bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐velocity tails on the velocity distribution of solar wind ions
Authors: Ogilvie K. W. , Geiss J. , Gloeckler G. , Berdichevsky D. , Wilken B.
Published: February 2008

Recent observations of the solar wind using the SWICS instrument on the Ulysses spacecraft have shown the presence of high‐velocity “tails” on the velocity distribution of protons. Similar features have also been observed on the velocity distributions of helium and oxygen ions. Of the order of 1% of the solar wind density is involved in these tails, which are approximately exponential in shape and persist to V = VB + 10Vth or beyond, where VB is the bulk velocity and Vth the thermal velocity of the solar wind. This paper contains a preliminary description of the phenomenon. It is clear that it is ultimately connected with the passage of interplanetary shocks past the spacecraft and that particle acceleration at oblique shocks is involved.
Gravity and potential changes due to shear and tensile faults in a half‐space
Authors: Okubo Shuhei
Published: February 2008

Analytic expressions derived from a simplified model are invaluable because they often provide deep insight into geophysical phenomena. Such is the case with potential and gravity changes. Here we present expressions of potential and gravity changes caused by faulting on a finite rectangular plane buried in a homogeneous half‐space. The expressions can be used to evaluate coseismic changes in surface gravity and geoid height. Observed gravimetric data combined with our formulae thus provide constraints on the fault geometry and magnitudes of dislocation.
Paleomagnetic measurement of nonbrittle coseismic deformation across the San Andreas Fault at Pallett Creek
Authors: Salyards Stephen L. , Sieh Kerry E. , Kirschvink Joseph L.
Published: February 2008

Paleomagnetic data have been obtained to address a problem at the Pallett Creek paleoseismological site: the 9 mm/yr slip rate determined from three‐dimensional mapping of late Holocene offsets across discrete faults is only a quarter of the expected value. We suspected that nonbrittle deformation adjacent to the faults might account for the 26 mm/yr discrepancy. In our search for the missing slip we collected and analyzed 264 paleomagnetic samples from a 53‐m‐wide transect across the fault zone. Half the samples came from a unit deposited immediately after a large earthquake of about A.D. 1480; these samples were affected by two large earthquakes that involved rupture at the site in 1812 and 1857. We collected the other half of the samples from a slightly older bed, one that was deposited before the earthquake of about A.D. 1480. Relative to “control” groups composed of 10 samples and collected 50 m from the fault, samples closer to the fault display clockwise rotations of 30° or less. If interpreted as block rotations, the data from the older unit imply that it has sustained a total of 14.0 ± 2.9 m of dextral warp during the past three major earthquakes and that the younger unit has experienced a total of 8.5 ± 1.0 m of warp during the most recent two. Combining these values with the amounts of dextral slip across the mapped fault planes yields dextral offsets of 5.5, 6.25, and 6.25 m for the events of A.D. 1480, 1812, and 1857 and a slip rate of 35.6 ± 6.7 mm/yr. This slip rate, averaged over the past three complete seismic cycles, is consistent with published rates from other sites. Offsets associated with the past three events are remarkably similar. These amounts, however, appear independent of the length of interseismic cycles. These observations suggest (1) that this part of the San Andreas fault has a characteristic strength and (2) that conventional concepts of strain accumulation and relief (for example, time‐ and slip‐predictable models of earthquake occurrence) are unrealistic.
Seismic reflection constraints on the structure of the crust beneath the San Bernardino Mountains, Transverse Ranges, southern California
Authors: Li Yong‐Gang , Henyey Thomas L. , Leary Peter C.
Published: February 2008

A 30 km‐long N‐S seismic reflection line was shot by California Consortium for Crust Studies (CALCRUST) across the southern Mojave Desert and onto the northern flank of the San Bernardino Mountains in southern California. On the northern end of the seismic section, the reflectivity increases markedly in the midcrust at a depth corresponding to a two‐way travel time of 4 to 5 s (12–15 km), suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust. The high reflectivity disappears at about 8 s (24 km) and may be correlated with a change in seismic velocity in the lower crust from 6.3 km/s to 6.8 km/s. A band of reflectivity between 9.5 and 10 s (27–30 km) is believed to represent the Moho. The midcrustal relectivity transition and Moho both deflect downward toward the San Bernardino Mountains uplift over the entire length of the profile. The deflection of the midcrustal transition (12°) appears greater than that of the Moho (6°), resulting in a thinning of the lower crust to the south beneath the uplift. In addition, the midcrustal transition coincides with the base of the seismogenic zone (brittle‐ductile transition?) which is also dipping southward beneath the San Bernardino Mountains, while the Moho deflection is consistent with elastic flexure resulting from edge loading by the San Bernardino Mountains which have been thrust over the Mojave block. It is suggested that the thinning of the lower crust beneath the San Bernardino Mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust. Since a portion of the load is transmitted through the lower crust to the Moho, the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift (≥2 m.y.).
Nonlinear viscoelasticity and the formation of transverse ridges
Authors: Bercovici David , Dick Henry J. B. , Wagner Thomas P.
Published: February 2008

Transverse ridges axe regions of anomalously high uplift that run parallel to many of the transform faults at mid‐ocean ridges. Previous models of the formation of these ridges generally fail to explain the magnitude of uplift (between 1 and 8 km) or account for geophysical observations (which indicate that the ridges are dynamically uplifted by neutral or heavy, but not buoyant, material). However, the potentially large Maxwell relaxation time of the lithosphere and the high strain rates and finite deformation of transform faults imply that nonlinear viscoelasticity may play an important role in the formation of these ridges. We present simple theoretical and experimental models of viscoelastic flow beneath a transform fault to show that the purely horizontal motion of the fault can generate vertical uplift typical of transverse ridges. The theoretical model is of an infinite half‐space of fluid driven from above by two plates moving parallel and in opposite directions to one another. The constitutive relation for the viscoelastic rheology is for a second‐order fluid; i.e., nonlinear viscoelastic effects are treated as perturbations to Newtonian, purely viscous flow. The theoretical model not only predicts the two transverse ridges that are often observed on either side of the transform fault, but the central transform valley that separates them as well. For typical lithospheric flexural rigidity and deformation associated with transverse ridges, the viscoelastic effect provides sufficient vertical stress to produce the observed uplift. Further, the dynamic uplift is a purely mechanical effect (i.e., it does not involve buoyant material) which corresponds to geophysical observations. The experimental model is comprised of a viscoelastic fluid (2% aqueous solution of high molecular weight carboxymethylcellulose) driven by a rotating plate. This is analogous to a transform fault because the boundary of a spinning circular tectonic plate (surrounded by stationary plates) is one continuous transform fault. The experiment produces a large ridge of fluid along the edge of the plate with a slight trough outside of the ridge; this result is qualitatively described by a cylindrical version of of the above theoretical model. In conclusion, both experimental and theoretical models show that when nonlinear viscoelasticity is accounted for, uplift characteristic of transverse ridges can be generated from purely horizontal motion typical of transform faults.
Vema Fracture Zone (central Atlantic): Tectonic and magmatic evolution of the median ridge and the eastern ridge‐Transform intersection domain
Authors: Lagabrielle Yves , Mamaloukas‐Frangoulis Vassilios , Cannat Mathilde , Auzende Jean‐Marie , Honnorez Jose , Mevel Catherine , Bonatti Enrico
Published: February 2008

The eastern Vema fracture zone intersection with the Mid‐Atlantic Ridge (MAR) axis was surveyed with the French submersible Nautile during the Vemanaute cruise in 1989. At this ridge‐transform intersection (RTI), an elongated, E‐W ridge, more than 50 km long, is present in the transform valley. This median ridge rises up to 1000 m above the surrounding seafloor. The crest of the median ridge is lower and presents an arcuate shape at the tip of the MAR axis. Dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation. This detrital cover consists of polymictic sedimentary breccias, sandstones and siltstones, composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle. Most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation. Disaggregation, mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls. This event could be related to the uplift of the southern wall of the fracture zone (the “transverse ridge”) which took place probably between 10 and 3 Ma ago. Since this uplift episode, the transverse ridge (which is now undergoing subsidence) and the detritus covered transform valley floor, separated by the transform fault zone, have migrated westward and eastward respectively. Vertical tectonics of the median ridge at the approach of the RTI can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors. A possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western RTI, is more than 1 km out of isostatic equilibrium. Recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the MAR axis have been recorded on the southern flank of the median ridge. Several stages in the very recent tectonic‐volcanic history of the eastern RTI, that is, roughly during the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shape of the median ridge east of 40°57′W are inferred to have resulted from tectonic extension during the creation of the nodal basin.
Very long period magnetotellurics at Tucson Observatory: Implications for mantle conductivity
Authors: Egbert Gary D. , Booker John R.
Published: February 2008

In a companion paper (Egbert et al., this issue) we describe the estimation of very long period (0.16 < T < 91 days) magnetotelluric (MT) impedances from 11 years of data collected at the Tucson geomagnetic observatory. Here we discuss the implications of these data for mantle conductivity. Using minimum norm (flattest and smoothest) inversions, we find simple one‐dimensional models of electrical conductivity in the depth range 0–1500 km. We use forward modeling, a linearized resolution analysis, and constrained one‐dimensional inverions to delineate the range of models which are consistent with the estimated impedances. Although the MT data have limited resolution, large‐scale vertical averages of mantle conductivity are well constrained. We reach the following conclusions concerning mantle conductivity beneath Tucson: (1) The upper 200 km has a conductance of order 104 Siemens (S). This anomalously high conductance may be concentrated in an aesthenospheric high conductivity layer, but the geometry of the conductive zone is not constrained. (2) Typical conductivities in the transition zone (400–700 km) are ≈ 0.1–0.3 S m−1. A step increase to reach this value at or near the 400 km olivine‐spinel phase transition is consistent with, but not required by, the data. An upper mantle which is resistive throughout (0.05 S m−1 or less) is not allowed by the data. (3) Resolvable large‐scale averages of conductivity increase from ≈ 0.2 S m−1 to ≈ 1.0 S m−1 between 600 and 900 km depth. A range of models, including those with step increases, and step decreases, at the 670 km seismic discontinuity are consistent with the data. (4) Between 900–1500 km, conductivity increases slowly. Average conductivities in this region are of the order of 1 S m−1, to within a factor of 2 or 3. While limited zones of highly resistive mantle are consistent with the data, a lower mantle which is resistive throughout is not. Models in which conductivity is always above 5 S m−1 below 1000 km can also be ruled out. In conjunction with improved laboratory estimates of electrical conductivities of mantle minerals at high temperatures and pressures, these constraints can provide important clues to the composition and physical state of the mantle.
Dynamically supported trench topography
Authors: Wdowinski Shimon
Published: February 2008

A viscous flow model of a subduction zone is used to calculate the near‐trench deformation and topography of the overriding plate in response to tectonic and buoyancy forces. The tectonic force, which is associated with global plate motion, arises from subduction of a cold oceanic lithosphere that shears the overriding lithosphere along the contact between the two plates. The buoyancy force arises in response to horizontal density variations and tends to relax the existing topography. The time evolution of the near‐trench topography is investigated via a finite element technique that solves for the flow field in the overriding plate. The results indicate that the near‐trench topography approaches a steady state configuration, in which the upper surface topography produces a buoyancy force that balances the tectonic force induced by the subducting slab. The model predicts that the steady state trench depth increases with the subduction rate, and hence explains the correlation between trench depth and subduction rate observed by Grellet and Dubois (1982). Transition from one steady state trench topography to another adds or removes material from the overriding plate; this may result from a change in subduction rate, angle of subduction, or subduction of a seamount. During a transition to a deeper steady state configuration, the near‐trench stress field has an extensional component, which agrees with the observed extension in regions of tectonic erosion. Similarly, during a transition to a shallower steady state configuration, the near‐trench stress field has a compressional component, which agrees with the observed compression of accretionary wedges. Hence, the model explains tectonic erosion and accretion at trenches as transitional features that indicate a change from one steady state configuration to another.
What information on the Earth model do reflection travel times provide?
Authors: Delprat‐Jannaud Florence , Lailly Patrick
Published: February 2008

Inversion of reflection travel times (or reflection tomography or tomographic inversion) allows determination of the structure of the subsurface from arrival times of reflected waves. Because the solution of a tomographic inversion is often underdetermined, it is essential to carry out an uncertainty analysis on the solution, which is classically done by linearizing the problem at the vicinity of the computed solution. Earlier uncertainty analyses gave results which strongly depend on the chosen discretization interval: they have no physical meaning. Our goal is to compute numerical uncertainties that approximate the physical uncertainties. Limiting our study to Hilbertian model spaces, we theoretically derive a necessary and sufficient condition which yields the desired result: the norm chosen in model space has to bind the Fréchet derivative of the forward map. Physically, this requires the velocity distribution and the reflectors to be smooth functions, an assumption which is quite compatible with the smoothness imposed to make use of ray theory. The required degree of smoothness depends on the space dimension (two‐ or three‐dimensional). This theoretical study yields a new method which allows, in the classical framework of a linearization, calculation of uncertainties that are almost independent of the chosen model discretization interval: they are thus intrinsically related to travel time information. Hence we can predict how travel times determine the slowness distribution and the reflector geometries. Our method is validated for a simple example. As a result, a very fine parameterization of the model is needed for an accurate estimate of the uncertainties on the solution.
Inferred depth of creep on the Hayward Fault, central California
Authors: Savage J. C. , Lisowski M.
Published: February 2008

A relation between creep rate at the surface trace of a fault, the depth to the bottom of the creeping zone, and the rate of stress accumulation on the fault is derived from Weertman's 1964 friction model of slip on a fault. A 5 ± 1 km depth for the creeping zone on the Hayward fault is estimated from the measured creep rate (5 mm/yr) at the fault trace and the rate of stress increase on the upper segment of the fault trace inferred from geodetic measurements across the San Francisco Bay area. Although fault creep partially accommodates the secular slip rate on the Hayward fault, a slip deficit is accumulating equivalent to a magnitude 6.6 earthquake on each 40 km segment of the fault each century. Thus, the current behavior of the fault is consistent with its seismic history, which includes two moderate earthquakes in the mid‐1800's.
Hydrothermal methane venting between 12°N and 26°N along the Mid‐Atlantic Ridge
Authors: Charlou Jean‐Luc , Donval Jean‐Pierre
Published: February 2008

Hydrographic surveys along the Mid‐Atlantic Ridge (MAR) between 12°N and 26°N, carried out from 1984 to 1990, show a variable pattern of CH4‐rich water column plumes. The vertical distribution of CH4 at stations located every 20–40 km is presented along this 1200‐km‐long section of the MAR. CH4 venting is clearly demonstrated. CH4‐enriched fluids rise from vents as plumes; spreading is confined to the axial valley due to the topography of the MAR. CH4 contents from 45 nmol to 675 nmol/kg are measured in the buoyant plumes above the two active hydrothermal sites (MARK 23°N; TAG 26°N) known at present, whereas CH4 anomalies up to 3.6 nmol/kg are typically observed in plumes emitted either on the inner floor, on the walls, and/or at the top of the rift mountains along the studied ridge section. CH4 concentrations (45 μmol to 144 μmol/kg) in MARK and TAG vent fluids are of the same order of magnitude as those found in the East Pacific Rise fluids. Even though CH4 is known to be unstable with respect to oxidation by dissolved oxygen, and in spite of its microbial oxidation in plumes, these results confirm CH4 to be a good indicator to track hydrothermal plumes and to map the variation of hydrothermal activity along mid oceanic ridges. Moreover, between 12° and 26°N along the MAR, CH4 results show that while hydrothermal activity is present everywhere along the ridge, it is predominant near fracture zones (FZ) (Kane FZ, 15°20′N FZ). Comparison of CH4 tracer with total dissolvable manganese (TDM) tracer in plumes allows us to differentiate subseafloor hydrothermal processes. The high TDM/CH4 found above TAG and MARK areas is indicative of basalt‐seawater interaction, while at 15°N the low TDM/CH4 provides evidence of fluid circulation in ultrabasic rocks. CH4 data confirm the association between mantle degassing, hydrothermal activity, and serpentinization along this 12°–26°N section of the Mid‐Atlantic Ridge.
P wave tomographic imaging of the crust and upper mantle beneath the Japan Islands
Authors: Zhao Dapeng , Hasegawa Akira
Published: February 2008

More than 50,000 high‐accuracy arrival times from 1200 shallow and deep earthquakes selected from the Japan University Network Earthquake Catalog are used to investigate the three‐dimensional (3‐D) seismic velocity structure beneath Japan. Travel times and ray paths are accurately calculated by using an efficient 3‐D ray‐tracing technique. P wave tomographic images of the crust and upper mantle to a depth of 500 km beneath the Japan Islands are determined with spatial resolutions of 25–50 km. The obtained P wave velocity in the crust and uppermost mantle is slightly higher and relatively homogeneous in western Japan, in contrast with an average lower velocity and a large heterogeneity in eastern Japan where many active volcanoes exist. This feature correlates well with the distributions of heat flow and gravity anomalies. Low‐velocity (low‐V) zones are imaged in the crust and uppermost mantle beneath active volcanoes. The low‐V zones in the mantle wedge dip toward the west and extend continuously to a depth of about 200 km. A high‐velocity (high‐V) zone corresponding to the subducted Pacific plate is delineated. The thickness of the high‐V zone is estimated to be 80–90 km. In southwestern Japan, the descending Philippine Sea plate is also delineated; its thickness is estimated to be 30 km.
The role of volatiles in the transition zone
Authors: Gasparik Tibor
Published: February 2008

Phase relations in the system MgO‐SiO2with low volatile contents of H2O, F2, and Cl2were experimentally investigated at 100–234 kbar pressures and 850° –2050°C temperatures using a split‐sphere anvil apparatus (USSA‐2000). Superphase B, Mg10Si3O14[OH,F]4, was observed at 160–234 kbar, fluorous phase B, Mg12Si4O19F2, at 178 kbar, and hydrous phase F, Mg4Si7O16[OH]4, also at 178 kbar. At lower pressures, the volatiles were present in phase A, Mg7Si2O8[OH,F]6, phase C, Mg(7−x)Si(2+x)O(9+x)[OH,F]4and phase E, Mg(2−x)Si(1+x)O(4+x−y)[OH,F]2y. With the exception of the fluorous phase B, all these phases can coexist with stishovite at pressures above 130 kbar. In a purely hydrous system, the dehydration temperatures of the assemblage superphase B + stishovite increase from 1200°C at 160 kbar to 1400°C at 234 kbar. Dehydration produces beta phase, Mg2SiO4, possibly containing up to 7 wt % of structural H2O. About 3 wt % of H2O in beta phase expands the stability of the assemblage beta phase + stishovite at the expense of ilmenite by 30 kbar. In more complex compositions, the volatile solidus is below 1350°C, thus at the temperatures which are much lower than the expected mantle temperatures at the relevant depths. Consequently, most of the mantle is in a partially molten state. The volatile‐bearing phases can only be present in the colder subducted slabs but not in the rest of the transition zone (400–670 km). Hydrous beta phase and stishovite are the most likely constituents of the slabs. Incongruent melting of the hydrous beta phase coexisting with stishovite, producing garnet and melt, could be the cause of deep‐focus earthquakes. Evidence for a mineralogically and chemically stratified upper mantle is reviewed and its structure is discussed in the context of the present results.
Paleomagnetism of alkaline complexes and remagnetization in the Kapuskasing Structural Zone, Ontario, Canada
Authors: Costanzo‐Alvarez Vincenzo , Dunlop David J. , Pesonen Lauri J.
Published: February 2008

The Shenango and Nemegosenda alkaline complexes of the southern Kapuskasing Structural Zone (KSZ), an uplifted crustal block in northern Ontario, preserve primary thermoremanent magnetizations, with mean directions D=112°, I=−46° (k=13, α95=13°, N=9 sites) and D=304°, I=+51° ( k=31, α95=10°, N=7 sites) for Shenango and D=124°, I=−57° ( k=13, α9520°, N=4 sites) and D=312°, I=+54° (k=17, α95=13°, N=7 sites) for Nemegosenda. Their average paleomagnetic pole is 45°N, 189°E (dp=6°, dm=8°), falling on the Keweenawan track of the North American apparent polar wander path (APWP) between 1098–1107 Ma reverse‐polarity (R) and 1087–1098 Ma normal‐polarity (N) poles of Lake Superior lavas and dikes. Unlike the Keweenawan paleopoles, which have a reversal asymmetry of ≈30°, the Shenango and Nemegosenda R and N poles are almost 180° reversed, which was found also by Lewchuk and Symons (1990b). This symmetry of R and N poles casts doubt on the previously accepted predominance of asymmetric reversals of the Earth's field in Keweenawan time. The Archean Shawmere anorthosite is overprinted by a B magnetization with mean direction D=97°, I=−68° ( k=12, α95= 10°, N=18 sites). The Shawmere B paleopole at 38°N, 226°E (dp=13°, dm=l6°) falls between the carbonatite poles and ≈1850 Ma cratonic poles, and the B magnetization seems to be a composite of middle and late Proterozoic overprints. Sites in the western part of the Shawmere body close to the “carbonatite line” of faults linking the various alkaline complexes are severely altered, with an abundance of secondary magnetite and hematite. Virtual geomagnetic poles for these sites fall close to the Keweenawan track and have an average pole of 189°, 53°N (dp=15°, dm=20°) suggesting that the B NRM at these sites is an ≈1100 Ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion. More easterly sites are less altered. Their VGPs resemble ≈1850 Ma cratonic poles and average to 235°E, 30°N (dp=15°, dm=17°) These B natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle Proterozoic hydrothermal activity associated with intrusion of early alkaline bodies, fault movement on the Ivanhoe Lake fault zone, or late‐stage uplift of the KSZ.
Structure and tectonic setting of the 77°E and 75°E Grabens, Kerguelen Plateau, South Indian Ocean
Authors: Munschy Marc , Rotstein Yair , Schlich Roland , Coffin Millard F.
Published: February 2008

The Central Kerguelen Plateau (South Indian Ocean) is characterized by abundant north‐south striking normal faults, which comprise two prominent north‐south rifts known as the 77°E and 75°E grabens. The 77°E Graben is a well‐defined structure which extends over some 800 km from the eastern margin of the Kerguelen Plateau to about 58.5°S. Over most of its length it is associated with a 10–30 km wide axial rift and with a 100–150 km wide uplift. The 75 °E Graben is less well documented, but the available data suggest that its dimensions and internal structure resemble that of the 77°E Graben. In the better documented 77°E Graben, six rift segments, 50–100 km long, are identified. Faulting is more developed at the northern and southern ends of the 77°E Graben, possibly resulting from the interaction with other rifts. To the north, the 77°E Graben abuts the highly faulted eastern margin of the Kerguelen Plateau and the northern part of an even larger rift zone, the Plate Boundary Rift Zone, which extends along the boundary with the Australian‐Antarctic Basin. To the south, the 77°E Graben adjoins the northwestern end of the Southern Kerguelen Plateau Rift Zone. The 77°E and 75°E grabens, and the other rift zones on the Kerguelen Plateau, appear to have been formed at approximately the same time, between 72 and 60 Ma. They are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the Plate Boundary Rift Zone into the Southeast Indian Ridge, between 46 and 43 Ma. The north‐south trend of the 77°E and 75°E grabens is different from that of the other rift zones, which are oriented northwest‐southeast. This geometry suggests that some strike‐slip motion may have occurred along the north‐south trending grabens as a result of extension on the northwest‐southeast trending rifts, particularly the Southern Kerguelen Plateau Rift Zone. However, since near‐surface extension estimates for the Southern Kerguelen Plateau Rift Zone are small, the strike‐slip motion along the 77°E Graben must be equivalently small. Also, the available seismic data from this graben do not show typical seismic characteristics of a strike‐slip environment, such as zones of compression or flower structures. Thus the results of this work are inconsistent with any model for the development of the South Indian Ocean which requires significant amount of transform motion on the 77°E or 75°E grabens. Finally, the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting. We find great similarities between the two processes, in the segmentation of the rifts, the asymmetric cross section, and the associated shoulder uplifts.
A new method for the validation of altimeter‐derived sea state parameters with results from wind and wave models
Authors: Guillaume Anne , Mognard Nelly M.
Published: February 2008

A new approach for the validation of altimeter‐derived sea state parameters using data from wind and wave models is proposed. Wind speeds and significant wave heights (SWH) acquired with the Geosat altimeter over the North Atlantic are compared with the wind analysis from the European Centre for Medium‐Range Weather Forecasts (ECMWF) atmospheric model and with wave hindcast from the VAG model for the week of October 12–19, 1987. The statistical results with this 1‐week data set agree with those of a 7‐month comparison study between Geosat and the 43 National Data Buoy Center (NDBC) buoys. An advantage with the models is the large number of comparison points which allows an investigation of high sea state conditions and statistical analyses by binning in various ways. The underestimation of SWH by the Geosat altimeter, compared with the VAG hindcast, is confirmed and underestimation is shown to be proportional to the SWH. In order to use model hindcasts to validate altimeter‐derived wind parameters, a method to improve the model wind data set is proposed. This method relies on the SWH comparisons between altimeter and model that are simultaneously available with the wind data. Tests on the 1‐week data set show the validity of this new method. Applied to the three algorithms, Brown, smooth Brown, and Chelton‐Wentz, these tests show that the statistical differences between the various algorithms are a function of wind speed range and indicate that the wind speeds derived with the Brown algorithm (which already agree best with buoy data) also agree best with the ECMWF 10‐m wind fields.
Comparison of velocity estimates from advanced very high resolution radiometer in the Coastal Transition Zone
Authors: Kelly Kathryn A. , Strub P. Ted
Published: February 2008

Two methods of estimating surface velocity vectors from advanced very high resolution radiometer (AVHRR) data were applied to the same set of images and the results were compared with in situ and altimeter measurements. The first method used an automated feature‐tracking algorithm and the second method used an inversion of the heat equation. The 11 images were from 3 days in July 1988 during the Coastal Transition Zone field program and the in situ data included acoustic Doppler current profiler (ADCP) vectors and velocities from near‐surface drifters. The two methods were comparable in their degree of agreement with the in situ data, yielding velocity magnitudes that were 30–50% less than drifter and ADCP velocities measured at 15–20 m depth, with rms directional differences of about 60°. These differences compared favorably with a baseline difference estimate between ADCP vectors interpolated to drifter locations within a well‐sampled region. High correlations between the AVHRR estimates and the coincident Geosat geostrophic velocity profiles suggested that the AVHRR methods adequately resolved the important flow features. The flow field was determined to consist primarily of a meandering southward flowing current, interacting with several eddies, including a strong anticyclonic eddy to the north of the jet. Incorporation of sparse altimeter data into the AVHRR estimates gave a modest improvement in comparisons with in situ data.
A free surface general circulation model for the tropical Pacific Ocean
Authors: Zhang R.‐H. , Endoh M.
Published: February 2008

A free surface tropical ocean model is presented for explicit prediction of the actual topography of sea surface and other variables, for further intercomparative studies among differently formulated oceanic general circulation models (OGCMs), and data assimilation of the observed sea level. Based on an OGCM without the rigid‐lid approximation developed at the Institute of Atmospheric Physics (Beijing, China), the dynamic formulation and numerics of the model have some unique features such as explicit treatment of sea level elevation as a prognostic variable, calculation of the departures of thermodynamic variables in time integration by subtracting the standard stratification, introduction of the σ vertical coordinate, total available energy consideration, and a barotropic‐baroclinic mode‐splitting algorithm for computational efficiency. The model covers the tropical Pacific Ocean between 30°N and 30°S with horizontal grid spacing of 1° in latitude and 2° in longitude. There are 14 vertical layers with 8 layers in the top 200 m. Prior to further applications of the model, simulations of the annual mean and seasonal cycle of the tropical Pacific Ocean have been carried out to examine performance of this free surface OGCM forced by climatological atmospheric fluxes. It is shown that with the appropriate treatments of the governing equations and numerical methods, the free surface baroclinic OGCM can be computed at least as efficiently as a rigid‐lid model. By eliminating the rigid‐lid approximation, keeping explicitly the available surface energy and energy conversion related with divergence of vertically integrated flow, and by subtracting the standard stratification, the model would be more accurate, reasonable, and effective in representing the energy cycle in the real ocean. The misrepresentation of energy conversion caused by the rigid‐lid approximation is shown and estimated on the basis of the simulated data. The simulated results show that the model is able to reproduce not only the observed current system and thermal structure and their variations but also the actual sea level in the tropical Pacific, which shares many observed sea surface features. The preliminary comparison with other existing simulations shows some noticeable differences in the simulated circulation in spite of using the same wind stress, which need to be investigated further.
Diurnal tides near the Yermak Plateau
Authors: Padman Laurie , Plueddemann Albert J. , Muench Robin D. , Pinkel Robert
Published: February 2008

Recent current measurements obtained from drifting platforms over the Yermak Plateau in the eastern Arctic Ocean confirm that the plateau is a region of greatly enhanced diurnal tidal currents. Modulation of the diurnal currents is clearly related to the plateau topography, as has been previously proposed. We show, however, that temporal variability due to spring‐neap modulation must also be considered in interpreting records from drifting platforms. We review simple models of tidal current amplification in this region and find that the previous assumption of near‐resonant, barotropic shelf waves propagating around the plateau's entire perimeter is inconsistent with the true topography. Instead, we propose that the diurnal variability is due to topographic shelf waves at the K1 and O1 tidal frequencies that are generated at points on the plateau's perimeter where the waves' group velocities are near zero. Observed cross‐slope variations in ellipticity, orientation, and magnitude of tidal oscillations are consistent with the presence of topographic waves generated in this manner. The topographic enhancement of the diurnal tide near the Yermak Plateau has important consequences for the sea ice cover, hydrography, and general circulation of this region. For example, the stress divergence applied by the tidal currents at the ice base greatly exceeds the typical divergence of the surface wind stress, and tides may therefore be important to local ice deformation. The strong cross‐slope tidal currents also appear to be responsible for the production of high‐frequency internal wave packets, which are associated with energetic diapycnal mixing in the pycnocline. We also consider the possibility that tidal rectification is responsible for a mean current transporting Atlantic Water clockwise around the plateau.
Observing the advection of sea ice in the Weddell Sea using buoy and satellite passive microwave data
Authors: Massom Robert A.
Published: February 2008

This study examines the progress and behavior of an area of sea ice as it drifts from the southwestern Weddell Sea by using data from four buoys tracked by Nimbus 6 and concurrent ice concentrations retrieved from Nimbus 7 scanning multichannel microwave radiometer data. It covers the entire growth season of 1980. The overall drift characteristics, and their relationship to ice edge displacement, are examined within the framework of four zones. Three phases are identified in the large‐scale behavior of the Weddell sea ice cover, namely, a rapid equatorward and eastward advance, a quasi‐equilibrium phase, and a period of rapid recession. A broad division is made between the sectors to the east and west of 35°–40°W in terms of the nature of ice edge advance. Transient departures from the mean northward advection are linked to the passage of cyclones; certain such events may be significant in terms of the formation and maintenance of the region of highly consolidated perennial ice observed in the western Weddell Sea. Mean daily drift rates (up to 0.9 m s−1) from within the central‐western Weddell pack are high in 1980, particularly early in the growth season and when compared with rates to the west of 50°W. Outbreaks of cold continental air alternate with incursions of relatively warm air from the north; warm conditions are recorded as far as 1200 km in from the ice edge in winter. Closed loops in the buoy trajectories, which are clockwise to the south of 63°S, reverse to become anticlockwise to the north. Mid‐ocean islands impede the eastward progress of the ice. A coherence is noted in the response of the buoys to the passage of storms, even though the buoys separated by a distance of over 100 km.
Modeling aerosol particle size distributions over the North Sea
Authors: Van Eijk Alexander M. J. , De Leeuw Gerrit
Published: February 2008

An empirical model has been developed to predict aerosol particle size distributions from a set of relatively simple meteorological parameters. The model is based on measurements taken at Meetpost Noordwijk (MPN) (North Sea) during the Humidity Exchange Over the Sea Main Experiment in 1986. The air masses at MPN contain a mixture of continental and marine aerosol. The relative contribution of each type of air mass strongly depends on the wind direction. Partitioning the aerosol data set into eight wind sectors based on geographical considerations of the sources of natural, anthropogenic, and industrial aerosols is supported by analysis based on the wind speed dependence of the particle concentrations. For each wind sector the aerosol concentrations in the 0.2–15‐μm diameter range were analyzed in terms of relative humidity, wind speed, air‐sea temperature difference, and water temperature, that is, those parameters that determine the production, dispersal, and deposition of the aerosol. The model predicts the aerosol concentrations to within a factor of 1.7 (68% confidence limit). Model performance is reduced during frontal passages and in periods of rain and high relative humidity. This is illustrated with two case studies.
Recirculation and seasonal change of the Kuroshio from altimetry observations
Authors: Qiu Bo
Published: February 2008

Surface height fluctuations across an eastward flowing boundary current, such as the Gulf Stream and the Kuroshio, can be caused by changes both in the upstream inflow and in the neighboring recirculation gyres. Clarifying these causes is important in understanding the time‐dependent nature of the boundary currents and their relationship to the surface wind and buoyancy forcing. Using satellite altimetry data to identify these causes, however, requires mean surface height field information, which is not readily available from the altimetry observation owing to lack of accurate geoid data. The present study presents a method to estimate the mean surface height profile across a boundary current system (including recirculation gyres) by combining altimetrically measured residual height data and historical hydrographic data. Applying this method to the Kuroshio revealed that the absolute surface height profiles thus estimated agree well with the results from quarterly, in‐situ hydrographic observations. By separating the signals of the surface height fluctuations into those of the eastward flowing jet and the recirculation gyres, we found that the seasonal cycle in the surface transport of the Kuroshio, which has a maximum in July and August, is primarily due to the seasonal change in the intensity of the recirculation gyre south of the Kuroshio.
Velocity observations above a rippled bed using laser Doppler velocimetry
Authors: Agrawal Y. C. , Aubrey D. G.
Published: February 2008

Velocity observations are reported in a plane across a regular nonmigrating ripple in 2.5 m of water depth. The forcing was weak, primarily wave‐induced near‐bed orbital motion. The data were acquired using a two‐axis submerged laser Doppler velocimeter. Synchronous records of pressure below the surface were also obtained. The instrumentation is described. The pressure information was used to separate wave‐coherent and turbulence motion in the observed velocity. Turbulent kinetic energy and dissipation rates are obtained over a range of distances above the bed from 1.5 mm above the ripple to 10 cm. Velocity time series are smooth except at high‐velocity magnitudes, indicating the formation of a lee vortex during peak velocities only. The data show convective acceleration of the flow over the ripple and a lack of significant shear in the rms horizontal motion farther from the bed. Velocity spectra at one location, 1.5 mm from the bed show second and third harmonics of the incident wave, which are interpreted to be due to convective acceleration over the ripple and vortex shedding. High‐frequency velocity spectra show uniformly decreasing turbulence intensity away from the bed. We conclude that the boundary layer observed here was transitional, i.e., laminar through small velocity periods, becoming turbulent for short periods during which the breakup of the vortex shed by the ripples occurred. Laminar estimates of maximum bottom friction are consistent with the threshold for initiation of motion criterion for 0.2 mm sand and the observed ripple steepness.
Analysis of acoustic measurements of suspended sediments
Authors: Thorne Peter D. , Hardcastle Peter J. , Soulsby Richard L.
Published: February 2008

Estuarine measurements of suspended sediments over sand waves have been taken with the objective of clarifying the role of turbulence in suspension transport, sand waves development, and migration. To discern the mechanisms of transport, observations of the turbulent flow were taken and records of the response of the suspended sediment concentration were obtained using acoustic backscattering (ABS). As an intermediate goal of the overall objective an analysis of the ABS data has been conducted with the aim of assessing and quantifying the acoustic technique. Recent theoretical descriptions of the interaction of sound with suspensions are presented, and predictions are compared with a laboratory study on suspensions of spheres and noncohesive sediments. The laboratory data show the theoretical approach to be broadly correct, and it has been implemented to interpret the estuarine observations. Acoustic estimates of the suspended sediment concentration have been compared with pumped sample data and shown to be similar. Comparison of the laboratory and estuarine estimates for the form function and total scattering cross section for the estuarine sediments is presented, and some unresolved differences are considered. The effects of the sediment attenuation and the random amplitude of the backscattered signal are also highlighted, and their implications regarding acoustic estimates of the suspended sediment concentration are addressed. Finally, after assessing the accuracy of the acoustic approach, high‐resolution estimates of the suspended concentration are presented with some speculation on the origins of the concentration variability observed.
Trade wind fetch‐related variations in equatorial undercurrent depth, speed, and transport
Authors: McPhaden M. J.
Published: February 2008

A dynamical model is developed to examine the dependence of Equatorial Undercurrent depth, speed, and volume transport on trade wind fetch. The model ocean is linear, continuously stratified, vertically diffusive, and formulated on the equatorial beta plane. Analytical solutions are derived in closed form. For realistic choices of model parameters, results indicate that the undercurrent would be significantly shallower and weaker in the Atlantic than in the Pacific because of the shorter Atlantic trade wind fetch.
Variability in the western equatorial Pacific Ocean during Japanese Pacific climate study cruises in 1989 and 1990
Authors: Kuroda Yoshifumi , McPhaden Michael J.
Published: February 2008

Data are examined from two cruises of the R/V Natsushima in the region 5°N to 5°S, 160°E to 160°W during January‐February 1989 and January‐February 1990. These cruises were conducted in the climatically sensitive region of the central equatorial Pacific cold tongue and the western equatorial Pacific warm pool during extremes of the Southern Oscillation index (SOI). Associated with high SOI values in January‐February 1989, anomalously strong trade winds prevailed along the equator between 160°E and 160°W, the cold tongue penetrated to west of the date line, surface dynamic height (relative to 1000 dbar) sloped up to the west along the equator by 17 dyn cm between 160°W and 160°E, and the depth of the equatorial thermocline increased from 100 m at 160°W to 150 m at 160°E. Flow in the surface layer was to the west at speeds up to 60–80 cm s−1 in the South Equatorial Current, and volume transport in the upper 100 m (300 m) was 52 Sv (98 Sv) to the west across the date line between 5°N and 5°S. Flow and hydrographic conditions were markedly different 1 year later when the SOI was at a negative extreme. Winds became westerly west of the dateline at speeds up to 10 m s−1, the zonal slope of the upper thermocline and of surface dynamic height reversed, the surface layer warmed by 2°–3°C between 160°W and 160°E, and the cold tongue disappeared from the western Pacific. Zonal currents reversed and flowed eastward at speeds of 20–60 cm s−1 in the upper 75 m along the equator between 155°E and 175°E. Conversely, east of 180° easterly surface winds and westward surface flow prevailed, so that near the date line both surface zonal winds and currents were convergent. Associated with this convergence, surface dynamic height rose by 12 dyn cm, and a relatively sharp zonal gradient in water mass properties developed in the surface layer. Net westward volume transport across the dateline between 5°N and 5°S in the upper 100 m (300 m) was reduced to 29 Sv (47 Sv) in early 1990, or about 50% of the transports observed the previous year. Diagnosis of the depth integrated zonal momentum balance for both cruises indicated that the changes in upper ocean transport could not be interpreted as a linear equilibrium response to local wind forcing.