bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of enriched‐type mid‐ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N
Authors: Niu Yaoling , Collerson Ken D. , Batiza Rodey , Wendt J. Immo , Regelous Marcel
Published: September 2002

The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid‐ocean ridge basalts (E‐MORB) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two‐component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched domains, whereas the recycled mantle/peridotitic residues are the most depleted matrix. On Pb‐Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume‐ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume‐ridge interactions” seen along the Mid‐Atlantic Ridge. The so‐called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic lithologies. The positive Na8‐Si8/Fe8 and negative Ca8/Al8‐Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.
Inherited crustal features and tectonic blocks of the Transantarctic Mountains: An aeromagnetic perspective (Victoria Land, Antarctica)
Authors: Ferraccioli F. , Bozzo E.
Published: September 2002

Aeromagnetic images covering a sector of the Transantarctic Mountains in Victoria Land as well as the adjacent Ross Sea are used to study possible relationships between tectonic blocks along the Cenozoic and Mesozoic West Antarctic rift shoulder and prerift features inherited mainly from the Paleozoic terranes involved in the Ross Orogen. The segmentation between the Prince Albert Mountains block and the Deep Freeze Range‐Terra Nova Bay region is related to an inherited NW to NNW ice‐covered boundary, which we name the “central Victoria Land boundary.” It is interpreted to be the unexposed, southern continuation of the Ross age back arc Exiles thrust system recognized at the Pacific coast. The regional magnetic high to the west of the central Victoria Land boundary is attributed to Ross age calc‐alkaline back arc intrusives forming the in‐board Wilson “Terrane,” thus shifting the previously interpreted Precambrian “shield” at least 100 km farther to the west. The high‐frequency anomalies of the Prince Albert Mountains and beneath the Polar Plateau show that this region was extensively effected by Jurassic tholeiitic magmatism; NE to NNE trending magnetic lineations within this pattern could reflect Cretaceous and/or Cenozoic faulting. The western and eastern edges of the Deep Freeze Range block, which flanks the Mesozoic Rennick Graben, are marked by two NW magnetic lineaments following the Priestley and Campbell Faults. The Campbell Fault is interpreted to be the reactivated Wilson thrust fault zone and is the site of a major isotopic discontinuity in the basement. To the east of the Campbell Fault, much higher amplitude magnetic anomalies reveal mafic‐ultramafic intrusives associated with the alkaline Meander Intrusive Group (Eocene‐Miocene). These intrusives are likely genetically linked to the highly uplifted Southern Cross Mountains block. The NW‐SE trends crossing the previously recognized ENE trending Polar 3 Anomaly offshore of the Southern Cross Mountains are probably linked to Cenozoic reactivation of the Paleozoic Wilson‐Bowers suture zone as proposed from recent seismic interpretations. The ENE trend of the anomaly may also be structural, and if so, it could reflect an inherited fault zone of the cratonal margin.
Theoretical analysis of cross‐joint geometries and their classification
Authors: Bai Taixu , Gross Michael R.
Published: September 2002

One commonly observed bedrock joint pattern consists of cross joints that terminate against an older set of longer systematic joints. Dyer [1988] showed that unique cross‐joint geometries may develop in response to a combination of remote stresses and local perturbations of the stress field in the vicinity of preexisting joints. We expand upon Dyer's analysis by providing the general solutions for cross‐joint geometry as a function of remote principal stress orientations and ratios. Specific cross‐joint geometries are determined by solving for the local cross‐joint angle as a function of distance from the preexisting joint. On the basis of theoretical derivations, cross‐joint geometries are grouped into five main categories: curving parallel, curving perpendicular, quasi‐curving parallel, quasi‐curving perpendicular and noncurving geometries. Furthermore, by discussing the stress state in the vicinity of the preexisting joint, a general expression for the dimension of compressive zone is derived, and a more detailed classification of cross‐joint geometry is provided. Results from this study provide a method to estimate the relative magnitudes of remote principal stresses simply based on the observed cross‐joint geometry, which may help constrain the tectonic history of a region. In addition, knowledge of precise cross‐joint curvature and intersection geometries may provide important insights into their connectivity and fluid flow characteristics.
Short‐period P wave constraints on D″ reflectivity
Authors: Reasoner Colin , Revenaugh Justin
Published: September 2002

We present evidence of a P wave reflector in D″ beneath the central and eastern equatorial Pacific obtained from nearly 7000 short‐period seismic recordings of earthquakes in the Tonga‐Fiji and South America subduction complexes. Both regions overlap previous observations of an S wave triplication at similar depths. Double‐array waveform stacking places the reflector 190 ± 20 km above the core‐mantle boundary and allows for a velocity transition zone no thicker than 50 km. The data admit a wide range of compressional (vP) and shear wave velocities (vS); the 2–3% increase in vS suggested by earlier long‐period S wave studies, however, necessitates only a 0.5–0.6% increase in vP. The data stacks represent spatial averages over large areas of the lower mantle, such that reflector intermittence and topography decrease stack amplitude and lower the apparent vP jump. Our results constrain these processes to act over length scales of 10–100 km.
Isotope geochemistry of the Darwin Rise seamounts and the nature of long‐term mantle dynamics beneath the south central Pacific
Authors: Janney Philip E. , Castillo Paterno R.
Published: September 2002

The Darwin Rise region in the west central Pacific Ocean contains an extraordinarily large number of Cretaceous seamounts with isotopic compositions spanning a range nearly equal to that of the global ocean island basalt data set. These edifices formed in what is now the south central Pacific, which is characterized by anomalously shallow lithosphere and concentrated intraplate volcanism. We present Sr, Nd, and Pb isotopic data for Darwin Rise seamount basalts recovered by Ocean Drilling Program Leg 143 and Deep Sea Drilling Project Leg 7. Comparison of chemical and isotopic data for Darwin Rise and French Polynesian volcanoes confirms that the same range of distinct sources has been tapped by intraplate volcanism in the south central Pacific since the Early Cretaceous. Moreover, the increased data coverage reveals that most of the isotopic variation within Darwin Rise volcanic chains can be explained by shallow mixing between plumes and asthenosphere or oceanic lithosphere. On the basis of geophysical and geochemical evidence we propose an alternative model for volcanism in the south central Pacific from the Cretaceous to the present. Instead of the conventional hypothesis involving a few long‐lived mantle plumes, we propose that numerous hotspots formed and died out in the region over the past 130 Myr. We envision that these hotspots are the surface expression of “plumelets” continually separating from a shallow thermochemical boundary layer at the top of a large heterogeneous body of material which rose from the lower mantle. These plumelets rise and melt beneath the south central Pacific lithosphere, producing chains of volcanic edifices which terminate when they become exhausted of melt. The surfacing of multiple plumelets in proximity can explain the complex temporal, spatial, and geochemical patterns of volcanism observed in most volcanic chains in the south central Pacific and Darwin Rise.
Constraints on the proposed Marie Byrd Land‐Bellingshausen Plate Boundary from seismic reflection data
Authors: Heinemann J. , Stock J. , Clayton R. , Hafner K. , Cande S. , Raymond C.
Published: September 2002

Single‐channel and multichannel marine seismic data off the coast of West Antarctica collected during two Nathaniel B. Palmer cruises (NP92‐8 and NP96‐2) in the vicinity of 65°S to 71°S, 220°E to 250°E, reveal a NNW trending graben. We interpret this graben to be part of the paleodivergent plate boundary between the Marie Byrd Land and Bellingshausen plates. This graben coincides with a −520 nT magnetic anomaly to the NNW and a −720 nT anomaly to the SSE, as well as a 20 mGal negative gravity anomaly. Seismic profiles subparallel to the graben (22 km/Ma half‐spreading rate) reveal greater seafloor roughness to the NE, where seafloor spreading was slower, than to the SW (27 km/Ma half‐spreading rate). These data allow the position of the Marie Byrd Land‐Bellingshausen plate boundary to be constrained more precisely than has previously been possible, with a trend of N17°W from 68.52°S, 233.65°E to 68.41°S, 233.56°E. The sediment‐filled graben has normal separation of sedimentary layers varying from 740±30 m to 580±20 m imaged in seafloor of age A33y (74 Ma).
Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements
Authors: Crawford Wayne C. , Webb Spahr C. , Hildebrand John A.
Published: September 2002

Seafloor compliance measurements across the East Pacific Rise at 9°48′N reveal low shear velocities throughout the crust and at the crust‐mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9°48′N, the shear velocity is less than 20 m/s in the shallow on‐axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on‐axis melt lens 5.4 ± 1 km beneath the seafloor, with shear velocities slower than 50 m/s. This “deep” melt lens may be created by melt pooling at a permeability or density barrier at the crust‐mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5–18% melt. Melt persists in the lower crust to at least 10 km off‐axis, where the top of the lower crustal low‐velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on‐axis to 0.58 by 10 km off‐axis, indicating that there are abundant thin cracks in the sheeted dikes on‐axis and that these cracks close away from the rise axis. High on‐axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.
Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments
Authors: Xu Wenyue , Ruppel Carolyn
Published: September 2002

Using a new analytical formulation, we solve the coupled momentum, mass, and energy equations that govern the evolution and accumulation of methane gas hydrate in marine sediments and derive expressions for the locations of the top and bottom of the hydrate stability zone, the top and bottom of the zone of actual hydrate occurrence, the timescale for hydrate accumulation in sediments, and the rate of accumulation as a function of depth in diffusive and advective end‐member systems. The major results emerging from the analysis are as follows: (1) The base of the zone in which gas hydrate actually occurs in marine sediments will not usually coincide with the base of methane hydrate stability but rather will lie at a more shallow depth than the base of the stability zone. Similarly, there are clear physical explanations for the disparity between the top of the gas hydrate stability zone (usually at the seafloor) and the top of the actual zone of gas hydrate occurrence. (2) If the bottom simulating reflector (BSR) marks the top of the free gas zone, then the BSR should occur substantially deeper than the base of the stability zone in some settings. (3) The presence of methane within the pressure‐temperature stability field for methane gas hydrate is not sufficient to ensure the occurrence of gas hydrate, which can only form if the mass fraction of methane dissolved in liquid exceeds methane solubility in seawater and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. These critical flux rates can be combined with geophysical or geochemical observations to constrain the minimum rate of methane production by biogenic or thermogenic processes. (4) For most values of the diffusion‐dispersion coefficient the diffusive end‐member gas hydrate system is characterized by a thin layer of gas hydrate located near the base of the stability zone. Advective end‐member systems have thicker layers of gas hydrate and, for high fluid flux rates, greater concentrations near the base of the layer than shallower in the sediment column. On the basis of these results and the very high methane flux rates required to create even minimal gas hydrate zones in some diffusive end‐member systems, we infer that all natural gas hydrate systems, even those in relatively low flux environments like passive margins, are probably advection dominated.
Hydrothermal activity and the evolution of the seismic properties of upper oceanic crust
Authors: Grevemeyer Ingo , Kaul Norbert , Villinger Heinrich , Weigel Wilfried
Published: September 2002

In order to investigate the impact of off‐axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust (layer 2A), we performed an extensive geophysical survey on the eastern flank of the East Pacific Rise at 14°S. Seismic refraction and heat flow data were obtained along a 720‐km‐long and 25 to 40‐km wide corridor, covering thinly sedimented seafloor created since 8.5 Ma. The seismic data yield a seismic velocity of ∼2.9 km/s at the top of 0.5‐m.y.‐old basement rocks. Within about 8 m.y. the velocity increases gradually to a value of mature oceanic crust (∼4.3 km/s). Heat flow data, derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements, suggest that an open hydrothermal circulation system persists for at least 6–7 m.y. In crust older than 7 Ma, regional heat flow is close to values predicted by plate cooling models, suggesting that hydrothermal circulation is going to cease. Considering published dating of alteration minerals, it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10–15 m.y. This idea may explain why seismic velocities in the Pacific ocean have not changed significantly in igneous crust older than 8–10 Ma. In regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different; velocities increase rapidly and reach values of mature oceanic crust within 1–2 m.y. We therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust [Stephen and Harding, 1983; Rohr, 1994].
How continents break up: Insights from Papua New Guinea
Authors: Taylor Brian , Goodliffe Andrew M. , Martinez Fernando
Published: September 2002

The Woodlark Basin in the western Pacific forms a continuous system of active continental rifting evolving to well‐developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin. Magnetic isochrons indicate a single Euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiated geologically synchronously (at ∼6 Ma) along the length of the protomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion from east to west within this zone of deformation. Spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the Solomon and Coral Seas. The transition to spreading occurred after a rather uniform degree of continental extension: 200±40 km. Both early and late stage rifting involved high‐ and low‐angle normal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures. Continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating. The initial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults between initially nontransform spreading segment offsets, produced rift and spreading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direction but, as the protomargins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within the protomargins. Thus stepwise spreading nucleation in order to remain within a rheologically weak zone, rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis, is the dominant form of the rifting‐to‐spreading transition in the Woodlark Basin.
Mixing and compositional stratification produced by natural convection: 2. Applications to the differentiation of basaltic and silicic magma chambers and komatiite lava flows
Authors: Jellinek A. Mark , Kerr Ross C.
Published: September 2002

The petrogenesis of igneous rocks can be controlled significantly by the mixing of dissimilar magmas. Within the contexts of basaltic and silicic magma chambers and komatiite lava flows we identify circumstances in which the extent to which contrasting magmas are mixed by natural convection potentially controls their differentiation. To evaluate the amount of mixing in each context, we apply the experimental results from part 1 [Jellinek et al., this issue] of this study, in which we quantified the conditions under which fluids could be mixed by convection at large Rayleigh numbers (> 1011). When our laboratory results are applied to basaltic magma chambers, we find that convection driven by compositionally buoyant magma released during floor crystallization or floor dissolution will produce partial to nearly complete mixing of the ascending fluid in chambers that are tens of meters to kilometers high, respectively. We also conclude that substantial floor melting (with extensive mixing) is expected only for basaltic chambers emplaced in the deep crust. During the turbulent flow of Archean komatiites, underlying sediments melted by forced convective heat transfer are predicted to have been mixed nearly completely into the overriding lavas. During the replenishment of silicic magma chambers by basaltic magmas we predict that the convection of buoyant silicic magma overrun by a spreading injection of denser basalt will cause little mixing. However, after emplacement, heat transfer from a basalt layer will gradually melt and mobilize its felsic floor, producing a small flux of buoyant felsic liquid that will be mixed extensively.
Observations of Mediterranean flow into the Black Sea
Authors: Di Iorio Daniela , Yüce Hüseyin
Published: September 2002

Mediterranean Sea water inflow into the Black Sea is investigated using acoustic and oceanographic data obtained in the Black Sea exit region. The path of Mediterranean water and the resulting spreading on the continental shelf are observed with multibeam echo‐sounding (SWATH) bottom bathymetry measurements, high‐resolution echo‐sounding images, and conductivity‐temperature‐depth profiles. It is found that the saline Mediterranean water is reduced by 6 practical salinity units as it flows and spreads on the shelf before reaching the continental slope, where it sinks to a depth appropriate to its density. Temporal and spatial variability in the flow and their relation to atmospheric and sea level changes are documented. It is shown that blockage of the Mediterranean undercurrent occurs when the estimated Black Sea water transport exceeds 25 × 103 m3 s−1, which corresponds to a relative sea level difference greater than 40 cm. Mediterranean flow into the Black Sea is a high Reynolds, low Richardson number flow, resulting in a turbulent bottom boundary layer. Measurements of the path‐averaged turbulent kinetic energy dissipation rate give values ranging from 1 × 10−6 to 1 × 10−4 W kg−1.
Upper layer currents in the western tropical North Atlantic (1989–1991)
Authors: Bourles B. , Molinari R. L. , Johns E. , Wilson W. D. , Leaman K. D.
Published: September 2002

Shipboard acoustic Doppler current profiler (ADCP) measurements and hydrographic observations of temperature, salinity, and dissolved oxygen are used to examine the upper water column flow field in the North Brazil Current (NBC) retroflection region of the western tropical Atlantic Ocean. Observations are presented from six cruises, one conducted in August 1989 and the other five conducted during the Western Tropical Atlantic Experiment between January 1990 and September 1991. The upper water column is divided into two layers, an upper thermocline layer located between the surface and the 24.5 σθ isopycnal surface and a lower subthermocline layer located between the 24.5 and 26.75 isopycnals. In the upper layer the NBC retroflects north of the equator to form the eastward flowing North Equatorial Countercurrent (NECC). During the six cruises the retroflection appeared complete. However, data coverage did not extend shoreward of the 200‐m isobath, so the possibility of a continuous flow over the shelf still remains. There were also indications of several NBC rings that had apparently separated from the NBC retroflection and drifted to the northwest toward the eastern Caribbean Sea. North of the NBC retroflection and the NECC, the North Equatorial Current (NEC) flows west as the southern limb of the subtropical gyre. Part of the NEC is observed to retroflect cyclonically to join the eastward NECC flow. In the lower layer, beneath the NBC, the North Brazil Undercurrent retroflects to feed the eastward North Equatorial Undercurrent (NEUC). To the north a deeper component of the NEC recurves to also contribute to the NEUC.
NSCAT tropical wind stress maps: Implications for improving ocean modeling
Authors: Kelly Kathryn A. , Dickinson Suzanne , Yu Zuojun
Published: September 2002

Using wind vectors from the NASA scatterometer (NSCAT), daily maps of pseudostress have been constructed for the tropical Pacific Ocean and compared with pseudostress maps derived from the European Centre for Medium‐Range Weather Forecasts (ECMWF) 10‐m wind product. The map resolution for the NSCAT pseudostress maps was selected using both a statistical measure of the expected mapping errors and tests on realistic wind fields. The selected map resolution is 5 days and 2°, which minimizes residual effects from the NSCAT sampling pattern, while maximizing temporal and spatial resolution. Comparisons with the ECMWF maps showed significant differences in most regions, corresponding to mean wind speeds of 2–3 m s−1, particularly in the Intertropical Convergence Zone (ITCZ) and at 20°S and 20°N. A canonical correlation analysis between NSCAT and ECMWF fields showed a high degree of correlation of temporal variations and systematic differences in spatial structure. In the NSCAT fields the ITCZ is narrower, stronger, and is located 1–2° latitude farther south than in the ECMWF fields. The high degree of correlation between the two fields suggests that “hybrid” fields can be generated. The dynamical implications of the differences in wind forcing are illustrated using estimates of the Sverdrup stream function and the Ekman pumping. A simple reduced‐gravity, linear vorticity model, forced by both the ECMWF and hybrid winds to examine predicted differences in ocean response, showed higher skill for the NSCAT winds.
A multiyear hourly sea surface skin temperature data set derived from the TOGA TAO bulk temperature and wind speed over the tropical Pacific
Authors: Zeng Xubin , Zhao Ming , Dickinson Robert E. , He Yanping
Published: September 2002

A theoretical relationship is derived to estimate the sea surface skin temperature from near‐surface wind speed and the diurnal variation of sea surface bulk (or bucket) temperature. Coefficients in the relation are determined using the R/V Franklin data during the Tropical Ocean‐Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE). In contrast to previous methods, surface energy flux data are not explicitly required but rather are implied by the temporal variation of bulk temperature. A multiyear hourly skin temperature data set is obtained using data of bulk temperature at 1‐m depth and wind speed from the TOGA Tropical Atmosphere‐Ocean (TAO) moored buoys spanning the tropical Pacific Ocean from 95°W in the eastern Pacific to 137°E in the western Pacific between 9°N and 8°S. The diurnal amplitude of skin temperature reaches its maximum of about 2.8 K for daily averaged wind speed between 1–2 m s−1 and skin temperature between 20°–21°C and decreases with greater wind speeds. The most frequent amplitude is about 0.5 K, the average amplitude is 0.65 K, and the accumulated frequency for amplitudes greater than 1 K is 10% within the parameter space of daily averaged wind speed between 1 and 15 m s−1 and daily averaged skin temperature between 18° and 34°C.
Bound waves and Bragg scattering in a wind‐wave tank
Authors: Plant William J. , Keller William C. , Hesany Vahid , Hara Tetsu , Bock Erik , Donelan Mark A.
Published: September 2002

We present optical and microwave measurements that show the presence of bound waves traveling at the speed of the dominant wave in a wind‐wave tank. We suggest that when these bound waves are much shorter than the dominant waves, they are preferentially located on the leeward face of the dominant wave and hence have a mean tilt. We hypothesize that the turbulence associated with these bound waves suppresses freely propagating, wind‐generated waves where bound waves are present so that we may divide the rough water surface into patches containing free and patches containing bound waves. This model is shown to account for the observed histograms of slope measured in the tank and, at least qualitatively, for the observed decrease in the probability of finding bound waves with increasing wind speed. Furthermore, if we add these bound, tilted waves to the free waves of the standard Bragg/composite‐surface scattering model for microwave scattering from rough water surfaces, then the model can account for many otherwise unexplained features of the scattering. Principal among these features are the rapid decrease in polarization ratio and rapid increase in the first moment of the microwave Doppler spectrum with increasing wind speed when the antenna is directed upwind, features that occur to a much lesser extent when the antenna looks downwind.
Bubble shattering: Differences in bubble formation in fresh water and seawater
Authors: Slauenwhite David E. , Johnson Bruce D.
Published: September 2002

Breaking waves result in very different populations of bubbles in fresh water and seawater. The differences in sizes and numbers of bubbles in the two media cause significant differences in many natural processes including gas transfer and generation of aerosols as well as affect sound and light propagation and ambient sound production. Observed differences in bubble populations in seawater and fresh water have in the past been attributed to coalescence inhibition in seawater, but bubble‐shattering studies described here suggest that other factors may be important. When air bubbles are expanded through an orifice at constant pressure drop, they can be made to shatter into clouds of smaller bubbles. We report here measurements showing that the number and size distribution of resulting populations depend on the physical and chemical characteristics of the water samples. In particular, bubbles were found to break up into 4–5 times more small bubbles in seawater than in fresh water. The number of bubbles produced in shattering was found to be a function of salt concentration but was especially sensitive to the types of ions present. In addition, medium from the marine diatom Phaeodactylum tricornutum was found to further double production numbers, and a decrease in temperature from 20°C to 3°C was found to increase bubble production in seawater by nearly 50%. These effects appear to be separate from coalescence inhibition, the factor usually cited for differences in bubble populations in fresh water and sea‐water.
Spatial regression and multiscale approximations for sequential data assimilation in ocean models
Authors: Chin Toshio M. , Mariano Arthur J. , Chassignet Eric P.
Published: September 2002

Effects of spatial regularity and locality assumptions in the extended Kalman filter are examined for oceanic data assimilation problems. Biorthogonal wavelet bases are used to implement spatial regularity through multiscale approximations, while a Markov random field (MRF) is used to impose locality through spatial regression. Both methods are shown to approximate the optimal Kalman filter estimates closely, although the stability of the estimates can be dependent on the choice of basis functions in the wavelet case. The observed filter performance is nearly constant over a wide range of values for the scalar weights (uncertainty variances) given to the model and data examined here. The MRF‐based method, with its inhomogeneous and anisotropic covariance parameterization, has been shown to be particularly effective and stable in assimilation of simulated TOPEX/POSEIDON altimetry data into a reduced‐gravity, shallow‐water equation model.
Velocity structure, turbulence and fluid stresses in experimental gravity currents
Authors: Kneller Benjamin C. , Bennett Sean J. , McCaffrey William D.
Published: September 2002

Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock‐exchange tank. Laser‐Doppler anemometry was used to construct a two‐dimensional picture of the velocity structure. The velocity maximum within the gravity current occurs at y/d ≈ 0.2. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed, and this largely undiluted fluid becomes rapidly mixed with ambient fluid in the wake region. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness. Turbulent kinetic energy reaches a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum near the velocity maximum where fluid shear is low.
Deep inflow into the Mozambique Basin
Authors: Read J. F. , Pollard R. T.
Published: September 2002

More than 200 conductivity‐temperature‐depth (CTD) stations were worked around the Southwest Indian Ridge and Del Caño Rise as part of the World Ocean Circulation Experiment. A selection of these data provides information about the inflow of bottom water into the Mozambique Basin. The basin is closed below 3000 m, yet the inflow is significantly large, of order 1 Sv (1 Sv = 106 m3 s−1). Estimates of the basin‐scale upwelling at 4000 m suggest that the vertical velocity is also large, 10 × 10−5 cm s−1 or more, an order of magnitude greater than global ocean estimates. Examination of the characteristics of the bottom water in the Mozambique and Agulhas Basins and the Prince Edward Fracture Zone shows that bottom water enters the Mozambique Basin from the Agulhas Basin and also directly from the Enderby Basin. Most of the transport enters the Mozambique Basin via the Agulhas Basin, where two regions of northward flow below 4000 m are found. The major flow, on the eastern flank of the Mozambique Ridge, is through and above the deep, extending (5900 m) trench that connects the Agulhas and Mozambique Basins. The second, weaker flow enters the Transkei Basin along the deep eastern flank of the Agulhas Plateau, then turning east into the Mozambique Basin. The only source of bottom water to the Agulhas Basin is the Enderby Basin, but a more direct route between the Enderby and Mozambique Basins exists via the Prince Edward fracture, which extends deeper than 4000 m throughout its length and links the two basins directly across the Southwest Indian Ridge. Full depth CTD stations trace the changing characteristics of the deep and bottom water in the fracture, and moored current meter data show the strength and persistence of the throughflow. Strong mixing with the overlying deep water elevates the salt content of the bottom water by comparison with the other water in the Mozambique Basin. Thus two distinct bottom waters of the Mozambique Basin originate in the same place (the Enderby Basin), and their different characteristics are solely a function of the routes they have taken and the processes encountered along the different pathways.
An extended Kalman filter to assimilate satellite altimeter data into a nonlinear numerical model of the tropical Pacific Ocean: Method and validation
Authors: Verron J. , Gourdeau L. , Pham D. T. , Murtugudde R. , Busalacchi A. J.
Published: September 2002

A new implementation of the extended Kalman filter scheme is developed for the purpose of assimilating observations into a high‐resolution nonlinear numerical model of the tropical Pacific Ocean. It is characterized by two successive stages: (1) approximation of the error covariance matrix by a singular low‐rank matrix which leads to making corrections only in those directions for which the error is not naturally attenuated by the system (in this respect it is quite similar to the approach adopted by Cane et al. [1996]), and (2) modification of these directions over time according to the model dynamics, thus reflecting the evolutive nature of the filter. The filter is initialized by a method based on the empirical orthogonal functions obtained from free runs of the model. The reduction of the error covariance matrix avoids the overwhelming burden of computing the temporal evolution of the prediction error with all the degrees of freedom of the full state vector. This assimilation scheme is implemented in the Gent and Cane [1989] primitive equation reduced gravity model. This model has the advantage of using a σ coordinate in the vertical which allows a geographical refinement of the vertical resolution for thermocline gradients. It is forced by the Florida State University database winds, and the Chen et al. [1994] hybrid scheme parameterization is assumed for the mixed layer. The domain of application is the tropical Pacific Ocean basin between 120°E and 80°W longitude and 30°S and 30°N latitude. The resolution is finest at the equator and steadily increases away from the equator. In a first application, validation twin experiments are conducted in which observations are assumed to be synthetic altimeter data sampled according to the TOPEX/POSEIDON mission features. The method is shown to be efficient under various conditions and, in particular, to be capable of transferring information in the vertical from the surface‐only altimeter data to the deepest ocean layers and therefore of adequately constraining the thermal and the velocity profiles within the 300 or 400 m upper ocean. In a second application, real TOPEX/POSEIDON data are used and the results evaluated against independent in situ data, namely, Tropical Ocean‐Global Atmosphere (TOGA)/TAO mooring observations. Improvements in the model behavior are clear in terms of the time variability of the tropical ocean. It therefore seems that the reduced state Kalman filter with empirical orthogonal function (EOF) initialization can be applied with success to such a primitive equation model of the Pacific for the assimilation of altimeter data. However, at this stage the evolutivity of the filter seems to have a limited impact on the performances, which was not the case for tests in the strongly nonlinear midlatitudes [Pham et al., 1998a].
Influence of melting icebergs on distribution, characteristics and transport of marine particles in an East Greenland fjord
Authors: Azetsu‐Scott Kumiko , Syvitski James P. M.
Published: September 2002

A detailed study of distribution of suspended sediments in an East Greenland fjord with a high iceberg production rate reveals the existence of intermediate nepheloid layers (INLs). Observed INLs extended from the head to the mouth of the fjord at water depths between 100 and 400 m. Particulate organic carbon and nitrogen, chlorophyll a, and nutrient measurements and observation by microscope and energy dispersive X‐ray analyses were used to characterize marine particles. Particles in the INLs are composed of glacial flour including >50 μm quartz and feldspar grains with angular and sharp edges, considered to be released from melting icebergs. In situ photographs show large aggregates (>1 mm) at high concentration (>300 particles per liter) in and below INLs. These aggregates, in comparison with the dispersed particle size distribution, demonstrate that smaller size particles (e.g., clay) settle effectively along with larger single size grains. In Kangerlugssuaq Fjord, the mass transport of marine particles was governed by the subsurface iceberg melting, producing observed INLs, rather than the surface meltwater plume. This suggests that the subsurface water temperature controls release of iceberg debris and the existence of warm subsurface water, as well as the spread of cold and fresh water in the surface layer, needs to be considered to evaluate the occurrence of ice‐rafted debris layers, including Heinrich layers. This study provides the field evidence of a modern analogue on ocean conditions that could form iceberg‐rafted layers.
Approximating the energy transport across oceanic sections
Authors: Warren Bruce A.
Published: September 2002

A straightforward thermodynamic integration shows that the transport of the product of density in situ, potential temperature, and specific heat at constant pressure (the last two evaluated at sea level atmospheric pressure) across oceanic sections at which both the mass and the salt transports are zero approximates the internal energy transport to an accuracy of a few percent and the total energy transport to order 0.1%. It is misleading to call this water‐property transport “heat flux.”
Directional spreading of waves in the nearshore
Authors: Herbers T. H. C. , Elgar Steve , Guza R. T.
Published: September 2002

Observations of surface gravity waves shoaling between 8‐m water depth and the shoreline on a barred beach indicate that breaking results in an increase in the directional spread of wave energy, in contrast to the directional narrowing with decreasing depth predicted by refraction theory (Snell's law). During low‐energy wave conditions, when breaking‐induced wave energy losses over the instrumented transect are small, the observed mean propagation direction and spread about the mean both decrease with decreasing depth, consistent with the expected effects of refraction. Nonlinearity causes high‐frequency components of the spectrum to become directionally aligned with the dominant incident waves. During high‐energy wave conditions with significant wave breaking on the sand bar, the observed mean directions still decrease with decreasing depth. However, the observed directional spreads increase sharply (nominally a factor of 2 for values integrated over the swell‐sea frequency range) between the outer edge of the surf zone and the crest of the sand bar, followed by a decrease toward the shoreline. Observations on a nonbarred beach also show directional broadening, with spreads increasing monotonically from the outer edge of the surf zone to a maximum value near the shoreline. Although the mechanism is not understood, these spatial patterns of directional broadening suggest that wave breaking causes significant scattering of incident wave energy into obliquely propagating components.
Measurements of Reynolds stress profiles in unstratified tidal flow
Authors: Stacey Mark T. , Monismith Stephen G. , Burau Jon R.
Published: September 2002

In this paper we present a method for measuring profiles of turbulence quantities using a broadband acoustic doppler current profiler (ADCP). The method follows previous work on the continental shelf and extends the analysis to develop estimates of the errors associated with the estimation methods. ADCP data was collected in an unstratified channel and the results of the analysis are compared to theory. This comparison shows that the method provides an estimate of the Reynolds stresses, which is unbiased by Doppler noise, and an estimate of the turbulent kinetic energy (TKE) which is biased by an amount proportional to the Doppler noise. The noise in each of these quantities as well as the bias in the TKE match well with the theoretical values produced by the error analysis. The quantification of profiles of Reynolds stresses simultaneous with the measurement of mean velocity profiles allows for extensive analysis of the turbulence of the flow. In this paper, we examine the relation between the turbulence and the mean flow through the calculation of u*, the friction velocity, and Cd the coefficient of drag. Finally, we calculate quantities of particular interest in turbulence modeling and analysis, the characteristic lengthscales, including a lengthscale which represents the stream‐wise scale of the eddies which dominate the Reynolds stresses.
Tides and tidal torques of the world ocean since the last glacial maximum
Authors: Thomas Maik , Sündermann Jürgen
Published: September 2002

Peltier's [1994] paleobathymetries are used to investigate tides and corresponding balances of torques of the world ocean since the last glacial maximum 21,000 years before present. Simulations with a numerical tidal model point out that variable ice loads at the Earth's surface and rearrangements in the interior led to considerable modifications of the resonance behavior of the oceans. Associated evident changes of the tidal range were not confined to the vicinity of formerly ice‐covered areas. In contrast to the tidal oscillation system that was subject to almost uniform changes since the beginning of ice retreat, the dissipation rate of the M2 tide increased distinctly during the principal melting time from 15 to 8 kyr B. P. and reached a local extreme eight thousand years ago though surface and volume of the world ocean were already very similar to present conditions. Postglacial deformations during the following two thousand years caused the decrease of tidal dissipation from the maximum value to the present‐day level, indicating the sensibility of tidal torques and dissipation rates to comparatively small variations of ocean's geometry. Since the vicinity of the tidal oscillation system to a resonance situation is more decisive with respect to total dissipation than the extension of shelves where most of the tidal dissipation is expected to take place, states of extreme dissipation rates do not necessarily coincide with extreme bathymetric conditions.
Variability of Fram Strait ice flux and North Atlantic Oscillation
Authors: Kwok R. , Rothrock D. A.
Published: September 2002

We estimate the winter sea ice export through the Fram Strait using ice motion from satellite passive microwave data. Sea ice motion (October‐May) is obtained by tracking the displacement of common features in sequential 85 and 37 GHz brightness temperature fields. The average winter area flux over the 18‐year record (1978–1996) is 670,000 km2, ∼7% of the area of the Arctic Ocean. The winter area flux ranges from a minimum of 450,000 km2 in 1984 to a maximum of 906,000 km2 in 1995. The daily, monthly, and interannual variabilities of the ice area flux are high. There is an upward trend in the ice area flux over the 18 year record. The average winter volume flux over the winters of October 1990 through May 1995 is 1745 km3 ranging from a low of 1375 km3 in the 1990 flux to a high of 2791 km3 in 1994. The sea level pressure gradient across the Fram Strait explains more than 80% of the variance in the ice flux over the 18 year record. We use the coefficients from the regression of the time series of area flux versus pressure gradient across the Fram Strait and ice thickness data to estimate the summer area and volume flux. The average 12 month area flux and volume flux are 919,000 km2 and 2366 km3. We find a significant correlation (R = 0.86) between the area flux and positive phases of the North Atlantic Oscillation (NAO) index over the months of December‐March. Correlation between our 5 years of volume flux estimates and the NAO index gives R = 0.56. During the high NAO years a more intense Icelandic low increases the gradient in the sea level pressure by almost 1 mbar across the Fram Strait, thus increasing the atmospheric forcing on ice transport. Correlation is reduced during the negative NAO years because of decreased dominance of this large‐scale atmospheric pattern on the sea level pressure gradient across the Fram Strait.