Diffusion in silicate melts: II. Multicomponent diffusion in CaOAl2O3SiO2 at 1500°C and 1 GPa

Chemical diffusion profiles in molten CaO---Al₂O₃---SiO₂ have been measured over a large range of compositions at 1500°C and l GPa. The diffusion profiles have been inverted for effective binary diffusion coefficients (EBDCs) and for the chemical diffusion matrix. The EBDCs are shown to depend strongly on both composition and direction of diffusion in composition space. The dependence of EBDCs on direction in composition space, which for the system studied here can be as large as a factor of seven, severely limits the applicability of EBDCs to interdiffusion in any direction other than the one used to derive the EBDCs.

The chemical diffusion matrix for molten CaO---Al₂O₃---SiO₂ was determined using diffusion profiles from two or three mutually orthogonal diffusion couples in the ternary composition space. All features of the diffusion profiles shown in this work can be reproduced by representing the chemical fluxes in the three-component system as a linear combination of concentration gradients via a 2 × 2 diffusion matrix. Chemical diffusion in molten CaO---Al₂O₃---SiO₂ shows clear evidence of strong diffusive coupling among the components. This can be seen in the uphill diffusion profiles of components that were initially uniform, in the fact that the apparent rate of diffusion of some components is a strong function of direction in composition space, and most quantitatively in the magnitude of off-diagonal elements of the diffusion matrix relative to the magnitude of the diagonal elements. SiO₂ for example, is found to be strongly coupled with CaO in relatively silicic melts, whereas Al₂O₃ is strongly coupled with CaO in less silicic melts. Furthermore, the coupling of CaO with either Al₂O₃ or Si02 reverses sign between more and less polymerized compositions. Interdiffusion profiles in natural melts have numerous features that suggest similar coupling between Al₂O₃ and CaO and between SiO₂ and CaO.     

The effect of organic compounds in the oxidation kinetics of Cr(III) by H2O2

The oxidation of Cr(III) has been studied in NaCl solutions in the presence of two siderophore models, acetohydroxamic acid (Aha) and benzohydroxamic acid (Bha), the natural siderophore Desferal (DFOB) and the synthetic aminocarboxilate (ethylenedinitrilo)-tetra-acetic acid (EDTA) as a function of pH (8-9), ionic strength (0.01-2 M) and temperature (10-50 °C), at different Cr(III)-organic compound ratios. The addition of Aha and Bha caused the rates to increase at low ligand/Cr(III) ratios and decrease at high ratios. The variation of the pseudo first order rate constant (k₁) as a function of the ligand concentration has been attributed to the formation of three Cr(III)-organo species (1:1, 1:2, 1:3), which can form in the presence of monohydroxamic acids. A kinetic model has been developed that gives a value of 600 (min⁻¹) for the pseudo first order rate constant k_1CrAha²⁺ and values approaching zero for and k_1CrAha3. These kinetic results demonstrate that these monohydroxamic acids are able to bind with Cr(III) under experimental conditions that may occur in natural waters and can increase the oxidation rates of Cr(III) with H₂O₂ by a factor of 3.5 at an Aha/Cr(III) ratio of about 50-100.The monohydroxamic acids also affect the rates on aged products of Cr(III), suggesting that these ligands are able to affect the oxidation rates by releasing reactive Cr(III). DFOB and EDTA do not have a great effect on the oxidation of Cr(III) with H₂O₂. This is thought to be due to the much longer times they need to form complexes with Cr(III) compared to Aha and Bha. The rates for the formation of DFOB and EDTA complexes with Cr(III) are not competitive with the rates of the formation of aged Cr(III). After allowing Cr(III) and DFOB to react for 5 days to form the complex, reaction rates of Cr(III) with H₂O₂ appear to be lowered probably because of steric hindrance of the chelated Cr(III).     

Determination of the mass-dependence of cadmium isotope fractionation during evaporation

Mass fractionation laws relate the fractionation factor α_A for one isotope ratio to the fractionation factor α_B for a second isotope ratio of the same element, with a fractionation exponent β such that α_A = α_B^β. The exponent β defines the mass-dependence of the mass fractionation law and thus determines the slope of a mass fractionation line in linearized three isotope space. The generalized power law (GPL) defines β as a function of a variable exponent n. The laws that aim to describe equilibrium and kinetic isotope fractionations are special cases of the GPL with n = −1 and n → 0, respectively.Large isotope fractionations (up to 10% for ¹⁰⁶Cd/¹¹⁴Cd) were found to accompany the evaporation of molten Cd into vacuum at about 180°C. The slopes of the fractionation lines (β-values) were obtained by analyzing the Cd isotope compositions of the evaporation residues relative to the starting material with two different multiple collector-ICPMS instruments. For the most fractionated sample, the difference between the theoretical β-values, that describe kinetic and equilibrium isotope fractionation, is 10 to 20 times larger than the measurement uncertainty. A mass-dependence with n = −0.35 was determined for this sample. This result differs significantly from the value that would be expected for simple kinetic evaporation (n → 0), which is governed by the diffusion of monatomic Cd from the melt into vacuum. The observed “non-kinetic” mass-dependence probably results from partial recondensation (back reaction) of Cd vapor into the melt phase. This interpretation requires that equilibrium evaporation of Cd at about 180°C is associated with significant isotope fractionation.The present study demonstrates that the mechanism of isotope fractionation can be investigated by studying the associated mass-dependence, which can be determined by measuring the isotope ratios of a fractionated product relative to the starting material. The quantification of mass fractionation line slopes with the GPL should aid the interpretation of mass-dependent and small mass-independent isotope effects.     

Sequence stratigraphic framework for mixed aeolian,peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.     

Future changes in internal variability of the Atlantic Meridional Overturning Circulation

The response of the internal variability of the Atlantic Meridional Overturning Circulation (MOC) to enhanced atmospheric greenhouse gas concentrations has been estimated from an ensemble of climate change scenario runs. In the model, enhanced greenhouse forcing results in a weaker and shallower MOC with reduced internal variability. At the same time at 55°N between 0 and 1,000 m the overturning increases as a result of a change in the area of convection. In a warmer world, new regions of deepwater formation form further north due to the poleward retreat of the sea-ice boundary. The dominant pattern of internal MOC-variability consists of a monopole centered around 35°N. Due to anthropogenic warming this monopole shifts poleward. The shift is associated with a stronger relation between MOC-variations and heat flux variations over the subpolar gyre. In old convective sites (Labrador Sea) convection becomes more irregular which leads to enhanced heat flux variability. In new convective sites heat flux variations initially are related to sea-ice variations. When the sea-ice coverage further decreases they become associated with (irregular) deepwater formation. Both processes act to tighten the relation between subpolar surface heat flux variability and MOC-variability, resulting in a poleward shift of the latter.     

中国南京与美国德克萨斯稻田甲烷排放的比较(英文)

稻田甲烷排放试验分别在南京与德克萨斯水稻生长季实施，观测期内测定甲烷排放通量、上壤温度和水稻生物量。结果表明：南京稻田土镶温度的季节性变幅为１５．３℃，甲烷排放通量与土壤温度成非线性正相关而与水稻生物量无关；德克萨斯稻田土壤温度的季节性变幅为的２．９℃，甲烷排放通量与土壤温度无关而与水稻生物量成线性正相关。由此得出结论：在持续淹水和无外源有机碳施加的条件下，土壤温度变幅大的地区驱动稻田甲烷排放季节性变化的关键因子为土壤温度，土壤温度变幅小的地区其关键驱动因子则为水稻的生长量。     

Bashikaogong-Shimierbulake granitic complex, north Altun, NW China:Geochemistry and zircon SHRIMP ages

The Bashikaogong-Shimierbulake granitoid complex is about 30 km long and 2―6 km wide, with an area of 140 km2, located at the north margin of the Bashikaogong Basin in the north Altun terrain. It intruded into schist, metapelite and metatuff of Precambrian ages. This granitoid complex consists of darkish quartz diorite, grey granite, pink granite and pegmatite. Geochemically, the quartz diorite has I-type granite affinity and belongs to Calc-alkaline sereies, and the other gran- ites have S-type affinity and to high-K calc-alkaline series. Zircon SHRIMP U-Pb dating shows that the quartz diorite has a bigger age than those of other granites, which is 481.6±5.6 Ma for quartz diorite, 437.0±3.0 Ma―433.1±3.4 Ma for grey granite and 443±11 Ma―434.6±1.6 Ma for pink granite, re- spectively. Combined with regional geology, we think that the quartz diorite formed in tectonic envi- ronment related to oceanic crust subduction and the granites in post-collision.     

A continuous seismic profile survey of Windermere

Continuous seismic profile devices, modified for use in freshwater have been used on Windermere to determine the total thickness of lakebed sediments and the morphology of the underlying bedrock. The survey detected two overdeepened irregularly shaped rock basins. The northerly one approaches 110 feet (34 m) below sea level and contains 70 feet (21 m) of sediment. The corresponding dimensions of the Southern basin are 120 feet (37 m) and 130 feet (40 m) respectively. Glacial scouring is postulated as the origin of the rock basins. Some profiles show two types of sediment; the higher parts of which are known to be Post Glacial.     

Estimating the Impact of Global Change on Flood and Drought Risks in Europe: A Continental, Integrated Analysis

Most studies on the impact of climate change on regional water resources focus on long-term average flows or mean water availability, and they rarely take the effects of altered human water use into account. When analyzing extreme events such as floods and droughts, the assessments are typically confined to smaller areas and case studies. At the same time it is acknowledged that climate change may severely alter the risk of hydrological extremes over large regional scales, and that human water use will put additional pressure on future water resources. In an attempt to bridge these various aspects, this paper presents a first-time continental, integrated analysis of possible impacts of global change (here defined as climate and water use change) on future flood and drought frequencies for the selected study area of Europe. The global integrated water model WaterGAP is evaluated regarding its capability to simulate high and low-flow regimes and is then applied to calculate relative changes in flood and drought frequencies. The results indicate large ‘critical regions’ for which significant changes in flood or drought risks are expected under the proposed global change scenarios. The regions most prone to a rise in flood frequencies are northern to northeastern Europe, while southern and southeastern Europe show significant increases in drought frequencies. In the critical regions, events with an intensity of today's 100-year floods and droughts may recur every 10–50 years by the 2070s. Though interim and preliminary, and despite the inherent uncertainties in the presented approach, the results underpin the importance of developing mitigation and adaptation strategies for global change impacts on a continental scale.     

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.