bluuebunny/crossref_metadata_2025 huggingface dataset:
Dormant state of rifting below the Byrd Subglacial Basin, West Antarctica, implied by magnetotelluric (MT) profiling
Authors: Wannamaker Philip E. , Stodt John A. , Olsen Steven L.
Published: February 2004

During the 1994–1995 austral summer field season, we collected twelve, high‐quality MT soundings over the Byrd Subglacial Basin of central West Antarctica (82°36′S lat., 118°14′W long, approx.) in the period range 0.01 s to 400 s. Ten equispaced sites in a 54 km profile cross regional aeromagnetic trends and complement seismic reflection and refraction results collected by others. Our purposes were to prove such measurements were feasible over the 2 km thick interior ice sheet, and to show from deep electrical resistivity whether the Byrd Basin comprises an active rift environment. The difficult acquisition of electric field data on ice was overcome using a custom electrometer system, with preamplifiers located at the electrode sites to buffer the high contact impedances of the ice as close to the source as possible. Two‐dimensional modeling of the profile shows that resistivity of the deep crust and upper mantle is about 2000–3000 ohm‐m to 100 km depth or more. This is much higher than observed in active extensional regimes, suggesting that the current state of rifting, at least in this part of central West Antarctica, is dormant.
Latitudinal dependence of the radial IMF component: Coronal imprint
Authors: Suess S. T. , Smith E. J.
Published: February 2004

Measurements by Ulysses have confirmed that there is no significant gradient with respect to heliomagnetic latitude in the radial component, Br, of the interplanetary magnetic field. In the corona, the plasma β is ≪ 1, except directly above streamers, so longitudinal and latitudinal gradients in field strength will relax due to the transverse magnetic pressure gradient force as the solar wind carries magnetic flux away from the Sun. This happens quickly enough so that the field is essentially uniform by 5–10 R⊙, apparently remaining so as it is carried to beyond 1 AU. Here, we illustrate the coronal relaxation with a qualitative physical argument and by reference to a detailed MHD simulation.
Ring current ion flows and convection electric field as expected from observations by SAC‐B/ISENA
Authors: Milillo A. , Orsini S. , Daglis I. A. , Candidi M.
Published: February 2004

In the present paper, ENA energy spectra at the location at the SAC‐B satellite are simulated by using AMPTE/CCE/CHEM ion distributions as originating source, with the purpose of analysing the SACB/ISENA response at different magnetic local times and latitudes. The shape of the ENA spectra simulated along different lines of sight on the equatorial plane is analysed to show that an instantaneous estimate of the global convection electric field will be possible with the ISENA data every few minutes. This unprecedented time resolution for the measurement of such a crucial parameter of the Earths magnetosphere will allow testing of many theories, like tail convection, pressure balance, global circuits, ring‐current shielding, etc,
Non‐gravitational effects and LAGEOS' rotation
Authors: Vokrouhlický David
Published: February 2004

The space orientation of the LAser GEO‐dynamics Satellite (LAGEOS) spin axis evolves slowly in time due to a permanent influence of perturbing torques. We extend a theoretical model of this evolution by including two, previously neglected, effects: (i) a radiation torque due to an asymmetric reflectivity of the satellite surface, and (ii) a perturbing torque related to a possible miscentering between the mass and geometric centers of the satellite. We demonstrate that both effects will contribute importantly to the scenario of the LAGEOS spin axis evolution at the end of this decade, and could be detected by both direct measurements of the LAGEOS spin axis and orbit analysis.
Nitrous oxide emissions from the Arabian Sea
Authors: Bange Hermann W. , Rapsomanikis Spyridon , Andreae Meinrat O.
Published: February 2004

Dissolved and atmospheric nitrous oxide (N2O) were measured on the legs 3 and 5 of the R/V Meteor cruise 32 in the Arabian Sea. A cruise track along 65°E was followed during both the intermonsoon (May 1995) and the southwest (SW) monsoon (July/August 1995) periods. During the second leg the coastal and open ocean upwelling regions off the Arabian Peninsula were also investigated. Mean N2O saturations for the oceanic regions of the Arabian Sea were in the range of 99–103% during the intermonsoon and 103–230% during the SW monsoon. Computed annual emissions of 0.8–1.5 Tg N2O for the Arabian Sea are considerably higher than previous estimates, indicating that the role of upwelling regions, such as the Arabian Sea, may be more important than previously assumed in global budgets of oceanic N2O emissions.
Permeability reduction in a dilating rock: Network modeling of damage and tortuosity
Authors: Zhu Wenlu , Wong Teng‐Fong
Published: February 2004

There are fundamental differences between low‐ and high‐porosity rocks in the evolution of permeability in the brittle faulting regime. In a rock with porosity less than ∼5%, dilatancy and permeability enhancement were observed prior to the peak stress. In porous rocks, the behavior is more complex. Before the onset of dilatancy C', both permeability and porosity decrease with increasing effective mean stress. Beyond C', permeability may decrease while the pore space dilates. After the peak stress has been attained, the development of a relatively impermeable shear band caused an accelerated decrease of permeability. A network model (with pore space statistics constrained by microstructural data) was developed to simulate the interplay of pre‐existing tubular pores and stress‐induced cracks in influencing the permeability evolution. The simulations show that while the accumulation of damage in form of microcracks may dilate the pore space, it causes the flow path to be more tortuous and as a result, the permeability may actually decrease.
Detailed morphology of the TAG Active Hydrothermal Mound: Insights into its formation and growth
Authors: Humphris Susan E. , Kleinrock Martin C.
Published: February 2004

The detailed structure and morphology of the TAG active hydrothermal mound (Mid‐Atlantic Ridge) are investigated using near‐bottom, high‐resolution bathymetry data and digital photographic imagery. The mound comprises two distinct circular platforms with one superposed asymmetrically on the other. Areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales (years). We propose a model for the growth of the TAG active mound in which changes in the near‐surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed.
Loss of H and O from Mars: Implications for the planetary water inventory
Authors: Lammer Helmut , Stumptner Willibald , Bauer Siegfried J.
Published: February 2004

The evolution of the Mars atmosphere, with regard to water, is influenced by non‐thermal escape of oxygen atoms. In our evaluation of the various escape processes we recognized that the widely quoted escape rates for oxygen atoms originating from dissociative recombination (exospheric O) are too high by an order of magnitude. Thus the originally postulated coupling of H and O escape in the ratio of 2∶1 can no longer be maintained. This has important implications for the stability of the Martian CO2 atmosphere. This study takes into account temperature dependent photodissociation coefficients, optical depths, ozone chemistry, hydrogen and oxygen escape fluxes for conditions of an ancient dense Mars atmosphere. Since the only source of hydrogen is water, the calculated hydrogen escape flux represents an upper limit for an equivalent water loss to space. Under this constraint, we find much lower oxygen escape rates originating from water than current estimates, implying a loss of H2O to space corresponding to an equivalent depth ≤5 meter over the last 3.5 billion years. Our results cast serious doubt on previous estimates that as much as 50–80 meters of H2O were lost to space during this period.
The 14C “bomb spike” determines the age spread and age of CO2 in Law Dome firn and ice
Authors: Levchenko V. A. , Francey R. J. , Etheridge D. M. , Tuniz C. , Head J. , Morgan V. I. , Lawson E. , Jacobsen G.
Published: February 2004

We report a precise, model‐independent determination of the age and age spread of CO2 in air trapped in ice. A large pulse of atmospheric radiocarbon, generated in the atmosphere by nuclear tests, peaked in the early‐to‐mid 1960's. We measure the profile of the radiocarbon “bomb spike” in firn air and ice bubbles from high snow‐accumulation sites drilled in 1987 and 1993 on Law Dome, East Antarctica, by employing high precision AMS (Accelerator Mass Spectrometry). Large 14C atmospheric growth rates and a high signal‐to‐noise ratio lead to a direct and precise determination of the CO2 age and age‐spread in the ice. A least‐squares comparison with the atmospheric history gives a mean CO2 age of 8.9±0.5 years at the bottom of the firn (where vertical gas diffusion ceases) with an age spread in the ice (full width of a moving average smoothing window) of 12.5±1.5 years. These results confirm the possibility of examining decadal trace gas variations prior to direct instrumental measurements.
Strange attractor characteristics of earthquakes in Shillong Plateau and adjoining regions
Authors: Srivastava H. N. , Bhattacharya S. N. , Sinha Ray K. C.
Published: February 2004

Strange attractor dimensions were determined for two concentric areas of about 220 km (2°) and 440 km (4°) radii around WWSSN station at Shillong based on earthquake data during the period 1964 to 1992. Existence of deterministic chaos for both the areas was supported by the same fractal dimension of 6.1 and positive value of the largest Lyapunov exponent. The results have provided justification for delineating precursory seismicity patterns based on earthquakes up to epicentral distance of 440 km on different fault systems.

Conspicuous higher value of the strange attractor dimension in Hindukush, Himalaya and northeast India than that in Koyna region in the peninsular India suggests a new criteria for distinction between interplate and intraplate earthquakes. Differences with subduction zone in Pacific vis‐à‐vis stress drop in great earthquakes (M≥8) and implications of the chaotic approach in earthquake hazard analysis have also been brought out.
The potential impact of soot particles from aircraft exhaust on cirrus clouds
Authors: Jensen Eric J. , Toon Owen. B.
Published: February 2004

Homogeneous freezing of sulfate aerosols may dominate ice nucleation in cirrus, implying that large supersaturations are required for cirrus cloud initiation at low temperatures. However, insoluble particles from the surface or soot particles injected directly into the upper troposphere by jet aircraft may act as heterogeneous ice nuclei. If the soot particles are sufficiently effective ice nuclei, then they will allow ice nucleation at lower supersaturations than those required for homogeneous freezing, resulting in an increase in the areal coverage of cirrus clouds. Simulations using a detailed ice cloud model indicate that cirrus driven by slow, steady lifting (a few cm‐s−1) will be transient, precipitating clouds if only pure sulfate haze aerosols are present. However, if effective heterogeneous nuclei are present, then extensive, persistent, diffuse cirrus should form. In addition, heterogeneous ice nucleation on insoluble particles may modify the number of ice crystals nucleated in cirrus, resulting in alterations of the cloud evolution and radiative properties. Heterogeneous freezing on relatively few insoluble particles (Ninv <0.1 cm−3) should result in fewer ice crystals nucleating than if homogeneous freezing were to occur. However, if large numbers of insoluble particles are present, the ice crystal number density may be increased.
Bow shock ‘splitting’ in bi‐ion flows
Authors: Sauer Konrad , Dubinin Eduard , Baumgärtel Klaus , Bogdanov Alexander
Published: February 2004

This paper discusses modifications of a bow shock ahead of an obstacle in the solar wind (SW) which can occur when the flow consists of a proton plasma and a secondary ion populations. The secondary species may be composed of alpha particles, which are a natural part of the ambient SW, or of heavier particles which are picked up by the solar wind in source regions, such as at comets or Mars. By using a 2D collisionsless bi‐ion fluid model which treats protons and heavy ions as distinct and which assumes that the two fluids communicate with each other by means of electromagnetic forces only, it is shown that for high enough value of the heavy ion mass density a ‘splitting’ of the bow shock takes place. Downstream from the proton bow shock, where differential streaming between ion species arises, a second discontinuity is formed which resembles a shock‐like transition for the heavy ion flow. This plasma boundary, called the heavy‐ion discontinuity (HID), causes also a distinct deflection of the proton flow and significant magnetic field variation. The results seem to be of importance for different types of SW obstacles, especially for planetary objects where massloading of the SW plays a dominant role in bow shock formation, as at comets and probably at Mars. It is suggested that the ‘massloading boundary (MLB)' found in the magnetosheath of Mars and the ‘mysterious boundary’ detected in the cometosheath of Halley and Grigg‐Skejllerup are HID's of the described nature.
Helium‐3 emission in and around Teide Volcano, Tenerife, Canary Islands, Spain
Authors: Pérez Nemesio M. , Nakai Shun'ichi , Wakita Hiroshi , Hernández Pedro A. , Salazar José M.
Published: February 2004

Air corrected ³He/4He ratios of fluid samples collected in and around Teide volcano, Tenerife, Canary Islands, ranged from 5.97 to 8.06 Ra. Helium‐3 emission spatial distribution has a quite different pattern from other stratovolcanoes which show commonly a decreased trend for the ³He/4He ratio with respect to distance from the summit crater. This finding might be related to the volcano‐tectonic setting differences between subduction‐type and oceanic island‐type statrovolcanoes. Radiogenic helium production is a significant process for the dilution of uprising derived‐mantle fluids in and around subduction‐type stratovolcanoes while it can be almost negligible for polygenetic volcanoes at oceanic islands.
The reactions of ozone with alkenes: An important source of HOx in the boundary layer
Authors: Paulson Suzanne E. , Orlando John J.
Published: February 2004

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals. Organic peroxy radicals (RO2) probably accompany the production of OH. In this paper, we draw attention to the potential importance of these reactions in the primary production of HOx (HOx = OH, HO2 and RO2) radicals in various regions of the boundary layer. The reactions of ozone with anthropogenic alkenes are shown to be the most important source of HOx in many urban settings during the day and evening, and a significant source at night. The majority of HOx comes from trace quantities of alkenes with internal double bonds. Reaction of O3 with isoprene and terpenes can be an important source of HOx in forested regions; we show that these reactions are the dominant radical source in the late afternoon and into the night. This additional HOx source is expected to increase predicted OH concentrations compared to those calculated by models that do not include the O3‐alkene source.
An assessment of the accuracy of 14.5 years of Nimbus 7 TOMS version 7 ozone data by comparison with the Dobson network
Authors: McPeters R. D. , Labow G. J.
Published: February 2004

A Version 7 algorithm and calibration have been applied to the 14.5 year Nimbus 7 TOMS ozone record (1978–1993). The ozone retrieval algorithm has been significantly improved for cloudy conditions and for high solar zenith angles, and the radiative transfer used in the algorithm is more accurate. New calibration techniques have been used that produce a very stable data set even after 1990 when TOMS degradation became significant. TOMS ozone now agrees with average ozone from an ensemble of 30 northern hemisphere ground stations (Dobsons and Brewers) to within ±1% throughout most of the 14.5 year record. The time‐dependent drift relative to Dobson is 0.29% per decade through the end of the data record. There is almost no solar zenith angle dependence in the comparison for angles below about 80°, but data should be used with caution for larger solar zenith angles. There is also a residual total ozone dependence in the TOMS‐Dobson difference, of about 1% per 100 DU.
Effect of shot interval on ocean bottom seismograph and hydrophone data
Authors: Christeson G. L. , Nakamura Y. , McIntosh K. D. , Stoffa P. L.
Published: February 2004

Data collected by 18 ocean bottom receivers for a seismic line shot at both 50‐m (∼24 s shot interval) and 125‐m (∼58 s shot interval) shot spacing provide a direct field comparison of the effect of shot interval on marine wide‐angle seismic data. Our results indicate that both shot spacings produce high‐quality refraction data in shallow water (<1000 m) on hydrophone and vertical channel data. In deeper water, the data quality of the 50‐m line is adequate for the vertical channel, but it is often poor at large offsets for the hydrophone channel in comparison to the 125‐m shot spacing data. A theoretical model to explain these observations provides further information useful for designing an experiment using ocean‐bottom receivers.
The symmetry of garnets on the pyrope (Mg3Al2Si3O12)‐majorite (MgSiO3) join
Authors: Parise John B. , Wang Yanbin , Gwanmesia Gabriel D. , Zhang Jianzhong , Sinelnikov Yegor , Chmielowski Josef , Weidner Donald J. , Liebermann Robert C.
Published: February 2004

Garnets with compositions between majorite and pyrope, Mj38, Mj48, Mj75 and Mj79 were synthesized at high pressures and temperatures in a 2000‐ton uniaxial split‐sphere apparatus (USSA‐2000) and investigated using high resolution synchrotron X‐ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj75. The cubic‐tetragonal structural phase transition in garnet along the majorite‐pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earth's mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.
Intense bursts of electron cyclotron harmonic waves near the dayside magnetopause observed by GEOTAIL
Authors: Matsumoto Hiroshi , Usui Hideyuki
Published: February 2004

We report GEOTAIL/PWI observations of intense bursts of electron cyclotron harmonic (ECH) waves detected at the dayside equatorial magnetopause region on October 17, 1992. Along a magnetopause skimming orbit of the GEOTAIL spacecraft, the intense ECH emissions were observed intermittently and sporadically in time, superimposed on weak diffuse ECH waves. On account of the spectral shape of the dynamic spectra, we name this burst of ECH waves “Totem Pole” emission (TP emission). The duration time of the TP emission is less than a few minutes and its maximum electric field intensity is approximately
. Although not always observed, the TP emission has harmonic components even above the local plasma frequency. This type of emission is often observed along the magnetopause crossing orbits. It is also found that the TP emission often has two discrete spectral components in the first harmonic band between fce and 2fce, where fce denotes the electron cyclotron frequency.
Energetic neutral atoms in the outer magnetosphere: An upper flux limit obtained with the HEP‐LD Spectrometer on board GEOTAIL
Authors: Wilken B. , Daglis I. A. , Milillo A. , Orsini S. , Doke T. , Livi S. , Ullaland S.
Published: February 2004

The energetic particle spectrometer HEP‐LD on board the Japanese mission GEOTAIL is the first instrument in the energy range above 77 keV which can discriminate between energetic neutral atoms (ENA) and charged particles. The principle for particle identification is based on a time‐of‐flight (T) and energy (E) measurement which uniquely determines the particle species. A rather compact and highly anisotropic active collimation system in front of the (E,T)‐detector head acts as an effective charged particle rejector (CPR) in the ENA mode. The resulting high‐pass filter function for charged particles allows the detection of ENAs with energies below 200 keV. This report concentrates on ENA observations in the outer magnetosphere on the dayside (GSE x=−3, y=+12, z=+1 RE) and establishes an upper limit for the ENA flux in quiet conditions. The measurements agree reasonable well with predictions, however, the statistical uncertainties are considerable.
A general circulation model simulation of the Martian polar warming
Authors: Wilson R. John
Published: February 2004

This paper reports on a successful general circulation model simulation of a rapid warming phenomenon observed in the Martian winter polar atmosphere during global dust storm conditions. The model includes a self‐consistent simulation of the dust distribution which is forced with a prescribed surface source. The warming is shown to be largely a response to the development of a pole‐to‐pole solstitial Hadley circulation resulting from the greatly increased dust loading. A crucial aspect of the simulation is a sufficiently deep computational domain that allows for the full development of this circulation. These simulations indicate that the thermal tides play a contributing role by providing zonal momentum flux divergence at the winter pole and by the advection of aerosol.
Laboratory reproduction of arecibo experimental results: HF wave‐enhanced Langmuir waves
Authors: Lee M. C. , Riddolls R. J. , Vilece K. D. , Dalrymple N. E. , Rowlands M. J. , Moriarty D. T. , Groves K. M. , Sulzer M. P. , Kuo S. P.
Published: February 2004

Laboratory experiments at MIT using the Versatile Toroidal Facility (VTF) have produced “cascading” and “frequency‐upshifted” spectra of HF wave‐enhanced Langmuir waves resembling the spectra observed in Arecibo experiments. The VTF experiments are well‐explained using the source mechanism proposed by Kuo and Lee [1992] to interpret observed Langmuir wave spectra at Arecibo, Puerto Rico. This mechanism is referred to as a nonlinear scattering of parametric decay instability (PDI)‐excited Langmuir waves by “pre‐existing” lower hybrid waves to preferentially produce anti‐Stokes (i.e., frequency‐upshifted) Langmuir waves. Recent radar spectral observations of anti‐Stokes Langmuir waves at Arecibo with improved range and time resolution [Sulzer and Fejer, 1994] can be reasonably understood in terms of this mechanism.
New oceanographic data from beneath Ronne Ice Shelf, Antarctica
Authors: Nicholls K. W. , Makinson K. , Johnson M. R.
Published: February 2004

Oceanographic data have been obtained via an access hole made through Ronne Ice Shelf. The site, which is the third in a series of similar studies, lies 17 km west of Korff Ice Rise where 825 m of ice overlies a 485‐m deep water column. Measurements included conductivity and temperature profiles, and an instrument mooring was deployed for long‐term measurements of currents, temperature and conductivity. At the sea floor there was a 150‐m layer of well‐mixed water with a potential temperature and salinity of −1.97°C and 34.72. The water cooled and freshened towards the ice‐shelf base, ultimately reaching −2.41°C and 34.51. The hydrographic and water current data imply a flow into the deepest part of the sub‐ice shelf cavity of about 200,000 m³ s−1 of the deeper, relatively warm water, which would be able to power an average basal melt rate of 0.2 m a−1 for the western portion of Ronne Ice Shelf.
Hyperproductivity of the Ross Sea (Antarctica) polynya during austral spring
Authors: Smith Walker O. , Gordon Louis I.
Published: February 2004

Although satellite data of surface layer pigments have suggested that the daily productivity in the Ross Sea is among the largest found in any marine system, no modern oceanographic cruise has entered the Ross Sea polynya to quantitatively assess the austral spring productivity over time scales of days to weeks. We conducted a cruise to the Ross Sea polynya in November–December, 1994 to measure the contribution of phytoplankton during the austral spring to the annual productivity of the region and found markedly enhanced levels of phytoplankton biomass. Chlorophyll concentrations were greater than 3 µg l−1 in mid‐November, a time when the polynya was covered by a thin (ca. 20–30 cm) layer of ice. Particulate matter concentrations increased through time, and by early December chlorophyll and particulate carbon concentrations exceeded 10 µg l−1 and 53 µmol l−1, respectively. Primary productivity also increased through time: the mean productivity in early December equaled 3.53 g C m−2 d−1, and maximum measured rates exceeded 6 g C m−2 d−1. Productivity based on nitrate disappearance averaged 1.52 g C m−2 d−1 (with a maximum rate of 2.49 g C m−2 d−1), suggesting that the bloom's new production was also substantial. The Ross Sea polynya is the most southerly location in the Antarctic where phytoplankton growth is initiated this early and which supports such high standing stocks by early December. Inclusion of this production in a carbon budget for the region suggests that this area supports an annual production of 200 g C m−2, the largest of any region in the Southern Ocean, and confirms its hyperproductive nature.
Mountain uplift and surface temperature changes
Authors: Kitoh Akio
Published: February 2004

Mountain uplift significantly affects both sea surface temperature (SST) and land surface temperature. This was studied using a coupled atmosphere/mixed layer ocean model with and without mountains. The global mean SST dropped 1.4°C with mountain uplift, mainly due to increased lower tropospheric clouds in the subtropical eastern Pacific. Increased clouds hinder solar radiation into the ocean and lower SST. The increased frequency of stratus incidence is related to subtropical anticyclones intensified by a strong temperature contrast between the continent and ocean in mountainous regions. Land surface temperature drops due to the lapse‐rate effect. When this effect is eliminated, the continental interior does not become as cool with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport. Southern Asia becomes cooler because monsoon‐induced precipitation wets the ground and increases clouds. Our result showed greater northern high latitude temperature changes than the previous study, indicating the importance of cloud‐related feedback in paleoclimate modeling.
Preliminary results from a field investigation of aeolian sand transport using high resolution wind and transport measurements
Authors: Jackson Derek W. T. , McCloskey John
Published: February 2004

Preliminary results are presented from two field experiments which employed a new high‐sampling‐frequency aeolian sand trap together with a single anemometer and wind vane. This system allows simultaneous assessment of wind velocity and sand transport rate every second. Results indicate an almost instantaneous response of transport to wind accelerations which is modified by a short (<6s) memory. The wind velocity/transport relationship is found to be well constrained by a second order polynomial in velocity with a threshold. Transport predictions based on this equation give a high correlation with observation; discrepancies possibly being due to differences in unquantified environmental variables between the two experiments. It is suggested that this approach could be improved to include the effect of other environmental variables, and that useful predictions of transport rate might then be made from existing meteorological databases.
Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S
Authors: Grevemeyer Ingo , Weigel Wilfried
Published: February 2004

One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [Houtz and Ewing,1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45–50% within only 0.5–1 m.y., double within 5 m.y. or less and remain constant thereafter.
Models of Hawaiian volcano growth and plume structure: Implications of results from the Hawaii Scientific Drilling Project
Authors: DePaolo D. J. , Stolper E. M.
Published: February 2004

The shapes of typical Hawaiian volcanoes are simply parameterized, and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate. The dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the Hawaiian plume, with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit. The timescale of volcano growth is t = 2R/νplate where R is the radius of the melting zone of the (circular) plume and νplate is the velocity of the Pacific plate. The growth progress of a volcano can be described by a dimensionless time t′ = tνplate/2R, where t′ = 0 is chosen to be the start of volcano growth and t′ = 1 approximates the end of “shield” growth. Using a melt generation rate for the whole plume of 0.2 km3/yr, a plume diameter of 50 km, and a plate velocity of 10 cm/yr, we calculate that the lifetime of a typical volcano is 1000 kyr. For a volcano that traverses the axis of the plume, the “standard” dimensions are a volume of 57,000 km3, a summit thickness of 18 km, a summit elevation of 3.6 km, and a basal radius of 60 km. The volcano first breaches the sea surface at t′ ≈ 0.22 when it has attained only 5% of its eventual volume; 80% of the volume accumulates between t′ = 0.3 and t′ = 0.7. Typical lava accumulation rates start out over 50 m/kyr in the earliest stages of growth from the seafloor, and level out at ∼35 m/kyr from t′ ≈ 0.05 until t′ = 0.4. From t′ = 0.4 to t′ = 0.9, the submarine lava accumulation rates decrease almost linearly from 35 m/kyr to ∼0; subaerial accumulation rates are about 30% lower. The lava accumulation rate is a good indicator of volcano age. A volcano that passes over the plume at a distance 0.4R off to the side of the plume axis is predicted to have a volume of about 60% of the standard volcano, a lifetime about 8% shorter, and lava accumulation rates about 15–20% smaller. The depth‐age data for Mauna Kea lavas cored by the Hawaii Scientific Drilling Project are a good fit to the model parameters used, given that Mauna Kea appears to have crossed the plume about 15–20 km off‐axis. The lifetime of Mauna Kea is estimated to be 920 kyr. Mauna Loa is predicted to be at a stage corresponding to t′ ≈ 0.8, Kilauea is at t′ ≈ 0.6, and Loihi is at t′ ≈0.16. The model also allows the subsurface structure of the volcanoes (the interfaces between lavas from different volcanoes) to be modeled. Radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross‐sectional area of the plume; this inference is qualitatively born out by available isotopic data. The model predicts that new Hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano's flank.
High‐resolution geochemical stratigraphy of Mauna Kea flows from the Hawaii Scientific Drilling Project core
Authors: Albarède Francis
Published: February 2004

Trace elements have been measured in the deepest 101 lava flows from the Hawaii Scientific Drilling Project (HSDP) by inductively coupled plasma mass spectrometry. In order to compensate for the lack of a precise timescale for the HSDP samples and in spite of some probable long stratigraphic hiatuses, a constant eruption rate is assumed that provides a timescale for the geochemical dynamics of Mauna Kea. Periods of nearly continuous variations in incompatible element ratios suggest that geochemical variability is controlled by reservoirs larger the shallow magma chambers inferred from seismicity and deflation data. The principal component analysis of the differenced data clearly separates three components associated with (1) olivine + spinel phenocryst abundance, (2) weathering, and (3) melting conditions and source mineralogy. The picritic character of some lavas does not seem associated with a particular mantle source or melting process. The weathering component appears to discriminate lateral from vertical transfer. A recurrence of the melting conditions or source heterogeneities on a scale time of ∼30 kyr is inferred from the segmented record of incompatible element distributions.
Phase transitions in one‐dimensional steady state hydrothermal flows
Authors: Young R. M.
Published: February 2004

The paper sets out necessary conditions for the existence of a phase transition in a steady state one‐dimensional vertical hydrothermal flow. The entropy inequality (reflecting the presence of capillarity) and a thermodynamical inequality at the phase boundary are combined to delineate the region of permissible flow states in the mass‐energy flow plane. In particular, it is shown that for small mass flow/permeability ratios the configuration consisting of single‐phase liquid lying over a vapor‐dominated two‐phase fluid is not stable. These and other results are discussed in the context of several field examples, including Kawah Kamojang (Indonesia) and Wairakei (New Zealand).
Global surface wave phase velocity variations
Authors: Zhang Yu‐Shen , Lay Thorne
Published: February 2004

Global Love and Rayleigh wave phase velocity variations from 85 to 250 s are determined using about 30,000 seismograms from earthquakes with M ≥ 6.0. The phases used are G1, R1, G2, and R2 arrivals. The data were obtained from the Chinese Digital Seismic Network, GEOSCOPE, and Global Seismic Network archives for the time period January 1980 to June 1992. All seismograms underwent careful quality control in the time and frequency domains. We consider the inversion damping and model parameterization issues, examining the data statistics and assuming that errors in data and model spaces are Gaussian random distributions with zero mean. It is not stable to invert for a model with resolution higher than about spherical harmonic degree, l = 22, given the path sampling and scatter of our data set, and the low frequency data only warrant expansion up to about l = 12. We find that a block model parameterization, with its local basis functions, has fewer artifacts than a high order spherical harmonic inversion which must be heavily damped. However, spherical harmonic functions provide intrinsically superior recovery of the long‐wavelength structure. Our final models involve a hybrid parameterization, in which an initial iteration retrieves the low order spherical harmonic components that are used as an aspherical reference model for performing a final block model inversion. The optimal block size decreases with frequency. This approach exploits the separate attributes of global and local basis functions for resolving different components of the heterogeneity, allows for variable resolution for different frequencies, and provides a natural framework for embedding localized high resolution regional inversions into the global model. We perform inversions both with and without corrections for shallow crustal structure. At the shorter periods the phase velocity maps strongly reflect the surface tectonics, correlating with continental shields, mid‐ocean ridges, and tectonically active areas. The tectonic correlation diminishes gradually with period.
A seismodynamical model of lithosphere deformation: Development of continental and oceanic rift networks
Authors: Heimpel M. , Olson Peter
Published: February 2004

Seismicity and lithosphere extension on the scale of a tectonic plate are investigated in two horizontal dimensions and time using a dynamical model for a thin plate overlying a viscoelastic channel. The plate consists of two layers, a brittle elastic‐plastic crust that deforms as a result of earthquakes along faults and a strong viscoelastic layer representing the aseismic, creeping lower lithosphere. Numerical experiments of lithosphere rifting are performed for one‐ and two‐layer lithosphere models using two different rupture criteria. In all the calculations, earthquakes and faults of various sizes may form anywhere in the numerical domain as a result of the constitutive formulation. The differences between the one‐ and two‐layer model results suggest a mechanical explanation for the difference between continental and oceanic rifting. When the lower lithosphere is absent, a small number of faults dominate the deformation, and a relatively simple, narrow rift zone develops corresponding to oceanic rifting. When a highly viscous lower lithosphere is included, a complex, distributed network of faults develops, similar to continental rifting. The complexity of the fault system also depends on the yield criterion in the crustal layer. In our simulations, complexity is greater for a yield criterion with strong shear stress dependence than for one with mainly extensional stress dependence. Whether a result of lower lithosphere stretching, crustal heterogeneity, or the yield criterion, higher complexity increases the timescale for fault development, increases self‐similarity in fault system geometry and stress and strain time series, and results in power law moment‐frequency earthquake size scaling.
Numerical modeling of subglacial sediment deformation: Implications for the behavior of the Lake Michigan Lobe, Laurentide Ice Sheet
Authors: Jenson John W. , MacAyeal Douglas R. , Clark Peter U. , Ho Carlton L. , Vela Julio C.
Published: February 2004

We apply a numerical model of the late Wisconsin (circa 20,000 years B.P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate how fine‐grained subglacial sediment might influence lobe behavior, particularly rapid millennial‐scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (“hard bedded”) basal boundary condition. In areas overlain by fine‐grained sediment (“soft bedded”), the base of the ice is coupled to a deformable sediment layer, using a rate‐dependent stress‐strain law. Geotechnical tests of clay‐rich till deposited by the LML provide control for sediment rheologic parameters. Simulated cross‐sectional profiles are consistent with reconstructions from geologic evidence. Time‐dependent simulations suggest that a soft‐bedded lobe could have reached steady state in about 20,000 years or less, in contrast to 50,000 to 60,000 years for an otherwise identical hard‐bedded lobe. A soft‐bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial‐scale shifts in accumulation or ablation as a nonsliding hard‐bedded lobe, but in both cases the response is slower than that indicated by the geologic record. Results suggest that while strong millennial‐scale changes in accumulation and ablation can produce responses in hard‐bedded or soft‐bedded ice that are consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for millennial‐ and submillennial‐scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior.
Electrical conductivity and Paleo‐Proterozoic foredeeps
Authors: Boerner D. E. , Kurtz R. D. , Craven J. A.
Published: February 2004

Sedimentary facies associated with Paleo‐Proterozoic foredeeps are apparently highly conductive. The conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies. The shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt. Carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400°C. Although foredeep facies develop diachronously, the most conductive unit is usually found in the core of the fold and thrust belt, suggesting that deformation may be essential for the complete graphitization of the carbon. The foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in Laurentia, and possibly throughout the world. The euxinic facies represent highly specific, tectonic marker horizons that are readily delineated by using electromagnetic methods.
Gravity anomalies, crustal thickness, and the pattern of mantle flow at the fast spreading East Pacific Rise, 9°–10°N: Evidence for three‐dimensional upwelling
Authors: Wang Xuejin , Cochran James R. , Barth Ginger A.
Published: February 2004

Gravity, bathymetry, and crustal structure data from the 9° to 10°N segment of the fast spreading northern East Pacific Rise are used to investigate density variations in the mantle in order to determine the pattern of mantle upwelling and melt distribution beneath the segment. Mantle Bouguer gravity anomaly contours are nearly parallel to the ridge axis, and there is only 3–4 mGal variation in axial mantle Bouguer anomalies along the entire segment. However, inclusion of the gravity effects of variations in crustal thickness and structure results in a significant change in the pattern of gravity anomalies. The “subcrustal” gravity anomalies show a very distinct gravity low centered over the segment axial depth minimum near 9°50′N. Axial subcrustal gravity anomalies increase by ∼10 mGal between 9°50′N and 9°23′N, a gradient of 0.2 mGal/km. Since the ridge axis has a uniform cross‐sectional shape throughout the entire segment with abundant evidence of magmatic activity along its entire length, it is unlikely that there are significant variations in crustal thermal structure other than at the ridge tips. We thus attribute the subcrustal gravity anomalies to density variations within the mantle. The distinct gravity low centered over the depth minimum provides qualitative evidence that mantle upwelling is focused or enhanced at that location. The effects of lithospheric cooling and subsidence away from the axis were removed from the bathymetry and free‐air gravity anomalies to isolate residual gravity and gravity anomalies associated with the axis. The residual anomalies were modeled by considering the axial bathymetric high to be a flexural feature resulting from the upward buoyant load of a region of low‐density material beneath the axis. After accounting for the isostatic effects of variations in crustal thickness, the remaining mass deficiency necessary to support the axial high and satisfy the gravity anomalies is not evenly distributed along the axis but is 35% greater beneath the axial depth minimum at 9°50′N than between 9°17′ and 9°27′N. Mantle upwelling in this segment thus appears focused or enhanced in the area of the depth minimum.
Paleomagnetism of Paleozoic strata, Brabant and Ardennes Massifs, Belgium: Implications of prefolding and postfolding Late Carboniferous secondary magnetizations for European apparent polar wander
Authors: Garza Roberto S. Molina , Zijderveld J. D. A.
Published: February 2004

Paleozoic carbonate rocks of the Brabant Massif, Namur Basin, and Dinant Synclinorium, Belgium, are characterized by two secondary Late Carboniferous magnetizations of uniform reverse polarity: a postfolding low unblocking temperature (300–375°C) component (LT) and a prefolding high unblocking temperature (400–550°C) component (HT). In the Namur Basin, LT magnetizations postdate late Westphalian deformation (∼305 Ma); in the Dinant Synclinorium, HT magnetizations predate early Westphalian deformation (∼316 Ma). LT magnetizations for the combined Brabant Massif and Namur Basin sites give a south paleopole at 42.3°S, 339.6°E (dp = 1.9°, dm = 3.8°), consistent with the European cratonic reference. The LT magnetizations in the Dinant Synclinorium give a paleopole at 36.2°S 321.0°E (dp = 2.4°, dm = 4.7°). The HT magnetization gives paleopoles at 34.1°S, 333.4°E (dp = 2.6°, dm = 5.1°; Brabant and Namur) and at 25.9°S, 321.5°E (dp = 3.5°, dm = 6.8°;, Dinant Synclinorium). Declinations indicate ∼15° clockwise rotation of the Dinant Synclinorium with respect to the Brabant Massif after acquisition of the LT magnetization. Devonian red beds yield near univectorial apparent synfolding magnetizations of distributed unblocking temperature that we interpret as artifacts of the remanence acquisition or deformation processes. The HT paleopole for the Brabant Massif is indistinguishable from other early Late Carboniferous European paleopoles and is similar to poles interpreted to be derived from remagnetizations in the British Isles. Age constraints on the time of remanence acquisition of secondary magetizations allow a refined calibration of the European apparent polar wander path. Inclinations of the LT and HT components indicate about 6–8° (∼700 km) northward motion during the Late Carboniferous.
Detection of transient motions with the Global Positioning System
Authors: Elósegui P. , Davis J. L. , Johansson J. M. , Shapiro I. I.
Published: February 2004

To assess the capability of Global Positioning System (GPS) phase measurements for the determination of transient velocity, we have made measurements with a GPS antenna on a moving platform. The antenna was translated in the horizontal plane at a constant velocity of 1 mm h−1for a period of somewhat more than 24 hours while GPS data were recorded simultaneously. Other stationary antennas at distances of 10 m to ∼1000 km were also simultaneously recording GPS data. We calculated an average velocity of the moving antenna by modeling its time‐dependent position as a random walk and fitting a straight line to the stochastic estimates. We have found that the accuracy of the resulting velocity estimates is dependent on the observing period and the baseline length. For 24‐hour data time spans, rms horizontal velocity errors were less than 0.2 mm h−1for all baseline lengths; for similar time spans, rms vertical velocity errors were 0.3–0.9 mm for lengths between 100 and 1000 km, and ≲0.2 mm for baselines ≤1000 m. We found it convenient to define a quantity ξ, which we term the dynamic resolution, equal to the ratio of the rms velocity variation to the mean velocity. For a random walk process, ξrwcan be used to calculate the variance per unit time σ2rwrequired by filter‐based analysis software. We also investigated the power spectral density (PSD) of our estimates of time‐dependent position and found that for the frequency range sampled (0.07–16 mHz), the PSD could be well modeled by να, where ν is the frequency and the spectral index α depends on the value of ξ. For strongly constrained (yet unbiased) estimates (obtained by choosing ξrw= 10 and σrw= 0.05 mm h−½), the resultant value for α is −4, indicating a strong filtering of high‐frequency noise.