bluuebunny/crossref_metadata_2025 huggingface dataset:
Is lithostatic loading important for the slip behavior and evolution of normal faults in the Earth's crust?
Authors: Kattenhorn Simon A. , Pollard David D.
Published: September 2002

Normal faults growing in the Earth's crust are subject to the effects of an increasing frictional resistance to slip caused by the increasing lithostatic load with depth. We use three‐dimensional (3‐D) boundary element method numerical models to evaluate these effects on planar normal faults with variable elliptical tip line shapes in an elastic solid. As a result of increasing friction with depth, normal fault slip maxima for a single slip event are skewed away from the fault center toward the upper fault tip. There is a correspondingly greater propagation tendency at the upper tip. However, the tall faults that would result from such a propagation tendency are generally not observed in nature. We show how mechanical interaction between laterally stepping fault segments significantly competes with the lithostatic loading effect in the evolution of a normal fault system, promoting lateral propagation and possibly segment linkage. Resultant composite faults are wider than they are tall, resembling both 3‐D seismic data interpretations and previously documented characteristics of normal fault systems. However, this effect may be greatly complemented by the influence of a heterogeneous stratigraphy, which can control fault nucleation depth and inhibit fault propagation across the mechanical layering. Our models demonstrate that although lithostatic loading may be an important control on fault evolution in relatively homogeneous rocks, the contribution of lithologic influences and mechanical interaction between closely spaced, laterally stepping faults may predominate in determining the slip behavior and propagation tendency of normal faults in the Earth's crust.
The crustal thickness of Australia
Authors: Clitheroe G. , Gudmundsson O. , Kennett B. L. N.
Published: September 2002

We investigate the crustal structure of the Australian continent using the temporary broadband stations of the Skippy and Kimba projects and permanent broadband stations. We isolate near‐receiver information, in the form of crustal P‐to‐S conversions, using the receiver function technique. Stacked receiver functions are inverted for S velocity structure using a Genetic Algorithm approach to Receiver Function Inversion (GARFI). From the resulting velocity models we are able to determine the Moho depth and to classify the width of the crust‐mantle transition for 65 broadband stations. Using these results and 51 independent estimates of crustal thickness from refraction and reflection profiles, we present a new, improved, map of Moho depth for the Australian continent. The thinnest crust (25 km) occurs in the Archean Yilgarn Craton in Western Australia; the thickest crust (61 km) occurs in Proterozoic central Australia. The average crustal thickness is 38.8 km (standard deviation 6.2 km). Interpolation error estimates are made using kriging and fall into the range 2.5–7.0 km. We find generally good agreement between the depth to the seismologically defined Moho and xenolith‐derived estimates of crustal thickness beneath northeastern Australia. However, beneath the Lachlan Fold Belt the estimates are not in agreement, and it is possible that the two techniques are mapping differing parts of a broad Moho transition zone. The Archean cratons of Western Australia appear to have remained largely stable since cratonization, reflected in only slight variation of Moho depth. The largely Proterozoic center of Australia shows relatively thicker crust overall as well as major Moho offsets. We see evidence of the margin of the contact between the Precambrian craton and the Tasman Orogen, referred to as the Tasman Line.
Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central
Authors: Riisager Janna , Perrin Mireille , Riisager Peter , Ruffet Gilles
Published: September 2002

Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1°N, 171.2°E (A95 = 8.6°, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9° +6.5°/−3.5°, is close to expected, suggesting that the paleomagnetic pole represents a time‐averaged field. Moreover, five new 40Ar/39Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine‐grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second “pTRM tail” check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single‐domain grains.
Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle
Authors: Yoshikawa Masako , Nakamura Eizo
Published: September 2002

Major and trace element compositions and isotopic ratios of Sr and Nd were determined for bulk rocks and their constituent clinopyroxenes from the Horoman peridotite complex, Japan. Al2O3, CaO, and heavy rare earth elements (HREE) contents of peridotites generally decrease from plagioclase lherzolite through spinel lherzolite to spinel harzburgite, indicating simple melt extraction from a single source. However, the extremely large variations in isotopic (87Sr/86Sr = 0.7019 to 0.7066, εNd = +110 to −10) and trace element compositions ([Ce/Yb]N = 0.006 to 4.0) cannot be explained by a simple melt extraction mechanism. The samples can be divided into two groups: one suite has depleted isotopic and light REE (LREE) characteristics (DP), while the other suite shows enriched isotopic and LREE signatures (EP). Sm‐Nd isotope systematics of whole‐rock DP samples yield an isochron age of 833±78 Ma with an initial 143Nd/144Nd ratio of 0.5119±2, which is identical to the isotopic composition of mid‐ocean ridge basalt (MORB) source mantle at that time. The relationship between MgO and Yb abundances of whole rocks shows that melt extraction was initiated at pressures near the garnet and spinel lherzolite transition. Peridotites that formed at different depths presently occur in close proximity to each other, sometimes within tens of meters. The chemical and isotopic signatures of the EP samples can be explained by mixing between mantle residue and an isotopically and more incompatible element enriched fluid derived from a subducted slab. These observations suggest that the small‐scale compositional layering observed in the complex may have formed in a wedge mantle by water‐enhanced thinning and folding of metasomatized peridotites which had previously developed large‐scale simple stratification as a result of melt extraction beneath a mid‐ocean ridge.
A convection model to explain anisotropy of the inner core
Authors: Wenk H.‐R. , Baumgardner J. R. , Lebensohn R. A. , Tomé C. N.
Published: September 2002

Seismic evidence suggests that the solid inner core of the Earth may be anisotropic. Several models have been proposed to explain this anisotropy as the result of preferred orientation of crystals. They range from a large annealed single crystal, growth at the melt interface, to deformation‐induced texture. In this study texture development by deformation during inner core convection is explored for ε‐iron (hcp) and γ‐iron (fcc). Convection patterns for harmonic degree two were investigated in detail. In the model it is assumed that traces of potassium are uniformly dispersed in the inner core and act as a heat source. Both for fee and hep iron, crystal rotations associated with intracrystalline slip during deformation can plausibly explain a 1–3% anisotropy inPwaves with faster velocities along the N‐S axis and slower ones in the equatorial plane. The effect of single crystal elastic constants is explored.
Transition from oblique subduction to collision in the northern Luzon arc‐Taiwan region: Constraints from bathymetry and seismic observations
Authors: Kao Honn , Huang Gwo‐Ching , Liu Char‐Shine
Published: September 2002

We systematically study the detailed bathymetry, seismicity, and source parameters of large‐ and moderate‐sized earthquakes that occurred in the northern Luzon arc‐Taiwan region between 1964 and 1996. The purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features, the characteristics of seismogenic structures, and the corresponding state of strain. To the south of 21.5°N, the existence of a subduction zone is clearly shown by both bathymetry and seismicity. The subducted Eurasia slab reaches a maximum depth of ∼200 km, and the depth decreases sharply as it approaches Taiwan. Focal mechanisms indicate that the subducted slab is in downdip extension for depths <150 km but switches to downdip compression at greater depths. To the north of 23°N, collision is clearly the predominant process with broad deformation on both sides of the suture. In between, the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism, closure of the forearc basin, and the right‐lateral NE‐SW striking Taitung canyon fault zone (TCFZ). These structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward. The present‐day plate boundary follows the axis of the Manila trench in the south, broadens significantly in the zone of transition, and finally becomes a zone of several tens of kilometers in width across eastern Taiwan. The deformation front, on the other hand, is located between the Western Foothills and Coastal plain in Taiwan, then connects to the Manila trench to the south. Seismic strain tensors indicate that the region to the north of TCFZ has a compression rate ∼15 times larger than that to the south. The northern Luzon arc shows characteristics of oblique subduction, but the effects of collision are not as significant within the subducted Eurasian lithosphere, as comparing to the subducted Philippine Sea plate beneath the southernmost Ryukyu arc.
A thermodynamic assessment of the potential synthesis of condensed hydrocarbons during cooling and dilution of volcanic gases
Authors: Zolotov Mikhail Y. , Shock Everett L.
Published: September 2002

The possibility for abiotic synthesis of condensed hydrocarbons in cooling/diluting terrestrial volcanic gases has been evaluated on the basis of the consideration of metastable chemical equilibria involving gaseous CO, CO2, H2 and H2O. The stabilities of n‐alkanes and polycyclic aromatic hydrocarbons (PAHs) have been evaluated for several typical volcanic gas compositions under various conditions for cooling/diluting of quenched volcanic gas. The modeling shows that n‐alkanes and PAHs have a thermodynamic potential to form metastably from H2 and CO below ∼250°C within the stability field of graphite. Despite the predominance of CO2 in volcanic gases, synthesis of hydrocarbons from CO2 and H2 is less favored energetically than from CO and H2. Both low temperature and a high H/C atomic ratio in volcanic gas generally favor stability of hydrocarbons with higher H/C ratios. PAHs are thermodynamically stable at temperatures ∼10°–50°C higher than large n‐alkanes; however, at lower temperatures, PAHs and n‐alkanes have similar stabilities and are likely to form metastable mixtures. Both the energetic drive to form hydrocarbons and possible temperatures of formation increase as the oxidation state (ƒO2) of the volcanic gases decreases and as the cooling/dilution ratios of volcanic gases increase. Synthesis of hydrocarbons is energetically more likely in cooling trapped gases than in ashcloud eruptive columns. Mechanisms for hydrocarbon formation may include Fischer‐Tropsch‐type synthesis catalyzed by magnetite from solid volcanic products. On the early Earth, Mars, and Jupiter's satellite Europa, several factors would have provided more favorable conditions for hydrocarbon synthesis in volcanic gases than under current terrestrial conditions and might have contributed to the production of organic compounds required for the emergence of life.
Buckling of the lithosphere in western Himalaya: Constraints from gravity and topography data
Authors: Caporali Alessandro
Published: September 2002

The deformation of the lithosphere in western Himalaya and Karakoram is analyzed by correlating Bouguer gravity anomalies and topographic heights sampled along profiles in the direction of the Indo‐Asian collision. The spectral features of the coherence, linear admittance, and correlation coefficients between topography and deformation at depth support the hypothesis that the flexural mode of lithospheric deformation is complemented by a folding mode, with typical wavelengths concentrated in bands centered at 250 and 120 km. When combined with realistic models of a rheologically stratified lithosphere with two strong layers (upper crust and upper mantle) sandwiching a weak, very ductile layer (the lower crust), the data are consistent with a model of response to horizontal deformation in which a coupled, biharmonic folding instability develops: The lithosphere as a whole warps with the longer wavelengths and the upper layer alone also with the shorter wavelengths. Major faults, such as the Main Boundary Thrust, or the Main Karakoram Thrust and the computed folds of the upper layer are phased, as if there were a causal relationship between the position of the faults and the zones of highest strain in the crust.
The Bouvet triple junction, 20 to 10 Ma, and extensive transtensional deformation adjacent to the Bouvet and Conrad transforms
Authors: Mitchell Neil C. , Livermore Roy A. , Fabretti Paola , Carrara Gabriela
Published: September 2002

The Bouvet triple junction has been proposed to have evolved as a ridge‐fault‐fault (RFF) type between 20 and 10 Ma, connecting the southernmost Mid‐Atlantic Ridge (SMAR) with the Bouvet and Conrad transforms, to the east and west, respectively. We surveyed immediately north of these two transforms with side‐scan and multibeam sonars, on seafloor that would have originally been created at the SMAR close to its junction with the two transforms. The sonar data reveal that SMAR fabrics on the Bouvet and Conrad sides, when corrected for plate rotation, are parallel to each other, so they were most likely formed at the same spreading ridge and confirm that the triple junction was indeed RFF. Our second major result is that the SMAR fabrics are extensively crosscut by normal faults, over most of the 400 km surveyed along both transforms and most intensely north of the Bouvet transform. Growth faults and faults affecting the sediment surface in multichannel seismic images show that the deformation has been long‐lived and is probably ongoing. Since the orientations of the crosscutting faults are similar to those of shear zone tension fractures, we interpret these areas to be transtensional zones. This extent of deformation adjacent to major oceanic transforms is rare, and we develop a number of ideas to explain its origin.
Seafloor spreading on the Amsterdam‐St. Paul hotspot plateau
Authors: Conder James A. , Scheirer Daniel S. , Forsyth Donald W.
Published: September 2002

The Amsterdam‐St. Paul (ASP) platform on the intermediate rate Southeast Indian Ridge (SEIR) is the only oceanic hotspot plateau outside the Atlantic Ocean containing an active, mid‐ocean ridge spreading axis. Because the ASP hotspot is small and remotely located, it has been relatively unstudied, and the ridge axis location in many places near the ASP plateau was previously unknown or ambiguous. We mapped the SEIR out to 1 Ma crust (Jaramillo anomaly) both on and near the ASP platform. We located the spreading center to within a few kilometers, based on side‐scan sonar reflectivity. Recent off‐platform magnetic anomalies and lineated abyssal hill topography are consistent with a simple spreading history. Off‐platform full spreading rates increase from ∼63 km/Myr on segment H to the north of the platform to ∼65.5 km/Myr on segment K to the south. In contrast, inversions of seafloor magnetization based on uniform and variable thickness magnetic source layers reflect a complex on‐platform tectonic history with ridge jumps, off‐axis volcanism, and propagating rifts. On one section of the ASP plateau the spreading location has stabilized and is beginning to rift the plateau apart, generating symmetric magnetic anomalies and lineated topography for the last several hundred thousand years. The larger, more stable, spreading segments of the ASP platform are aligned with major volcanic edifices, suggesting that along‐axis magma flow away from plume‐fed centers is an important influence on spreading geometry. Many complex tectonic features observed on the ASP plateau, such as ridge jumps, en echelon, oblique spreading centers, and transforms oblique to the spreading direction, are comparable to features observed on Iceland. The similarities suggest that moderate crustal thickening at an intermediate rate spreading center may have similar effects to pronounced thickening at a slow rate spreading center.
Effect of varying enstatite content on the deformation behavior of fine‐grained synthetic peridotite under wet conditions
Authors: McDonnell R. D. , Peach C. J. , van Roermund H. L. M. , Spiers C. J.
Published: September 2002

The effect of varying enstatite content on the deformation behavior of synthetic, fine‐grained (1 to 2 μm) forsterite‐enstatite rock with ∼0.5 wt% added water was investigated at temperatures of 900° to 1000°C, strain rates between 10−7 and 10−5 s−1, and a confining pressure of ∼600 MPa. The samples exhibited approached steady state flow at stresses ≤60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sharp decrease in flow strength in the range 0–2.5% vol% enstatite, with little further change up to 20 vol%. The observed power law n value of ∼1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt% enstatite and are consistent with a water‐enhanced grain boundary sliding (GBS) dominated deformation mechanism. Significantly, a negative correlation was found between grain size and enstatite content, indicating that enstatite content played a role in controlling the grain size of the starting materials. Moreover, a high correlation between measured flow strength and grain size was found, consistent with a grain size exponent of −3 in a conventional grain size sensitive flow equation. A water‐enhanced deformation mechanism involving GBS accommodated probably by grain boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control. Other effects of second‐phase content, such as weakening caused by interphase boundary diffusion and/or migration processes, seem to be unimportant under the conditions investigated.
Conditions for plumes to penetrate the mantle phase boundaries
Authors: Marquart Gabriele , Schmeling Harro , Ito Garrett , Schott Bertram
Published: September 2002

At a depth of ∼660 km in the Earth's mantle the spinel‐perovskite phase boundary is a prominent barrier for mantle convection. This is due to the negative Clapeyron slope of the phase equilibrium curve which leads to an elevation of the phase boundary within hot upwellings causing negative buoyancy forces. We have investigated the conditions for rising plumes either to penetrate and pass the spinel‐perovskite phase boundary or to stick and spread below it by studying the fundamental physics of this process. The plume heads were simply modeled as hot three‐dimensional (3‐D) spheres or 2‐D cylinders. A simple calculation balancing the positive thermal and the negative phase bouyancy forces leads to a better parameterization using two dimensionless quantities. In addition to the phase buoyancy parameter, we defined a deflection parameter, relating the elevation of the phase boundary to the plume head radius to account for the geometrical shape of a plume head. This parameterization is further tested with numerical models that include the effects of thermal diffusion, latent heat, the olivine‐spinel phase boundary at a depth of 410 km, and temperature and/or phase‐dependent viscosity structure. For laboratory estimates of the slope (−3 MPa/K) and density increase at the spinel‐perovskite phase boundary (250 kg/m3) our models predict that plumes with excess temperatures of 50°–600°C will stick at the top of the lower mantle if their head radii are less than ∼100 km. Plumes will penetrate into the upper mantle if plume head radii exceed 100 km. While the style of plume penetration or spreading at the top of the lower mantle strongly depends on the viscosity structure, the conditions for penetration do not. All rising hot volumes with nonpenetrating conditions stick at the top of the lower mantle and spread laterally, independent of their viscosity structure. For weakly nonpenetrating conditions, heat diffusion increases the radius of the hot volume and leads to penetration during a secondary stage. For strongly nonpenetrating conditions, spreading at the top of the lower mantle drives a mechanically coupled counterflow in the upper mantle, which is stable for a very long time. For volumes with an excess temperature of more than 350°C heat diffusion across the phase boundary will eventually inhibit this counter‐flow and stabilize a thermally coupled flow which might entrain some material of the hot volume. However, our results suggest that the spinel‐perovskite phase boundary is unlikely to inhibit the penetration of mantle plumes of the size thought to have generated many flood basalt provinces and hotspot chains.
Chemistry of hydrothermal vent fluids from 9°–10°N, East Pacific Rise: “Time zero,” the immediate posteruptive period
Authors: Von Damm K. L.
Published: September 2002

In 1991, vent fluids were sampled from 10 high temperature hydrothermal vent sites within weeks of a volcanic eruption/dike intrusion on the East Pacific Rise, between 9° and 10° north latitude, and from eight high temperature vents south of the eruptive area. As a result of the eruption north of 9°45′N, the vent fluids from this area have unique compositional characteristics compared to the present global database of hydrothermal vent fluid compositions, including unusually low concentrations of Li, Na, K, Ca, Sr, Si, Cl, and Br, low pH, low alkalinity and high concentrations Of H2S, and temperatures up to 403°C. These compositions reflect subcritical phase separation at very shallow depths within the oceanic crust (≤300 m) and minimal water‐rock interaction. Dramatic changes in composition also occurred in the vent fluid compositions on time scales as short as a week, our minimum resampling interval. Our data suggest that the fluids that were generated were not pre‐existing brines that were resident in the oceanic crust before the eruption/intrusion. Some of the fluids may be influenced by the dissolution of halite, assumed to be formed at the time of the eruption. Concentrations of Fe in these fluids are unusually high, both on an absolute and especially on a Cl‐normalized basis, which we attribute to the more reducing nature of these fluids. Fluids south of 9°45′N, out of the area where the eruption occurred, are generally higher in Cl and have compositions more similar to those observed elsewhere. There is evidence to suggest that at least some of the vents south of the eruption area may also have been influenced by the eruption/intrusion, although no young lavas were observed at the surface at these other vent sites.
Time‐resolved study of charge generation and propagation in igneous rocks
Authors: Freund Friedemann
Published: September 2002

Electrical resistivity changes, ground potentials, electromagnetic (EM), and luminous signals preceding or accompanying earthquakes have been reported many times, in addition to ground uplift and tilt and other parameters. However, no concept exists that would tie these diverse phenomena together into a physically coherent model. Using low‐to medium‐velocity impacts to measure electrical signals with microsecond time resolution, it is observed that when gabbro and diorite cores are impacted at relatively low velocities, ≈ 100 m/s, highly mobile charge carriers are generated in a small volume near the impact point. They spread through the rocks, causing electric potentials exceeding +400 mV, EM, and light emission. As the charge cloud spreads, the rock becomes momentarily conductive. When a granite block is impacted at higher velocity, ≈ 1.5 km/s, the propagation of the P and S waves is registered through the transient piezoelectric response of quartz. After the sound waves have passed, the surface of the granite block becomes positively charged, suggesting the same charge carriers as observed during the low‐velocity impact experiments, expanding from within the bulk. During the next 2–3 ms the surface potential oscillates, indicating pulses of electrons injected from ground and contact electrodes. The observations are consistent with positive holes, e.g., defect electrons in the O2‐ sublattice, traveling via the O 2p‐dominated valence band of the silicate minerals. The positive holes propagate as charge clouds rather than as classical EM waves. Before activation, they lay dormant in form of electrically inactive positive hole pairs, PHP, chemically equivalent to peroxy links, O3X/OO//XO3 with X = Si4+, Al3+, etc. PHPs are introduced into the minerals by way of hydroxyl, O3X‐OH, which all nominally anhydrous minerals incorporate when crystallizing in H2O‐laden environments. The fact that positive holes can be activated by low‐energy impacts, and their attendant sound waves, suggests that they can also be activated in the crust by microfractures during the dilatancy phase. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that can account for earthquake‐related electrical signals and EM emission. Wherever such charge clouds intersect the surface, high fields are expected, causing electric discharges and earthquake lights.
Variations in the electrical conductivity of the upper mantle beneath North America and the Pacific Ocean
Authors: Neal Scott L. , Mackie Randall L. , Larsen Jimmy C. , Schultz Adam
Published: September 2002

Variations in the electrical conductivity of the mantle beneath Carty Lake in the Canadian Shield, Tucson in the southwest United States, and Honolulu and Midway in the north central Pacific were determined through the inversion of long‐period magnetotelluric and geomagnetic depth sounding data. Inversion of computed response functions is carried out using a minimum structure, regularized approach. The upper mantle beneath Carty Lake is approximately an order of magnitude more resistive than the upper mantle beneath Tucson and nearly 1.5 orders of magnitude more resistive than Honolulu and Midway Island. Inversions were also carried out where the minimum structure constraint was removed at known upper mantle discontinuities. These models show a jump in conductivity of ∼1.5 orders of magnitude across the 660 km discontinuity, a result that is consistent with laboratory experiments on realistic mantle assemblages. Mantle conductivity profiles at Carty Lake are significantly more resistive than those at Tucson, Honolulu, and Midway to depths of ∼300–400 km. These observations likely reflect differing thermal states, the presence (or absence) of partial melt and volatiles, and may also be related to chemical differences between depleted and undepleted upper mantle. The observed conductivity variations may be interpreted as lateral variations in temperature, partial melt, and/or dissolved hydrogen in olivine.
Heat transport in McMurdo Sound first‐year fast ice
Authors: Trodahl H. J. , McGuinness M. J. , Langhorne P. J. , Collins K. , Pantoja A. E. , Smith I. J. , Haskell T. G.
Published: September 2002

We have monitored the temperature field within first‐year sea ice in McMurdo Sound over two winter seasons, with sufficient resolution to determine the thermal conductivity from the thermal waves propagating down through the ice. Data reduction has been accomplished by direct reference to energy conservation, relating the rate of change of the internal energy density to the divergence of the heat current density. Use of this procedure, rather than the wave attenuation predicted by the thermal diffusion equation, avoids difficulties arising from a strongly temperature dependent thermal diffusivity. The thermal conductivity is an input parameter for ice growth and climate models, and the values commonly used in the models are predicted to depend on temperature, salinity, and the volume fraction of air. The present measurements were performed at depths in the ice where the air volume is small and the salinity is nearly constant, and they permit the determination of the absolute magnitude of the thermal conductivity and its temperature dependence. The weak temperature dependence is similar to that predicted by the models in the literature, but the magnitude is smaller by ∼10% than the predicted value most commonly used in climate and sea ice models. In the first season we find an additional scatter in the results at driving temperature gradients larger than ∼10–15 °C/m. We suggest that the scatter arises from a nonlinear contribution to the heat current, possibly associated with the onset of convective motion in brine inclusions. Episodic convective events are also observed. We have further determined the growth rate of the ice and compared it with the rate explained by the heat flux from the ice‐water interface. The data show a sudden rise of growth rate, without a rise in heat flux through the ice, which coincides in time and depth with the appearance of platelet ice. Finally, we discuss the observation of radiative solar heating at depth in the ice and demonstrate that the absorption exceeds that in the ice alone; dust or algae must contribute to the absorption.
Modification of NO, PO, and NO/PO during flow across the Bering and Chukchi shelves: Implications for use as Arctic water mass tracers
Authors: Cooper Lee W. , Cota Glenn F. , Pomeroy Lawrence R. , Grebmeier Jacqueline M. , Whitledge Terry E.
Published: September 2002

The NO and PO tracers (9[NO3−] + O2 and 135[PO4−] + O2, respectively,) and their derivative NO/PO have found increasing use in Arctic water mass analyses for identifying the specific basin or shelf areas from which surface waters originate, based upon assumed differences in Pacific‐ and Atlantic‐derived content and basin‐to‐basin differences within the Arctic. Following shipboard sampling in June‐September 1993 and May‐June 1994, both north and south of Bering Strait, we have found evidence that Pacific‐derived waters flowing north to Bering Strait do not necessarily have any unique NO, PO, or NO/PO identity that would permit unequivocal use as a water mass tracer. In particular, NO/PO ratios in the Bering Sea continental shelf (<150 m) waters varied from 0.7 to 1.1, which encompasses ratios previously reported for Arctic continental shelf and Atlantic origin waters in the Arctic Ocean. The highest NO/PO ratios (∼1) in the Bering Sea were observed to the southwest of St. Lawrence Island, close to where high nutrient waters are first upwelled onto the shelf, and seasonally early in the biological production cycle. By contrast, later in the summer, north of Bering Strait, at the depth of the Arctic Ocean nutrient maximum, the highest concentrations of silica (∼60 μM) were associated with low NO/PO ratios (∼0.7). Apparent increases in the proportions of sea ice melt in these waters, inferred from 18O and salinity regressions, were associated with lower NO/PO ratios. This pattern, the potential for sea‐air exchange, and a significant relationship between decreases in nitrate/phosphate ratios and both NO/PO ratios and silica concentrations indicate that biological and physical processes north and south of Bering Strait affect the fidelity of these nutrient‐based tracers. These results indicate the need for consideration of shelf‐based processes before NO/PO ratios and other nutrient‐based tracers can be successfully applied as Arctic circulation tracers.
Local data assimilation in the estimation of barotropic and baroclinic open boundary conditions
Authors: Shulman Igor , Lewis James K. , Mayer John G.
Published: September 2002

The problem of data assimilation in the specification of open boundary conditions for limited area models is addressed in this paper. Optimization approaches are detailed, which are based on combining available data on an open boundary with the physics of the hydrodynamical model. In our case the physics is in terms of the flux of energy through the open boundary. These optimized boundary conditions, for both barotropic and baroclinic situations, interpreted physically as special linearizations of the Bernoulli equation for each normal mode. Because of the complexity of decomposing variables into normal modes for open boundaries with varying bathymetry, we present two alternative approaches. The first is a simplification of the optimized baroclinic boundary condition based on normal modes. The second makes use of empirical orthogonal functions instead of normal modes. The results of testing and comparisons of these approaches are presented for coupling coarse‐ and fine‐resolution models. In this case our approach is in assimilating values and variables from a large‐scale model (along the open boundaries of a limited area model). In the proposed coupling schemes the energy fluxes are estimated either from coarse or from fine‐grid model results. With the progress of oceanographic observing systems we would like to explore ways of combining model outputs with the oceanographic measurements in order to estimate energy fluxes used in optimized open boundary conditions.
Variability of the simulated meridional heat transport in the North Atlantic for the period 1951–1993
Authors: Häkkinen Sirpa
Published: September 2002

A 43‐year ocean model simulation for the period 1951–1993 is analyzed, with a focus on the meridional heat transport (MHT) as a proxy for the strength of meridional overturning cell (MOC) at 25°N. The surface heat flux forcing associated with the North Atlantic Oscillation (NAO) pattern is related to the variability in MHT both on interannual and longer timescales. The manifestation of the interannual and decadal oceanic response to NAO is nonlocal, as evidenced by the concentrated heat content and sea level variability at the Gulf Stream and North Atlantic Current regions and shown by the comparison of the model sea level variability to the altimeter data between low MHT years in mid‐1980s and high MHT years in early 1990s. The same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level. MHT and time series of the leading sea level mode are highly correlated, which reflects the importance of the MOC in determining the variability of the heat content of the whole basin. Also, the model suggests that the MHT/MOC has entered a very strong phase since the mid‐1980s and this trend has continued up to the end of 1993; this behavior follows the general trend in the NAO index.
The role of winter intermediate waters in the spring‐summer circulation of the Balearic Sea: 2. A sensitivity numerical study
Authors: Pinot J.‐M. , Álvarez A. , Fernández V. , Riera M.
Published: September 2002

A primitive equation model is used to investigate the role of winter intermediate waters in the surface circulation of the Balearic Sea in spring‐summer. The interior solution is forced by restoring to thermohaline fields in four reservoirs schematizing the adjacent basins. Several simulations are carried out with different initial and boundary conditions, which evidence the sensitivity of the circulation to the thermohaline structure in the basin and to the inflows through the channels. The accumulation of winter intermediate waters to the north of the Ibiza Channel in spring is found to have a drastic effect on the circulation and on the water exchange through the channels: the Continental Current is deflected away from the slope toward the Balearic Islands, hence reducing the southward transport through the Ibiza Channel while enhancing the transport through the Mallorca Channel. A simultaneous northward energetic inflow of modified Atlantic waters through the Balearic Channels happens to amplify this effect and can lead to a major blocking of the Continental Current, which veers back to the north, feeding the Balearic Current during several months. Model results compare favorably with observations and help interpret some data sets.
Satellite‐derived sea surface temperature fronts on the continental shelf off the northeast U.S. coast
Authors: Ullman David S. , Cornillon Peter C.
Published: September 2002

The distribution and variability of sea surface temperature (SST) fronts, over the shelf and slope along the east coast of North America from Cape Hatteras to Nova Scotia, are studied using a 12 year time series (1985–1996) of advanced very high resolution radiometer (AVHRR) images. After the masking of cloud‐contaminated pixels, an edge‐detection algorithm identifies surface temperature fronts in each image. Maps of the seasonal probability of detecting a front indicate substantial spatial and temporal variability in the occurrence of SST fronts. Over the continental shelf south of New England during spring through autumn, surface fronts are rare and observed only in scattered locations. North of Nantucket Shoals, both tidal mixing fronts and fronts associated with the Eastern Maine Coastal Current occur during the summer. A major finding of this study is the observation of fronts in winter over the inner and middle shelf from Cape Hatteras to the Bay of Pundy. These fronts, peaking during January‐March, are characterized by cold SST on their inshore (shallow) side and appear to result from the influence of surface cooling on shallow nearshore waters. The shelfbreak front is found to vary strongly with season, being detected most frequently during spring and autumn. South of Hudson Canyon, it essentially disappears during the summer, while from Hudson Canyon to Northeast Channel, it weakens during summer but nevertheless remains detectable in SST.
Chlorophyll enhancement and mixing associated with meanders of the shelf break front in the Mid‐Atlantic Bight
Authors: Ryan J. P. , Yoder J. A. , Barth J. A. , Cornillon P. C.
Published: September 2002

Meanders of the shelf break front in the Mid‐Atlantic Bight (MAB) during April and May of 1997 were associated with chlorophyll enhancement along a hydrographic and a topographic feature. The hydrographic feature was the surface outcrop of the front, which ranged from ∼10 to >100 km seaward of the shelf break owing to the meanders. The topographic feature was the shelf break (100‐m isobath). Chlorophyll enhancement was observed by a satellite instrument, the ocean color and temperature sensor, and by a fluorometer in situ. It developed in near‐surface waters typically nutrient depleted during late spring, thus local nutrient enrichment of near‐surface waters was probable. Observations of sufficient resolution to define processes were available only for the region of shelf break chlorophyll enhancement. Along two meander troughs (shoreward extremities near the shelf break), we observed shoaling of cold shelf water. Shelf water shoaled >20 m along frontal isopycnals, and phytoplankton absorption maxima coincided directly with the shoaled water. Thus local nutrient enrichment by along‐isopycnal upwelling was the supported mechanism of chlorophyll enhancement at the shelf break. The basis for along‐isopycnal upwelling was seaward flow of shelf water forced by meander circulation near the shelf break. Strong cross‐isobath flow and mixing developed as these meanders propagated along the shelf break front of the MAB at a relatively constant rate of ∼9 km day−1.
Investigation of the stability of the ERS 1 range bias through tide gauge augmented altimetry
Authors: Moore P. , Ehlers S. , Murphy C. M. , Reynolds M. D.
Published: September 2002

Studies of sea level rise or ice sheet deformation from altimeter missions require careful calibration if systematic errors are not to be aliased into the result. For satellite missions that have distinct phases or when the repeat period is too long for repeat pass studies, altimetric crossover differences provide the mechanism for examination of residual systematic effects. In this study, altimetry, augmented with sea‐surface variability time series as provided by the global network of tide gauges, is used to examine the stability of the ERS 1 range bias from April 1992 to August 1995. This 3‐year period covered the first multidisciplinary phase, the geodetic phase, and the first four cycles of the second multidisciplinary phase. A correlation analysis using TOPEX/Poseidon and ERS 1 was first employed to identify a subset of island tide gauges where the time series is representative of that observed by the altimeter in the open ocean. Tide gauge augmented crossovers and all available repeat pass data were analyzed in the vicinity of these gauges. In particular, crossovers linking the ERS 1 multidisciplinary and geodetic phases permitted the range bias to be monitored continuously. Dual‐crossover residuals between ERS 1 and TOPEX/Poseidon were utilized to gain insight into the characteristics of the temporal variation in the long‐wavelength, geographically correlated error for the 3‐year period. Failure to incorporate this geographically correlated error over the 3‐year period led to a spurious drift of some 2–3 mm/yr. However, its inclusion reduces the drift to the extent that the observed rate is not statistically different from zero.
Close interactions between two pairs of heton‐like vortices under sea ice
Authors: Chao Shenn‐Yu , Shaw Ping‐Tung
Published: September 2002

Close interactions between two pairs of vertically stacked counterrotating eddies under sea ice were investigated in numerical experiments. The numerical model contains a stratified ocean capped by an ice layer. Under the ice layer a shallow brine source produces a top cyclone and a submerged anticyclone, while a shallow freshening source generates a top anticyclone and a submerged cyclone. Ice‐exerted friction dissipates the top eddies in time, leaving submerged ones in lone existence. In this work, winning vortices are sought from a brine‐brine pair, a freshening‐brine pair, and a freshening‐freshening pair. Brine and freshening sources are made equal in strength to level the playing field. When closely paired, fission, an increase in the number of subsurface vortices, occurs from a brine‐brine pair and a freshening‐brine pair but not from a freshening‐freshening pair. Consequences of fission invariably increase the number of submerged anticyclones. A heuristic argument is given to explain this polarity. The strong interaction among closely packed eddies operates in timescales of tens of days, helping to explain the predominance of submerged anticyclones under the Arctic sea ice.
Role of ocean currents and bottom pressure variability on seasonal polar motion
Authors: Ponte Rui M. , Stammer Detlef
Published: September 2002

Changes in the ocean angular momentum (OAM) components about the equatorial axes, either due to fluctuations in currents or bottom pressure (mass redistribution), can induce movements of the Earth's pole of rotation, commonly referred to as polar motion or wobble. Output from a 1° resolution ocean model is used to calculate the effective equatorial OAM functions χ1O and χ2O, corresponding to polar motion excitation about the equatorial axis pointing to the Greenwich and 90°E meridians, respectively. Time series of χO are combined with similar atmospheric series χA, computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses, to interpret the observed low‐frequency polar motion excitation for the period 1985–1996. Results indicate that the oceans are a very important excitation source for the Chandler (∼433 days), annual, and semiannual wobbles, providing for much better amplitude and phase agreement with the observed excitation at these periods, in comparison with what is obtained when only the atmosphere is considered. Both oceanic mass and motion terms are found to be important but with mass signals having somewhat larger amplitudes. The role of regional variability in ocean currents and bottom pressure in contributing to χO signals is quantified. Midlatitude regions (∼30°–70°) figure prominently as places of strong local oceanic excitation signals. The North Pacific basin is found to be generally important for χ1O excitation, while the Southern Ocean is important for both χ1O and χ2O. The largest positive covariances of local with global χ1O signals occur in the Kuroshio region near the western boundary of the North Pacific for χ1O and southwest of Australia for χ2O.
Annual and interannual sea level variations in the Indian Ocean from TOPEX/Poseidon observations and ocean model simulations
Authors: Basu Sujit , Meyers Steven D. , O'Brien James J.
Published: September 2002

Sea level variations relative to a 4‐year mean in the Indian Ocean north of 10°S are examined during 1993–1996 using both a numerical reduced gravity model with realistic coastline geometry and wind stress and sea level measurements from the TOPEX/Poseidon altimeters. The annual signal is found to be composed of propagating as well as nonpropagating features. The propagation speeds in the model and altimetry generally agree to within 25% or less. Complex empirical orthogonal function (CEOF) decomposition yields a separation between the annual and semiannual cycles (46 and 30% of the respective variance for the model, and 40 and 26% for the altimetric measurements, respectively). The propagation of these signals across the ocean basin is indicated by the spatial phase functions. Both temporal phase functions are steady for the annual cycle, though the amplitudes are modulated in time. The results for the semi‐annual cycles are similar, but the temporal phase functions are disrupted for ∼12 months starting in 1994. This may be due to an unusually strong monsoon during that time. The correlation between model sea level variation and those measured by altimetry is highly variable in both space and time. Low‐frequency filtering of the sea level anomalies, obtained by summing the two largest CEOF modes (the annual and semiannual cycles), improves the correlation. The filtered anomalies correlate in time as high as 0.9 in the western Arabian Sea and as high as 0.7 south of the equator and in the eastern Bay of Bengal. There are pockets of poor correlation (as low as −0.4) in the eastern Arabian Sea, central Bay of Bengal, and central equatorial region. These areas tend to contain recurring Rossby wave interactions as represented by the 1.5‐layer model. Each area is associated with a “phase nexus” (analogous to an amphidromic point in tidal theory) or a strong gradient of the model spatial phase functions. The spatial correlation between the filtered anomalies is typically 0.6 over much of the observation period but contains unexplained declines as low as 0.2 during a few months in both 1995 and in 1996.