Study of celestite flotation efficiency using sodium dodecyl sulfonate collector: factorial experiment and statistical analysis of data

Flotation tests of synthetic mixtures of celestite (SrSO₄) and calcite (CaCO₃) minerals using sodium dodecyl sulfonate as collector were carried out using a factorial experimental design 2³. The independent experimental variables included celestite grade in the feed (50% and 90% SrSO₄), conditioning pH (6.5 and 9) and sodium silicate depressant concentration (10⁻⁵ and 10⁻³ M). All experiments were performed at a constant collector concentration (10⁻⁴ M) and conditioning time of 15 min. The response variables were recovery and grade of SrSO₄ in the concentrate.The factors that had the greatest effect on the grade and recovery of celestite were the celestite grade in the feed (L) and the depressant concentration (C); also the combination of these factors significantly affected the response variables. The highest celestite yield (96% recovery, 98% grade) was obtained when a 90% of celestite grade in the feed was used.     

Geochemistry of shallow ground water in coastal plain environments in the southeastern United States: implications for aquifer susceptibility

Ground-water chemistry data from coastal plain environments have been examined to determine the geochemical conditions and processes that occur in these areas and assess their implications for aquifer susceptibility. Two distinct geochemical environments were studied to represent a range of conditions: an inner coastal plain setting having more well-drained soils and lower organic carbon (C) content and an outer coastal plain environment that has more poorly drained soils and high organic C content. Higher concentrations of most major ions and dissolved inorganic and organic C in the outer coastal plain setting indicate a greater degree of mineral dissolution and organic matter oxidation. Accordingly, outer coastal plain waters are more reducing than inner coastal plain waters. Low dissolved oxygen (O₂) and nitrate (NO₃⁻) concentrations and high iron (Fe) concentrations indicate that ferric iron (Fe (III)) is an important electron acceptor in this setting, while dissolved O₂ is the most common terminal electron acceptor in the inner coastal plain setting.The presence of a wide range of redox conditions in the shallow aquifer system examined here underscores the importance of providing a detailed geochemical characterization of ground water when assessing the intrinsic susceptibility of coastal plain settings. The greater prevalence of aerobic conditions in the inner coastal plain setting makes this region more susceptible to contamination by constituents that are more stable under these conditions and is consistent with the significantly (p<0.05) higher concentrations of NO₃⁻ found in this setting. Herbicides and their transformation products were frequently detected (36% of wells sampled), however concentrations were typically low (<0.1 μg/L). Shallow water table depths often found in coastal plain settings may result in an increased risk of the detection of pesticides (e.g., alachlor) that degrade rapidly in the unsaturated zone.     

Atmospheric palaeo-CO2 estimates based on the carbon isotope and stomatal data of Cheirolepidiaceae from the Lower Cretaceous of the Jiuquan Basin,Gansu Province

The stable carbon isotope compositions and the stomatal parameters (stomatal density and stomatal index) of four Cheirolepidiaceae species, Brachyphyllum ningxiaensis, Brachyphyllum obtusum, Pseudofrenelopsis dalatzensis and Pseudofrenelopsis gansuensis, were analyzed to recover the late Early Cretaceous atmospheric CO₂ levels. The fossil plants were collected from 5 consecutive sedimentary members of the uppermost Zhonggou Formation. Based on the stomatal data, the estimated palaeo-atmospheric CO₂ concentrations in the Jiuquan Basin during the late Early Cretaceous were 1060–882 ppmv based on the carboniferous standardization and were 641–531 ppmv based on the recent standardization; the pCO₂ values present at first a decreasing and then an increasing trend within the sedimentary time of the five members. The δ¹³Cp values based on the 21 Brachyphyllum specimens showed a large variation, which ranged from −20.98‰ to −25.69‰, with an average of −24.2‰. The values also identified a C3 photosynthetic pathway for the Brachyphyllum specimens. The predicted δ¹³Ca values varied from −2.1‰ to −6.38‰, with an average of −5.03‰. These two proxies were irregular within the different members; therefore, the correlation with the change in atmospheric CO₂ concentrations was not significant. Moreover, a water-stressed environment was proposed based on the δ¹³C values of the present fossil plants, a proposal that was also supported by the previous palaeobotanical, palynological and stratigraphical evidence. In the present study, an inconsistent relationship between the stable carbon isotope and the stomata values was apparent, which most likely indicated that the stomata numbers of the plant were more sensitive to the variation in the concentration of the atmospheric CO₂, whereas the δ¹³C values were sensitive to the moisture conditions.     

Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters

The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo₆₇Fa₃₃, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, K _D=(Fe⁺²/Mg) _M1/(Fe⁺²/Mg) _M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe⁺² and Mg is nearly random at low temperatures, whereas Fe⁺² shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine.     

Impacts of zerovalent iron application on the adsorption behavior of alachlor and metalaxyl in water and soil systems

Alachlor and metalaxyl contaminations of environmental and agricultural water and soil systems cause potential threats to human health. However, information on the pesticide adsorption quantity–intensity (Q/I) relationships in water and soil systems is limited. Therefore, adsorption behavior and the fate of alachlor and metalaxyl in the systems as influenced by the application of zerovalent iron (ZVI, Fe⁰) were investigated using the pesticide adsorption Q/I relationships. After treating ZVI in the systems, the concentration of alachlor rapidly decreased within a few days and then it disappeared at approximately 5–7 experiment days; whereas metalaxyl concentration was reduced by approximately 40–45% during the 28 day experimental period. In particular, ZVI adsorbed more metalaxyl in the aqueous system than in the soil-solution system. The alachlor concentration in the water and soil solution drastically decreased with increasing ZVI treatment, while metalaxyl concentration was relatively slow in its decrease. Adsorption mechanism of the pesticides in the soil-solution system was shown as multiple-site adsorption Q/I fitting. Buffering capacity (BC) of the pesticides increased with ZVI treatment in all sorption sites. The BC values in sandy soil were escalated with increasing ZVI application rates, so that the values were rather higher in sandy soil than in clayey soil. In addition, changes in alachlor concentration with applying ZVI were due to both dechlorination and adsorption; whereas metalaxyl concentration was dependent upon adsorption reaction. Thus, the pesticide adsorption Q/I relationships in different soil-solution systems were critically affected by the ZVI treatment.     

Study of Colombian coals using photoacoustic Fourier transform infrared spectroscopy

Five of the most important Colombian coals were studied using Photoacoustic Fourier transform infrared (PA-FTIR) spectroscopy. The PA-FTIR spectra were used to obtain structural parameters and to determine the analytical potential of this technique. The molecular structural parameters, CH₂/CH₃, f_a, H_al/H_ar, and (R/C)_u were correlated with fixed carbon contained in the samples to determine aliphatic hydrogen, H_al, and the aromatic hydrogen, H_ar in coals studied here. These parameters enabled the determination of aromaticity, f_a, which was found to be between 0.66 and 0.98. The aliphatic and aromatic regions were considered between 3005–2760 cm^?1 and 912–670 cm^?1, respectively. Finally, using the molecular structural parameters, the grade of maturation (rank) of coal samples studied was determined.     

A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt

We have developed an idealized mathematical model to understand the isotopic variability of the mantle and its relation to the observed variations in isotopic ratios ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr, ¹⁷⁶Hf/¹⁷⁷Hf, ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, and ²⁰⁷Pb/²⁰⁴Pb measured on mid-ocean ridge basalt (MORB). We consider a simple box model of mantle processes. A single melt region produces a melt fraction F of melt, and the average time since a given parcel of mantle material last visited this region is given by the time scale τ_melt. The melt region fractionates the parent/daughter ratios. Over time this leads to variations in the mantle isotopic ratios as the parent decays to the daughter. Key assumptions are that the half-life of the parent isotope is large compared with τ_melt, that the flow is strongly stirring, and that the mantle has reached a statistical steady state. This enables us to neglect the specifics of the underlying flow. Sampling from our model mantle is dealt with by averaging over a large number N of samples to represent the mixing after melting.The model predicts a probability density for isotopic ratios in MORB which, with exception of the Pb isotopes, are consistent with measurements. Fitting the MORB data to this model gives estimates of the model parameters F, τ_melt, and N. Small melt fractions with F around 0.5% are essential for a good fit, whereas τ_melt and N are less well constrained. τ_melt is estimated at around 1.4 to 2.4 Ga, and N is of the order of hundreds. The model predicts a larger variability for the Pb isotopes than that observed. As has been stated by many previous authors, it appears that fundamental differences exist between the dynamics of Pb isotopes and those of Nd, Sr and Hf isotopes.     

The isotopic composition of carbonaceous matter in a metamorphic profile from the Swiss Alps

44 Δ¹³C-values for carbonaceous matter in a metamorphic profile from the Swiss Alps have been determined. The analyzed samples range from unmetamorphosed sediments to staurolite schists. The carbon isotopic composition is more-or-less constant with δ-values around ?25%. in the unmetamorphosed sediments, but shifts towards higher ¹³C-content with increasing grade of metamorphism. δ¹³C values of around ?11%. were measured in the rocks of the highest metamorphic grade.     

Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics

The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (P_O2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (M_R), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe_R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe_R increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn_R also increased gradually similar to Fe_R but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe²⁺ and Mn²⁺ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled P_O2 variations during the Paleoproterozoic by applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols, which enabled us to derive an Fe²⁺ oxidation term referred to as ψ. Possible changes in temperature and P_CO2 during this time window and their effects on resulting models of P_O2 evolution have been also considered. We assumed four cases for the calculations of P_O2 variations between 2.5 and 2.0 Ga: no change in either temperature or P_CO2, long-term change in only P_CO2, long-term changes in both temperature and P_CO2, and short-term fluctuations of both temperature and P_CO2 during the possible, multiple global-scale glaciations. The calculations indicate that P_O2 increased gradually, linearly on the logarithmic scale, from <∼10⁻⁶ to >∼10⁻³ atm between 2.5 and 2.0 Ga. Our calculations show that the P_O2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.     

The role of metal sulfates in thermochemical sulfate reduction (TSR) of hydrocarbons: Insight from the yields and stable carbon isotopes of gas products

The mechanism of thermochemical sulfate reduction (TSR) was investigated by separately heating n-C₂₄ with three different sulfates (CaSO₄, Na₂SO₄, MgSO₄) in sealed gold tubes at 420 °C and measuring the stable carbon isotope values of hydrocarbon (C₁-C₅) and non-hydrocarbon (CO₂) products. Extensive TSR was observed with the MgSO₄ reactant as reflected by increasing concentrations of H₂S, ¹³C depleted CO₂ and relatively low concentrations of H₂ (compared to the control). H₂S yields were already very high at the first monitoring time (12 h) when the temperature had just reached 420 °C, suggesting that TSR had commenced well prior to this temperature. Only trace amounts of n-C₂₄ and secondary C₃-C₅ alkanes were detected at 12 h, reflecting the efficient TSR utilization of the reactant and lower molecular weight alkane products. Ethane levels were still relatively high at 12 h, but declined thereafter as it was subject to TSR in the absence of higher molecular weight alkanes which had already been utilized. Methane yields were consistently high throughout the 48 h MgSO₄ treatment. The temporal decrease in the concentrations of alkanes available for TSR may also contribute to the sharp enhancement of CO₂ after 36 h. Absence or dampening of the molecular and isotopic trends of MgSO₄ TSR was observed with Na₂SO₄ and CaSO₄ respectively, directly reflecting the levels of TSR reached using these sulfate treatments.For all treatments, the δ¹³C values of C_1-5n-alkanes showed an increase with both molecular weight and treatment time. MgSO₄ TSR led to a 5-10‰ increase in the δ¹³C values of the C₁-C₅ hydrocarbons and a 20‰ decrease in the δ¹³C value of CO₂. The significant ¹³C depletion of the CO₂ may be due to co-production of ¹³C enriched MgCO₃, although this remains unproven as the δ¹³C of MgCO₃ was not measured. The difference in the δ¹³C values of ethane and propane (Δδ¹³C_EP) increased in magnitude with the degree of TSR, and this trend could be used to help evaluate the occurrence and extent of TSR in subsurface gas reservoirs.     

EPR measurement of the effect of glass composition on the oxidation states of europium

The techniques of electron paramagnetic resonance (EPR) were used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in quenched silicate liquids as a function of their compositions. The compositional end members were CaAl₂Si₂O₈ and either MSiO₃ or M₂Si0₄, M = Mg, (Ca_0.5, Mg_0.5), and Ca. All of the liquids were quenched from 1650 ± 25°C, 10^?6.9±0.6 atm of oxygen, and 10^?6.1±0.6 atm total pressure. For a particular choice of M, the ratio of Eu²⁺ to Eu³⁺ increased as much as a factor of 24 with increasing atomic ratio (Al + Si)/(O); for a constant value of (Al + Si)/(O), the ratio of Eu²⁺ to Eu³⁺ increased in the order Mg > (Ca_0.5,Mg_0.5) >Ca. In order to interpret the compositional dependence of the redox equilibrium of Eu in a systematic manner, the concept of a solvent coefficient was introduced.     

The variations of stable carbon isotope ratio of land plant-derived n-alkanes in deep-sea sediments from the Bering Sea and the North Pacific Ocean during the last 250,000 years

Two piston cores, one located far from the continents (The North Pacific Ocean: ES core), and another located comparatively closer to the continents (The Bering Sea: BOW-8a core) were investigated to reconstruct environmental changes on source land areas. The results show significant contribution of terrestrial organic matter to sediments in both cores. The δ¹³C values of n-C₂₇, n-C₂₉, and n-C₃₁ alkanes in sediments from the North Pacific ES core show significant glacial to interglacial variation whereas those from the Bering Sea core do not. Variations of δ¹³C values of land plant n-alkanes are related to the environmental or vegetational changes in the source land areas. Environmental changes, especially, aridity, rainfall, and pCO₂ during glacial/interglacial transitional periods can affect vegetation, and therefore C₃ / C₄ plant ratios, resulting in δ¹³C changes in the preserved land plant biomarkers. Maximum values of δ¹³C as well as maximum average chain length values of long chain n-alkanes in the ES core occur mostly at the interglacial to glacial transition zones reflecting a time lag related to incorporation of living organic matter into soil and transportation into ocean basins via wind and/or ability of C₄ plants to adapt for a longer period before being replaced by C₃ plants when subjected to gradual climatic changes. Irregular variations with no clear glacial to interglacial trends in the BOW-8a core may result from complex mixture of aerosols from westerly winds and riverine organic matter from the Bering Sea catchments. In addition, terrestrial organic matter entering the Bering Sea could originate from multiple pathways including eolian, riverine, and ice rafted debris, and possibly be disturbed by turbidity and other local currents which can induce re-suspension and re-sedimentation causing an obliterated time relation in the Bering Sea biomarker records.     

Calcification rate and the stable carbon, oxygen, and nitrogen isotopes in the skeleton, host tissue, and zooxanthellae of bleached and recovering Hawaiian corals

We tested the effectiveness of stable isotopes as recorders of physiological changes that occur during coral bleaching and recovery. Montipora capitata and Porites compressa fragments were bleached in outdoor tanks with seawater temperature raised to 30 °C (treatment corals) for one month. Additional fragments were maintained at 27 °C in separate tanks (control corals). After one month, (0 months recovery), buoyant weight was measured and a subset of fragments was frozen. Remaining fragments were returned to the reef for recovery. After 1.5, 4, and 8 months, fragments were collected, measured for buoyant weight, and frozen. Fragments were analyzed for stable carbon and oxygen isotopic compositions of the skeleton (δ¹³C_s; δ¹⁸O_s) and nitrogen and carbon isotopic compositions of the host tissue (δ¹⁵N_h; δ¹³C_h) and zooxanthellae (δ¹⁵N_z; δ¹³C_z). δ¹³C_s decreased immediately after bleaching in M. capitata, but not in P. compressa. δ¹⁸O_s of both species failed to record the warming event. During the remaining months of recovery, δ¹³C_s and δ¹⁸O_s were more enriched in treatment than control corals due to decreases in calcification and metabolic fractionation during that time. Increased δ¹⁵N_h of treatment P. compressa may be due to expelled zooxanthellae during bleaching and recovery. Increased δ¹⁵N_z at 1.5 months in treatment fragments of both species reflects the increased incorporation of dissolved inorganic nitrogen to facilitate mitotic cell division and/or chl a/cell recovery. Changes in δ¹³C_h and δ¹³C_z at 1.5 months in treatment M. capitata indicated a large increase in heterotrophically acquired carbon relative to photosynthetically fixed carbon. We experimentally show that isotopes in coral skeleton, host tissue and zooxanthellae can be used to verify physiological changes during bleaching and recovery, but their use as a proxy for past bleaching events in the skeletal record is limited.     

Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H2O-CO2-CaCO3 solution and the related isotopic composition of calcite in stalagmites

The isotopic composition of carbon and oxygen in a calcite precipitating CO₂-H₂O-CaCO₃ solution is preserved in the calcite precipitated. For the interpretation of isotopic proxies from stalagmites knowledge of the evolution of δ¹³C and δ¹⁸O in the solution during precipitation is required. A system of differential equations is presented from which this evolution can be derived. Both, irreversible loss of carbon and oxygen from the solution with precipitation time τ and exchange of oxygen in the carbonates with the oxygen in the water with exchange time T are considered. For carbon, where no exchange is active, a modified equation of Rayleigh-distillation is found, which takes into account that precipitation stops at c_eq, the saturation concentration of DIC with respect to calcite, and that c_eq as well as the precipitation time τ is slightly different for the heavy and the light isotope. This, however, requires introducing a new parameter γ = (A_eq/B_eq)/(A₀/B₀), which has to be determined experimentally. (A_eq/B_eq) is the isotopic ratio for the heavy (A) and the light isotope (B) at both chemical and isotopic equilibrium and (A₀/B₀) is the initial isotopic ratio of the solution. In the case of oxygen, where exchange is present, the isotopic shifts are reduced with increasing values of the precipitation time τ. For τ ? T the solution stays in isotopic equilibrium with the oxygen in the water during the entire time in which precipitation is active. The isotopic ratios in a calcite precipitating solution R(t)/R₀ = (1 + δ(t)/1000) for carbon are plotted versus those of oxygen. R₀ is the isotopic ratio at time t = 0, when precipitation starts and δ(t) the isotopic shift in the solution after time t. These show positive correlations for the first 50% of calcite, which can precipitate. Their slopes increase with increasing values of τ and they closely resemble Hendy-tests performed along growth layers of stalagmites. Our results show that stalagmites, which grow by high supply of water with drip times less than 50 s, exhibit positive correlations between δ¹³C and δ¹⁸O along a growth layer. But in spite of this the isotopic composition of oxygen in the solution at the apex is in isotopic equilibrium with the oxygen in the water, and therefore also that of calcite deposited at the apex.     

Density variations of Cr-rich garnets in the upper mantle inferred from the elasticity of uvarovite garnet

The thermoelastic parameters of Ca₃Cr₂Si₃O₁₂ uvarovite garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1100 K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch–Murnaghan (BM3) EoS yielded K₀ = 164.2 ± 0.7 GPa, V₀ = 1735.9 ± 0.3 ų (K’₀ fixed to 4.0). P–V–T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K₀ = 163.6 ± 2.6 GPa, K’₀ = 4.1 ± 0.5, its temperature derivative (?K_0,T/?T)_P = –0.014 ± 0.002 GPa K^?1, and the thermal expansion coefficients a₀ = 2.32 ± 0.13 ×10^?5 K^?1 and b₀ = 2.13 ± 2.18 ×10^?9 K^?2 (K’₀ fixed to 4.0). Our results showed that the Cr³⁺ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.     

Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules

The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature, partial pressure of oxygen, and sizes of the spheres. The Na₂O content in the sphere is uniform in each run. but it decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization, and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing P_O2 and size of the spheres. The relation of the Na₂O content in the liquid sphere with time and its size indicate that the amount of Na₂O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na₂O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and P_o2. The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 1450–1600 C at 10^?9,2 atm p_O2; the activation energy for the sodium volatilization is approximately 100 kcal-mole^?1. The rate is also approximately proportional to within the range of p_O2 from 10^?10.2 to 10^?5.0 atm at about 1500° C. Based on the present results and the Na₂O contents in chondrules. it is suggested that they experienced an instant heating with maximum temperature of 1400–2200° C followed by an immediate cooling.     

The spontaneous fission rate of U-238 and fission track dating

The rate of spontaneous fission decay of uranium-238 (λ_f²³⁸) was determined in 4π-geometry by the fission track method. Uranium glasses of known age of manufacture were used. Spontaneous tracks have accumulated since the time of manufacture and induced tracks to determine the uranium content were produced by thermal neutron irradiations. Spontaneous tracks in all glasses were found to be partially annealed. By correcting for this annealing effect, a (λ_f²³⁸) = 8.7 ± 0.6 × 10^?17yr^?1 was obtained. Uncertainty in the neutron dose is the largest source of error.     

Pressure effect on the divalent cation distribution in nonideal solid solution of forsterite and fayalite

The divalent cation distribution in olivine (Mg, Fe)₂SiO₄ under high pressure and temperature was studied to clarify the detailed state of olivine in the mantle. Single crystal samples were heated for a sufficient period of time for the cations to migrate and quenched fast enough to preserve the equilibrated state under high pressures, and the crystal structure was determined with X-ray method. The pressure effect on the distribution coefficient K _D[= (Fe/Mg) _M1/(Fe/Mg) _M2] was determined for the first time; dK _D/dP?0.02 GPa^?1. A set of five thermodynamic parameters required to describe the regular solution model was determined from data concerning the pressure dependence and the known temperature and compositional effects. As a result we have shown how K _D depends on pressure, temperature, and composition. The notable feature clarified is the very large contribution of nonideality in the olivine solid solution. The K _D of olivine crystals in the mantle is predicted; K _D increases to ～ 2.2 at the depth of 400 km, in contrast to 0.9 ～ 1.2 of natural samples available at the surface of the Earth.     

Experimental and numerical studies of the O exchange between CO2 and water in the atmosphere-soil invasion flux

The ¹⁸O/¹⁶O ratio of CO₂ is a potentially powerful tracer of carbon dioxide fluxes from the soil to the atmosphere, which is influenced by complex interactions involving both biotic and abiotic soil processes. We use a simplified experimental approach and numerical simulations to examine in isolation the ¹⁸O exchange between CO₂ and soil water associated with the abiotic invasion of atmospheric CO₂ into soil. This allowed us to verify, in particular, whether the ¹⁸O of the retro-diffusion flux of CO₂ from the soil reflects ¹⁸O equilibration with water at the soil surface, or at some depth. Sterile soil samples with known water isotopic composition were placed in a closed box attached to a specially designed flow chamber and the changes in δ¹⁸O of CO₂ between the chamber inlet and outlet, due only to invasion effects, were determined. Numerical simulations constrained by the laboratory gas exchange measurements indicated that between the two commonly used diffusion models [Penman, H.L. (1940). Gas and vapor movements in soil, 1: the diffusion of vapors through porous solids. Int. J. Agric. Sci.30, 437-462; Moldrup, P., Olesen, T., Yamaguchi, T., Schjonning, P., Rolston, D.E. (1999). Modeling diffusion and reaction in soils, IX, the Backingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil. Soil Sci.164, 542-551], only the former provided good agreement with the measurements over a wide range of soil water contents. Based on the model calculations constrained by experimental data, and on comparison of characteristic diffusion/reaction times, we conclude that the depth required for full CO₂-water ¹⁸O equilibration ranges between 2 and 8.5 cm. The depth depends, in order of importance, on (1) soil moisture content; (2) temperature, which dominates the rate of hydration isotopic exchange; (3) CO₂ residence time, which is determined by the time of replacement of the column air above the soil; and (4) soil structure, including porosity, tortuosity and grain size, with the later probably influencing the water surface area exposed to CO₂ exchange. Using field data from a semi-arid forest site in Israel, numerical simulations indicated that the ¹⁸O full equilibrium depth varied at this site between 4 cm (January) and 8 cm (November), being sensitive mostly to temperature and soil water content. Deepening of the equilibration depth as the soil dries should limit the effects of ¹⁸O evaporative enrichment at the surface on the isotopic composition of the soil-atmosphere CO₂ flux.     

Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada

Modelling of tree-ring δ¹³C and δ¹⁸O data from the Columbia Icefield area in the eastern Rocky Mountains of western Canada provides fuller understanding of climatic and hydrologic variability over the past 1000 yr in this region, based on reconstruction of changes in growth season atmospheric relative humidity (RH_grs), winter temperature (T_win) and the precipitation δ¹⁸O-T_win relation. The Little Ice Age (~ AD 1530s-1890s) is marked by low RH_grs and T_win and a δ¹⁸O-T_win relation offset from that of the present, reflecting enhanced meridional circulation and persistent influence of Arctic air masses. Independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies. In contrast, the early millennium was marked by higher RH_grs and T_win and a δ¹⁸O-T_win relation like that of the 20th century, consistent with pervasive influence of Pacific air masses because of strong zonal circulation. Especially mild conditions prevailed during the “Medieval Climate Anomaly” ~ AD 1100-1250, corresponding with evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighbouring western USA.