The Origin of Chondrules and Chondrites     

Alan E. Rubin 《Geochimica et cosmochimica acta》2005,69(19):4745-4746

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Tidal wetting and drying on shore platforms: An experimental study of surface expansion and contraction     

Alan Trenhaile   《Geomorphology》2006,76(3-4):316-331

Laboratory experiments have been conducted to measure rock expansion and contraction induced by wetting and drying on intertidal shore platforms in eastern Canada. Almost 800 rock cores and cubes were subjected to between 800 and 1000 tidal cycles. The rocks were put into tidal simulators and inundated in de-ionized water for 1, 6, or 11 h of each 12 h cycle, replicating conditions at the high, mid-, and low tidal levels, respectively. Micro-erosion meter (MEM) stations were also installed in slabs of argillite, sandstone, and basalt. The slabs were subjected to conditions at the high, mid-, and low tidal levels, and a traversing MEM was used to measure rock expansion and contraction as the slabs dried in air with variable temperature and humidity during low tide. The slabs were also exposed for long periods to air with variable humidity. Cores and cubes often gained in weight when they absorbed or adsorbed water and retained it for periods ranging from 1 month to 1 year or more, although this did not appear to be a prelude to rock breakdown. Tidal wetting and drying and exposure to humid air caused the argillite and basalt slabs to expand and contract by up to 0.14 and 0.04 mm, respectively. Wetting and drying did not induce expansion or contraction of the sandstone, but the sandstone did respond slightly to changes in air humidity. The greatest amount of expansion and contraction was at the high tidal level in the argillite and at the mid-tidal level in the basalt, which is consistent with previously measured rates of downwearing.  相似文献   

The Impact of U.S. Government Antiterrorism Policies on Canada‐U.S. Cross‐Border Commerce: An Exploratory Study from Western New York and Southern Ontario     

Alan D. MacPherson  James E. McConnell  Anneliese Vance  Vida Vanchan 《The Professional geographer》2006,58(3):266-277

This article examines the extant and potential impact of U.S. antiterrorism policies on Canada‐U.S. cross‐border commerce. Particular attention is focused on the cross‐border trade that takes place between southern Ontario (Canada) and western New York (United States). Evidence from a survey of Canadian and U.S. exporters suggests that U.S. antiterrorism measures have inflated the business costs of exporters on both sides of the border. These measures have also created shipment delays that ultimately imply lost revenues for producers, as well as higher prices for consumers. Security‐related initiatives motivated by a genuine concern for the well‐being of U.S. citizens may nevertheless act as nontariff barriers to bilateral trade. We argue that a potential long‐run consequence of these additional costs is trade diversion. The article concludes with a brief discussion of the implications of the empirical findings for the geography of Canada‐U.S. bilateral trade.  相似文献   

Economy, Politics and Institutions: From Adaptation to Adaptive Management in Climate Change     

Alexander Thompson  Paul Robbins  Brent Sohngen  Joseph Arvai  Tomas Koontz 《Climatic change》2006,78(1):1-5

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Human Strategies for Coping with El Niño Related Drought in Amazônia     

Emilio F. Moran  Ryan Adams  Bryn Bakoyéma  Stefano Fiorini T.  Bruce Boucek 《Climatic change》2006,77(3-4):343-361

This article reports on findings of a research project examining farmers' coping strategies in the Brazilian Amazon in response to El Niño related weather events. We examine the extent of vulnerability of small and large farmers to these events in a tropical rainforest environment. Little attention has been given to the impact of ENSO events in Amazônia, despite evidence for devastating fires during ENSOs. Although we found a range of locally developed forecasting techniques and coping mechanisms, farmers have sustained significant losses, and we suggest that increased access to scientific forecasts would greatly enhance the ability of the farmers in our study area to cope with El Niño related weather events. In Amazônia the El Niño phase of the El Niño-Southern Oscillation (ENSO) climate pattern leads to an extended period of reduced rainfall (Hobbs et al., 1998). This period of reduced rainfall can result in significant agricultural losses for farmers and ranchers in the area and in increased forest flammability. We have found that the majority of our study population uses several methods of forecasting, coping with, and adapting to drought events – and they recognize the economic losses they can experience and the loss of forests through the accidental spread of fire. The poorest farmers in our study area experience El Niño related drought events as a serious threat to their livelihoods. Their vulnerability is heightened during extreme climate events and our observations revealed that all of the farmers in our study would benefit from increased availability of improved forecast information relevant to their locality and their current farming strategies. This paper examines the availability and use of forecasts, the occurrence of accidental fires and techniques to prevent fire related losses, and the coping mechanisms for dealing with El Niño related drought in the agricultural regions surrounding the cities of Altamira and Santarém, in Pará State, Brazil. Distribution of an El Niño Prediction Kit at the end of the study and a series of workshops may lead to better local information on rainfall variability and create a farmer-maintained grid of collecting stations to sensitize farmers to the variability of precipitation in the region, and on their property.  相似文献   

Structure of flow over alluvial bedforms: an experiment on linking field and laboratory methods     

Alexander N. Sukhodolov  Juan J. Fedele  Bruce L. Rhoads 《地球表面变化过程与地形》2006,31(10):1292-1310

This paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time–space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time–space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities   总被引：1，自引：0，他引：1  

Alexandre Schubnel  Philip M. Benson  Ben D. Thompson  Jim F. Hazzard  R. Paul Young 《Pure and Applied Geophysics》2006,163(5-6):947-973

Crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive crack effective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic properties. Using this same tool, we show that both the anisotropy and shear-wave splitting of elastic waves can be derived. Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range of validity of the non-interactive effective medium theory model extends to a total crack density of approximately 0.5, considering symmetries up to orthorhombic. In the isotropic case, Kachanov's (1994) non-interactive effective medium model was used in order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and give coherent results in terms of crack density and aperture evolution. Crack density variations can be interpreted in terms of crack growth and/or changes of the crack surface contact areas as cracks are being closed or opened respectively. More importantly, the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio (and therefore aperture) with pressure, which has broader geophysical implications, in particular on fluid flow. The recovered evolution of aspect ratio is also consistent with current mechanical theories of crack closure. In the anisotropic cases—both transverse isotropic and orthorhombic symmetries were considered—anisotropy and saturation patterns were well reproduced by the modelling, and mean crack fabric orientations we recovered are consistent with in situ geophysical imaging. Our results point out that: (1) It is possible to predict damage, anisotropy and saturation in terms of a crack density tensor and mean crack aspect ratio and orientation; (2) using well constrained wave velocity data, it is possible to extrapolate the contemporaneous evolution of crack density, anisotropy and saturation using wave velocity inversion as a tool; 3) using such an inversion tool opens the door in linking elastic properties, variations to permeability.  相似文献   

The subsurface radial gradient of solar angular velocity from MDI f-mode observations     

T. Corbard  M.J. Thompson 《Solar physics》2002,205(2):211-229

We report quantitative analysis of the radial gradient of solar angular velocity at depths down to about 15 Mm below the solar surface for latitudes up to 75° using the Michelson Doppler Imager (MDI) observations of surface gravity waves (fmodes) from the Solar and Heliospheric Observatory (SOHO). A negative outward gradient of around –400 nHz/R , equivalent to a logarithmic gradient of the rotation frequency with respect to radius which is very close to –1, is found to be remarkably constant between the equator and 30° latitude. Above 30° it decreases in absolute magnitude to a very small value at around 50°. At higher latitudes the gradient may reverse its sign: if so, this reversal takes place in a thin layer extending only 5 Mm beneath the visible surface, as evidenced by the most superficial modes (with degrees l>250). The signature of the torsional oscillations is seen in this layer, but no other significant temporal variations of the gradient and value of the rotation rate there are found.  相似文献   

Environmental changes in an alpine lake (Gossenköllesee, austria) over the last two centuries – the influence of air temperature on biological parameters     

Karin A. Koinig  Christian Kamenik  Roland Schmidt  Anna Agustí-Panareda  Peter Appleby  Andrea Lami  Miroslava Prazakova  Neil Rose  Øyvind A. Schnell  Richard Tessadri  Roy Thompson  Roland Psenner 《Journal of Paleolimnology》2002,28(1):147-160

Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.  相似文献   

Influence of fracture anisotropy on ground water ages and chemistry,Valley and Ridge province,Pennsylvania   总被引：5，自引：0，他引：5  

Burton WC  Plummer LN  Busenberg E  Lindsey BD  Gburek WJ 《Ground water》2002,40(3):242-257

Model ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium-3 (³H/³He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding-plane parting and east striking, steeply dipping axial-plane spaced cleavage. In the area of the piezometer arrays, which trend north-south on the north limb of the fold, north dipping bedding-plane parting is a more dominant fracture set than is steeply south dipping axial-plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding-plane parting has younger ³H/³He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.  相似文献