bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral identification of geological units on the surface of Mars related to the presence of silicates from Earth‐based near‐infrared telescopic charge‐coupled device imaging
Authors: Pinet Patrick , Chevrel Serge
Published: February 2008

During the September 1988 Mars opposition, we obtained new high spatial (100–150 km) and spectral (Δλ/ λ = 1%) resolution near‐IR telescopic charge‐coupled device images of Mars from Pic‐du‐Midi Observatory. These images allow the association of spectral units with morphologic surface units on Mars, especially within the dark regions which exhibit much greater variability than the bright regions. Mineralogical interpretation of the data leads to a global description of the surface state of alteration consistent with the spatial distribution of bright and dark regions, with the bright regions being more altered than the dark. Within the less altered regions, Fe2+ crystal field absorption bands are detected, indicative of the presence of mafic minerals (Opx, Cpx, Ol) in agreement with a likely crustal basaltic composition. The most conspicuous Fe2+ absorption features are clearly related to the volcanic regions of the Syrtis Major Shield and Hesperia Planum unit. The strongest observed absorptions due to olivine and clinopyroxene are spatially associated with the restricted central caldera complex of Nili‐Meroe Paterae (within Syrtis Major) and the Tyrrhena Patera unit (within Hesperia Planum) and indicate an ultramafic composition.
Additions and corrections to the absorption coefficients of CO2 Ice: Applications to the Martian south polar cap
Authors: Calvin Wendy M.
Published: February 2008

Reflectance spectra of carbon dioxide frosts were calculated using the optical constants provided by Warren (1986) for the wavelength region 2–6 μm. In comparing these calculated spectra to spectra of frosts observed in the laboratory and on the surface of Mars, problems in the optical constants presented by Warren (1986) became apparent. Absorption coefficients for CO2 ice have been derived using laboratory reflectance measurements and the Hapke (1981) model for calculating diffuse reflectance. This provides approximate values in regions where no data were previously available and indicates where corrections to the compilation by Warren (1986) are required. Using these coefficients to calculate the reflectance of CO2 ice at varying grain sizes indicates that a typical Mariner polar cap spectrum is dominated by absorptions due to CO2 frost or ice at grain sizes that are quite large, probably of the order of millimeters to centimeters. There are indications of contamination of water frost or dust, but confirmation will require more precise absorption coefficients for solid CO2 than can be obtained from the method used here.
Plasma observations of the solar wind interaction with Mars
Authors: Vaisberg O. L. , Luhmann J. G. , Russell C. T.
Published: February 2008

Measurements with the plasma analyzers on the Mars‐2, 3 and 5 spacecraft show that Mars deflects a large fraction of the incoming solar wind flow to form a strong bow shock. The bow shock is about 1.41 Rm from the center of the planet at the subsolar point and about 2.40 Rm at the terminator. These distances are similar to those for Venus at times of moderate solar activity. The inferred effective obstacle altitude is about 400–700 km. An ion cushion has been found which is similar in its properties to the Venus magnetic barrier. The formation of this cushion appears to cause the deflection of the solar wind. Inside the cushion but well above the ionosphere is found a region where the ions are at the background, the electrons are cool and the magnetic pressure dominates. This region may resemble a planetary magnetosphere.
Faulting mechanism and complexity of the November 23, 1980, Campania‐Lucania Earthquake, inferred from surface observations
Authors: Pantosti Daniela , Valensise Gianluca
Published: February 2008

The lack of a well‐recognized relation between large earthquakes and their surface effects in Italy has motivated the detailed reconstruction of the surface expression of the 1980 Campania‐Lucania normal faulting event (Ms = 6.9). Instrumental investigations showed that it was a complex event, composed of four or more subevents occurring within 40 s of the origin time. This earthquake was the first in Italian history, and among the few in the Mediterranean basin, to produce sizable and coherent surface faulting. On the basis of field work we reconstructed three main strands which together form a 38‐km‐long, northwest trending fault scarp. Leveling observations have been used together with field observations to describe the faulting model; geological and geodetic estimates of the focal parameters inferred for each subevent have been compared with similar estimates on the basis of instrumental observations. A good consistency is found between the different models of the 1980 earthquake, with respect to both the magnitude and the geometry of the subevents. The total moment inferred from surface observations is comparable with that computed using long‐period waveform modeling and strong motion accelerograms and is 30% lower than the moment tensor inversion estimate. Much effort has been put into understanding the coseismic deformation‐surface topography interaction. Observations of back‐dipping scarps which tend to reverse the present topography indicate the young age of the presently active tectonic regime. The variability of the surface faulting and the diversion of parts of the scarp from the main trend have been interpreted in terms of intrinsic fragmentation of the characteristic earthquake‐generating 1980 segment; gaps in the scarp height and changes in the fault trend and relation to topography have been identified with persisting “relaxation geometric barriers,” which have been found coincident with the actual rupture initiation sites. The peculiarities of the 1980 earthquake and the youthfulness of its tectonic framework illustrate the first stages of landscape development in a tectonically active region. A correct understanding of this process in well‐monitored regions is essential to recognize long‐term surface features related to large earthquakes of the past and therefore to provide robust estimates for seismic hazard analyses.
Sensitivity of the telluric monitoring array in Parkfield, California, to changes of resistivity
Authors: Park Stephen K. , Fitterman David V.
Published: February 2008

Parkfield is the site of characteristic earthquakes which repeat every 22±5 years, so that the next one is due between 1983 and 1993. We have installed a telluric monitoring array to detect possible changes of resistivity prior to the next characteristic earthquake. However, the array detects only changes in the telluric response and an independent estimate of the geoelectrical structure is needed in order to locate the perturbed regions and to determine the magnitude of the change in resistivity. We have developed a three‐dimensional electrical model for Parkfield based on 12 shallow (<1500 m) Schlumberger soundings and 31 deep soundings using our array. The model is complex but shows that rock adjacent to and southwest of the San Andreas fault is conductive (less than 10 ohm m) to depths of at least 10 km. We infer that these low resistivities are due to brecciation from intense shearing along the fault. We have also used this geoelectric structure to predict the sensitivity of our telluric array to changes in resistivity along the fault. Changes in resistivity beneath the array at depths from 1.5 to 6 km produce the largest effects at the surface (1% change for a perturbation of 1% at depth). Changes of resistivity from depths of 6–12 km yielded lower sensitivities but are still detectable. Because the top of the seismogenic zone in Parkfield is at 5 km, the array should be sensitive to changes of resistivity prior to the next earthquake.
Modeling the consolidation of a porous aggregate of dry salt as isostatic hot pressing
Authors: Holcomb David J. , Zeuch David H.
Published: February 2008

Densification of porous aggregates is an important geophysical process. Salt is a convenient material to use in studying time‐dependent densification because extensive studies have been made of the creep behavior of intact salt, and its relatively low melting point enhances the effects of temperature at easily attained laboratory conditions. To obtain data on instantaneous and time‐dependent consolidation of granular salt, we measured the consolidation for times up to 400,000 s at temperatures from 21° to 100°C and hydrostatic pressures from 1.7 to 21 MPa. All tests were done under hydrostatic loading and nominally dry conditions. The only water present was the ∼0.2% water content of the salt. A model of the isostatic hot‐pressing process was used to describe the data, using the known creep behavior of intact salt for input. When brittle phenomena such as particle rearrangement and fracturing were neglected, the model successfully predicted late‐stage consolidation rates, but underpredicted the absolute densities attained in the tests. After a term was added to the model to account for brittle consolidation, the model predicted most of the observed behavior. The model makes specific predictions regarding the dependence of the consolidation rate on temperature and pressure, and these predictions are testable in the laboratory. Most importantly, the model predicts the consolidation behavior in terms of creep mechanisms known to operate in intact rock salt both in the laboratory and under geological conditions. Thus the model may be credibly extrapolated to physical conditions and long time spans beyond laboratory test capabilities.
Effect of pressure, temperature, and bulk composition on the structure and species distribution in depolymerized alkali aluminosilicate melts and quenched melts
Authors: Mysen Bjorn O.
Published: February 2008

The influence of pressure, temperature, and bulk composition on the structure of di‐ and tetra‐aluminosilicate melts and quenched melts have been studied with Raman spectroscopy. The melt polymerization and Al/(Al + Si) ranges represented are those typically found in quartz‐normative basaltic magmatic liquids. The reaction describing the equilibrium among coexisting coexisting units in the melts is T2O5 (2Q3) ⇔ TO3 (Q2)+TO2 (Q4), where T=Si+Al, and the superscripts on the alternative Q‐notations represent the number of bridging oxygen in the unit. The reaction shifts to the right with increasing temperature, pressure and Al/(Al+Si). The ΔG for the reaction (derived from molar abundances under the assumption of ideal mixing) decreases (becomes more negative), therefore, with increasing temperature, pressure, and bulk melt Al/(Al+Si). As the ionization potential of charge‐balancing and network‐modifying cations is increased at constant pressure, temperature, and Al/(Al+Si), the ΔG increases. The Al3+ exhibits a preference for the most polymerized of the coexisting structural units (TO2 or Q4) probably because among these units the TO2 units have the smallest average intertetrahedral angle. It is proposed that the difference in intertetrahedral angle between the coexisting units exerts an important control on the magnitude of the Al ‐ preference. The angle difference decreases as the network‐modifying cation becomes smaller and the Al3+ preference is less pronounced. The bulk compositional, temperature, and pressure effects leading to decreasing ΔG also weaken bridging oxygen bonds in the TO2 units. Transport properties (viscosity, diffusivity, and conductivity) are directly related to the strength and abundance of the bridging oxygen bonds. Increasing pressure results in enhanced cation diffusivity and decreased viscosity, and these trends will be more pronounced the larger the charge‐balancing and network‐modifying metal cation and the more aluminous the melt.
Clast sizes in terrestrial and Martian ignimbrite lag deposits
Authors: Wilson Lionel , Heslop Sally E.
Published: February 2008

Large‐scale pyroclastic flows (ignimbrites) are produced when a dense mixture of volcanic gas and pyroclasts is erupted into an atmosphere and forms a collapsed, fountain‐like structure (Sparks et al., 1978) over a vent. A physical model of the resulting fluid flow field can be devised (Heslop, 1987) for any planetary environment (Heslop and Wilson, 1988) which allows the pressure and velocity of the “dusty gas” mixture emerging from the vent to be deduced. Calculation of the drag forces exerted by the dusty gas on relatively large clasts allows us to deduce the sizes of the largest clasts of a given density which can just be transported through the vent to contribute to a proximal lag breccia deposit. We find that ignimbrites on Earth are potentially capable of transporting out of the vent significantly larger clasts than those commonly found in lag deposits; Martian ignimbrite lag breccia deposits should contain near‐vent clasts at least twice as large as those in terrestrial deposits produced in eruptions with the same volatile contents. Such clasts should be detectable from future Mars‐orbiting spacecraft. Furthermore, the near‐vent clast sizes in Martian ash fall deposits will be at least a factor of 100 times smaller than those in proximal ignimbrite deposits. When coupled with other observations of the morphology of near‐vent deposits, the detection of the presence of coarse clasts should provide a strong indication of the eruption mechanism which emplaced them.
Exact solutions for certain two‐dimensional problems of induction in thin nonuniform sheets
Authors: Abou‐Dina M. S. , Ashour A. A.
Published: February 2008

An inversion procedure, similar to that introduced by Ashour (1974), is given here for obtaining solutions in closed form to two‐dimensional electromagnetic induction in plane and cylindrical sheets. The integrated conductivities of these sheets vary as (a2+x2)−1, a≥ 0 and as (1+ε cos ϕ)−1, 0 ≤ ε ≤ 1, respectively, where x is the distance from a fixed line on the plane sheet and ϕ is the azimuthal angle of the cylinder (a and ε are constants). In each case, the conductivity decreases gradually as we go away from a fixed line in the sheet. The sheets are suitable models for certain geomagnetic applications. Numerical results for induction in the cylindrical shell by a uniform inducing field in the direction ϕ = π/2 are given when the time variations of the inducing field are either periodic or aperiodic.
Evidence for age and evolution of Corner Seamounts and Great Meteor Seamount Chain from multibeam bathymetry
Authors: Tucholke Brian E. , Smoot N. Christian
Published: February 2008

The Comer seamounts in the western North Atlantic and Great Meteor seamount “chain” in the eastern North Atlantic are thought to progress in age from Late Cretaceous through late Cenozoic. They both presumably formed by volcanism above the New England hotspot when first the North American plate, and then the Mid‐Atlantic Ridge axis and African plate, moved over the hotspot. High‐resolution, multibeam bathymetry of the seamounts shows geomorphic features such as guyots, terraces, and a base level plateau (Cruiser plateau) that we interpret to have formed at sea level. We have backtracked these features to sea level along the North Atlantic crustal age‐depth curve in order to estimate their ages. The derived age pattern of volcanism indicates formation of the Comer seamounts at ca. 80 Ma to 76 Ma, with migration of the Mid‐Atlantic Ridge plate boundary over the hotspot and formation of the Cruiser plateau about 76 Ma. Seamount ages suggest that subsequent volcanism on the African plate moved first northward, in the Late Cretaceous to early Cenozoic (Plato, Tyro, and Atlantis seamount groups), then southward to Great Meteor Seamount in the late Cenozoic. Recurrent volcanism appears to have occurred at some seamounts up to 20–30 m.y. after their initial passage over the hotspot. It would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away.
A Devonian paleomagnetic pole from red beds of the Tarim Block, China
Authors: Li Yianping , McWillams Michael , Sharps Robert , Cox Allan , Li Yongan , Li Qiang , Gao Zhengjia , Zhang Zhengkun , Zhai Yongjian
Published: February 2008

We present new Devonian paleomagnetic results from 59 sites in three stratigraphic sections exposed on the northwestern margin of the Tarim Block. At one section, 1998 m of red sandstone is continuously exposed; the other two sections can be correlated with the first on both magnetostratigraphic and lithologic grounds. Progressive thermal demagnetization reveals three characteristic magnetizations. One is a Late Permian overprint which is isolated below 578°C at sites near contacts with Late Permian dikes. The other two components are approximately antipodal, have a much higher unblocking temperature than the overprint, and are directionally distinct from the Late Permian overprint. We believe that these are characteristic Devonian magnetizations. Within the continuous 1998 m section, our results denote a 395 m reversed polarity zone overlying a 1603 m normal polarity zone. One reversed event is recorded at the base of the section. A paleomagnetic pole calculated by averaging results from 47 normal and reversed sites lies at λp = 16.5°N, ϕp = 165.0°E, K = 25, and A95 = 4.3°. This pole is statistically distinct from a previously reported Late Devonian pole for Tarim. On the basis of field geologic and rock magnetic studies, we believe that the previously reported pole, in addition to results from one of our three sections, reflects at least partial contamination in the form of a thermal overprint caused by Late Permian igneous activity. An analysis of Devonian to Late Carboniferous polar wander suggests that the Tarim Block was attached to a subducting plate, and moved northward and rotated clockwise during the late Paleozoic. Comparison of the Devonian paleolatitudes of Siberian, Kazakhstan, Tarim and South China Blocks indicates that both the Tarim and South China Blocks were located in equatorial regions and were far south of the northern Angaran plate (Siberia and Kazakhstan).
The Columbia River Basalt Group in the Cascade Range: A Middle Miocene reference datum for structural analysis
Authors: Beeson Marvin H. , Tolan Terry L.
Published: February 2008

The distributional patterns of individual Columbia River Basalt Group (CRBG) unite and structures recorded in the CRBG place the following constraints on Neogene tectonics of the northern Oregon Cascade Range: (1) A north–east trending topographic low of regional extent, the Columbia transarc lowland, transected the Miocene northern Oregon Cascade Range. The Columbia transarc lowland existed prior to the eruption of the CRBG and served to funnel CRBG flows across the Miocene Cascade Range. The lowland was apparently devoid of active volcanic centers from before 17 to ∼14.5 Ma. After 15 Ma, volcanic centers began to develop in the southern and central parts of the lowland. (2) Folds and associated faults of the Yakima Fold Belt extend through the western Columbia Plateau and far into the Cascade Range. These structures were active between 16.5 and S Ma and represent N–S shortening. (3) The Portland Hills‐Clackamas River structural zone is a northwest trending dextral wrench fault zone that became active prior to the incursion of CRBG flows (∼16.5 Ma) and limited the distribution of some CRBG units. This structural zone appears to mark the western termination of the Yakima Fold Belt. Other similarly trending, dextral strike‐slip faults are common in the northern Oregon Cascades but did not affect the distribution of CRBG flows and therefore developed in post‐CRBG time. (4) Mount Hood is located near the axis of a major Yakima Fold Belt syncline in the CRBG, in the general location of earlier major andesitic volcanic activity related to the Rhododendron Formation.
Volcanic and tectonic evolution of the Cascade Volcanic Arc, central Oregon
Authors: Priest George R.
Published: February 2008

From 22 to 0 Ma, σ3 in the Cascade arc rotated clockwise from approximately N–S to E–W. σ1 rotated from subhorizontal to vertical at about 7 Ma, producing an extensional stress regime at 7–0 Ma. Rotation of σ1 and σ3 was likely a response to decreasing influence of ENE to NE compression at the Juan de Fuca plate‐North American plate (JDFP‐NAP) boundary relative to N‐S compression and attendant continental extension produced at the Pacific plate (PP)‐North American plate boundary. Decreases in orthogonal convergence rate, convergence angle, and length of the convergent margin relative to the NAP‐PP transform boundary caused the stress rotations. Volcanic production decreased by a factor of 3 from the interval 35–17 Ma to 16.9–7.4 Ma, probably reflecting a decrease in convergence rate. Volcanic production increased at 7.4–0 Ma even though convergence rate continued to decrease. The extensional stress regime at 7–0 Ma promoted mafic volcanism that caused the increased volcanic production. Volcanic production is therefore a function of convergence rate and upper plate stress regime. The volcanic front migrated progressively eastward from 35 to 0 Ma as the volcanic belt narrowed. The narrowing was caused primarily by steepening slab dip at depths greater than 100 km. Eastward migration was likely caused by decreasing shallow (0–100 km) slab dip resulting from thinning of the NAP. Uplift of the Western Cascades province in the early Pliocene may have been caused by vigorous flow into the mantle wedge accommodating an increase of free rollback rate of the subducted plate at 4 Ma.
Tomographic image of the southern California mantle
Authors: Humphreys Eugene D. , Clayton Robert W.
Published: February 2008

P wave teleseismic travel time delays recorded by the Southern California Array are inverted by the method of backprojection tomography to obtain images of variations in the P wave velocity structure to a depth of 750 km. Two major upper mantle features arc resolved: one beneath the Transverse Ranges region and another beneath the Salton Trough region. The Transverse Ranges feature appears as a curtainlike, east trending, high‐velocity anomaly. This feature is ∼60 km thick, extends most deeply on its eastern end (to ∼250 km), and attains a maximum velocity −3% greater than average southern California mantle of the same depth. The Salton Trough feature, which is not as well resolved as the Transverse Ranges feature, is composed of low velocities in the upper 70–100 km. These P wave velocities arc depressed 3–4% compared to average southern California mantle. Tests of the inversion indicate the major aspects of the imaged structure are authentic.
Inversion for sources of crustal deformation and gravity change at the Yellowstone Caldera
Authors: Vasco D. W. , Smith R. B. , Taylor C. L.
Published: February 2008

The 3100 km2 Yellowstone caldera, Yellowstone National Park, was formed in the latest of three explosive eruptions of rhyolites and ash flow tuffs totalling 3700 km3 at 2, 1.2, and 0.6 m.y. before present. Its youthful volcanic history, widespread hydrothermal activity, intense seismicity, and extremely high heat flow, in excess of 30 times the continental average, marks the Yellowstone volcanic system as a giant caldera at unrest Orthometric height increases of the caldera of up to 76 cm, measured from precise leveling surveys from 1923 to 1975–1977, were inverted to determine volume expansion source models for the caldera‐wide deformation. For the 1923 to 1977 uplift episode, two regions of expansion were found: (1) in the northern part of the caldera near the Sour Creek resurgent dome of ∼0.37 km3, and (2) in the southern part of the caldera, near the Mallard Lake resurgent dome of ∼0.41 km3. Both bodies occur in the upper crust from near‐surface depths to 6.0 km, but the largest volume expansions were found in the 3.0–6.0 tan depth range. The southern caldera source volume, near the Mallard Lake dome, may extend down to 9.0 km. The data are, however, unable to resolve if these two resurgent domes, separated laterally by ∼40 km, are connected. Prom 1976 to 1987, nearly simultaneous measurements of elevation and gravity changes were made on a profile across the northern caldera during a period of net uplift. Models of the temporal gravity variation infer that the volume increase for the northern caldera source must lie above 9.0 km and involved a density perturbation greater than +0.002 g/cm3. The modeled volumetric sources are in the same general locations as bodies of low P wave velocities, high seismic attenuation, and large negative Bouguer gravity anomalies. In view of the intense Quaternary volcanism, the anomalously high heat flow, and the correlation of the source volumes with the regional geophysical anomalies, it is likely that the modeled volumetric increases were caused by migration of magmas and/or the introduction of large volumes of hydrothermal fluids into the upper crust.
A seismic refraction investigation of the Namaqualand Metamorphic Complex, South Africa
Authors: Green R. W. E. , Durrheim R. J.
Published: February 2008

The Namaqualand Metamorphic Complex forms part of the Precambrian shield of southern Africa, and hosts several major base metal deposits. A long line seismic refraction experiment traversing the Complex was conducted in order to complement geological and geophysical studies of the structure and evolution of this region. Seismograms were obtained at approximately 10 km intervals along the 290 km profile. Seismic ray‐tracing techniques were used to model the travel‐time and amplitude data. It was found that the Namaqualand lower crust is comprised of a substantial proportion of intermediate velocity (6.2 to 6.9 km/s) rocks. These intermediate velocity rocks approach the surface near the midpoint of the profile. Moho was found to be at a depth of 42 km. No correlation was found between the seismic and electrical resistivity models of the crust. When the seimic velocity structure of the Middle Proterozoic Namaqualand Metamorphic Complex is compared with that of adjacent geological provinces, it is found to differ significantly from the Archaean Kaapvaal Province, but resemble the Late Proterozoic Damara Province. These differences might be attributed to variations in the principal mechanisms of crustal growth between Archaean and Proterozoic times.
Constraints on the tectonics of the Mule Mountains Thrust System, southeast California and southwest Arizona
Authors: Tosdal Richard M.
Published: February 2008

The Mule Mountains thrust system crops out discontinuously over a 100‐km‐strike length in the Blythe‐Quartzsite region of southeast California and southwest Arizona. Along the thrust system, middle and upper crustal metamorphic and plutonic rocks of Proterozoic and Mesozoic age are thrust north‐northeastward (015° to 035°) over a lower plate metamorphic terrane that formed part of the Proterozoic North American craton, its Paleozoic sedimentary rock cover, overlying Mesozoic volcanic and sedimentary rocks, and the intruding Jurassic and Cretaceous granitic rocks. Stratigraphic, petrologic, and Pb isotopic ties for Jurassic granitoids and for Jurassic(?) and Cretaceous sedimentary rocks across the various parts of the thrust system indicate that related crustal blocks are superposed and preclude it from having large displacements. The thick‐skinned thrust system is structurally symmetrical along its length with a central domain of synmetamorphic thrust faults that are flanked by western and eastern domains where lower plate synclines underlie the thrusts. Deformation occurred under low greenschist facies metamorphic conditions in the upper crust. Movement along the thrust system was probably limited to no more than a few tens of kilometers and occurred between 79±2 Ma and 70±4 Ma. The superposition of related rocks and the geometry of the thrust system preclude it from being a major tectonic boundary of post‐Middle Jurassic age, as has been previously proposed. Rather, the thrust system forms the southern boundary of the narrow zone of Cretaceous intracratonic deformation, and it is one of the last tectonic events in the zone prior to regional cooling.
TrigonometricK riging: A New Methodf or Removingt he Diurnal Variation From Geomagnetic Data
Authors: Seguret Serge , Huchon Philippe
Published: February 2008

In order to remove the effect of the diurnal fluctuation of the magnetic field from magnetic measurements recorded at sea, we propose a new method: trigonometric kriging. This method is a particular case of the classical kriging method, but in this case we estimate locally a drift function of the form D(ti) = Ai cos (ωti) + Bi sin (ωti) or a truncated Fourier series, using a kriging neighborhood both in time and space. This method does not require any crossing point, which is a major advantage over previously published methods. After presenting the theoretical basis, we show how to put it into practice, i.e., how to choose, both in space and time, the set of points used for the estimation (kriging neighborhood) and how to determine experimentally the model for the generalized covariance which was used in the estimation. Trigonometric kriging has some interesting properties, such as to be exact at track crossings (if any), to allow an only “almost” periodic drift to be estimated, and finally to make possible filtering and gridding in one operation. Trigonometric kriging has been applied successfully to several sets of geomagnetic data recorded at sea near the equator, and we show two actual examples in Peru and in Indonesia.
High pressure‐temperature electrical conductivity of magnesiowustite as a function of iron oxide concentration
Authors: Li Xiaoyuan , Jeanloz Raymond
Published: February 2008

The electrical conductivity of (Mg,Fe)O magnesiowüstite containing 9 and 27.5 mol % FeO has been measured at simultaneously high pressures (30–32 GPa) and temperatures using a diamond anvil cell heated with a continuous wave Nd:YAG laser and an external resistance heater. The conductivity depends strongly on the FeO concentration at both ambient and high pressures. At the pressures and temperatures of about 30 GPa and 2000 K, conditions expected in the lower mantle, the magnesiowüstite containing 27.5% FeO is 3 orders of magnitude more conductive than that containing 9% FeO. The activation energy of magnesiowüstite decreases with increasing iron concentration from 0.38 (±0.09) eV at 9% FeO to 0.29 (±0.05) eV at 27.5% FeO.
Summary of seismological constraints on the structure of the Earth's core
Authors: Masters T. G. , Shearer P. M.
Published: February 2008

We review the current state of the seismological constraints on core structure. The evidence supports a simple two‐layer model of the core. The fluid outer core seems to have properties which are consistent with those of a vigorously convecting region. The inner core has a sharp boundary characterised by a 0.5–0.6 g/cc density jump, a 0.6–0.7 km/s jump in P velocity, and a jump in shear velocity to 2.5 km/s or greater. There is weak evidence for an anomalously steep shear velocity gradient at the top of the inner core. Finally, the attenuation of both body waves and free oscillations indicate that the inner core is strongly attenuating near its surface though the data suggest that the attenuation is frequency dependent
Electric fields produced by Florida thunderstorms
Authors: Livingston John M. , Krider E. Philip
Published: February 2008

Electric fields produced by air mass thunderstorms have been recorded at 25 field mill sites at the NASA Kennedy Space Center (KSC) during the summers of 1975 and 1976. Time‐ and area‐averaged fields produced by individual storms are typically −0.8 to −2.1 kV m−1 during periods of intense lightning activity and usually 2–4 times larger (−2.3 to −4.3 kV m−1) in the final, less active storm periods. The total number of lightning discharges generated by individual storms ranged from 8 to 1987. The average flashing rate per storm was 0.3–9.3 discharges per minute. Large storms tend to evolve through an initial, an active, and a final phase of electrical activity. The average point discharge computed during the active period is 3–4 times less than that computed during the final phase. In 1975, about 71% of all lightning discharges occurred during active storm periods, which in turn represented only about 27% of the total storm durations. In general, the structures of the electrostatic fields produced by lightning discharges in Florida were found to be similar to those in other geographical locations. During active storm periods, 42–52% of all lightning discharges were to ground, and during a final storm period only about 20% were to ground. The fraction of discharges to ground which contained continuing currents ranged from 29 to 46%; the mean duration of 239 ground flashes was about 420 ms. The average density of lightning flashes to ground during 1974 and 1975 at KSC was about 6 km−2 month−1 during the months of June and July.
Tropospheric N2O mixing ratio measurements
Authors: Goldan P. D. , Bush Y. A. , Fehsenfeld F. C. , Albritton D. L. , Crutzen P. J. , Schmeltekopf A. L. , Ferguson E. E.
Published: February 2008

Seventy‐five ground level and six parachute descent grab samples of tropospheric air have been made at a variety of locations throughout the world over a 6‐month period. The N2O mixing ratios in the samples have been determined by electron capture gas chromatography to an absolute accuracy of about 5%, an estimate that is based on the results of several interlaboratory comparisons of calibration standards. The mean ground level N2O mixing ratio from these measurements was 326 ppbv (parts per billion by volume), referenced to ‘dry air,’ and was found to be remarkably constant. The residual standard deviation was 5.3 ppbv. This worldwide mean is consistent with other recent measurements but is considerably larger than many of the earlier measurements. The present data indicate that the N2O residence time in the troposphere is very likely to be greater than 11 years. Furthermore, the data separately show that the possibility of a current annual increase in the N2O mixing ratio by more than 4% seems to be excluded. The parachute descent data, taken just below the tropopause, have a slightly smaller mean, 318 ppbv, but support the contention that the present tropospheric N2O mixing ratio is well above 300 ppbv.
The 18.6‐year cycle of sea surface temperature in shallow seas due to variations in tidal mixing
Authors: Loder John W. , Garrett Christopher
Published: February 2008

The semidiurnal and diurnal lunar tidal forces are modulated by a few percent over the 18.61‐year nodal cycle. A simple model of vertical mixing in coastal seas indicates that a fractional variation ε in the strength of tidal currents should lead to a variation in sea surface temperature (SST) of at least
times the average temperature difference from surface to bottom. Thus we hypothesize that long‐term records of SST might show a significant nodal cycle, and we find evidence for this at some North American locations.
Double‐diffusive intrusions into a density gradient
Authors: Turner J. S.
Published: February 2008

It is now accepted that double‐diffusive processes play a part in the mixing of heat and salt in the ocean and that relevant information about them can be obtained from laboratory experiments. There is still a considerable gap, however, between the laboratory work and the conditions under which the phenomena occur in the ocean. Most of the laboratory experiments have been one dimensional, whereas in the ocean, strong double‐diffusive layering is often associated with large horizontal gradients of T and S, especially where water with one set of T‐S properties intrudes at its own density level into an environment with different properties. The main purpose of the present paper is to report a series of laboratory experiments which are related to the latter case. The sugar‐salt system and shadowgraph photography have been used to explore various situations of increasing complexity. The double‐diffusive effects are compared with the behavior of a simple source of salt solution, which intrudes horizontally as a thin layer into a salinity gradient at its own density level. In strong contrast to this a source of sugar in the same salinity gradient produces vigorous vertical convection near the source, followed by spreading at several levels. The extending fluid moves out at an angle to the horizontal, rising or falling across isopycnals in a manner which can be related to the composition of the environment and source fluids and the double‐diffusive transports across the boundaries of the intrusions. When the ambient fluid is set up with opposing gradients, so that potential energy is stored in one of the components, there is a more rapid spreading both horizontally and vertically, and strong shears are produced. The laboratory results are compared with published oceanic data to suggest explanations of some existing observations and to predict what might be measurable in future work.
Variability of the Antarctic polar frontal zone in the Drake Passage—summer 1976–1977
Authors: Sievers Hellmuth A. , Emery William J.
Published: February 2008

Expendable bathythermograph, wind, and sea surface temperature data collected along 12 crossings of the Drake Passage were analyzed to describe variations in the structure of the antarctic polar frontal zone within a single austral summer (1976–1977). Collected from icebreakers, research and supply vessels traveling to and from the Antarctic, these data show the development of cold features within the antarctic polar frontal zone, which are most likely expressions of eddies or meanders formed at the polar front. These cold features appear to widen the antarctic polar frontal zone by intensifying the subantarctic front and moving It to the north. The majority of the temperature sections contain such cold features, whose signature can be identified as an inflection in average profiles of 450‐m heat content and sea surface temperature. Changes in air temperature and wind direction over the antarctic polar frontal zone observed when cold features are present may reflect the influence of the ocean on the overlying atmosphere.
Tropospheric NO2
Authors: Noxon J. F.
Published: February 2008

Optical absorption measurements show the vertical column abundance of NO2 in the troposphere to be less than 3 × 1014 cm−2 in the Colorado mountains when urban pollution does not contribute; when it does contribute, the abundance often exceeds 1016 cm−2. Elsewhere in the western hemisphere the clean air limit is below 1015 cm−2; with a 2‐km scale height for NO2 this corresponds to a mixing ratio of 0.2 ppb at the surface. Owing to the uncertain effects of urban pollution, we can make only a very crude estimate of 1015 cm−2 as a suggested mean for the NO2 column abundance in the troposphere over North America. Additional measurements of NO2 production during electrical storms suggest that lightning strokes, not corona discharge, are by far the dominant source.
Light hydrocarbons in recent Texas continental shelf and slope sediments
Authors: Bernard Bernie B. , Brooks James M. , Sackett William M.
Published: February 2008

The distributions of the concentrations of methane, ethene, ethane, propene, and propane in twelve 1‐to 2‐m‐long gravity cores for two transects from nearshore to midslope off the southwest Texas Gulf Coast are reported. Methane profiles exhibit maxima in the top 40 cm of sediment on the shelf, in contrast to downward increasing gradients in the slope region. Nearshore surface methane concentrations ranging from 50 to 400 μl (normal temperature and pressure) per liter pore water are apparently due to microbial production in sulfate‐free microenvironments such as fecal pellets in a near‐seawater sulfate environment. A decrease in sediment methane levels to less than 5 μl/l pore water in downslope sediments is attributed to reduced microbial activity due to lower organic contents and temperatures. Profiles of the saturated and unsaturated C2 and C3 hydrocarbons suggest that these gases are also microbially produced.
Climatic characteristics of the Antarctic Polar Front zone
Authors: Taylor H. W. , Gordon A. L. , Molinelli E.
Published: February 2008

The thermal structures for the upper 500 m presented in four long meridional sections across the subantarctic and antarctic regimes of the Southern Ocean display a number of similar features as well as some differences. The similarities are associated with the latitudinal variation of depth and temperature of the temperature minimum (T min) layer and the common occurrence of much vertical relief of isotherms north of the polar front, which is believed to reflect the presence of transient features, sucfy as meanders and eddies. The differences seem to be related to the details of the thermal structure of the polar front, the continuity of the T min layer at its northern extent, and the presence of strong thermal fronts north of the polar front. Neither local sea‐air flux of heat nor the wind‐induced Ekman drift is consistent with the presence of the polar front. Hence the front is not locally generated. The sea‐air heat flux in the vicinity of the front reverses the large‐scale trend of increased heat loss of the ocean as latitude increases: the heat flux is from ocean to atmosphere to the immediate north of the polar front and from atmosphere to ocean to the immediate south. Northward Ekman drift of surface waters is near the circumpolar maximum at the polar front, which means that the front divides the convergence region to the north from the divergence region to the south. The Antarctic Polar Front is quite similar to the Subantarctic Front of the North Pacific in nearly all respects.
Preferred crystal orientations in the fast ice along the margins of the Arctic Ocean
Authors: Weeks W. F. , Gow A. J.
Published: February 2008

Field observations of the growth fabrics of the fast and near‐fast ice along the coasts of the Beaufort and Chukchi seas show that at depths of more than 60 cm below the upper ice surface the sea ice crystals show striking alignments within the horizontal plane. At one site this alignment was well developed at a depth of 15 cm, and in all cases the degree of preferred orientation increased with depth. In general, the c axes of the crystals were aligned roughly E‐W parallel to the coast. In the vicinity of islands, alignment roughly paralleled the outlines of the islands, and in narrow passes between, islands the alignment paralleled the channel. Our observations, as well as similar observations made in the Kara Sea by Cherepanov, can be explained if it is assumed that the c axes of the crystals are aligned parallel to the ‘long‐term’ current direction at the sea ice‐seawater interface. The alignments are believed to be the result of geometric selection among the growing crystals, the most favored orientation being that in which the current flows normal to the (0001) plates of ice that make up the dendritic ice/water interface characteristic of sea ice. It is hypothesized that current flow in this direction reduces the thickness of the solute boundary layer as well as the salinity in the liquid at the interface. This lowered salinity allows crystals in the favored orientation to extend farther into the melt than neighboring crystals with less favored orientations. In addition, the current tends to induce a continuous flux of supercooled seawater against the sides of the crystals that extend ahead of the interface. This favors their lateral growth. The aligned crystal aggregate that forms has the overall characteristics of a single crystal. The development of such crystal alignments results in pronounced anisotropy in the mechanical, thermal, and electrical properties of fast ice. It is suggested that such crystal orientations can be used as an aid in determining current patterns in perennially ice‐covered areas such as the Canadian Archipelago.
Wind‐induced current flow in the upper Hudson Shelf Valley
Authors: Nelsen Terry A. , Gadd Peter E. , Clarke Thomas L.
Published: February 2008

Drawing from wind and current meter data, an empirical, semiquantitative model has been developed for wind‐induced current flow in the New York Bight apex portion of the Hudson Shelf Valley. Data have shown that winds from 270°T (±50°), blowing for at least 9 hours at speeds of greater than 5 m/s, can cause northward (upchannel) bottom flow in the shelf valley at velocities in excess of 40 cm/s. Southern (downchannel) flow is initiated by winds from 75°T (±35°) blowing for at least 6 hours at speeds of 4 m/s or more. Seasonal variation in the wind field results in predominant upchannel flow during October–April with downchannel flow throughout the rest of the year.
Poleward propagating perturbations in currents and sea levels along the Peru Coast
Authors: Smith Robert L.
Published: February 2008

In 1976 and 1977 current measurements were made on the Peru continental shelf and slope as part of the Coastal Upwelling Ecosystems Analysis expedition Joint II. The fluctuations in current were not well correlated with the local wind, in contrast to observations from coastal upwelling regions off Oregon and northwest Africa. Data from current meter arrays at separations up to 700 km are available for periods of 40–75 days. Subtidal (0.05–0.25 cpd) fluctuations propagate coherently at 200 km d−1 in a poleward direction. Analysis of current, water temperature, and sea level time series suggests that these fluctuations have an offshore scale of 70 km and represent baroclinic Kelvin waves, whose origin is equatorward of 10°S and which propagate to at least 15°S.
Acoustic monitoring of industrial chemical waste released at deep water dump site 106
Authors: Orr Marshall H. , Hess Frederick R.
Published: February 2008

Some industrial chemical wastes form flocs or particles upon being introduced into seawater. High‐frequency acoustic backscattering systems have been used to study the dispersion of the particulate phase of industrial chemical waste in a water column (depths of >1500 m) with a sharp seasonal thermocline. The particles have been found to remain trapped for extended periods (at least 74 hours) in the mixed layer, to accumulate on the density structure found between the mixed layer and the seasonal thermocline, to accumulate on the density structure associated with the seasonal thermocline, and on occasion, to penetrate the seasonal thermocline and form a diffuse cloud in the main thermocline. The effluent phase of the chemical waste, which was tagged with rhodamine WT dye, has been found to remain in the vicinity of the particulate phase of the waste.
The fine structure of lightning return stroke wave forms
Authors: Weidman Charles D. , Krider E. Philip
Published: February 2008

Lightning discharges to ground produce radiation fields which have structures that are surprisingly similar for different return strokes in the same discharge and also for different discharges. The first stroke begins with an initial portion or front which rises slowly for 2–8 μs to about half of the peak field amplitude. Return strokes subsequent to the first have initial fronts which last 0.5–1.0 μs, on the average, and which rise to about 20% of the peak field. Following the front, both first and subsequent stroke wave forms rise abruptly to peak with 10–90% rise times of 0.2 μs or less when the field propagation is over seawater. After the initial peak most strokes have a small second peak or shoulder within about 2 μs, and all first strokes have several large subsidiary peaks at intervals of 10–30 μs following the first peak. The physical processes which produce these structures are not well understood, but the data suggest that there may be rather large currents in the upward connecting discharges that occur just prior to first strokes and that all return stroke currents contain a large submicrosecond component. The small second peak may be produced by a current oscillation or by a reflection of a traveling wave in the return stroke just after its onset. The large subsidiary peaks in first strokes are probably produced by the effects of branches. The similarity of subsequent stroke fields within a flash suggests that the currents and velocities of different subsequent strokes in the same discharge are often very nearly the same.