bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction to the Ulysses Encounter With Jupiter
Authors: Smith Edward J. , Wenzel Klaus‐Peter
Published: February 2008

The Ulysses spacecraft encountered Jupiter in February 1992, passing within 6.31 radii of the planet. For approximately 8 days it was inside the Jovian magnetosphere, and for several days before and after that, Ulysses was in the interaction regions formed by the solar wind (the magnetosheath and boundary layer). The inbound trajectory was at a ≈ 1000 LT and the outbound trajectory was at 1800 LT, that is, dusk, a unique feature of the flight path. Three regions interior to the magnetosphere were identified as on previous missions both inbound and outbound. In addition, the spacecraft twice penetrated a cusplike region at high latitude in the inner magnetosphere. Following closest approach, Ulysses traversed the Io plasma torus in basically a north‐south direction. Although Ulysses is a heliospheric mission, the experiments were well suited to an investigation of Jupiter's magnetosphere and have returned much new information. TMs introduction to the accompanying articles by the Ulysses investigators provides basic information on the experiments, the spacecraft, and the trajectory. In addition, the scientific context of the encounter is reviewed on the basis of the preliminary analyses of the Ulysses observations and a rudimentary comparison with the earlier Pioneer and Voyager results. Some important scientific questions raised by the encounter, along with some tentative answers, are presented.
Paleomagnetism of the Ordovician Bluffer Pond Formation: Paleogeographic implications for the Munsungun terrane of northern Maine
Authors: Potts Stephen S. , van der Pluijm Ben A. , Van der Voo Rob
Published: February 2008

The Munsungun area of northern Maine is an early Paleozoic terrane within the Central Mobile Belt (CMB) of the northern Appalachians. The Bluffer Pond Formation of the Munsungun Terrane is composed primarily of pillow basalt and gabbro with minor intercalated sediments and has a maximum thickness of 1230 m. This unit is of early‐Late Ordovician (Caradocian) age, based on paleontological data, and folding most likely occurred during the Devonian (Acadian Orogeny). This region has been metamorphosed to subgreenschist facies. Samples for paleomagnetic study have been obtained from 15 sites of pillow basalt and seven sites of gabbro within the Bluffer Pond Formation. Natural remanent magnetization (NRM) directions are similar to the expected present‐day field direction and are interpreted to be strongly affected by a viscous overprint. Detailed stepwise thermal demagnetization fails to adequately resolve a partial overprint direction with intermediate blocking temperature, but stable endpoints combined with great circles do isolate a characteristic high‐temperature component carried by magnetite as indicated by unblocking temperatures and rock magnetic experiments. After tilt correction the formation mean calculated from pillow basalt sites only is D = 339°, I = −33° (k = 12, α95 = 14°, n = 11), with a corresponding paleopole of 23°N, 133°E (δp = 9°, δm = 16°), calculated for an average site location of 46.5°N, 291°E. When both pillow basalt and gabbro sites are used to calculate a tilt‐corrected formation mean, the result is D =339°, I = −28° (k = 12, α95 = 12°, n = 14), and this characteristic component passes a tectonic tilt test. The calculated paleopole (26°N, 134°E; δp = 7°, δm = 13°) from pillows and gabbros falls near (but not on) the Ordovician apparent polar wander path segment for North America and yields a paleolatitude (18°± 9°S) similar to that of the Lunksoos Terrane which is located farther to the southeast within the CMB of Maine. The Laurentian margin at this time was located at paleolatitudes of 15–20°S. The paleogeographic implication of this result is that the Munsungun Terrane was located near the Laurentian margin within the Iapetus Ocean and this near‐continent location is consistent with a marginal or back arc basin tectonic environment for the extrusion of the Bluffer Pond volcanics.
Spatio‐temporal complexity of slip on a fault
Authors: Rice James R.
Published: February 2008

Three‐dimensional analyses are reported of slip on a long vertical strike‐slip fault between steadily driven elastic crustal blocks. A rate‐ and state‐dependent friction law governs motion on the fault; the law includes a characteristic slip distance L for evolution of surface state and slip weakening. Because temperature and normal stress vary with depth, frictional constitutive properties (velocity weakening/strengthening) do also. Those properties are taken either as uniform along‐strike at every depth or as perturbed modestly from uniformity. The governing equations of quasi‐static elasticity and frictional slip are solved on a computational grid of cells as a discrete numerical system, and a viscous radiation damping term is included to approximately represent inertial control of slip rates during earthquake‐like instabilities. The numerical results show richly complex slip, with a spectrum of event sizes, when solved for a grid with oversized cells, that is, with cell size h that is too large to validly represent the underlying continuous system of equations. However, in every case for which it has been feasible to do the computations (moderately large L only), that spatio‐temporally complex slip disappears in favor of simple limit cycles of periodically repeated large earthquakes with reduction of cell size h. Further study will be necessary to determine whether a similar transition occurs when the elastodynamics of rupture propagation is treated more exactly, rather than in the radiation damping approximation. The transition from complex to ordered fault response occurs as h is reduced below a theoretically derived nucleation size h* which scales with L but is 2 × 104 to 105 larger in cases considered. Cells larger than h* can fail independently of one another, whereas those much smaller than h* cannot slip unstably alone and can do so only as part of a cooperating group of cells. The results contradict an emergent view that spatio‐temporal complexity is a generic feature of mechanical fault models. Such generic complexity does apparently result from models which are inherently discrete in the sense of having no well‐defined continuum limit as h diminishes. Those models form a different class of dynamical systems from models like the present one that do have a continuum limit. Strongly oversized cells cause the model developed here to mimic an inherently discrete system. It is suggested that oversized cells, capable of failing independently of one another, may crudely represent geometrically disordered fault zones, with quasi‐independent fault segments that join one another at kinks or jogs. Such geometric disorder, at scales larger than h*, may force a system with a well‐defined continuum limit to mimic an inherently discrete system and show spatio‐temporally complex slip at those larger scales.
Nature of seismic coupling along simple plate boundaries of the subduction type
Authors: Pacheco Javier F. , Sykes Lynn R. , Scholz Christopher H.
Published: February 2008

The downdip width of the seismogenic zone is defined for 19 subduction zones. This width is measured from the base of the accretionary prism to the maximum depth of nucleation of thrust events along the plate boundary. Those two points are taken to define the upper and lower depth transitions from stable to unstable frictional sliding. The lower depth transition is found to be between 35 and 70 km. The dip angle of the thrust zone is also reevaluated. We find a linear increase in the dip angle as a function of depth, the slope of which varies between 0.2° and 0.6° km−1. The downdip width obtained, which is generally narrower than previously determined by most other authors, varies from about 50 to 150 km. We also determine the ratio of the rate of slip that occurs in earthquakes to the rate of relative plate motion. This ratio is defined as the seismic coupling coefficient (a). We obtain two different estimates of the seismic coupling coefficient: an average value from 90 years of seismicity and a value obtained using the slip‐predictable recurrence model for large earthquakes. We find a large variation in the computed values of a along and among subduction zones. For most of the subduction zones a is much less than 1.0; for several it is less than a few percent. Worldwide, we find no significant correlation between either the seismic coupling coefficient or the width of the seismogenic zone and subduction parameters such as the age of the oceanic lithosphere that is being subducted, plate convergence rates or absolute velocity of the upper plate in the hot spot reference frame. Such correlation exists only for a few individual subduction zones where other parameters do not vary as much. The observed variations in seismic coupling could be explained as differences in the frictional behavior of materials at the plate interface. Some of these differences may be attributed to the subduction of large bathymetric features, the roughness of topography, the presence of unstable triple junctions and active‐spreading ridges, and sediment composition.
Mechanics of wedge‐shaped fault blocks: 1. An elastic solution for compressional wedges
Authors: Yin An
Published: February 2008

An elastic model is developed to investigate the initiation of normal and thrust faults in thrust wedges. The model assumes frictional sliding along the base and a linear variation of shear and normal tractions with depth along the rear of the thrust wedge. Using this model, the roles of basal friction, pore fluid pressure, and wedge configuration in controlling the mechanics of thrust wedges were evaluated. The model predicts coeval development of normal and thrust faults in the same thrust wedge and a listric shape for both thrust and normal faults. In particular, lower friction favors dominantly horizontal compression in a thrust wedge, whereas higher friction can produce dominantly horizontal extension. Wedges bounded by steep thrusts (> 45°) are more likely to experience extension regardless of their sizes and the boundary conditions applied. For wedges bounded by shallow‐dipping thrusts (< 15°) and under the same boundary conditions, shorter wedges are more dominated by compression than longer wedges. Using this model, the length of a Hubbert‐Rubey thrust toe (defined as the length of the unfractured, frontal portion of a thrust wedge) is calculated. With the same boundary conditions and mechanical properties, a thinner wedge, in general, favors a longer Hubbert‐Rubey toe than a thicker wedge. This simple model is applied to explain the initiation of the short‐lived Miocene normal fault system in the Higher Himalaya as a consequence of a rapid release of pore fluid pressure along the Main Central Thrust during the formation of two‐mica leucogranites in the hanging wall. It also simulates the occurrence of normal faults associated with the El Asnam thrust‐type earthquake.
Hydration swelling effects on time‐dependent deformation of zeolitized tuff
Authors: Blacic James D.
Published: February 2008

Long‐duration creep deformation experiments were performed on zeolitized tuffs from Yucca Mountain, Nevada, at temperatures of 35°–75°C, confining pressure of 5 MPa, and water pore pressure of 0.1 MPa. The test samples were initially loaded to a relatively small fraction of the uniaxial compressive failure stress, and allowed to creep; then the stress was increased periodically until creep failure occurred. The strain versus time creep curves exhibit the classical phenomenological phases of primary, secondary, and tertiary creep analogous to those for ductile metals, although the tuffs deform by dilational microcracking and fail by brittle fracture. Mechanical “steady state” or secondary creep strain rates as low as 1 × 10−10 s−1 were observed in the tests. Tertiary creep axial failure strains averaged about 6 × 10−3. The early, water saturation phases of the tests are characterized by large hydration swelling strains associated with water absorption by the zeolite minerals in the tuffs. The hydration was often incomplete after one month of immersion, and slow, continued hydration affected the creep curves so that the samples appeared to lengthen against axial differential stresses as high as 55 MPa. This strong chemical‐mechanical coupling suggests that in situ hydration state changes could generate stresses of similar magnitude.
Three‐dimensional velocity structure, seismicity, and fault structure in the Parkfield Region, central California
Authors: Eberhart‐Phillips Donna , Michael Andrew J.
Published: February 2008

This study examines the three‐dimensional velocity structure in a 60‐ by 80‐km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three‐dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across‐fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4‐km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low‐velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20‐km‐long high‐velocity slice is imaged northeast of the SAF near Gold Hill. Its along‐fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right‐lateral strike‐slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.
Do layer 3 rocks make a significant contribution to marine magnetic anomalies? In situ magnetization of gabbros at Ocean Drilling Program hole 735B
Authors: Pariso Janet E. , Johnson H. Paul
Published: February 2008

We estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies. Results from Ocean Drilling Program Hole 735B show that the drilled crustal section has a mean effective remanent magnetization of 2.5 A/m. Olivine gabbros, which make up 60% of the section, have an average effective magnetization between 1 and 2 A/m. We used two‐dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface. These results indicate that if polarity boundaries within oceanic layer 3 are near‐vertical, crustal sections similar to 735B would contribute from 25% to 75% of the overlying marine magnetic anomalies at the sea surface. The shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers.
Computer simulation of defects and diffusion in perovskites
Authors: Wright Kate , Price Geoffrey D.
Published: February 2008

We have used computer simulation techniques to study the defect and diffusion processes in the perovskites SrTiO3, CaTiO3, and MgSiO3. The prediction of oxygen diffusion activation enthalpies in SrTiO3 agrees well with experimentally determined values thus justifying the use of such methods for materials with the perovskite structure. Calculations on defects and diffusion in MgSiO3 have been carried out at both zero and simulated high pressure in order to estimate the activation volumes for defect formation and diffusion. Our simulations predict that at pressures above 60 GPa, the nonstoichiometric MgO Schottky defect will to be the most favorable and that Mg will replace Si as the slowest moving ionic species. The activation volume for Si diffusion is found to be negative. The magnitude of activation enthalpies for intrinsic defect processes is large, suggesting that extrinsic processes may well dominate defect formation and diffusion in MgSiO3 perovskite at depth in the Earth's mantle.
Elastic properties of diopside, anorthite, and grossular glasses and liquids: A Brillouin Scattering study up to 1400 K
Authors: Askarpour Vahid , Manghnani Murli H. , Richet Pascal
Published: February 2008

The compressional Vpand shear Vs wave velocities of diopside (CaMgSi2O6), anorthite (CaAl2Si2O8), and grossular (Ca3Al2Si3O12) glasses and liquids have been measured by high‐resolution Brillouin spectroscopy up to 1200 or 1400 K. For all three materials, marked breaks in the wave velocity‐temperature relationships are observed at temperatures consistent with the glass transition temperatures observed in viscosity and calorimetry experiments. From comparisons between high‐temperature ultrasonic (relaxed Vp) data and extrapolated Brillouin (unrelaxed Vp and Vs) values, the configurational compressibility of diopside and anorthite liquids has been determined. The relative importance of vibrational and configurational contributions can be markedly different for the compressibility and heat capacity of silicate melts.
Temporal and geochemical variability of volcanic products of the Marquesas Hotspot
Authors: Desonie D. L. , Duncan R. A. , Natland J. H.
Published: February 2008

The Marquesas archipelago is a short, NW‐SE trending cluster of islands and seamounts that formed as a result of volcanic activity over a weak hotspot. This volcanic chain lies at the northern margin of a broad region of warm and compositionally diverse mantle that melts to build several other subparallel volcanic lineaments. Basalts dredged from submerged portions of volcanoes along the Marquesas lineament decrease in age from northwest to southeast. The new sample age distribution yields a volcanic migration rate significantly slower than that expected for Pacific plate motion over a stationary Marquesas hotspot. This and the aberrant orientation of the chain indicate deflection of the plume by westward upper mantle flow. The interaction of this weak plume with upper mantle flow accounts for the temporal and spatial patterns in Marquesan volcanism. The compositions of subaerial and submarine basalts reflect the mixing of at least two mantle sources, distinguished by Sr, Nd, and Pb isotope and trace element compositions. There is a consistent evolutionary pattern at each volcano, from early tholeiitic to later alkalic basalt eruptions. Tholeiitic and transitional lava compositions can be derived by variable degrees of partial melting of a source composed of depleted mid‐ocean ridge basalt mantle (DMM) and mantle characterized by radiogenic Pb (HIMU). Alkalic lava compositions appear to be dominantly the result of smaller degrees of melting of a more radiogenic mantle source (EM II). Large‐scale melting of the lower lithosphere or upper mantle (DMM+HIMU) entrained within a sheared, thermally buoyant plume (EM II) could produce the tholeiitic and transitional basalts found in the main shields of the volcanoes, while alkalic basalts could result from melting of mantle of EM II composition at the edges of the hotspot.
Seismic propagation across the East Pacific Rise: Finite difference experiments and implications for seismic tomography
Authors: Wilcock William S. D. , Dougherty Martin E. , Solomon Sean C. , Purdy G. M. , Toomey Douglas R.
Published: February 2008

We use a full‐waveform acousto‐elastic finite difference technique to investigate seismic propagation across the East Pacific Rise at 9°30′N for a two‐dimensional velocity model based on that proposed by Vera and others (1990). The primary feature of the model is an upper crustal low‐velocity region, corresponding to the axial magma chamber, which includes a small magma body located 1.6 km beneath the seafloor at the rise axis. The high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber. A qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9°30″N shows generally good agreement. For paths that cross the rise axis, the first arrival in the finite difference solutions diffracts above the magma chamber. This phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment. In such cases, the first arrival with significant energy is a diffraction from below the magma chamber. A high‐amplitude Moho‐turning (PmP) phase which results from the large velocity change across the Moho beneath the rise axis is apparent in both the finite difference solutions and the observations. Ray‐theoretical calculations of the paths of the diffracted arrivals are very unstable, and for the diffractions above the magma chamber no solution can be found with a single‐precision algorithm. Synthetic delay‐time inversions using an approximate ray‐tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body. Synthetic inversions of spectral estimates of t* show that Q−1 models are compromised not only if the ray paths are incorrect but also if t* estimates include significant contributions from more than one phase. Deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t*, but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation.
Convection and melting at mid‐ocean ridges
Authors: Cordery Matthew J. , Morgan Jason Phipps
Published: February 2008

We present a thermodynamically self‐consistent model of plate and buoyancy driven flow and melt generation beneath mid‐ocean ridges. Mantle flow is driven by a rigid lithosphere and by buoyancy forces resulting from melting, depletion, and melt extraction. Melt is generated using the solidus of Kinzler and Grove (1992a). Constant viscosity models without melt buoyancy forces show that no significant narrowing of the melting region or pressure gradient focusing of the melt is achieved. Temperature‐dependent viscosity models show that pressure gradients in a high viscosity lithosphere are insufficient for focusing melt to the ridge axis. Constant viscosity models with melt buoyancy forces show that significant narrowing of the melting region is possible at slow spreading rates but not at fast spreading rates. Melt buoyancy forces cause the crustal thickness to decrease with spreading rate. Aggregate melts are similar to those required to form mid‐ocean ridge basalt (MORB) but are too depleted in incompatible elements. They are also insensitive to spreading rate and melt region shape but are sensitive melting rate distribution. Model residuum trends are similar to those in abyssal peridotites and imply that abyssal peridotites result from partial melting and not refertilization. Massif peridotites appear to result from refertilization of harzburgite with MORB primary melt. Vertically integrated melts show very similar trends to data from the off‐axis Lamont seamounts on the flanks of the East Pacific Rise. This implies the melting region at fast spreading ridges is at least 80–100 km wide.
Can remanence anisotropy detect paleomagnetic inclination shallowing due to compaction? A case study using Cretaceous deep‐sea limestones
Authors: Hodych Joseph Paul , Bijaksana Satria
Published: February 2008

We studied 35 Cretaceous limestone specimens from five Pacific plate Deep Sea Drilling Project sites. Inclination IN of the natural remanence is on average 17° shallower than the average 44° expected paleofield inclination I. Anhysteretic remanence (ARM) applied identically to various axes was found to be weakest (ARMmin) perpendicular to bedding and strongest (ARMmax) parallel to bedding. The average ARMmin/ARMmax of 0.87 as well as the inclination shallowing of 17° likely originated from sediment compaction rotating the long axes of magnetite grains toward the bedding plane. This origin is theoretically and experimentally consistent with the average fractional compaction of 0.6 experienced by our sediments (estimated from their porosity). A compaction origin is also supported by the significant correlation found between tan IN/tan I and ARMmin/ARMmax. The correlation line's slope of 2.3±0.7 agrees with theory, taking into account our observation that ARM given perpendicular to the long axes of magnetite grains has on average ∼ 0.37 times the intensity of ARM given axially. These results suggest that compaction‐induced inclination shallowing may be detected in a suite of fine‐grained magnetite‐bearing sediments by looking for a correlation between tan IN and ARMmin/ARMmax (having shown that ARM anisotropy is foliated in the bedding plane). This correlation line's prediction of IN when ARMmin/ARMmax = 1 should estimate I corrected for inclination shallowing.
Structure of the Dead Sea pull‐apart basin from gravity analyses
Authors: Ten Brink U. S. , Ben‐Avraham Z. , Bell R. E. , Hassouneh M. , Coleman D. F. , Andreasen G. , Tibor G. , Coakley B.
Published: February 2008

Analyses and modeling of gravity data in the Dead Sea pull‐apart basin reveal the geometry of the basin and constrain models for its evolution. The basin is located within a valley which defines the Dead Sea transform plate boundary between Africa and Arabia. Three hundred kilometers of continuous marine gravity data, collected in a lake occupying the northern part of the basin, were integrated with land gravity data from Israel and Jordan to provide coverage to 30 km either side of the basin. Free‐air and variable‐density Bouguer anomaly maps, a horizontal first derivative map of the Bouguer anomaly, and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin. The basin is a long (132 km), narrow (7–10 km), and deep (≤10 km) full graben which is bounded by subvertical faults along its long sides. The Bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends, suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends. The surface expression of the basin is wider at its center (<16 km) and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin. These blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened. The collapse was probably facilitated by movement along the normal faults that bound the transform valley. We present a model in which the geometry of the Dead Sea basin (i.e., full graben with relative along‐axis symmetry) may be controlled by stretching of the entire (brittle and ductile) crust along its long axis. There is no evidence for the participation of the upper mantle in the deformation of the basin, and the Moho is not significantly elevated. The basin is probably close to being isostatically uncompensated, and thermal effects related to stretching are expected to be minimal. The amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1.19. If the rate of crustal stretching is similar to the rate of relative plate motion (6 mm/yr), the basin should be ∼3.5 m.y. old, in accord with geological evidence.
Fast assimilation of xenoliths in magmas
Authors: Sachs Peter M. , Stange Stefan
Published: February 2008

Analytical and numerical modeling indicates that melting of xenoliths in magmas may be controlled by viscous flow. The xenolith melt will be removed immediately by the host magma if flow is present and new xenolith melt can be produced. This melting mechanism is referred to as ablation. Ablation dominates the melting process in earlier experiments in the system diopside. We propose that complex phases (solid solutions) undergo ablation when the heat flow into a xenolith is high. Ablation is found to be controlled by a nearly linear rate law after a short heating up period. The average computed ablation rates coincide with earlier experiments of diopside. We predict steady state ablation rates at 20 K superheating for β‐cristobalite (3.8 × 10−4 m/s), high‐sanidine (1.6 × 10−4 m/s), calcite (4.9 × 10−5 m/s), clinoenstatite (4.0 × 10−5 m/s), and forsterite (3.2 × 10−5 m/s). A cold xenolith might cool down the host liquid next to it below the glass transition temperature and to accrete a glassy crust. Such chilled selvage accretion will modify the total assimilation rates of granulites by less than 10% for xenolith radii <20 cm. Ablation appears to be more effective as compared to diffusion controlled assimilation. This could explain the worldwide predominance of mafic and ultramafic (mantle and lower crust) compared to felsic lower crustal xenoliths.
A model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with GLORIA sidescan sonar
Authors: Mitchell Neil C.
Published: February 2008

Where the seafloor consists of a highly backscattering surface covered with a drape of fine‐grained sediment, and where the sediment does not contribute significantly to the backscattered signal, the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer. A first‐order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments. Data collected with Geological Long‐Range Inclined Azdic (GLORIA) are used to develop and test this model. At an acoustic frequency of 6.5 kHz, the range of sediment thickness that the sonar can potentially recover is 0–20 m, a range that is often below the resolution of vertical profilers. In the first example, the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation, which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer. The surface is a lava flow north of Hawaii covered by 1–2 m of fine‐grained sediments (Clague et al., 1990), and GLORIA data analyzed with the model suggest an attenuation coefficient of 0.2–0.4 dB/m, a value consistent with results from laboratory and field measurements of sediment attenuation. In the second example, a rocky surface is buried by varying amounts of sediment, which is analyzed at almost constant grazing angle. The surface is the volcanic basement of the Southeast Indian Ridge, and GLORIA data reveal the thickening of the sediment cover with distance away from the spreading center. This is used to derive an average sedimentation rate of 6 mm/ka, which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions. Furthermore, the model is used to calculate, from image amplitude distributions, the sediment thickness distribution, which represents the accumulation of sediments in ponds and the exposure of abyssal hills. These sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age. I explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes (the first‐order model assumes a uniform thickness at the subfootprint scale).
A numerical study of haline convection beneath leads in sea ice
Authors: Smith David C. , Morison James H.
Published: February 2008

Leads are openings in sea ice in polar oceans which result from divergence in ice drift. Although they represent a small percentage of the total ice cover, they are important sites for large air‐sea heat flux. Cooling of the upper ocean results in the refreezing of the lead and associated brine rejection in the upper water column. Depending on the ice‐water velocity difference, the brine is mixed away by convective sinking or by turbulent mixing associated with ice‐water momentum flux. In this study, a two‐dimensional numerical model is used to examine the upper ocean response to buoyancy and momentum flux associated with leads in sea ice. The results indicate that the ocean response can be quantified in terms of the ratio of these two forcing factors. For typical buoyancy forcing and ice‐water velocities it is found that both of these regimes should be typically operative in the Arctic Ocean. The results also illustrate that geostrophic adjustment produces along‐lead surface and subsurface jets. Comparison with laboratory results, however, suggests that rotation does not play a major role in the turbulent adjustment.
The Arctic Ocean ice balance: A Kalman smoother estimate
Authors: Thomas D. R. , Rothrock D. A.
Published: February 2008

The methodology of Kalman filtering and smoothing is used to integrate a 7‐year time series of buoy‐derived ice motion fields and satellite passive microwave observations. The result is a record of the concentrations of open water, first‐year ice, and multiyear ice that we believe is better than the estimates based on the microwave data alone. The Kalman procedure interprets the evolution of the ice cover in terms of advection, melt, growth, ridging, and aging of first‐year into multiyear ice. Generally, the regions along the coasts of Alaska and Siberia and the area just north of Fram Strait are sources of first‐year ice, with the rest of the Arctic Ocean acting as a sink for first‐year ice via ridging and aging. All the Arctic Ocean except for the Beaufort and Chukchi seas is a source of multiyear ice, with the Chukchi being the only internal multiyear ice sink. Export through Fram Strait is a major ice sink, but we find only about two‐thirds the export and greater interannual variation than found in previous studies. There is no discernible trend in the area of multiyear ice in the Arctic Ocean during the 7 years.
Flocculation of fine‐grained sediments due to differential settling
Authors: Lick Wilbert , Huang Hening , Jepsen Richard
Published: February 2008

The flocculation of fine‐grained particles depends on collisions due to Brownian motion, fluid shear, and differential settling. Previous experimental work on the flocculation of fine‐grained sediments has emphasized the effects of fluid shear. These effects are significant in high‐turbulence regions. However, as the turbulence and fluid shear decrease, as, for example, in open waters away from shore, differential settling becomes the dominant mechanism for flocculation. In the present article, previous work on the effects of fluid shear is reviewed. However, the emphasis is on recent experimental work on the effects of differential settling on the flocculation of fine‐grained, primarily inorganic particles. The transition in effects between situations where fluid shear is dominant and the other extreme, where differential settling is dominant, was also investigated and is discussed. The sediments used in these studies were natural bottom sediments from the Detroit River inlet to Lake Erie. The tests were initiated with disaggregated sediments and were continued as the particles aggregated and formed flocs. These flocs then grew until a steady state size distribution was reached. In order to reach a steady state the differential settling tests sometimes continued for as long as 30 days; they were done in both freshwater and seawater at sediment concentrations from 1 mg/L to 200 mg/L and with and without treatment to remove organic matter. Floe size distributions as a function of time were determined. From the experiments it is shown that the times to steady state and the steady state median diameters are much larger when differential settling is the dominant mechanism for flocculation than when fluid shear is the dominant mechanism for flocculation. It is also shown that the effects of sediment concentration and salinity are qualitatively similar; i.e., as these quantities increase, both the time to steady state and the steady state floe size decrease. Settling speeds of the flocculated particles were also measured; the settling speeds of flocs are much larger and increase more rapidly with floc diameter when produced by differential settling than when fluid shear is dominant.
An investigation of Ekman upwelling in the North Atlantic
Authors: McClain Charles R. , Firestone James
Published: February 2008

Surface winds from the Fleet Numerical Oceanography Center (FNOC) for 1979–1986 were used to compute the monthly mean Ekman up welling fields 〈we〉m, the seasonal and annual climatologies of 〈we〉m (〈we〉s and 〈we〉a, respectively), and the basin wide monthly mean vertical Ekman velocities 〈we〉mb in the North Atlantic Ocean (the analyses excluded the zone within ±5° of the equator). The annual climatology compares very well with the climatology of Leetmaa and Bunker (1978). The analyses show a pronounced minimum in 〈we〉mb during 1981–1982. Additional analyses using 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts exhibit a similar pattern in 〈we〉mb. Further examination of the FNOC 〈we〉m fields shows that the primary source of the interannual signal was the region off NW Africa in the vicinity of the Guinea Dome. Other sectors of the basin experienced no significant interannual trends. Hydrographic data and sea surface temperature data from the NW Africa sector for 1981–1986 indicate a cooling trend beginning in late 1982, consistent with increased upwelling. Also, the fall and winter seasons' mixed layers at the center of the Guinea Dome were deeper in 1984 and 1985 than in previous years. Finally, the potential impact of large interannual variations in Ekman upwelling on basin wide primary productivity is discussed.
Hydrography and vertical fluxes of heat and salt northeast of Svalbard in autumn
Authors: Steele Michael , Morison James H.
Published: February 2008

Temperature and salinity data collected by a SALARGOS buoy during October 1988 northeast of Svalbard, Norway, as part of the Coordinated Eastern Arctic Research Experiment are analyzed. As the buoy drifted across the continental slope, the data show a transition from central Arctic conditions, where the thermocline lies below the halocline, to shelf conditions, where the two coincide. Surface waters on the shelf were above the freezing point, losing heat to sea ice advected southward by the wind. The Atlantic Inflow Current was also observed on the continental slope, with a maximum temperature above 3°C. A storm caused upwelling and entrainment of this current, leading to strong ice‐ocean heat fluxes and ice melting. The resulting flux of fresh water stabilized the surface layer and shut off further entrainment. This episodic process can lead to bigger heat losses from the Atlantic Inflow Current than previously considered for ice‐covered conditions.
Microstructure activity within a minifilament in the coastal transition zone
Authors: Dewey Richard K. , Moum James N. , Caldwell Douglas R.
Published: February 2008

This paper presents observations of a dynamic minifilament and summarizes the findings of three microstructure surveys across and within the meandering current structures of the Coastal Transition Zone (CTZ). An actively turbulent minifilament is discussed and possible consequences of the small‐scale processes with regard to frontal dynamics are examined. Four different frontal structures were observed as part of the CTZ microstructure program, two resembling meanders and two characterized as filaments. Microstructure measurements included continuous rapid sampling vertical profiler (RSVP) (Caldwell et al., 1985) and acoustic Doppler current profiles (ADCP) profiling along meridional lines bisecting the frontal features. Common features of the two meanders observed in 1986: (Moum et al., 1988) and 1987: (Dewey and Moum, 1990) include offshore and onshore flow patterns (consistent with meridional sections through meanders), rapid temporal evolution of the frontal structures between consecutive (10–24 hour) transects, no elevated turbulent activity below the pycnocline, despite the enhanced current shears (quiescent core within central region where pycnocline is near surface), and approximate geostrophy of the dominant currents at scales > 10 km. A large filament, or jetlike feature observed in 1988 (Dewey et al., 1991) was characterized by off‐shore geostrophic flow, no enhanced turbulent activity, and significant asymmetry in the temperature, salinity and current fields, with stronger vorticity on the south (upwelled) side of the velocity maximum. An actively turbulent minifilament was also observed in 1988, and exhibited some characteristics not common to the larger meanders or filaments. A strongly divergent velocity signature was confined to the near‐surface layer (< 60 m), with a cool, uplifted core, possibly a result of localized upwelling and subsurface turbulent mixing. Below the surface mixed layer, elevated turbulent dissipation rates were coincident with regions of high shear and correspondingly low Ri. However, the buoyancy flux from turbulent mixing was estimated to be too low to have formed the cool surface signature of the minifilament. The estimated Rossby number (R0 = 1.3) for the minifilament suggests that significant departures from geostrophy are likely. However, due to the limited temporal and zonal resolution of the minifilament, no definitive ageostrophic analysis can be performed. Corresponding satellite images indicate that the observed structure, although narrow (10 km), may be as long as 100 km, be evolving over periods of days, and a common submesoscale feature in the swirling patterns revealed by advenced very high resolution radiometer (AVHRR).
Derivation of satellite wind model functions using operational surface wind analyses: An altimeter example
Authors: Freilich Michael H. , Dunbar R. Scott
Published: February 2008

One year of global surface wind products from multiple operational numerical weather prediction (NWP) forecast/analysis systems are used as comparison data to derive an empirical wind speed model function for the Geosat altimeter. The resulting model function is nearly identical to the modified Chelton‐Wentz model for wind speeds from 4.5 to 15 m/s. The large quantity of comparison data at each wind speed allows detailed examination of the data scatter (and hence, model function uncertainty) as a function of wind speed. Highly skewed distributions at low wind speeds are consistent with specular reflections and antenna mispointing errors hypothesized by others on the basis of extremely limited data. Mesoscale variability in the wind field and synoptic scale errors in the NWP products are shown to account for ∼ 30% of the observed scatter of σ0 at each wind speed. The remaining scatter is largest at low winds, and decreases to a nearly constant value of ∼ 12% at speeds greater than 7 m/s. Model function uncertainty expressed more traditionally in units of wind speed is examined for historical model functions as well as the present NWP‐based model. The historical models have significant biases at high wind speeds owing to the lack of comparison in situ data used in their construction. Although the narrow swath, sensitivity to mispointing errors at low wind speeds, and small variation of backscatter with winds at high speeds limit the scientific utility of radar altimeter wind measurements, the present study demonstrates that modern operational NWP surface wind products are sufficiently accurate to allow development of fully empirical model functions and associated error analyses.
Determination of the major groups of phytoplankton pigments from the absorption spectra of total particulate matter
Authors: Hoepffner Nicolas , Sathyendranath Shubha
Published: February 2008

The contributions of detrital particles and phytoplankton to total light absorption are retrieved by nonlinear regression on the absorption spectra of total particles from various oceanic regions. The model used explains more than 96% of the variance in the observed particle absorption spectra. The resulting absorption spectra of phytoplankton are then decomposed into several Gaussian bands reflecting absorption by phytoplankton pigments. Such a decomposition, combined with high‐performance liquid chromatography data on phytoplankton pigment concentrations, allows the computation of specific absorption coefficients for chlorophylls a, b, and c and carotenoids. The spectral values of these in vivo absorption coefficients are then discussed, considering the effects of secondary pigments which were not measured quantitatively. We show that these coefficients can be used to reconstruct the absorption spectra of phytoplankton at various locations and depths. Discrepancies that do occur at some stations are explained in terms of particle size effect. These coefficients can be used to determine the concentrations of phytoplankton pigments in the water, given the absorption spectrum of total particles.
A modification of the response method of tidal analysis
Authors: Hyde Richard A. , Pettigrew Neal R.
Published: February 2008

An easily implemented extension of the standard response method of tidal analysis is outlined. The modification improves the extraction of both the steady and the tidal components from problematic time series by calculating tidal response weights uncontaminated by missing or anomalous data. Examples of time series containing data gaps and anomalous events are analyzed to demonstrate the applicability and advantage of the proposed method.
Seasonal variability in the southwestern Atlantic
Authors: Matano Ricardo P. , Schlax Michael G. , Chelton Dudley B.
Published: February 2008

The circulation of the southwestern Atlantic Ocean is dominated by the Subtropical Gyre and the confluence of the Brazil and Malvinas currents. Observations indicate that the latitude of this confluence changes seasonally, lying farther north during the austral winter than during the summer. This phenomenon has important consequences for the local climate and marine population, as the latitude of the confluence also marks the boundary between the warm waters of the subtropical gyre and the cold waters of the Antarctic Circumpolar Current. We present evidence that these seasonal migrations may be related to changes in the transport of both the Brazil and Malvinas currents. A numerical model forced by climatological wind stress indicates that the transport of the Brazil Current decreases during winter months and increases during summer months. Geosat altimeter data corroborate the model results and also indicate that the transport of the Malvinas Current undergoes a seasonal cycle with phase opposite to that of the Brazil Current. Our hypothesis is that during the austral summer, a southward displacement of the latitude of the confluence is coincident with an acceleration of the flow in the subtropical gyre and a weakening of the transport of the Malvinas Current. This situation reverses during the winter when the Malvinas Current grows stronger, the Brazil Current transport decreases, and the latitude of the confluence of these two currents moves northward.