bluuebunny/crossref_metadata_2025 huggingface dataset:
Slab temperature and thickness from seismic tomography: 2. Izu‐Bonin, Japan, and Kuril subduction zones
Authors: Deal Michael M. , Nolet Guust
Published: September 2002

Delay times from teleseismic and local P wave arrivals are used to invert for a high‐resolution three‐dimensional velocity model beneath the northwest Pacific. The model shows high‐velocity slabs with average velocity anomalies of the order of 3–4%. Assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly, the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model. Temperature variations between the ambient mantle and the interior of the slab are converted to P wave velocity perturbations using dVp/dT ≈ 4.8 × 10−4 km s−1 °C−1. A nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature. Using 1180±100°C as the potential temperature, thickness estimates of 88±8 km, 85±8 km, and 84±8 km are obtained for the Izu‐Bonin, Japan, and Kuril slabs, respectively. A correlation exists between slab thickness and age, which is strong if mantle temperature variations along the slab strike can be ruled out. In the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum‐norm tomographic result. The initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null‐space components. The use of the null‐space components guarantees that the enhanced solution will satisfy the original seismic delay times. The enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results.
Thermal convection in a vertical permeable slot: Implications for hydrothermal circulation along mid‐ocean ridges
Authors: Rabinowicz Michel , Sempéré Jean‐Christophe , Genthon P.
Published: September 2002

Hydrothermal vents along unsedimented portions of mid‐ocean ridges are fed by flow confined to a 1–3 km high percolating system of fissures subparallel to the spreading center. We have solved the coupled thermal and flow equations in such a system. We assume that hydrothermal circulation occurs in a vertical porous slot 1.5 km high, 2.3 km long, and at most 100 m wide. In this model we take into account the heat transported by the hydrothermal flow inside the porous slot as well as the heat conducted through the surrounding impermeable crustal layer. The fluid is free to enter or leave the top of the slot. The fluid enters the slot at a temperature of 2°C, while the bottom of the porous slot is held at a constant temperature of 420°C. We first consider a vertical slot with a horizontal base. Our calculations show the development of tall and narrow unsteady convective cells. Because of the nonlinear relationship of the viscosity and density of seawater with temperature, the upper thermal boundary layer is much thicker than the lower one. The fluid advected in the hot plumes tends to remain confined inside the slot because the upper boundary is thick and viscous. In high Rayleigh number experiments, some very active hot plumes are able to thin and pierce the upper cold boundary layer, and vent hot fluids at the seafloor. The maximum temperature at the exit of these plumes reaches a value at most equal to 270°C. The presence of an axial magma chamber along most of the length of the East Pacific Rise may justify the use of a horizontal base to model hydrothermal systems along fast spreading ridges. However, where the hydrothermal systems develop above a short‐lived magmatic intrusion a few kilometers long, the lower interface of the percolating system of fissures is inclined. In that case our calculations show that the convective flow tends to be unicellular and steady state with a wide region of downwelling and a narrow upwelling zone. The temperature of the fluid exiting the slot at the top reaches a maximum value of 360°C. These results lead us to suggest that hydrothermal plumes generated inside a fracture with a horizontal base give rise to vents which wander along the fracture plane with a maximum temperature of about 270°C. However, when the base of the fracture plane is inclined, a large hydrothermal steady plume is generated which can give rise, at its exit, to a black smoker with fluid temperatures in excess of 350°C.
A microearthquake survey of the high‐temperature vent fields on the volcanically active East Pacific Rise (9°50′N)
Authors: Sohn Robert A. , Hildebrand John A. , Webb Spahr C.
Published: September 2002

A 3 month deployment of nine ocean bottom seismometers (OBS) on the axis of the East Pacific Rise at 9°50′N detected 283 local microearthquakes in the spring of 1995. The earthquakes exhibit small, uniform seismic moments of 1014–1016 dyne‐cm (). Accurate locations were determined for 147 of the earthquakes, with hypocenters clustered beneath two high‐temperature hydrothermal fields (Bio9/P and Tube Worm Pillar/Y) at depths <1.4 km beneath the seafloor. Waveform cross‐correlation techniques relocated 76 events, 65 of which had lateral standard errors <100 m. Relocated hypocenters lie along two vertical columns from 0.7 to 1.1 km depth. A continuous level of seismic activity was observed beneath the Tube Worm Pillar and Y vent fields, while activity beneath the Bio9 and P vent fields was dominated by a vigorous swarm of 162 events during 3 hours that triggered a 7°C temperature increase in Bio9 exit fluids. The close correlation between the hydrothermal systems and the observed seismicity suggests the microearthquakes were generated by thermal strain associated with cooling of the shallow crust and that the water‐rock reaction zone is also a seismogenic zone. The earthquake depths suggest that brittle deformation on the rise axis is restricted to the upper kilometer of crust.
Emplacement of the Resurrection Peninsula ophiolite in the southern Alaska forearc during a ridge‐trench encounter
Authors: Kusky Timothy M. , Young Charles P.
Published: September 2002

The 57±1 Ma Resurrection Peninsula ophiolite of southern Alaska's Chugach terrane formed in a near‐trench environment as the Kula‐Farallon ridge was being subducted beneath North America. The magmatic sequence includes pillow lavas, sheeted dikes, gabbros, trondhjemites, and a poorly‐exposed ultramafic section. The lavas show mid‐ocean ridge basalt and arc‐like geochemical signatures, interpreted to reflect compositionally diverse melts derived from near‐fractional melting of a variably depleted mantle source, mixed with variable amounts of assimilated continentally‐derived flysch. A sedimentary sequence overlying the ophiolite, here named the Humpy Cove Formation of the Orca Group, preserves a continuous 3.6 plusmn;1.4 Ma record of turbidite sediments that were deposited on the ophiolite as it was transported to North America and emplaced in the Chugach accretionary prism. A transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent. Chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt, signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent. Hydrothermal metals are not resolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite, suggesting that they were diluted by turbidite sedimentation. The top of the section is truncated by the Fox Island shear zone, a 1 km thick, greenschist‐facies, west‐over‐east thrust related to the emplacement of the ophiolite into the accretionary wedge. The Fox Island shear zone is intruded by a 53.4±0.9 Ma granite, showing that the ophiolite formed, was transported to the North American continent, overthrust by a major accretionary prism‐related thrust, and intruded by granite all within 3.6±1.4 Ma. Using these estimates for the timing of formation and emplacement of the ophiolite, with palinspastic reconstructions and plate motion vector triangles, we suggest that the Resurrection Peninsula ophiolite was transported up to 500 km (461±79 km) relative to North America from the place of its formation at the Kula‐Farallon ridge prior to its emplacement into the North American margin. Since the best estimates of the amount of translation of the Chugach terrane come from paleomagnetism of the Resurrection Peninsula ophiolite, current estimates of margin‐parallel strike slip need to be reduced considerably.
Magnetic power spectrum of the ocean crust on large scales
Authors: O'Brien Michael S. , Parker Robert L. , Constable Catherine G.
Published: September 2002

The geomagnetic power spectrum Rl is the squared magnetic field in each spherical harmonic degree averaged over a spherical surface. Satellite measurements have given reliable estimates of the spectrum for the part that originates in the core, but above l = 15, where the geomagnetic field arises primarily from crustal magnetization, there is considerable disagreement between various estimates derived from observation. Furthermore, several theoretical models for the spectrum disagree with each other and the data. We have examined observations from a different source, 5000‐km‐long Project Magnet aeromagnetic survey lines; we make new estimates of the spectrum which overlap with the wavelength interval accessible to the satellites. The usual way the spectrum is derived from observation is to construct a large spherical harmonic decomposition first, then square, weight, and add the Gauss coefficients in each degree, but this method cannot be applied to isolated flight lines. Instead, we apply a statistical technique based on an idea of McLeod and Coleman which relates the geomagnetic spectrum to the power and cross spectra of magnetic field components measured on the survey lines. Power spectra from the 17 aeromagnetic surveys, all of which were conducted over the oceans, are averaged together to improve geographic coverage and reduce variance, and the average spectra are then inverted for the geomagnetic spectrum Rl. Like most of the theoretical models, our spectrum exhibits a maximum, but at a wavelength of 100 km, about a factor of 2 smaller than the closest theoretical prediction. Our spectrum agrees quite well with the most recent estimates based on satellite observations in the range 20≤l≤50, but above l = 50, our values increase slowly, while all the satellite data suggest a sharply rising curve. In this wavelength range we believe our measurements are more trustworthy. Further work is planned to confirm the accuracy of our spectrum when continental survey paths are included.
An accurate and efficient method for including the effects of topography in three‐dimensional elastic models of ground deformation with applications to radar interferometry
Authors: Williams Charles A. , Wadge Geoff
Published: September 2002

Topography has a large effect on the results predicted by elastic surface deformation models in regions of significant relief. In some cases, topography may have more influence on the predicted deformation field than do model source parameters. We have developed an approximate analytical technique for including topographic effects that retains most of the computational simplicity of elastic half‐space models while providing an accurate representation of topographic effects. We use a series expansion of the elastic half‐space solution with a small slope approximation, yielding a set of higher‐order corrections. The integrated effect of these corrections is evaluated using Fourier methods. We investigate the effectiveness of our method by comparing predicted results for a tilted triaxial ellipsoid with those predicted by finite element models. The resulting displacements and displacement gradients are in good agreement with finite element results both for relatively smooth topography (synthetically generated) and for the greater relief in the vicinity of Mount Etna volcano. We then compare the results of our method with those predicted by traditional elastic half‐space models using different reference elevations, and with a previously proposed method of estimating topographic effects. Our new method provides a significantly better fit to the finite element results than do the other methods. Our method is able to accurately portray both the number and the horizontal pattern of fringes in a synthetic interferogram when compared with finite element results. In particular, the method accurately represents the broadening of fringes that is observed in regions of high relief, as well as reproducing the location of the fringe center and the topographically induced deviation of the fringes from a regular pattern. Elastic half‐space models typically show a fringe center that is displaced with respect to the finite element results, indicating that parameter inversions based on such a model would provide incorrect estimates of the horizontal source position. The lack of topographically generated fringe distortions in elastic half‐space results would also likely lead to inaccuracies in the predicted magnitude and orientation of proposed deformation sources. Our new technique over‐comes these limitations and should be useful when evaluating either traditional geodetic results or the greater areal coverage provided by interferometric observations.
Gravitational stability of three‐dimensional stratovolcano edifices
Authors: Reid Mark E. , Christian Sarah B. , Brien Dianne L.
Published: September 2002

Catastrophic flank collapses have occurred at many stratovolcanoes worldwide. We present a three‐dimensional (3‐D) slope stability analysis for assessing and quantifying both the locations of minimum edifice stability and the expected volumes of potential failure. Our approach can search the materials underlying a topographic surface, represented as a digital elevation model (DEM), and determine the relative stability of all parts of the edifice. Our 3‐D extension ofBishop's[1955] simplified limit‐equilibrium analysis incorporates spherical failure surfaces, variable material properties, pore fluid pressures, and earthquake shaking. Although a variety of processes can trigger collapse, we focus here on gravitationally induced instability. Even homogeneous rock properties strongly influence the depth and volume of the least stable potential failure. For large failures in complex topography, patterns of potential instability do not mimic local ground surface slope alone. The May 18, 1980, catastrophic failure of the north flank of Mount St. Helens provides the best documented case history to test our method. Using the undeformed edifice topography of Mount St. Helens in an analysis of dry, static slope stability with homogeneous materials, as might be conducted in a precollapse hazard analysis, our method identified the northwest flank as the least stable region, although the north flank stability was within 5% of the minimum. Using estimates of the conditions that existed 2 days prior to collapse, including deformed topography with a north flank bulge and combined pore pressure and earthquake shaking effects, we obtained good estimates of the actual failure location and volume. Our method can provide estimates of initial failure volume and location to aid in assessing downslope or downstream hazards.
Crustal structure of the Altiplano from broadband regional waveform modeling: Implications for the composition of thick continental crust
Authors: Swenson Jennifer L. , Beck Susan L. , Zandt George
Published: September 2002

We have modeled the full waveforms from six intermediate‐depth and two shallow earthquakes recorded at regional distances by the BANJO Broadband Andean Joint Experiment (BANJO) and Seismic Exploration of the Deep Altiplano (SEDA) portable seismic networks in the central Andes. In this study we utilize data from those BANJO and SEDA stations located within the Altiplano and Eastern Cordillera. We used reflectivity synthetic seismograms and a grid search to constrain four parameters of the Altiplano‐Eastern Cordillera lithosphere: crustal thickness, average crustal velocity (Vp), and crustal and upper mantle Poisson's ratios (σcrust and σmantle). Using our grid search, we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths. Robust models for the Altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average Vp of 5.75–6.25 km/s, crustal thicknesses of 60–65 km, σcrust = 0.25, and σmantle = 0.27–0.29. We find a north‐south variation in the structure of the Altiplano, with the crust south of the BANJO transect characterized by either lower than average crustal P wave velocities or a slightly higher σcrust relative to crust north of the BANJO transect. These results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust, rather than by underplating or magmatic intrusion from the mantle.
Subcontinental mantle dynamics: A further analysis based on the joint constraints of dynamic surface topography and free‐air graviy
Authors: Pari Giovanni , Peltier W. Richard
Published: September 2002

The depth extent, density, and dynamical role of apparent subcontinental keels are investigated by using constraints provided by the very long‐wavelength representations of several geophysical fields. We first consider local cross correlations between the nonhydrostatic free‐air gravity field, the map of the ice sheets at Last Glacial Maximum, and the surface expression of continental cratons. We initially confine our analyses to North America and observe that equally good local correlations exist between the gravity field and either of the others. The case of Eurasia is also considered. We observe that correlation analyses of this type cannot be employed to unambiguously infer the cause of long‐wavelength continental gravity anomalies; therefore we revert to explicit postglacial rebound and tomography‐based viscous flow modeling of the gravity field. For viscosity profiles that optimally reconcile relative sea level constraints from the Laurentide platform, we find that the rebound process accounts for only 10% of the observed free‐air gravity low over Hudson Bay. We consider mantle convection as the more likely source of this gravity anomaly and alternatively investigate the implications of assuming that seismically fast, deep structure imaged tomographically beneath the continent represents either negatively, neutrally, or positively buoyant material. In addition to the gravity constraint we introduce the independent constraint of continental dynamic surface topography. We infer this new datum by using the Crust 5.1 global model of crustal structure. Remarkably, continents are found to systematically reside in topographic depressions of the order of 1–2 km. Within the context of our modeling assumptions we find that optimal model descriptions of the joint gravity and dynamic surface topography constraints over the continents require deep and dense subcontinental undercurrents.
Monte Carlo simulation of multiple scattering of elastic waves
Authors: Margerin Ludovic , Campillo Michel , Van Tiggelen Bart
Published: September 2002

We study multiple scattering of elastic waves with a Monte Carlo method. We take into account the mode conversions and the polarization of the S waves. Some important physical parameters relevant to the description of the polarization are recalled, such as the definition and properties of the elastic Stokes vector. We briefly derive the scattering and Mueller matrices, as well as the differential and total scattering cross sections for one spherical inclusion embedded in a homogeneous matrix. The results of the single‐scattering problem are used as a building block for multiple scattering. A Monte Carlo method to simulate the propagation of full elastic waves is presented. We pay a special attention to the convergence toward the diffusive regime which exhibits the equilibration of the P and S energy densities. Our simulations show the shear energy to become very rapidly dominant in the coda and the S to P energy density ratio to tend to 10.4 for a Poisson solid, as predicted by the equipartition theorem. However, the typical timescale and length scale to reach equipartition heavily depend on the scattering parameter kpa, where kp is the P wave number and a is the sphere radius. For Rayleigh scattering (kpa ≪ 1) we find a smooth evolution of energy density with time and a slow convergence toward the equilibration, mainly because of the large difference between the P and S scattering mean free paths in this case. On the other hand, for Rayleigh‐Gans scattering (kpa ∼ 1.2, 1.6) a peak of energy associated with the forward scattered waves is observed, followed by a slow decay according to the diffusion approximation. We find that after only a few mean free times, equipartition is reached in spite of the strong anisotropy of the scattering in this regime. As the scattering parameter kpa increases, we find that equipartition is again delayed because the transport mean free paths become quite large. We find that a large source‐station distance favors a rapid equilibration. This effect is seen to be very pronounced for Rayleigh scatterers. When a source of P waves is considered, the equipartition time can be twice as long as compared with a shear source. The time evolution of the Ep/Es ratio could be used as a marker for the different scattering mechanisms.
Strain offsets with monotonous damped oscillations during the 1986 Izu‐Oshima volcano eruption
Authors: Fujita Eisuke , Fukao Yoshio , Kanjo Kenji
Published: September 2002

The sequential process of the 1986 Izu‐Oshima volcano eruption, Japan, was recorded by a volumetric strain meter near the erupted summit. The records show occasional strain offsets accompanying long‐period damped oscillations, which we interpret as responses of the volcanic system to perturbations in magmatic pressure. The polarities of strain offsets were contractive for all events before the enormous ground deformation of November 21 due presumably to the intensive dike intrusion. On the contrary, all events after that had expansive polarities. The characteristic frequencies of oscillations were ∼42.6 mHz for the events before, changing abruptly to half that after the ground deformation. In our model, which is a one‐dimensional magma‐filled conduit, the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion. Before the magma escape from the reservoir as a dike, magma was episodically supplied from the reservoir to the conduit, and thus contraction events were generated. After the magma escape, magma left in the conduit was episodically drained back to the reservoir to generate expansion events. This episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit. We solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe. Physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation. The conduit length and fluid acoustic velocity are estimated to be ∼10 km and 430 m/s, respectively, suggesting that the fluid is composed of a gas‐liquid mixture. If the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation, the fluid viscosity is 108–109 Pa s, a reasonable estimate for the basaltic magma. The applied pressure changes at thé bottom of the conduit are in a range from 105 to 106 Pa with risetimes of the order of 4 to 100 s.
Neotectonics of the Macquarie Ridge Complex, Australia‐Pacific plate boundary
Authors: Massell C. , Coffin M. F. , Mann P. , Mosher S. , Frohlich C. , Duncan C. S. , Karner G. , Ramsay D. , Lebrun J.‐F.
Published: September 2002

New marine geophysical data along the Macquarie Ridge Complex, the Australia‐Pacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia‐Pacific plate boundary along the Macquarie Ridge Complex. We interpret a ∼5–10 km wide Macquarie Fault Zone between the two plates along a bathymétrie high that extends nearly the entire length of the Australia‐Pacific plate boundary south of New Zealand. We conclude that this is the active Australia‐Pacific strike‐slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ∼50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike‐slip deformation, although teleseismic evidence overwhelmingly indicates strike‐slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present‐day strike‐slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia‐Pacific crust in the region. Our conceptual model for the transition from seafloor spreading to strike‐slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.
Stability of dike intrusion along preexisting fractures
Authors: Ziv Alon , Rubin Allan M. , Agnon Amotz
Published: September 2002

We investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress. Relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma (the dike) results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress concentration near the dike tip. This could lead to the production of new tensile cracks, oblique to the parent dike, that could capture the flow. If the shear stress resolved on the fracture plane is perpendicular to the fracture front (mode I‐II), the front may deviate along its entire length; if the shear stress is parallel to the fracture front (mode I‐III) the front may splay into segments. For mode I‐II dikes the maximum tensile stress occurs at the dike tip and is parallel to the dike. If the effective tensile stress exceeds the rock tensile strength, then the intruding magma, rather than dilating the existing fracture, is expected to propagate into a self‐generated crack analogous to the “wing cracks” observed to form at the tips of pure mode II fractures. For mode I‐III dikes the maximum tensile stress lies within the plane of the dike and is oriented at some angle that depends upon the far‐field boundary conditions. Even if the magma pressure exceeds the ambient normal stress, it appears to be very difficult for dikes to intrude into preexisting fractures unless one or more of the following conditions is satisfied: (1) the fracture is nearly perpendicular to the least compressive stress; (2) the resolved shear stress on the fracture is small compared to the excess magma pressure (i.e., the ratio of shear to opening of the dike walls is small); (3) the effective ambient dike‐normal stress is small compared to the rock tensile strength. This indicates that it may be quite difficult for dikes emerging from midcrustal to lower crustal depths to follow faults.
Laminar flow of fine sediment‐water mixtures
Authors: Battaglia M. , Borgia A.
Published: September 2002

Flows of sediment‐water mixture (suspensions) are common superficial processes in nature. To investigate the possibility of predicting the behaviour of such flows, we study the free surface flow of a clay‐water suspension down a slope, which may represent a simple realistic model for the flow of some concentrated sediment‐water mixture in a natural environment. The hydrodynamic lubrication equations governing the flow are derived from the full Navier‐Stokes equations, including an appropriate tensorial expression for the constitutive equation of the suspension (a Herschel‐Bulkley fluid with a hysteresis in the yield strength). Results have been obtained for the free surface and length of an unsteady, laminar, isothermal flow of the suspension. The theory is in good agreement with laboratory experiments with fine clay suspensions.
Flux measurements of nonvolcanic CO2 emission from some vents in central Italy
Authors: Rogie John D. , Kerrick Derrill M. , Chiodini Giovanni , Frondini Francesco
Published: September 2002

In central Italy, nonvolcanic CO2 is discharged from many areas of focused degassing (vents and associated strong diffuse emission) and from high‐pCO2 groundwater. We estimate that there are at least 100 CO2 vents in central Italy, many of which have a history of lethality to animals and humans. The gas vents are extremely efficient pathways for transporting large amounts of deeply derived CO2 from gas reservoirs to the surface. As CO2 fluxes from these vents were heretofore unmeasured, several complementary methods were developed to measure the CO2 flux from seven of these vents, which cumulatively discharge 3.1×109 mol yr−1 CO2 (472 t d−1). Considering all of the vents in central Italy, the total CO2 flux from vents alone may be similar to the CO2 emission from the Taupo Volcanic Zone in New Zealand (∼1010 mol yr−1; ∼1200 t d−1) and other areas of high heat flow. Summing the CO2 discharged from vents, areas of diffuse CO2 degassing, and degassing of high‐pCO2 groundwater, we provisionally estimate that the total nonvolcanic CO2 flux to the atmosphere in central Italy may be >1011 mol yr−1 (∼12,000 t d−1).
A magnetotelluric model of the Mana Pools basin, northern Zimbabwe
Authors: Bailey D. , Whaler K. A. , Zengeni T. , Jones P. C. , Gwavava O.
Published: September 2002

The Mana Pools sedimentary basin lies within the Zambezi mobile belt in northern Zimbabwe. New and preexisting magnetotelluric data and the available seismic reflection data are used to constrain the basin structure and the depth to the electrical basement. Long‐period magnetotelluric (LMT) data were collected at five stations along a 60 km north‐south profile across the Mana Pools basin and onto the southern escarpment. These data augment an existing audiofrequency (AMT) data set from 11 sites in the same area. The subsurface apparent resistivities measured at periods sampling the basin are very low (a few Ωm). After processing both data sets, the estimated impedance tensor is decomposed, showing that the resistivity structure of the Mana Pools basin can be modeled two dimensionally. The ρ+ algorithm is used to show that there is no systematic offset in magnitude between the AMT and LMT data sets before they are combined. Minimum structure resistivity models of the Mana Pools basin compare well with the information from reflection seismic data and support its previous description as a half graben basin of ∼7 km depth. The excellent conductor in the Mana Pools basin is quite different to those seen elsewhere in the orogenic belt in that it is a feature of the sedimentary fill rather than the basement. The resistivity of the basement is low but no localized good conductor is observed; these low resistivities may result from a high degree of either chemical or tectonic alteration to the underlying rocks due to metamorphic processes and tectonic disruption during rift formation.
Streaming potential in volcanic rocks from Mount Pelée
Authors: Jouniaux Laurence , Bernard Marie Lise , Zamora Maria , Pozzi Jean Pierre
Published: September 2002

Streaming potential and electric conductivity have been measured in a laboratory on 11 consolidated samples coming from five deposits of the different evolutionary stages of Mount Pelée volcano. The streaming potential coupling coefficient ranges from −35 to −4905 mV MPa−1and increases with increasing permeability. This increase is mainly due to the dependency of rock effective conductivity with permeability. The permeability of the samples varies from 0.146×10−12to 34×10−12m2. The zeta potential, at pH = 7 and water conductivity of 2.1×10−4S m−1, is relatively small for the majority of the samples. It ranges from −4 to −19 mV. According to water conductivity analysis on Mount Pelée, streaming potential coupling coefficients of −25 to −406 mV MPa−1can be expected for this volcano.
Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike‐slip plate boundary, New Zealand
Authors: Audoine Etienne , Savage Martha K. , Gledhill Ken
Published: September 2002

Shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone (lower half of the North Island of New Zealand), and its transition to oblique transform faulting (Marlborough area, northern South Island). In Mar !borough, delay times show almost no increase with depth, and it is most likely that the higher‐frequency phases used in this study respond mainly to lithospheric anisotropy. In the central Marlborough Fault System (MFS), fast polarizations are subparallel to the faults. Anisotropy is attributed to the presence of metamorphosed schist (eclogite), of 30±10 km thickness and located 50–80 km beneath the MFS. On the edges of the MFS, fast polarizations are parallel to the maximum compressive stress direction, consistent with crack‐induced anisotropy in the crust. The shear zone, which is as wide as the island in the mantle as inferred from SKS phases, seems to occur in a narrower zone in the crust. In the lower half of the North Island, fast polarizations from events at all depths are oriented parallel to the strike of the Hikurangi subduction zone as well as to the faults. Polarisations are similar to those of SKS phases, which mainly sample the mantle. This suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike‐slip shear. We calculate 1.2±0.3% velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth. In order to match the SKS delay times, this result requires the presence of anisotropic material down to 580±100‐km depth, or a change in anisotropy with depth, or frequency dependent splitting.
Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand
Authors: Boyd Philip , LaRoche Julie , Gall Mark , Frew Russell , McKay R. Michael L.
Published: September 2002

Phytoplankton processes in subantarctic (SA) waters southeast of New Zealand were studied during austral autumn and spring 1997. Chlorophyll a (0.2–0.3 μg L−1) and primary production (350–650 mg C m−2 d−1) were dominated by cells <2 μm (cyanobacteria) in both seasons. The photochemical efficiency of photosystem II (Fυ/Fm) of cells was low (0.3), indicating physiological stress. Dissolved Fe (DFe) levels in surface waters were subnanomolar, and the molecular marker flavodoxin indicated that cells were iron stressed. In contrast, Subtropical Convergence (STC) and subtropical waters had higher algal biomass/production levels, particularly in spring. In these waters, DFe levels were >1 nmol kg−1, there was little evidence of Fe‐stressed algal populations, and Fυ/Fm approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with caution. In vitro Fe enrichment incubations in SA waters resulted in a switch from flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6‐day experiment, iron‐mediated increases in chlorophyll a (in particular, increases in large diatoms) were of similar magnitude to those observed in a concurrent Si/Fe enrichment; ambient silicate levels were 4 μM. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, resulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In contrast, during summer, as mean irradiance increases and silicate levels decrease, Fe limitation, Fe/Si colimitation, or silicate limitation may determine diatom growth.
Mesoscale variations of biogeochemical properties in the Sargasso Sea
Authors: McGillicuddy D. J. , Johnson R. , Siegel D. A. , Michaels A. F. , Bates N. R. , Knap A. H.
Published: September 2002

A mesoscale resolution biogeochemical survey was carried out in the vicinity of the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series Study (BATS) site during the summer of 1996. Real‐time nowcasting and forecasting of the flow field facilitated adaptive sampling of several eddy features in the area. Variations in upper ocean nutrient and pigment distributions were largely controlled by vertical isopycnal displacements associated with the mesoscale field. Shoaling density surfaces tended to introduce cold, nutrient‐rich water into the euphotic zone, while deepening isopycnals displaced nutrient‐depleted water downward. Chlorophyll concentration was generally enhanced in the former case and reduced in the latter. Eddy‐induced upwelling at the base of the euphotic zone was affected by features of two different types captured in this survey: (1) a typical mid‐ocean cyclone in which doming of the main thermocline raised the near‐surface stratification upward and (2) a mode water eddy composed of a thick lens of 18°C water, which pushed up the seasonal thermocline and depressed the main thermocline. Model hindcasts using all available data provide a four‐dimensional context in which to interpret temporal trends at the BATS site and two other locations during the 2 weeks subsequent to the survey. Observed changes in near‐surface structure at the BATS site included shoaling isopycnals, increased nutrient availability at the base of the euphotic zone, and enhanced chlorophyll concentration within the euphotic zone. These trends are explicable in terms of a newly formed cyclone that impinged upon the site during this time period. These observations reveal that eddy upwelling has a demonstrable impact on the way in which the nitrate‐density relationship changes with depth from the aphotic zone into the euphotic zone. A similar transition is present in the BATS record, suggesting that eddy‐driven upwelling events are present in the time series of upper ocean biogeochemical properties. The variability in main thermocline temperature and nitrate in this synoptic spatial survey spans the range observed in these quantities in the 10‐year time series available at BATS to date (1988–1998).
Pacific subtropical‐tropical thermocline water exchange in the National Centers for Environmental Prediction ocean model
Authors: Huang Boyin , Liu Zhengyu
Published: September 2002

Pacific subtropical‐tropical thermocline water exchange is estimated using the National Centers for Environmental Prediction oceanic general circulation model in which observed temperatures are assimilated. In the main thermocline (23<σt<26.2), the equatorward exchange transports are about 17 Sv (1 Sv = 106 m3 s−1) and 26 Sv at 10°N and 10°S, respectively. Consistent with observations and other model simulations, the water exchange has a Low‐Latitude Western Boundary Current (LLWBC) pathway and an interior pathway. The equatorward LLWBC and interior transports are about 14 and 3 Sv, respectively, at 10°N and are about 15 and 11 Sv, respectively, at 10°S. The time required for subtropical water to penetrate into the equatorial ocean is usually less than 10 years. Exchange windows are also identified in both the North and South Pacific.
Three‐dimensional isoneutral potential vorticity structure in the Indian Ocean
Authors: McCarthy Mary C. , Talley Lynne D.
Published: September 2002

The three‐dimensional isoneutral potential vorticity structure of the Indian Ocean is examined using World Ocean Circulation Experiment and National Oceanic and Atmospheric Administration conductivity‐temperature‐depth data and historical bottle data. The distribution of the potential vorticity is set by the Indian Ocean's source waters and their circulation inside the basin. The lower thermocline has a high potential vorticity signal extending westward from northwest of Australia and a low signal from the Subantarctic Mode Water in the south. The Antarctic Intermediate Water inflow creates patches of high potential vorticity at intermediate depths in the southern Indian Ocean, below which the field becomes dominated by planetary vorticity, indicating a weaker meridional circulation and weaker potential vorticity sources. Wind‐driven gyre depths have lower potential vorticity gradients primarily due to same‐source waters. Homogenization and western shadow zones are not observed. The β‐effect dominates the effect of the Somali Current and the Red Sea Water on the potential vorticity distribution. Isopleths tilt strongly away from latitude lines in the deep and abyssal waters as the Circumpolar Deep Water fills the basins in deep western boundary currents, indicating a strong meridional circulation north of the Antarctic Circumpolar Current. The lower‐gradient intermediate layer surrounded vertically by layers with higher meridional potential vorticity gradients in the subtropical Indian Ocean suggests that Rossby waves will travel ∼1.3 times faster than standard theory predicts. To the south, several pools of homogenized potential vorticity appear in the upper 2000 m of the Southern Ocean where gyres previously have been identified. South of Australia the abyssal potential vorticity structure is set by a combination of the Antarctic Circumpolar Current and the bathymetry.
Interannual variability in the Southern Ocean from an ocean model forced by European Centre for Medium‐Range Weather Forecasts reanalysis fluxes
Authors: Bonekamp Hans , Sterl Andreas , Komen Gerbrand J.
Published: September 2002

Anomaly patterns in the Southern Ocean in response to variability in the atmospheric forcing are investigated. To this end we forced the Hamburg large‐scale geostrophic ocean general circulation model with surface fluxes from the European Centre for Medium‐Range Weather Forecasts reanalysis (ERA). ERA covers the period January 1979 through February 1994. First, the atmospheric variability of sea level pressure and the associated wind stress anomalies within the ERA data set are analyzed. An Antarctic Circumpolar Wave type pattern is identified. In the ocean response, sea water temperature and salinity anomalies are found to vary with similar periods. The anomalies advect with the Antarctic Circumpolar Current. High amplitudes occur in the southeast Indian and Pacific Oceans. Sensitivity studies are made, pinning down wind stress and heat flux as the dominant factors generating these anomalies. The oceanic interannual variations are explained in terms of enhanced oceanic convection resulting from anomalous Ekman pumping and anomalous heat fluxes, both operating in a standing pattern dominated by wavenumbers 2 and 3.
Role of diurnal processes in the seasonal evolution of sea ice and its snow cover
Authors: Hanesiak John M. , Barber David G. , Flato Gregory M.
Published: September 2002

Comparisons between hourly forced data simulations and daily average forced simulations using a one‐dimensional thermodynamic sea ice model show that diurnal changes in the surface energy balance that directly affect the snow depth, albedo, and surface temperature ultimately affect modeled seasonal ice evolution. Hourly forcing produces earlier onset of snowmelt, and open water duration is increased by 21 days. These differences are due to nonlinearities associated with absorbed shortwave radiation as well as latent and sensible heat fluxes. The accuracy of model parameterizations of downwelling shortwave (K↓) and longwave (L↓) fluxes and albedo over diurnal timescales is assessed by comparison to field observations made during the Seasonal Ice Monitoring and Modeling Site (SIMMS) 1992–1993 field experiments near Resolute Bay, Northwest Territories. Mean K↓ errors (modeled ‐ observed) were −23 W m−2, with a standard deviation of 89 W m−2, and mean L↓, errors were −1 W m−2, with a standard deviation of 18.8 W m−2. Modeled dry/new snow and wet snow albedos were about 0.1 and 0.05, respectively, lower than those observed, allowing greater amounts of shortwave energy absorption into the snowpack. Using SIMMS 1992 on‐ice field data as replacement for model K↓, L↓, and albedo parameterizations shows very different spring period simulations compared with using hourly forcing from Resolute land‐based observations. Simulated net surface flux, surface temperature, and snow‐ice ablation were significantly improved.
Where do ocean eddy heat fluxes matter?
Authors: Wunsch Carl
Published: September 2002

A quasi‐global compilation of current meter and temperature records, along with some of the published literature, is used to assess the importance of the meridional eddy heat flux in the ocean circulation, and a comparison is made to a uniform global estimate from altimetric data. Eddy fluxes are found to be important, relative to estimated total heat fluxes, in western boundary current areas of all oceans. Eddy heat flux divergences are potentially important in many other locations, but the mooring coverage is inadequate to be more quantitative. The eddy heat flux in the Southern Ocean is of only modest magnitude compared with that in the western boundary currents, but because of the large circumpolar distance, the integrated flux is very important and probably dominant. All these results are generally consistent with independent estimates made from satellite altimeter variability, but the mooring coverage is not adequate to assess the apparent importance of eddy heat fluxes in the tropics as implied by the altimeter. With the exception of the extreme northeastern North Atlantic, where the local wind dominates the variability, there is no evidence for upgradient fluxes. Climate change models must apparently accurately model the eddy mean‐flow interactions near western boundary currents, in the high‐latitude North Atlantic, in the tropics, and in the Southern Ocean. One can infer that models describing other physical properties such as carbon or nutrients must also adequately represent the eddy contributions to those fluxes as well.