bluuebunny/crossref_metadata_2025 huggingface dataset:
The interaction of two closely spaced cracks: A rock model study
Authors: Lin Peng , Logan John M.
Published: February 2008

The influence of preexisting fractures on fracture development and the geometric patterns of crack growth have been investigated experimentally. Rectangular specimens of Berea sandstone with two parallel thin cuts were compressed along their longest dimension at a confining pressure of 60 MPa, a pore pressure of 20 MPa, an average displacement rate of 2×10−4 cm/s, and room temperature. The precuts are inclined 35° to the loading direction. They are en echelon with each other, each extending only part way from the specimen boundary. When the sense of shear ( left‐ or right‐lateral), overlap (left or right stepping), and the amount of separation are changed, significant difference of fracture interaction occurs. An extensional fracture zone forms between left‐stepping precuts subjected to left‐lateral shear (or right‐stepping precuts under right‐lateral shear). Secondary fractures tend to develop from one precut and toward the other precut. The sequence of the fracture development is as follows: a fractures which are about 60° to the precuts, β fractures which are about 30° to the precuts, and γ fractures for large overlap pattern which are about 70° to the precuts but opposite in direction. The fracture zone is confined by precuts and β fractures. Therefore, the larger the overlap, the larger the fracture zone that develops. For large overlap patterns, a set of α fractures forms nearly parallel and equally spaced within the fracture zone. In cross sections, fractures are normal faults with surface depression. Microscopic study shows that microfractures are highly concentrated in the area immediately adjacent to the macrofractures and at angles less than 30° to the macrofracture. Grains away from macrofractures remain unfractured. For right‐stepping precuts subject to left‐lateral shear (or left‐stepping precuts under right‐lateral shear), a compression zone forms between the precuts. The ζ fractures propagate at precut tips and diverge away. The η fractures develop within the compression zone and are bounded by precuts and ζ fractures. The η fractures develop within the compression zone and are bounded by precuts and ζ fractures. The η fractures have reverse offsets in cross section.
Monte Carlo estimation and resolution analysis of seismic background velocities
Authors: Koren Zvi , Mosegaard Klaus , Landa Evgeny , Thore Pierre , Tarantola Albert
Published: February 2008

The complete solution to an inverse problem, including information on accuracy and resolution, is given by the a posteriori probability density in the model space. By running a modified simulated annealing, samples from the model space can be drawn in such a way that their frequencies of occurrence approximate their a posteriori likelihoods. Using this method, maximum likelihood estimation and uncertainty analysis of seismic background velocity models are performed on multioffset seismic data. The misfit between observed and synthetic waveforms within the time windows along computed multioffset travel times, is used as an objective function for the simulated annealing approach. The real medium is modeled as a series of layers separated by curved interfaces. Lateral velocity variations within the layers are determined by interpolation from specified values at a number of sampling points. The input data consists of multioffset seismic data. Additionally, zero‐offset times are used to migrate the reflectors in time to the depth domain. The multioffset times are calculated by an efficient ray‐tracing algorithm which allows inversion of a large number of seismograms. The a posteriori probability density for this problem is highly multidimensional and highly multimodal. Therefore, the information contained in this distribution cannot be adequately represented by standard deviations and covariances. However, by sequentially displaying a large number of images, computed from the a posteriori background velocity samples and the data, it is possible to convey to the spectator a better understanding of what information we really have on the subsurface.
TOPEX orbit determination and gravity recovery using global positioning system data from repeat orbits
Authors: Wu Jiun‐Tsong , Yunck Thomas P.
Published: February 2008

A covariance analysis is presented for satellite tracking and gravity recovery with a differential Global Positioning System‐based technique to be demonstrated on TOPEX in the early 1990s. The technique employs data from an ensemble of repeat ground tracks to recover a unique satellite epoch state for each track and a set of invariant positional parameters common to all tracks. The positional parameters represent the effect of mismodeled gravitational field on the satellite orbit. At an altitude of 1336 km, where gravity modeling is the dominant systematic error, averaging of random error over many arcs and adjustment of the gravity model reduce the final satellite position error. The positional parameters can then be used to produce a refined global gravity model. The analysis indicates that errors ranging from 5 to 8 cm in TOPEX altitude and 0.05 to 0.2 mGal for the gravity field can be achieved, depending on the number of repeat arcs used.
The effects of fluid escape on accretionary wedges 1. Variable porosity and wedge convexity
Authors: Breen Nancy A. , Orange Daniel L.
Published: February 2008

Fluid flow out of accretionary wedges is a consequence of the porosity reduction of wedge material both with depth and with distance from the toe of the wedge. Changes in porosity result in changes in bulk density, because bulk density is a function of solid density and porosity. These variations in bulk density in turn affect the critical taper of the wedge. The critical taper of a wedge with spatially varying strength, pore pressure, and/or bulk density is dependent not only on the value of these parameters, but on their first derivatives as well [Dahlen, 1990]. Using the equations derived by Dahlen we find that the arc ward decrease in porosity results in the commonly observed convex wedge surface. A hypothetical model using a density‐depth function generated from a synthesis of accretionary wedge data [Bray and Karig, 1985] demonstrates that the surface angle can decrease between 0.5 and 1° in the first 60 km from the toe of the wedge. Steeper wedges exhibit a greater degree of convexity (greater changes in slope). The amount of change in slope is relatively insensitive to the internal strength of the wedge. Wide angle seismic data from the Barbados accretionary wedge [Bangs et al., 1990] imaged velocity variations from the surface down to the décollement for a distance of 100 km from the toe. These data can be used to map the spatial variations in bulk density within the Barbados accretionary wedge. Holding strength and pore pressure values constant, and allowing only bulk density to vary, results in a model that closely matches the observed bathymetry both in overall convexity and in shorter wavelength bathymetrie features. The model allows a limited range of acceptable basal pore pressure values for the wedge. The ability of the model to accurately match the majority of the observed bathymetry suggests that bulk density variations are of first order importance for determining not only the wedge shape but also the shorter wavelength features. The model, however, does not produce the most pronounced changes in slope at the ten kilometers nearest the toe. This suggests that other factors, such as variations in pore pressure, sediment strength, or cohesion, are more important influences on the wedge taper nearest the toe.
Dynamics of magma storage in the summit reservoir of Kilauea Volcano, Hawaii
Authors: Johnson Daniel J.
Published: February 2008

An eruption of Kilauea Volcano, Hawaii, normally results in a decrease in the internal pressure of a summit magma reservoir as it partly drains. The interior of the volcano responses to the decrease in pressure (1) by elastic expansion of remaining magma and gas, (2) additional exsolution of gas dissolved in the magma, and (3) by contraction of the volcanic edifice. This process is modeled here using gravity, geodetic, and observational data for recent events at Kilauea. During magma removal, the ratio of pressure change in the magma reservoir to the volume of associated summit subsidence ranges from less than 0.22 Pa/m3 to more than 0.43 Pa/m3. Lower ratios occur during large subsidences because of a large effective volume of the reservoir. Small events may involve source volumes of only 2 km3. In contrast, a volume of near 13 km3 is associated with larger events because the boundary between plastic and elastic behavior may migrate outward with increased time and strain. The aggregate compressibility of Kilauea's reservoir is significantly increased by volumetric changes of the CO2 gas phase by bulk compression and solution with pressure. This effect is more pronounced for relatively shallow sources because the bulk modulus of gas is proportional to pressure. The average reservoir CO2 weight fraction may be near 0.0005 to 0.0009. CO2 appears to accumulate in the upper portion of the reservoir where the concentration reaches 0.0030 weight fraction. Gradual long‐term subsidences are different from the short‐term deflations associated with specific eruptions. During long‐term subsidence, no net change in mass or pressure occurs within the summit reservoir. Rifting of the summit area, which may tend to shorten the reservoir as it widens, or slow degassing and contraction of stored magma may cause long‐term subsidence.
Self‐reversal of thermoremanent magnetization in the ilmenite‐hematite System: Order‐disorder, symmetry, and spin alignment
Authors: Hoffman Kenneth A.
Published: February 2008

A model of the process by which certain members of the ilmenite‐hematite solid solution series acquire reverse thermoremanent magnetization (reverse TRM) is proposed. It is based on the observation that the cation order‐disorder transformation at work in this mineral system produces cation‐ordered regions, or domains, antiphase with respect to cation positions. Given the fundamental assumption that the so‐called “x phase”, the weakly magnetic phase known to control self‐reversal, is in fact Fe‐rich cation‐disordered boundary material between the two types of cation‐ordered domains, symmetry considerations help eliminate the possibility that the direction of magnetic remanence associated with the boundaries lies parallel to the line of Fe spin. Rather, the net magnetization of the boundary material need be essentially at right angles to the orientation of spin, as is the case for pure hematite. Results of thermomagnetic experiments further suggest that superexchange with the Fe‐rich boundary material produces a femmagnetic structure within each cation‐ordered domain such that its net moment lies along the direction of spin, that is, perpendicular to the net boundary moment. At this stage the moments of each pair of cation‐ordered domains (antiphase in composition) cancel. We propose that reverse TRM arises during cooling in an applied field through rotation of particular Fe spins within each cation‐ordered domain, causing the spin arrangement to become progressively noncollinear. Competing superexchange interactions within the cation‐ordered regions may be responsible for such a spin rotation. According to this model, each cation domain provides a component of magnetization opposite to the net magnetization associated with the boundary material. Provided that there exists in a given grain equal fractions of each type of cation domain, the grain will fully self‐reverse. Synthesized polycrystalline samples as well as rocks containing ferrian ilmenites having compositions in the self‐reversing range will acquire reverse TRM.
Morphology and tectonics of the Mid‐Atlantic Ridge (25°–27°30′S) from Sea Beam and magnetic data
Authors: Grindlay N. R. , Fox P. J. , Vogt P. R.
Published: February 2008

High‐resolution Sea Beam bathymetry and sea surface magnetic data have been collected between 25°S and 27°30′S on the Mid‐Atlantic Ridge. Analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three‐dimensional inversion of sea surface magnetic data are presented. This 227‐km‐long portion of the Mid‐Atlantic Ridge is offset 42 km by the right‐lateral Rio Grande Transform at 25°40′S and 9.5 km by the left‐lateral nontransform Moore Discontinuity at 26°30′S. A third nontransform discontinuity, the Midway Discontinuity, exists near 26°12′S and is defined by a small displacement of the axial magnetization distribution and the disruption in the along‐strike continuity of ridge‐parallel terrain elements. Although the detailed survey presented in this paper extends out to only 5‐ to 7‐m.y.‐old crust, a regional compilation of magnetic data from this area by Cande et al. (1988) indicates that the relative positions and dimensions of the spreading cells have characterized this part of the Mid‐Atlantic Ridge for at least the past 55 m.y. The offsets dimensions and geometries, however, have changed markedly because of prolonged differential asymmetric spreading between segments. Between and within the accretionary units that are spreading at uniform rates (35 mm/yr total opening rate), the morphology of the ridge axis varies dramatically, from a well‐defined rift valley (20–25 km wide, 1–1.5 km deep) to an axial swell that rises 1500 m above the adjacent valley floor. The axial magnetic structure is also highly variable along‐strike. The inversion solution shows a series of generally linear, short‐wavelength (<10 km) magnetization highs within the central Brunhes anomaly over each of the four segments in the survey area. These anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world's mid‐ocean spreading centers. The distinct axial magnetization highs can extend out to 2‐m.y.‐old crust at ridge‐discontinuity intersections, suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks, e.g., highly fractionated and strongly magnetized basalts enriched in iron and titanium. The magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area. Both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed. In the case of an axial swell centered at 26°S, attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and/or a zone of substantial alteration. Taken together, the bathymetric data and magnetic modeling document strong along‐axis gradients in crustal and upper mantle properties.
The natural thermoluminescence of meteorites 4. Ordinary chondrites at the Lewis Cliff Ice Field
Authors: Benoit Paul H. , Sears Hazel , Sears Derek W. G.
Published: February 2008

Natural thermoluminescence (TL) measurements have been made on 302 meteorites from the vicinity of the Lewis Cliff in the Beardmore region of Antarctica. The data provide information on terrestrial age and unusual radiation and thermal histories, which, in turn, are helpful in identifying fragments of a single fall and in understanding ice sheet movements and the mechanisms by which meteorite concentration occurs at this site. The present data with data for induced TL, class, find location, hand‐specimen descriptions and mineral composition have enabled 70 of the present samples to be assigned to 27 groups of “paired” meteorites, with between 5 and 2 meteorites in each group. The distribution of meteorites on the ice, the shape of the fields of “paired” meteorites, and trends in the natural TL data indicate that there is a western component to the movement of the ice at this location, as well as the previously supposed movement to the north. This western vector probably explains the concentration of meteorites along the western edge of the ice tongue. Meteorites at the northern end of the tongue (the Lower Ice Tongue), and Meteorite Moraine to the east, have relatively high natural TL, and therefore young terrestrial ages, while those on the upper tongue show a broad range of ages including a great many large ages. Details of the meteorite concentration are different on the Upper and Lower Tongue and it might be that these two parts of the ice sheet are unrelated. These new natural TL data identify several recent falls and several meteorites which probably had unusually small perihelia immediately prior to capture by the earth.
Three‐dimensional seismic imaging of the Costa Rica accretionary prism: Structural diversity in a small volume of the lower slope
Authors: Shipley Thomas H. , McIntosh Kirk D. , Silver Eli A. , Stoffa Paul L.
Published: February 2008

Conventional two‐dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms. We undertook a three‐dimensional seismic reflection survey of a small part of a prism arcward of the Middle America Trench off Costa Rica to more correctly image structure and to use the improved structural information to examine the processes of accretion. This survey reveals small features, with dimensions of hundreds of meters, while also defining features thousands of meters in lateral extent, both of which were underappreciated in conventional two‐dimensional data from the same area. We have imaged active off scraping at the trench and both duplexing and out‐of‐sequence faulting a few kilometers arcward of the trench. Fault spacing and reflector geometry vary dramatically over a space of several hundred meters. Some of these variations are related to visible changes in morphology of the underlying oceanic basement, but others are not so easily documented. Fault surface reflections define an architecture which may control gross fluid motion through the prism. This architecture is apparently formed by duplexing and out‐of‐sequence faulting and has been maintained by periodic motion on some of the out‐of‐sequence faults. The slope sediment apron records multiple phases of deformation. Abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism. A primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism.
A mechanism for the formation of methane hydrate and seafloor bottom‐simulating reflectors by vertical fluid expulsion
Authors: Hyndman Roy D. , Davis Earl E.
Published: February 2008

Bottom‐simulating reflectors (BSR) are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation. They are believed to correspond to the deepest level at which methane hydrate (clathrate) is stable. We present a model in which BSR hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field. In this model, most of the methane is generated below the level of hydrate stability, but not at depths sufficient for significant thermogenic production; the methane is primarily biogenic in origin. The model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid. The model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field, for the source of the large amount of methane contained in the hydrate, and for BSRs being common only in special environments. Strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes, so BSRs are uncommon. Upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation. In these areas hydrate BSRs are common. The most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field. The critical boundary in the phase diagram between the fluid‐plus‐hydrate and fluid‐only fields is not well constrained. However, the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep‐sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production. In most fluid expulsion regimes, the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation. Thus, as observed in borehole data, there are no large chloride or isotope anomalies remaining in the local pore fluids. The differences in the concentration of methane and probably of CO2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions.
Tracking the dispersal of hydrothermal plumes from the Juan de Fuca Ridge using suspended matter compositions
Authors: Feely Richard A. , Massoth Gary J. , Baker Edward T. , Lebon Geoff T. , Geiselman Terri L.
Published: February 2008

In 1988 and 1989, particles from on‐ and off‐axis neutrally buoyant plumes over the Juan de Fuca Ridge were sampled to study their changing composition with distance from the ridge crest. Plume depth and particle composition in off‐axis hydrothermal plumes were used to identify plume sources and trajectories. Iron and phosphorus concentrations and, in particular, P/Fe molar ratios were found to be sensitive indicators of hydrothermal phases in the suspended matter. Phosphorus, vanadium, and arsenic are scavenged from solution where dissolved iron from hydrothermal vents oxidizes in seawater and forms submicrometer‐sized particles of Fe oxyhydroxide. The P/Fe molar ratio (∼0.23) is essentially the same for vent fields along Cleft and Endeavour segments, indicating that the scavenging processes are the same for both regions. High P/Fe ratios are observed over the South and North Cleft Segment vent fields and to the west along the Vance Seamount Chain. The seamounts evidently function as a barrier, bathymetrically steering the hydrothermal plumes to the west away from the ridge crest. Since particles from the Endeavour vent field are greatly enriched in copper relative to hydrothermal plumes from other vent fields along the Juan de Fuca Ridge, copper concentrations in the plumes can be used to distinguish plume sources. Based on water column depth and elemental composition, hydrothermal plume particles from specific vent fields on the Juan de Fuca Ridge may be tracked as much as several tens of kilometers from their source using particle compositions.
Rates and patterns of groundwater flow in the Cascade Range Volcanic Arc, and the effect on subsurface temperatures
Authors: Ingebritsen S. E. , Sherrod D. R. , Mariner R. H.
Published: February 2008

The central Oregon section of the Cascade Range volcanic arc is characterized by relatively high Quaternary volcanic extrusion rates and hot‐spring discharge rates, and by high conductive heat flow. However, a large area of near‐ zero near‐surface conductive heat flow occurs in the younger volcanic rocks, due to downward and lateral flow of cold groundwater. Alternate models for the high heat flow observed in older rocks on the flanks of the Cascade Range involve (1) a laterally extensive midcrustal heat source or (2) a narrower, spottier deep heat source that is confined to the Quaternary arc and is flanked by relatively shallow conductive heat flow anomalies caused by regional groundwater flow. We simulated groundwater flow and heat transport through two cross sections west of the Cascade Range crest: one in the Breitenbush area, where there is no major arc‐parallel normal faulting, and one in the McKenzie River drainage, where major graben‐bounding faults exist. Measured temperature profiles, hot‐spring discharge rates, and geochemical inferences constrain the results. The numerical simulations provide some estimates of regional‐scale permeabilities; simulated bulk permeabilities of ∼10−14 m2 in the youngest (0–2.3 Ma) rocks and ∼10−17 m2 in the oldest (18–25 Ma) rocks allow the thermal observations to be matched. In general, permeability decreases downsection, but for rocks of any age, permeability at very shallow (<50 m) depths is probably much higher than the bulk permeability values required by the thermal observations: this is indicated by high recharge rates in 0–7 Ma rocks (>1 m yr−1) and well‐test data from domestic wells in rocks older than 7 Ma (which indicate permeability values of about 10−14 to 10−12 m2). In the simulations, the alternate conceptual models for the deep thermal structure were represented as wide or localized deep heat sources. We found that either model can satisfy the observations. Thermal observations in the Breitenbush area seem to require significant advective heat transfer, whereas the sparser observations in the McKenzie River area can be satisfied with either advection‐ or conduction‐dominated simulations. Available regional gravity, magnetic, and electrical geophysical data do not clearly favor either of the two alternate models. Deep drilling in areas of high heat flow in the older rocks would be the most definitive test. The actual thermal structure is probably more complex than either of the models considered here.
Effects of surficial modification processes on thermal infrared signatures in the arid southwestern United States
Authors: Weitz Catherine M. , Farr Tom G.
Published: February 2008

Thermal infrared spectra from both laboratory and remote sensing instruments were used to distinguish different age lava flows with varying amounts of surficial modification at Cima and Lunar Crater volcanic fields. The modification processes affecting the lava flows cause spectral differences that can be seen in remotely sensed emittance spectra from the thermal infrared multispectral scanner. Important influences on the thermal infrared spectra include aeolian mantling, vegetation, lava flow roughness, and rock varnish. Laboratory thermal infrared reflectance spectra of the subaerially exposed surfaces of 1–15 cm basalt fragments at Cima volcanic field show a feature attributable to rock varnish on the younger flows (0.14–0.75 m.y.), that is lacking on the older flows (>0.8 m.y.). This suggests that rock varnish is thinner on the older flows at Cima volcanic field. Scanning electron microscope images and thin sections of the different age flows at Cima volcanic field support the conclusion that rock varnish is thinner and may be eroding on the older flow surfaces.
Integrating samplers for the deep sea
Authors: Ledwell James R. , Watson Andrew J. , Doherty Kenneth W. , Berelson William M.
Published: February 2008

Two new types of mechanical integrating samplers have been tried in the deep sea, one driven by a constant force spring, the other by a spring‐driven hydraulic system. The samplers are triggered simultaneously with the Niskin bottle to which they are attached and fill in about 90 min. They are useful for sampling poorly mixed tracers, and have been used successfully in tracer release experiments at 800 m depth in Santa Monica Basin in 1985, and to depths of 1900 m in Santa Cruz Basin in 1988.
The role of the turbulent stress divergence in the equatorial Pacific zonal momentum balance
Authors: Hebert D. , Moum J. N. , Paulson C. A. , Caldwell D. R. , Chereskin T. K. , McPhaden M. J.
Published: February 2008

From a comprehensive set of upper ocean measurements made during a moderate El Niño in boreal spring 1987, we reassess the role of turbulence in transporting momentum vertically at the equator. An examination of the terms in the vertically integrated zonal momentum equations indicates that on short time scales the zonal pressure gradient is not balanced by the surface wind stress despite an apparent balance of these terms on longer (seasonal) time scales. The vertical redistribution of zonal momentum is complex. The strength of the wind determines both the magnitude and, likely, the mechanisms of momentum transport between the surface and the core of the undercurrent. During low wind conditions in April 1987 the turbulent stress divergence was significantly different in magnitude and vertical structure from that found during strong winds in November 1984. In November 1984 the turbulent stress divergence was much too large above 40 m to balance the residual term in the zonal momentum budget of Bryden and Brady (1985, 1989) and decayed exponentially with depth from the wind stress value at the surface. In April 1987 the turbulent stress divergence was smaller than that required by Bryden and Brady and decayed linearly from the surface wind stress. For a proper comparison with Bryden and Brady's zonal momentum balance, it is necessary to determine the annual average turbulent stress divergence.
Correlation of nearshore ice movement with seabed ice gouges near Barrow, Alaska
Authors: Shapiro Lewis H. , Barnes Peter W.
Published: February 2008

Nearshore patterns of ice motion near Barrow, Alaska, were monitored between 1973 and 1979 with a sea ice radar system. Side scan sonar surveys of the seafloor were made at the same location during the summers of 1977 and 1978. The two data sets provide information on ice motion, ice gouging, and the rate and character of seabed deformation by ice. Four ice stages (open water, freeze‐up, winter, and breakup) can be defined on the basis of the frequency and patterns of ice motion and ice cover processes as recorded by the radar system. In all four ice stages the dominant ice motion was nearly parallel to the coast. Side scan sonar surveys show that the principal ice gouge directions nearshore were at a high angle to the coast and to the predominant direction of ice drift in both years. Gouge density was greatest in a narrow zone centered about the 10‐m isobath which reflects the distribution of deep ice keels and the pattern of reworking of the seafloor by waves and currents. We believe the majority of observed gouges were formed by keels of multiyear ice floes during an event that occurred in July 1975, 2 years before the first seabed survey. Persistence of the gouge pattern from 1975 until 1977 is attributed to persistence of the ice cover and limited reworking by waves and currents, or additional gouging. The observed decrease in gouge density between 1977 and 1978 is attributed to (1) seafloor reworking and gouge infilling by storms and (2) the absence of ice conditions that were conducive to creating fresh gouges.
Topographic effects on the seasonal circulation of the Indian Ocean
Authors: Kindle John C.
Published: February 2008

The relative effects of islands and topography on the seasonal circulation of the southern Indian Ocean are examined. The numerical simulations use a one‐active layer reduced gravity model and a two‐layer model with full bottom topography forced by the Hellerman and Rosenstein wind stress climatology. The reduced gravity experiments show that the absence of a seasonal cycle of transport off the north coast of Madagascar is not caused by the presence of islands to the east. The two‐layer experiments indicate that the presence of abrupt topography associated with the Mascarene Ridge and the Providence Bank north of Madagascar decrease the annual barotropic signal of the North Madagascar Current and increase the seasonal variation of the Southeast Madagascar Current. Furthermore, the two‐layer experiments predict the existence of a significant seasonal variation of transport north of the Saya de Malha Bank along the Mascarene Ridge; the signal (12–16 Sv in amplitude) is sufficiently large that the model results may be tested by future observations. Finally, the differences between the seasonal variation of the Indian and North Atlantic oceans are discussed.
Structure and dynamics of the Sea of Okhotsk marginal ice zone from “ocean” satellite radar sensing data
Authors: Mitnik Leonid M. , Kalmykov Anatoly I.
Published: February 2008

Radar images of the Sea of Okhotsk ice cover are analyzed. The images were obtained by side looking radars (SLR) installed on the USSR “Ocean” series of satellites since September 1983. SLR operates at wavelength 3.15 cm at vertical polarization. A swath width is 460 km at a spatial resolution of 1–3 km. Satellite optical images, air reconnaissance, and meteorological information are used to interpret sea ice radar contrasts. The radar images permit monitoring of the formation and transformation of the ice cover and its displacement under the influence of winds and currents. Radar contrast is enhanced in the marginal ice zone on account of the great difference in the scattering properties of pancake ice and grease ice. In the marginal ice zone, different regular motions are registered for ice edge waves, ice bands and jets, eddies, and mushroomlike structures with dimensions ranging between 10–20 and 100–120 km.
The transport of anthropogenic carbon dioxide into the Weddell Sea
Authors: Anderson Leif G. , Holby Ola , Lindegren Roger , Ohlson Mats
Published: February 2008

Total carbonate data from the southern Weddell Sea, collected during the Swedish Antarctic Expedition in 1988–1989, has been investigated with respect to its content of anthropogenic carbon dioxide. The measured data was corrected for decay of organic matter by the use of the apparent oxygen utilization and the ratio of carbon to oxygen in organic matter. Correction due to precipitation or dissolution of metal carbonate was not found to be necessary from alkalinity and calcium data. With a combination of the water mass mixing conditions and the atmosphere‐ocean carbon dioxide exchange, the anthropogenic input of carbon dioxide has been estimated. The anthropogenic carbon dioxide mean concentration varies in the different water masses as a result of the mixing conditions, from about 36 μmol/kg in the Ice Shelf Water to 8 μmol/kg in the Antarctic Bottom Water. The data were combined with oceanographic information in order to elucidate the ventilation of carbon dioxide in the Weddell Sea. With the outflow of Ice Shelf Water from the Filchner Depression equal to 0.7 × 106 m3/s (Foldvik et al., 1985), the annual transport of anthropogenic carbon amounts to 8 × 1012 g for this special area. There are indications of other areas with significant deep water formation, but it is unlikely for these to add more than 2 times that of the Filchner Depression. This results in a total sequestering in the Weddell Sea of less than 1% of the annual anthropogenic emission of about 5.5 Gt C.
Probable errors in width distributions of sea ice leads measured along a transect
Authors: Key J. , Peckham S.
Published: February 2008

The degree of error expected in the measurement of widths of sea ice leads along a single transect are examined in a probabilistic sense under assumed orientation and width distributions, where both isotropic and anisotropic lead orientations are examined. Methods are developed for estimating the distribution of “actual” widths (measured perpendicular to the local lead orientation) knowing the “apparent” width distribution (measured along the transect), and vice versa. The distribution of errors, denned as the difference between the actual and apparent lead width, can be estimated from the two width distributions, and all moments of this distribution can be determined. The problem is illustrated with Landsat imagery and the procedure is applied to a submarine sonar transect. Results are determined for a range of geometries, and indicate the importance of orientation information if data sampled along a transect are to be used for the description of lead geometries. While the application here is to sea ice leads, the methodology can be applied to measurements of any linear feature.
Saline outflow from the Arctic Ocean: Its contribution to the deep waters of the Greenland, Norwegian, and Iceland seas
Authors: Aagaard K. , Fahrbach E. , Meincke J. , Swift J. H.
Published: February 2008

Since 1985 various investigators have proposed that Norwegian Sea deep water (NSDW) is formed by mixing of warm and saline deep water from the Arctic Ocean with the much colder and fresher deep water formed by convection in the Greenland Sea (GSDW). We here report on new observations which suggest significant modification and expansion of this conceptual model. We find that saline outflows from the Arctic Ocean result in several distinct intermediate and deep salinity maxima within the Greenland Sea; the southward transport of the two most saline modes is probably near 2 Sv. Mixing of GSDW and the main outflow core found over the Greenland slope, derived from about 1700 m in the Arctic Ocean, cannot by itself account for the properties of NSDW. Instead, the formation of NSDW must at least in part involve a source which in the Arctic Ocean is found below 2000 m. The mixing of various saline outflows is diapycnal. While significant NSDW production appears to occur in northern Fram Strait, large amounts of saline Arctic Ocean outflow also traverse the western Greenland Sea without mixing and enter the Iceland Sea. During the past decade, deep convection in the Greenland Sea has been greatly reduced, while deep outflow from the Arctic Ocean appears to have continued, resulting in a markedly warmer, slightly more saline, and less dense deep regime in the Greenland Sea.
Frequency dependence of electromagnetic bias in radar altimeter sea surface range measurements
Authors: Walsh E. J. , Jackson F. C. , Hines D. E. , Piazza C. , Hevizi L. G. , McLaughlin D. J. , McIntosh R. E. , Swift R. N. , Scott J. F. , Yungel J. K. , Frederick E. B.
Published: February 2008

Range measurements made by satellite radar altimeters experience a bias toward the troughs of ocean waves. A series of aircraft flights during February–April 1989 measured this electromagnetic (EM) bias at three radar frequencies and the UV under a variety of wind and wave conditions, and provided the first airborne open‐ocean measurements at the 13.6‐GHz and 5.3‐GHz operating frequencies of the NASA altimeter on the TOPEX/Poseidon satellite. The data suggest that the mean EM bias decreases linearly with increasing radar frequency between 5.3 and 36 GHz, according to the expression: EM bias (% of significant wave height) = (3.0–0.0617 F)(1±0.5), where F is in gigahertz. EM bias is fairly constant over a mesoscale region on a given day but can fluctuate significantly from one day to another. It shows a strong increase at all radar frequencies with increasing wind speed, although other sea state conditions, such as the wind direction relative to the wave direction, are also factors.
Thermohaline catastrophe in a simple four‐box model of the ocean climate
Authors: Joyce Terrence M.
Published: February 2008

A four‐box model of the ocean is examined in which a surface and deep layer of equal thickness and large volume are connected to two polar reservoirs of smaller volume. While the four reservoirs are thoroughly mixed, there is no significant mixing between them; exchange is due to flow between reservoirs with the flow out of or into the polar reservoirs driven by pressure gradients. Newtonian boundary conditions are imposed on the temperature and salinity of the surface layer and the two polar reservoirs; the deep water is forced only by flows into it from other boxes. The temperature and salinity forcing is applied with a different time scale for each variable. When the time scales are nearly equal, the system responds as a simple component fluid with a single solution for each forcing. When the salinity time scale is long compared with temperature, up to four multiple equilibria can be present for a given forcing. These equilibria are unstable to finite amplitude perturbations, and numerical solutions exhibit catastrophic transitions from one mode to another. When the deep water formation is totally inhibited as a result of a transition, the system can go into a long‐term climatic oscillation, quasi‐periodic in nature. The system possesses four “attractors” corresponding to deepwater formation in either, both, or neither of the two polar reservoirs.
Effect of a lower layer current on wind‐driven upwelling
Authors: Wenju Cai
Published: February 2008

The conventional two‐layered, alongshore independent, wind‐driven upwelling model is extended to incorporate an initial subsurface current, through the advective term involving cross‐stream shear. Under the condition that the thickness of the upper layer and the equilibrium displacement of the interface are both much smaller than the thickness of the second layer, explicit solutions are obtainable for cases of exponential and parabolic profiles of the initial interface. In the presence of the initial current in the lower layer, the upwelling is reduced near the coast, and increased offshore compared to a case without the initial current. The modification is achieved through an abatement and an enhancement in divergence near the coast and offshore, respectively, as a result of the net effect from the continuity condition and the advection of the offshore velocity in terms of the cross‐stream shear. The mechanism provides one of the possible explanations for an exceptionally wide cold water zone observed in an upwelling system influenced by an ambient large‐scale ocean current.
Scaling of marine microlayer film surface pressure‐area isotherms using chemical attributes
Authors: Frew Nelson M. , Nelson Robert K.
Published: February 2008

Heterogeneity in the chemical composition of sea surface films due to variation in sources of surfactant materials and in physical dynamics during film formation may lead to spatial and temporal variability in interfacial physical properties. Alternatively, slick physical properties may be postulated to be largely averaged and therefore similar across a variety of oceanic regimes. In order to assess interslick variability in regimes of different productivity, surface pressure‐area (π‐A) isotherms have been measured for a suite of microlayers sampled at stations in productive coastal and mesotrophic waters of the southern California Bight. Surface‐active organics isolated from the microlayers by solid phase extraction on C18 Sep‐Pak cartridges were respread on clean seawater, and the π‐A isotherms of the reconstituted films were determined. Several bulk chemical parameters including dry weight, UV absorbance, and carbon, nitrogen, and lipid content have been examined in an attempt to scale the isotherms; the latter were shown not to collapse to a single well‐constrained envelope when compared on a specific area basis. However, for several parameters there was a clear separation of the slicked microlayer isotherms into two distinct film types composed primarily of inshore and offshore samples, respectively. Area scaling and virial equation modeling also clustered the isotherms into two groups with different elastic properties suggesting the presence of distinct “end‐member” components. Surfactants from an unslicked microlayer appeared to produce films that exhibited much lower elastic moduli than those from slicked microlayers. The observed interslick variability in π‐A characteristics was larger than the uncertainties inherent in the methodology, and it was concluded that the variability was related to significant differences in chemical composition of films. The inferred compositional differences must be taken into account in order to correctly define interfacial boundary conditions in modeling of wind‐wave interactions.
Seasonal variations in the western tropical Atlantic: Surface circulation from Geosat altimetry and WOCE model results
Authors: Didden Norbert , Schott Friedrich
Published: February 2008

Sea level variations and geostrophic circulation in the western tropical Atlantic are studied in an intercomparison of Geosat altimetry and the World Ocean Circulation Experiment community model effort high‐resolution model forced with climatological windstress. Overall, the annual cycles of geostrophic current fields of both products compare very well. Special comparison areas are the western North Equatorial Countercurrent (NECC) and the North Brazil Current (NBC) region. Meridional profiles of zonal velocity anomalies show a seasonal meridional migration of the NECC core centered at 5°N and a weaker eastward maximum during fall at 9°N in both products. The Geosat and model seasonal cycles of the NECC core velocity in the region 35°–45°W are highly correlated and agree with respect to the onset of eastward current acceleration and deceleration in May and December, respectively. Geosat time series from November 1986 to June 1989 show year to year differences, in particular an anomalous early NECC acceleration phase in 1987. In the NBC region 54°–58°W, flow anomalies from both Geosat and the model have two westward maxima, in March and June, which appear to be associated with eastward anomalies further offshore.
Shear instability of longshore currents: A numerical study
Authors: Putrevu Uday , Svendsen Ib A.
Published: February 2008

Temporal variations of longshore currents have recently been identified on natural beaches (Oltman‐Shay et al., 1989). Termed shear waves, these variations appear to be strongly linked to the strength of the mean longshore current. Bowen and Holnan (1989) proposed a simple model based on the shear instability of a steady bilinear longshore current profile over a constant depth to explain these observations. In this work we generalize Bowen and Holman's model. A numerical solution technique is employed to study the characteristics of shear waves generated by instabilities of smooth longshore current profiles on general bottom topographies. We find that the stability characteristics are extremely sensitive to variations in the bottom topography. The “dispersion” relationship of the shear waves is almost linear and does not seem to be very sensitive to the topography. We find that the presence of a nearshore bar greatly enhances the strength of the instability and, in certain situations, tends to trap the shear wave shoreward of the bar crest. The results also suggest that the shear on the seaward face of the longshore current profile is an important parameter in the stability problem. A Fjortoft condition is established for the instability equation. Estimates of length and time scales indicate that shear waves should be detectable in laboratory experiments. An order of magnitude analysis indicates that shear waves may be a plausible mixing mechanism in the nearshore region.
Observed growth of Langmuir circulation
Authors: Smith Jerome A.
Published: February 2008

Surface velocity patterns and upper ocean density profiles are presented from a period following a sudden increase in wind. A prior wind speed of 8 m/s failed to produce visible signs of Langmuir circulation. After the wind speed increased to 13 m/s, Langmuir circulation developed within 15 min. The initial scale observed was about 16 m, streak to streak, and may have been restricted by the depth scale of the measurements. This spacing is about two thirds of the dominant wavelength (4‐s‐period waves). The streak spacing (two cell widths) increased at roughly 40 m/h for the next hour. The density measurements indicate a depth of mixing which increased over the same period at a rate of 20 m/h; thus the vertical to horizontal aspect ratio was about 1:1. Prior stratification was weak (buoyancy frequency of about 1.5 cph), and probably did not affect the circulation initially.