Quantification of the ferric/ferrous iron ratio in silicates by scanning transmission X-ray microscopy at the Fe L2,3 edges

Estimation of Fe³⁺/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe³⁺/ΣFe ratio in silicates using Fe L_2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe³⁺/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe³⁺ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L₃-edge. This method allows mapping of Fe³⁺/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L₂ maximum for Fe³⁺, which is compared to the total integral intensity of the Fe L₂-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe³⁺/ΣFe ratios at the submicrometre scale in geological samples.     

Estimating phytolith influx in lake sediments

So far, no phytolith extraction protocols have been tested for accuracy and repeatability. Here we aim to display a phytolith extraction method combining the strengths of two widely used protocols, supplemented with silica microspheres as exogenous markers for quantifying phytolith concentrations. Phytolith concentrations were estimated for samples from two sedimentary sequences in which numerical age–depth models make it possible to calculate phytolith influxes (phytolith numbers per cm²per yr). Analysis of replicates confirmed the statistical robustness, the repeatability and the very few biases of our extraction technique for small phytoliths, since the relationship between grass silica short cells and microspheres was kept stable. Furthermore, we demonstrated that silica microspheres are robust exogenous markers for estimating phytolith concentrations. The minimum number of items (i.e., phytoliths plus silica microspheres) that must be counted to estimate phytolith concentrations and thus influxes depends on the ratio of phytoliths to microspheres (R) and is minimized when R = 1. Nevertheless, we recommend using ratios R ≤ 1 in order to avoid having the counting process become excessively time-consuming, because microspheres are easier to identify and count than phytoliths.     

Criteria for the identification of ventifacts in the geological record: A review and new insights

Ventifacts (wind-worn stones) are typical of terrestrial environments remained very long without any vegetation, under hot or cold climates. Therefore, within the sedimentary record, they can allow recognizing desert conditions, even where no aeolian dune deposits are preserved. There seems that, in the recent literature, pebbles and cobbles from various palaeoenvironments were mistaken for ventifacts. This may partly be explained by the scatter and relative scarcity of illustrations to which refer. The aim of this paper is to help recognizing ventifacts in the sedimentary record, based on a critical review of the diagnostic properties generally used, and on new studies permitting to suggest additional criteria. After an evaluation of the sedimentary contexts favourable to preserving ventifacts, the distinctive characters that could be seen on each one are treated in order of increasing alteration of the original appearance: surface features, medium-scale features (new types of pit especially), and general shape. Finally, the problem of distinguishing between ventifacts and aquafacts is approached.     

Dust motions in quasi-statically charged binary asteroid systems

In this paper, we discuss dust motion and investigate possible mass transfer of charged particles in a binary asteroid system, in which the asteroids are electrically charged due to solar radiation. The surface potential of the asteroids is assumed to be a piecewise function, with positive potential on the sunlit half and negative potential on the shadow half. We derive the nonautonomous equations of motion for charged particles and an analytic representation for their lofting conditions. Particle trajectories and temporary relative equilibria are examined in relation to their moving forbidden regions, a concept we define and discuss. Finally, we use a Monte Carlo simulation for a case study on mass transfer and loss rates between the asteroids.     

Simulation Study of a Robust Algorithm for Soil Moisture and Surface Roughness Estimation Using L‐Band Radar Backscatter

Abstract

Various inversion algorithms have been developed to obtain estimates of soil moisture and surface roughness parameters from multifrequency, multiangle, and multipolarization radar reflectances. Since the penetration depth for radar signals increases with wavelength, an inversion algorithm using widely separated frequencies does not yield comparable probing depths. Furthermore, existing algorithms assume a linear relationship between the radar backscatter coefficient (in dB) and soil parameters, such as the volumetric soil moisture, soil surface roughness and surface slope. This assumption is valid only over a narrow range of soil parameters, thereby restricting its operational use under realistic conditions. Our research specifically explored the use of inversion algorithms based on L‐Band radar reflectances at 1 GHz and 2 GHz frequencies in order to retain relatively consistent probing depths. In order to extend the range of applicability, a non‐linear exponential‐type relationship was developed between radar reflectance at a specified frequency, polarization and incidence angle combination, and soil parameters of interest, viz., soil moisture, surface roughness, and surface slope. An over‐constrained inversion algorithm using a six‐parameter combination was found to yield relatively accurate estimates of soil parameters over a wide range of soil conditions even in the presence of system error.     

A Multilevel Analysis of Agricultural Trade and Socioeconomic Inequality in Rural Mozambique

This study uses a mixed methodology to investigate the relationship between agricultural trade and socioeconomic inequality in Mozambique at multiple geographical scales. The quantitative analysis examines all rural regions of the country and a nationally representative sample of rural households, whereas the qualitative analysis focuses primarily on two case study villages. Research findings suggest that social and geographical context greatly influence the relationships between agricultural trade and regional inequality. Trade-related effects on economic inequality—and the social status of traders—vary across Mozambican regions. Results suggest that trade increases inequality in regions where traders have low social status, but reduces inequality in areas where they have high social status. These findings suggest that social and economic factors work together to shape landscapes of inequality.     

Non-stationary frequency analysis of heavy rainfall events in southern France

Abstract

Heavy rainfall events often occur in southern French Mediterranean regions during the autumn, leading to catastrophic flood events. A non-stationary peaks-over-threshold (POT) model with climatic covariates for these heavy rainfall events is developed herein. A regional sample of events exceeding the threshold of 100 mm/d is built using daily precipitation data recorded at 44 stations over the period 1958–2008. The POT model combines a Poisson distribution for the occurrence and a generalized Pareto distribution for the magnitude of the heavy rainfall events. The selected covariates are the seasonal occurrence of southern circulation patterns for the Poisson distribution parameter, and monthly air temperature for the generalized Pareto distribution scale parameter. According to the deviance test, the non-stationary model provides a better fit to the data than a classical stationary model. Such a model incorporating climatic covariates instead of time allows one to re-evaluate the risk of extreme precipitation on a monthly and seasonal basis, and can also be used with climate model outputs to produce future scenarios. Existing scenarios of the future changes projected for the covariates included in the model are tested to evaluate the possible future changes on extreme precipitation quantiles in the study area.

Editor Z.W. Kundzewicz; Associate editor K. Hamed

Citation Tramblay, Y., Neppel, L., Carreau, J., and Najib, K., 2013. Non-stationary frequency analysis of heavy rainfall events in southern France. Hydrological Sciences Journal, 58 (2), 280–294.     

Soil CO2 flux trends with differences in soil moisture among four types of land use in an Ecuadorian páramo landscape

Abstract

Páramo grasslands are important carbon sinks in the Ecuadorian Andes. Although carbon content of páramo Andisols is correlated with high water retention, the effects of differences in soil moisture under different types of land use on soil carbon processes have not been explicitly tested in the Ecuadorian Andes. This study assessed the relationship between soil moisture and soil CO₂ flux among a mature páramo grassland, recently burned páramo grassland, native montane forest, and pine plantation in an Ecuadorian páramo landscape. Soil CO₂ flux was greater in the forest sites compared to the grassland sites. Thus, a shift from grassland to forest cover may have significant implications for soil carbon loss via CO₂ flux. Our results suggest that although soil moisture plays a significant role in differences of soil CO₂ flux rates among land-use types, more investigation into mechanisms for soil carbon loss and how they are driven by land-use change is needed. To our knowledge, these are the first soil CO₂ flux rates reported for the Ecuadorian páramo.     

Linear and weakly nonlinear internal wave theories applied to “morning glory” waves

Abstract

Detailed comparisons are made between the predictions of Benjamin's weakly nonlinear theory for internal solitary waves in fluids of great depth, with observational data on solitary wave-type disturbances in the lower atmosphere associated with the “morning glory” phenomenon.

It is shown that, while the theory is not wholly unreasonable, neither is it completely satisfactory. In particular, although the calculated wave speeds are generally close to those observed, they are no improvement on those based on linear long wave theory; at the same time the predicted wave half-widths are too large by a factor of two to three. The limitations of the theory appear to be associated with the requirement that wave half-widths are much less than the total fluid depth, a condition not satisfied in the atmospheric case. However, the alternative theory for shallow fluids, based on the Korteweg-de Vries equation is found to be even more unsuitable.

Our analyses highlight some of the problems in comparing theory with observations and bring to the fore some of the present limitations of the data for such purposes.     

A system dynamics approach for urban water reuse planning: a case study from the Great Lakes region

Water reclamation and reuse practices are recently receiving growing attention due to increasing water scarcity, concerns about the effect of wastewater discharges on receiving water, and availability of high-performing and cost-effective water reuse technologies. However, incorporation of water reuse schemes into water/wastewater infrastructure systems is a complex decision making process, involving various economical, technological, and environmental criteria. System dynamics (SD) allows modeling of complex systems and provides information about the temporal and feedback behavior of the system. In this sense, a SD model of the existing water/wastewater system in Kalamazoo-Michigan, an urban area in the Great Lakes region, was created with the hypothetical incorporation of water reuse. The model simulates and optimizes the overall water system cost (including water, wastewater and water reuse components), accounting for future scenarios of population, economic growth and climate change. Results indicate significant levels of water reuse after an infrastructure build delay. The model also indicates that a decision to implement water reuse yields remarkably lower water withdrawals and lower water treatment costs even in a location with a relatively abundant water supply like Kalamazoo. This study emphasizes the fact that a true understanding of the practice of water reuse cannot be achieved without taking regional and climatic parameters into account.