Crystal field and charge transfer spectrum of (Mg, Fe)SiO3 majorite     

Hans Keppler  Catherine A. McCammon 《Physics and Chemistry of Minerals》1996,23(2):94-98

Majorite of bulk composition Mg_0.86Fe_0.15SiO₃ was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO₂ buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm^?1. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm^?1 due to the ⁵E_g → ⁵T_2g transition of Fe²⁺ in a distorted dodecahedral site. (2) A band at 9340 cm^?1 due to the transition ⁵T_2g → ⁵E_g of octahedral Fe²⁺. (3) A band at 22784 cm^?1 resulting from Fe³⁺, probably in an octahedral site (⁶A_1g → ⁴A_1g, ⁴E_g). (4) A very intense system of Fe²⁺ → Fe³⁺ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm^?1. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe³⁺ in two different octahedral sites. The crystal field splitting Δ of Fe²⁺ in dodecahedral coordination is 5717 cm^?1. If a splitting of the ground state in the order of 1000 cm^?1 is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm^?1, comparable to the CFSE of Fe²⁺ in pyrope-rich garnet. However, the splitting of ⁵T_2g is significantly higher than in pyrope. This would be consistent with Fe²⁺ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe²⁺, Δ= 9340 cm^?1 and CFSE=3736 cm^?1, assuming negligible splitting of the ground state.  相似文献   

The Sm-Nd age of Kambalda volcanics is 500 Ma too old!     

Catherine Chauvel  Bernard Dupré  George A. Jenner 《Earth and Planetary Science Letters》1985,74(4):315-324

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Development of a high resolution runoff routing model, calibration and application to assess runoff from the LMD GCM   总被引：3，自引：0，他引：3  

Agns Ducharne  Catherine Golaz  Etienne Leblois  Katia Laval  Jan Polcher  Emmanuel Ledoux  Ghislain de Marsily 《Journal of Hydrology》2003,280(1-4):207-228

Large-scale runoff routing models (RRMs) are important as a validation tool for GCMs, and to close the hydrological cycle in fully-coupled climate models. The model RiTHM was developed to simulate the discharge of large rivers from the total runoff simulated by the LMD GCM. It uses a 1024×800 grid, nested in the 64×50 grid of the LMD GCM. The runoff simulated in a GCM grid cell is uniformly distributed over the underlying cells, where a series of two reservoirs accounts for the delay related to infiltration through the unsaturated zone and aquifers. The resulting riverflow is routed assuming pure translation along the drainage network, extracted with a GIS from a 5 min DEM. The transfer time from a cell to the outlet depends on topography, and on a basin-wide parameter, the time of concentration. RiTHM was calibrated in 11 river basins, using a realistic runoff forcing (computed by the land surface model SECHIBA from reanalyzed meteorological forcing). This led to a very satisfactory reproduction of observed hydrographs. The main problems were related to hydraulic processes neglected in RiTHM (reservoirs, diversion of riverflow because of flooding or irrigation). These results helped to validate SECHIBA, except for its snow processes, shown to be too simple. With the same parameters, RiTHM was also forced with runoff from the LMD GCM. This induced an important degradation of the simulated hydrographs, regarding both volume and timing. It was largely explained by errors in precipitation, and more generally climate, in the GCM. The direct calibration of RiTHM under the GCM-runoff forcing markedly improved the timing of simulated discharge, which could be interesting for land–atmosphere–ocean coupling. This work demonstrated that the usefulness of RRMs for GCMs strongly depends on their adequate calibration.  相似文献   

Thermal convection in a heterogeneous mantle     

Anne Davaille  Michael Le Bars  Catherine Carbonne 《Comptes Rendus Geoscience》2003,335(1):141-156

Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics.  相似文献   

Geographical Linked Data: The Administrative Geography of Great Britain on the Semantic Web   总被引：1，自引：0，他引：1  

John Goodwin  Catherine Dolbear  Glen Hart 《Transactions in GIS》2008,12(Z1):19-30

Ordnance Survey, the national mapping agency of Great Britain, is investigating how semantic web technologies assist its role as a geographical information provider. A major part of this work involves the development of prototype products and datasets in RDF. This article discusses the production of an example dataset for the administrative geography of Great Britain, demonstrating the advantages of explicitly encoding topological relations between geographic entities over traditional spatial queries. We also outline how these data can be linked to other datasets on the web of linked data and some of the challenges that this raises.  相似文献   

The Oxford–Dartmouth Thirty Degree Survey – II. Clustering of bright Lyman break galaxies: strong luminosity-dependent bias at z= 4     

Paul D. Allen  Leonidas A. Moustakas  Gavin Dalton  Emily MacDonald  Chris Blake  Lee Clewley  Catherine Heymans  Gary Wegner 《Monthly notices of the Royal Astronomical Society》2005,360(4):1244-1256

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Occupational paths towards commercial agriculture: The key roles of farm pluriactivity and the commons     

Ward Anseeuw  Catherine Laurent 《Journal of Arid Environments》2007,70(4):659

In South Africa, “black” and “coloured” farmers engaged in several occupational activities are often considered as being “not interested in farming” or “not serious”. However, in Namaqualand the large majority of “coloured” farmers and especially the most successful ones are pluriactive and have off-farm incomes. Investigations were therefore carried out in that region to better understand the role of pluriactivity. Observations focus on middle-income households from the mining sector entering into small-scale commercial farming. Based on surveys in both non-farm and farm sectors, the results show that, for many households, access to sufficient farm assets depends strongly on the employment situation of the sector of origin and requires a level of investment which can be reached only progressively. This obliges these households to rely on off-farm incomes and communal land at least during the early transitional stage. These results stress the importance of taking into account the diversity of paths and of differentiating the various stages in the transitional period between two occupational activities. Finally, the paper emphasises the importance of taking these specific situations into consideration in the management of the commons.  相似文献   

An assessment of coefficient accuracy in linear regression models with spatially varying coefficients   总被引：1，自引：3，他引：1  

David C. Wheeler  Catherine A. Calder 《Journal of Geographical Systems》2007,9(2):145-166

The realization in the statistical and geographical sciences that a relationship between an explanatory variable and a response variable in a linear regression model is not always constant across a study area has led to the development of regression models that allow for spatially varying coefficients. Two competing models of this type are geographically weighted regression (GWR) and Bayesian regression models with spatially varying coefficient processes (SVCP). In the application of these spatially varying coefficient models, marginal inference on the regression coefficient spatial processes is typically of primary interest. In light of this fact, there is a need to assess the validity of such marginal inferences, since these inferences may be misleading in the presence of explanatory variable collinearity. In this paper, we present the results of a simulation study designed to evaluate the sensitivity of the spatially varying coefficients in the competing models to various levels of collinearity. The simulation study results show that the Bayesian regression model produces more accurate inferences on the regression coefficients than does GWR. In addition, the Bayesian regression model is overall fairly robust in terms of marginal coefficient inference to moderate levels of collinearity, and degrades less substantially than GWR with strong collinearity.  相似文献   

Export fluxes of biogenic matter in the presence and absence of seasonal sea ice cover in the Chukchi Sea     

Catherine Lalande  Jacqueline M. Grebmeier  Paul Wassmann  Lee W. Cooper  Mikhail V. Flint  Valentina M. Sergeeva 《Continental Shelf Research》2007

Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll-a (Chl-a), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition (δ¹³C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m⁻² d⁻¹) and in the absence of ice cover (442±203 mg C m⁻² d⁻¹) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m⁻² d⁻¹) than in the presence of ice cover (44±29 mg C m⁻² d⁻¹) at the EHS stations. The C/N ratios and δ¹³C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl-a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl-a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions.  相似文献   

Present and past bio-available phosphorus budget in the ultra-oligotrophic Lake Brienz     

Beat Müller  David Finger  Michael Sturm  Volker Prasuhn  Thomas Haltmeier  Peter Bossard  Catherine Hoyle  Alfred Wüest 《Aquatic Sciences - Research Across Boundaries》2007,69(2):227-239

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