Water Concentration Gradient Research Articles

Extraction of Thickness and Water-Content Gradients in Hydrogel-Based Water-Backed Corneal Phantoms Via Submillimeter-Wave Reflectometry

Absolute thickness and free-water-content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature and aqueous backing were extracted via coherent submillimeter-wave reflectometry at 220–330 GHz. Fourier-domain-based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and 0.1°, respectively, for signal-to-noise ratios (SNRs) between 40 and 50 dB. Total 12 phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical-coherence tomography-based sphericity metrics via image segmentation. The extracted free-water volume fraction varied less than 5% in the five phantoms whose fabrication yielded the most spherical geometry. Submillimeter-wave-based thickness accuracy was better than 111 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m (∼λ/9) with an average of 65 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m (∼λ/17) and standard deviation of 44 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m (∼λ/25) for phantoms with physiologically relevant geometry. Monte-Carlo simulations of measurement noise and uncertainty limits agree with the experimental data analysis and indicate a lower thickness accuracy limit of 33 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m, and water-content sensitivities of 0.5% and 11.8% for the anterior and posterior segments, respectively. Numerical analysis suggests that the measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produces statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique, which is able to extract both the thickness and water-content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.

Convective Transport of Ce3+ in Nafion® Membrane

Proton exchange membrane fuel cells (PEMFC) are often loaded with various metal cations (Ce3+, Co2+, etc.) within the ion-conducting membrane. These proton-displacing cations can be introduced at beginning-of-life (BOL) for chemical degradation mitigation purposes or may passively accumulate over run time via Pt-alloy catalyst dissolution or other contamination sources.1 Within the proton conducting ionomer phase of the membrane electrode assembly (MEA), the metal cations ion pair with the sulfonate anions of the polymer. During operation over less than 100 hours, the metal cations are observed to migrate or redistribute on cm-length scales in the plane of the MEA. The movement of cations within the MEA plane is governed by cation charge, ionic radius, local water content (λ = n H2O/-SO3 -) and water content gradients (Δλ). Previous studies have shown that the cation isotropic concentration-gradient diffusion coefficients vary strongly with the membrane hydration level.2 The resulting redistribution of cations can have a significant impact on cell performance and mitigation of chemical degradation processes. Accordingly, there is great interest in understanding and quantifying the factors that govern cation movement to optimize fuel cell efficiency and durability.We will report the in-plane convective migration of Ce3+ and Co2+ in Nafion® membranes at 80 °C under the influence of controlled λ gradients. The cation migration studies were performed using a two-chamber RH cell as shown in Figure 1a. The test design can investigate the migration of both discreet (positions 2-4) and continuous uniform cation distributions (position 1), allowing a comprehensive understanding of the migration physics.Figure 1b depicts the convective migration of Ce3+ in Nafion® membrane (NR211, 12 mol%) over 288 hours of treatment in the two-chamber cell at 80 °C. Significant net migration of Ce3+ from the wet (95%RH) to the dry chamber (50% RH) is observed, including wet-side Ce3+ depletion and dry-side concentration. In addition to the convective migration, isotropic concentration gradient diffusion coefficient measurements were obtained from positions 3 and 4 (Figure 1a).Using the fundamental diffusion coefficients determined at various RH values, a robust 1-D transient model of the convective transport has been developed that captures experimental observations of cation migration across a wide range of membrane hydration gradients and cation concentrations. References A. Kusoglu and A. Weber, Chem Rev. 117, 987, (2017)A. M. Baker, S. K. Babu, K. Chintam, A. Kusoglu, R. Mukundan, and R. L. Borup, ECS Trans., 92 (8), 429 (2019) Figure 1. Depiction of the convective cation migration apparatus and representative data. The two-chamber device (a) accommodates four test samples that can support a range of t=0 cation distributions. Experimental 80°C time series data of a uniform 12 mol% Ce3+ Nafion® sample subjected to 95 and 50% RH in the wet and dry chambers, respectively. Figure 1

Anomalous Electronic and Protonic Conductivity of 2D Titanium Oxide and Low‐Temperature Power Generation Using Its Protonic Conduction

AbstractIn this study, a titanium‐oxide film is fabricated by stacking 2D nanosheets with a thickness of 1 nm layer‐by‐layer. The electrical properties of the titanium‐oxide film are investigated under various temperatures (25–300 °C) and a water vapor atmosphere. As a result, it is found that protonic conduction is dominant at low temperatures (&lt;100 °C), while electronic conduction is highest at high temperatures (&gt;200 °C). The electrical conductivity is about one billion times higher than that of conventional 3D titanium‐oxide. Protons, dominant charge carriers at low temperature, are transported along the pathway between layers of nanosheets. The protonic conductivity is as high as 10−4−5 S cm−1 due to the increased interfacial density of the film. This anomalous electronic and protonic conduction is expected to be available in various applications. In this study, the developed 2D titanium‐oxide with high protonic conductivity is employed in the solid electrolyte of a solid oxide fuel cell using water as a fuel. A competitive power density of ≈0.22 mW cm−2 is obtained at low temperature (75 °C) under a water concentration gradient.

True driving force and characteristics of water transport in osmotic membranes

Diffusion cannot be a major water transport mechanism in osmotic membranes because of the lack of true water concentration gradient within the membrane. Due to the semipermeable property of osmotic membranes, water concentration in the membrane is virtually constant because of the absence of salts. The recently confirmed porous structure of the skin layer of osmotic membranes cannot support the basis to exclude bulk water flow in the membrane as assumed in the classic solution-diffusion model. Herein we demonstrate that the concentration difference of water at the membrane-solution interface manifests itself as a negative hydraulic pressure in the membrane. Hence, the only possible driving force for water movement in osmotic membranes is hydraulic pressure gradient. Osmotically driven membrane processes are characterized with negative pressure within the membrane below the water vapor pressure, inevitably leading to the formation of vapor or small bubbles within the membrane matrix. This phenomenon is expected to markedly reduce the effectiveness of osmotic pressure as a driving force for water transport. Delineation of the breakdown and possible restoration of water continuity under negative pressure is essential for proper understanding of the principles governing water transport in osmotic membranes.

A Computational Model of Water Migration in a Closed System of Unsaturated Expansive Clay

Expansive clay behaves like mechanical performances of multi-fissures, and deterioration with the changing moisture. To ensure the safe operation of foundation, the effects of intensive water migration on earth constructions founded on semiarid area, have therefore been received much more attention together with the global warming and climate changes. Herein, analytical models considering respectively liquid water and water vapor migration were presented to address changes in moisture content of unsaturated expansive clay under various water content gradients. Moreover, measured data obtained from migration experiments performed under various water content gradients in a closed system, were utilized to check the validity of analytical equations. Results indicate that the migration of both water vapor and mixed water shows nonlinear relationships with time and location. For the expansive clay with low moisture content, major components of migration water was water vapor, which accounted for 83.61–88.62%; while for the clay with higher water content, the liquid water was dominant migration of water due to more free water around the soil particles. The proposed model is helpful to investigate moisture migration law in the unsaturated expansive clay.

Age-Dependent Changes in Total and Free Water Content of In Vivo Human Lenses Measured by Magnetic Resonance Imaging.

PurposeTo use magnetic resonance imaging (MRI) to measure age-dependent changes in total and free water in human lenses in vivo.MethodsSixty-four healthy adults aged 18 to 86 years were recruited, fitted with a 32-channel head receiver coil, and placed in a 3 Tesla clinical MR scanner. Scans of the crystalline lens were obtained using a volumetric interpolated breath-hold examination sequence with dual flip angles, which were corrected for field inhomogeneity post-acquisition using a B1-map obtained using a turbo-FLASH sequence. The spatial distribution and content of corrected total (ρlens) and free (T1) water along the lens optical axis were extracted using custom-written code.ResultsLens total water distribution and content did not change with age (all P > 0.05). In contrast to total water, a gradient in free water content that was highest in the periphery relative to the center was present in lenses across all ages. However, this initially parabolic free water gradient gradually developed an enhanced central plateau, as indicated by increasing profile shape parameter values (anterior: 0.067/y, P = 0.004; posterior: 0.050/y, P = 0.020) and central free water content (1.932 ms/y, P = 0.022) with age.ConclusionsMRI can obtain repeatable total and free water measurements of in vivo human lenses. The observation that the lens steady-state free, but not total, water gradient is abolished with age raises the possibility that alterations in protein-water interactions are an underlying cause of the degradation in lens optics and overall vision observed with aging.

Evidence of foliar water uptake in a conifer species

Foliar water uptake (FWU) is the main mechanism used by herbaceous plants in arid areas to absorb small amounts of precipitation. It has an important ecological and hydrological effect for alleviating plant drought stress. However, it is unknown whether woody plants can absorb water through the leaves when water absorption via roots is insufficient. This study explored whether FWU exists and the FWU conditions in semi-humid rock mountainous areas. Isotopes were labeled to detect FWU under different soil water content (SWC) gradients (3.9–6.5%, 6.5–9.1%, 9.1–15.6%, 15.6–20.8%, and 20.8–26.0%) and different precipitation gradients (1 mm/h, 5 mm/h, 10 mm/h, and 15 mm/h) in simulated precipitation experiments with indoor potted plants. The results showed that FWU occurred in each treatment if the SWC ≤ 21.9% no matter the precipitation amount. Water absorption via FWU increased with the increase of precipitation intensity but decreased with the increase of SWC. The greatest ratios of FWU were 2.77% and 9.52% of precipitation intensity of 1 mm/h and 15 mm/h, respectively, in the 3.9–6.5% treatment. The precipitation absorbed by the leaves can be transported to the xylem or root system along the water potential gradient of leaves–branches–roots. The precipitation with reverse migration in branches and roots increased with the increase of the water potential gradient of leaves–branches–roots. These findings suggest that Platycladus orientalis can uptake water through roots and leaves, effectively alleviating drought stress. This study provides new insights into water use patterns and water migration in trees.

Watershed spatial heterogeneity of soil saturated hydraulic conductivity as affected by landscape unit in the critical zone

The spatial heterogeneity and related driving factors of the saturated hydraulic conductivity (Ks) in the Earth’s critical zones (CZs) have been documented widely in the literature. However, information regarding the effect of distinct landscape units (i.e., Slope-land, Gully-land, and Tableland) on such heterogeneity at the watershed scale in CZs characterized by complex topographic conditions remains scant. In the current study, we collected undisturbed and disturbed soil samples from the surface soil layer (0–5 cm) at 1195 sampling sites across the ShuangChaGou watershed (0.22 km2) in the Chinese Loess Plateau (CLP). According to classical and geostatistical analyses combined with the structural equation model (SEM), the mean Ks followed the sequence Slope-land (0.25 cm·min−1) > Gully-land (0.187 cm·min−1) > All samples (0.186 cm·min−1) > Tableland (0.14 cm·min−1) (p < 0.05), with the Ks values exhibiting moderate variations except for the Gully-land (strong variation). The LogKs spatial variation characteristics (i.e., the anisotropy ratio, effective range, and C/(C0 + C)) considering the influence of topography varied among the landscape units and the entire watershed. The SEM identified that soil bulk density, saturated soil water content, silt content, plan curvature, NDVI, and slope gradient were the controlling factors in sequence affecting the Ks variation for All samples (AS). In contrast, factors controlling Ks variations were different under the Tableland, Gully-land, Slope-land, and AS. The magnitudes, spatial variations and controlling factors of the surface Ks were greatly affected by the landscape units. Our study provided a robust assessment of the spatial variation of Ks in CZs involving complicated topographic features, highlighting the utility of dividing distinct landscape units when evaluating the Ks variation. A good understanding of this information is helpful for developing distributed hydrological models and for evaluating water and solute transportation in CZs of the CLP and similar regions around the world.

An x-ray microanalytical method for measuring in vivo element and water concentrations, relating to osmoregulation, in cells and tissues of the posterior eye.

Osmoregulation is critical for cell and tissue survival yet there are relatively few methods available to determine osmotic gradients from water and elemental concentration either in single cells or across multiple cellular layers of tissue. X-ray microanalysis of frozen-hydrated preparations in a scanning electron microscope is one such powerful, sensitive, nondestructive technique. Here we use x-ray microanalysis to quantitatively analyse intracellular element concentrations and oxygen concentrations, as a proxy for water concentrations, in selected individual cells of the posterior eye. Using frozen-hydrated preparations of the retinal complex of chicken eyes, it is shown that structural preservation is sufficient to identify cell layers and individual cells. The quantitative analysis of selected areas in the photoreceptor layer, inner nuclear layer and ganglion cell layer, where specific cell types were known to be present, provided measurements of intracellular element concentrations comparable with the analysis of individual cells. It is also shown that in the cells of the retinal pigment epithelium and outer photoreceptor segments elemental analyses were reasonably consistent at the cellular level in different depth levels of the same sample. Comparison of oxygen concentrations, as a proxy for water concentration, at two accelerating voltages (15 and 5 kV) indicated that at 15 kV oxygen concentration was largely derived from intracellular water. Water concentrations could be calculated and concentrations of diffusible elements (Na, K) could be defined in mmol/L. From the latter it is possible to calculate osmotic concentrations of individual cells and osmotic gradients across the tissue. LAY DESCRIPTION: Understanding many cellular processes, in both healthy and diseased states, depends on knowing how the water content of cells and their surrounding fluids is controlled. The transport of water is generally down its concentration gradient or against the osmotic concentration gradient defined by solutes such as sodium, potassium and chloride dissolved in the water. We have refined a microanalytical method, that detects the x-rays emitted from specific elements when they are bombarded by electrons in a scanning electron microscope, to apply it to the analysis of the retina of the eye. The method facilitates the measurement of the elemental composition, water and osmotic concentration gradients of cells and tissues in the eye, that may be involved in the development of myopia, or short sightedness, a condition that afflicts many people including some 80 - 90% of children in Asia.

Self-Assembled Micelles from Thermoresponsive Poly(methyl methacrylate)-b-poly(N-isopropylacrylamide) Diblock Copolymers in Aqueous Solution

The temperature-dependent phase behavior and self-assembly behavior in aqueous solution of the thermoresponsive amphiphilic diblock copolymer PMMA21-b-PNIPAM283 made of a short permanently hydrophobic poly(methyl methacrylate) block and a long thermoresponsive poly(N-isopropylacrylamide) are studied. Turbidimetry, dynamic light scattering (DLS), and synchrotron small-angle X-ray scattering (SAXS) provide temperature-dependent structure information. A lower critical solution temperature (LCST) behavior with a cloud point TCP = 31.1 °C in dilute solution is observed. Below TCP, spherical core–shell micelles are present, having a small PMMA core and a hydrated PNIPAM shell with a radial gradient of water content. Below TCP, the micelles are interpenetrated and show a weak correlation with each other. As the temperature approaches TCP, the micellar core shrinks and thus, the aggregation number decreases, revealing that the PMMA blocks are still mobile. Moreover, the micellar shell dehydrates above TCP and the micelles notably contract. They form clusters, which grow and transform into large compact aggregates as temperature is raised far above TCP.

Study on Heat and Mass Transfer Performance of Cathode Membrane Humidifier in Fuel Cell System

The experimental study on the heat and mass transfer performance of membrane humidifier in high-power atmospheric fuel cell system cathode side was carried out, and the main factors affecting the performance of membrane humidifier were determined. The mathematical model of heat and mass transfer of membrane humidifier was established to study these factors. Mechanism of influence on membrane humidifier performance is studied with a humidifier heat and mass transfer model. Results show that reducing the operating pressure of the humidifier, increasing the air flow rate, and increasing the intake air temperature are beneficial to increase the diffusion coefficient; reducing the air flow rate and lowering the dry air inlet temperature are beneficial to reducing the water content gradient in the membrane.

Transport and Retention of Sulfidized Silver Nanoparticles in Porous Media: The Role of Air‐Water Interfaces, Flow Velocity, and Natural Organic Matter

AbstractThe sulfidation and aging of silver nanoparticles (Ag‐NPs) with natural organic matter (NOM) are major transformation processes along their pathway in wastewater treatment plants and surface waters. Although soils appear to be a sink for disposed Ag‐NPs, the impact of variable saturation on the transport and retention behavior in porous media is still not fully understood. We studied the behavior of sulfidized silver nanoparticles (S‐Ag‐NPs, 1 mg L−1) in saturated and unsaturated sand columns regarding the effects of (i) the presence of NOM (5 mg L−1) in the aquatic phase on retention, transport, and remobilization of S‐Ag‐NPs and (ii) the distribution and quantity of air‐water and solid‐water interfaces for different flow velocities determined via X‐ray microtomography (X‐ray μCT). Unsaturated transport experiments were conducted under controlled conditions with unit gradients in water potential and constant water content along the flow direction for each applied flux. It was shown that (i) NOM in S‐Ag‐NP dispersion highly increased the NP‐mobility; (ii) differences between saturated and unsaturated transport were increasing with decreasing flux and, consequently, decreasing water contents; (iii) both, solid‐water and air‐water interfaces were involved in retention of S‐Ag‐NPs aged by NOM. Using numerical model simulations and X‐ray μCT of flow experiments, the breakthrough of Ag‐NP could be explained by a disproportional increase in air‐water interfaces and an increasing attachment efficiency with decreasing water content and flow velocity.

Processing kinetics, quality and thermodynamic evaluation of mulberry juice concentration process using Ohmic heating

This study was conducted to investigate the effect of Ohmic heating method (OHM) on different aspects of black mulberry juice concentration process, and results compared with the conventional heating method (CHM). The total phenol content of OHM is 3–4.5 times higher than the CHM. The higher voltage gradient caused less change in pH and total phenol content. The consumed energy by OHM (3.33–3.82MJ/kg water) is 4.6–5.3 times lower than the energy consumption by CHM (17.50MJ/kg water). The concentration performance of OHM is 38–46% more than the CHM. Electrical conductivity increased with increasing the sample concentration ratio and modeled as a function of variables process (phase heating, water content, sample temperature, and voltage gradient). Our results showed that the OHM provided better quality and significantly saved the engineering parameters than the CHM during the concentration process of black mulberry juice.

Explosive Volcanic Eruptions and Spinodal Decomposition: A Different Approach to Deciphering the Tiny Bubble Paradox

AbstractBubbles in magmas drive explosive volcanic eruptions. The spatial distribution of bubble nucleation sites in an ascending, decompressing, and supersaturating magma is one of the primary controls on ash morphologies and volcanic hazards. The mechanism of bubble formation is important because it ultimately determines the spatial distribution of bubbles in the fragmenting magma. The initial nucleation of bubbles in a homogeneous magma is problematical because excessive surface tension pressure in very small, nascent bubbles should drive exsolved volatiles back into the melt. This thermodynamic barrier to bubble viability confounds understanding of homogeneous bubble nucleation, yet very small bubbles form, grow, and ultimately drive explosive volcanic eruptions. We refer to this as “the tiny bubble paradox.” Classical nucleation theory typically explains bubble formation and growth, but we propose that a spectrum of bubble‐forming mechanisms may include both homogeneous nucleation and spinodal decomposition (the spontaneous unmixing of phases by uphill diffusion) as end‐member processes. As spinodal decomposition progresses, regularly sized and regularly spaced quasi‐spherical zones form with increasingly high concentration of dissolved water at the centers. Bubble formation occurs as the concentration of water in the interior of the water‐rich zones approaches 100% and the concentration of melt approaches zero. The presence of a broad, diffuse, concentration gradient of water rather than a narrow water‐melt interface means that there is no surface, per se, for surface tension to arise. This is the crux of the solution of the tiny bubble paradox.

Investigation of Power Performance of a PEM Fuel Cell Using MATLAB Simulation

Fuel cell based power generation systems have gained remarkable interest in this modern age, due to its high conversion efficiency and reliability. Among the different types of fuel cells, PEM fuel cells are achieving more significance due to its fast start up time and low operating temperature. This paper studies the mathematical model of proton exchange membrane of fuel cell (PEMFC) using Matlab/SIMULINK software. The paper consists of the calculation of cell voltage, stack current, ohmic loss, activation loss. This model is used to research the fuel cell behavior and the characteristic of output values at different parameters. The model consists of the cathode gas channel, gas diffuser, catalyst layer, and the membrane. In order to composite shape of the gas diffuser and for its gradient in liquid water content, the gas diffuser is modeled as a series of parallel layers with different porosity. It represents in terms of the physical and thermodynamic parameters of the fuel cell. The curve of polarization is expressed parametrically as a function of the surface over potential. This paper expresses for cathode internal as well as overall effectiveness factors, active fraction of the catalyst layer resistance, catalyst layer, limiting current density, and the slope of the polarization curve.

Secador de camada fixa: uma adaptação para melhorar a secagem de café natural no Brasil

A secagem de café por meio de secadores de camada fixa tem sido amplamente utilizada por cafeicultores brasileiros. Entretanto, frequentemente são relatados elevados gradientes de teor de água formados nestes secadores, bem como o aparecimento de microrganismos na massa de café. Desenvolveu-se o presente trabalho com a finalidade de monitorar a performance destes secadores e seu efeito na qualidade final do produto, comparando à uma adaptação feita em secadores de camada fixa convencional. Durante a secagem foram monitorados em diversos pontos na câmara de secagem as seguintes características: temperatura do ar de secagem, do ar intergranular, do ar de exaustão e do ar ambiente; umidade relativa do ar de secagem, do ar ambiente e do ar de exaustão e teor de água do produto em diferentes pontos e alturas na câmara de secagem (0,12; 0,36 e 0,60 m). Dentre as avaliações do produto seco, foram realizadas: análise de cor e análise microbiológica. De acordo com os resultados obtidos, a secagem com inversão do fluxo de ar resultou em um gradiente de teor de água de 0,4% (b.u.), enquanto a secagem convencional ocasionou um gradiente de 8% (b.u.). Em relação à coloração do produto nos diferentes manejos de secagem, a secagem com inversão resultou em produtos mais claros. Para a análise microbiológica, foram encontrados fungos dos gêneros Cladosporium, Fusarium e Aspergillus, dentre outros. Sendo que a média da incidência destes fungos foi maior nas amostras da secagem convencional que na secagem de camada fixa com inversão do fluxo do ar.

Dissipative Particle Dynamics Modeling of Polyelectrolyte Membrane-Water Interfaces.

Previous experiments of water vapor penetration into polyelectrolyte membrane (PEM) thin films have indicated the influence of the water concentration gradient and polymer chemistry on the interface evolution, which will eventually affect the efficiency of the fuel cell operation. Moreover, PEMs of different side chains have shown differences in water cluster structure and diffusion. The evolution of the interface between water and polyelectrolyte membranes (PEMs), which are used in fuel cells and flow batteries, of three different side-chain lengths has been studied using dissipative particle dynamics (DPD) simulations. Higher and faster water uptake is usually beneficial in the operation of fuel cells and flow batteries. The simulated water uptake increased with the increasing side chain length. In addition, the water uptake was rapid initially and slowed down afterwards, which is in agreement with the experimental observations. The water cluster formation rate was also found to increase with the increasing side-chain length, whereas the water cluster shapes were unaffected. Water diffusion in the membranes, which affects proton mobility in the PEMs, increased with the side-chain length at all distances from the interface. In conclusion, side-chain length was found to have a strong influence on the interface water structure and water penetration rates, which can be harnessed for the better design of PEMs. Since the PEM can undergo cycles of dehydration and rehydration, faster water uptake increases the efficiency of these devices. We show that the longer side chains with backbone structure similar to Nafion should be more suitable for fuel cell/flow battery usage.

Water-in-olivine magma ascent chronometry: Every crystal is a clock

The syneruptive decompression rate of basaltic magma in volcanic conduits is thought to be a critical control on eruptive vigor. Recent efforts have constrained decompression rates using models of diffusive water loss from melt embayments, olivine-hosted melt inclusions and clinopyroxene phenocrysts; however, these techniques are difficult to apply because of the rarity of pyroxene crystals or melt embayments suitable for analysis and the complexities associated with modeling water loss from melt inclusions. We have developed a new magma ascent chronometer based on syneruptive diffusive water loss from olivine phenocrysts.We have found water zonation in every olivine phenocryst we have measured, from explosive eruptions of Seguam, Fuego, Kilauea and Cerro Negro volcanoes. The majority of the olivine phenocrysts were polished to expose a central plane normal to the crystallographic ‘b’ axis and volatile concentration profiles were measured along ‘a’ and ‘c’ axes by secondary ion mass spectrometry (SIMS). Profiles are compared to 1D and 3D finite-element models of diffusive water loss from olivine, whose boundaries are in equilibrium with a melt undergoing closed-system degassing.Least-squares fitting of measured water concentration gradients in olivine to a 1D Monte Carlo model produces constraints on magma decompression rates that are in good agreement with independent constraints from melt embayment studies and modeling of water loss from olivine-hosted melt inclusions at Fuego, Seguam, Kilauea, and Cerro Negro. Such agreement confirms the accuracy and sensitivity of the water-in-olivine chronometer over a range of decompression rates (dP/dt) spanning ~2 orders of magnitude (from 0.007 to 0.45 MPa/s). We find that the assumption of a zero-water boundary condition (in which the water concentration at the edges of the olivine phenocrysts is fixed at 0 ppm throughout their ascent) leads to an overestimation of the decompression rate by an order of magnitude compared to the closed-system degassing boundary condition assumed in our model, thereby highlighting the sensitivity of the water-in-olivine chronometer to the host magma degassing path.At Seguam, a wide range of best-fit values of dP/dt is obtained both from the water-in-olivine chronometer (0.04–0.23 MPa/s) and from melt embayments (0.02–0.13 MPa/s). We find systematically higher dP/dt values in the melt embayments that appear to have stalled or crystallized at the shallowest depths. Together, these observations are suggestive of magma acceleration during ascent.A strength of the water-in-olivine chronometer is the prevalence of olivine in mafic to intermediate magmas. This new technique yields many values of dP/dt from a single eruption, providing insight into the diversity of ascent records carried by the crystal cargo and possibly defining changes in dP/dt through time and space. This data density offers a more detailed window into syneruptive conduit processes than has been possible using other techniques for constraining dP/dt. In theory, each crystal is a clock, with the potential to record variable ascent in the conduit, over the course of an eruption, between eruptions, and among volcanic systems.

State of the in situ Febex test (GTS, Switzerland) after 18 years: a heterogeneous bentonite barrier

The in situ Full-scale Engineered Barriers Experiment or Febex was a full-scale test reproducing the near field of a nuclear waste repository. It was performed in a gallery excavated in granite at the Grimsel Test Site in Switzerland, with a heater whose surface temperature was set to 100°C, simulating the waste canister and a bentonite barrier composed of highly compacted blocks. The test was completely dismantled after 18 years of operation. Numerous samples of bentonite were taken for the on-site determination of dry density and water content. The on-site measurements showed that the physical state of the barrier was very much affected by the processes to which it had been subjected, namely, hydration with the granite groundwater and/or thermal gradient. Although the degree of saturation of the bentonite was overall quite high, there were important water content and dry density gradients everywhere in the barrier, but the gradients were steeper around the heater. These gradients did not impair the performance of the barrier, but imply that the barrier can be irreversibly inhomogeneous.

Estimates of the remineralization and burial of organic carbon in Lake Baikal sediments

Sediment cores collected from several stations throughout Lake Baikal in water depths from 100 m off the Selenga River delta to the deepest basin of the lake (~1640 m), have been analyzed for sedimentary organic carbon, nitrogen, and the remineralized components in pore water. The organic carbon content of surface sediments generally varied from 2.3 to 3.2% by weight, and profiles typically showed an exponential decrease in both organic carbon and nitrogen in the upper 20–30 cm of the sediment column. Steady state models of organic matter diagenesis yield first order decomposition rate constants which range from 0.0009 to 0.022 y−1. The calculated residence times for the metabolizable fraction of the organic matter in these sediments increases roughly with increasing water depth and is on the order of 50–300 years. Pore water concentration profiles were determined for dissolved inorganic carbon, dissolved organic carbon (DOC), methane, and dissolved ammonium. At depth (25–30 cm) methane concentrations ranged from 50 to 800 μmol Lpw−1 and DOC from 400 to 900 μmol Lpw−1. Estimation of carbon recycling rates based upon diffusion along pore water concentration gradients at the sediment-water interface, indicate that combined DOC and methane fluxes generally contribute <15% of the overall turnover of sedimentary organic carbon. Comparisons to Laurentian Great Lakes environments show trends in sediment deposition, organic matter remineralization, and the time scales of carbon recycling across nearly two orders of magnitude with the fraction of organic content buried generally decreasing with decreasing sedimentation rates.