The effects of solid solution and diffusion on infiltration-driven metamorphism with application to metaperidotite,Val d’Efra,Central Swiss Alps

The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (X _i) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between X _i and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξ_ss ≤ ξ_max) that depends on input fluid composition (ξ_max is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with X _i homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξ_ss in all domains except upstream from the slowest moving front where ξ = ξ_ss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξ_max) are best reproduced by down-temperature infiltration of a disequilibrium fluid with X_\textCO2 =  0. 1 9 6 X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 into a multi-domain medium with uniform X_\textCO2 X_{{{\text{CO}}_{2} }} at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm² rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature.     

Assessing sound exposure from shipping in coastal waters using a single hydrophone and Automatic Identification System (AIS) data

Underwater noise from shipping is a growing presence throughout the world's oceans, and may be subjecting marine fauna to chronic noise exposure with potentially severe long-term consequences. The coincidence of dense shipping activity and sensitive marine ecosystems in coastal environments is of particular concern, and noise assessment methodologies which describe the high temporal variability of sound exposure in these areas are needed. We present a method of characterising sound exposure from shipping using continuous passive acoustic monitoring combined with Automatic Identification System (AIS) shipping data. The method is applied to data recorded in Falmouth Bay, UK. Absolute and relative levels of intermittent ship noise contributions to the 24-h sound exposure level are determined using an adaptive threshold, and the spatial distribution of potential ship sources is then analysed using AIS data. This technique can be used to prioritize shipping noise mitigation strategies in coastal marine environments.     

Measuring gravel transport and dispersion in a mountain river using passive radio tracers

Random walk models of fluvial sediment transport recognize that grains move intermittently, with short duration steps separated by rests that are comparatively long. These models are built upon the probability distributions of the step length and the resting time. Motivated by these models, tracer experiments have attempted to measure directly the steps and rests of sediment grains in natural streams. This paper describes results from a large tracer experiment designed to test stochastic transport models. We used passive integrated transponder (PIT) tags to label 893 coarse gravel clasts and placed them in Halfmoon Creek, a small alpine stream near Leadville, Colorado, USA. The PIT tags allow us to locate and identify tracers without picking them up or digging them out of the streambed. They also enable us to find a very high percentage of our rocks, 98% after three years and 96% after the fourth year. We use the annual tracer displacement to test two stochastic transport models, the Einstein–Hubbell–Sayre (EHS) model and the Yang–Sayre gamma‐exponential model (GEM). We find that the GEM is a better fit to the observations, particularly for slower moving tracers and suggest that the strength of the GEM is that the gamma distribution of step lengths approximates a compound Poisson distribution. Published in 2012. This article is a US Government work and is in the public domain in the USA.     

Ventifacts on Earth and Mars: Analytical, field, and laboratory studies supporting sand abrasion and windward feature development

Terrestrial ventifacts – rocks that have been abraded by windblown particles – are found in desert, periglacial, and coastal environments. On Mars, their abundance suggests that aeolian abrasion is one of the most significant erosional processes on the planet. There are several conflicting viewpoints concerning the efficacy of potential abrasive agents, principally sand and dust, and the relationships between wind direction and ventifact form. Our research, supported by a review of the literature, shows that sand, rather than dust or other materials, is the principle abrasive agent on Earth and Mars. Relative to dust, sand delivers about 1000× the energy onto rock surfaces on a per particle basis. Even multiple dust collisions will do little or no damage because the stress field from the impact is much smaller than the spacing of microflaws in the rock. The abrasion profiles of terrestrial ventifacts are consistent with a kinetic energy flux due to saltating sand, not airborne dust. Furthermore, Scanning Electron Microscope images reveal surfaces that are fractured and cleaved by sand grain impact. With respect to their distribution, ventifacts are found in regions that contain sand or did so in the past, but are not found where only dust activity occurs. Contrary to some published reports, our evidence from field studies, analytical models, and wind tunnel and other experiments indicates that windward, not leeward, abrasion is responsible for facet development and feature formation (pits, flutes, and grooves). Leeward abrasion is confined to fluvial conditions, in which the high viscosity and density of water are able to entrain sand-size material in vortices. Therefore, ventifacts and abraded terrain provide an unambiguous proxy for the direction of the highest velocity winds, and can be used to reconstruct palaeowind flow.     

Bank erosion of legacy sediment at the transition from vertical to lateral stream incision

Streambank erosion is a primary source of suspended sediments in many waterways of the US Atlantic Piedmont. This problem is exacerbated where banks are comprised of fine sediment produced by the intensive land use practices of early European settlers. A stream in this region, Richland Creek incises into banks comprised of three stratigraphic layers associated with historic land use: pre‐European settlement, early European agriculture and development, and water‐powered milldam operation. This study aims to identify the bank processes along a reach of Richland Creek that is eroding towards its pre‐disturbance elevation. The volume of material that has eroded along this stream since the milldam breached was calculated by differencing a reconstructed surface of the pond bed and an aerial lidar digital terrain model (DTM). Immediately downstream from the study reach, the channel is floored by bedrock and immediately upstream the rate of channel erosion approximately doubled along the longitudinal profile of Richland Creek, which indicate that the study reach spans the transition from a channel dominated by vertical incision in the upstream direction to horizontal widening in the downstream direction. The combined hydrometeorological conditions and dominant processes causing reach‐scale cut bank erosion were investigated with analyses of stream stage, precipitation, and streambank volumetric and surfaces change that was measured during nine terrestrial lidar surveys in 2010–2012. The spatial variability of erosion during a simulated precipitation event was examined in a field‐based experiment. Erosion was greatest where mill pond sediment columns detached along vertical desiccation and horizontal seepage cracks. This sediment accumulated on the bank toe throughout the study and was a source of readily‐entrained fine sediment contrary to the upper reaches where depositional accommodation space is more limited. Findings suggest that hotspots of sediment excavation progress upstream, indicating that restoration efforts should focus upon stabilizing banks at these locations. Copyright © 2015 John Wiley & Sons, Ltd.     

Burning molten metallic spheres: One class of ball lightning?

Abrahamson and Dinniss [2000. Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil. Nature 403, 519–521] proposed a theory of ball lighting in which silicon nanoparticles undergo slow oxidation and emit light. Paiva et al. [2007. Production of ball-lightning-like luminous balls by electrical discharges in silicon. Physical Review Letters 98, 048501] reported that an electric arc to silicon produced long-lasting luminous white spheres showing many characteristics of ball lightning. We show experimentally that these consist of burning molten silicon spheres with diameters in the 0.1–1 mm range. The evidence of our experiments leads us to propose that a subset of ball lightning events may consist of macro-scale molten spheres of burning metallic materials likely to be ejected from a conventional lightning strike to earth.     

Slab dehydration and basalt petrogenesis in subduction systems involving very young oceanic lithosphere

Compositions of post-Miocene basalts erupted in the Garibaldi and Central America volcanic arcs exhibit significant correlations with the age of the subducted plate. In general, SiO₂, Al₂O₃, CaO, V, and (Sr/P)_N decrease and FeO, MgO, TiO₂ and Na₂O increase as the age of the subducted plate decreases. Variations in CaO/Al₂O₃, SiO₂, (Sr/P)_N, and Ba are compatible with lesser slab input, and hence less hydrous melting conditions in the mantle wedge in segments of the arcs overlying the youngest oceanic lithosphere. This interpretation is supported by comparison with peridotite melting experiments, which suggest higher melt pressures and temperatures in the mantle wedge above very young oceanic lithosphere. These observations point to a model in which dehydration of the downgoing slab occurs at shallow depths in subduction systems involving oceanic lithosphere younger than about 20 Ma. Because young oceanic lithosphere is relatively warm, little post-subduction heating is required to produce metamorphic reactions that release slab volatiles. Geodynamic models indicate most volatile-liberating reactions will occur within the seismogenic zone in oceanic lithosphere younger than 20 Ma, thus limiting the volatile flux beneath the arc and encouraging drier, higher temperature and higher pressure melting conditions in the mantle wedge in comparison to typical arc systems. Liberation of volatiles in the downgoing plate is strongly dependant on the shear stress on the fault, but is predicted to occur within the seismogenic zone for shear stresses greater than 33 MPa. Similarly, early loss of volatiles is predicted over a wide range of convergence rates, plate dips, and convergence angles. These results are shown to be robust for realistic ranges of slab dip, convergence angle, and shear stress, suggesting that volatile-poor melt generation is a characteristic of modern and ancient arc systems that involve subduction of young oceanic lithosphere.     

Temperature variation in Guangdong Province, China

1IntroductionThereisgrowingawarenessofclhatechange,andnumerousresearchonclhatictrendsandvallabilityatglobalorregionalscalehasbenconductedMoStstudieshaveconcen~onthelong-tenntrendsintemperatUI'eandPeripiboonpiousstudiesonhendSPherictempo~bendshavebeenconducted.AngellandKorshover(1977ac1978)StUdiedthetemperatUresinthenodsmhelinSPhereandstatedthattherewasadeClineoftemperatUreSinthe1960sandaslightw~gwasObservedafter197ill;l.OtherauthorssuchasLamb(1982)andJonesetal.(1982and1986a)SUPPOrted…     

全球化与城市国际化:国际城市的一项实证研究

在当今全球化时代,许多城市被确认为世界城市或全球城市,但本文认为世界上只有少数一些城市真正达到“世界城市”的标准,具有控制世界经济的功能。而许多其他城市,无论是支配或被支配的城市,只显示出国际化的若干特征。本文利用全球264个城市与69家高级生产者服务公司的资料,从国际城市的角度研究世界上的重要城市。研究发现不同的城市有不同的国际化水平,除了少数高度国际化的一流城市达到世界城市的标准,许多其他城市只达到局部国际化,且他们的分布在地域上并无集中的趋势。同时研究发现世界上只有少数高级服务公司拥有全球城市服务网络,大部分公司的服务网络只集中在部分地区。