Temperature Dependence Of Fraction Research Articles

Mechanism of NO-SCR by methane over Co,H-ZSM-5 and Co,H-mordenite catalysts

Results of X-ray photoelectron spectroscopic (XPS) examination and temperature-programmed reduction measurements by H2 (H2-TPR) showed that the Co-zeolite catalysts, which were found most active in the selective catalytic reduction of NO by methane to N2 in the presence of excess O2 (NO-SCR), contain both Co2+/[Co-OH]+/H+ exchange cations, Co-oxo species and cobalt oxide clusters. Using operando Diffuse Reflectance Infrared Fourier Transform Spectroscopic method (DRIFTS method) the NO-SCR reaction was shown to proceed in consecutive steps via bifunctional mechanism over active sites (i) promoting the oxidation of NO by O2 to NO2 (NO-COX reaction), and sites (ii) whereon disproportionation and charge separation of 2NO2 generates activated surface intermediate NO3−/NO+ ion pair. Latter process was found to require Co2+ zeolite cations. The NO-COX reaction was shown to proceed over Co-oxo species and cobalt oxide, if present, and also over Brønsted acid sites but at a significantly lower rate. In the reaction of methane and the NO3−/NO+ ion pair CO2, H2O, and N2 was formed and the active Co2+ sites were recovered (CH4/NO-SCR reaction). The surface concentration of the NO3−/NO+ ion pair must have been controlled by the relative magnitude of the apparent rate constants of the consecutive NO-COX and CH4/NO-SCR reactions. Below about 700K reaction temperature latter reaction governed the rate of the consecutive NO reduction process. Above about 700K combustion became the main reaction of methane. Because of the low equilibrium NO2 concentration at these high temperatures the NO-COX reaction took over the control over the rate of the NO-SCR process. Under steady state reaction conditions a temperature-dependent fraction of the Co2+ active sites was always poisoned by adsorbed H2O formed in the CH4 oxidation reaction.

In-situ Fe isotope ratio determination in Fe–Ti oxides and sulfides from drilled gabbros and basalt from the IODP Hole 1256D in the eastern equatorial Pacific

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe–Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (>300°C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower δ56Fe values by 0.2‰–0.5‰ as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals.We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of <0.1‰ (2σ level) at micrometer-scale. We have found significant variations of the δ56FeIRMM-014 values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of δ56Femagnetite in 1256D rocks ranges from −0.12 to +0.64‰, and of δ56Feilmenite from −0.77 to +0.01‰. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive δ56Fe values between +0.29 and +0.56‰, whereas pyrite in the other samples has generally negative δ56Fe values from −1.10 to −0.59‰.One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite–ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including β-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the δ56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite–ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures.Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Biology and air–sea gas exchange controls on the distribution of carbon isotope ratios (δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C) in the ocean

Abstract. Analysis of observations and sensitivity experiments with a new three-dimensional global model of stable carbon isotope cycling elucidate processes that control the distribution of δ13C of dissolved inorganic carbon (DIC) in the contemporary and preindustrial ocean. Biological fractionation and the sinking of isotopically light δ13C organic matter from the surface into the interior ocean leads to low δ13CDIC values at depths and in high latitude surface waters and high values in the upper ocean at low latitudes with maxima in the subtropics. Air–sea gas exchange has two effects. First, it acts to reduce the spatial gradients created by biology. Second, the associated temperature-dependent fractionation tends to increase (decrease) δ13CDIC values of colder (warmer) water, which generates gradients that oppose those arising from biology. Our model results suggest that both effects are similarly important in influencing surface and interior δ13CDIC distributions. However, since air–sea gas exchange is slow in the modern ocean, the biological effect dominates spatial δ13CDIC gradients both in the interior and at the surface, in contrast to conclusions from some previous studies. Calcium carbonate cycling, pH dependency of fractionation during air–sea gas exchange, and kinetic fractionation have minor effects on δ13CDIC. Accumulation of isotopically light carbon from anthropogenic fossil fuel burning has decreased the spatial variability of surface and deep δ13CDIC since the industrial revolution in our model simulations. Analysis of a new synthesis of δ13CDIC measurements from years 1990 to 2005 is used to quantify preformed and remineralized contributions as well as the effects of biology and air–sea gas exchange. The model reproduces major features of the observed large-scale distribution of δ13CDIC as well as the individual contributions and effects. Residual misfits are documented and analyzed. Simulated surface and subsurface δ13CDIC are influenced by details of the ecosystem model formulation. For example, inclusion of a simple parameterization of iron limitation of phytoplankton growth rates and temperature-dependent zooplankton grazing rates improves the agreement with δ13CDIC observations and satellite estimates of phytoplankton growth rates and biomass, suggesting that δ13C can also be a useful test of ecosystem models.

Sorption of antibiotic sulfamethoxazole varies with biochars produced at different temperatures

Sorption of sulfonamides on biochars is poorly understood, thus sulfamethoxazole (SMX) sorption on biochars produced at 300–600 °C was determined as a function of pH and SMX concentration, as well as the inorganic fractions in the biochars. Neutral SMX molecules (SMX0) were dominant for sorption at pH 1.0–6.0. Above pH 7.0, although biochars surfaces were negatively-charged, anionic SMX species sorption increased with pH and is regulated via charge-assisted H-bonds. SMX0 sorption at pH 5.0 was nonlinear and adsorption-dominant for all the biochars via hydrophobic interaction, π–π electron donor–acceptor interaction and pore-filling. The removal of inorganic fraction reduced SMX sorption by low-temperature biochars (e.g., 300 °C), but enhanced the sorption by high-temperature biochars (e.g., 600 °C) due to the temperature-dependent inorganic fractions in the biochars. These observations are useful for producing designer biochars as engineered sorbents to reduce the bioavailability of antibiotics and/or predict the fate of sulfonamides in biochar-amended soils.

Experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600 to 800 °C

Magnesium isotopes are potentially powerful tracers for high-temperature geochemical processes if relevant fractionation factors are known. However, experimental data for Mg isotope fractionation are lacking at high temperatures. We performed experiments at 600, 700, and 800°C and 1GPa to establish the equilibrium magnesium isotope partitioning between forsterite (Mg2SiO4) and magnesite (MgCO3) and between spinel (MgAl2O4) and magnesite, making use of the carbonate as an isotope exchange medium to overcome sluggish diffusion-limited magnesium isotope exchange between spinel and forsterite. Using the three-isotope method, the magnitudes of exchange between forsterite and magnesite, and between spinel and magnesite, were determined at the three temperatures for varying lengths of time, allowing equilibrium isotope partitioning to be established. Results are as follows: Δ26MgFo–Mgs=0.04±0.04‰ at 800°C, 0.11±0.10‰ at 700°C, and 0.44±0.10‰ at 600°C; and Δ26MgSpl–Mgs=0.90±0.28‰ at 800°C, 1.10±0.27‰ at 700°C, and 1.73±0.38‰ at 600°C. From these experimentally determined equilibrium fractionation values, we derive the temperature-dependent equilibrium fractionation between spinel and forsterite by difference, yielding Δ26MgSpl–Fo values of 0.86±0.29‰ at 800°C, 0.99±0.29‰ at 700°C, and 1.29±0.39‰ at 600°C. These data agree well with first-principles estimates of equilibrium magnesium isotope fractionation between spinel and forsterite at high temperatures. The data allow the calculation of an experimentally determined equation for the temperature dependence of 26Mg/24Mg fractionation between spinel and olivine: Δ26MgSpl–Fo=0.96(±0.21)×106/T2. This first high-T experimental calibration of Mg isotope fractionation of mantle minerals is consistent with expectations based on the crystal chemical environment of Mg in these phases.

Otolith δ18O of Pacific bluefin tuna Thunnus orientalis as an indicator of ambient water temperature

To detect the relationship between ambient temperature and otolith stable oxygen isotope (δ18O), Pacific bluefin tuna Thunnus orientalis larvae were reared at 6 different temperatures (between 23 and 28°C at 35 psu; mean δ18O value, +0.31‰) for 2 to 8 d after hatching. For the first time, otolith δ18O was measured using a continuous-flow isotope ratio mass spectrometry system. The linear relationship between otolith δ18O and temperature was determined as follows: δotolith – δwater = –2.70 (T °C) + 5.193 (r2 = 0.806, p < 0.01). The temperature-dependent fractionation of otolith δ18O was close to that reported for inorganic aragonite, indicating that the vital effects of isotopes are small. The otolith δ18O values of juveniles reared at a mean of 26.5°C were not significantly different from those of larvae reared at 26°C. With regard to the 2 pairs of otoliths within an individual fish, the differences in δ18O values between the left and right otoliths were subtle. The otolith δ18O values of larvae reared in δ18O-depleted seawater (24 to 26°C at 32 psu; δ18O, −0.17‰) were lower, but were not significantly different from those of larvae reared in +0.31‰ water, which is the δ18O value of the water in which the spawners were raised. These results suggest that the sample size for the 32 psu experiment was too small and that the rearing durations were too short to affect the otoliths completely. Our results demonstrate that otolith δ18O values of Pacific bluefin tuna larvae can provide precise and accurate estimates of their ambient temperature experience.

Finite element simulation of butt welded 2·25Cr–1·6W steel pipe incorporating bainite phase transformation

This article presents a newly developed global optimisation method for the finite element simulation of welding process considering bainite transformation. In this method, the pattern search algorithm was applied to determine kinetic parameters in Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation during a continuous cooling process. Meanwhile, the JMAK equation was modified into an explicit form as a function of welding temperature field to improve calculation efficiency in the optimisation process. This methodology improves the accuracy as calculating the temperature dependent volume fraction of bainite transformation in finite element simulation. The calculated welding residual stresses considering phase transformation effects exhibited better agreement with the measured results than those calculated without phase transformation. The influences of variable cooling rates on welding residual stresses were also investigated.

Finite element thermal stress analysis of solid oxide fuel cell cathode microstructures

Two-dimensional images of solid oxide fuel cell (SOFC) cathode microstructures (50:50 wt.% LSM:YSZ) are used to generate three-dimensional finite element (FE) models. An approximate, heuristic scheme is developed to derive a microstructure of 30:70 wt.% LSM:YSZ composition from the original, real microstructures. The derived model is validated by calculating three-phase boundary (TPB) and phase surface area densities by comparing with data available in the literature. Construction of such derived microstructures will provide insights on the effects of phase compositions on the mechanics of electrode structures. The heuristic scheme is not proposed as a replacement for rigorous approaches such as the random packing model, but rather as a simplified approach for deriving reasonably realistic microstructures of different compositions within a limited range of validity. The models are analyzed using finite elements to estimate thermal stresses and probability of failure using Weibull analysis. The effects of temperature-dependent material properties and phase volume fractions on probability of failure of the cathode are discussed.

Stable carbon and oxygen isotopic compositions of wood ash: an experimental study with archaeological implications

Wood ash, composed mainly of the mineral calcite, is an important component in many archaeological sites. Identification of wood ash in the archaeological record is often difficult due to mixing of ash with other calcitic components of geogenic origin and/or due to diagenetic changes. A recent empirical study using the stable isotope compositions of carbon (δ13C) and oxygen (δ18O) in wood ash enabled the identification of mixtures of wood ash with geogenic calcite and to follow diagenetic changes due to partial dissolution and re-precipitation of ash in two prehistoric cave sites in Israel. Little however is known about the processes responsible for the isotopic compositions of wood ash in relation to formation at various temperatures and the influence on isotopic composition of ash from a variety of plant species. Here we present an experimental study of wood ash formed by burning three C3 tree species and one C4 desert bush at different temperatures. The results indicate that there are significant differences in the isotopic compositions of carbon and oxygen between wood ash that forms by combustion at a relatively low temperature (500 °C) and at a higher temperature (900 °C). In addition, we show that the isotopic composition of carbon and oxygen in high temperature wood ash approaches equilibrium over a period of several months and that the carbon isotopic composition of low temperature wood ash may reflect the photosynthetic pathway of the burnt woody species. Lastly, we show that the isotopic compositions obtained from wood ash prepared at different temperatures do not reflect a temperature dependent fractionation process, but a mixing line between calcite that formed by low temperature combustion and calcite formed by high temperature combustion which later underwent re-carbonation with atmospheric CO2. In addition, we suggest that exchange processes may possibly occur during combustion between decomposing calcium-oxalate and atmospheric O2, CO2 and CO. The archaeological implications of this study are discussed in relation to identification of wood ash in the archaeological record, identification of fuel sources and burning temperatures, and diagenetic changes expected in karstic cave environments. The method presented here can be applied at any archaeological site.

Lithium defects and diffusivity in forsterite

Lithium is an important geochemical tracer used to infer the thermal and chemical evolution of minerals in the Earth’s upper mantle. Knowledge of point defect chemistry and diffusion is critical for the interpretation of Li distribution in minerals. Using quantum mechanical methods we show that in forsterite Li will be incorporated as bound interstitial–substitutional pairs. Furthermore, there will be temperature dependent fractionation of its two isotopes between the different sites. The fractionation decreases dramatically from 87.1‰ at 300K to 1.0‰ at 3000K. Diffusion is predicted to occur via two inter-related mechanisms: Mg–Li exchange, and a second, vacancy assisted interstitial mechanism. This behaviour is complex, facilitates migration of the heavier isotope and offers insights into observations of Li mobility and zoning in olivine, the most volumetrically important upper mantle mineral.

Relaxation mechanisms in magnetic colloids studied by stroboscopic spin-polarized small-angle neutron scattering

Stroboscopic small-angle neutron scattering by using polarized neutrons is presented as a technique for investigations of relaxation phenomena on a nanometer scale. Applying a cyclic perturbation to the sample by an oscillating magnetic field allows the polarization-dependent scattering cross-sections to be extracted as time-dependent response. The potential of this special type of ``physical'' contrast variation technique is demonstrated with three examples of ferrofluids of different particle sizes and viscosities. The dynamics of magnetic moment reorientations turned out to be governed by different time scales. For a temperature-dependent fraction of individual mobile particles, a frequency-dependent phase shift and damping of the intensity modulation have been precisely determined and quantitatively explained using a Debye-like relaxation model. The characteristic relaxation times, \ensuremath{\tau}, in the liquid state of all samples scale with the particle volume and viscosity confirming a Brownian-relaxation mechanism. For the remaining fraction of particles that form locally ordered domains the orientation distribution of the dynamically arrested static moments has been quantified by an order parameter ${S}_{1}$${}^{\mathrm{arr}}$, which is not accessible with nonpolarized neutrons. In a high viscous Co and a magnetite-based ferrofluid, ${S}_{1}$${}^{\mathrm{arr}}$ corresponds to maximal alignment of the static moments in the liquid state and to a gradual decrease at lower temperatures when the solvent is frozen. In a low viscosity Co-ferrofluid almost no preferred alignment of the arrested particle moments was observed. In the magnetite-based ferrofluid with the highest dipolar attraction energy an additional phase shift has been detected which increases with frequency and magnitude of the magnetic field. This indicates clearly that the set-up of local particle ordering in distorted hexagonal arrangement is delayed with respect to the moment alignment in the time varying magnetic field.

Exploring the calcium isotope signature of Arctica islandica as an environmental proxy using laboratory- and field-cultured specimens

The calcium-isotope composition (δ44/40Ca) of the aragonitic bivalve Arctica islandica grown in laboratory and field cultures was investigated in terms of environmental and biological controls to explore its potential as a palaeoceanographic proxy. While we found no significant effect of salinity on Ca-isotope fractionation, the relationship between 44Ca/40Ca ratios of A. islandica and temperature was best explained by a generalised additive model, which describes the temperature versus Ca-isotope relationship as a nonlinear, second-degree polynomial with an optimum temperature of 15°C. Since 44Ca/40Ca ratios clearly increase with increasing temperature up to 15°C, the temperature versus Ca-isotope relation can be approximated through a linear fit up to 15°C. The slope of this linear temperature dependent Ca-isotope fractionation resembles previous calibrations of inorganic and biogenic aragonites, providing insights into fractionation processes during bivalve biomineralization. Our findings suggest that the observed biologically-induced Ca-isotope fractionation occurs when Ca is transported from seawater to the site of mineralization, most likely during the passage through the epithelial mantle. The observed evidence for strong vital effects indicates limited use of Ca isotopes in A. islandica as a temperature proxy.

Experimental study of the role of physicochemical surface processing on the IN ability of mineral dust particles

Abstract. During the measurement campaign FROST 2 (FReezing Of duST 2), the Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to investigate the influence of various surface modifications on the ice nucleating ability of Arizona Test Dust (ATD) particles in the immersion freezing mode. The dust particles were exposed to sulfuric acid vapor, to water vapor with and without the addition of ammonia gas, and heat using a thermodenuder operating at 250 °C. Size selected, quasi monodisperse particles with a mobility diameter of 300 nm were fed into LACIS and droplets grew on these particles such that each droplet contained a single particle. Temperature dependent frozen fractions of these droplets were determined in a temperature range between −40 °C ≤T≤−28 °C. The pure ATD particles nucleated ice over a broad temperature range with their freezing behavior being separated into two freezing branches characterized through different slopes in the frozen fraction vs. temperature curves. Coating the ATD particles with sulfuric acid resulted in the particles' IN potential significantly decreasing in the first freezing branch (T&gt;−35 °C) and a slight increase in the second branch (T≤−35 °C). The addition of water vapor after the sulfuric acid coating caused the disappearance of the first freezing branch and a strong reduction of the IN ability in the second freezing branch. The presence of ammonia gas during water vapor exposure had a negligible effect on the particles' IN ability compared to the effect of water vapor. Heating in the thermodenuder led to a decreased IN ability of the sulfuric acid coated particles for both branches but the additional heat did not or only slightly change the IN ability of the pure ATD and the water vapor exposed sulfuric acid coated particles. In other words, the combination of both sulfuric acid and water vapor being present is a main cause for the ice active surface features of the ATD particles being destroyed. A possible explanation could be the chemical transformation of ice active metal silicates to metal sulfates. The strongly enhanced reaction between sulfuric acid and dust in the presence of water vapor and the resulting significant reductions in IN potential are of importance for atmospheric ice cloud formation. Our findings suggest that the IN concentration can decrease by up to one order of magnitude for the conditions investigated.

High-temperature Si isotope fractionation between iron metal and silicate

The Si stable isotope fractionation between metal and silicate has been investigated experimentally at 1800, 2000, and 2200 °C. We find that there is a significant silicon stable isotope fractionation at high temperature between metal and silicate in agreement with Shahar et al. (2009). Further we find that this fractionation is insensitive to the structure and composition of the silicate as the fractionation between silicate melt and olivine is insignificant within the error of the analyses. The temperature-dependent silicon isotope fractionation is Δ 30Si silicate–metal = 7.45 ± 0.41 × 10 6/ T 2. We also demonstrate the viability of using laser ablation MC-ICPMS as a tool for measuring silicon isotope ratios in high pressure and temperature experiments.

Experimental determination of carbon isotope fractionation between iron carbide melt and carbon: 12C-enriched carbon in the Earth's core?

We report here new experimental data on equilibrium carbon isotope fractionation between graphite/diamond and iron carbide melt at 5 and 10 GPa and in the temperature range between 1200 and 2000 °C. Carbon isotope equilibrium was tested using morphological features of graphite and also by performing a longer duration experiment, both of which suggested that equilibrium carbon isotope fractionation is present. The results suggest that iron carbide melt will preferentially accumulate 12C rather than 13C. An equilibrium temperature dependent fractionation between iron carbide melt and graphite/diamond is proposed based on the relation Δ 13 C ( gr / dia – iron carbide melt ) = 8 . 85 1 0 6 / T 2 K + 0. 99 . Our results are consistent with the carbon isotope distribution between graphite and cohenite (Fe 3C) observed in iron meteorites. We propose that temperature-dependent fractionation of carbon isotopes between iron carbide melt and graphite/diamond might have created a “ 12C-enriched core” with a significant difference in the distribution of carbon isotopes between the carbon in the metallic core and bulk silicate Earth during the accretion and differentiation of early Earth. Recent findings of low δ 13C carbonados and diamonds of deep mantle origin supports the presence of a 12C enriched source. The possible presence of a reservoir of 12C-enriched carbon in the Earth's core implies that it can generate large perturbations in the surface and shallow carbon-isotope system by the flux of lighter carbon from the core–mantle boundary.

Reorientational Dynamics of the Dodecahydro-closo-dodecaborate Anion in Cs2B12H12

Rapid reorientational motions of the B(12)H(12)(2-) icosahedral anion, a key intermediate in borohydride dehydrogenation, are revealed by quasielastic neutron scattering (QENS) measurements of Cs(2)B(12)H(12) between 430 and 530 K. At 430 K, over the range of momentum transfers collected, the elastic incoherent structure factor (EISF) is consistent with a model for reorientational jumps about a single molecular axis. At temperatures of 480 K and higher, however, the EISF suggests the emergence of multiaxis reorientation by dynamically similar, independent jumps about two axes, on average, preserving crystallographic order. Alternatively, if one assumes that the anions are undergoing temperature-dependent rotational trapping, then the EISF is also consistent with a jump model involving a temperature-dependent mobile fraction of anions statistically tumbling between discrete crystallographic sites. Although neutron vibrational spectra demonstrate that the anion torsional modes soften dramatically with increasing temperature, the QENS-derived activation energy of 333 meV for reorientation clearly shows that the anions are not undergoing isotropic rotational diffusion.

The Medieval Climate Anomaly in Greenland ice core data

Temperature signals in ice core data Greenland ice core data can be used to derive information on past climatic conditions in the Greenland area. Recently, Vinther et al. (2010) demonstrated a high correlation between δ18O data and Greenland coastal temperature observations even on seasonal timescales, reaffirming the ability of ice core δ18O to capture local temperatures, first proposed by Dansgaard (1954). The Vinther et al. (2010) analysis of the seasonal ice core δ18O data showed that the winter δ18O contains the strongest temperature signal, and that even annual average Greenland temperatures are more accurately captured in the winter δ18O rather than annual average δ18O data. This surprising fact is partly explained by the observation that Greenland winter temperatures are much more variable than summer temperatures and thus dominate the annual average variability (Vinther et al., 2010). Past Greenland climatic conditions can also be derived directly from Greenland ice core borehole temperature observations, by solving the equation of heat conduction in moving ice and firn (Dahl-Jensen et al., 1998). Using a Monte Carlo inversion technique, Dahl-Jensen et al. (1998) derived temperature histories from both southern and central Greenland from high precision borehole temperature measurements obtained at the DYE-3 and GRIP ice core drill sites. A third way of estimating past Greenland temperature conditions is to measure isotopes on nitrogen and argon trapped in air-bubbles within the ice (Severinghaus et al., 1998). These gases undergo a temperature dependent mass fractionation in the snow column before the air-bubbles are formed in the ice, thus retaining a retrievable temperature signal.

The role of the glassy state in production and storage of freeze-dried starter cultures

The inactivation rates k of the model microorganism F19 (Chr. Hansen A/S, Hørsholm, Denmark) as well as the spin- spin relaxation time, T2,were determined in the area around the glass transition temperature Tg. By keeping cell-sugar lyophilisates at defined temperatures and water activities, storage above and below Tg could be realized. In order to characterize the temperature dependence of T2 and k, the results were fitted using the Williams-Landel-Ferry (WLF) and Arrhenius models. T2 was found to be solely dependent on the temperature interval between Tg and the storage temperature, Tp. In case of inactivation kinetics this temperature interval could not fully explain the results. In contrast to molecular mobility, the microbial inactivation was supposed to be influenced additionally by the absolute storage temperature. This is due to fact that microbial inactivation is a highly complex process which is dependent on a number of physico-chemical reactions with some being more while some are less diffusion limited. In terms of inactivation kinetics and molecular mobility, Tg did not act as an absolute threshold but could divide the results into weakly and strongly temperature-dependent fractions. Within the freeze-drying process the glassy state of the sample matrix did not show a protective effect.

Coupled silicon–oxygen isotope fractionation traces Archaean silicification

Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (−0.64‰<δ30Si<−0.01‰ and +8.6‰<δ18O<+11.9‰) to silicified basalts (δ30Si and δ18O values as high as +0.81‰ and +15.6‰, respectively). Cherts generally have positive isotope ratios (+0.21‰<δ30Si<+1.05‰ and +10.9<δ18O<+17.1), except two cherts, which have negative δ30Si values, but high δ18O (up to +19.5‰).The pronounced positive correlations between δ30Si, δ18O and SiO2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18O and 30Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.

Zinc Isotopic Composition of Particulate Matter Generated during the Combustion of Coal and Coal + Tire-Derived Fuels

Atmospheric Zn emissions from the burning of coal and tire-derived fuel (TDF) for power generation can be considerable. In an effort to lay the foundation for tracking these contributions, we evaluated the Zn isotopes of coal, a mixture of 95 wt % coal + 5 wt % TDF, and the particulate matter (PM) derived from their combustion in a power-generating plant. The average Zn concentrations and δ(66)Zn were 36 mg/kg and 183 mg/kg and +0.24‰ and +0.13‰ for the coal and coal + TDF, respectively. The δ(66)Zn of the PM sequestered in the cyclone-type mechanical separator was the lightest measured, -0.48‰ for coal and -0.81‰ for coal+TDF. The δ(66)Zn of the PM from the electrostatic precipitator showed a slight enrichment in the heavier Zn isotopes relative to the starting material. PM collected from the stack had the heaviest δ(66)Zn in the system, +0.63‰ and +0.50‰ for the coal and coal + TDF, respectively. Initial fractionation during the generation of a Zn-rich vapor is followed by temperature-dependent fractionation as Zn condenses onto the PM. The isotopic changes of the two fuel types are similar, suggesting that their inherent chemical differences have only a secondary impact on the isotopic fractionation process.