bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of the Walvis Ridge‐Rio Grande Rise Hot Spot System: Implications for African and South American Plate motions over plumes
Authors: O'Connor John M. , Duncan Robert A.
Published: February 2008

Crystallization ages of volcanic rocks, dredged or drilled from the Walvis Ridge (ten sites) and the Rio Grande Rise (one site), have been determined by the 40Ar/39Ar incremental heating technique. The fundamentally age‐progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of Tristan da Cunha, along the Walvis Ridge and Rio Grande Rise, and for the formation of the continental flood basalts located in Namibia (Africa) and Brazil (South America). The Walvis Ridge‐Rio Grande Rise volcanic system evolved along a section of the South Atlantic spreading‐axis, as the African and South American plates migrated apart, astride, or in close proximity to, an upwelling plume. Reconstructions of the spatial relationship between the spreading‐axis, the Tristan hot spot, and the evolving Walvis Ridge‐Rio Grande Rise volcanic feature show that, at about 70 Ma, the spreading‐axis began to migrate westward, away from the hot spot. The resulting transition to intraplate hot spot volcanism along the Walvis Ridge (and associated termination of Rio Grande Rise formation) also involved a northward migration of previously formed African seafloor over the hot spot. Rotation parameters for African motion over fixed hot spots (i.e., absolute motion) have been recalculated such that the predicted trail of the Tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the Walvis Ridge. African absolute motion has been extended to the South and North American plates, by the addition of relative motion reconstruction poles.
A case study of the seismic source function: Salmon and sterling reevaluated
Authors: Denny Marvin D. , Goodman Dennis M.
Published: February 2008

As part of a larger joint effort by the Defense Advanced Research Project Agency and the Department of Energy to study the seismic source problem, a comprehensive reevaluation of the 1964 Salmon and 1966 Sterling nuclear explosions in dome salt was carried out. The Sterling source function originally estimated by Springer et al. (1968) conveys the impression that the cavity was badly overdriven; on reexamination this does not appear to be the case. The work of Glenn et al. (1987) on the Sterling free‐field data is expanded upon, confirming that the cavity response was close to the theoretical expectation. Sterling's source function is estimated and is found to be comparable to Patterson's (1966) slightly weakened salt model. A source model for Salmon is derived from the Sterling source model and the five seismic stations that recorded both events. The new source model has a reduced displacement potential ψ∝ of about half that previously estimated. A temporary nonlinear two‐wave system developed during the Salmon explosion as the compressional wave evolved from a shock wave; the separation of these two waves resulted in a high‐frequency roll‐off of the reduced velocity potential of ω−3. In addition, it is shown that the comer frequency is much higher and is created much closer to the cavity than the eigenfrequency. For both Salmon and Sterling the radial stresses are approximately a low‐passed damped sinusoid superimposed on a small step function. The decoupling value of 72 obtained by Springer et al. (1968) is confirmed. A revision of Patterson's (1966) partial decoupling curve shows that the value for full decoupling in a shot‐generated cavity would be only slightly higher. Contrary to previous studies, decoupling as a function of frequency for the surface waves is found to be the same as for the P waves. A new definition of decoupling appropriate to threshold test‐ban treaty monitoring is also proposed.
A kinematic model for the Late Cenozoic development of southern California crust and upper mantle
Authors: Humphreys Eugene D. , Hager Bradford H.
Published: February 2008

A model is presented for the young and ongoing kinematic deformation of southern California crust and upper mantle that accounts for both the known late Cenozoic geologic record and upper mantle structure. In this model, the high‐velocity upper mantle Transverse Ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere. Using typical scaling relations, we estimate this anomaly to be ∼500°C colder and 1% more dense than average southern California mantle of the same depth. We infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly. The low‐velocity upper mantle anomaly beneath the Salton Trough region is attributed to high temperatures and 1–4% partial melt related to adiabatic decompression during mantle upwelling. Our model has crustal motions in the central Transverse Ranges region differing from mantle motions: While the subcrustal lithosphere south of the Transverse Ranges moves toward and sinks beneath the Transverse Ranges, the corresponding crust largely avoids convergence by adopting a more westerly direction of motion. The differing velocity fields necessitates of a decoupling zone immediately south of the Transverse Ranges near the Moho. We interpret the upper mantle anomalies as results of small‐scale convection beneath southern California, to which we attribute much of the recent tectonic activity in the region.
Block rotation in western Turkey: 1. Observational evidence
Authors: Westaway Rob
Published: February 2008

Western Turkey is a region of rapid continental extension, where extension rate increasing westward causes counterclockwise vertical vorticity and hence counterclockwise rotation around vertical axes on a regional scale. Field observations and seismicity of the past century are used to deduce positions and slip senses of major active oblique normal faults in this region, which take up extension in the upper crustal brittle layer and allow fault‐bounded blocks to rotate around vertical axes. The principal active normal faults in western Turkey strike roughly east‐west near the Aegean coast, turning gradually toward westnorthwest‐eastsoutheast strike across ∼200 km distance inland to the east. Slip vector azimuths indicate typical extension direction on these faults is ∼S18°±8W. They truncate a second set of faults with smaller displacement and roughly orthogonal strike and slip vector azimuth. As well as subdividing the brittle layer into blocks that show little internal deformation, some major oblique normal faults subdivide it into domains that show different Neogene rotation around vertical axes. Some domains contain sets of major active oblique normal faults that bound blocks that are elongated, angular, and shaped like tilted dominoes. These faults take up extension, and can also take up rotation around vertical axes provided they and the blocks between them rotate around vertical axes at the same rate. Where adequate paleomagnetic observations are available, they indicate these domains have rotated counterclockwise, as expected, by up to ∼40°: roughly equal to the change in strike of the principal set of active faults between the Aegean coast and the eastern edge of the actively extending zone. However, evidence is scarce for the right‐lateral strike slip expected in some kinematic models on a set of oblique normal faults that takes up extension and counterclockwise rotation; instead, each fault appears to have the component of strike slip necessary to enable it, given its local strike, to slip in Ue direction in which the region is extending. Two domains that have rotated clockwise are approximately circular, in plan view with diameter ∼90 km, and are bounded by faults with variable orientation; each comprises a single block with no internal major active faults.
Two‐dimensional inverse modeling of crustal thermal regime with application to East European geotraverses
Authors: Shen P. Y. , Wang K. , Beck A. E.
Published: February 2008

The modeling of crustal thermal regimes is basically the inference of subsurface distributions of heat flow density and temperature by continuing downward the near—surface measurements of heat flow density, temperature, thermal conductivity, and heat production. To account for the uncertainties in the near‐surface data and in the subsurface distributions of thermal conductivity and heat production, and to provide explicit estimates for the uncertainties in the resulting models, we have employed a Bayesian inverse formulation that incorporates the a priori information on model parameters in the form of a joint gaussian probability density function and that gives the inverse solution in the form of the most probable estimates and the a posteriori covariance matrix for the model parameters. To illustrate the method, we apply it to the modeling of thermal regimes along East European geotraverses EEGT 5 and EEGT 1, using both a linear 5‐point finite difference and a quadrilateral isoparametric finite element discretization. Two features are emphasized. First, we introduce an a priori autocovariance function for the heat flow density at model base to help stabilize the solution. Second, we demonstrate the improvement in solution accuracy and resolution when a priori temperature information is available and incorporated in the inversion. For the geotraverses studied, the finite difference and finite element formulations yield essentially identical results, although there is a substantial difference in the extent of their parameterization. In general, the surface and basal heat flow densities are similar in shape, implying a relatively uniform total crustal heat production across the geotraverses. Both the heat flow densities and the crustal temperatures exhibit strong lateral variations that appear to correlate with the tectonic age: the heat flow densities and temperatures are low in the Precambrian East European Platform, relatively higher in the Variscan units, high in the Alps, and very high in the Pannonian Basin. The results support the presence of lateral variations of up to 40 m W m−2 in the Moho heat flow density, provided that one accepts as reasonable an a priori standard deviation of 25% for the surface heat flow density and an uncertainty of a factor of two for the heat production. Finally, the a posteriori and the observed surface heat flow densities match well but exceptions occur in localized regions, suggesting the presence of perturbing effects that cannot be accounted for in a steady state, purely conductive model.
Structural evidence for northeastward movement on the Chocolate Mountains Thrust, southeasternmost California
Authors: Dillon John T. , Haxel Gordon B. , Tosdal Richard M.
Published: February 2008

The Late Cretaceous Chocolate Mountains thrust of southeastern California and southwestern Arizona places a block of Proterozoic and Mesozoic continental crust over the late Mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive Orocopia Schist. The Chocolate Mountains thrust is interpreted as a thrust (burial, subduction) fault rather than a low‐angle normal (exhumation, unroofing, uplift) fault. An important parameter required to understand the tectonic significance of the Chocolate Mountains and related thrusts is their sense of movement. The Chocolate Mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds. Fabric relations, supported by regional geologic evidence, indicate that these folds are an integral part of and coeval with the thrust zone. On a lower hemisphere equal‐area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities, spread over an area 60 by 10 km, Z folds plot northwest of and S folds plot southeast of a northeast‐southwest striking vertical plane of overall monoclinic symmetry. The only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast. Asymmetric microstructures studied at several localities also indicate top to the northeast movement. Paleomagnetic data suggest that the original sense of thrusting, prior to Neogene vertical axis tectonic rotation related to the San Andreas fault system, was northward. The essential point is that movement of the upper plate of the Chocolate Mountains thrust evidently was continentward. Continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern California. The suture predicted by this model is elusive; but the Chocolate Mountains thrust and underlying Orocopia Schist themselves may represent the suture, at the present level of exposure. Alternative tectonic models involving subduction of the Orocopia Schist eastward beneath continental southern California circumvent the suture problem but are presently not supported by any direct structural evidence.
Composition of the lower crust in west central Arizona from three‐component seismic data
Authors: Goodwin Erik B. , McCarthy Jill
Published: February 2008

New P and S wave seismic data from the Basin and Range province—Colorado Plateau transition zone allow a better determination of the composition of the lower crust than would be possible from P wave data alone. The data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the U.S. Geological Survey's Pacific to Arizona Crustal Experiment. Significant seismic observations about the lower crust include (1) strong P wave reflections at its top, (2) moderate‐strength S wave reflections from the entire lower crust, (3) an average P wave velocity of 6.6±0.15 kms−1, and (4) an average Poisson's ratio of σ = 0.27±0.02. Using these constraints and physical property measurements from laboratory rock samples, we conclude that the average composition of the lower crust in this region is intermediate‐to‐rnafic and that mafic underpiating has been minor. Consequently, the Cenozoic‐age mafic and ultramafic xenoliths that are found in the Transition Zone must have resided in the upper mantle or in a thin transition zone at the base of the crust, since these rocks have higher P wave velocities and Poisson's ratio than observed seismically.
The Bouse Formation and bracketing units, southeastern California and western Arizona: Implications for the evolution of the Proto‐Gulf of California and the lower Colorado River
Authors: Buising Anna V.
Published: February 2008

Mio‐Pliocene sediments of the lower Colorado River area represent the northernmost well documented extent of the proto‐Gulf of California, a tectonically enigmatic marine incursion that occupied much of what is now the Gulf of California region—including the lower Colorado River area and the Salton Trough—as much as 8 m.y. prior to the onset of spreading‐and transform‐related subsidence in that area. Fanglomerate and volcanic rocks informally referred to here as the Osborne Wash strata interfinger with the conformably overlying Bouse Formation. The Bouse Formation includes carbonate and coarse terrigenous‐clastic basin margin deposits that interfinger laterally with basin fill material; basin fill strata comprise a basal estuarine carbonate unit overlain by fine‐grained terrigenous‐clastic deltaic deposits. The upper portion of the Bouse interfingers with overlying cobble conglomerates here referred to as the Colorado River gravels. Syndepositional folding of the Bouse Formation and bracketing units is believed to reflect slumping on oversteepened slopes, perhaps exacerbated by episodic tectonic activity. Syndepositional faults show both normal and reverse separation; outcrop relations allow but do not prove a strike‐slip component of motion on some structures. Contemporaneous minor faults seem to reflect mutually incompatible stress orientations; this suggests that they record either localized stress fields or localized anomalous responses to regional stresses. Alternatively, they may reflect extension in two directions in the sediment pile overlying the subsiding basin floor. The Bouse Formation and bracketing units record four stages in the evolution of the northern proto‐Gulf/lower Colorado River area: (1) dissection of preexisting, detachment fault‐controlled topography and localized, interior‐drainage alluvial deposition (about 14–9 Ma); (2) regional subsidence and proto‐Gulf transgression (perhaps as early as about 8 Ma; not later than about 5.5 Ma); (3) progradation of ancestral Colorado River delta into the northern end of the proto‐Gulf basin (prior to about 4.3 Ma); and (4) arrival of throughgoing Colorado fluvial channel (prior to about 3.5–4 Ma). Outcrop relations between the Osborne Wash strata and the Bouse Formation, and the relationship of these units to modern landforms, indicate that topography in the lower Colorado River area has not changed significantly since middle Miocene time. Subsidence of the Bouse basin is believed to have occurred via broad regional downwarping, which may have represented a sag formed as the locus of active proto‐Gulf extension propagated northward into the lower Colorado River area from the block‐faulted southern portion of the proto‐Gulf. Timing suggests that incipient proto‐Gulf extension in the lower Colorado River area was arrested by the approximately 5‐Ma shift to the modern transtensional regime.
Geochemistry of crustally derived leucocratic igneous rocks from the Ulugh Muztagh Area, Northern Tibet and their implications for the formation of the Tibetan Plateau
Authors: McKenna L. W. , Walker J. D.
Published: February 2008

Igneous rocks collected from the Ulugh Muztagh, 200 km south of the northern rim of the Tibetan Plateau (36°28′N, 87°29′E), form intrusive and extrusive bodies whose magmas were produced by partial melting of upper‐crustal, primarily pelitic, source rocks. Evidence for source composition includes high initial 87Sr/86Sr ratios (∼0.711 to 0.713), 206Pb/204Pb ratios of 18.72, 207Pb/204Pb of 15.63 and 208Pb/204Pb of 38.73. The degree of melting in the source region was increased by significant heating via in situ decay of radioactive nuclides; a reasonable estimate for the heat production rate in the source is 3.9×10−6 W/m3. The crystallization ages and cooling ages [Burchfiel et al., 1989] of the earliest intrusive rocks within the suite suggest crustal thickening began in the northern Tibetan Plateau before 10.5 Ma, with maximum average unroofing rates in this part of the Tibetan Plateau for the period between 10.5 and 4 Ma at approximately < 2 mm/yr. The Ulugh Muztagh flows are at the northern edge of a widely distributed field of Plio‐pliestocene volcanic rocks in the north‐central Tibetan Plateau. The crustally derived rocks described here are an endmember component of a wide mixing zone of hybrid magmas; the other end‐member forms mantle‐derived, potassic basanites and tephrites exposed in the central section of the Plio‐Pleistocene field. The compositional trends in these belts strike east‐west, at high angle to the N30E strike of the Plateau itself. Consideration of the chemical data and published geophysical data argue that the sub‐Plateau mantle is mechanically detached from the overlying continental lithosphère, and that in this section of the plateau the thermal structure of the asthenosphere is not responsible for the formation or maintenance of the plateau's topography.
A model for Trondhjemite‐Tonalite‐Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons
Authors: Drummond Mark S. , Defant Marc J.
Published: February 2008

The petrogenesis of trondhjemite‐tonalite‐dacite (TTD) involves all major petrologic models in various tectonic settings. A specific subtype of TTD, high‐Al type, is the one most commonly associated with Archean gneiss terranes. During the Archean, continental crust formation was operating at an elevated rate relative to the Phanerozoic, and the generation of high‐Al TTD played an integral role in its nucleation and growth. High heat flow, rapid convection, and subduction of hotter, smaller plates were unique tectonic elements to the Archean which optimized conditions required for transformation of subducted oceanic crust into sial via partial melting. Anatexis of Archean mid‐ocean ridge basalt (MORB) under eclogitic to garnet amphibolitic conditions produced weakly peraluminous to metaluminous high‐Al TTD with low heavy rare earth elements (HREE), Y, Nb, K/Rb, and Rb/Sr and high La/Yb and Sr/Y. This study demonstrates that Archean TTD crustal generation processes are also present in selected high‐Al Phanerozoic TTD terranes. The Cenozpic high‐Al TTD suites are commonly found in tectonic settings which are thought to recreate the elevated Archean thermal gradients, i.e., at sites of young, hot oceanic plate subduction. These relationships imply a petrologic continuity of TTD generation through time. A fertile zone of melting is envisioned at 23–26 kbar (75–85 km) and 700–775°C, where wet partial melting of the subducting slab occurs concurrently with dehydration reactions. At this depth, the converting mantle wedge continuously feeds hot mantle material to the wedge‐slab interface, creating strong temperature gradients, intraslab fluid migration, and slab melting. In summary, in modern arc terranes where young (< 20–30 Ma) oceanic crust is being subducted, high‐Al TTD generation via slab melting is considered likely; whereas when older (>30 Ma) oceanic crust is subducted, mantle‐derived magmas are dominant, giving rise to basaltandesite‐dacite‐rhyolite (BADR) fractionation suites. This study introduces the importance of subducted oceanic crustal age on arc petrogenesis.
Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus
Authors: Gillis Kathryn M. , Robinson Paul T.
Published: February 2008

Alteration patterns in the lavas and dykes of the Troodos Ophiolite, Cyprus, record a complex history of axial hydrothermal alteration, crustal aging, and subsequent uplift and emplacement of the ophiolite. Field mapping shows that distribution of five alteration zones, each with distinct mineralogical, geochemical, and hydrologie characteristics, is influenced by igneous stratigraphy, structure, and the nature and thickness of the overlying sediments. Paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off‐axis. Along spreading axes, low temperatures (≤50°C) were maintained by the rapid flow of seawater in and out of the lavas, and only minimal alteration took place. In contrast, lower water/rock ratios and higher temperatures (>200°C) in the dykes promoted extensive seawater‐rock interaction. Although the sharp rise in temperature between the two regimes generally coincides with the lava‐dyke transition, late‐stage intrusions or hydrothermal upwelling zones locally cause high‐temperature alteration to extend upward into the lavas. As a segment of crust moved off‐axis, temperatures remained low in the lavas and progressively decreased, from >250° to <80°C, in the dykes. High permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations show that alteration has a consistent vertical pattern in Troodos, the alteration zones are not laterally continuous, and the stratigraphie depth of their boundaries varies considerably.
Analytical expressions for the permeability of random two‐dimensional Poisson fracture networks based on regular lattice percolation and equivalent media theories
Authors: Hestir Kevin , Long Jane C. S.
Published: February 2008

The permeability of random two‐dimensional Poisson fracture networks can be studied using a model based on percolation theory and equivalent media theory. Such theories are usually applied on regular lattices where the lattice elements are present with probability p. In order to apply these theories to random systems, we (1) define the equivalent to the case where p = 1, (2) define p in terms of the statistical parameters of the random network, and (3) define the equivalent of the coordination number z. An upper bound for permeability equivalent to the case of p = 1 is found by calculating the permeability of the fracture network with the same linear fracture frequency and infinitely long fractures. The permeability of networks with the same linear fracture frequency and finite fractures can be normalized by this maximum. An equivalent for p is found as a function of the connectivity ζ, which is defined as the average number of intersections per fracture. This number can be calculated from the fracture density and distributions of fracture length and orientation. Then the equivalent p is defined by equating the average run length for a random network as a function of ζ to the average run length for a lattice as a function of p. The average run length in a random system is the average number of segments that a fracture is divided into by intersections with other fractures. In a lattice, it is the average number of bonds contiguous to a given bond. Also, an average coordination number can be calculated for the random systems as a function of ζ. Given these definitions of p and z, expressions for permeability are found based on percolation theory and equivalent media theory on regular lattices. When the expression for p is used to calculate the correlation length from percolation theory, an empirical formula for the size of the REV can be developed. To apply the models to random length systems, the expression for ζ must be modified to remove short fractures which do not contribute to flow. This leads to a quantitative prediction of how permeability decreases as one removes shorter fractures from a network. Numerical studies provide strong support for these models. These results also apply to the analogous electrical conduction problem.
Thermal infrared spectral character of Hawaiian basaltic glasses
Authors: Crisp Joy , Kahle Anne B. , Abbott Elsa A.
Published: February 2008

Multispectral thermal infrared remote sensing of Hawaiian basalts younger than a few hundred years old can be used to discriminate basalt lava flows that are difficult to map with visible and near‐infrared images. Laboratory thermal infrared reflectance spectra of the exposed surfaces of Mauna Loa and Kilauea basalts show systematic changes with age. Fresh glass collected from active lava flows shows a broad reflectance feature between 8 and 12 μm, indicative of a strong degree of disorder. After a few weeks of exposure to the environment the spectra of the top surfaces of samples begin to exhibit spectral features suggestive of ordering into silicate chainlike and sheetlike units. With progressive aging, features of apparent sheetlike structures become the preferred mode. As soon as 3 years after emplacement, a silica‐rich coating can begin to develop on the surfaces, adding another spectral feature to the laboratory reflectance measurements. The Thermal Infrared Multispectral Scanner (TIMS) with six channels between 8 and 12 μm has been used to take advantage of these spectral differences in basalts to map flows in Hawaii. Remote sensing instruments with increased spectral resolution in the thermal infrared would aid in further discrimination of basalts with more subtle differences in age, quenching, and weathering histories.
Active faulting and block rotations in the Western Transverse Ranges, California
Authors: Jackson James , Molnar Peter
Published: February 2008

The geometry of faulting, slip vectors of major earthquakes, and results of very long baseline interferometry (VBLI) are consistent with active clockwise rotation of crustal blocks in the western Transverse Ranges about vertical axes. Oblique left‐lateral thrust faults trending roughly east‐west across the western Transverse Ranges apparently separate long east‐west blocks of upper crust, and the large components of lea‐lateral shear on east striking planes suggest that the blocks rotate with respect to the San Andreas fault in the Mojave region (between the western Transverse Ranges and the Mojave Desert). Paleomagnetic declinations indicate some 35° (and possibly 60°) of clockwise rotation since about 10 Ma, and more importantly, the relative velocity between the only two, frequently measured, VBLI sites in the western Transverse Ranges is consistent with a rotation rate of 6°/Ma (±3°/Ma), during a 5‐year period in the 1980s. Azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the San Andreas fault across the western Mojave Desert, indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation (extension) of material parallel to that segment of the fault. Plate motions, Late Quaternary offsets on the San Andreas and San Jacinto faults, and geodetic measurements in the Mojave region yield a calculated direction of relative motion between the San Gabriel Mountains and the Pacific plate (across the western Transverse Ranges) that also is perpendicular to the San Andreas fault in the Mojave region. Rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates. The rotation rate of 2.8–5.3°/Ma calculated assuming that the long, east‐west blocks rotate in response to a uniform compression of a viscous substratum to the western Transverse Ranges, however, overlaps the range of observed average rotation rates. Thus, the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it.
River‐induced currents in a Fjord Lake
Authors: Hamblin P. F. , Carmack E. C.
Published: February 2008

Models based on stream tube geometry and geostrophy are used to describe the passage of a strong river through a long narrow (fjord‐type) lake. Analysis is specifically applied to the entry of the Thompson River into Kamloops Lake, British Columbia. Lake‐river interaction is considered in three stages of flow: a sinking plume stage during which the incoming river water sinks to a depth where its density matches that of the incoming river, a horizontal spreading stage during which the movement of river water is unconstrained by bottom and/or shoreline effects, and a shorebound stage during which the flow of river water downlake occurs principally in a balance between cross‐stream pressure gradients and the Coriolis force. Exchanges of mass and momentum are parameterized by entrainment and friction coefficients evaluated from field data. Results of this study show, in general, that river‐induced currents influence or even dominate circulation patterns within a fjord lake.
Geographical variations of NO and O3 in the lower stratosphere
Authors: Loewenstein M. , Borucki W. J. , Savage H. F. , Borucki J. G. , Whitten R. C.
Published: February 2008

Nitric oxide and ozone concentrations in the lower stratosphere have been measured from a high‐altitude research aircraft using in situ measuring techniques. Results of several geographical surveys are presented along with predictions of two two‐dimensional stratospheric models. Meridional and zonal data were obtained in June 1974 and in June, July, and August 1975. At longitudes 122°–158°W the meridional data taken between 5° and 80°N latitude show an increasing NO concentration with latitude, by a factor of 4 at 21‐km altitude and a less marked increase at 18 km. The minimum NO concentration at 21 km is observed at 5°N latitude and is about 6 × 108 cm−3. Zonal data at latitudes 22°–38°N taken from 55° to 176°N longitude show little variation of the NO and O3 concentrations with longitude.
Large‐scale relative dynamic topography of the Southern Ocean
Authors: Gordon A. L. , Molinelli E. , Baker T.
Published: February 2008

The large‐scale or climatic distribution of relative dynamic topography south of 30°S using a selective set of hydrographic stations indicates significant deviations from a purely latitudinal dependence of the isopleths. The geostrophic flow relative to 1000‐dbar level shows the Antarctic Circumpolar Current to vary greatly in its intensity and latitude during its course around Antarctica. High‐intensity flow occurs over the northern slope of the midocean ridge from 105°E eastward to the fracture zones near 145°W, significant perturbations being introduced in the vicinity of the Macquarie Ridge and Campbell Plateau. A strong expression of the Antarctic Circumpolar Current is found in the Drake Passage downstream of weak baroclinicity in the southeast Pacific. East of the Drake Passage strong meridional trends of the dynamic height isopleths occur with significant latitudinal offset (about 1100 km) of the main axis of geostrophic flow. In the southwest Indian Ocean, maximum baroclinicity is associated with the Agulhas Return Current near 40°S rather than the more continuous branch of Circumpolar flow to the south near 50°S. The two axes may combine east of Kerguelen Island. The dynamic height anomaly to the south of the main circumpolar current indicates the presence of weak baroclinic gyres in the western Weddell Basin and to the north and northeast of the Ross Sea and much meridional trend of the isopleths over the south eastern flank of the Kerguelen Plateau. North of the main circumpolar flow dynamic height anomaly also indicates weak baroclinicity, though a relatively strong axis of northward geostrophic flow occurs over the eastern flank of Campbell Plateau.
Hydrodynamic instability in the surf zone as a mechanism for the formation of horizontal gyres
Authors: Miller Christopher , Barcilon Albert
Published: February 2008

We model analytically the wave‐induced horizontal circulation on a mildly sloping beach. We are specifically interested in the cellular flow that is induced when monochromatic waves come in normal to straight, parallel, and nondeformable bottom contours, and we wish to determine the longshore dimension of the observed uniformly spaced cells as a function of the physical parameters of the problem. The energy for these motions is derived from the potential energy stored in the wave‐induced sea surface setup. We postulate that infinitesimal perturbations of the proper wavelength will extract the energy associated with this setup and convert it into the circulation velocities of the gyres. A steady, linear stability analysis is performed, based on an appropriate nondimensional set of depth‐integrated and time‐averaged equations which contain parameterizations for the effects of bottom friction and wave breaking. Lateral friction effects are not incorporated. Two distinct dynamic regions are considered: the zones seaward and shoreward of the breaker line. They are linked together by matching conditions at the breaker line. We find that the eigen‐wave numbers are a function of the rate at which the wave energy is dissipated across the surf zone (which is controlled by the beach slope) and of the magnitude of the bottom friction coefficient. The eigenvalues decrease with increasing beach slope and increase with an increasing bottom friction factor. The range of eigenvalues computed agrees with field observations, although the data are not complete enough to verify the parametric trends.
A note on the topographically controlled Oceanic Polar Front in the Barents Sea
Authors: Johannessen Ola M. , Foster L. A.
Published: February 2008

A mesoscale investigation of the temperature structure of the Oceanic Polar Front in the Barents Sea established that the position of the front as indicated by strong horizontal temperature gradients was well correlated with the bathymetric features.
On tidal propagation in shallow rivers
Authors: LeBlond Paul H.
Published: February 2008

A reexamination of the momentum balance in shallow rivers, with scaling appropriate to the Saint Lawrence and the Fraser, shows that frictional forces exceed accelerations over most of the tidal cycle. Consequently, tidal propagation in shallow rivers is more properly envisaged as a diffusion than as a wave propagation phenomenon. The long time lags associated with low waters, unexplainable in terms of a simple wave propagation model, are easily accounted for by an equally simple diffusive model.
Radar anisotropy of sea ice due to preferred azimuthal orientation of the horizontal c axes of ice crystals
Authors: Kovacs Austin , Morey Rexford M.
Published: February 2008

Results of impulse radar, ice crystal c axis, and subice current measurements on the fast ice near Narwhal Island, Alaska, are presented. The crystal structure of the ice was found to have a horizontal crystal c axis with a preferred azimuthal orientation. This orientation was found to align with the direction of the current at the ice‐water interface. Impulse radar reflection measurements revealed that the preferred orientation of the sea ice crystal structure behaved as a microwave polarizer. It was observed that when the antenna E field was oriented parallel with the c axis of the crystal platelets, a strong reflection of the radar signal from the bottom of the ice was obtained. However, when the antenna E field was oriented perpendicular to the c axis, no bottom reflection was detected. The results of this study fully support earlier reports of sea ice inhomogeneity and anisotropy in reference to both structure and electromagnetic energy transmission.
A time‐dependent model of a coastal boundary layer with friction
Authors: Birchfield G. E. , Lunde Albert T.
Published: February 2008

The time‐dependent development of a coastal frictional boundary layer forced by a wind stress impulse is investigated. Although because of mathematical nonuniformities in space and time a solution is found restricted to an inner region dominated by bottom friction, some details of the coastal response are found. In the context of a coastal response of a lake of the size of Lake Ontario it is found that maximum surface displacements of several centimeters occur in the left upwind and right downwind quadrants of the basin. In the adjustment to steady state these propagate counterclockwise around the coastal region. The coastal jets with maxima in the same quadrants do not move significant distances in the approach to steady state. Several aspects of the model appear in a forced response observed on the south shore of Lake Ontario.
Temporal sampling requirements for long‐term atmospheric pollutant dispersion studies
Authors: Sheih C. M. , Hess G. D.
Published: February 2008

The statistical sampling theory given by Lumley and Panofsky (1964) can be used to compute the temporal sampling interval and the total sampling duration required for statistical studies of the longterm horizontal dispersion of pollutants. To illustrate the technique, the release of a series of tracer particles from St. Louis has been simulated, and trajectories have been computed from observed winds. An analysis of trajectories according to season indicates that the minimum interval required for statistical independence of samples varies from a minimum value of 20 hours for winter to a maximum value of 49 hours for spring. If the trajectories are computed for the entire year without separation, the minimum sampling interval required for statistical independence is 35 hours. To limit the statistical error (ε) due to insufficient averaging time to 10%, the corresponding required durations of sampling are 1300 and 2900 hours for winter and spring, respectively. If the year is examined as a whole, a sampling duration of 2400 hours is required. These sampling durations are highly dependent upon ε, and if accuracy is limited to within 30%, they may be decreased by about an order of magnitude.
A new evaluation of the wind stress coefficient over water surfaces
Authors: Amorocho J. , DeVries J. J.
Published: February 2008

An analysis of data from numerous investigators, as well as information obtained directly by the authors, indicates that a large portion of the difficulties encountered in the past in establishing a relationship between the wind stress coefficient C10 and the wind velocity U10 can be attributed to computationally induced scatter of the data points. However, plots of the shear velocity u* against U10 reveal clear trends which show that three regions exist in the development of the wind shear stress: (1) a lower region in which the wind waves have not begun to break, and for which C10 is approximately constant; (2) a transitional region, after the onset of breakers, for which C10 varies nonlinearly with U10 and (3) a limiting region for which C10 tends again toward a constant value, and corresponds to a condition of breaker saturation. A single general equation to express C10 as a function of U10 is proposed, which agrees with the above findings. It is shown that in contrast with the perception of previous investigators, Charnock's coefficient α = z0g/u*2, where z0 is the roughness length and g is the acceleration due to gravity, is not constant anywhere in the range of wind velocities 0 < U10 < 40 m/s. Finally, the data indicate that the wind flow boundary in each one of the three regions described above can be classified as having ‘low roughness,’ ‘transitional,’ and ‘high roughness,’ respectively.
Long‐period gravity waves in ice‐covered sea
Authors: Bates Howard F. , Shapiro Lewis H.
Published: February 2008

A floating ice sheet under compressive stress is modeled as a laterally compressed thin linearly elastic plate, floating on a compressible liquid of constant depth over a rigid bottom. This system is analyzed as a wave guide for plane waves. Sinusoidal traveling waves having vertical planes of constant phase are sought for the impulse response of the system. Two bands of gravity waves are found: (1) flexural and (2) floating‐membrane. The two bands join at the critical frequency of the system, at which the impulse response is unbounded. The long‐period floating‐membrane gravity waves exist at all frequencies below the flexural gravity wave band. For long‐period gravity waves the coupling force between plate and liquid is found to be a Hooke's Law force; the effective foundation modulus is directly proportional to the difference between the square of the phase velocity of a wave in the floating‐plate system and that of a free‐surface gravity wave at the same wavelength. The analogy of an elastic plate supported on an elastic foundation is apt for these long‐period waves. It is concluded that the impulsive release of stored elastic energy when the compressed ice breaks supplies adequate energy to produce measurable long‐period waves in floating ice. The results suggest that long‐period wave motion accompanies lateral compression of the ice sheet; hence wave measurements may provide a means of detecting the buildup of widespread compressive stress in the ice.
The evolution of a turbulent Ekman layer
Authors: Csanady G. T. , Shaw P. T.
Published: February 2008

The evolution of shear flow at the sea surface, following the sudden application of wind stress, is modeled as a wall layer—outer layer combination of turbulent flow. The outer layer eddy viscosity is proportional to eddy length scale and hence to the penetration of the shear flow, so that it grows initially, then approaches a constant asymptote. The analytical solution for time‐dependent eddy viscosity has a structure very similar to the classical Ekman‐Fredholm solution for constant viscosity. In physical terms the spontaneous depth limitation of the shear layer at the surface of a rotating viscous fluid remains effective in turbulent flow even though the viscosity is not constant in space or time. When a sharp density interface limits the downward penetration of a surface shear layer at a ‘mixed’ layer depth h, the same rotating viscous fluid effect controls the shear across the interface and hence the entrainment rate. Therefore the equilibrium depth of a mixed layer remains proportional to u*/ƒ, as the shear layer in a homogeneous fluid (u* is the friction velocity; ƒ is the Coriolis parameter). A detailed model of the equilibrium mixed layer may be constructed by a combination of a free surface wall layer, an outer layer, and an interface wall layer. The behavior of this model is similar overall to the classical model of Rossby and Montgomery (1935) and leads to bulk formulae of identical structure.
An observation of a precipitation echo intensification associated with lightning
Authors: Szymanski E. W. , Szymanski S. J. , Holmes C. R. , Moore C. B.
Published: February 2008

A lightning echo detected by an 11‐cm radar was followed immediately in the same volume by the development of a modest precipitation echo that intensified and rapidly fell out. The event transpired too quickly to be observed easily by conventional, cloud physics radars.
Computation of acoustic‐gravity waves, Kelvin‐Helmholtz instabilities,and wave‐induced eddy transport in realistic atmospheric models
Authors: Klostermeyer J.
Published: February 2008

A variety of motions associated with gravity waves and Kelvin‐Helmholtz instabilities have recently been detected by high‐power VHF and UHF radars at tropospheric, stratospheric, and mesospheric heights. The simultaneously observed mean temperature and wind profiles differ considerably from those used particularly in theoretical studies of wave‐mean flow interactions. Therefore a linear operational model has been developed describing acoustic‐gravity waves and Kelvin‐Helmholtz instabilities in compressible atmospheres with arbitrary vertical profiles of the mean temperature, wind, and viscosity coefficient. The linearized hydrodynamic equations are solved by the nonstandard multilayer method and a Gauss‐Seidel group iteration. Results from test computations of wave‐critical layer interactions and a Kelvin‐Helmholtz instability show good quantitative agreement with known theoretical results from simple incompressible models. Wave‐induced instabilities are parameterized by mean eddy viscosity and thermal conductivity, assuming that the wave‐perturbed gradient Richardson number is nowhere smaller than some critical value. A new iterative method is used to obtain the height profiles of the eddy transport coefficients. At mesospheric heights, wave‐induced dissipation limits the amplitudes of internal gravity waves so that no significant nonlinearities are introduced in the hydrodynamic equations.
Vertical mixing of lake sediments by tubificid oligochaetes
Authors: Fisher J. B. , Lick W. J. , McCall P. L. , Robbins J. A.
Published: February 2008

Vertical mixing of lake sediments by tubificid oligochaetes was studied in laboratory experiments by using a radioactive (cesium 137 labeled sediment) marker horizon. Results from these experiments were used to develop and test a mathematical model describing tubificid sediment mixing as a dominantly advective process. Tubificids were found to mix sediments to a depth of 6–9 cm. The rate of tubificid sediment processing observed in the experiments agrees well with those reported by other workers. Extrapolation of laboratory results to Lake Erie shows that the feeding activity of tubificids alone may result in significant sediment mixing throughout the lake.
Emissions and concentrations of hydrogen sulfide in the air of the tropical forest of the Ivory Coast and of temperate regions in France
Authors: Delmas Robert , Baudet Jean , Servant Jean , Baziard Yves
Published: February 2008

Atmospheric H2S was measured by a fluorometric method (sensitivity 10 ng). In France, for aerated soils the emissions were between 0.8 and 27 μg m−2 h−1 H2S (average 5 μg m−2 h−1). The soil temperature is an important factor governing this emission. The H2S concentrations in the air ranged from 0.017 to 0.17 μg m−3 (average 0.080 μg m−3). In the Ivory Coast the H2S emissions were estimated between 30 and 300 μg m−2 h−1. The measured concentrations of H2S in the air at ground level ranged from 0.10 to 8.7 μg m−3. The relative importance of the measured emissions for anoxic soils of the humid equatorial forests in the global S cycle is discussed.