Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>     

J.?MajzlanEmail author  R.?Stevens  J.?Boerio-Goates  B. F.?Woodfield  A.?Navrotsky  P. C.?Burns  M. K.?Crawford  T. G.?Amos 《Physics and Chemistry of Minerals》2004,31(8):518-531

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).  相似文献   

Fire and vegetation history during the last 3800 years in northwestern Montana   总被引：1，自引：0，他引：1  

Mitchell J. Power  Cathy Whitlock  Patrick Bartlein  Lora R. Stevens 《Geomorphology》2006,75(3-4):420

Foy Lake in northwestern Montana provides a record of annual-to-decadal-scale landscape change. Sedimentary charcoal and pollen analyses were used to document fire and vegetation changes over the last 3800 years, which were then compared to similar records from AD 1880 to 2000. The long-term record at Foy Lake suggests shifts between forest and steppe as well as changes in fire regime that are likely the result of climate change. Fire activity (inferred from the frequency of charcoal peaks) averaged 18 fire episodes/1000 years from 3800 to 2125 cal year BP, and increased from 16 fire episodes/1000 years at 2125 cal year BP to 22 episodes/1000 years at 750 cal year BP, a period when the pollen data suggest that steppe vegetation yielded to increasing patches of forest cover. Between 2125 and 750 cal year BP, increased forest cover produced more background charcoal than before and after this period, when vegetation was dominated by steppe. Between 750 and 75 cal year BP steppe has expanded and fire episode frequency averaged 33 episodes/1000 years, increasing to a maximum of 40 episodes/1000 years at ca. 300 cal year BP and then decreasing to present levels. Since AD 1880, the pollen record indicates an increase in shrubs and grasses from AD 1895 to 1960 as a result of vegetation changes associated with timber harvesting and livestock grazing. No fires have been documented in the Foy Lake watershed since AD 1880. Charcoal from the extralocal fires of AD 1910, burning over 4,111,249 ha in Idaho, Montana, and Wyoming, however, is present in Foy Lake. Between AD 1970 and 2000, increased arboreal pollen in the record is consistent with observations that the forest has become more closed. The activities of Euro–Americans have led to a decline in forest cover between AD 1880 and 1970, followed by a recent increase as trees are now growing in areas previously occupied by steppe. Euro–Americans are likely the cause of a reduction in fire activity in watershed since AD 1880.  相似文献   

Wind forcing of internal waves in a long narrow stratified lake     

Craig Stevens  Gregory Lawrence  Paul Hamblin  Eddy Carmack 《Dynamics of Atmospheres and Oceans》1996,24(1-4)

A vertically integrated approach to the analysis of thermistor chain and wind data from a long, narrow, stratified lake indicates that forcing events are short relative to the response time. Correlations between forcing and isotherm tilts clearly indicate stratification response times dependent on internal wave speed. In addition, it is possible to infer basin-scale internal wave damping where over 80% of the potential energy in the internal wave setup is lost in one internal wave period.  相似文献   

Sensitivity to open boundary forcing in a fine-resolution model of the Iberian shelf-slope region     

I. G. Stevens  J. A. Johnson 《Annales Geophysicae》1997,15(1):113-123

A fine-resolution primitive equation numerical model is constructed for the Iberian continental shelf and slope region, with open boundaries to the north, south and west. The model is forced by climatological wind fields and relaxed at the surface to climatological temperature and salinity fields. A series of numerical experiments is conducted to investigate the influence of the open boundary conditions. The numerical results include coastal upwelling in summer and a poleward current in winter. The effects of advection of Mediterranean Water and eastern North Atlantic Central Water feature in the circulation. Qualitative comparisons are made with observations.  相似文献   

Harbours – silting and environmental sedimentology   总被引：1，自引：0，他引：1  

Rodney L. Stevens 《Environmental Geology》2003,43(4):432-433

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Fluid compositions in equilibrium with silica-undersaturated magmas in the system Na<Subscript>2</Subscript>O–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O: clues to the composition of fenitizing fluids     

Robin F. Preston  Gary Stevens  Terence S. McCarthy 《Contributions to Mineralogy and Petrology》2003,144(5):559-569

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A Numerical Study Of Nocturnal Wavelike Motion In Forests     

Xinzhang Hu  Xuhui Lee  David E. Stevens  Ronald B. Smith 《Boundary-Layer Meteorology》2002,102(2):199-223

In this paper, we use a two-dimensional eddy-resolved model to investigate the instability of a parallel shear flow in a stably stratified boundary layer whose lower domain is occupied by a canopy. The results support our contention that wave motion in the canopy is initiated by shear in an air layer near the treetops. Significant modification by the wave motion of the mean velocity and temperature fields is found even before the wave reaches saturation. The wave fluxes of momentum and heat are not constant with height. Downwind tilting braids are found at the finite amplitude stage of the wave growth and could persist after wave breaking; these downwind tilting structures are believed to be the same as the temperature microfronts reported in the literature. We also present an analysis of the velocity and temperature fields of an observed wave event in the time-height domain and show that the simulation has captured the broad features of the observation.  相似文献   

Constraints on Infrasound Scaling and Attenuation Relations from Soviet Explosion Data     

J. L. Stevens  I. I. Divnov  D. A. Adams  J. R. Murphy  V. N. Bourchik 《Pure and Applied Geophysics》2002,159(5):1045-1062

—?The Institute for the Dynamics of the Geospheres (IDG) in Moscow, Russia, contains an archive of infrasound recordings from Soviet atmospheric nuclear tests that were conducted in 1957 and 1961, and has digitized the highest quality records from this data set. We have measured the infrasound signals from these records and compared them with previously developed scaling and attenuation relations. We find that the data are in best agreement with a scaling and attenuation relation developed by the Los Alamos National Laboratory (LANL) which can be written as logP = 3.37 + 0.68 logW? 1.36logR where P is zero to peak pressure amplitude in Pascals, W is the yield in kilotons, and R is the source to receiver distance in kilometers. We use the scaling relations to define an infrasound magnitude, and to estimate the detection capability of the International Monitoring System (IMS) being developed as part of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The detection threshold for the proposed 60-station IMS network is estimated to be slightly higher than the CTBT design goal of 1 kiloton in some locations.  相似文献   

Cosmopolitan urbanism - Edited by Jon Binnie, Julian Holloway, Steven Millington and Craig Young     

Quentin Stevens 《Area》2007,39(3):407-408

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Study of near-surface models for large-eddy simulations of a neutrally stratified atmospheric boundary layer     

Inanc Senocak  Andrew S. Ackerman  Michael P. Kirkpatrick  David E. Stevens  Nagi N. Mansour 《Boundary-Layer Meteorology》2007,124(3):405-424

In large-eddy simulations (LES) of the atmospheric boundary layer (ABL), near-surface models are often used to supplement subgrid-scale (SGS) turbulent stresses when a major fraction of the energetic scales within the surface layer cannot be resolved with the temporal and spatial resolution at hand. In this study, we investigate the performance of both dynamic and non-dynamic eddy viscosity models coupled with near-surface models in simulations of a neutrally stratified ABL. Two near-surface models that are commonly used in LES of the atmospheric boundary layer are considered. Additionally, a hybrid Reynolds- averaged/LES eddy viscosity model is presented, which uses Prandtl’s mixing length model in the vicinity of the surface, and blends in with the dynamic Smagorinsky model away from the surface. Present simulations show that significant portions of the modelled turbulent stresses are generated by the near-surface models, and they play a dominant role in capturing the expected logarithmic wind profile. Visualizations of the instantaneous vorticity field reveal that flow structures in the vicinity of the surface depend on the choice of the near-surface model. Among the three near-surface models studied, the hybrid eddy viscosity model gives the closest agreement with the logarithmic wind profile in the surface layer. It is also observed that high levels of resolved turbulence stresses can be maintained with the so-called canopy stress model while producing good agreement with the logarithmic wind profile.  相似文献