bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic wave scattering by anisotropic obstacles: Application to fractured volumes
Authors: Gibson Richard L. , Ben‐Menahem Ari
Published: February 2008

The Born approximation is frequently applied to isotropic media to determine wave fields scattered from heterogeneous regions. We extend this theory to the general case of an anisotropic obstacle embedded within an anisotropic matrix and show that a perturbation to any of the 21 independent elastic constants acts as a secondary moment tensor source which radiates energy as it is encountered by the incident wave. We consider the case of an anisotropic obstacle in an isotropic background medium in more detail, since the well‐known Green's tensor for isotropic, homogeneous media and the Born approximation allow an expression of the radiation patterns, P and S wave, of Rayleigh scattering due to a perturbation to any elastic constants. The wave fields scattered from an anisotropic obstacle differ significantly from the isotropic case in dependence on both the direction of observation and the direction of the incident wave. In order to provide a concrete example of these results, we examine the case of a small fractured volume. If the fractures within the volume are randomly oriented, the resulting material is isotropic, but if the cracks are aligned, the material is anisotropic with five independent elastic constants. In the latter case, the value of C44, analogous to the isotropic rigidity, is unchanged from the background value (δC44 = 0) when the cracks are aligned perpendicular to the x; axis. We present the P, SV and SH radiation patterns for the two scatterers and discuss the implications for observation of fractured volumes. The most important result is that the the zero perturbation δC44 in the aligned crack case causes the scattered displacement field to vanish completely for incident shear waves polarized parallel to the crack plane. Determination of this direction for observations of a fractured zone would allow important insights into the nature of the fracturing.
Evaluation of visible and near‐infrared and thermal‐infrared reflectance spectra for studying thermal alteration of Pierre Shale, Wolcott, Colorado
Authors: Rowan Lawrence C. , Salisbury John W. , Kingston Marguerite J. , Vergo Norma , Bostick Neely H.
Published: February 2008

Analyses of visible and near‐infrared (0.4–2.5 μm) and thermal‐infrared (2.5–25.0 μm) spectral reflectance of whole rock and kerogen concentrates of dike‐intruded Pierre Shale indicate that the degree of thermal alteration of contained organic matter (OM) can be estimated by use of spectral reflectance. With increasing thermal alteration of the OM, whole rock, and kerogen spectra show the following trends: (1) visible and near‐infrared (VNIR) and thermal‐infrared (TIR) reflectance decreases progressively, with the largest change in the VNIR occurring between 0.90 and 1.50 μm; and (2) the intensities of absorption features in the 3‐ to 5‐μm region, which are related to C‐H stretching vibrations, decrease. The VNIR and TIR spectral reflectance trends are highly correlated with H/C ratios and vitrinite reflectance data. The largest changes in all these parameters occur at a distance that is equivalent to about one‐half the dike thickness, where the temperature range is estimated to have been 240°–350° C. The VNIR and TIR spectral reflectance variations are related to thermally induced composition and structural changes in the OM. Landsat Thematic Mapper channels 3, 4, and 5 should have excellent potential for remote detection of OM maturity. Analysis of in situ VNIR and TIR measurements may provide a rapid method for estimating thermal maturity in the field. In addition, laboratory measurements of TIR spectral reflectance are a more sensitive and convenient means of measuring the maturity of kerogen than the traditional transmittance method. Further work is necessary to examine the effects of variable OM abundance and type, mineralogic composition, grain size, and weathering on spectral reflectance characteristics.
Chemical bonding in the outer core: High‐pressure electronic structures of oxygen and sulfur in metallic iron
Authors: Sherman David M.
Published: February 2008

From its density the outer core is believed to be an alloy of iron and a light element such as sulfur or oxygen. The nature of the light element in the core is an important constraint for theories of the Earth's formation. In this paper the electronic structures of oxygen and sulfur impurities in metallic iron are investigated to determine if pressure, temperature, and composition‐induced changes in bonding might affect phase equilibria along the Fe‐FeS and Fe‐FeO binaries. The electronic structure of sulfur in metallic iron is consistent with the miscibility between Fe and FeS liquids. Volume compression strengthens the Fe‐S bond, and it is expected that at sufficiently high pressure, sulfur can substitute for Fe and give solid solution behavior between Fe and FeS. In contrast, the electronic structure of oxygen in metallic iron shows that oxygen cannot act as a substitutional impurity (replacing Fe). This explains the observed miscibility gap on the Fe‐FeO binary at 1 atm pressure. Volume compression does not greatly change the electronic structure if oxygen substitutes for iron in bcc and fcc iron. Iron‐oxygen bonding does occur, however, if oxygen occupies interstitial sites. Insofar as the molar volume of FeO incorporated as interstitial oxygen in metallic iron is smaller than that of pure FeO, the incorporation of oxygen into metallic iron may be favored under the pressures of the Earth's core.
Finite element modeling of lower crustal flow: A model for crustal thickness variations
Authors: Marquart Gabriele
Published: February 2008

Small‐scale convection beneath continental lithosphere is likely to initiate viscous flow in the ductile lower crust. Positive lithospheric bending will develop above upwelling mantle flow leading to a lateral squeezing out of crustal material from elevated areas which results in significant crustal thickness variations in its final stage. This process was studied with a finite element approach for viscous material for a number of different viscosity contrasts between lower crust and lithospheric mantle. Significant Moho undulations of the order of at least 5 km within a time period of about 50 m.y. can only be reached if the lower crustal viscosity is less than 1021 Pa s or if the lithospheric mantle viscosity is less than 1024 Pa s. The maximum topography is a function of the viscosity contrast and is generally of the order of a few hundreds of meters with a relaxation time between 5 and 103 m.y., strongly dependent on the crustal viscosity. In conclusion, this process might be important to explain crustal thinning and thickening of the order of about 10 km in areas of enhanced thermal gradient.
The Giant Crater Lava Field: Geology and geochemistry of a compositionally zoned, high‐alumina basalt to basaltic andesite eruption at Medicine Lake Volcano, California
Authors: Donnelly‐Nolan Julie M. , Champion Duane E. , Grove Timothy L. , Baker Michael B. , Taggart Joseph E. , Bruggman Peggy E.
Published: February 2008

The Giant Crater lava field consists of >4 km3 of basaltic lava, compositionally zoned from first‐erupted calc‐alkaline basaltic andesite to last‐erupted primitive high‐alumina basalt. On the FeO*/MgO (where FeO* is total Fe calculated as FeO) versus SiO2 discrimination diagram commonly used to distinguish tholeiitic from calc‐alkaline series lavas the compositionally zoned eruption crosses from the tholeiitic field to the calc‐alkaline field. The lavas erupted in a brief span of time about 10,500 years ago from several closely spaced vents on the south flank of Medicine Lake volcano in the southern Cascade Range. Six chemical‐stratigraphic groups were mapped. Lower K2O, higher MgO groups always overlie higher K2O, lower MgO groups. Group 6 lavas erupted last and are aphyric, have high contents of MgO and Ni, and contain as little as 0.07% K2O. Group 1 lavas are porphyritic and have as much as 1.10% K2O. Major element contents of primitive group 6 Giant Crater basalt are very similar to a subset of primitive mid‐ocean ridge basalts (MORB). Group 6 lava is more depleted in middle and heavy rare earth elements (REE) and Y than is primitive MORB, but it is enriched in large ion lithophile elements (LILE). These LILE enrichments may be a result of fluid from the subducting slab interacting with the mantle beneath Medicine Lake volcano. The group 6 REE pattern is parallel to the pattern of normal‐type MORB, indicating a similar although perhaps more depleted mantle source. The location of Medicine Lake volcano in an extensional environment behind the volcanic front facilitates the rise of mantle‐derived melts. Modification of the primitive group 6 basalt to more evolved compositions takes place in the upper crust by processes involving fractional crystallization and assimilation. The group 1 calc‐alkaline Giant Crater basaltic andesite produced by these processes is similar to other Cascade basaltic andesites, implying that a similar high‐alumina basalt may be parental.
General patterns of flow field growth: Aa and blocky lavas
Authors: Kilburn Christopher R. J. , Lopes Rosaly M. C.
Published: February 2008

Lava flow fields consist of one or more flows. Four ideal emplacement regimes are recognized: (a) that for single flows and (b) that for flow fields dominated by (1) widening, (2) thickening, or (3) lengthening, as a result of generating new flows. Most aa and blocky lavas belong to the flow field widening or single‐flow regimes. These two regimes are analyzed assuming advance is controlled by the distal core of a flow, where motion is treated as steady, uniform, and laminar. Because of low deformation rates, the distal core is also approximated to a Newtonian fluid. Widening and, possibly, lengthening are ultimately limited by crustal resistance. After a critical cooling interval, new flows are generated from the upper reaches of the flow field. A simple relation is derived linking flow field dimensions and underlying slope to eruption duration, independent of terms involving gravity or lava chemistry and rheology. The relation well describes field data from several volcanoes (involving lava compositions from K phonolitic tephrite to dacite). This supports the premise that the overall growth of aa and blocky flow fields is systematic and also suggests that such growth may be predictable at time scales greater than, or similar to, the emplacement times of major flows.
On the structural stress field of the longitudinal bend fold and its distribution with respect to the fault system
Authors: Li Yangjian , Chen Yancheng
Published: February 2008

Until now the structural stress field of the longitudinal bend fold has been understood only qualitatively, and only a few stresses are known in the longitudinal bend fold; therefore the distribution of the fault system cannot be explained exactly and comprehensively. We draw on the conception of the longitudinal bend bar in mechanics of materials to research systematically and quantitatively the various stress distributions and the evolution of longitudinal bend folds. We explain the cause of formation for every type of fault system in the different positions and developmental stages of the longitudinal bend folds, and we discover an important new cross‐shear stress and a new, fan‐type thrust fault.
Crustal stress regime in Fennoscandia from focal mechanisms
Authors: Gregersen Søren
Published: February 2008

An extensive data set of earthquake focal mechanisms is now available for all of northern Europe and especially for Fennoscandia. These mechanisms are considered to provide representative coverage of the stress field. The maximum horizontal compressional stress orientations are internally very consistent over the area of northern Europe. The dominating NW‐SE compressional stresses appear tied to relative plate motion, with Mid‐Atlantic Ridge spreading and European‐African collision in southern Europe. For Fennoscandia some exceptions to the regional stress pattern exist. The causes of these local anomalies have been investigated. No correlations with geological provinces or province boundaries were found. In addition, there does not appear to be any clear correlation between anomalous stress directions and postglacial uplift in the present earthquake activity. This lack of correlation is in sharp contrast to geological evidence in the form of large faults indicative of large postglacial earthquakes occurring right after the end of the latest ice age, 9000 years ago. Taken together, this evidence suggests a tremendous change of stress field in Holocene time, from one dominated by the postglacial unloading right after the ice age to one dominated by the present plate motion today.
Seismicity and aeromagnetic features of the Gulf of Aqaba (Elat) Region
Authors: Alamri Abdullah M. , Schult Frederick R. , Bufe Charles G.
Published: February 2008

Recent seismicity in the Gulf of Aqaba region has been examined in relation to pull‐apart tectonics and structures indicated by bathymetry, surface geology, and new analyses of aeromagnetic data. Forty earthquakes occurring in the region from 1985 to 1989 were located using data from a network of five telemetered seismographs in the Midyan region of Saudi Arabia east of the Gulf of Aqaba and from a network north of the gulf in Israel. Interpretation of aeromagnetic data from the Gulf of Aqaba and the adjacent Midyan region confirms previous estimates of offsets across the zone of transform faulting in the gulf. Aeromagnetic anomaly patterns, obtained by reduction‐to‐the‐pole techniques, and earthquake locations provide evidence for continuation of the faulting regime from the gulf northeastward into the Midyan region. The vicinity of the Elat Deep (primarily the southwest flank) probably is undergoing active normal faulting. Other seismicity and first‐motion polarity data confirm the regional pattern of NNE trending sinistral strike‐slip faulting.
Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California
Authors: Bateman P. C. , Dodge F. C. W. , Kistler R. W.
Published: February 2008

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10−2 SI units in the east and central parts of the batholith and drop abruptly to less than 10−3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10−3 to 10−2) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ18O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87Sr/86Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe3+ to Fe2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.
High‐stress consolidation experiments and their geologic implications
Authors: Karig Daniel E. , Hou Gang
Published: February 2008

Although uniaxial consolidation of sediments in basins defines one of the most common and simple of geologic stress paths, there is remarkably little information concerning lateral stresses and elastic moduli associated with this process. Short‐term relationships explored by geotechnical engineers show that ratios of effective horizontal stress (σh′) to effective vertical stress (σν′), or Ko, are constant for a given sediment over the low‐stress range, but there is no consensus as to whether these ratios remain constant at the higher stresses and longer durations of geologic conditions. A series of uniaxial consolidation tests on a silty clay and a fine sand, including several elastic unload‐reload cycles, were carried out to explore consolidation and elastic relationships to a σν′ of 35 MPa. Over this stress range, Ko remained constant for the clay (0.62) but increased slightly for the sand (0.44 to 0.53). The unload‐reload cycles showed that the vertical Young's modulus (Eν) increases markedly with consolidation stress (σc′). These cycles also demonstrated that σh′/σν′ (elastic) is greater for clay (0.35) than for sand (0.21), but this ratio shows no variation with σc′. This implies that Poisson's ratios (v) did not vary significantly with σν′ or σc′ over the stress range of the tests. Comparison with natural sediments suggests that cementation results in increases of E of up to an order of magnitude above that of an analogous uncemented sediment, but does not seem to affect v. The effect of geologic time on Ko was explored using results of sparse applicable in situ stress measurements; these suggest not only that clay‐rich sediments can support significant differential stresses, but also that stress ratios may even retain their short‐term values for 106 yr or more.
Paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine Shirahama Group (Upper Miocene‐Pliocene), Japan
Authors: Tamura Yoshihiko , Koyama Masato , Fiske Richard S.
Published: February 2008

The cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations. Six to 13 samples were cored and measured from six essential andesite blocks 1–5 m in diameter. Thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps, each representing different blocking temperatures (Tb). Each sample has three components of remanent magnetization (C1, C2, and C3). C1 is characterized by a Tb of 250°C and an orientation coincident with the present geomagnetic field. C2 has a Tb of 250°–500°C and shows a consistent direction with reversed polarity. C3 has a Tb of 500°–600°C, and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts. On the basis of the remanence direction of each component, the differing Tb, and the petrography of the samples, C1 is interpreted as secondary viscous remanence, whereas C2 and C3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement. C3 was probably acquired while the blocks were rolling or sliding downslope, and C2 formed during continued cooling of the blocks after they came to rest. Heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted. If the eruptive vent was only 2–3 km distant, as suggested by field relations, a downslope transport rate of less than 36 km/h is implied.
Reconstruction of subduction zone paleogeometries and quantification of upper plate material losses caused by tectonic erosion
Authors: Lallemand Serge E. , Schnurle Philippe , Manoussis Stratis
Published: February 2008

We propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins. We first test the method, using isostatic and elastic models, applying it to 15 different well‐constrained convergent margins. The main conclusion is that most of them are presently close to the isostatic equilibrium. Consequently, we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation. Then, we apply this model to compute the geometry of the paleoslab knowing the paleotopography of the margin. The comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge. Two cases were studied for paleoreconstructions: Japan and Peru. The results show that a considerable amount of material was removed during the Neogene along both margins. A detailed interpretation of the Neogene history of the Japan margin is proposed on the basis of our results.
Global Positioning System Radiation Force Model for geodetic applications
Authors: Fliegel H. F. , Gallini T. E. , Swift E. R.
Published: February 2008

To generate the highly precise ephemerides of Global Positioning System (GPS) satellites necessary for modern geodetic applications, one must have an accurate force model that includes the pressure of solar radiation and spacecraft thermal emission. We present the dimensions and optical parameters of Block I and Block II GPS satellites, show how they are used to form the models of the solar force, and compare predictions of these models with values estimated from tracking data. Simple approximating functions are given for the solar/thermal radiation pressure, and the problem of estimating a smaller, unmodeled force called Y bias is discussed. A simple model is given for the effect of earthshine on GPS spacecraft. Two kinds of discrepancy are noted between model predictions and actual performance. For Block II satellites, beginning soon after launch, a transient force decaying exponentially with time, perhaps due to outgassing, aliases into estimates of the solar force. Also, at the beginning of eclipse season and for at least some satellites, Y bias estimates vary rapidly for reasons not presently understood. Therefore the geodetic worker should not only use the best available force models but also should avoid using a satellite in those critical time periods when unexplained model errors are present.
Hydrothermal venting and the apparent magmatic budget of the Juan de Fuca Ridge
Authors: Baker Edward T. , Hammond Stephen R.
Published: February 2008

The fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large‐scale geological controls on the distribution of hydrothermal venting along the mid‐ocean ridge. We undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the Juan de Fuca Ridge, a medium‐rate spreading center with segments of markedly differing morphology. Hydrothermal anomalies were detected by towing a high‐precision conductivity‐temperature‐depth‐transmissometer (CTDT) package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment. These surveys, conducted between 1985 and 1989, have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid‐ocean ridge. Hydrothermal discharge is strongest on those segments, or portions of segments, where the apparent magmatic budget is highest, as indicated by the degree of along‐axis inflation and other morphological characteristics. This correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber. We propose that the probability of discharge occurring at any point on the mid‐ocean ridge is proportional to the spreading rate. This probability, scaled by the observation that significant plumes presently emanate from ∼20% of the Juan de Fuca Ridge, is expected to range from ∼5% on the slowest‐spreading ridge to ∼50% on the fastest‐spreading ridge.
Seismic mapping of a magma chamber beneath the Valu Fa Ridge, Lau Basin
Authors: Collier Jenny S. , Sinha Martin C.
Published: February 2008

We analyze a dense grid of seismic normal incidence and wide‐angle data collected over an active back arc spreading center in the Lau Basin of the southwest Pacific. Our survey area covers the whole of a 35‐km‐long morphological segment and a small overlapping spreading center. A bright reflector, which is coincident with a velocity inversion at a depth of 3.2 ± 0.2 km below the seafloor, is observed on every one of the 40 across‐axis profiles. We interpret this reflector as being the roof of a crustal magma chamber. The widest magma chamber reflector occurs beneath the overlapping spreading center, where it extends up to 4 km, being imaged beneath both ridges and, in places, beneath the overlap basin. Elsewhere the width of the reflector varies between 0.6 and 2.3 ±0.4 km. The narrowest reflectors are observed beneath deviations (devais) of the ridge axis. In addition to being observed on each of the kilometer‐spaced across‐axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis. The reflection coefficient of the event is between −0.34 and −0.65. This requires the melt at the top of the chamber to have a P wave velocity less than 2.7 km/s if the interface is planar or less than 3.8 km/s if the interface is layered.
Primary magmas of mid‐ocean ridge basalts 2. Applications
Authors: Kinzler Rosamond J. , Grove Timothy L.
Published: February 2008

Variable initial mantle composition and extent of depletion during dynamic melting processes strongly influence compositions of primary basaltic magmas. The descriptions of the equilibria that pertain to melting in the upper oceanic mantle presented in the companion paper (Kinzler and Grove, this issue) are used to estimate the major element compositions and temperatures of aggregate primary magmas of mid‐ocean ridge basalt (MORB) generated in the adiabatically upwelling mantle beneath oceanic spreading centers. Primary MORB magmas with high Na2O abundances that are produced from more fertile mantle compositions or represent initial melts of a depleted spinel‐lherzolite have higher SiO2 and Al2O3 and lower MgO, FeO, and CaO abundances, relative to low‐Na2O primary magmas. Na2O abundance variation in the mantle source during polybaric, near‐fractional melting processes causes melt compositions to vary significantly. The total extents of depletion achieved by the decompression melting process to yield the observed variation in major elements of MORB range from ∼ 6 to 18%; the range of mean pressures of melting is relatively narrow, 8–15 kbar; the total range modeled for the adiabatic, near‐fractional melting process is 4 to 25 kbar. Aggregate primary magmas of MORB are not picritic, nor do they resemble sampled primitive MORB (MORB with MgO > 9.0 wt%). Much of the variation in major element composition observed in sampled MORB can be explained by melting a similar depleted MORB‐mantle source. The ambient temperature range of the upper mantle beneath the global ridge system required to explain the observed chemical variations is 1475°–1315°C.
Subtidal frequency estuary‐shelf interaction: Observations near Delaware Bay
Authors: Garvine Richard W.
Published: February 2008

Interaction between a large estuary and the adjacent inner continental shelf at subtidal frequencies occurs through a variety of physical processes. I use recent field observations to study these processes near the weakly stratified estuary of Delaware Bay on the east coast of the United States. The primary observations were shipboard hydrography and long time series of current, temperature, and conductivity. The observations revealed three distinct spatial regions for the coupled circulation between estuary and shelf: the estuary mouth, the inner shelf where the mean flow is landward, and a buoyancy‐driven coastal current. The coastal current is the principal discovery of the work. It begins near the estuary mouth as lighter water from Delaware Bay exits the mouth on the right side when viewed to seaward. Initially, the current is about one internal Rossby radius in width, but it broadens as it flows seaward to reach a width of about 20 km off the coast of Delaware. Observed mean currents were 3–5 cm s−1 there. In all three regions temporal variability in current and salinity was induced by variations in alongshore wind and river discharge into the estuary, the latter occurring mainly at very long periods of several weeks and longer.
Wind wave cast in the Mediterranean Sea
Authors: Cavaleri Luigi , Bertotti Luciana , Lionello Piero
Published: February 2008

An advanced, third‐generation wave model has been repeatedly applied to the Mediterranean Sea. We have studied the accuracy of the results and the factors that control it. The grid resolution, when less than 0.5°, is shown to have in general negligible effect in the open sea. The related accuracy of description of the coastal border can have a relevant effect on the wave field on the side and on the wake of the coastal details, for a distance of a few grid points. The difficulty of correctly evaluating the wind in the Mediterranean basin is dramatically shown by comparing the results obtained using wind fields with different resolution. The crucial role of the orography in shaping the wind fields and the need for higher‐resolution models is established. It is found that owing to the strong dependence of wave height on wind speed, for resolution coarser than 70 km the loss of accuracy of the wind fields, even if meteorologically acceptable, makes them useless for an efficient evaluation of the wave conditions. The exception, explained by the local absence of relevant mountain ridges, is the case of a southerly wind in the eastern Mediterranean. The accuracy of wave forecasting is obtained by comparing analysis and forecast fields for a very severe storm. In connection with the preferential west to east flow of most of the Mediterranean storms, and with the scarcity of information on the Atlantic Ocean, the equality of the meteorological forecast rapidly deteriorates beyond 1 or 2 days of forecast. This leads to an even faster deterioration of the wave forecast, whose practical limits in the western Mediterranean can at present be established at 1 day.
A new measurement of CO2 eddy flux in the nearshore atmospheric surface layer
Authors: Smith S. D. , Anderson R. J. , Jones E. P. , Desjardins R. L. , Moore R. M. , Hertzman O. , Johnson B. D.
Published: February 2008

Fluxes of CO2 in the atmospheric surface layer have been measured at a shoreline site by the eddy correlation method using a new CO2 sensor with greater sensitivity than the ones used in earlier studies. In this experiment, preliminary to more extensive measurements at an offshore site, 13 upward and downward fluxes were observed to respond to differences in pCO2 between water and air. As in an earlier study, a negative correlation of CO2 flux was found with hourly change in wind speed.
Effects of atmospheric stability on ocean ripples: A comparison between optical and microwave measurements
Authors: Wu Jin
Published: February 2008

Fine structures of the sea surface responding instantaneously to the wind influence greatly air‐sea exchange processes. Much has been learned on the generation of these structures by the wind, while little has been studied on effects of the atmospheric stability. Reported “direct” optical measurements of, and “indirect” microwave returns from, the sea surface are analyzed to show a similar quantitative suppression of ocean ripples under stable atmospheric conditions. Both mean square slopes and radar returns are magnified under unstable conditions, but their quantitative modifications are less certain.
Variation of upper layer dynamics during breakup of the seasonal ice cover in Hudson Bay
Authors: Lepage Serge , Ingram R. Grant
Published: February 2008

The present study describes circulation and stratification changes associated with the melt and breakup of the seasonal ice cover in the coastal waters of southeast Hudson Day. Field work was carried out at a station located 25 km north of the Great Whale River. Buoyancy fluxes and dissipation rates were calculated as well as changes in potential energy. Surface velocity data were partitioned into frequency bands and complex demodulated. Throughout the sampling period, most of the current energy was found to be in the semi‐diurnal tidal band. After ice breakup, however, low frequency and inertial motions became relatively more important in response to direct wind forcing at the sea surface. Changes in amplitudes and phases of the major tidal constituents occurred and are related to the presence of the sea ice cover. Between early April and mid‐June, semi‐diurnal current amplitude doubled while its phase shifted by 45 to 60 degrees. In early June, the ice cover was sufficiently dispersed to allow the surface turbulence to overcome the buoyancy flux and mix the upper water column.
Small air bubbles in reagent grade water and seawater: 1. Rise velocities of 20‐ to 1000‐μm‐diameter bubbles
Authors: Detsch Richard M.
Published: February 2008

The terminal rise velocities of 20‐ to 1000‐μm‐diameter air bubbles in water and seawater were measured at atmospheric pressure and room temperature. The velocities were the same in both liquids for bubbles with diameters less than 450 μm. Above 450 μm the rise velocities were higher in water than in seawater. For all diameters tested, the measured velocities fell between those expected for bubbles with “frozen” surfaces and those expected for bubbles with fluid surfaces which exhibit internal circulation.
The comparison of a numerical model to observations of velocity and scalar data in Burrard Inlet and Indian Arm, British Columbia
Authors: Stacey Michael W. , Pond Stephen , Cameron Andrew N.
Published: February 2008

A laterally integrated, time‐dependent, numerical model of Burrard Inlet and Indian Arm is compared to time series of cyclesonde velocity and scalar data that have 10 m vertical resolution. As long as deep water renewal is not occurring, the model can reproduce the observed M2 and K1 tidal response in the basin of Indian Arm. Also, although the rate of freshwater inflow into Indian Arm and Burrard Inlet is not large, the simulated tidal response is noticeably improved if freshwater inflow is taken into account. When deepwater renewal is observed to occur, the model also simulates renewal, but it does so poorly, and the ability of the model to accurately simulate the M2 and K1 tide degrades significantly.
Estimation of time averages from irregularly spaced observations: With application to coastal zone color scanner estimates of chlorophyll concentration
Authors: Chelton Dudley B. , Schlax Michael G.
Published: February 2008

A formalism is presented for quantifying the sampling error of an arbitrary linear estimate of a time‐averaged quantity constructed from a time series of irregularly spaced observations at a fixed location. The method is applicable to any irregularly sampled time series; it is applied here to satellite observations of chlorophyll from the coastal zone color scanner (CZCS). The two specific linear estimates considered here are the composite average formed from the simple average of all observations within the averaging period and the optimal estimate formed by minimizing the mean squared error of the temporal average based on all of the observations in the time series. The formalism requires a priori knowledge of the variances and correlation functions of the chlorophyll signal and CZCS measurement error. In the usual absence of the necessary detailed information on these parameters, values obtained here from in situ measurements of chlorophyll and fluorescence off the coast of southern California can be used. The resulting estimates are referred to here as “suboptimal estimates,” which are optimal only if the assumed values for the parameters are correct. Suboptimal estimates are shown to be much more accurate than composite averages. Moreover, suboptimal estimates are also shown to be nearly as accurate as optimal estimates obtained using the correct signal and measurement error variances and correlation functions for realistic ranges of these parameters. Suboptimal estimation is thus a very useful and practical alternative to the composite average method generally used at present.
Middepth flow near hydrothermal venting sites on the southern Juan de Fuca Ridge
Authors: Cannon G. A. , Pashinski D. J. , Lemon M. R.
Published: February 2008

Long‐term moored current measurements around the southern end of the Juan de Fuca Ridge show that flow parallels the ridge and may be intensified by its presence. The mean flow is northward on the west side and southward on the east. Thus plumes emanating from the ridge probably are advected along the ridge before flowing away from the ridge. Other topographic features may be responsible for westward or eastward turning. Along the west side of the ridge, we hypothesize a regional flow pattern with southward flow north of Axial Seamount, northward flow south of Axial, and a convergence somewhere south of Axial resulting in westward flow. Along the east side, the flow may be southward everywhere. Above the ridge crest the long‐term mean flow is weaker and more variable, and it includes tidal, inertial, and a dominant broadband 4‐day oscillation. This 4‐day energy is maximum in the rift valley (where the inertial is least), decreases upward and away from the ridge, and is more energetic in winter. This oscillation is predominately a clockwise rotating feature which, along with the presence of inertial oscillations, suggests that it may be storm‐induced from the sea surface. These observations along with those by Cannon and Pashinski (1990) suggest northward propagation. Flow variations across the ridge lasting one to a few months have mean speeds of 1–3 cm/s and are capable of transporting plumes off axis to the west or east into the northerly or southerly flows, respectively.
Horizontal transport and the distribution of nutrients in the Coastal Transition Zone off northern California: Effects on primary production, phytoplankton biomass and species composition
Authors: Chavez Francisco P. , Barber Richard T. , Kosro P. Michael , Huyer Adriana , Ramp Steven R. , Stanton Timothy P. , Rojas de Mendiola Blanca
Published: February 2008

Conductivity‐temperature‐depth surveys during 1988 encountered strong baroclinic jets that were evident in acoustic Doppler current profiler and hydrographic data. During June and July 1988 a filament with high surface nitrate, high chlorophyll, abundant populations of neritic centric diatoms, and higher rates of primary production was evident perpendicular to the coast between Point Arena and Point Reyes. However, the high‐nutrient and phytoplankton regions were not in the baroclinic jets but were south and inshore of them. Surface water transported offshore by the strong baroclinic jets was found to have relatively low nutrient content, suggesting that the jets themselves do not carry significant levels of coastally upwelled, high‐nutrient water to the ocean interior. The low nutrient and salinity content of the jet suggests that the water originated several hundred kilometers upstream. Although the jets themselves do not appear to transport significant levels of nutrients directly from the coastal regime to the oceanic regime, dynamic processes associated with a meandering jet are likely responsible for high surface nutrients found several hundred kilometers offshore. Processes such as upwelling along the southern edge of the seaward jet result in significant enrichment of the coastal transition zone and in large blooms of neritic diatoms. During 1988 the high‐nutrient, high‐phytoplankton filament was present when the survey sequence began but then decayed after a month. The surface and subsurface nitrate fields were coherent with the dynamic topography field throughout the survey sequence; however, the surface and integrated chlorophyll fields were coherent only through the first two surveys. A decrease in phytoplankton biomass began during the third survey coincident with physical changes which occurred in that time frame: (1) an intensification of the undercurrent and (2) changes in the surface circulation from predominantly offshore to predominantly longshore. Understanding the processes responsible for the uncoupling between biology and physics is paramount for realistic biological models of this region.
Phytoplankton and photosynthetic light response in the Coastal Transition Zone off northern California in June 1987
Authors: Hood Raleigh R. , Abbott Mark R. , Huyer Adriana
Published: February 2008

In June 1987 the geostrophic flow in the coastal transition zone off northern California (between 50 and 150 km off the coast from Point Reyes to just north of Cape Mendocino) was dominated by a well‐defined, southward‐meandering current. Three vertical sections are presented that show the hydrographic structure of the current down to 100 m and its relationship to the distribution of phytoplankton biomass. The sections show that the geostrophic adjustment brought cold, saline, deep water up to the surface on the low steric height, or cold side of the flow, and that this upwelled water supported a relatively large diatom biomass (chlorophyll‐a concentrations between 1 and 10 mg m−3). We present particle size spectra and photomicrographs of the phytoplankton that show that the diatom biomass was dominated by chain‐forming species (e.g., Skeletonema costatum, Chaetoceros spp., Thalassiosira spp., and Rhizosolenia alata, but also a single‐celled Actinocyclus sp.). Photosynthetic light response measurements reveal that these diatom communities were capable of high photosynthetic rates (Pmax between 5 and 25 mg C mg Chla−1 h−1). Although most of the diatoms were located in cold, slow‐moving water on the low side of the current, some were being carried downstream. High chlorophyll concentrations were observed at depths > 75 m in and along the cold edge of the flow in all of our sections; we show evidence that in two out of three cases this was the result of water mass subduction.
Effect of seasonal surface freshwater flux on sea surface temperature in the tropical Atlantic Ocean
Authors: Carton James A.
Published: February 2008

In addition to its relationship to evaporative cooling and the atmosphere's radiative balance, anomalies of freshwater flux at the ocean surface have a potential impact on the stratification of the tropical mixed layer. Under light wind conditions when the thermocline is deep, excess rainfall can cause a low‐salinity layer to form, inhibiting vertical convection and mixing. It has previously been hypothesized that under conditions where the heat entering the mixed layer is balanced by cooling at the base of the mixed layer, such as occurs in upwelling regions, this effect could raise sea surface temperatures (Miller, 1976). Conversely, excess evaporation in such a region could destabilize the mixed layer, enhancing convection and lowering sea surface temperature. Here we report on a numerical experiment to test these ideas. We employ a model of the seasonal tropical Atlantic circulation which includes full representation of salinity and heat transport. Two simulations are compared. In one the surface boundary conditions include complete hydrological forcing by river discharge, rainfall, and evaporation. In the other simulation the vertical flux of water at the surface is set to zero. We find that seasonal changes in net freshwater flux at the surface do alter sea surface temperature by ≈±0.1°C. This change is too small to significantly alter the exchange of heat across the surface of the ocean. However, the effect is larger in areas strongly affected by river discharge and at the base of the mixed layer.
Observations of the Antarctic Circumpolar Current and deep boundary current in the southwest Atlantic
Authors: Whitworth T. , Nowlin W. D. , Pillsbury R. D. , Moore M. I. , Weiss R. F.
Published: February 2008

Fourteen‐month velocity and temperature records from an array of 14 moorings north and west of the Falkland Plateau and supporting hydrographic and tracer data reveal a narrow boundary current that carries dense Antarctic waters. The current flows west along the northern flank of the Falkland Plateau with mean speeds of more than 10 cm s−1 at 5000 m and more than 30 cm s−1 at 2500 m. The westward flow extends from the bottom to at least 1000 m, but the upper portion of the current is a branch of the Antarctic Circumpolar Current (ACC) following the only deepwater route between the Scotia Sea and the Argentine Basin. Waters colder than 0.2°C are too cold to be associated with the ACC at Drake Passage and must ultimately derive from the Weddell Sea as part of the deep thermohaline circulation. The westward transport of water colder than 0.2°C is 8.2 × 106 m3 s−1. In the mean the bottom boundary current is similar to that predicted by the Stommel‐Arons model, but considerable variability is introduced by the meandering of the overlying ACC. Chlorofluorocarbon data suggest that new Antarctic water from the Georgia Basin enters the Argentine Basin via the deep boundary current, which passes beneath the ACC; some new water is also advected east after being entrained in the ACC. Most of the water in the deep boundary current is recirculated water that has been in residence in the Argentine Basin for some time. Water colder than −0.2°C is relatively new to the basin and comprises about 2.5 × 106 m3 s−1 of the westward flow of the boundary current.
A relationship for the penetration of ultraviolet B radiation into the Norwegian Sea
Authors: Højerslev Niels , Aas Eyvind
Published: February 2008

A linear relation between the vertical attenuation coefficients of UV‐B and blue irradiances is obtained for the Norwegian Sea. The Atlantic waters in this region are on average characterized by a deeper 1% depth of UV‐B irradiance than the surrounding waters, the depth being situated between 23 and 33 m.