bluuebunny/crossref_metadata_2025 huggingface dataset:
An unconventional approach to modeling spatial and temporal variability of local shear stress in stream segments
Authors: Lamouroux Nicolas , Statzner Bernhard , Fuchs Ulrike , Kohmann Fritz , Schmedtje Ursula
Published: February 2004

Measuring and especially modeling the spatial and temporal variability of local bottom shear stress in streams are major issues in concepts of hydraulic engineering and cotic ecology and becomes very complicated with increasing channel complexity and/or decreasing channel size. Using Fliesswasserstammtisch (FST) hemispheres, an unconventional method for near‐bottom flow measurements, we determined the parameters of shear stress frequency distributions of German mountainous streams by a maximum likelihood procedure. The resulting model is robust and generates information on the spatial and temporal variability of local shear stress in stream segments from easily obtained input variables (discharge, mean stream width and depth).
Spatial analysis of soil hydraulic conductivity in a tropical rain forest catchment
Authors: Elsenbeer Helmut , Cassel Keith , Castro Jorge
Published: February 2004

The topography of first‐order catchments in a region of western Amazonia was found to exhibit distinctive, recurrent features: a steep, straight lower side slope, a flat or nearly flat terrace at an intermediate elevation between valley floor and interfluve, and an upper side slope connecting interfluve and intermediate terrace. A detailed survey of soil‐saturated hydraulic conductivity (Ksat)‐depth relationships, involving 740 undisturbed soil cores, was conducted in a 0.75‐ha first‐order catchment. The sampling approach was stratified with respect to the above slope units. Exploratory data analysis suggested fourth‐root transformation of batches from the 0–0.1 m depth interval, log transformation of batches from the subsequent 0.1 m depth increments, and the use of robust estimators of location and scale. The Ksat of the steep lower side slope decreased from 46 to 0.1 mm/h over the overall sampling depth of 0.4 m. The corresponding decrease was from 46 to 0.1 mm/h on the intermediate terrace, from 335 to 0.01 mm/h on the upper side slope, and from 550 to 0.015 mm/h on the interfluve. A depthwise comparison of these slope units led to the formulation of several hypotheses concerning the link between Ksat and topography.
Spatial patterns of throughfall and soil water dynamics in a Douglas fir stand
Authors: Bouten W. , Heimovaara T. J. , Tiktak A.
Published: February 2004

Measured spatial patterns of water uptake were found to be related to measured throughfall patterns around trees, especially in dry months. To simulate these lateral feedback mechanisms, the one‐dimensional soil water model SWIF was modified to a quasi‐three‐dimensional model allowing preferential water uptake from wet sites. Input‐output relations, linking soil physical input parameters to simulation results, showed that soil water contents at field capacity and those after a dry period in summer could be used to parametrize the soil physical characteristics. To assess parameter values for the 2.5‐ha research area, soil water contents were mapped using data from vertically installed time domain reflectometry sensors. Model results indicated that the spatial distributions of yearly water uptake and percolation fluxes were strongly affected by throughfall patterns, whereas soil water contents primarily depended on soil physical properties. Results also indicated that refined model parametrization can improve the reliability of model predictions of soil water dynamics at specific sites.
Decision making with imprecise probabilities: Dempster‐Shafer Theory and application
Authors: Caselton William F. , Luo Wuben
Published: February 2004

The information and data used to support decision making under uncertainty in water resources situations can often be characterized as being very limited or weak. The representation of knowledge in conventional Bayesian decision analysis is in the form of precisely specified distributions and is the same no matter how weak the information source for this knowledge. A Bayesian analysis therefore may inadvertently impart too much precision to the input information and to the results. The concept of imprecise probability addresses this problem of excessive precision and a number of methods incorporating this concept have emerged. One such method, developed by Dempster and Shafer, accommodates greater imprecision by allowing the specification of probabilities on intervals. Theoretical aspects of the Dempster‐Shafer methodology and its application to inference and decision analysis are described. A water resources example of an application of the Dempster‐Shafer approach is presented, and the results contrasted with those obtained from the closest equivalent Bayesian scheme.
Redfield ratios of remineralization determined by nutrient data analysis
Authors: Anderson Laurence A. , Sarmiento Jorge L.
Published: February 2004

A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of “preformed” concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time‐mean, basin‐wide Redfield ratios calculated. It is found that the P/N/Corg/−O2 ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitrification) are approximately constant with depth and basin, at a value of 1/16 ± 1/117 ± 14/170 ± 10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast‐sinking matter produced by sporadic, high‐productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 ± 2 between 1000 and 3000 m. In the Indian and Pacific basins the Corg/Cinorg regeneration ratio decreases from approximately 7 ± 3 at 400 m to 3 ± 1 at 1000 m and to 1 ± 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in the Indian and Pacific oceans.
The variation of available chlorine, Cly, in the Arctic Polar Vortex during EASOE
Authors: Schmidt Ulrich , Bauer Reimar , Engel Andreas , Borchers Reinhard , Lee Julia
Published: February 2004

Whole air samplers were flown on large balloons that were launched from Kiruna (67.9°N, 21.1°E) in the period from 30 November 1991 until 20 March 1992 during EASOE. Thirteen vertical profiles of the CCl2F2 mixing ratio were obtained from the analyses of stratospheric air samples that were collected at altitudes between about 10 and 31 km. An empirical correlation function derived from simultaneous profile observations of CCl2F2 and the five other most abundant organic chlorine species was used to determine the vertical distribution of total organic chlorine (Σ Clorg) for each flight. These profiles allowed to estimate vertical profiles of the mixing ratio of total available chlorine, Cly, and the temporal variation of its column abundance.
Column measurements of stratospheric trace species over Åre, Sweden in the winter of 1991–1992
Authors: Bell W. , Martin N. A. , Gardiner T. D. , Swann N. R. , Woods P. T. , Fogal P. F. , Waters J. W.
Published: February 2004

Total vertical column amounts of stratospheric HCl, HF, ClONO2 and HNO3 are reported from high resolution infrared solar absorption spectra taken during the European Arctic Stratospheric Ozone Experiment in the winter of 1991–1992. These ground based measurements were made near Åre in North Sweden (63.4°N, 13.1°E) at an altitude of 800 m using a Fourier transform spectrometer and tunable diode laser heterodyne spectrometer. On 9th January 1992 the HCl vertical column dropped to 1.2 × 1015 molecules cm−2 from a November average of 3.6 × 1015 molecules cm−2. The HCl drop occurred at a time when Åre was below the polar vortex and the lower stratospheric ClO column above Åre was elevated to ∼2 × 1015 molecules cm−2 as measured by the Microwave Limb Sounder experiment on the Upper Atmosphere Research Satellite. These measurements indicate conversion of lower stratospheric chlorine from reservoir to chemically active forms.
Variability of solar UV‐B radiation at high and middle latitudes during EASOE 1991/92
Authors: Bais A. F. , Zerefos C. S. , Meleti C. , Ziomas I. C. , Tourpali K. , Karaouza V. , Balis D.
Published: February 2004

Measurements of ultraviolet solar spectral irradiance from ground‐based observations made at Reykjavik (64°N) and Thessaloniki (40°N) are used to calculate erythemally‐weighted UV‐B dose during EASOE. It is shown that the largest values of UV‐B dose observed were about double those expected with an unperturbed ozone amount under cloudy conditions. At middle latitudes UV‐B doses were within expectation throughout the period of the campaign.
First composition measurements of stratospheric negative ions and inferred gaseous sulfuric acid in the winter Arctic vortex: Implications for aerosols and hydroxyl radical formation
Authors: Krieger A. , Arnold F.
Published: February 2004

Negative ion composition measurements and inferred gaseous sulfuric acid concentrations were for the first time obtained in the winter arctic vortex. The observations were made by a balloon‐borne quadrupole mass spectrometer, between 24 and 30 km altitude, on 18 January 1992. The H2SO4 data provide strong evidence for an additional OH source, other than photochemical OH‐production from O(¹D), operative in the winter arctic stratosphere. The additional OH source might be due to ion‐molecule reactions involving ambient ions, formed by galactic cosmic rays. This mechanism seems to explain the presence of gaseous sulfuric acid in the winter arctic vortex and may even influence H2SO4 abundances at midlatitudes during nighttime.
Obliquity‐oblateness feedback: Are climatically sensitive values of obliquity dynamically unstable?
Authors: Bills Bruce G.
Published: February 2004

A new model is presented for feedback between rotational and climatic variations, operative on time scales of 104–107 years. Due to the combined effect of planetary perturbations to the Earth's orbit plane and luni‐solar torques on the oblate figure of the Earth, the obliquity varies by ∼1° on a 4·104 year time scale. Associated changes in the seasonal and latitudinal pattern of incident solar radiation cause major glaciations. Mass transport from the oceans to the polar ice sheets during these glaciations can change the gravitational oblateness of the Earth by amounts approaching 1%. As the rate of spin axis precession is directly proportional to the oblateness, the climatically forced mass transport can by dynamically significant. A simple parameterization of us coupled orbital‐rotational‐climatic system suggests that there is a strong tendency for the system to evolve away from climatically sensitive values of the obliquity. This may explain the mid‐Pleistocene transition from an obliquity dominated regime to the present regime in which most climatic variability is concentrated at longer (105 year) periods.
Aircraft measurements of CLO and HCL during EASOE 1991/92
Authors: Crewell S. , Künzi K. , Nett H. , Wehr T. , Hartogh P.
Published: February 2004

As part of the European Arctic Stratospheric Ozone Experiment (EASOE), performed in the winter period 1991/92, stratospheric chlorine monoxide (ClO) and hydrochloric acid (HCl) have been observed using the Submillimeter Atmospheric Sounder (SUMAS). The instrument measures the thermal emission of the atmosphere in the frequency range 620–650 GHz. Due to strong tropospheric water vapor absorption in this frequency range the radiometer has to be operated on‐board a high‐flying aircraft. During EASOE several flight missions were performed over northern Europe in the periods 10–13 December 1991, 5–14 February 1992 and 7–13 March 1992 using the research aircraft FALCON operated by the German Air and Space Organization (DLR). We report on two flights in February and two in March. From a first analysis it is found that the HCl column content as observed during the March flight, increased by about 20–30% compared to the results for the mid‐February flight. On the other hand, high ClO amounts, particularly at lower altitudes, were observed in February. From the observed trends for the ClO and HCl abundances we assume that some HCl had been converted to reactive chlorine indicating a chemically disturbed Arctic vortex in February.
A case study in forecasting the stratospheric vortex during EASOE
Authors: Carver G. D. , Norton W. A. , Pyle J. A.
Published: February 2004

Different high resolution versions of the UGAMP GCM are used to forecast the position of the stratospheric vortex for a period in January 1992. The sensitivity of these forecasts to changes in the vertical resolution, gravity wave drag parametrization scheme and horizontal diffusion are presented. The forecasts are compared to the ECMWF 5 day forecast and ECMWF analyses. For this case period, all runs produce a good 5 day forecast. Increased vertical resolution and reduced horizontal diffusion improve the 7 day forecast. The forecasts are also sensitive to the parametrization of gravity wave drag.
Multiwavelength lidar observation of thin cirrus at the base of the Pinatubo stratospheric layer during the EASOE Campaign
Authors: Del Guasta Massimo , Morandi Marco , Stefanutti Leopoldo , Stein Bernhard , Kolenda Jurgen , Rairoux Patrick , Wolf J. P. , Matthey Renaud , Kyro Esko
Published: February 2004

Multiwavelength lidar measurements carried out in Sodankylä (Finland, 66°N) during EASOE campaign showed high cirrus clouds growing at the base or within Pinatubo aerosol layer. The temperature was generally below −35°C. The mean peak depolarization at 532 nm for all the campaign was 28%, comparable to the values measured in polar cirrus clouds, observed in Dumont d'Urville (Antarctica, 62°S) during 1989, well before the Pinatubo eruption. Optical depth at 532 nm was smaller than 0.3 (thin cirrus). A wavelength dependency was observed in EASOE cirrus backscattering, suggesting a major presence of submicron and micron‐sized particles. Such a presence could explain the low depolarization values.
Some case studies of chlorine activation during the EASOE Campaign
Authors: Pyle J. A. , Carver G. D. , Schmidt U.
Published: February 2004

A combination of meteorological data and measurements of the CFCs is used to assess the chlorine activation in January 1992 during the EASOE campaign. Large activation can be expected when the coldest region occurs inside the polar vortex, as during early January. In mid January activation on the edge of the vortex, where the available chlorine is lower, should also have occurred. We consider the subsequent fate of this activated air, a small fraction of which clearly mixes into middle latitudes. A situation where extra‐vortex air mixes into the vortex is also considered.
A re‐evaluation of magnetocrystalline anisotropy and magnetostriction constants
Authors: Ye Jun , Newell Andrew J. , Merrill Ronald T.
Published: February 2004

The standard theory of magnetocrystalline anisotropy for a rigid ferromagnetic body, with cubic symmetry, makes use of a “zero strain” anisotropy constant K1. If the effect of magnetostriction is taken into account, this constant must be replaced by a “zero stress” anisotropy constant K′1 which differs from K1 by a term involving magnetostriction constants and elastic constants. There are also zero strain and zero stress versions of higher order constants (K2, etc) for cubic symmetry, as well as for other symmetries. The constant K′1 also appears in dynamic behavior of ferromagnets driven by an applied field, unless the field changes too rapidly for the system to remain in equilibrium. It is predicted that ferromagnetic resonance (FMR) experiments give a direct measurement of K1.
Ozone response to enhanced heterogeneous processing after the eruption of Mt. Pinatubo
Authors: Rodriguez Jose M. , Ko M. K. W. , Sze N. D. , Heisey C. W. , Yue G. K. , McCormick M. P.
Published: February 2004

Increases in aerosol loading after the Pinatubo eruption are expected to cause additional ozone depletion. Even though aerosol loadings were highest in the winter of 1991–1992, recent analyses of satellite and ground‐based ozone measurements indicate that ozone levels in the winter of 1992–1993 are the lowest recorded in recent years, raising the question of the mechanisms responsible for such behavior. We have incorporated aerosol surface areas derived from the Stratospheric Aerosol and Gas Experiment II (SAGE‐II) measurements into our two‐dimensional model. Inclusion of heterogeneous chemistry on these enhanced aerosol surfaces yields maximum ozone reductions during the winter of 1992–1993 in the Northern Hemisphere, consistent with those derived from observations. This delayed behavior is due to the combination of the non‐linear nature of the impact of heterogeneous reactions as a function of aerosol surface area, and the long time constants for ozone in the lower stratosphere. If heterogeneous mechanisms are primarily responsible for the low 1992–1993 ozone levels, we expect ozone concentrations to start recovering in 1994.
Some results of water vapor, ozone and aerosol balloon borne measurements during EASOE
Authors: Khattatov V. , Yushkov V. , Khaplanov M. , Zaitzev I. , Rosen J. , Kjome N.
Published: February 2004

As part of the European Arctic Stratospheric Ozone Experiment (EASOE) in the northern winter of 1991/92, regular measurements of the vertical distribution of ozone and aerosols were carried out from two Russian polar stations, Heiss Island (81N, 58E) and Dikson Island (73N, 81E). In addition measurements of the vertical distribution of water vapor and aerosols were made from Esrange (68N, 21E), near Kiruna in Sweden. The instruments used were electrochemical ozone sondes (ECC‐4A), a fluorescence hygrometer, and the University of Wyoming backscattersonde. Following the eruption of Mt.Pinatubo, in the Philippines, in June 1991, volcanic aerosol had reached Arctic latitudes at altitudes below 19 km by September. At all three sites it was observed on every flight. Polar stratospheric clouds were encountered above the volcanic aerosol on two flights from Esrange. There were no indications of dehydration in the Arctic stratosphere. On all flights the minimum mixing ratio of water vapor was observed 2 to 3 km above the tropopause. Total ozone was much lower than the climatological mean, over Dikson Island from the January 27, and over Heiss Island from mid‐February, until the end of EASOE. Ozone profiles over these stations showed rapid increases in partial pressure immediately above the peak values of backscatter ratio when the volcanic aerosol was especially dense.
Age variation in the physical properties of oceanic basalts: Implications for crustal formation and evolution
Authors: Johnson H. Paul , Semyan Scott W.
Published: February 2004

The time‐dependent variability of the physical properties of upper oceanic crustal rocks have been characterized by compiling acoustic velocity, density, porosity, iron oxidation state, and H2O+ data from the upper 50 m of all Deep Sea Drilling Project and Ocean Drilling Program drill holes which recovered basalt samples from normal oceanic basement. This data compilation is interpreted in terms of the geological processes responsible for these changes as the seafloor ages. Drill site averages of both acoustic velocity and bulk density show a decrease with increasing age for crustal ages less than 100 Ma. Similarly, chemical data from the same drill cores indicate the presence of two distinct alteration processes. Oxidation of ferrous to ferric ions within the rock‐forming minerals proceeds rapidly within the first 5 Ma after formation, slows in the subsequent 5‐ to 20‐m.y. period, and then appears to stop. Hydration of the silicate minerals also appears to generally increase with age over the full 150‐m.y. age range of the samples, although at a slower rate than oxidation. The observed distribution of the physical properties of the upper crustal rocks for the first 100 Ma can be explained by three processes: (1) a large initial variability of Vp and density, caused by the range of original porosity of the rocks, (2) an age‐progressive alteration associated with oxidation that systematically reduces both Vp and density with increasing age for the first 5 m.y., and (3) an additional, nonoxidative alteration that increases the hydration of the silicate minerals, with a resulting major impact on the physical properties. Still unexplained, however, is the lack of an expected trend of decreasing porosity with increasing age and alteration and the large, nonsystematic scatter in the physical property data for samples older than 100 Ma.
Mapping lava flow hazards using computer simulation
Authors: Wadge G. , Young P. A. V. , McKendrick I. J.
Published: February 2004

Computer simulations of the paths of flowing lava are achieved using a program, FLOWFRONT, that describes the behavior of flow and digital models of the terrain. Two methods of application of simulations to the hazards posed by lava flows are described. The first, deterministic, method requires that program parameters such as vent position, minimum flow thickness, and thickness/slope relationship be based on the ambient eruptive conditions so that the future course of a specific lava flow can be simulated. This is illustrated using retrospective modeling of the first 21 days of the eruption of an andesitic lava flow at Lonquimay volcano, Chile, in 1988–1989. The usefulness of this method for real‐time predictive modeling is likely to be limited by the lack of accurate field data on flow characteristics, the simple nature of the model, and the sensitivity to parameter choice of the final planimetric form of the model flow. The second application is probabilistic in nature and creates a map of the likelihood of inundation by lava flows that is useful for long‐term land use planning. This method uses the historical record of past eruptions to constrain a series of Monte Carlo simulations and is illustrated using data from Etna volcano in Sicily. A multivariate statistical analysis of nine parameters for the 1763–1989 eruption catalog using simulated annealing permitted a classification of Etna's flank eruptions into two types: A and B. Type A eruptions are short‐lived and produce linear lava flows; type B eruptions are long‐lived, and produce lava flows that are much broader in shape, and their vents are restricted to the eastern flank of the volcano. The simulation method consists of creating a probability surface of the location of future eruption vents and segmenting the region according to the most likely historical eruption on which to base the simulation. Analysis of the autocorrelation of the historical eruptions shows that type A eruptions are strongly autocorrelated but type B eruptions are only weakly autocorrelated. A library is created of simulation parameters that give reasonable retrospective fits to each of the historical lava flows. Monte Carlo sampling of the simulation space using this library and the above spatial constraints produces a map of probability of lava inundation.
Neural networks and inversion of seismic data
Authors: Röth Gunter , Tarantola Albert
Published: February 2004

Neural networks can be viewed as applications that map one space, the input space, into some output space. In order to simulate the desired mapping the network has to go through a learning process consisting of an iterative change of the internal parameters, through the presentation of many input patterns and their corresponding output patterns. The training process is accomplished if the error between the computed output and the desired output pattern is minimal for all examples in the training set. The network will then simulate the desired mapping on the restricted domain of the training examples. We describe an experiment where a neural network is designed to accept a synthetic common shot gather (i.e., a set of seismograms obtained from a single source), as its input pattern and to compute the corresponding one‐dimensional large‐scale velocity model as its output. The subsurface models are built up of eight layers with constant layer thickness over a homogeneous half‐space, 450 examples are used to train the network. After the training process the network never computes a subsurface model which perfectly fits the desired one, but the approximation of the network is sufficient to take this model as starting model for further seismic imaging algorithms. The trained network computes satisfactory velocity profiles for 80% of the new seismic gathers not included in the training set. Although the network gives results that are stable when the input is contaminated with white noise, the network is not robust against strong, i.e., correlated, noise. This application proves that neural networks are able to solve nontrivial inverse problems.
Radial orbit error reduction and mean sea surface computation from the Geosat altimeter data
Authors: Houry S. , Minster J. F. , Brossier C. , Dominh K. , Gennero M. C. , Cazenave A. , Vincent P.
Published: February 2004

Using the altimeter data from the Geosat Exact Repeat Mission, we have produced yearly averaged mean profiles and a global mean sea surface. The radial error of each 6‐day orbital arc computed with the GEM‐T2 geopotential was first estimated by calculating the amplitude and phase of the nine dominant frequencies of the difference between the altimetric profiles and the mean sea surface obtained when adding the permanent sea surface topography (computed from the Levitus' Climatological Atlas) to the GEM‐T2 geoid. We show that this operation is little affected by the choice of the geoid or by its formal error. The resulting correction has been subtracted from each individual arc. Yearly mean profiles were then obtained by averaging the corrected altimetric data of each repeat cycle on a yearly basis. Their noise level is 1 to 2 cm and their resolution is 20 km, but the differences of the altimetric heights at crossovers of ascending and descending tracks are still 30 cm nns. The latter can be reduced to 7 cm rms by a crossover analysis. In addition to the mean values, standard deviations were computed at each point of the repeat cycle. This “yearly along‐track variability” is of the order of 10 cm rms and is dominated by the ocean mesoscale variability. A global yearly mean sea surface has been derived by bilinear interpolation. Its resolution ranges approximately from 160 km to 80 km, depending on the latitude. It is shown to be much less noisy than those deduced from GEOS 3 and Seasat data.
Seismic attenuation structure beneath the Juan de Fuca Ridge from tomographic inversion of amplitudes
Authors: White D. J. , Clowes R. M.
Published: February 2004

Tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the Juan de Fuca Ridge to further constrain a velocity structure previously obtained by travel time‐velocity tomography and to obtain a model for attenuation variations within the shallow crust. Amplitude analysis has identified an additional boundary, defined by a decrease in vertical velocity gradient, at ∼2.0 km subbottom which was not required by the travel time data. This transition zone likely represents the oceanic layer 2‐layer 3 boundary. The analysis also identified zones of relatively high attenuation (QE ∼ 20–100) at subbottom depths of 0.3–2.0 km beneath the ridge axis which correlate well with low‐velocity zones (velocities reduced by <0.4 km/s) in the velocity model. The subaxial zone is interpreted as a region of increased fracture porosity and/or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex.
Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls on the spatial and temporal evolution of mid‐ocean ridge basalt
Authors: Smith Matthew C. , Perfit Michael R. , Jonasson Ian R.
Published: February 2004

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along‐strike and off‐axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3‐km‐wide axial valley floored by ponded sheet flows and bisected by a 30‐ to 50‐m‐wide cleft. Farther north at the “Young Sheet Flow” site, the ridge axis is characterized by a distinct 500‐m‐wide inner graben that is largely covered by distinctly younger looking sheet flows. The northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillowed lavas at the “Young Pillow Mound” site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exist between degrees of fractionation, relative ages of lavas, and latitude; lavas are progressively younger looking and more mafic to the north. The compositional range of mid‐ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35–40% low‐pressure fractional crystallization of relatively primitive, but not primary, depleted (N‐type) melts. Scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small‐scale mantle heterogeneities. Furthermore, the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along‐axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were derived from discrete magma chambers or lenses, each in a different stage of evolution. The youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds. The oldest stage (∼40% additional crystallization of the most mafic composition) is associated with focused hydrothermal activity and tectonic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical and tectonomagmatic features observed along the Cleft segment are similar to those recently documented along the East Pacific Rise from 9°30′N–10°N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments.
Surface wave profiling of the lithosphere beneath the Mojave Desert using TERRAscope data
Authors: Wang Jin , Teng Ta‐Liang
Published: February 2004

We have applied surface wave analysis to the Mojave Desert in southern California to extract the regional lithosphere velocity structure. Seismograms from teleseismic sources recorded by the TERRAscope stations, a recently established network of broadband instruments, were used to determine the phase velocity dispersion data of Rayleigh waves for wave paths crossing the Mojave Desert. An inversion analysis on the dispersion data for each path gave the shear wave velocity structure beneath the path. Inversion results show that there is a low‐velocity layer at the middle and lower crust, which is particularly pronounced for paths traversing the western part of the Mojave Desert. This low‐velocity layer at the midcrust may reflect the existence of a transition zone from the brittle to the ductile regimes, where there is an absence of earthquake occurrence.
Fracture and permeability analysis in magma‐hydrothermal transition zones in the Samail ophiolite (Oman)
Authors: Nehlig P.
Published: February 2004

Many numerical models have stressed the key importance of processes operative within magma‐hydrothermal transition zones of spreading centers, but few studies have attempted a geologic characterization of the key physical parameters of this zone. With this in mind, fracture‐related data have been collected in the Samail ophiolite (Oman) within gabbro/dike transition zones and major plagiogranite plutons, and at each site the nature, abundance, orientation, filling, and attitude of the fractures were determined. Two main hydrothermal vein groups can be distinguished: an amphibole vein system and a quartz‐epidote‐sulfide vein system. Amphibole veins are restricted to the gabbro, whereas quartz‐epidote‐sulfide veins mainly appear in the sheeted‐dike complex and plagiogranite. The amphibole vein system is strongly anisotropic (perpendicular to the layering and subparallel to the sheeted dikes) and heterogeneous. The quartz‐epidote‐sulfide veins, too, are subparallel to the sheeted dikes. They are characterized by a sharp density decrease of macroscopic veins: from an average 0.5 m−1(sheeted dikes) to 0.2 m−1(transition zone) to less than 0.0l m−1(gabbro). In plagiogranite, where no concentric or radiating vein systems were seen, they are 0.1 to 0.3 m−1. The ubiquitous presence of ridge parallel hydrothermal veins implies a major contribution of regional tectonic stresses associated with spreading in addition to other mechanisms involved in fracture generation: thermal cracking, volatile‐rich magma expansion, differential expansion of pore fluids.
Temporal evolution of mantle sources during continental rifting: The volcanism of Djibouti (Afar)
Authors: Deniel Catherine , Vidal Philippe , Coulon Christian , Vellutini Pierre‐Jean , Piguet Patrick
Published: February 2004

Magmatism occurred almost continuously over the past 25 m.y. in the Republic of Djibouti. Lavas are mainly basic to intermediate with some rhyolites. Large chemical and isotopic variations among the volcanic series are interpreted in terms of mantle source heterogeneity. Crustal contribution is only evidenced in the oldest rhyolites emplaced during the initial stages of rifting. Excluding these old rhyolites, a clear evolution through time of the mantle sources is observed in relation to rifting. Three sources were involved in the genesis of these lavas: (1) an old subcontinental lithospheric component (87Sr/86Sr ≈ 0.706,206Pb/204Pb ≈ 17.9), mainly observed in the oldest lavas (25 to 10 Ma), (2) an HIMU (high U/Pb ratio)‐type reservoir, and (3) a depleted mantle. As rifting goes on, there is an increasing contribution of an HIMU‐type mantle source. It is attributed to the influence of a mantle diapir (Afar plume) thermally eroding the subcontinental lithosphere. The geochemical characteristics of 9 to 1 Ma old lavas, erupted after the strong increase of spreading rate in Afar, reflect this evolution of mantle sources. The influence of the mantle plume is most prominent in the northern youngest lavas (<1 Ma), particularly Manda, characterized by the strongest HIMU signature (87Sr/86Sr ≈ 0.7035,206Pb/204Pb ≈ l9.2). The contribution of the depleted mantle component originating from the asthenosphere is best recognized in the young (<4 Ma) lavas, particularly Tadjoura and Asal lavas (3 to 1 Ma). The evolution of Djibouti lava sources through time may be accounted for by the recent models developed for plume structure.
The root zones of oceanic hydrothermal systems: Constraints from the Samail ophiolite (Oman)
Authors: Nehlig Pierre , Juteau Thierry , Bendel Valérie , Cotten Joseph
Published: February 2004

The Cretaceous Samail ophiolite in Oman exposes an almost complete, 500‐km‐long, along‐axis section of oceanic crust, providing a unique opportunity to study the geometry, physical conditions, and effects of the hydrothermal circulation that fed the volcanic‐hosted massive‐sulfide deposits. These fossil discharge zones are rooted in the sheeted‐dike complex, down to the transition zone with the plutonic sequence. The sheeted‐dike complex as a whole was affected by greenschist‐facies metamorphism (albite, actinolite, chlorite, quartz, sphene). Diabase dikes are commonly altered into three major alteration end‐members termed spilite, mineralized spilite, and epidosite. These alteration patterns usually follow dikes, thus resulting in a well‐marked alongstrike vertical anisotropy. Their vertical distribution is also anisotropic: epidosites occur mainly in the basal sheeted‐dike complex, and mineralized spilites are usually limited to its top and to the transition zone with the volcanic extrusives. The alongstrike distribution of the different alteration patterns also shows a well‐marked correlation with the occurrence of massive‐sulfide deposits. The sheeted‐dike complex underlying these deposits is characterized by a sharp increase in the volume of epidosite and mineralized spilite. The stratigraphic position of the epidosite zones just above the magma chamber and in the lower sheeted‐dike complex, their attitude parallel to the margins of the dikes, the recorded high trapping temperatures, the high water/rock ratios, their textural reconstitution and base metal depletion suggest that they formed in a focused upflow portion of a vertical, along‐strike convecting, high‐temperature (subcritical) hydrothermal system a short time after emplacement of the dikes. The common occurrence of epidosite bands devoid of veins, and the absence of major fracturing or listric faulting, indicates that formation of epidosite does not first require tectonic extension.
Perovskite at high P‐T conditions: An in situ synchrotron X ray diffraction study of NaMgF3 perovskite
Authors: Zhao Yusheng , Weidner Donald J. , Ko Jiadong , Leinenweber Kurt , Liu Xing , Li Baosheng , Meng Yue , Pacalo Rosemary E. G. , Vaughan Michael T. , Wang Yanbin , Yeganeh‐Haeri Amir
Published: February 2004

The lattice distortion and structural phase transition of NaMgFa perovskite (Neighborite) have been studied using synchrotron X ray powder diffraction at high pressure and temperature. Changes in the unit cell dimensions of the perovskite are determined by conventional peak indexing and least squares routines. The stress field within the high‐pressure cell assembly is analyzed, and the yield strength of the NaMgF3 perovskite is determined at high P and T. The pressure‐ and temperature‐induced dimensional changes of the NaMgF3 perovskite structure are expressed empirically as a combination of compression/expansion of the [Mg‐F] bond length and tilting of the MgF6 octahedral framework. The linear thermal expansions of the NaMgF3 perovskite observed at different pressures show significant anisotropy with αa > αc > αb which reflects the decrease of structural distortion and the development of a phase transition in the perovskite with increasing temperature. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero, in a manner expected for a ferroelastic phase transition, as the temperature approaches the transition point Tc. The apparent [Mg‐F] bond lengths of the MgF6 octahedra experience drastic shrinkage with increasing temperature just prior to the transition. Despite a 12% change in volume due to compression, the experimental results on NaMgF3 perovskite show that the thermal expansivity is independent of pressure, i.e., dα/dP ≈ 0, and, compatibly, that the compressibility is independent of temperature, i.e., dβ/dT 0. However, the dominant compression mechanism is the compression of the octahedral bond length, whereas the dominant mechanism for thermal expansion is the diminishing of octahedral tilting. The Earth's mantle may be isochemical if the thermal expansion of MgSiO3 perovskite at high pressure behaves like NaMgF3, of which the Anderson‐Grüneisen parameter is near zero, i.e., δs ≈ 0. It is observed that crystal structure of the NaMgF3 perovskite transforms directly from the orthorhombic Pbnm phase to the cubic
phase at all pressures. The transition temperature is observed to increase with increasing pressure with a positive slope of 45 K/GPa.
A geopotential model from satellite tracking, altimeter, and surface gravity data: GEM‐T3
Authors: Lerch F. J. , Nerem R. S. , Putney B. H. , Felsentreger T. L. , Sanchez B. V. , Marshall J. A. , Klosko S. M. , Patel G. B. , Williamson R. G. , Chinn D. S. , Chan J. C. , Rachlin K. E. , Chandler N. L. , McCarthy J. J. , Luthcke S. B. , Pavlis N. K. , Pavlis D. E. , Robbins J. W. , Kapoor S. , Pavlis E. C.
Published: February 2004

An improved model of Earth's gravitational field, GEM‐T3, has been developed from a combination of satellite tracking, satellite altimeter, and surface gravimetric data. GEM‐T3 provides a significant improvement in the modeling of the gravity field at half wavelengths of 400 km and longer. This model, complete to degree and order 50, yields more accurate satellite orbits and an improved geoid representation than previous Goddard Earth Models. GEM‐T3 uses altimeter data from GEOS 3 (1975–1976), Seasat (1978) and Geosat (1986–1987). Tracking information used in the solution includes more than 1300 arcs of data encompassing 31 different satellites. The recovery of the long‐wavelength components of the solution relies mostly on highly precise satellite laser ranging (SLR) data, but also includes TRANET Doppier, optical, and satellite‐to‐satellite tracking acquired between the ATS 6 and GEOS 3 satellites. The main advances over GEM‐T2 (beyond the inclusion of altimeter and surface gravity information which is essential for the resolution of the shorter wavelength geoid) are some improved tracking data analysis approaches and additional SLR data. Although the use of altimeter data has greatly enhanced the modeling of the ocean geoid between 65°N and 60°S latitudes in GEM‐T3, the lack of accurate detailed surface gravimetry leaves poor geoid resolution over many continental regions of great tectonic interest (e.g., Himalayas, Andes). Estimates of polar motion, tracking station coordinates, and long‐wavelength ocean tidal terms were also made (accounting for 6330 parameters). GEM‐T3 has undergone error calibration using a technique based on subset solutions to produce reliable error estimates. The calibration is based on the condition that the expected mean square deviation of a subset gravity solution from the full set values is predicted by the solutions' error covariances. Data weights are iteratively adjusted until this condition for the error calibration is satisfied. In addition, gravity field tests were performed on strong satellite data sets withheld from the solution (thereby ensuring their independence). In these tests, the performance of the subset models on the withheld observations is compared to error projections based on their calibrated error covariances. These results demonstrate that orbit accuracy projections are reliable for new satellites which were not included in GEM‐T3.
Rotated paleomagnetic poles from the Sassamansville syncline, Newark basin, southeastern Pennsylvania
Authors: Kodama K. P. , Hedlund C. , Gosse J. , Strasser J.
Published: February 2004

A paleomagnetic fold test was conducted at eight sites collected from the Sassamansville syncline which folds the Norian age Passaic Formation along the border fault of the Newark basin in southeastern Pennsylvania. Thermal demagnetization isolated two components of magnetization; a synfolding age, intermediate unblocking temperature, normal polarit B component with intermediate inclinations and northwesterly declinations and a prefolding age, high unblocking temperature, dual polarity C component with shallow inclinations and northwesterly‐southeasterly declinations. The paleomagnetic poles calculated from these magnetizations are both rotated ≈20°±6° counterclockwise from contemporary paleopoles isolated from other parts of the Newark basin. A paleomagnetic study of the Sassamansville diabase (Kodama and Mowery, 1993) indicates a 15°±5° counterclockwise rotation, so the 20° rotation of the Passaic Formation remanence at the Sassamansville syncline is interpreted to be due to a 15° block rotation of the Sassamansville area. The remainder although not statistically distinguishable may be due to grain‐scale strain based on rock and magnetic fabric measurements. Anisotropy of magnetic susceptibility, low temperature:high temperature susceptibility ratios and X ray pole figure goniometry results suggest that a tectonic fabric developed in the phyllosilicates, which has vertical minimum axes and maximum extensional axes parallel to the fold axis, could have caused the small (≤5°) grain‐scale rotation of the B and C component magnetizations. Fry analysis of quartz grains, however, does not record a significant finite strain in the rocks and may remain the best method to identify if a large grain‐scale rotation of remanence has occurred. If local block rotations also affected some of the sites collected from the nearby Jacksonwald syncline, the effect on the B and C component paleopoles reported by Witte and Kent (1991) and Witte et al.(1991) from the Newark basin is small enough that the discrepancy between Newark basin paleopoles and coeval poles from the southwestern United States is not appreciably reduced. When the 20° rotations are removed from the Sassamansville paleomagnetic data the B component magnetization yields a high latitude paleomagnetic pole which supports the Newark B remagnetization pole of Witte and Kent (1991) and the Moat Volcanics paleopole of Van Fossen and Kent (1990). The C component magnetization yields a pole consistent with the Norian age pole from the Passaic Formation reported by Witte et al. (1989).
Geochemistry of north Cleft segment vent fluids: Temporal changes in chlorinity and their possible relation to recent volcanism
Authors: Butterfield David A. , Massoth Gary J.
Published: February 2004

Hydrothermal vent fluids from the North Cleft segment of the Juan de Fuca Ridge between 44°54′ and 45°00′N were sampled in 1988, 1990, 1991, and 1992. In 1988, chloride‐depleted (relative to seawater) diffuse fluids with low dissolved metal concentrations (relative to other mid‐ocean ridge (MOR) fluids) were sampled over a distance of ∼10 km along axis. In 1990 and 1991, both high‐temperature (>200°C) and diffuse vents were consistently metal‐ and chloride‐enriched. The end‐member compositions of high‐temperature vents and nearby diffuse vents were very similar. There were small but significant correlated changes in diffuse and high‐temperature composition from 1990 to 1991. The large‐scale but temporary venting of low‐chlorinity fluids over the entire North Cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride‐enriched fluids. It is possible that a volcanic event along the North Cleft segment prior to 1987, for which there is firm geologic evidence, initially caused a boiling event which resulted in the preferential venting of vapor‐enriched fluids through 1988, followed by a transition to brine‐enriched fluids by 1990. High iron, low sodium, and low Sr/Ca ratios in the high‐chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event. The absence of chloride‐depleted fluids from 1990 onward, and the systematics of lithium, boron, and manganese with chloride in the high‐temperature fluids from North Cleft suggest that the evolution toward lower chlorinity at Monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater.