Extreme Gradients Research Articles

The role of entrained sediments in sea ice in the distribution of aluminium and iron in the surface waters of the Arctic Ocean

Abstract The distribution of reactive Al and Fe was determined in surface water samples collected by hand from ice floes and by shipboard rosette at 28 stations occupied during the 1994 US–Canadian Arctic Ocean Section. The data show significant variations in the concentration of both species over short spatial scales superimposed on a general trend of higher values in the Canada Basin and lower values in the Eurasian Basin. Detailed examination of the distributions indicates that the highest concentrations correspond to regions in which shipboard observations indicate the presence of significant amounts of sea-ice containing entrained sediments “dirty ice”. In many of these regions, strong vertical gradients in trace element concentrations are also observed. The observed correlation of high trace element concentrations and dirty ice suggests that ice-rafted sediment may be an important transport mechanism for supplying reactive trace elements such as Al and Fe to the surface waters of the central Arctic Ocean. Input of this sedimentary material through seasonal ice melt into strongly stratified surface waters leads to enrichment of properties in near surface layers which may have biogeochemical consequences for organisms living on the undersides of ice floes. Extreme concentration gradients in the upper 1–2 m though are likely to be missed or destroyed by conventional shipboard rosette sampling.

Rolling Contact Testing of Vapor Phase Lubricants—Part IV: Diffusion Mechanisms©

Auger depth profile data, obtained from vapor-lubricated T15 bearing steel, were modeled using the solution for diffusion in a semi-infinite pair. A tertiary-butyl phenyl phosphate was used as the vapor lubricant. The primary species undergoing diffusion are iron and carbon. The objective of the experiment was to determine the order of magnitude of the diffusion coefficient and to qualitatively assess what types of diffusion mechanisms are involved. The experimental results indicate that the diffusion profile travels at a velocity equal to the bearing wear rate under dynamic conditions. This is possible if iron diffusions at a faster rate than carbon, i.e., the Kirkendall effect. Analyses of the data were performed using Darken's equations. The results indicate that the diffusion coefficient of iron is of the order of 1 × 10−14 cm2/s at test temperatures of 370° and 430°C. Diffusion is thought to occur via the migration of iron cations through an anionic lattice of polyphosphate and phosphite, i.e., cation diffusion. The diffusion mechanism provides a satisfactory explanation of how several hundred monolayers containing iron can exist in vapor deposition film grown from organophosphorus lubricants. The chemical gradient, ionic attraction, and the extreme stress gradients of the Hertzian contact are thought to contribute to diffusion under dynamic conditions. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998

Late Quaternary precessional cycles of terrigenous sediment input off the Norte Chico, Chile (27.5°S) and palaeoclimatic implications

The palaeoclimatic conditions during the Last Glacial Maximum (LGM) of southern South America and especially latitudinal shifts of the southern westerly wind belt are still discussed controversially. Longer palaeoclimatic records covering the Late Quaternary are rare. A particularly sensitive area to Late Quaternary climatic changes is the Norte Chico, northern Chile, because of its extreme climatic gradients. Small shifts of the present climatic zonation could cause significant variations of the terrestrial sedimentary environment which would be recorded in marine terrigenous sediments. To unveil the history of shifting climatic zones in northern Chile, we present a sedimentological study of a marine sediment core (GeoB 3375-1) from the continental slope off the Norte Chico (27.5°S). Sedimentological investigations include bulk- and silt grain-size determinations by sieving, Atterberg separation, and detailed SediGraph analyses. Additionally, clay mineralogical parameters were obtained by X-ray diffraction methods. The 14C-dated core, covering the time span from approximately 10,000 to 120,000 cal. yr B.P., consists of hemipelagic sediments. Terrigenous sedimentological parameters reveal a strong cyclicity, which is interpreted in terms of variations of sediment provenance, modifications of the terrestrial weathering regimes, and modes of sediment input to the ocean. These interpretations imply cyclic variations between comparatively arid climates and more humid conditions with seasonal precipitation for northern Chile (27.5°S) through the Late Quaternary. The cyclicity of the terrigenous sediment parameters is strongly dominated by precessional cycles. For the palaeoclimatic signal, this means that more humid conditions coincide with maxima of the precession index, as e.g. during the LGM. Higher seasonal precipitation for this part of Chile is most likely derived from frontal winter rain of the Southern Westerlies. Thus, the data presented here favour not only an equatorward shift of this atmospheric circulation system during the LGM, but also precession-controlled latitudinal movements throughout the Late Quaternary. Precessional forcing of latitudinal movements of the westerly atmospheric circulation system may be conceivable through teleconnections to the Northern Hemisphere monsoonal system in the Atlantic Ocean region.

Terminology and Predominant Processes Associated with the Formation of Weak Layers of Near-Surface Faceted Crystals in the Mountain Snowpack

Although recent observations indicate that weak layers of near-surface faceted crystals are widely associated with snow avalanches, little research has addressed these layers. Further, current research has been hindered by an absence of a framework with which to discuss their formation. This paper proposes terminology and describes three predominant processes observed in mid-latitude mountains which result in extreme near-surface temperature gradients, thereby forming near-surface faceted crystals: radiation recrystallization, melt-layer recrystallization, and diurnal recrystallization. It is hoped that this framework will improve scientific discussion and theory-building related to the formation and spatial distribution of near-surface faceted crystals.

The distribution of iodine in the earth's crust

Analytical data by ICP-MS measurements after pyrohydrolysis on a reasonably large series of samples representing about 300 rocks of major units and subunits of the earth's crust, and a few organic materials, are provided for a genetic discussion. Biogenic accumulation and volatilization of iodine from metamorphosed sediments and magmatic rocks has caused extreme gradients of concentrations towards the top of the oceanic and continental crust. Transporting agents for recycling into surface water were hydrothermal fluids and partial volatilization from surface waters which occurred as methyl iodide. Because of the low iodine concentrations in magmatic and metamorphic rocks such samples exposed at the earth's surface not far from the seashore could have undergone contamination by iodine which escaped from the oceans. The high average concentrations of 30 ppm I in deep-sea carbonates and of 2.5 ppm I in continental limestones were accumulated by planktonic and shallow sea organisms, respectively. Deep-sea clays (3.9 ppm I) and continental shales (1.8 ppm I) contain the residual iodine after dissolution and oxidation of a major fraction of carbonates and organic carbon, respectively. Shales vary more in iodine than deep-sea clays because of greater variations in the environments of deposition and diagenesis. The I/C weight ratio increases from 10 −3 to 2×10 −1 from reduced to oxygenated recent nearshore sediments. Iodine is more stable than carbon during diagenesis of biogenic sediments. Greywackes and sandstones contain on average 150 and 120 ppb I, respectively. Metasedimentary gneisses, mica schists and granulites have as little as 12 to 25 ppb I and have lost from 75 to >95% of their iodine at metamorphic temperatures. Granites, granodiorites, tonalites and basalts are even lower in iodine and contain 4 to 9 ppb I almost independent of the species of magmatic rock. The continental crust, the oceanic crust (including seawater) and bulk Earth's crust contain 119 ppb, 777 ppb and ∼300 ppb I, respectively. Nearly 70% of I is calculated to exist in ocean sediments. The Cl/I weight ratios of the continental, oceanic and bulk Earth's crust are 3800, 4500 and 4300, respectively, to be compared with 1210 and 403 in the Orgueil and Ivuna Cl chondrites.

Vapor transport, grain growth and depth-hoar development in the subarctic snow

AbstractMeasurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layered structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2–3. while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10−7 kg m−2 s−1, though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten limes higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.

Vapor transport, grain growth and depth-hoar development in the subarctic snow

AbstractMeasurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layered structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2–3. while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10−7kg m−2s−1, though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten limes higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.

Water Balance, Climate Change and Land-use Planning in the Pearl Harbor Basin, Hawai'i

The Pearl Harbor basin on the tropical oceanic island of O'ahu, Hawaii, exhibits extreme climatic gradients, rapid land-use change and groundwater use near sustainable yield. The basin's water usage and groundwater recharge, and hence water yield, are strongly influenced by the spatial distribution of land use. Current recharge is expected to drop by about one-eighth with the demise of the remaining sugar-cane and pineapple. Evidence suggests that lower rainfall and increased evaporation may well accompany warmer periods in Hawaii, and water-balance simulations indicate many scenarios having a significant decrease in available water. Land-use planners can use such results in tandem with multiobjective optimization models to generate alternative land-use plans and show trade-offs among objectives.

Catalyst testing at a lab scale in mild oxidation: Can you control the reaction temperature?

Recent trends in catalysis have emerged using new processes and new raw materials in order to reduce the number of steps leading from petroleum distillates to chemical intermediates. Selective activation of the strong C H bonds present in alkanes by catalytic action of oxygen is a stimulating challenge and new efficient catalysts have appeared in some cases. Nevertheless, the selective materials exhibit very low activities and low selectivities at high hydrocarbon conversion. Extreme conditions (partial pressures, residence time) are, thus, chosen to improve the production rate, which can have dramatic consequences on the real working conditions of the catalytic surface, when laboratory tests are performed. Catalytic oxidation of isobutane forming methacrylic acid was chosen as an example and compared to oldest and ‘classical’ mild oxidations. It is shown that in micro-reactors, in apparent isothermal and differential operation, extreme thermal gradients between the catalyst surface and fluid can develop. Moreover, whatever the kinetics and thermal exchange, the search for productivity announced in patents causes runaway in all cases without external evidence. These effects could lead to an erroneous interpretation of results in terms of kinetics and mechanisms, to drastic decreases in selectivities and therefore to the rejection of the most active catalysts due to a low apparent selectivity which could be a consequence of poor testing conditions. Simulation of different reactor geometry shows that industrial-like pilot plants could be more stable and more selective than micro-reactors. The increase of catalytic bed volume appears as a major positive factor in this objective. The screening of catalysts for these new reactions must thus be carried out with extreme care in the choice of experimental parameters.

A device to estimate the light gradients relevant to phototropism in the laboratory and the field

An apparatus has been devised to measure the irradiance falling on a central point from each of 48 points of the compass. The apparatus has been used to estimate the magnitude and spatial distribution of light gradients in a variety of situations which are known to cause phototropism.Measurements show that the typical laboratory treatments used to induce phototropism produce extreme light gradients which are not typical of the light gradients likely to be experienced by seedlings growing in the natural environment. Diffuse light which is always present in the natural environment, ensures that all flanks of seedlings will be subject to potentially physiologically significant light stimulation. Such findings need to be taken into account when devising and testing models of phototropic detection in plants.

A model to predict the through-thickness distribution of heat generation in cross-ply carbon-fiber composites subjected to alternating magnetic fields

A theory of local and global mechanisms of heat generation and distribution in carbon-fiber-based composites subjected to an alternating magnetic field has been proposed. A model is developed which predicts the strength and distribution of thermal generation through the thickness of carbon-fiber-based laminated composites. Earlier work established the distribution of point voltages in the plane of the laminate which exist in the form of potential differences between fibers in adjacent plies in a cross-ply or angle-ply laminate system. In the present work, a capacitive layer microstructure is formulated which models the actual fiber-reinforced-polymer microstructure from a square-packing assumption to a series of conductive parallel plates. From this capacitive layer analogy, an effective parameter of heating, γ, is defined which establishes the distribution of heating through the thickness. Extreme gradients in this thermal source can exist with peaks occurring at the interfaces of ply/ply orientation changes. An optimization study establishes the effect of various micro- and macrostructural parameters on the parameter γ. Several parametric studies are performed on a computer algorithm which calculates γ to further analyze these effects.

Direct modeling of flow of FENE fluids

Direct simulations of macromolecular fluids are carried out for flows between parallel plates and in expanding and contracting channels. The macromolecules are modeled as FENE dumbbells with soft disks or Lennard-Jones dumbbell-dumbbell interactions. The results are presented in terms of profiles and contour plots of velocity, pressure, temperature, density, and flow fields. In addition the data for potential energy, shear stress, and the normal components of the stress tensor are collected. In general, an excellent agreement is found between the simulated profiles and the well-known flow structures, such as flow separation and formation of viscous eddies, indicating that micro-hydrodynamics is a viable tool in linking macroscopic phenomena with the underlying physical mechanisms. The simulations are performed in the Newtonian regime, for medium-size systems comprising up to 3888 dumbbells. This number is sufficiently large to control boundary and particle number effects. The flow is induced by gravity. The traditional stochastic (thermal) and periodic boundary conditions are employed. Also, diffusive boundary conditions, which could include a stagnant fluid layer and repulsive potential walls, are developed. The scaling problems, which are related to the application of a large external force in a microscopic system (of the size of the order 100 A), result in extreme pressure and temperature gradients. In addition, the viscosity and thermal conductivity coefficients obtained from velocity and temperature profiles of the channel flow are presented. These results are confirmed independently from modeling of Couette flow by the SLLOD equations of motion and from the Evans algorithm for thermal conductivity.

Luminal perfusion of isolated gastric glands

We have extended to rabbit gastric glands the technique for perfusing single isolated renal tubules. We isolated glands by hand dissection and used concentric glass pipettes to hold them and perfuse their lumina. Parietal cells (PCs), which tended to be located toward the gland opening, were identified by their pyramidal shape, large size, and autofluorescence. Chief cells (CCs) were identified by their round shape and smaller size. In some experiments, we perfused the lumen with hydroxypyrenetrisulfonate, a pH-sensitive fluorophore, at pH 7.4 and used digital image processing to monitor luminal pH (pH1). Solutions were buffered with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid to pH 7.4 at 37 degrees C. With fast perfusion, we found no evidence of decreased pH1, even with stimulation by 10 microM carbachol. With slow perfusion, pH1 often fell below the dye's sensitive range (pH < 5), especially at low perfusate buffering power. In other experiments, we loaded cells with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein and monitored intracellular pH (pHi) in multiple individual PCs and CCs in a single gland. Mean pHi was 7.21 +/- 0.02 (n = 136 cells) for PCs and 7.27 +/- 0.03 (n = 103) for CCs. To examine the response to decreased pH1 and basolateral pH (pHb), we lowered pHb to 6.4 or lowered pH1 to 3.4 or 1.4. Lowering pHb to 6.4 for approximately 1 min caused pHi to fall reversibly by 0.39 +/- 0.05 (n = 53) in PCs and 0.58 +/- 0.03 (n = 50) in CCs. Lowering pH1 to 3.4 or 1.4 caused no significant pHi changes in PCs (n = 38 and 82) or in CCs (n = 44 and 77). Carbachol did not affect the response to changes in pH1 or pHb. We conclude that the apical surfaces of PCs and CCs are unusually resistant to extreme pH gradients.

Effect of activity differences on hydrogen migration in dissimilar titanium alloy welds

The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency for hydrogen migration within dissimilar metal welds. The alloys were: Ti-CP, Ti-3A1-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-lTa, Ti-6A1, Ti-6A1-4V, and Ti-6Al-2Nb-lTa-0.8Mo. Hydrogen pressure-hydrogen concentration relationships were determined for temperatures from 600 ‡C to 800 ‡C and hydrogen concentrations up to approximately 3.5 at. pct (750 wppm). Fusion welds were made between Ti-CP and Ti-CP and between Ti-CP and Ti-6A1-4V to observe directly the hydrogen redistribution in similar and dissimilar metal couples. Hydrogen activity was found to be significantly affected by alloying elements, particularly Al in solid solution. At a constant Al content and temperature, an increase in the volume fraction ofΒ reduced the activity of hydrogen in α-@#@ Β alloys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen activity was much greater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen redistribution should be expected within similar metal couples subjected to extreme temperature gradients, such as those peculiar to fusion welding. Significant hydrogen redistribution in dissimilar alloy weldments also can be expected for many of the compositions in this study. Hydride formation stemming from these driving forces was observed in the dissimilar couple fusion welds. In addition, a basis for estimating hydrogen migration in titanium welds, based on hydrogen activity data, is described.

The physiology of Lactococcus lactis subsp. lactis biovar. diacetylactis immobilized in hollow-fibre bioreactors: glucose, lactose and citrate metabolism at high cell densities

Lactococcus lactis subsp. lactis biovar. diacetylactis was selected to study the physiological influences of immobilization and growth to high cell densities. Cells were cultivated on glucose or lactose medium in the presence and absence of citrate. With excess glucose the cells produced mainly lactate as the fermentation product (homofermentative) providing that not all of the substrate was consumed. The population so cultivated was exposed to extreme gradients of pH and lactate concentrations. When the glucose concentration was reduced the population showed a mixed product profile with half of the glucose being fermented to lactate, the remainder to formate, acetate, ethanol and 2,3-butanediol. Inclusion of citrate in the medium shifted the population to homofermentation, with respect to the amount of glucose or lactose consumed. The citrate was metabolized via the pyruvate-formate lyase and α-acetolactate synthase routes. The pH of the medium was shown to strongly influence the product profile from citrate, presumably by affecting the activity of the key enzymes of pyruvate metabolism. The lactococci immobilized at high cell densities show product profiles typical of carbohydrate limitation at low dilution rates.

Enhanced sputtering of one species in the processing of multielement thin films

We discuss two mechanisms for enhanced selective sputtering which may strongly influence the composition of thin films deposited under energetic particle bombardment. The first mechanism occurs on sloping substrate surfaces which experience a higher fraction of material resputtered than flat surfaces. 500 eV Ar+ ion bombardment during deposition of Al–5 at. % Cu causes an increase in Cu concentration by up to a factor of 2 on sloping surfaces. This effect is modeled with the dynamic Monte Carlo code T-DYN, which simulates film growth under ion bombardment and confirms the selective sputtering of Al. The second mechanism occurs if diffusion of the film constituents is significant, and the film is situated on top of an underlying material containing one of the film components. Enhanced sputtering of one component occurs if that component maintains a high surface concentration by diffusion or segregation. Raising the temperature of TiSi2 on (100) Si to the range in which Si atoms are mobile (500–700 °C) increases the sputtering yield ratio of Si to Ti under 300 eV Ar+ bombardment to a value of 19 from the room temperature value of 2. During high rate sputter deposition, both of these mechanisms of enhanced sputtering of one species may occur, leading to phase separation and extreme concentration gradients on topographically structured surfaces.

A HYBRID SOLUTION OF THE SEMICONDUCTOR DEVICE EQUATIONS

State of the art programs for the solution of the drift diffusion semiconductor equations are based on finite difference techniques, or on certain combinations of finite elements and finite differences. The extreme gradients which occur in semiconductor devices have motivated several attempts to exploit perturbation analysis in order to improve the numerical scheme. Selberherr, Markowich et. al. showed the singular perturbation nature of the drift diffusion equations. Their analytic approximations of diodes influenced some aspects of the Minimos and Bambi simulators. Asymptotic analyses of MOSFET were presented by Brews and by Ward. These solutions are mainly valid for long channels, and their accuracy is limited.

Underplating of the crust and xenolith geotherms in Australia

Xenolith data indicate extreme geothermal gradients at shallow depth in eastern Australia contrary to results obtained from models based on thermal conduction. Consequently mechanisms involving partial melting accompanied by underplating must be assumed to supply additional heat unrelated to radiogenic profiles. A mid‐crustal heat‐source is required to satisfy the xenolith data suggesting a layered structure consistent with episodic underplating at maximum rates approaching 900m/Myr in eastern Australia.

Viscous sintering: the surface-tension-driven flow of a liquid form under the influence of curvature gradients at its surface

A boundary-element method is applied to solve the equations describing the deformation of a two-dimensional liquid region under the influence of gradients of the curvature of its outer boundary. This research is motivated by a desire to obtain a better understanding of viscous sintering processes in which a granular compact is heated to a temperature at which the viscosity of the constituent material becomes low enough for surface tension to cause adjacent particles to deform and coalesce. The boundary-element method is capable of showing how a moderately curved initial shape transforms itself into a circle. Initial shapes showing more extreme curvature gradients, which are relevant in the initial stages of a sintering process, cannot be dealt with by the boundary-element method in its present form. The numerical solution of the continuous model shows a tendency to create oscillations in the outer boundary of the liquid region. On the other hand, an analytical small-amplitude analysis shows that rapid oscillations vanish exponentially fast.

Aerodynamic Torque Characteristics of Butterfly Valves in Compressible Flow

The results of an experimental investigation of the aerodynamic torque characteristics of butterfly valves under compressible flow conditions are reported. Both three-dimensional prototype valves and two-dimensional planar models have been studied at choked and unchoked operating points. Other parameters investigated include the operating pressure ratio across the valve, the valve disk angle, and the disk shape. The results demonstrate the importance of flow separation and reattachment phenomena on the valve aerodynamic torque characteristics, the importance of disk shape at intermediate angles, and the sensitivity of the torque to the valve disk geometry near the leading and trailing edges where extreme pressure gradients can occur.