Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures     

Mark A. van Zuilen  Marc Chaussidon  Claire Rollion-Bard  Bernard Marty 《Geochimica et cosmochimica acta》2007,71(3):655-669

Carbonaceous matter occurring in chert deposits of the 3.4-3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ¹³C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ¹³C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ¹³C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer-Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.  相似文献   

Experimental study of D/H isotopic fractionation factor of water adsorbed on porous silica tubes     

Thierry Richard  Lionel Mercury  Marc Massault  Jean-Luc Michelot 《Geochimica et cosmochimica acta》2007,71(5):1159-1169

This paper presents D/H isotopic fractionation factor measurements of water adsorbed on porous silica tubes in isotopic equilibrium with water vapor. The fractionation factor is measured as a function of vapor pressure ranging from 10% to 80% RH (relative humidity) at room temperature. It is shown that the fractionation factor between the adsorption film or nanometrically confined water and vapor is smaller than that between bulk liquid water and vapor. A qualitative analysis relates this deuterium depletion to a modification of the zero-point energy of the water isomers in the adsorbed/confined state. Furthermore, the behavior of the fractionation factor with RH shows two different linear trends. The transition between the two (at 60% RH) may indicate the transition from a two-dimensional adsorbed water to a three-dimensional water network.  相似文献   

Assessing the reliability of the modified three-component spatial autocorrelation technique   总被引：3，自引：0，他引：3  

Andreas Köhler  Matthias Ohrnberger  Frank Scherbaum  Marc Wathelet  Cecile Cornou 《Geophysical Journal International》2007,168(2):779-796

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High-end aerial digital cameras and their impact on the automation and quality of the production workflow     

Nicolas Paparoditis  Jean-Philippe Souchon  Gilles Martinoty  Marc Pierrot-Deseilligny 《ISPRS Journal of Photogrammetry and Remote Sensing》2006,60(6):400

The IGN digital camera project was established in the early 1990s. The first research surveys were carried out in 1996 and the digital camera was first used in production in 2000. In 2004 approximately 10 French departments (accounting for 10% of the territory) were covered with a four-head camera system and since summer 2005 all IGN imagery has been acquired digitally. Nevertheless the camera system is still evolving, with tests on new geometric configurations being continuously undertaken. The progressive integration of the system in IGN production workflows has allowed IGN to keep the system evolving in accordance with production needs. Remaining problems are due to specific camera characteristics such as CCD format, the optical quality of off-the-shelf lenses, and because some production tools are ill-adapted to digital images with a large dynamic range. However, when considering the pros and cons of integrating these images into production lines, the disadvantages are largely balanced by the numerous benefits this technology offers.  相似文献   

An Ensemble Seasonal Forecast of Human Cases of St. Louis Encephalitis in Florida Based on Seasonal Hydrologic Forecasts     

Jeffrey Shaman  Jonathan F. Day  Marc Stieglitz  Stephen Zebiak  Mark Cane 《Climatic change》2006,75(4):495-511

We present a method for the ensemble seasonal prediction of human St. Louis encephalitis (SLE) incidence and SLE virus transmission in Florida. We combine empirical relationships between modeled land surface wetness and the incidence of human clinical cases of SLE and modeled land surface wetness and the occurrence of SLE virus transmission throughout south Florida with a previously developed method for generating ensemble, seasonal hydrologic forecasts. Retrospective seasonal forecasts of human SLE incidence are made for Indian River County, Florida, and forecast skill is demonstrated for 2–4 months. A sample seasonal forecast of human SLE incidence is presented. This study establishes the skill of a potential component of an operational SLE forecast system in south Florida, one that provides information well in advance of transmission and may enable early interventions that reduce transmission. Future development of this method and operational application of these forecasts are discussed. The methodology also will be applied to West Nile virus monitoring and forecasting.  相似文献   

Increasing Aridity is Enhancing Silver Fir Abies Alba Mill.) Water Stress in its South-Western Distribution Limit     

Marc Macias  Laia Andreu  Oriol Bosch  J. Julio Camarero  Emilia Gutiérrez 《Climatic change》2006,79(3-4):289-313

Tree populations located at the geographical distribution limit of the species may provide valuable information about the response of tree growth to climate warming across climatic gradients. Dendroclimatic information was extracted from a network of 10 silver-fir (Abies alba) populations in the south-western distribution limit of the species (Pyrenees, NE Iberian Peninsula). Ring-width chronologies were built for five stands sampled in mesic sites from the Main Range in the Pyrenees, and for five forests located in the southern Peripheral Ranges where summer drought is more pronounced. The radial growth of silver-fir in this region is constrained by water stress during the summer previous to growth, as suggested by the negative relationship with previous September temperature and, to a lesser degree, by a positive relationship with previous end of summer precipitation. Climatic data showed a warming trend since the 1970s across the Pyrenees, with more severe summer droughts. The recent warming changed the climate-growth relationships, causing higher growth synchrony among sites, and a higher year-to-year growth variation, especially in the southernmost forests. Moving-interval response functions suggested an increasing water-stress effect on radial growth during the last half of the 20th century. The growth period under water stress has extended from summer up to early autumn. Forests located in the southern Peripheral Ranges experienced a more intense water stress, as seen in a shift of their response to precipitation and temperature. The Main-Range sites mainly showed a response to warming. The intensification of water-stress during the late 20th century might affect the future growth performance of the highly-fragmented A. alba populations in the southwestern distribution limit of the species.  相似文献   

Alignment Trends of Velocity Gradients and Subgrid-Scale Fluxes in the Turbulent Atmospheric Boundary Layer     

Chad W. Higgins  Marc B. Parlange  C. Meneveau 《Boundary-Layer Meteorology》2003,109(1):59-83

Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model.  相似文献   

Present-day velocity and stress fields of the Amurian Plate from thin-shell finite-element modelling     

Carole Petit  Marc Fournier 《Geophysical Journal International》2005,160(1):358-370

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Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data   总被引：2，自引：0，他引：2  

Elia d'Acremont  Sylvie Leroy  Marie-Odile Beslier  Nicolas Bellahsen  Marc Fournier  Cécile Robin  Marcia Maia  Pascal Gente 《Geophysical Journal International》2005,160(3):869-890

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Using a Computational Grid for Geographic Information Analysis: A Reconnaissance     

Marc P. Armstrong  Mary Kathryn Cowles  Shaowen Wang 《The Professional geographer》2005,57(3):365-375

High performance computing has undergone a radical transformation during the past decade. Though monolithic supercomputers continue to be built with significantly increased computing power, geographically distributed computing resources are now routinely linked using high‐speed networks to address a broad range of computationally complex problems. These confederated resources are referred to collectively as a computational Grid. Many geographical problems exhibit characteristics that make them candidates for this new model of computing. As an illustration, we describe a spatial statistics problem and demonstrate how it can be addressed using Grid computing strategies. A key element of this application is the development of middleware that handles domain decomposition and coordinates computational functions. We also discuss the development of Grid portals that are designed to help researchers and decision makers access and use geographic information analysis tools.  相似文献