CHANGES IN THE MORPHOLOGY OF THREE AMERICAN CENTRAL BUSINESS DISTRICTS 1952–1966     

David C. Weaver 《The Professional geographer》1969,21(6):406-410

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Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties   总被引：1，自引：0，他引：1  

Elena Piñero  Eulàlia Gràcia  Francisca Martínez-Ruiz  Juan Cruz Larrasoaña  Alexis Vizcaino  Gemma Ercilla 《Geo-Marine Letters》2007,27(2-4):279-288

Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.  相似文献   

The Creation of a National Agricultural Land Use Dataset: Combining Pycnophylactic Interpolation with Dasymetric Mapping Techniques     

Alexis Comber  Chris Proctor  Steve Anthony 《Transactions in GIS》2008,12(6):775-791

Agricultural Census data is summarised over spatially coarse reporting units for reasons of farm confidentiality. This is problematic for research at a local level. This article describes an approach combining dasymetric and volume preserving techniques to create a national land use dataset at 1 km² resolution. The results for an English county are compared with contemporaneous aggregated habitat data. The results show that the accurate estimates of local agricultural land use (Arable and Grass) patterns can be estimated when individual 1 km squares are combined into blocks of > 9 squares, thereby providing local estimates of agricultural land use. This in turn allows more detailed modelling of land uses related to specific livestock and cropping activities. The dataset created by this work has been subject to extensive external validation through its incorporation into a number of other national models: nitrate leaching (e.g. MAGPIE, NEAP‐N), waste, and pathogen modelling related to agricultural activity.  相似文献   

Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry,Québec,Canada: petrological and stable isotope data     

Ian Cartwright  Tamie R. Weaver 《Contributions to Mineralogy and Petrology》1993,113(4):533-544

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of 0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO₂0.15. In the metasediments, ¹⁸O values of calcite (Cc) are as low as 8.6, suggesting that the fluids were in isotopic equilibrium with the syenites (¹⁸O =8.8–10.2). Marble ¹³C(Cc) values are-0.1 to-3.2; the lack of correlation between ¹³C(Cc) and ¹⁸O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m³/m² (4×10⁶ mol/m²), corresponding to actual fluxes of 3×10^-11 to 3×10^-12 m³/m²-s and intrinsic permeabilities of 10^-18 to 10^-20 m² for fluid flow lasting 0.1-1Ma. Marble ¹⁸O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble ¹⁸O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. ¹⁸O(Br) and ¹⁸O(Scrp) values correlate with ¹⁸O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that ¹⁸O(Cc-Br) and ¹⁸O(Cc-Serp) values are higher in marbles that have lower ¹⁸O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite ¹⁸O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.This paper is a contribution to IGCP 304, Lower Crustal Processes  相似文献   

The evolution of groundwater in the Tyrrell catchment, south-central Murray Basin, Victoria, Australia     

B. Petrides  I. Cartwright  T. R. Weaver 《Hydrogeology Journal》2006,14(8):1522-1543

The Tyrell catchment lies on the western margin of the Riverine Province in the south-central Murray Basin, one of Australia’s most important groundwater resources. Groundwater from the shallow, unconfined Pliocene Sands aquifer and the underlying Renmark Group aquifer is saline (total dissolved solids up to 150,000 mg/L) and is Na-Cl-Mg type. There is no systematic change in salinity along hydraulic gradients implying that the aquifers are hydraulically connected and mixing during vertical flow is important. Stable isotopes (¹⁸O+²H) and Cl/Br ratios indicate that groundwater is entirely of meteoric origin and salts in this system have largely been derived by evapotranspiration of rainfall with only minor halite dissolution, rock weathering (mainly feldspar dissolution), and ion exchange between Na and Mg on clays. Similarity in chemistry of all groundwater in the catchment implies relative consistency in processes over time, independent of any climatic variation. Groundwater in both the Pliocene Sands and Renmark Group aquifers yield ages of up to 25 ka. The Tyrrell Catchment is arid to semi-arid and has low topography. This has resulted in relatively low recharge rates and hydraulic gradients that have resulted in long groundwater residence times.  相似文献   

用MODIS遥感资料分析四川盆地气溶胶光学厚度时空分布特征   总被引：25，自引：1，他引：25       下载免费PDF全文

李成才  刘启汉等 《应用气象学报》2003,14(1):1-7

利用MODIS卫星遥感光学厚度产品，分析了四川盆地光学厚度分布和季节变化特征。由于受沙尘天气的影响，春季四川盆地具有最大的平均光学厚度。盆地内几个大值区中，西部成都一带的中心常年维持，季节变化小；南部中心位于宜宾到重庆沿长江流域一带；东部南充到重庆间的大值中心，季节变化大，在夏季消失。光学厚度分布和季节变化的数据结果为研究区域气候变化提供了依据。  相似文献   

On the role of flux adjustments in an idealized coupled climate model     

A. F. Fanning  A. J. Weaver 《Climate Dynamics》1997,13(10):691-701

 The effect of employing flux adjustments on the climatic response of an idealized coupled model to an imposed radiative forcing is investigated with two coupled models, one of which employs flux adjustments. A linear reduction (to the planetary longwave flux) of 4 W/m² is applied over a 70 y period and held constant thereafter. Similar model responses are found (during the initial 70 y period) for global-scale diagnostics of hemispheric air temperature due to the nearly linear surface-air temperature response to the radiative forcing. Significant regional scale differences do exist, however. As the perturbation away from the present climate grows, basin-scale diagnostics (such as meridional overturning rates) begin to diverge between flux adjusted and non-flux adjusted models. Once the imposed radiative forcing is held constant, differences in global mean air temperature of up to 0.5 °C are found, with large regional-scale differences in air temperature and overturning rates within the North Atlantic and Southern Ocean. Two additional experiments with the flux adjusted model (beginning from points further along the control integration) suggest that the elimination of much of the coupling shock before the radiative forcing is applied leads to results slightly closer to the non-flux adjusted case, although large differences still persist. In particular a dipole structure indicating an enhanced warming within the Pacific sector of the Southern Ocean, and cooling within the Atlantic sector is not reproduced by the flux adjusted models. This disparity is intimately linked to the Southern Ocean overturning cell along with the flux adjustments employed as well as the drift arising from coupling shock. If a similar form of sensitivity exists in more realistic coupled models, our results suggest: (1) perturbation experiments should not be undertaken until after the coupled model control experiment is carried out for several hundred years (thereby minimizing the coupling shock); (2) care should be exercised in the interpretation of regional-scale results (over the ocean) in coupled models which employ flux adjustments; (3) care should also be taken in interpreting even global-scale diagnostics in flux adjusted models for large perturbations about the present climate. Received: 15 November 1996 / Accepted: 4 June 1997  相似文献   

Improved methods for solving for the magnetic field in E-polarization induction problems with fixed and staggered grids     

J. T. Weaver  X. H. Pu  A. K. Agarwal 《Geophysical Journal International》1996,126(2):437-446

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The effect of sea-ice extent in the North Atlantic on the stability of the thermohaline circulation in global warming experiments   总被引：1，自引：1，他引：1  

O. A. Saenko  M. Eby  A. J. Weaver 《Climate Dynamics》2004,22(6-7):689-699

Different climate models simulate different behavior of the Atlantic meridional overturning circulation (MOC) under the same global warming scenario. We propose a plausible explanation for this and argue that a proper simulation of the present-day climate in the subpolar North Atlantic is important. This is illustrated using results from idealized global warming experiments, in which both the radiative forcing scenario and the model employed are the same, with the only major difference being the initial subpolar North Atlantic climate. The initial conditions are made progressively colder, with more extensive sea-ice cover in the northern North Atlantic.The key result is that starting from conditions which are too cold in the North Atlantic and with sea-ice that is too extensive leads to an MOC that is more stable to the radiative forcing. Furthermore, under considerably underestimated sea surface temperatures in subpolar regions, the MOC can even intensify. A reduction of freshwater flux associated with the reduction of sea-ice melt is shown to be important for such unusual behavior of the MOC. Other mechanisms are also considered, but not deemed as important in explaining published inter-model differences.  相似文献   

Phases greater than 90° in MT data: Analysis using dimensionality tools     

F.E.M. Lilley  J.T. Weaver 《Journal of Applied Geophysics》2010,70(1):9-16

An example of magnetotelluric data is discussed which has distinctive phase values out of quadrant, and for which the Z_xx element of the measured tensor is greater than the Z_xy element in both real and quadrature parts. Mohr circle plots and principal value decompositions of the real and quadrature parts of the tensor clarify its understanding. An analysis of rotational invariants of the data suggests a case of galvanic distortion of a 2D structure. Results from phase tensor analysis are included, and the example is seen to be a dramatic instance of phase tensor analysis reducing an apparently 3D example to 2D characteristics.The Mohr circles of this example do not capture the axes origin, although there are phases out of quadrant. Extra intrigue is thus added to the question, noted in Lilley, F.E.M., 1998. Magnetotelluric tensor decomposition: part I, theory for a basic procedure. Geophysics 63, 1885–1897 and arising independently in Caldwell, T.G., Bibby, H. M., Brown, C., 2004. The magnetotelluric phase tensor. Geophys. J. Int. 158, 457–469 and Bibby, H.M., Caldwell, T.G., Brown, C., 2005. Determinable and non-determinable parameters of galvanic distortion in magnetotellurics. Geophys. J. Int. 163, 915–930, whether the determinant values taken separately of the real and quadrature parts of a magnetotelluric tensor should both never be negative.For data whose Mohr circles do not capture the axes origin, simple conditions are derived regarding phase. These conditions govern whether or not it is formally possible for observed phases of Z_xy to exceed 90°, for any rotation of the observing axes.  相似文献