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211.
Impacts of the grounding of the oil tanker Jessica off San Cristóbal island, Galápagos, included both effects of oil on biota and also mechanical effects associated with a shallow furrow 50 m x 30 m gouged across the rocky seafloor and wreckage strewn over 7500 m(2). The wreckage represented a minor but potentially chronic source of pollution to the surrounding environment through delayed releases of oil, antifouling compounds and other toxic chemicals, and a possible source of exotic marine taxa. Investigation at the wreck site indicated that impacts on subtidal plant and macro-invertebrate communities were largely confined within 100 m of the wreck site itself. Observed population effects included significant increases in cover of opportunistic algae (filamentous green algae, filamentous red algae and Ulva sp.) and the hydroid Ectopleura media adjacent to the wreck, while densities of the green sea urchin Lytechinus semituberculatus significantly decreased from 0.3 m(-2) adjacent to the wreck to 11 m(-2) at 100 m distance. 相似文献
212.
In contrast to local perceptions, the impact on the local Galápagos artisanal fishery of the 16 January 2001 grounding of the Jessica and subsequent oil spill was relatively minor. No significant changes in fishing effort, total fishing catches or catch-per-unit effort were detected after the spill based on analyses of fisheries monitoring data. Nevertheless, large boats tended to move away from sites near the path of the spill following the grounding in 2001, with no fishing recorded from the oil-affected regions of Floreana and southern Isabela in February 2001. The total fishing effort of small boats operating from the Jessica-grounding island of San Cristóbal also declined immediately after the spill, probably in part because such boats were used in clean up operations. During 2001, prices paid to fishers remained stable at levels higher than in 2000, with the notable anomaly that prices fell precipitously to 30% of previous levels during a 1-2 week period in early February 2001. Fish exports remained at similar levels for the years 2000 and 2001; however, as in the previous year, little fish product was exported from Galápagos in the month following the spill, with most fish product dried and stored for up to two months prior to transport to the continent. 相似文献
213.
Although recognized as important, measures of connectivity (i.e. the existence of high-conductivity paths that increase flow and allow for early solute arrival) have not yet been incorporated into methods for upscaling hydraulic conductivities of porous media. We present and evaluate a binary upscaling formula that utilizes connectivity information. The upscaled hydraulic conductivity (K) of binary media is determined as a function of the proportions and conductivities of the two materials, the geometry of the inclusions, and the mean distance between them. The use of a phase interchange theorem renders the formula equally applicable to two-dimensional media with inclusions of low K and high K as compared with the matrix. The new upscaling formula is tested on two-dimensional binary random fields spanning a broad range of spatial correlation structures and conductivity contrasts. The computed effective conductivities are compared to what is obtained using self-consistent effective medium theory, the coated ellipsoids approximation, and to a streamline approach. It is shown that, although simple, the proposed formula performs better than available methods for binary upscaling. The use of connectivity information leads to significantly improved behavior close to the percolation threshold. The proposed upscaling formula depends exclusively on parameters that are obtainable from field investigations. 相似文献
214.
D.W. Graham L.M. Larsen B.B. Hanan M. Storey A.K. Pedersen J.E. Lupton 《Earth and Planetary Science Letters》1998,160(3-4):241-255
Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have 3He/4He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic 3He/4He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic 3He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high 3He/4He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high 3He/4He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology. 相似文献
215.
Parametric-historic Procedure for Probabilistic Seismic Hazard Analysis Part I: Estimation of Maximum Regional Magnitude mmax 总被引:1,自引:0,他引:1
—A new methodology for probabilistic seismic hazard analysis (PSHA) is described. The approach combines the best features of the "deductive" (Cornell, 1968) and "historical" (Veneziano et al., 1984) procedures. It can be called a "parametric-historic" procedure.¶The maximum regional magnitude m max is of paramount importance in this approach and Part I of our work presents some of the statistical techniques which can be used for its evaluation. The work is an analysis of parametric procedures for the evaluation of m max, when the form of the magnitude distribution is specified. For each of the formulae given there are notes on its origin, assumptions made of its derivation, and some comparisons. The statistical concepts of bias and variance are considered for each formula, and appropriate expressions for these are also given. Also, following Knopoff and Kagan (1977), we shall demonstrate why there must be a finite upper bound to the largest seismic event if the Gutenberg-Richter frequency-magnitude relation is accepted. 相似文献
216.
Colin M. Graham John W. Valley John M. Eiler Hideki Wada 《Contributions to Mineralogy and Petrology》1998,132(4):371-389
Using a recently developed ion microprobe technique, a detailed oxygen isotope map of calcite grains in a coarse-grained
marble has been constructed, supported by trace element (Mn, Sr, Fe) analysis and cathodoluminescence (CL) imaging, in order
to constrain scales of oxygen isotope equilibrium, timescales and mechanisms of metamorphic fluid infiltration, and fluid
sources and pathways. Results are compared with a previous study of this sample (Wada 1988) carried out using a cryo-microtome
technique and conventional oxygen isotope analysis. The marble, from the high temperature/low pressure Hida metamorphic belt
in north-central Japan, underwent granulite facies followed by amphibolite facies metamorphic events, the latter associated
with regional granite intrusion. The CL imaging indicates two types of calcite, a yellow luminescing (YLC) and a purple luminescing
(PLC) variety. The YLC, which occupies grain boundaries, fractures, replacement patches, and most of the abundant deformation
twin lamellae, post-dates the dominant PLC calcite and maps out fluid pathways. Systematic relationships were established
between oxygen isotope and trace element composition, calcite type and texture, based on 74 18O/16O and 17 trace element analyses with 20–30 μ m spatial resolution. The YLC is enriched in Mn and Fe, and depleted in 18O and Sr compared to PLC, and is much more 18O depleted than is indicated from conventional analyses. Results are interpreted to indicate infiltration of 18O-depleted (metamorphic or magmatic) fluid (initial δ18O = 9‰–10.5‰) along grain boundaries, fractures and deformation twin lamellae, depleting calcite grains in Sr and enriching
them in Mn and Fe. The sample is characterised by gross isotopic and elemental disequilibrium, with important implications
for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks.
Areas of PLC unaffected by “short-circuiting” fluid pathways contain oxygen diffusion profiles of ∼10‰/∼200 μm in grain boundary
regions or adjacent to fractures/patches. When correction is made for estimated grain boundary/fracture and profile orientation
in 3D, profiles are indistinguishable within error. Modelling of these profiles gives consistent estimates of Dt (where D is the diffusion coefficient and t is time) of ∼0.8 × 10−8 m2, from which, using experimental data for oxygen diffusion in calcite, timescales of fluid transport along grain boundaries
at amphibolite facies temperatures of ∼103 to ∼104 years are obtained. These short timescales, which are much shorter than plausible durations of metamorphism, imply that
rock permeabilities may be transiently much higher during fluid flow than those calculated from time integrated fluid fluxes
or predicted from laboratory measurements. The preservation of 18O/16O profiles requires either rapid cooling rates (∼100–600 °C/million years), or, more plausibly, loss of grain boundary fluid
such that a dry cooling history followed the transient passage of fluid. The δ18O/trace element correlations are also consistent with volume diffusion-controlled transport in the PLC. Fluid transport and
element exchange occurred by two inter-related mechanisms on short timescales and on different lengthscales – long-distance
flow along cracks, grain boundaries and twin lamellae coupled to ∼200 μm-scale volume diffusion of oxygen.
Received: 8 December 1997 / Accepted: 18 May 1998 相似文献
217.
The pressure and temperature dependence of the elastic properties of single-crystal fayalite Fe2SiO4
E. K. Graham J. A. Schwab S. M. Sopkin H. Takei 《Physics and Chemistry of Minerals》1988,16(2):186-198
The nine adiabatic elastic stiffness constants of synthetic single-crystal fayalite, Fe2SiO4, were measured as functions of pressure (range, 0 to 1.0 GPa) and temperature (range, 0 to 40° C) using the pulse superposition ultrasonic method. Summary calculated results for a dense fayalite polycrystalline aggregate, based on the HS average of our single-crystal data, are as follows: Vp = 6.67 km/s; Vs = 3.39km/s; K= 127.9 GPa; μ = 50.3 GPa; (?K/?P)T = 5.2; (?μ/?P)T=1.5;(?K/?T)P= ?0.030 GPa/K;and,(?/?T)P =-0.013 GPa/K (the pressure and temperature data are referred to 25° C and 1 atm, respectively). Accuracy of the single-crystal results was maintained by numerous cross and redundancy checks. Compared to the single-crystal elastic properties of forsterite, Mg2SiO4, the fayalite stiffness constants, as well as their pressure derivatives, are lower for each of the on-diagonal (C ij for which i=j) values, and generally higher for the off-diagonal (C ij for which i≠j) data. As a result, the bulk moduli (K) and dK/dP for forsterite and fayalite are very similar, but the rigidity modulus (μ) and dμ/dP for polycrystalline fayalite are much lower than their forsterite counterparts. The bulk compression properties derived from this study are very consistent with the static-compression x-ray results of Yagi et al. (1975). The temperature dependence of the bulk modulus of fayalite is somewhat greater (in a negative sense) than that of forsterite. The rigidity dependencies are almost equivalent. Over the temperature range relevant to this study, the elastic property results are generally consistent with the data of Sumino (1978), which were obtained using the RPR technique. However, some of the compressional modes are clearly discrepant. The elastic constants of fayalite appear to be less consistent with a theoretical HCP model (Leibfried 1955) than forsterite, reflecting the more covalent character of the Fe-O bonding in the former. 相似文献
218.
Following the success of the symposium held by the Photogrammetric Society on 31st March, 1987, a second one-day symposium was convened to give further support to the rapidly growing interest in the development of small format photography for aerial survey. This was held at Burlington House, London on 1st November, 1988. Over 70 people attended. Six of the papers are reproduced here, dealing with a range of topics including specifications for and navigation of small format aerial photography, the creation of digital terrain models from 35 mm photography, wildlife surveys in the Arctic and survey photography from model aircraft and tethered balloons. 相似文献
219.
An experimental study of the diffusion of oxygen in quartz and albite using an overgrowth technique 总被引:1,自引:0,他引:1
S. C. Elphick P. F. Dennis C. M. Graham 《Contributions to Mineralogy and Petrology》1986,92(3):322-330
Diffusion rates of18O tracer in quartz ( c, 1 Kb H2O) and Amelia albite ( 001, 2 Kb H2O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:-quartz ( c),
,Q=98±7 KJ mol–1 (600–825° C, 1 Kb); Amelia albite ( 001),
,Q=85±7 KJ mol–1, (400–600° C, 2 Kb). Measured quartz18O diffusivities decrease discontinuously at the- transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted. 相似文献
220.
L. H. Bowen E. DeGrave D. A. Reid R. C. Graham S. B. Edinger 《Physics and Chemistry of Minerals》1989,16(7):697-703
Celadonite from the northwestern Mojave Desert area of California was examined by detailed Mössbauer spectroscopy at temperatures from 10 K to 400 K. In addition to the predominant Fe3+ doublet with isomer shift 0.4 mm s–1 and quadrupole splitting 0.4 mm s–1, another Fe3+ doublet with 0.4, 1.2 mm/s and two Fe2+ doublets with 1.1, 1.7, 2.7 mm s–1 at 300 K were distinguished. The minor Fe3+ component is ascribed to dehydroxylated surface sites. Most of the remaining Fe(90%) is M2 cis-OH octahedral in an ordered M+–M2+ array. However, about 10% is M1 trans-OH Fe2+. Isomer shift vs. T gives Debye temperatures of 570 K for Fe3+ in M2 and 380 K for both Fe2+ sites, indicating greater vibrational freedom for Fe2+. Quadrupole splitting vs. T for Fe2+ gives a valence electronic energy splitting of 760 cm–1 between the ground and first excited state for M2. The M1 sites have a more drastic variation in vs. T which indicates not only a lower first excited state but a rhombic distortion at these sites. A proposed explanation is a neighboring M2 site vacancy. The soil clay formed from this celadonite, which is mostly Fe-rich smectite, was also studied by Mössbauer spectroscopy. About half the Fe2+ has been oxidized in the clay, but the isomer shifts and quadrupole splittings are essentially the same as in the original celadonite. A texture orientation in the clay absorber was detected by measuring the absorber at 55° to the source radiation. This texture effect produces asymmetric doublets in the usual 90° measurement. 相似文献