies have also contributed to the deeper erosion in the Ebro Basin. We conclude from lithostratic data and field evidence that the W half of the Rioja was part of the Duero in earlier times and that the escarpment retreated westward through the Rioja in four subsequent episodes of erosion. The tilt of the NW Duero is a consequence of isostatic rebound to this erosion. 相似文献
269.
Guangjie Chen Daniel T. Selbie Katherine Griffiths Jon N. Sweetman Morgan Botrel Zofia E. Taranu Sébastien Knops Jennifer Bondy Neal Michelutti John P. Smol Irene Gregory-Eaves 《Journal of Paleolimnology》2014,52(3):185-200
Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted. 相似文献
270.