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881.
Martin Novak Jitka Mikova Jan Kosler Eva Prechova Daniela Fottova 《Geochimica et cosmochimica acta》2010,74(15):4207-4218
Annual growth rings of a common hardwood species, Picea abies L., were investigated as a potential archive of past atmospheric Pb pollution. Wide distribution of trees in terrestrial settings and straightforward chronology are two advantages of this potential geochemical archive, but several processes described in the literature may obscure the trends in past Pb deposition. These confounding factors include, e.g., radial post-depositional mobility of Pb in xylem, and ecosystem acidification leading to higher bioavailability of Pb. One- to five-year annual wood increments were analyzed for Pb concentrations and 206Pb/207Pb ratios at Jezeri (JEZ), Uhlirska (UHL) and Na Lizu (LIZ), three sites in the Czech Republic, differing in atmospheric Pb loads. Three to four trees per site were included in the study. Distinct Pb concentration maxima between 1960 and 1990 at the two heavily polluted sites (JEZ and UHL) coincided with historical Pb emissions known from inventories of industrial production. No Pb concentration maxima were found at one site, LIZ, situated in a national park 150 km from major pollution sources. Spruce tree rings from JEZ, located just 5 km from coal-burning power stations, contained a large proportion of coal-derived Pb (a high-206Pb/207Pb ratio of 1.19). A coal-related maximum in 206Pb/207Pb in JEZ tree rings was found using two different analytical techniques, laser-ablation multi-collector ICP MS, and single-collector sector-field ICP MS. In a three-isotope graph (206Pb/207Pb vs. 208Pb/207Pb), tree-ring data plotted into the field of ombrotrophic (i.e., rain-fed) peat bogs, suggesting negligible contribution of bedrock-derived Pb in the xylem. We concluded that none of the potential confounding factors played a major role at our sites. Annual growth rings of P. abies in Central Europe faithfully recorded historical changes in atmospheric Pb depositions. 相似文献
882.
Laboratory testing of rectangular beams using a synthetic rock was used to investigate the onset of dilation and spalling.
The beams are axially compressed and subjected to 4-point bending to provide non-uniform compressive stresses. The maximum
tangential stress occurs at the top of the beam and rapidly decreases with distance from the top of the beam. This stress
distribution was used to simulate the maximum tangential stress distribution found around circular excavations. The results
showed using this beam test configuration that the onset of dilation based on beam displacement and visually observed spalling
began at approximately the same stress level. Discrete element numerical analyses (particle flow code) were used to evaluate
the stress path at various locations in the beams. The analyses revealed that spalling and dilation in the beams occurred
well below the peak strength failure envelope determined from conventional laboratory tests. The findings suggest that the
onset of dilation in laboratory tests appears to be a good indicator for assessing the stress magnitudes required to initiate
spalling. 相似文献
883.
Céline Martin Stéphanie Duchêne Béatrice Luais Philippe Goncalves Etienne Deloule Céline Fournier 《Contributions to Mineralogy and Petrology》2010,159(4):437-458
The geochemical and isotopic characterization of an eclogite and the associated retrogressive amphibolite at Vårdalsneset, WGR, Norwegian Caledonide was undertaken to investigate the mobility of REE and Hf and the behavior of Lu–Hf and Sm–Nd geochronometers during metamorphic dehydration/rehydration. Eclogitic garnets display a distinct core–rim chemical zoning. Thermodynamic modeling indicates that both cores (13–22 kbar, 500–580°C) and rims (>16 kbar, 610–660°C) crystallized under eclogite-facies conditions. The core–rim zoning corresponds to the dehydration of the system. This petrographic disequilibrium is associated with Lu–Hf and Sm–Nd disequilibrium, which prevents dating of the eclogitic stages. At the rock scale, the incoming fluid responsible for eclogite–amphibolite retrogression brought in Sm and Nd, leached Lu, and had no influence on Hf. At the grain scale, mass balance shows that Sm and Nd were stored in clinozoisite since the first eclogitic stage, whereas Lu and Hf, which were more thoroughly redistributed among minerals during retrogression, enable the dating of the amphibolitic facies at 378 ± 17 Ma. 相似文献
884.
885.
Fragment properties at the catastrophic disruption threshold: The effect of the parent body’s internal structure 总被引:1,自引:0,他引:1
Numerical simulations of asteroid breakups, including both the fragmentation of the parent body and the gravitational interactions between the fragments, have allowed us to reproduce successfully the main properties of asteroid families formed in different regimes of impact energy, starting from a non-porous parent body. In this paper, using the same approach, we concentrate on a single regime of impact energy, the so-called catastrophic threshold usually designated by , which results in the escape of half of the target’s mass. Thanks to our recent implementation of a model of fragmentation of porous materials, we can characterize for both porous and non-porous targets with a wide range of diameters. We can then analyze the potential influence of porosity on the value of , and by computing the gravitational phase of the collision in the gravity regime, we can characterize the collisional outcome in terms of the fragment size and ejection speed distributions, which are the main outcome properties used by collisional models to study the evolutions of the different populations of small bodies. We also check the dependency of on the impact speed of the projectile.In the strength regime, which corresponds to target sizes below a few hundreds of meters, we find that porous targets are more difficult to disrupt than non-porous ones. In the gravity regime, the outcome is controlled purely by gravity and porosity in the case of porous targets. In the case of non-porous targets, the outcome also depends on strength. Indeed, decreasing the strength of non-porous targets make them easier to disrupt in this regime, while increasing the strength of porous targets has much less influence on the value of . Therefore, one cannot say that non-porous targets are systematically easier or more difficult to disrupt than porous ones, as the outcome highly depends on the assumed strength values. In the gravity regime, we also confirm that the process of gravitational reaccumulation is at the origin of the largest remnant’s mass in both cases. We then propose some power-law relationships between and both target’s size and impact speed that can be used in collisional evolution models. The resulting fragment size distributions can also be reasonably fitted by a power-law whose exponent ranges between −2.2 and −2.7 for all target diameters in both cases and independently on the impact velocity (at least in the small range investigated between 3 and 5 km/s). Then, although ejection velocities in the gravity regime tend to be higher from porous targets, they remain on the same order as the ones from non-porous targets. 相似文献
886.
Karl R. Atkinson John C. Zarnecki Martin C. Towner Timothy J. Ringrose Axel Hagermann Andrew J. Ball Mark R. Leese Gunter Kargl Mark D. Paton Ralph D. Lorenz Simon F. Green 《Icarus》2010,210(2):843-851
The Huygens probe landed on the then unknown surface of Titan in January 2005. A small, protruding penetrometer, part of the Surface Science Package (SSP), was pushed into the surface material measuring the mechanical resistance of the ground as the probe impacted the landing site. We present laboratory penetrometry into room temperature surface analogue materials using a replica penetrometer to investigate further the nature of Titan’s surface and examine the sensor’s capabilities. The results are then compared to the flight instrument’s signature and suggest the Titan surface substrate material consists of sand-sized particles with a mean grain size ~2 mm. A possible thin 7 mm coating with mechanical properties similar to terrestrial snow may overlie this substrate, although due to the limited data we are unable to detect any further layering or grading within the near-surface material. The unusual weakening with depth of the signature returned from Titan has, to date, only been reproduced using a damp sand target that becomes progressively wetter with depth, and supports the suggestion that the surface may consist of a damp and cohesive material with interstitial liquid contained between its grains. Comparison with terrestrial analogues highlights the unusual nature of the landing site material. 相似文献
887.
888.
<正>Mushroom Mountain karren with predominantly flat tops that formed along the stylolites and bedding planes dictated the special features of this karst landscape.Their features and rock relief clearly reflect the geological conditions and development.The subsoil karren with conical tops dissected by subsoil rock relief were exposed from beneath the soil.Below tree vegetation,the subsoil karren are to a great extent covered by moss and lichen,under which they acquire their characteristic shape.On the bare surface they were reshaped by rainwater that carves flutes and solution pans.The old cave that opens below the top of one of the cones reveals the period before its dissection into hills and cones when this part of the karst aquifer was still deep under the water table. 相似文献
889.
890.
Herbert Lutz Uwe KaulfuSS Torsten Wappler Werner L?hnertz Volker Wilde Dieter F. Mertz Jens Mingram Jens L. Franzen Herbert Frankenh?user Martin Koziol 《《地质学报》英文版》2010,84(4):984-1009
<正>To mark the occasion of the 175th anniversary of the Rheinische Naturforschende Gesellschaft in 2009 and of the centennial of the Mainz Natural History Museum in 2010,we present a short account of our present knowledge of the Eckfeld Maar after 20 years of continuous research.This paper does not attempt to include all of the detailed results on the geology of the Eckfeld site or its biota.To date,nearly 250 papers and books have been published since the start of our project.An up-to-date list of these publications can be found at www.eckfeldermaar. de. 相似文献