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排序方式: 共有155条查询结果,搜索用时 234 毫秒
41.
Albin Hammerle Alois Haslwanter Michael Schmitt Michael Bahn Ulrike Tappeiner Alexander Cernusca Georg Wohlfahrt 《Boundary-Layer Meteorology》2007,122(2):397-416
Carbon dioxide, latent and sensible heat fluxes were measured by means of the eddy covariance method above a mountain meadow
situated on a steep slope in the Stubai Valley in Austria, based on the hypothesis that, due to the low canopy height, measurements
can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order
to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were
subjected to a rigorous testing procedure using a series of quality control measures established for surface-layer flows.
The resulting high quality dataset comprised 36% of the original observations, the substantial reduction being mainly due
to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition
periods. The validity of the high quality dataset was further assessed by two independent tests: (i) a comparison with the
net ecosystem carbon dioxide exchange measured by means of ecosystem chambers, and (ii) the ability of the eddy covariance
measurements to close the energy balance. The net ecosystem CO2 exchange measured by the eddy covariance method agreed reasonably well with ecosystem chamber measurements. The assessment
of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on
the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated
by 28% and 29%, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements
made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain. 相似文献
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The stable copper isotope composition of 79 samples of primary and secondary copper minerals from hydrothermal veins in the Schwarzwald mining district, South Germany, shows a wide variation in δ65Cu ranging from −2.92 to 2.41‰. We investigated primary chalcopyrite, various kinds of fahlores and emplectite, as well as supergene native copper, malachite, azurite, cuprite, tenorite, olivenite, pseudomalachite and chrysocolla. Fresh primary Cu(I) ores have at most localities copper isotope ratios (δ65Cu values) of 0 ± 0.5‰ despite the fact that the samples come from mineralogically different types of deposits covering an area of about 100 by 50 km and that they formed during three different mineralization events spanning the last 300 Ma. Relics of the primary ores in oxidized samples (i.e., chalcopyrite relics in an iron oxide matrix with an outer malachite coating) display low isotope ratios down to −2.92‰. Secondary Cu(I) minerals such as cuprite have high δ65Cu values between 0.4 and 1.65‰, whereas secondary Cu(II) minerals such as malachite show a range of values between −1.55 and 2.41‰, but typically have values above +0.5‰. Within single samples, supergene oxidation of fresh chalcopyrite with a δ value of 0‰ causes significant fractionation on the scale of a centimetre between malachite (up to 1.49‰) and relict chalcopyrite (down to −2.92‰). The results show that—with only two notable exceptions—high-temperature hydrothermal processes did not lead to significant and correlatable variations in copper isotope ratios within a large mining district mineralized over a long period of time. Conversely, low-temperature redox processes seriously affect the copper isotope compositions of hydrothermal copper ores. While details of the redox processes are not yet understood, we interpret the range in compositions found in both primary Cu(I) and secondary Cu(II) minerals as a result of two competing controls on the isotope fractionation process: within-fluid control, i.e., the fractionation during the redox process among dissolved species, and fluid-solid control, i.e., fractionation during precipitation involving reactions between dissolved Cu species and minerals. Additionally, Rayleigh fractionation in a closed system may be responsible for some of the spread in isotope compositions. Our study indicates that copper isotope variations may be used to decipher details of natural redox processes and therefore may have some bearing on exploration, evaluation and exploitation of copper deposits. On the other hand, copper isotope analyses of single archeological artefacts or geological or biological objects cannot be easily used as reliable fingerprint for the source of copper, because the variation caused by redox processes within a single deposit is usually much larger than the inter-deposit variation. 相似文献
44.
In 1995, mapping and classification of riparian vegetation along the Mojave River in southern California revealed an 8-km reach in which riparian cottonwoods (Populus fremontii Wats.) were stressed or dying. We tested a set of predictions based on the inference that cottonwood decline was an indirect result of lowered water-table levels following flood-related channel incision. Comparisons of topographic cross-sections from 1963 and 1997, indicated a net change in channel elevation between −0·71 and −3·6 m within zones of cottonwood stress and mortality. Ages of young cottonwood and willow stems adjacent to the present channel and radial stem growth of surviving cottonwoods were consistent with the inference that channel incision, associated with sustained flooding in January and February of 1993, lowered channel elevations throughout the affected reach. Well records and soil redoximorphic features indicate that channel incision caused net water-table declines 1·5 m on portions of the adjacent flood plain where cottonwood stand mortality ranged between 58 and 93%. In areas where water-table declines were estimated to be <1·0 m, stand mortality was 7–13%. 相似文献
45.
Sergey V. Marchenko Gregor Rauw Eleonora A. Antokhina Igor I. Antokhin Dominique Ballereau Jacques Chauville Michael F. Corcoran Rafael Costero Juan Echevarria Thomas Eversberg Ken G. Gayley Gloria Koenigsberger Anatoly S. Miroshnichenko Anthony F. J. Moffat Nidia I. Morrell Nancy D. Morrison Christopher L. Mulliss Julian M. Pittard Ian R. Stevens Jean-Marie Vreux Jean Zorec 《Monthly notices of the Royal Astronomical Society》2000,317(2):333-342
With the objective of investigating the windwind collision phenomenon and supporting contemporaneous X-ray observations, we have organized a large-scale, coordinated optical monitoring campaign of the massive, highly eccentric O9 III+B1 III binary Iota Orionis. Successfully separating the spectra of the components, we refine the orbital elements and confirm the rapid apsidal motion in the system. We also see strong interaction between the components during periastron passage and detect phase-locked variability in the spectrum of the secondary star. However, we find no unambiguous signs of the bow shock crashing on the surface of the secondary, despite the predictions of hydrodynamic simulations. Combining all available photometric data, we find rapid, phase-locked variations and model them numerically, thus restricting the orbital inclination to 50° i 70°. 相似文献
46.
Centrifuge model test on the face stability of shallow tunnel 总被引:2,自引:2,他引:0
This paper is an investigation of face stability on a small-scale tunnel model in a geotechnical centrifuge. By making use
of symmetry, half of the tunnel cross section was considered. The support at excavation face was provided by a piston, which
was adjusted during flight. Some aspects on the collapse at tunnel face are investigated for different overburden pressures:
failure mechanism, surface settlement, stress acting at tunnel face, and the required face support counteracting the earth
pressure. Ground deformation was observed through a transparent wall and measured by digital image correlation. The results
from centrifuge model tests were compared with theoretical models. 相似文献
47.
Gregor J. Golabek Bernard Bourdon Taras V. Gerya 《Meteoritics & planetary science》2014,49(6):1083-1099
The acapulcoite‐lodranite meteorites are members of the primitive achondrite class. The observation of partial melting and resulting partial removal of Fe‐FeS indicates that this meteorite group could be an important link between achondrite and iron meteorites, on the one hand, and chondrite meteorites, on the other. Thus, a better understanding of the thermomechanical evolution of the parent body of this meteorite group can help improve our understanding of the evolution of early planetesimals. Here, we use 2‐D and 3‐D finite‐difference numerical models to determine the formation time, initial radius of the parent body of the acapulcoite‐lodranite meteorites, and their formation depth inside the body by applying available geochronological, thermal, and textural constraints to our numerical data. Our results indicate that the best fit to the data can be obtained for a parent body with 25–65 km radius, which formed around 1.3 Ma after calcium‐aluminum‐rich inclusions. The 2‐D and 3‐D results considering various initial temperatures and the effect of porosity indicate possible formation depths of the acapulcoite‐lodranite meteorites of 9–19 and 14–25 km, respectively. Our data also suggest that other meteorite classes could form at different depths inside the same parent body, supporting recently proposed models (Elkins‐Tanton et al. 2011 ; Weiss and Elkins‐Tanton 2013 ). 相似文献
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