bluuebunny/crossref_metadata_2025 huggingface dataset:
A modified effective binary diffusion model
Authors: Zhang Youxue
Published: February 2008

Uphill diffusion, the diffusion of a component up its own concentration gradient, has often been observed in experimental studies of diffusion in synthetic multicomponent systems and natural silicate melts. To apply experimental data to natural processes, it is necessary to quantify this effect. A modified effective binary diffusion model is developed in this paper. The diffusive flux of a component is assumed to be proportional to its activity gradient (instead of concentration gradient). The activity coefficient is related to its concentration at quasi‐equilibrium distribution in a compositional continuum with the help of the two‐liquid partition concept. The concentration at quasi‐equilibrium distribution is approximately expressed as a simple function of SiO2 or SiO2+Al2O3 concentration. The diffusion of SiO2 or SiO2+Al2O3 is assumed to be effectively binary. With these assumptions, diffusion of a major or trace component can be described by two adjustable parameters, D (the intrinsic effective binary diffusivity) and Cƒ. (a parameter related to two‐liquid partition coefficients). The simple model fits well the experimental uphill diffusion profiles produced by diffusive crystal dissolution into an andesitic melt using constant diffusivities when compositional variation across the whole profile is small. The parameters fit to the profiles are generally physically meaningful. The model is able to predict the direction of diffusion (occurrence or absence of uphill diffusion) in previous experiments and can be used to simulate the development of chemical concentration and isotopic ratio profiles during diffusion. When compositional dependence of diffusivities is allowed, the model may be applied to systems with greater compositional variation. This model can be tested in several ways and should be useful in studying diffusion in a variety of natural systems.
A reexamination of the flexural deformation beneath the Hawaiian Islands
Authors: Wessel Pål
Published: February 2008

Seismically derived depth estimates to the top of the oceanic crust beneath the Hawaiian Islands indicate that the curvature of the deflected lithosphère is much larger than commonly believed. The conservative and model‐independent curvature estimates exceed 10−7 m−1 and are comparable in magnitude to curvatures at trenches and outer rise systems. The depth estimates are used to constrain both two‐dimensional (2‐D) and three‐dimensional (3‐D) flexural models. The curvature constraints require a 2‐D variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load. In an attempt to understand the nature of the yielding beneath the Hawaiian Islands we introduce two new 3‐D models. The first model combines a realistic yield strength based rheology with a new technique for 3‐D flexure calculations in which the elastic plate thickness is curvature‐dependent. The new variable rigidity model predicts an undeformed (mechanical) plate thickness of 44 km, decreasing to 33 km beneath the big island of Hawaii. The best‐fitting mechanical thickness corresponds approximately to the depth to the 600 °C isotherm in 90‐m.y.‐old lithosphere. The second model uses a broken plate, but here the crack is oriented along the weak Molokai fracture zone rather than along the island chain trend. This unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models. Both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts.
Application of fracture mechanics to the simulation of seismicity and recurrence of characteristic earthquakes on a fault
Authors: Yamashita Teruo
Published: February 2008

We present a fault model consisting of a large number of collinear cracks and intervening asperities for the simulation of seismicity and recurrence of large earthquakes on a fault. We apply fracture mechanics to the analysis of the interaction of cracks and to the consideration of fracture condition. Our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distribution of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths. The frequency distribution of event sizes is found to be bimodal with no intermediate size event in all the calculations. For the smaller events the frequency distribution shows a power law behavior, whereas the larger size events denote ones that rupture the whole fault. Statistically, the same frequency distributions are obtained for inhomogeneous distributions of crack lengths and fracture strengths. The occurrence of events that rupture the whole fault is shown to be periodic when the fault is isolated. The magnitudes of these large events generally increase with increasing recurrence period for given fault length and rate of remote loading. The events having a longer recurrence period have a shorter fault length for given magnitude and rate of remote loading. If there are fault interactions, the periodicity in the recurrence of the large events that would be observed for the isolated fault disappears and the recurrence of the large events becomes complex.
The electrical conductivity of lherzolite
Authors: Duba Al , Constable Steven
Published: February 2008

Electrical conductivity as a function of oxygen fugacity (ƒO2) between 10−5 Pa and 1 Pa, temperature between 700°C and 1200°C, and time over a period of 1700 hours are reported for a lherzolite nodule from Mount Porndon, Australia. Analysis of complex impedance collected at 100 Hz, 1 kHz, and 10 kHz indicates that there is significant frequency dispersion in this rock at temperatures below 900°C–1000°C. By choosing the resistance of an equivalent parallel RC network at the frequency having minimum phase, the effect of dispersion is largely avoided. Conductivity as a function of time, collected following changes in ƒO2 at 1010 and 1200°C, has been analyzed to determine the diffusivity of defects relating ƒO2 to the electrical conduction mechanism in olivine. The diffusivities so obtained are in remarkably good agreement with those determined from strain measurements during creep tests, which implies that magnesium vacancies are the rate‐limiting step for conductivity reequilibration after ƒO2 changes. A longer‐term process is observed in this rock in which the conductivity drifts upward or downward with a time constant of hundreds of hours after initial reequilibration to gas mix ( ƒO2) changes. We speculate that this long‐term drift could be related to equilibration of the iron distribution between coexisting olivine and pyroxene as a function of ƒO2. The sense is an increase in conductivity (olivine gains iron) at low ƒO2 and vice versa. The lherzolite conductivity data are not significantly different from measurements made on olivine single crystals and polycrystals, even though the rock contains about 34 modal % pyroxene. This consistency of laboratory measurements of electrical conductivity of olivines from many localities and geological settings supports the use of recent models relating mantle temperature with electrical conductivity in the interpretation of mantle geo/electromagnetic soundings.
Dynamics of Pacific plate crust in the South Island (New Zealand) zone of oblique continent‐continent convergence
Authors: Kamp Peter J. J. , Tippett J. Mark
Published: February 2008

Here we analyze topographic and fission track data to quantify the response of the surface of the Pacific plate in South Island, New Zealand, to late Cenozoic oblique continental convergence across the Alpine fault. Over the central 350 km length of the Southern Alps mountain chain we derive and map the rates of mean surface uplift, the rates of working associated with mean surface uplift during the late Cenozoic mountain building, the amounts and rates of denudation and consequent isostatic rebound, and the tectonic component of rock uplift. The rate of mean surface uplift ranges from <0.1 mm/yr adjacent to the Alpine fault to >0.3 mm/yr over most of the area east of the Main Divide. The highest rates of mean surface uplift occur to the southeast of the regions of highest mean elevation and relief. The rate of working against gravity during uplift of the mean surface ranges from ∼2.5 mW m−2 in the southwest to ∼10 mW m−2in the central eastern parts of the Alps. Areas of lower mean elevation uplifted most recently have received rates of energy input similar to that of areas of higher mean elevation where uplift started earlier. The amount of denudation is large compared with the mean surface uplift and ranges from ∼18 km adjacent to the Alpine fault to ∼2 km along the southeast margin of the Southern Alps. The rate of denudation ranges from ∼2.5 to ∼0.5 mm/yr with increasing distance from the Alpine fault across the Alps to the southeast. The amount of isostatic uplift ranges from a maximum of 14 km adjacent to the Alpine fault to ∼2 km along the southeast margin of the Alps. The tectonic component of uplift varies from ∼4 km along the Alpine fault to ∼1 km along the eastern margin of the Alps.
Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long‐wavelength magnetic anomalies
Authors: Kelso Paul R. , Banerjee Subir K. , Teyssier Christian
Published: February 2008

Rock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo‐single‐domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400°C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long‐wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanent magnetization along the present‐day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present‐day field of Central Australia. Susceptibility is nearly constant with temperature to within 30°C of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both remanent and induced components, that are large enough to account for most long‐wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high‐grade rocks in the Canadian Shield.
Automaton models of seismic fracture: Constraints imposed by the magnitude‐frequency relation
Authors: Lomnitz‐Adler Jorge
Published: February 2008

To obtain an understanding of the ingredients required in a realistic model of fault dynamics, we have constructed a number of models for the initiation and propagation of seismic fractures on a planar fault. The models are all of the cellular automaton‐type and fall into two broad categories which can be subdivided into 40 different classes. They differ in whether the fracture propagates as a crack or as a partial stress drop model; whether they are loaded homogenously or randomly; whether or not the models are asperity models; whether the characteristic time associated to the initiation of fracture is short or long; and whether or not the dynamic variable (e.g., stress or energy) is conserved on the fault plane. We restrict ourselves to the question whether models are capable of reproducing a Gutenberg‐Richter power‐law decay of event frequency with fracture dimensions, irrespective of the b value. We find that very few models can generate a power law which extends to all sizes, although more models can generate power laws that cover a broad range of sizes. Of these, only a few exhibit acceptable scaling behavior with system size. We conclude that, within the class of models studied, only a reduced subset of partial stress drop models is acceptable for the modeling of seismic fault dynamics.
The paradox of the axial profile: Isostatic compensation along the axis of the Mid‐Atlantic Ridge?
Authors: Neumann Gregory A. , Forsyth Donald W.
Published: February 2008

Mantle Bouguer gravity anomalies (MBA) and bathymetry on three profiles covering more than 1000 km along the axis of the Mid‐Atlantic Ridge (MAR) are highly correlated, suggesting that along‐axis topographic relief is locally compensated by variations in crustal thickness and/or mantle density structure. The quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin, narrow elastic strip, representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak, bounding normal faults. The paradox is that across‐axis profiles show that the median valley is an uncompensated feature, apparently created by a dynamic mechanism. New, extensive off‐axis coverage of the MAR from 31° to 36°S shows that the high correlation does not persist outside the axial zone. We suggest that the on‐axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along‐axis variations in crustal thickness, both of which contribute to the MBA. If the mechanism is extension of a brittle‐ductile lithosphere, the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis. A three‐dimensional (3‐D) thermal model incorporating passive mantle flow, hydrothermal circulation, the plate boundary geometry, and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography. We model the expected topography using cross‐axis sections of the 3‐D thermal model and a 2‐D finite element model of an extending lithosphere that incorporates temperature‐ and strain‐dependent rheology, as well as the flexural response of a thickening plate. The predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry. At the 18 mm yr−1 spreading half‐rate in our South Atlantic survey, the 2‐D models also reproduce the observed rapid transition along the axis between rift valley/no rift valley morphology. We conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the Southern MAR and the along‐axis correlation between MBA and bathymetry.
An inverse problem related to a finite part of a compact surface: Potential deduced from gravity gradients in space
Authors: Belikov M. V. , Groten E.
Published: February 2008

Mean values for gravity and potential at the surface of the Earth are derived for limited areas in terms of mean anomalies using the cross‐track and radial components of the Eötvös tensor derived from space‐borne gradiometer data at satellite altitude. Stability, resolution, and precision are investigated, and reliability criteria are established. The inverse boundary value problem is discussed for specific solutions for smaller and larger areas at the Earth's surface; discrete area means and point values are considered.
Velocity structure of the Long Valley Caldera from the inversion of local earthquake P and S travel times
Authors: Romero Arturo E. , McEvilly Thomas V. , Majer Ernest L. , Michelini Alberto
Published: February 2008

A high‐resolution tomographic study of the Long Valley region began in 1989 with the installation of a special fan array of eight three‐component borehole (80–160 m depths) receivers on the northwestern rim of the caldera to provide the data necessary for a joint inversion for hypocenters and the three‐dimensional P and S wave velocity structure in and below the caldera. The experiment specifically targeted the subsurface location of the previously inferred magma chamber beneath the Resurgent Dome. Additional coverage was provided by existing U.S. Geological Survey and University of Nevada, Reno seismographic networks. We employed the progressive inversion scheme of Thurber (1983), with cubic spline interpolation after Michelini and McEvilly (1991). The final model is based on data from 280 well‐distributed local earthquakes. Some 6900 arrival times were used, including about 700 S wave times from the three‐component stations. The resulting velocity structure reflects the known geology, defining the low‐velocity caldera fill to 2 km depth, contrasting markedly with the surrounding higher‐velocity Sierran block and highland terrane. No isolated distinct low‐velocity anomalies are revealed beneath the caldera floor, although a diffuse zone of reduced velocity persists to a depth of about 8 km. These lower velocities may be related to hydrothermal alteration and/or extensive fractures. The Vp/Vs structure contains significant lateral variation within and beneath the caldera to about 8 km depth, and these variations can be related to accepted models of the active geothermal system. The lack of a significant S wave velocity anomaly, along with the normal or low values of the Vp/Vs ratio, argues against the presence of a sizable and distinct magma body at shallow to midcrustal depth beneath Long Valley caldera, although a very low percentage of partial melt cannot be precluded.
Fault strain and seismic coupling on mid‐ocean ridges
Authors: Cowie Patience A. , Scholz Christopher H. , Edwards Margo , Malinverno Alberto
Published: February 2008

The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3°S and between 13°N and 15°N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5–10% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19°S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid‐ocean ridge faults it is shown that at fast spreading ridges (≥90 mm/yr) the seismic portion of a fault (Ws) is a small proportion of the total downdip width of the fault (Wƒ). The ratioWs/Wƒinterpreted as the seismic coupling coefficientX, and in this caseX≈ 0. In contrast, at slow spreading rates (≤40 mm/yr),Ws≈Wƒ, and thereforeX≈ 1, which is consistent with the occurrence of large‐magnitude earthquakes (mb= 5.0 to 6.0) occurring, for example, along the Mid‐Atlantic Ridge axis.
Isotopic and geochemical constraints on the origin and evolution of postcollapse rhyolites in the Valles Caldera, New Mexico
Authors: Spell Terry L. , Kyle Philip R. , Thirlwall Matthew F. , Campbell Andrew R.
Published: February 2008

Ring‐fracture rhyolites of the Valles Caldera (VC) were examined to determine the evolution of the magma system following eruption of the upper Bandelier Tuff (UBT) and subsequent caldera collapse. Volcanism began with eruption of Deer Canyon (DC), Redondo Creek, and Del Medio (DM) rhyolites during the interval 1140–1133 ka. Quartz δ18O for the UBT, (average +8.3 ‰), DC (+7.9 ‰), and DM (+8.7 ‰) rhyolites indicate no significant lowering of δ18O following caldera collapse. In contrast, DM rhyolites record low ϵNd of −3.6 to −3.8 relative to the UBT (−2.7) and variable initial 87Sr/86Sr (0.70923–0.71307). Del Abrigo (DA) and Santa Rosa I rhyolites (973–915 ka) exhibit lower ϵNd (−4.4 to −4.6) and initial 87Sr/86Sr (0.70707–0.71009), as well as shifts in compatible and incompatible trace element ratios. Seco, San Luis, and Santa Rosa II rhyolites (800–787 ka) have ϵNd (−4.0 to −4.3) similar to the 973 to 915 ka rhyolites but lower initial 87Sr/86Sr (0.70616–0.70747). After a hiatus of 230 ka, San Antonio (SA), South Mountain (SM), and La Jara (LJ) rhyolites (557–521 ka) were erupted with ϵNd of −3.7 to −4.3, distinctly lower initial 87Sr/86Sr of 0.70513–0.70553 and less evolved trace element compositions. The youngest rhyolites, the El Cajete‐Banco Bonito group (EC‐BB) (300–170 ka) are petrographically and geochemically distinct with the highest ϵNd (−2.7 to −3.0) and the lowest initial 87Sr/86Sr (0.70464–0.70478) of postcollapse rhyolites. The isotopic data indicate that Valles rhyolites are not direct melts of Proterozoic basement (ϵNd of −10 to −15) and indicate a significant mantle‐derived (basaltic) component. Elevated initial 87Sr/86Sr is restricted to rhyolites depleted in Sr (<10 ppm) and probably reflects minor upper crustal assimilation. Calculated magma δ18O values (+6.6 to +7.0 ‰) indicate no substantial supracrustal sediment in the source region, nor interaction with hydrothermal fluids (or altered wall rock) in the upper crust. The intermittent volcanic activity and the heterogeneity of petrographic, geochemical, and isotopic compositions from one eruptive interval to the next suggests that these rhyolites were vented from a series of temporally and spatially separated magma chambers. Parental melts supplied to these magma chambers were most probably produced by basalt‐induced fusion events in a hybridized deep crustal environment.
Shear‐promoted phase transitions in Fe2SiO4 and Mg2SiO4 and the mechanism of deep earthquakes
Authors: Wu T.‐C. , Bassett W. A. , Burnley P. C. , Weathers M. S.
Published: February 2008

Phase transformations in end‐member olivines have been investigated in the temperature range comparable to the interior of subducting slabs. This work constitutes the experimental evidence that the kinetics of transformation of silicate olivine (α phase) to modified spinel (β) and spinel (γ) phases is enhanced by shear deformation. Natural fayalite (α‐Fe2SiO4) subjected to a pressure gradient from 0 to 25 GPa at 380°C in the diamond anvil cell (DAC) developed a ring of γ phase where the pressure was in the stability field of the γ phase and shear stress was large enough to promote the α→γ transition. The sample inside the ring, despite being at higher pressure, remained dominantly as α phase. The outermost, lower‐pressure region of the sample also remained as α phase. In the Mg2SiO4 system, the transition from α to β was observed at 575°C in runs in which pressure covered the stability fields of β phase, γ phase and mixed oxides. These results show that the characteristic transformation temperature TCh can be lowered as much as ∼200°C by shear deformation. On the basis of these observations, we propose a nonhydrostatic kinetic boundary for the α→β and α→γ transitions in mantle olivine. For temperatures below this boundary, the transformations are kinetically inhibited, while above it, the transformations can be promoted by shear deformation. Therefore, olivine carried to a depth of several hundred kilometers in a subducting slab can remain as metastable α phase until shear deformation causes its transformation. We suggest that this region of shear‐promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes.
An automatic method for determining three‐dimensional normal fault geometries
Authors: Kerr Hugh G. , White Nicky , Brun Jean‐Pierre
Published: February 2008

We describe a general kinematic method for automatically determining the three‐dimensional (3‐D) geometry of normal faults at depth from the 3‐D geometry of deformed hanging wall horizons. The method should be applicable to normal faults which do not change shape as deformation proceeds. It is a generalization of two‐dimensional models and assumes that the hanging wall deforms by arbitrarily inclined bulk simple shear. Deformation in three dimensions is parameterized by the three Euler angles which describe the horizontal slip vector, the rake angle, and the inclination of shear planes. As in two dimensions, differential compaction can be included by assuming that the axis of shortening is parallel to the direction of shear. Three‐dimensional fault geometry and the three Euler angles can all be automatically determined provided the geometries of two or more hanging wall horizons are known. Here, the method is tested on both synthetic data and on 3‐D sand‐box models. Synthetic modeling shows that unique solutions can be found and more importantly that the horizontal slip vector can be constrained. Solutions obtained from sand‐box models are also encouraging: in most cases the method can predict automatically the correct direction of extension and a 3‐D fault geometry close to the fault geometry used. Hence arbitrarily inclined bulk simple shear appears to be a good approximation of hanging wall deformation in three dimensions. This general method will allow 3‐D palinspastic analysis to be undertaken on 3‐D seismic reflection data sets.
Early Oligocene geomagnetic field behavior from Deep Sea Drilling Project site 522
Authors: Hartl Paul , Tauxe Lisa , Constable Catherine
Published: February 2008

Hydraulic piston coring operations at Deep Sea Drilling Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3–4 cm intervals (∼3–5 kyr). Natural remanent magnetization demagnetization studies indicate a well‐behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo‐single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a ∼20–40 kyr interval of low field intensity accompanied by apparently random, low‐amplitude, short‐duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well‐defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of ∼70–90°W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undiminished “normal” secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low‐degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along ∼70–90° W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly “tiny wiggles”. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.
Boundary layer theory and subduction
Authors: Fowler A. C.
Published: February 2008

Numerical models of thermally activated convective flow in Earth's mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction, other than by the inclusion of artificial weak zones. Here we show, using a boundary layer argument, how the “ rigid lid ” style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction to occur through the visoc‐plastic failure of lithospheric rock. An explicit criterion for the failure of the lid is given, which is sensitive to the internal viscosity ηa below the lid. For numbers appropriate to Earth's mantle, this criterion is approximately ηa > 1021 Pa s.
High‐pressure infrared sepctra of ∝ ‐quartz, coesite, stishovite and silica glass
Authors: Williams Q. , Hemley R. J. , Kruger M. B. , Jeanloz R.
Published: February 2008

High‐pressure infrared absorption spectra of α‐quartz, coesite, stishovite, and SiO2 glass are consistent with the primary compression mechanism of the initially tetrahedrally bonded phases being the bending of the Si‐O‐Si angle at pressures less than 10–20 GPa. At higher pressures, up to 40 GPa, we observe a decline in the intensity of the infrared SiO4 asymmetric‐stretching vibrations (zero pressure frequencies between 1040 and 1220 cm−1) of all three phases, with an increase in the relative amplitude between 700 and 900 cm−1. This change in intensities is attributed to an increase in the average coordination number of silicon through extreme distortion of tetrahedra. At pressures above ∼20 GPa, the low‐pressure crystalline polymorphs gradually become amorphous, and the infrared spectra provide evidence for an increase in silicon coordination in these high‐density amorphous phases. The pressure‐amorphized samples prepared from quartz and coesite differ structurally both from each other and from silica glass that has been compressed, and the high pressure spectra indicate that these materials are considerably more disordered than stishovite under comparable pressure conditions. Average mode Grüneisen parameters calculated for quartz, stishovite and fused silica from both infrared and Raman spectra are compatible with the corresponding thermodynamic value of the Grüneisen parameter, however, that of coesite is significantly discrepant.
Reanalysis of S‐to‐P amplitude ratios for gross attenuation structure, Long Valley Caldera, California
Authors: Sanders Christopher O.
Published: February 2008

Because of the strong interest in the magmatism and volcanism at Long Valley caldera, eastern California, and because of recent significant improvements in our knowledge of the caldera velocity structure and earthquake locations, I have reanalyzed the local‐earthquake S‐to‐P amplitude‐ratio data of Sanders (1984) for the gross three‐dimensional attenuation structure of the upper 10 km of Long Valley caldera. The primary goals of the analysis are to provide more accurate constraints on the depths of the attenuation anomalies using improved knowledge of the ray locations and an objective inversion procedure. The new image of the high S wave attenuation anomaly in the west‐central caldera suggests that the top of the principal anomaly is at 7‐km depth, which is 2 km deeper than previously determined. Because of poor resolution in much of the region, some of the data remain unsatisfied by the final attenuation model. This unmodeled data may imply unresolved attenuation anomalies, perhaps small anomalies in the kilometer or two just above the central‐caldera anomaly and perhaps a larger anomaly at about 7‐km depth in the northwest caldera or somewhere beneath the Mono Craters. The central‐caldera S wave attenuation anomaly has a location similar to mapped regions of low teleseismic P wave velocity, crustal inflation, reduced density, and aseismicity, strongly suggesting magmatic association.
Variability in the Greenland Sea as revealed by a repeated high spatial resolution conductivity‐temperature‐depth survey
Authors: Budéus Gereon , Maul Andreas‐Alexander , Krause Gunther
Published: February 2008

This paper deals with the mesoscale variability of water masses in the Greenland Sea and its implications for bottom water formation. The results are based on two conductivity‐temperature‐depth surveys performed in successive years. The same section in the central Greenland Basin was sampled in 1989 and 1990; an additional transect across Fram Strait was carried out in 1990. The transects extended from shelf to shelf with a station spacing of 18 km throughout. The data set reveals a surprisingly strong horizontal variability of space scales between 20 and 60 km in the entire Greenland Sea, not only in the frontal zones. The area investigated is subdivided into four hydrographical regimes for which mesoscale variability is discussed in detail. A noteworthy result is the major change of the deep‐sea thermal structure within 1 year. The classical pattern with upward doming cold waters in the central basin was found in 1989 but was replaced by a “capped” structure with a warm intermediate layer in 1990. The implications of the observed changes are discussed with respect to deep water formation. A mechanism, based on differential compressibility, is proposed which is able to introduce negative heat input selectively into the bottom layer. It is shown that the fine structure of temperature profiles observed in summer can be used as a tracer for the occurrence of deep convection during the preceding winter. Convective depths are concluded of about 2200 m for 1989 and of only about 250 m for 1990.
Surface heat flux parameterizations and tropical Pacific Sea surface temperature simulations
Authors: Giese Benjamin S. , Cayan Daniel R.
Published: February 2008

Models of the tropical Pacific Ocean routinely use bulk formulae to estimate the surface fluxes of heat into and out of the ocean. The formulae generally require atmospheric variables such as cloud cover, surface wind speed, air‐sea temperature difference, and relative humidity. Since these quantities are not provided by most tropical Pacific Ocean models, modelers have resorted to simplifying parameterizations in terms of a few primary variables. Several of these parameterizations are tested by using observed data and comparing fluxes calculated using the simplified parameterizations with those from the full bulk formula. The impact of the various parameterizations is also assessed in a tropical Pacific Ocean general circulation model by comparing model sea surface temperature (SST) with the monthly means and anomalies of observed tropical Pacific SST during a 6‐year period that included both warm and cold episodes. The different flux parameterizations yield a broad mix of results. Overall, using a constant relative humidity of 0.80 does well, while using a constant downward radiation and a constant air temperature minus SST produces large discrepancies in comparison with the full flux parameterizations. In agreement with previous studies the model runs demonstrate that SST variability on the equator is dominated by wave processes whereas off the equator the surface flux of heat plays a crucial role in determining changes in SST.
Bubble excitation of surface waves and aerosol droplet production: A simple dynamical model
Authors: Dekker H. , De Leeuw G.
Published: February 2008

A novel theoretical analysis is presented of the dynamics of bubble‐mediated jet droplet production. The aerosol particle is recognized in its “bound state” in a capillary surface wave which is generated by a bursting bubble. The spatial excitation profile—as known from photographic data—is discussed by means of an argument involving a separation of time scales. By taking account of the dispersion relation for surface waves, the ensuing temporal behavior (of the local amplitude, vertical velocity, acceleration, and radius of curvature) is calculated analytically. The droplet is ejected just prior to the instant of maximum local (vertical) deceleration. Energy balance considerations applied to the transition from its bound state to its “free state” allow the prediction of the droplet size (as a function of bubble radius) and of the ejection velocity (as a function of droplet size). Numerical calculations support the analysis. The agreement with experimental data is shown to be good.
A preliminary study of the impact of the ERS 1 C band scatterometer wind data on the European Centre for Medium‐Range Weather Forecasts global data assimilation system
Authors: Hoffman Ross N.
Published: February 2008

A preliminary assessment of the impact of the ERS 1 scatterometer wind data on the current European Centre for Medium‐Range Weather Forecasts analysis and forecast system has been carried out. Although the scatterometer data results in changes to the analyses and forecasts, there is no consistent improvement or degradation. In this sense, the impact of the data is neutral. Our results are based on comparing analyses and forecasts from assimilation cycles which are identical in all respects except that the control experiment uses no scatterometer data. The experiment period runs from 0000 UT, December 26, 1991, through January 2, 1992. The two sets of analyses are very similar except for the low level wind fields over the ocean. Impacts on the analyzed wind fields are greater over the southern ocean, where other data are scarce. For the most part the mass field increments are too small to balance the wind increments. The effect of the nonlinear normal mode initialization on the analysis differences is quite small, but we observe that the differences tend to wash out in the subsequent 6‐hour forecast. In the northern hemisphere, analysis differences are very small, except directly at the scatterometer locations. Forecast comparisons reveal large differences in the southern hemisphere after 72 hours. Notable differences in the northern hemisphere do not appear until late in the forecast. Overall, however, the southern hemisphere impacts are neutral. The experiments described are preliminary in several respects. In light of expected refinements to the scatterometer model function, the ambiguity removal algorithm and the analysis procedures, we expect these data to ultimately prove useful for global data assimilation.
East Antarctic sea ice: Albedo, thickness distribution, and snow cover
Authors: Allison Ian , Brandt Richard E. , Warren Stephen G.
Published: February 2008

Characteristics of springtime sea ice off East Antarctica were investigated during a cruise of the Australian National Antarctic Research Expedition in October through December 1988. The fractional coverage of the ocean surface, the ice thickness, and the snow cover thickness for each of several ice types were estimated hourly for the region near the ship. These observations were carried out continuously during the 4 weeks the ship was in the ice. Thin and young ice types were prevalent throughout the region, and the observations show a systematic increase in the total area‐weighted pack ice thickness (including open water area) from only 0.2 m within 50 km of the ice edge to 0.45 m close to the coast. Ice thickness averaged over the ice‐covered region only is also relatively thin, ranging from 0.35 m near the ice edge to 0.65 m in the interior. These values are probably typical of average winter thickness for the area. The average snow cover thickness on the ice increased from 0.05 m near the ice edge to 0.15 m in the interior. Average ice concentration increased from less than 6/10 near the ice edge to 8/10 in the interior. The ship‐observed concentrations were in good agreement with concentrations derived from passive microwave satellite imagery except in some regions of high concentration. In these regions the satellite‐derived concentrations were consistently lower than those estimated from the ship, possibly because of the inability of the satellite sensors to discriminate the appreciable percentage of very thin ice observed within the total area. Spectral albedo was measured for nilas, young grey ice, grey‐white ice, snow‐covered ice, and open water at wavelengths from 420 to 1000 nm. Allwave albedo was computed by using the spectral measurements together with estimates of near‐infrared albedo and modeled spectral solar flux. Area‐averaged albedos for the East Antarctic sea ice zone in spring were derived from representative allwave albedos together with the hourly observations of ice types. These area‐averaged surface albedos increased from about 0.35 at the ice edge to about 0.5 at 350 km from the edge, remaining at 0.5 to the coast of Antarctica. The low average albedo is in part due to the large fraction of open water within the pack, but extensive fractions of almost snow‐free thin ice also play an important role.
The detectability of Arctic leads using thermal imagery under varying atmospheric conditions
Authors: Stone Robert S. , Key Jeffrey R.
Published: February 2008

Arctic leads (open or refrozen fractures in sea ice) provide for the exchange of heat between the ice/ocean surface and the atmosphere. Their influence on the energy budget is not easy to quantify, however, because little is known about their spatial and temporal distributions due to the difficulty in detecting and mapping leads using remote sensing techniques. The way in which thermal contrast between leads of varying widths and thicknesses can be distinguished from the background multiyear ice surface under varying atmospheric conditions is examined. The normalized brightness temperature difference between image pixels that include lead fractions and of the background ice is used in an attempt to determine thresholds of detection accounting for sensor field‐of‐view and various atmospheric phenomena that influence the Arctic radiation balance during winter. Brightness temperatures are derived from modeled top‐of‐the‐atmosphere radiances for three thermal channels (3, 4 and 5) of the Advanced Very High Resolution Radiometer. Surface temperatures are prescribed as a function of ice thickness and the effects of the intervening atmosphere are simulated by varying the optical depths of hypothetical cloud or haze layers varying in microphysical characteristics. Results indicate that the limits of lead detection may be determined as a function of pixel lead fraction and atmospheric optical depth if suitable values of normalized contrast are used as detection criteria. For example, given a pixel resolution of 1.0 km and the presence of a layer of ice crystals having a visible optical depth of 0.6 just above the surface, the minimum detectable lead width is estimated to be between 400 m and 750 m depending on what threshold criteria are used which, in turn, depends on the homogeneity of the multiyear ice surface. For the same conditions and range of threshold criteria but assuming a layer optical depth of 0.3, the minimum detectable lead width decreases to between 290 m and 560 m. Minimum detectable lead widths are also found to depend on the microphysical and physical properties of any intervening atmospheric layer. Our simulations indicate that narrower leads are detectable when hazy conditions exist than when stratiform water or ice clouds are present for any given layer optical depth.
Drag coefficients for winter Antarctic pack ice
Authors: Wamser Christian , Martinson Douglas G.
Published: February 2008

This paper presents air‐ice and ice‐water drag coefficients referenced to 10‐m‐height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP‐86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS‐89). The optimal values of the air‐ice drag coefficients, made from turbulent flux measurements, are C10 = (1.79 ± 0.06) × 10−3 for WWSP‐86 and (1.45 ± 0.09) × 10−3 for WWGS‐89. Neutral drag coefficient values are CN10 = 1.68 × 10−3 for WWSP‐86 and 1.44 × 10−3 for WWGS‐89. The slightly lower values for WWGS‐89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median
mm for WWSP‐86 and 0.27 mm for WWGS‐89). These values are consistent with Arctic measurements for 80–100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice‐water drag coefficient for both WWSP‐86 and WWGS‐89, estimated from periods of ice drift thought to represent free‐drift conditions (air‐ice stress balanced by ice‐water drag and Coriolis force), is (1.13 ± 0.26) × 10−3, and the ice‐water turning angle
. This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first‐year Arctic ice. Consistent with previous findings for WWSP‐86, the free‐drift form of the momentum balance can be used to describe the observed WWGS‐89 ice drift observations by using an “effective” drag coefficient and turning angle that subsume the influence of ice‐ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by ∼33%.
Effects of a three‐dimensional mean flow on intraseasonal Kelvin waves in the equatorial Pacific Ocean
Authors: Johnson Eric S. , McPhaden Michael J.
Published: February 2008

Intraseasonal Kelvin waves observed in the central and eastern equatorial Pacific Ocean have first vertical mode structures which are significantly different from first vertical modes in an otherwise motionless ocean. The observed mode has doubled temperature amplitudes relative to zonal velocity in the upper thermocline and shorter vertical scale structures in both zonal velocity and temperature. These departures are observed in the shallowest 200 m, where the mean currents are strong, and are related to interactions between these mean currents and the wave field. Modeled linear waves interacting with a zonally invariant zonal mean flow qualitatively explain some of the observed wave structures. However, the best quantitative assessment of wave variability is one that takes into account the full mean circulation and its gradients in all three dimensions. Specifically, by diagnosing the linearized budgets of temperature and zonal velocity for the observed and model waves we find that the observed doubling of wave temperature amplitudes is largely produced by the effects of mean vertical advection of wave temperatures. The increased wave temperature amplitudes lead to a wave pressure field which decays more rapidly with depth. Thus, for the same surface amplitudes the observed wave's pressure and velocity fluctuations in the deep ocean should be half the amplitude expected for a wave in the absence of mean flow. The effects of mean vertical advection are frequency dependent and will be stronger for longer‐period waves. We conclude that models of low‐frequency variability in the equatorial oceans must include the effects of a fully three‐dimensional mean flow in order to realistically reproduce the structures of first vertical mode Kelvin waves.
Finite‐floe wave reflection and transmission coefficients from a semi‐infinite model
Authors: Meylan Michael , Squire Vernon A.
Published: February 2008

A model to describe the reflection and transmission of ocean waves by a single ice floe is developed from the semi‐infinite model of Fox and Squire (1990, 1991). This is done by considering the coefficients for the transition from ice to water in the semi‐infinite case in terms of those from water to ice. Finite‐floe reflection and transmission coefficients, R and T, respectively, are then found as the solution of a set of four simple simultaneous equations. The properties of R and T are investigated, and examples of their absolute values are given for several geometries. |R| compares well with the predictions of a precise model in the case of deep water. These results suggest that the analytical model described has applications to defining the sea state within marginal ice zones, given the floe size and ice thickness distributions and the incoming sea wave spectrum.