An imaging,tunable magneto-optical filter

The Imaging, Tunable Magneto-Optical Filter (ITMOF) is a one- or two-cell magnetic birefringence filter designed to measure the Doppler shift of the solar potassium line (770 nm) with respect to a laboratory standard. Two gas vapor cells contain isotopically refined potassium and operate at temperature near 393 K. Hot cell windows are employed in a carefully controlled thermal environment to limit spurious birefringence in the pyrex cell and prevent condensation in the light path. Electromagnets provide a variable strength and direction longitudinal magnetic field of up to 5000 G on each cell. There is no rotating quarter-wave plate or other moving parts. The final image is detected with a CCD camera system.     

High-temperature carbon reduction of silica: A novel approach for oxygen isotope analysis of biogenic opal

A new technique has been developed for the determination of the oxygen isotope composition from biogenic silica. The iHTR method (inductive high temperature carbon reduction) is based on the reduction of silica by carbon with temperatures of up to 1830°C (maximum T 2200°C) to produce carbon monoxide for isotope analysis. Basically, samples of silica are mixed with graphite and filled into a sample holder made of a glassy carbon rod. The rod is introduced into a glassy carbon cylinder liner closed at the top which itself is enclosed by a double-walled glass vessel. The glassy carbon rod is inductively heated under vacuum to the temperature needed for quantitative conversion of the particular silica material to CO. The most critical process of dehydration (in the case of opal) and reduction to CO is routinely achieved in the iHTR device. Weakly bound oxygen and oxygen-containing contaminants, like hydroxyl groups, as well as remaining minor organic constituents are volatilized stepwise under high vacuum at temperatures of 850°C and 1050°C without isotopic exchange before the reduction of the silica. After completion of dehydration, the temperature is raised to the value needed for silica reduction. For both biogenic silica and quartz a temperature of 1550°C was found to be adequate. The technical design with a standard preparation routine and various test experiments is presented proving the reliability and capability of the new iHTR method, especially with respect to fresh diatom materials and diatom opal. The amount of sample material necessary at present is ∼1.5 mg of silica and the reproducibility achieved for natural samples is better than ±0.15‰. Replicate analysis of the quartz standard NBS28 resulted in a δ¹⁸O value of 9.62‰ ± 0.11‰ (n = 17).     

Distribution of clay minerals in surface sediments of the Aegean Sea: a compilation

Extensive published data sets and some new data on the clay mineral composition of surface sediments in the Aegean Sea, northeastern Mediterranean Sea, have been compiled in order to map the distribution patterns of clay mineral assemblages and to decipher source areas and transport paths. We distinguished six provinces, a Northwest Aegean Province, a Marmara-Dardanelles Province, a West Turkey Province, a Southeast Aegean Province, a Kithira Province and a Central Aegean Province. The clay mineral assemblages in the coastal and shelf areas carry the signature of the riverine sediment discharge from southeast Europe and Turkey, respectively. The southern Aegean Sea is probably influenced by the River Nile discharge and transport of clay minerals by surface currents. The clay minerals in the central Aegean Sea form a mixed assemblage comprising components of the other assemblages. A dispersion and dilution of clay minerals by surface currents is obvious.     

Quantification of the hydraulic diffusivity of a bentonite-sand mixture using the water head decrease measured upon sudden flow interruption

This paper presents a new modelling approach to quantify the hydraulic diffusivity of low-permeability unconsolidated porous media under confined saturated-flow conditions in the laboratory. The derived analytical solution for the transient variation of the hydraulic head after flow interruption was applied to experimental data obtained from continuous measurements of the water pressure at two locations in the soil column. Three soil samples made of a mixture of natural bentonite (at different mass fractions) and medium sand were studied during a series of stepwise constant flow rates of water. The numerical results well fit the experimentally measured decrease of the dimensionless hydraulic head. The study shows that the increase of the mass fraction of bentonite in the soil sample from 10 to 30% is accompanied by a strong decrease of the hydraulic diffusivity from 2.4 × 10⁻² to 1.1 × 10⁻³ m² s⁻¹, which is clearly due to the decrease of the hydraulic conductivity of the soil sample. The specific storages obtained for each of the three samples are in the same order of magnitude and seem to decrease with the increase of mass fraction of bentonite. However, they clearly reflect the predominant portion of the compressibility of the porous medium compared with that of water. Compared with reported literature values for clayey soils, the specific storage values in this study are slightly higher, varying within the range of 2 × 10⁻³ to 8.1 × 10⁻³ m⁻¹._. The experimental results also give insight into the limitations of the modelling approach. In the case of low-permeability soils (K < 2 × 10⁻⁶ ms⁻¹) and steady-flow conditions with low Reynolds numbers, for example, Re < 0.003, it is recommended to choose a time duration for flow interruption between subsequent flow rate steps of longer than 5 s. For high-permeability porous media, to increase the precision of the quantified hydraulic diffusivity, it might be useful to select a measuring frequency significantly higher than 1 Hz.     

Time scales of DNAPL migration in sandy aquifers examined via numerical simulation

The time required for dense nonaqueous phase liquid (DNAPL) to cease migrating following release to the subsurface is a valuable component of a site conceptual model. This study uses numerical simulation to investigate the migration of six different DNAPLs in sandy aquifers. The most influential parameters governing migration cessation time are the density and viscosity of the DNAPL and the mean hydraulic conductivity of the aquifer. Releases of between 1 and 40 drums of chlorinated solvent DNAPLs, characterized by relatively high density and low viscosity, require on the order of months to a few years to cease migrating in a heterogeneous medium sand aquifer having an average hydraulic conductivity of 7.4 x 10(-3) cm/s. In contrast to this, the release of 20 drums of coal tar (rho(D)= 1061 kg/m(3), micro(D)= 0.161 Pa.s) requires more than 100 years to cease migrating in the same aquifer. Altering the mean hydraulic conductivity of the aquifer results in a proportional change in cessation times. Parameters that exhibit relatively little influence on migration time scales are the DNAPL-water interfacial tension, release volume, source capillary pressure, mean aquifer porosity, and ambient ground water hydraulic gradient. This study also demonstrates that low-density DNAPLs (e.g., coal tar) give rise to greater amounts of lateral spreading and greater amounts of pooling on capillary barriers than high-density DNAPLs such as trichloroethylene or tetrachloroethylene.