bluuebunny/crossref_metadata_2025_split huggingface dataset:
Present and past regional ridge segmentation: Evidence in Geoid data
Authors: Calcagno P. , Cazenave A. 
Published: February 2008

Global geoid data from the Geosat altimeter satellite and topography data have been analysed in the Pacific and Atlantic oceans to study present (along‐axis) and past (off‐axis) regional mid‐ocean ridge segmentation. Along the Mid‐Atlantic Ridge, East Pacific Rise and Pacific‐Antarctic Ridge, axial geoid and depth variations are positively correlated at length scale 800–1000 km. Respective peak to peak amplitudes are in the range of 0.4–0.8 m and 200–400 m. Axial variations are correlated with the position of large offset transform faults which coincide with geoid and topography lows. Triple junctions are also located in geoid and topography lows. The observed admittance between geoid and depth is estimated to 2 ± 0.5 m/km, a value indicative of a dynamic (convective) origin for the along‐axis regional variability.Off‐axis analysis shows a regional pattern of geoid (topography) anomalies elongated in the direction of spreading. This pattern has also a length scale of 800–1000 km and possibly represents past regional segmentation. Large offset fracture zones coincide with off‐axis geoid (topography) lows.
A nested model study of the Sahelian climate response to sea‐surface temperature anomalies
Authors: Semazzi Fredrick H. M. , Lin Neng‐Huei , Lin Yuh‐Lang , Giorgi Filippo 
Published: February 2008

A nested high resolution atmospheric model is used to investigate the sensitivity of the Sahelian climate to large‐scale sea‐surface temperature (SST) anomalies. The nested system has realistic vegetation and detailed bottom orography. Two separate sets of northern hemispheric summer (June, July and August) numerical integrations are performed; one corresponding to the SST anomalies in 1950 when the Sahelian region was relatively much wetter than the long‐term average conditions and a second integration based on 1984 SST anomalies when one of the driest rain seasons in the last few decades was experienced.Although the low resolution (R15 ≈ 4.5° by 7.5° latitude by longitude) stand‐alone global climate model reasonably simulates the lower rainfall amounts in 1984 compared to 1950, the nested system yields more realistic regional climate because its forcing includes more detailed effects of topography, land‐sea contrasts, and land surface processes. In particular, two distinct rainfall maxima primarily anchored to the regions of highest terrain are simulated by the model. One corresponding to the highlands in Cameroon over the Adamawa Plateau and a second maxima over Guinea and Sierra‐Leone. Inspection of model circulation indicates that the weaker moist cross‐equatorial monsoon flow in the 1984 is responsible for the lower amounts of the Sahelian rainfall compared to 1950. Our results are in agreement with several diagnostic and modeling studies performed in the recent years which show that deficient sub‐Saharan rainy seasons tends to coincide with the southwesterly surface monsoon flow not extending as far north along the West African coast as in the wetter years (Lamb and Peppier, 1990, and others).
Introduction to Special Section on Physical Processes in Geodynamics
Authors: Schubert G. 
Published: February 2008

On May 16 and 17, 1992, a meeting was held at Cornell University as part of activities to honor Donald L. Turcotte on the occasion of his 60th birthday (which actually occurred on April 22, 1992). The papers in the following special section are a result of that meeting, which included invited presentations and posters representing the diverse areas of geology and geophysics to which Donald Turcotte has contributed.Three of the papers deal with aspects of mantle convection. A. C. Fowler addresses the problem of initiation of subduction by extending boundary layer theory to a viscoplastic rheology. Turcotte and Oxburgh [1967] pioneered such studies by showing how boundary layer theory could be used to describe the dependence of geophysical observables such as heat flux on distance from a midocean ridge or age of the oceanic crust. D. W. Sparks et al. use a model of three‐dimensional convection beneath a segmented ridge to explore how buoyancy effects and melt production beneath a spreading center influence ridge‐parallel variations in crustal thickness and gravity. Magma migration and the dynamics of midocean ridges have been major focuses of Donald Turcotte's research in papers dating back to 1978 [e.g., Turcotte and Ahern, 1978; Ahern and Turcotte, 1979; Turcotte, 1982a; Kenyon and Turcotte, 1987]. G. A. Glatzmaier and G. Schubert calculate models of three‐dimensional two‐layer mantle convection in a spherical shell and compare with whole layer, spherical shell, three‐dimensional convection models. These models represent the end‐member states of a spectrum of possible modes of mantle convection [Turcotte and Oxburgh, 1972; Oxburgh and Turcotte, 1978; Turcotte, 1979a; Silver et al., 1988].
Midlatitude seasonal measurements of stratospheric acidic chlorine vapor
Authors: Lazrus A. L. , Gandrud B. W. , Greenberg J. , Bonelli J. , Mroz E. , Sedlacek W. A. 
Published: February 2008

Stratospheric acidic chlorine vapors were sampled up to 36 km by means of base‐impregnated filters. The results do not reveal a measurable seasonal variability. The composite curve indicates a maximum of 0.8 ppbv at 24 km, decreasing to a minimum of 0.5 ppbv at about 31 km. The concentration then increases with altitude to 1.0 ppbv at 36 km.Laboratory simulation tests yield collection efficiencies of 80% or more for chlorine nitrate, and 100% for hydrochloric acid vapor. However, the concentration of acidic chlorine agrees well with spectrometrically determined concentrations of hydrochloric acid at 25 km and below. This suggests that large concentrations of chlorine nitrate are not present, though photochemical models predict similar concentrations for both chlorine nitrate and hydrochloric acid vapor at 25 km.
Limitations of ophiolite complexes as models for the magnetic layer of the oceanic lithosphere
Authors: Levi Shaul , Banerjee S. K. , Beske‐Diehl S. , Moskowitz B. 
Published: February 2008

We propose that only those ophiolite complexes whose magnetic properties and metamorphic grade correspond to comparable stratigraphic units of in‐situ oceanic crust should be used for modeling the magnetic properties of the oceanic lithosphere. Because drilling of the igneous oceanic crust by the Deep Sea Drilling Project (DSDP) has penetrated a maximum of 0.6 km, only the upper extrusives (pillow basalts and associated flows) of an ophiolite complex can be used in this direct comparison. Previously published and new data of submarine basalts are used for establishing minimum constraints for the magnetic properties of the extrusive rocks of ophiolite complexes. These criteria are:1) initial Curie points (Tc) should be between 100° ‐ 450°C (for Tc ⪞ 200° C the saturation magnetization versus temperature curves should be irreversible);2)
(intensity of magnetization) > 1×10−4 gauss (1×10−1 Am−1);3)
(ratio of remanent to induced magnetization) > 1;4) alteration of the upper extrusives should not exceed zeolite facies metamorphism.These necessary but not sufficient criteria were applied to five ophiolite complexes, of which only two satisfied all the requirements. The remaining ophiolite complexes were probably subjected to a metamorphic event either during or subsequent to their original emplacement as part of the oceanic lithosphere.
[Correction to] “Reply to 'Isotopic composition of the anomalous xenon in the Murchison Meteorite'” by Stephen P. Smith""
Authors: Srinivasan B. , Anders Edward 
Published: February 2008

This paper originally appeared in Vol. 6, No. 1, 59–61, 1979. Due to a production error in paste‐up, we have republished the first page. Our sincere apology to the authors and readers.Smith's reinterpretation of the data of Srinivasan and Anders is misleading, because it treats a 4‐component system as a 2‐component system, using a pseudocomponent whose existence, let alone constancy, is at variance with extensive data on Murchison and Allende. The derived "well defined" spectra are of doubtful significance, being based on the fit of a straight line to a curving set of 4 points.Our original identification of an s‐process component remains valid, but its purity ‐ never claimed by us, contrary to statements by Smith ‐ remains undetermined.
Implications of solar cycles 19 and 20 geomagnetic activity for magnetospheric processes
Authors: Feynman Joan 
Published: February 2008

The solar cycle variations of geomagnetic indices are compared for the last two solar cycles. It is found that the midlatitude aa index and the ring current index, Dst, exhibit a markedly different relationship to one another in the two cycles.During solar cycle 20 the yearly averaged aa has been shown to be linearly related to either the yearly averaged |Bz| v² or v², where Bz is the north‐south component of the interplanetary field and v is the solar wind velocity. However, the yearly averaged Dst is well represented by a linear function of Bzv and not by a function v². A combination of regression formulas for Dst and aa allows estimates to be made of the annual average v and Bz from 1957 until 1965, when in situ measurements of the solar wind became available. Most interesting, however, is the point that the dependence of aa and Dst on different functions of solar wind parameters implies that the ring current is not directly produced by substorm injections. This result is in agreement with models of magnetospheric processes in which the increased ring current and substorm activity are two separate aspects of increased magnetospheric convection.
Precipitation of energetic neutral hydrogen atoms at Arecibo during a magnetic storm
Authors: Tinsley Brian A. , Burnside Roger G. 
Published: February 2008

The production of ionization at upper E region levels at Arecibo and other low latitude sites during magnetic storms had been previously found to be associated with a few rayleighs of N2+ 4278Å emission, but it had not been possible to decide between energetic electrons, ions or neutrals as the primary particles involved.Observations of H Balmer β emission during such an event now demonstrate that energetic hydrogen atoms or protons are involved. Of these, H atoms from charge exchange of ring current protons are the more plausible explanation. Smaller contributions to the ionization and N2+ emission by other species may be present, and larger contributions to energy deposition by precipitating oxygen are probably present.
Boundary forces and plate velocities
Authors: Carlson Richard L. 
Published: February 2008

The notion that plate motions result from forces associated with plate boundaries implies that average plate velocities in a mantle (hot spot) frame of reference are systematically related to the nature and geometry of plate boundaries, and to the proportion of continental lithosphere included in the plates. Several simple, linear models relating plate velocity v to net ridge push (RP), slab pull (SP), trench suction (TS) and continental drag (CD) have been evaluated by multiple linear regression. Ridge push, slab pull and continental drag are found to be vitally important. Trench suction may be a real force, but because effective trench lengths are small and only three of the twelve plates considered are affected, it is not a statistically significant contributor to plate motions.The preferred relation, v(cm/yr) = (2.7±.4) + (5.2±1.9)RP + (13.2±1.6)SP − (4.7±.6)CD is a remarkably good predictor of average plate velocities, with an rms error of 0.50 cm/yr, and a correlation coefficient of 0.97. The implied balance between these net forces is generally consistent with the results of force‐balance models. The constant term includes all forces not explicitly considered, and suggests that the combined effects of such forces are about equal for all plates. Finally, because the parameters used for any particular plate are independent of all other plates, the success of this simple model strongly suggests that the forces which drive plate motions are intimately related to the plates themselves.
Magnetic charts of Canada derived from MAGSAT data
Authors: Newitt L. R. , Dawson E. , Coles R. L. , Nandi A. 
Published: February 2008

To evaluate the use of scalar and vector Magsat data for the production of regional magnetic charts, comparisons have been made between charts of the north, east, vertical and total fields derived from Magsat with those derived primarily from Canadian aeromagnetic data. Approximately 12000 satellite data were reduced to sea‐level using the spherical harmonic model MGST (4/81). Approximately 9900 aeromagnetic data from 1959 to 1976 were also reduced to sea‐level and updated to 1980. All data were gridded at 1deg; intervals of latitude and longitude, and charts contoured.Good agreement exists between charts produced from the two data sets; the overall rms difference is 150 nT. The poorest agreement occurs in the north component. In all components the largest differences occur in the northern United States and along the east coast of Canada. Over the rest of the map area there are only a few large, randomly distributed differences.
Parameters and a magnitude moment relationship from small earthquakes observed during hydraulic fracturing experiments in crystalline rocks
Authors: Pearson C. 
Published: February 2008

Using source parameters estimated from seismic spectra and magnitudes estimated from coda lengths, we demonstrate that the log‐linear relationship between moment and magnitude holds for events with magnitudes as low as −6. Using, as a data set, events induced by hydraulic fracturing experiments at the Fenton Hill, New Mexico, Hot Dry Rock (HDR) geothermal site, we find that the relationship between magnitude M and seismic moment (Mo) is

urn:x-wiley:00948276:media:grl1930:grl1930-math-0001
Moreover, the linear relationship between seismic moment and source radius (r) holds for the Fenton Hill microearthquakes. Analyses of the Fenton Hill data yield the following relationship.

urn:x-wiley:00948276:media:grl1930:grl1930-math-0002
Altitude variations in stratospheric aerosols of a tropical region
Authors: Goodman Jindra , Snetsinger K. G. , Ferry G. V. , Farlow N. H. , Lem H. Y. , Hayes D. M. 
Published: February 2008

To investigate the possibility that significant amounts of tropical tropospheric air may be convectively introduced into the stratosphere, aerosol samplings over Panama were made at various altitudes using a wire impactor collector.Percentage of particle sizes less than the mean mode decreases with height above the tropopause, suggesting depletion of small particles, possibly due to coagulation. Larger aerosols (greater than 0.3 µm in diam.) are more abundant farther above the tropopause, indicating growth, mainly by condensation. The total particle concentration decreases with increasing height above the tropopause, and also with increasing temperature.Aerosols containing smaller‐size particles are thus found closer to the tropopause, and larger‐size, more‐evolved aerosols occur at higher altitudes. These data indicate that convective activity at the ITCZ may be a source mechanism for stratospheric aerosols.
Pc 1 Pearl‐electron interactions on the L = 4.2 magnetic shell
Authors: Arnoldy R. L. , Kaufmann R. L. , Cahill L. J. , Lockheed S. B. Mende 
Published: February 2008

A number of examples of auroral light bursts correlated with Pc 1 "pearl" wave packets have been recorded at Siple, Antarctica (L = 4.2). The events are unique in that an individual light burst occurs at Siple not when the wave packet is recorded at Siple but rather when the wave is measured to be at the conjugate point over Roberval, Quebec. Although one can fit the observations to a model where ions are precipitated from an interaction region near the equator via cyclotron resonance, the number of ions required to produce the light and the above mentioned light burst‐ULF wave phase relationship both make the precipitation of electrons a more plausible interpretation.In an electron model, electrons are precipitated at Siple which have interacted with the Pc 1 wave packet over Roberval. Because of the South Atlantic magnetic anomaly, particle mirror points are nearly 200 km deeper in the atmosphere at Siple than at Roberval. Electrons of auroral energies (few keV) can be in longitudinal resonance with the Pc 1 waves over Roberval in the altitude range where the Alfvén speed rapidly decreases (2000 → 500 km). Low parallel velocities are possible for these electrons in this altitude range because the electrons are close to their mirror points. Those electrons in longitudinal resonance with the Pc 1 wave packet must have their mirror points lowered such that they do not reach the atmosphere at Roberval but are definitely precipitated at Siple. An interesting possibility is suggested in that these events represent the acceleration of electrons by a kinetic Alfvén wave.
Uppermantle anisotropy and the oceanic lithosphere
Authors: Anderson Don L. , Regan J. 
Published: February 2008

Rayleigh wave and Love wave dispersion data for oceanic paths cannot be satisfied by an isotropic uppermantle, and it is incorrect to invert these two datasets independently for separate isotropic structures. Available phase and group velocity data are inverted for oceanic structure as a function of age taking into account anelastic dispersion, sphericity, and anisotropy. The resulting models are quite different from previous results which ignore the above effects and the extra parameters involved in proper anisotropic inversion.The models have a high‐velocity nearly isotropic layer at the top of the mantle that thickens with age, and an anisotropic low‐velocity zone with an age‐dependent anisotropy. The LID, or seismic lithosphere, reaches a maximum thickness of 50 km which is about one‐half the thickness obtained in previous isotropic or pseudo‐isotropic inversions. The seismic lithosphere therefore may be comparable in thickness to the elastic or flexural lithosphere, raising the possibility that both are controlled by effects other than temperature, stress and time, such as mineralogy, crystal orientation or partial melting. The velocities in the low‐velocity zone are higher than inferred by previous surface wave studies.
Dimethyl sulfide in the Equatorial Pacific Ocean: A natural source of sulfur to the atmosphere
Authors: Cline Joel D. , Bates Timothy S. 
Published: February 2008

The most abundant volatile sulfur compound found in the surface layers of the central equatorial Pacific Ocean (148°W to 170°E) was dimethyl sulfide (DMS). Concentrations of DMS ranged from 100‐400 ng DMS/L; the highest concentrations occurred at 2°N and 148°W. DMS accounted for more than 97% of the total volatile sulfur species present, and was significantly correlated with chlorophyll ‘a’. Elevated concentrations of DMS near the equator are related to nutrient eutrophication stimulated by equatorial upwelling and the horizontal transport from the east. Vertical profiles taken at 9 stations indicated that maximum DMS concentration occurred at 20‐40 m, with concentrations decreasing to < 1 ng DMS/L at 150‐200 m.Flux calculations indicate that the central equatorial Pacific (6°N to 6°S between 148°W and 170°E) supplies about 1.0 Tg DMS/yr, assuming a mean surface concentration of 200 ng DMS/L and a piston velocity of 2.3 m/d.
Experimental evidence of non‐isotropic temperature distributions of ions observed by EISCAT in the auroral F‐region
Authors: Perraut S. , Bjørna N. , Brekke A. , Baron M. , Kofman W. , Lathuillière C. , Lejeune G. 
Published: February 2008

Many theoretical studies using different collision models have been devoted to calculating ion velocity distributions in the high latitude ionosphere. In the presence of strong convection electric fields, the ions can be accelerated to drift speeds which exceed the thermal speed of the neutral gas leading to velocity distribution functions distorted from the Maxwellian shape.With EISCAT we are able to measure three independent components of the ion velocity vector and temperature in the same scattering volume. The ion temperature observed at about 300 km during strong enhancements of the ion velocity is characterized by different temperatures parallel and perpendicular to the magnetic field with T⊥ > T⫽. For an electric strength of 50 mV/m, an anisotropy factor of about 1.5 was measured.
Observations of shear‐coupled P waves
Authors: Zandt George , Randall George E. 
Published: February 2008

Teleseismic SV waves couple to P waves in the lithosphere in several ways. P waves converted from discontinuities in the lithosphere precede the main SV arrival as small amplitude Sp precursors. Large amplitude P waves follow the main SV arrival after conversion by reflection at the free surface, and post‐critical reflection from a discontinuity in the lithosphere. SPdiff denotes a converted P wave diffracted along the underside of the crust when a teleseismic SV wave strikes it from below near the critical angle of incidence.We observed several of these shear‐coupled P waves on seismograms of deep earthquakes recorded at broadband seismic stations (RSTN) in North America. For these paths, large amplitudes relative to SV characterize the lithospheric multiple near 50°, with amplitudes decreasing rapidly beyond 60°. Calculation of synthetic seismograms indicate that a post‐critical reflection from a discontinuity at 70‐80 km depth where compressional velocity increases abruptly to about 8.5 km/s produce the largest amplitude P arrival. The observations of these shear‐coupled P waves presents a previously untapped source of information about the fine structure of the upper mantle.
Investigations and interpretations of the vertical distribution of U, TH, and K: South Africa and Canada
Authors: Schneider R. V. , Roy R. F. , Smith A. R. 
Published: February 2008

Investigations of the distribution of U, Th, and K in deep profiles exposed by Precambrian doming have been presented for Vredefort, South Africa and have recently been completed for a similar structure near Sudbury, Ontario, Canada. An evaluation of the Vredefort data shows evidence of a major crustal repetition of the vertical radioelement distribution with depth. A simple explanation is that the South African basement is geochemically layered, with repetition occurring from layer to layer. This implies a sawtooth, or “serrated” distribution with depth.Preliminary results from the Sudbury investigation show a rise in heat production with distance from the Sudbury Structure, corresponding to decreasing depth in the basement rocks. The repetition of the radioelement distribution observed at Vredefort does not appear in the Sudbury data. Although Th and K make a jump at nearly the same depth, the U data show no corresponding increase. U loss is suspected in the project area, affecting the overall distribution of heat production in the Sudbury basement. Given its complex history and geographic location, the material in the sample area may not be representative of the Superior Province.