Effects of Drying on the Low-Frequency Electrical Properties of Tournemire Argillites

Nearly water-saturated argillite samples (initial water content near 3.4 wt%) were cored from an undisturbed area of an underground facility of the French Institute for Radioprotection and Nuclear Safety (IRSN), located at Tournemire (Aveyron, France). These samples were subjected to the following desiccation path: (a) A desaturation phase during which the samples were dried at ambient temperature conditions, relative humidity equal to 43% in average and (b) a heating phase during which the same samples were heated at four temperature levels from 70°C up to 105°C. During both phases, the low-frequency complex resistivity (0.18Hz–12 kHz) was recorded by a four-electrode device. The amplitude of the complex resistivity was extremely sensitive to water content change. At the end of the isotherm desaturation phase, it was multiplied by a factor of 3 to 5. During the heating phase, the resistivity increased by more than two orders of magnitude compared to the initial state. The percentage of Frequency Effect shows a low sensitivity to water content changes during the desaturation stage while it increased by two orders of magnitude during the heating phase. This result confirms that low-frequency spectral signature is extremely sensitive to textural changes (i.e., thermal-induced microcracking in this case) that occurred during heating. Moreover, the complex resistivity of the samples shows a strong anisotropy (a ratio of 10 between both amplitudes measured in the perpendicular directions). The classical Cole-Cole model cannot be used to fit the experimental data obtained in the heating phase. A generalized formulation of this model is required and was successfully applied to represent the complex resistivity data.     

Modeling the risk of robbery in the city of Tshwane,South Africa

ABSTRACT

In this study, we model the risk of robbery in the City of Tshwane in South Africa. We use the collective knowledge of two prominent spatial theories of crime (social disorganization theory, and crime pattern theory) to guide the selection of data and employ rudimentary geospatial techniques to create a crude model that identifies the risk of future robbery incidents in the city. The model is validated using actual robbery incidences recorded for the city. Overall the model performs reasonably well with approximately 70% of future robbery incidences accurately identified within a small subset of the overall model. Developing countries such as South Africa are in dire need of crime risk intensity models that are simple, and not data intensive to allocate scarce crime prevention resources in a more optimal fashion. It is anticipated that this model is a first step in this regard.     

A computer simulation study of natural silicate melts. Part II: High pressure properties

The thermodynamic, structural and transport properties of natural silicate melts under pressure are investigated by molecular dynamics simulation with the help of a force field recently introduced by us [Guillot B. and Sator N. (2007) A computer simulation study of natural silicate melts. Part I: low pressure properties. Geochim. Cosmochim. Acta71, 1249-1265]. It is shown that the simulation reproduces accurately the bulk moduli of a large variety of silicate liquids as evaluated from ultrasonic studies. The equations of state (EOS) of the simulated melts are in good agreement with the density data on mid-ocean ridge basalt, komatiite, peridotite and fayalite as obtained either by sink/float experiments or by shock-wave compression. From the structural point of view it is shown that the population of ^[5]Al and ^[6]Al species increases rapidly upon initial compression (0-50 kbar) whereas for Si these highly coordinated species are found in a significant abundance (>5%) only above ∼50 kbar for ^[5]Si and ∼100-150 kbar for ^[6]Si. This increase of the coordination of network formers is not the only response of the melt structure to the densification: there is also a large redistribution of the T-O-T (T = Si, Al) bond angles with the pressure and noticeably upon initial compression in rhyolitic and basaltic liquids. Furthermore, a detailed analysis of the population of bridging oxygens (BO) and nonbridging oxygens (NBO) points out that the polymerization of the melt generally increases when the pressure increases, the magnitude of this polymerization enhancement being all the more important that the melt is depolymerized at low pressure. The role of triclusters (threefold coordinated oxygens to network former cations) is particularly emphasized in acidic and basaltic liquids. The pressure-induced redistribution of the oxygen atoms through the melt structure is also stressed. Finally, the simulation predicts a nonmonotonic behavior of the diffusivity of network former ions when the pressure increases, a feature with depends on the melt composition. This could have a counterpart in the electrical conductivity at sufficiently high temperature when the viscosity of the liquid is low.     

Projecting future drought in Mediterranean forests: bias correction of climate models matters!

Global and regional climate models (GCM and RCM) are generally biased and cannot be used as forcing variables in ecological impact models without some form of prior bias correction. In this study, we investigated the influence of the bias correction method on drought projections in Mediterranean forests in southern France for the end of the twenty-first century (2071–2100). We used a water balance model with two different atmospheric climate forcings built from the same RCM simulations but using two different correction methods (quantile mapping or anomaly method). Drought, defined here as periods when vegetation functioning is affected by water deficit, was described in terms of intensity, duration and timing. Our results showed that the choice of the bias correction method had little effects on temperature and global radiation projections. However, although both methods led to similar predictions of precipitation amount, they induced strong differences in their temporal distribution, especially during summer. These differences were amplified when the climatic data were used to force the water balance model. On average, the choice of bias correction leads to 45 % uncertainty in the predicted anomalies in drought intensity along with discrepancies in the spatial pattern of the predicted changes and changes in the year-to-year variability in drought characteristics. We conclude that the choice of a bias correction method might have a significant impact on the projections of forest response to climate change.     

Ocean feedback to tropical cyclones: climatology and processes

This study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air–sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air–sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect against storm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air–sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa °C^?1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics.     

Seasonal variability of the equatorial undercurrent termination and associated salinity maximum in the Gulf of Guinea

The termination of the Equatorial Undercurrent (EUC) in the eastern equatorial Atlantic during boreal summer and fall, and the fate of the associated saline water masses, are analyzed from in situ hydrological and currents data collected during 19 hydrographic cruises between 2000 and 2007, complemented by observations from Argo profiling floats and PIRATA moorings, and from a numerical simulation of the Tropical Atlantic Ocean for the period 1993–2007. An intense variability of the circulation and hydrological properties is evidenced from observations in the upper thermocline (24.5–26.2 isopycnal layer) between June and November. During early boreal summer, saline water masses are transported eastward in the upper thermocline to the African coast within the EUC, and recirculate westward on both sides of the EUC. In mid-boreal summer, the EUC weakens in the upper thermocline and the equatorial salinity maximum disappears due to intense mixing with the surface waters during the upwelling season. The extra-equatorial salinity maxima are also partially eroded during the boreal summer, with a slight poleward migration of the southern hemisphere maximum until late boreal summer. The upper EUC reappears in September, feeding again the eastern equatorial Atlantic with saline waters until boreal spring. During December–January, numerical results suggest a second seasonal weakening of the EUC in the Gulf of Guinea, with a partial erosion of the associated equatorial salinity maximum.     

Lipid biomarkers for anaerobic oxidation of methane and sulphate reduction in cold seep sediments of Nyegga pockmarks (Norwegian margin): discrepancies in contents and carbon isotope signatures

Distributions and carbon isotopic compositions of microbial lipid biomarkers were investigated in sediment cores from the G11 and G12 pockmarks in the Nyegga sector of the Storegga Slide on the mid-Norwegian margin to explore differences in depth zonation, type and carbon assimilation mode of anaerobic methane-oxidizing archaea (ANMEs) and associated sulphate-reducing bacteria responsible for anaerobic oxidation of methane (AOM) in these cold seep environments. While the G11 site is characterised by black reduced sediments colonized by gastropods and Siboglinidae tubeworms, the G12 site has black reduced sediments devoid of fauna but surrounded by a peripheral occurrence of gastropods and white filamentous microbial mats. At both sites, bulk sediments contained abundant archaeal and bacterial lipid biomarkers substantially depleted in ¹³C, consisting mainly of isoprenoidal hydrocarbons and dialkyl glycerol diethers, fatty acids and non-isoprenoidal monoalkylglycerol ethers. At the G11 site, down-core profiles revealed that lipid biomarkers were in maximum abundance from 10 cm depth to the core bottom at 16 cm depth, associated with δ¹³C values of ?57 to ?136‰. At the G12 site, by contrast, lipid biomarkers were in high abundance in the upper 5 cm sediment layer, associated with δ¹³C values of ?43 to ?133‰. This suggests that, as expected from the benthic fauna characteristics of the sites, AOM takes place mainly at depth in the G11 pockmark but just below the seafloor in the G12 pockmark. These patterns can be explained largely by variable fluid flow rates. Furthermore, at both sites, a dominance of ANME-2 archaea accompanied by their bacterial partners is inferred based on lipid biomarker distributions and carbon isotope signatures, which is in agreement with recently published DNA analyses for the G11 pockmark. However, the present data reveal high discrepancies in the contents and δ¹³C values for both archaeal and bacterial lipid profiles, implying the possible involvement of at least two distinct AOM-related microbial consortia at the inferred AOM depth zonation of G11 and G12 pockmark sediments. In both sediment cores, the δ¹³C profiles for most archaeal lipids suggest a direct assimilation of dissolved inorganic carbon (DIC) in addition to methane by ANMEs (chemoautotrophy); constant and highly depleted δ¹³C profiles for PMI:3, an archaeal lipid biomarker presumably related to ANME-2, suggest a direct assimilation of ¹³C-depleted methane-derived carbon via AOM (methanotrophy). Evidently, the common approach of investigating lipid biomarker contents and δ¹³C signatures in cold seep sediments does not suffice to precisely discriminate between the carbon assimilation mode for each ANME archaeal group and associated bacteria. Rather, this needs to be combined with further specific labelling studies including different carbon sources (methane carbon, methane-derived organic intermediates and DIC) in order to unravel the metabolic pathways of each microbial consortium involved in AOM (ANME-1 vs. ANME-2 vs. ANME-3 archaeal group and associated bacteria).     

Crustal‐scale fluid flow during the tectonic evolution of the Bighorn Basin (Wyoming,USA)

Stable isotope measurements (O, C, Sr), microthermometry and salinity measurements of fluid inclusions from different fracture populations in several anticlines of the Sevier‐Laramide Bighorn basin (Wyoming, USA) were used to unravel the palaeohydrological evolution. New data on the microstructural setting were used to complement previous studies and refine the fracture sequence at basin scale. The latter provides the framework and timing of fluid migration events across the basin during the Sevier and Laramide orogenic phases. Since the Sevier tectonic loading of the foreland basin until its later involvement into the Laramide thick‐skinned orogeny, three main fracture sets (out of seven) were found to have efficiently enhanced the hydraulic permeability of the sedimentary cover rocks. These pulses of fluid are attested by calcite crystals precipitated in veins from hydrothermal (T > 120 °C) radiogenic fluids derived from Cretaceous meteoric fluids that interacted with the Precambrian basement rocks. Between these events, vein calcite precipitated from formational fluids at chemical and thermal equilibrium with surrounding environment. At basin scale, the earliest hydrothermal pulse is documented in the western part of the basin during forebulge flexuring and the second one is documented in basement‐cored folds during folding. In addition to this East/West diachronic opening of the cover rocks to hydrothermal pulses probably controlled by the tectonic style, a decrease in ^87/86Sr values from West to East suggests a crustal‐scale partially squeegee‐type eastward fluid migration in both basement and cover rocks since the early phase of the Sevier contraction. The interpretation of palaeofluid system at basin scale also implies that joints developed under an extensional stress regime are better vertical drains than joints developed under strike‐slip regime and enabled migration of basement‐derived hydrothermal fluids.     

SARAL/AltiKa Global Statistical Assessment and Cross-Calibration with Jason-2

The CNES/ISRO mission SARAL/AltiKa was successfully launched on 25 February 2013. It reached its nominal orbit on 13 March 2013. AltiKa is the first altimeter using the Ka-band frequency. This article presents the results of the calibration and validation activities perfromed on the first year of the SARAL/AltiKa mission. The main objective of the article is to assess the SARAL/AltiKa data quality and to estimate the altimeter system performance using GDR products. To achieve this goal, we present mono-mission metrics and compare them with Jason-2 over the same period. Even if these missions do not have the same ground track, precise comparisons are still possible. They allow assessing parameter discrepancies and SSH consistency between both missions in order to detect geographically correlated biases, jumps or drifts. These results show that SARAL/AltiKa data quality is excellent: ocean data coverage is greater than 99.5%, standard deviation at cross-overs is 5.4 cm. The mission therefore fulfills the requirements of high precision altimetry and can be used (in conjunction with Jason-2) to monitor the global mean sea level, ensuring the continuity of the record over ERS/Envisat historical ground track. Possible improvements and open issues are also identified, foreseeing an even better mission performance.