Comparative influence of resuspended glacial sediment on physicochemical characteristics and primary production in two arctic lakes   总被引：2，自引：0，他引：2  

Stephen C. Whalen  Brian A. Chalfant  Eric N. Fischer  Kenneth A. Fortino  Anne E. Hershey 《Aquatic Sciences - Research Across Boundaries》2006,68(1):65-77

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Delivering pro-poor water and sanitation services: The technical and political challenges in Malawi and Zambia     

Eric Gutierrez 《Geoforum》2007,38(5):886-900

Meeting Millennium Development Goals on water and sanitation services in developing countries are fraught with difficulties, as can be seen most clearly from the experiences of Malawi and Zambia, two of the world’s poorest countries that have committed to meeting these goals. The challenges are not only technical, requiring programmatic or engineering solutions, but are also and most importantly political, because solutions will most often cause a rearrangement of the peculiar forms of power relations that have emerged within institutional and political environments of the two countries. The challenges include weak state support for water and sanitation provision, unreliable and contested indicators of coverage, poor sectoral co-ordination, and fragmented donor efforts. This field note examines these challenges in greater detail to cast new light as well as draw attention to possible solutions that can be implemented.  相似文献   

Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province)     

Payman Navabpour  Jacques Angelier  Eric Barrier   《Tectonophysics》2007,432(1-4):101-131

The Zagros fold-and-thrust belt of SW-Iran is among the youngest continental collision zones on Earth. Collision is thought to have occurred in the late Oligocene–early Miocene, followed by continental shortening. The High Zagros Belt (HZB) presents a Neogene imbricate structure that has affected the thick sedimentary cover of the former Arabian continental passive margin. The HZB of interior Fars marks the innermost part of SE-Zagros, trending NW–SE, that is characterised by higher elevation, lack of seismicity, and no evident active crustal shortening with respect to the outer (SW) parts. This study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Various strike–slip, compressional, and tensional stress regimes are thus identified with different stress fields. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields, considering the folding process. Results indicate that in the studied area, the main fold and thrust structure developed in a general compressional stress regime with an average N032° direction of σ₁ stress axis during the Miocene. Strike–slip structures were generated under three successive strike–slip stress regimes with different σ₁ directions in the early Miocene (N053°), late Miocene–early Pliocene (N026°), and post-Pliocene (N002°), evolving from pre-fold to post-fold faulting. Tensional structures also developed as a function of the evolving stress regimes. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal σ₁ axis since the onset of collision and a significant change in vertical stress from σ₃ to σ₂ since the late stage of folding and thrusting. A late right-lateral reactivation was also observed on some pre-existing belt-parallel brittle structures, especially along the reverse fault systems, consistent with the recent N–S plate convergence. However, this feature was not reflected by large structures in the HZB of interior Fars. The results should not be extrapolated to the entire Zagros belt, where the deformation front has propagated from inner to outer zones during the younger events.  相似文献   

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer   总被引：1，自引：0，他引：1  

Eric D. Skyllingstad  Dean Vickers  Larry Mahrt  Roger Samelson 《Boundary-Layer Meteorology》2007,123(2):219-237

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.  相似文献   

An SEM study of porosity and grain boundary microstructure in quartz mylonites, Simplon Fault Zone, Central Alps     

Neil S. Mancktelow  Djordje Grujic  Eric L. Johnson 《Contributions to Mineralogy and Petrology》1998,131(1):71-85

A backscattered and secondary electron SEM study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde Simplon Fault Zone established three characteristic components. (1) Fine isolated pores (≤?1?μm diameter) are scattered across two-grain interfaces, preferentially concentrated on surfaces in extension. Pores are uncommon on three-grain junctions and there is no evidence for fluid interconnectivity along three- and four-grain junctions. The fine porosity may develop by accumulation of original, mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding. Dynamic cavitation implies that the “ductile” mylonitic deformation is at least locally dilatant and therefore pressure sensitive. (2) Large “vug”-like pores (up to mm-scale) extend along multi-grain boundaries. Observed in all samples, they are most common in the higher initial temperature, coarse-grained samples with a microstructure dominated by grain boundary migration recrystallization. Grains bordering this connected porosity develop perfect crystal faces, undecorated by fine pores or pits. The irregular “lobate” optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces. The coarse porosity is probably due to accumulation during dynamic recrystallization of (CO₂-rich ?) fluid with a high wetting angle against quartz. (3) In one sample, interconnected sinuous ridges, ≤?0.2?μm high, are observed to follow three- and four-grain junctions and disjoint into more isolated worms and spheroidal globules. On two-grain interfaces, these are transitional to more branching vein-like or convoluted brain-like forms. The brain-like and globular forms have been observed, with varying frequency, through the range of samples, with the globules attaining sizes of up to 60?μm. Vein structures have also been observed on intragranular fractures. These topologies do not match across adjoining surfaces and must have developed into free space. The ridge-vein-brain-spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established. They could represent unstable meniscus necking of a thin grain-boundary phase of low viscosity, developed due to quasi-adiabatic shear and/or local stress-induced dilatancy during microcracking.  相似文献   

Variability associated with α accretion disc theory for standard and advection-dominated discs     

Eric G. Blackman 《Monthly notices of the Royal Astronomical Society》1998,299(4):L48-L52

The α turbulent viscosity formalism for accretion discs must be interpreted as a mean field theory, modelling a steady state only on spatial or time-scales greater than those of the turbulence. The extent of the scale separation determines the relative precision error (RPE) of the predicted luminosity L _ν. Turbulence and the use of α implies that (1) field line stretching gives a magnetic pressure  α²/6 of the total pressure generally, and a one-to-one relation between α and the pressure ratio for thin discs, and (2) large turbulent scales in advection-dominated accretion flows (ADAFs) predict a lower L _ν precision than thin discs for a given observation duration and central mass. The allowed variability (or RPE) at frequency ν increases with the size of the contributing region. For X-ray binary ADAFs, the RPE ∼ 5 per cent at R  ≤ 1000 Schwarzchild radii ( R _s) for averages over  1000 s. However, current data for galaxies like NGC 4258 and M87 give RPEs in L _ν of 50–100 per cent even at R  ≤ 100  R _S. More data are required, but systematic deviations from ADAF predictions are more significant than random deviations, and may constrain properties of the turbulence, the accretion mode, the assumption of a steady state or the accretion rate.  相似文献   

Reviews     

Daniel F. Merriam  R. A. Reyment  John W. Harbaugh  Eric C. Dahlberg  Marie Morisawa 《Mathematical Geosciences》1975,7(5-6):485-492

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The geologic history of the Nicoya Ophiolite Complex, Costa Rica, and its geotectonic significance     

Eric P. Kuijpers 《Tectonophysics》1980,68(3-4)

Costa Rica forms part of an intra-oceanic arc between the Pacific and Caribbean oceans; the Nicoya Ophiolite Complex is located along its Pacific border. In this study, evidence is given that the Nicoya Complex is composed of ridge-formed oceanic crust that suffered a strong compressional stress during Late Santonian times. As a result of this, isoclinal folding and large-scale nappe emplacement occurred at a shallow crustal depth. The principal component of this compressional stress was E-W-directed. It is also demonstrated that, from this time, the complex was situated between a subducting plate and a volcanic arc. From that Campanian until the Middle Eocene the zone was undulated, and generally at a great depth below sea level. During the Eocene—Oligocene epoch a new tectonic stress affected the area. It produced open folding with upthrusting in the ophiolite complex and overthrust folding of the overlying rock series. As a result of crustal thickening during this tectonic phase, the area was uplifted. From Miocene times, the zone was shaped into a dome and a synform. These undulations are attributed to compression of the subducting Coco Plate, west of the area.The Upper Santonian tectonic phase demonstrates how compressional stress produced the break-up of the Caribo-Pacific plate west of the study area, as a result of which, a Caribbean plate without an associated oceanic ridge and a Pacific plate originated. The compressional stress in question was presumably generated by the opposed spreading directions of the new Mid-Atlantic Ridge and an older ridge to the west of the study area.Furthermore, it is argued that the Cretaceous obduction of the ophiolite belt along the Pacific coast of the American continents, was produced by the directional change of these continents during the birth of the Mid-Atlantic Ridge. This created intra-plate compressional stress and converted originally passive continental margins into active zones, where thrusting of oceanic crust on to a continental margin (obduction) could occur. When the Mid-Atlantic Ridge started spreading, the obduction phase ended due to subduction of the oceanic plate below the leading edge of the continent.  相似文献   

Upper Limit of Iodine Oxide in the Lower Stratosphere     

Irene Pundt  Jean-Pierre Pommereau  Celine Phillips  Eric Lateltin 《Journal of Atmospheric Chemistry》1998,30(1):173-185

It has been suggested that iodine oxides, IOx, could play a significant role in the ozone destruction in the lower stratosphere. To investigate this suggestion, spectra from nine SAOZ uv-visible spectrometer balloon flights were examined for the IO absorption signature between 405 and 450 nm. IO was not detected, either at mid- or high latitude, in the morning or the evening, in summer or winter. An upper limit of 0.2 parts per trillion by volume (pptv) at 20 km and 0.1 pptv at 15 km at the 95% confidence level (2), was derived from the best measurements at 90° SZA at sunset and sunrise. Since a photochemical model shows that 70% of inorganic iodine should be in the form IO at that time, it is concluded that unless iodine chemistry is different from that assumed at the moment, the role of iodine in stratospheric ozone depletion is small.  相似文献   

Uncertainty assessment and analysis of the calibrated parameter values of an urban storm water quality model   总被引：1，自引：0，他引：1  

Eric Gaume  Jean-Pierre Villeneuve  Michel Desbordes 《Journal of Hydrology》1998,210(1-4):38-50

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