bluuebunny/crossref_metadata_2025 huggingface dataset:
The Jovian sodium nebula: Two years of ground‐based observations
Authors: Flynn Brian , Mendillo Michael , Baumgardner Jeffrey
Published: February 2004

The 1000‐RJ‐wide neutral sodium nebula of Jupiter has now been observed on four separate occasions from late 1989 to early 1992. The observations have consisted of both low‐dispersion spectroscopy and direct imaging. These observations have allowed us to establish some of the general features of the nebula, such as the largely equatorial confinement of emission, the r−l intensity behavior in the equatorial plane, and flaring of the emission with respect to the equator. Some additional evidence exists for more subtle morphology, such as a possible variable east‐west intensity asymmetry. In this paper, we present a summary of observational evidence of variability in overall intensity, as well as in some of the morphological features of the nebula, over timescales of days, months, and years.
Episodic large‐scale overturn of two‐layer mantles in terrestrial planets
Authors: Herrick David L. , Parmentier E. M.
Published: February 2004

It is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately. Possible buoyant overturn of the two mantle layers has not previously been considered. Such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers, reversing the relative sense of buoyancy. Once overturn has occurred, the chemically denser, but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy. If mixing is slow, this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large‐scale overturning. Thermal evolution calculations, for a two‐layer mantle over a wide range of parameter space, show that large‐scale overturn occurs cyclically with a well‐defined period. This period depends most strongly on the viscosity of the lower mantle, to which it is approximately proportional. Geologically interesting overturn periods on the order of 107 to 109 years result for lower mantle viscosities of 1022 to 1024 Pa s for the Earth and Venus, and 1021 to 1023 Pa s for Mars. The mantles of Mercury and the Moon are too thin to permit two‐layer convection, and therefore the model is not appropriate for them. Overturn cannot occur on Earth or Venus if the compositional density difference between the layers exceeds about 4%, or on Mars if it exceeds about 2%. Large‐scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the Earth or a major resurfacing event on Mars or Venus. Such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection.
Physics of interplanetary dust capture via impact into organic polymer foams
Authors: Anderson William W. , Ahrens Thomas J.
Published: February 2004

The physics of hypervelocity impacts into foams is of interest because of the possible application to interplanetary dust particle (IDP) capture by spacecraft. We present a model for the phenomena occurring in such impacts into low‐density organic polymer foams. Particles smaller than foam cells behave as if the foam is a series of solid slabs and are fragmented and, at higher velocities, thermally altered. Particles much larger than the foam cells behave as if the foam were a continuum, allowing the use of a continuum mechanics model to describe the effects of drag and ablation. Fragmentation is expected to be a major process, especially for aggregates of small grains. Calculations based on these arguments accurately predict experimental data and, for hypothetical IDPs, indicate that recovery of organic materials will be low for encounter velocities greater than 5 km s−1. For an organic particle 100 μm in diameter, ∼35% of the original mass would be collected in an impact at 5 km s−1, dropping to ∼10% at 10 km s−1 and ∼0% at 15 km s−l. For the same velocities the recovery ratios for troilite (FeS) are ∼95%, 65%, and 50%, and for olivine (Mg2SiO4) they are ∼98%, 80%, and 65%, demonstrating that inorganic materials are much more easily collected. The density of the collector material has only a second‐order effect, changing the recovered mass by <10% of the original mass.
Classification of asteroid spectra using a neural network
Authors: Howell E. S. , Merényi E. , Lebofsky L. A.
Published: February 2004

The 52‐color asteroid survey (Bell et al., 1988) together with the 8‐color asteroid survey (Zellner et al., 1985) provide a data set of asteroid spectra spanning 0.3–2.5 μm. An artificial neural network clusters these asteroid spectra based on their similarity to each other. We have also trained the neural network with a categorization learning output layer in a supervised mode to associate the established clusters with taxonomic classes. Results of our classification agree with Tholen's classification based on the 8‐color data alone. When extending the spectral range using the 52‐color survey data, we find that some modification of the Tholen classes is indicated to produce a cleaner, self‐consistent set of taxonomic classes. After supervised training using our modified classes, the network correctly classifies both the training examples, and additional spectra into the correct class with an average of 90% accuracy. Our classification supports the separation of the K class from the S class, as suggested by Bell et al. (1987), based on the near‐infrared spectrum. We define two end‐member subclasses which seem to have compositional significance within the S class: the So class, which is olivine‐rich and red, and the Sp class, which is pyroxene‐rich and less red. The remaining S‐class asteroids have intermediate compositions of both olivine and pyroxene and moderately red continua. The network clustering suggests some additional structure within the E‐, M‐, and P‐class asteroids, even in the absence of albedo information, which is the only discriminant between these in the Tholen classification. New relationships are seen between the C class and related G, B, and F classes. However, in both cases, the number of spectra is too small to interpret or determine the significance of these separations.
Glacial‐Holocene carbonate dissolution and sea surface temperatures in the south China and Sulu seas
Authors: Miao Qingmin , Thunell Robert C. , Anderson David M.
Published: February 2004

Sediment cores from the South China and Sulu seas have been used to study sea surface temperature changes in these two western equatorial Pacific basins during the last 25,000 years. Sea surface temperature (SST) estimates were derived using the planktonic foraminiferal transfer function FP‐12E previously developed by Thompson (1981). The water depths for the cores range from 500 m to more than 4,000 m and thus provide a good opportunity to evaluate the effect of carbonate dissolution on quantitative paleotemperature estimates. The sea surface temperature time series from shallow, well‐preserved cores indicate that average winter and summer temperatures during the Holocene were approximately 27°C and 29.5°C, respectively, for both the South China and Sulu seas. These estimates agree well with modern observations. During the last glacial maximum, summer sea surface temperatures were approximately 28.5°C in the South China Sea and 29°C in the Sulu Sea and thus were very similar to the Holocene. In contrast, glacial winter sea surface temperatures are estimated at approximately 21°C for the South China Sea and 24°C for the Sulu Sea. This decrease in glacial winter sea surface temperatures results in a much larger seasonality during the last glacial (5°‐8°C) compared to the Holocene (2°C). These seasonal contrasts are much greater than those estimated by Climate: Long‐Range Investigation, Mapping, and Prediction Members (1981) for this region of the western equatorial Pacific. Variation in intensity of the monsoon system and surface water exchange rates between these basins and the open ocean are the major factors controlling glacial‐interglacial SST fluctuations in the South China and Sulu seas. One factor influencing the accuracy of the SST estimates is the quality of preservation of the planktonic foraminiferal assemblages. Our results demonstrate that depth‐dependent increases in dissolution result in systematically cooler SST estimates. This is due to the fact that warm water planktonic foraminifera tend to be more solution susceptible, and as dissolution progresses, the assemblage becomes enriched in the more resistant, cooler water taxa. Since dissolution is more intense during interglacials than glacials in the Pacific, dissolution tends to reduce the amplitude of the glacial‐interglacial temperature difference.
Late Quaternary CaCO3 production and preservation in the Southern Ocean: Implications for oceanic and atmospheric carbon cycling
Authors: Howard William R. , Prell Warren L.
Published: February 2004

Recent geochemical models invoke ocean alkalinity changes, particularly in the surface Southern Ocean, to explain glacial age pCO2 reduction. In such models, alkalinity increases in glacial periods are driven by reductions in North Atlantic Deep Water (NADW) supply, which lead to increases in deep‐water nutrients and dissolution of carbonate sediments, and to increased alkalinity of Circumpolar Deep Water upwelling in the surface Southern Ocean. We use cores from the Southeast Indian Ridge and from the deep Cape Basin in the South Atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by Circumpolar Deep Water. This suggests that deep Southern Ocean carbonate ion concentrations were lower in glacial stages than in interglacials, rather than higher as suggested by the polar alkalinity model [Broecker and Peng, 1989]. Our observations show that changes in Southern Ocean CaCO3 preservation are coherent with changes in the relative flux of NADW, suggesting that Southern Ocean carbonate chemistry is closely linked to changes in deepwater circulation. The pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient‐depleted water (NADW) to the Southern Ocean and with an increase of nutrients in deep water masses. Carbonate mass accumulation rates on the Southeast Indian Ridge (3200–3800 m), and in relatively shallow cores (<3000 m) from the Kerguelen Plateau and the South Pacific were significantly reduced during glacial stages, by about 50%. The reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in CaCO3:Corg flux ratios, acting as another mechanism which would raise the alkalinity of Southern Ocean surface waters. The polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant. Even if overall productivity in the Southern Ocean were held constant, a decrease in the CaCO3:Corg ratio would result in increased alkalinity and reduced pCO2 in Southern Ocean surface waters during glacials. This ecologically driven surface alkalinity change may enhance deepwater‐mediated changes in alkalinity, and amplify rapid changes in pCO2.
Sedimentary records of relative paleointensity of the geomagnetic field: Theory and practice
Authors: Tauxe Lisa
Published: February 2004

Sediments have proved irresistible targets for attempts at determining the relative variations in the Earth's magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution. The assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited. Early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short, no dependence on magnetic field was expected. Nonetheless, a number of experiments showed that sedimentary magnetizations varied in accordance with the field, albeit not always in a simple, linear fashion. Experiments in which the sediments were stirred in the presence of a field (to simulate bioturbation) showed a reasonably linear relationship with the applied field, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences. Examination of existing paleointensity data sets allows a few general conclusions to be drawn. It appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the Earth's magnetic field. The dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences. Moreover, when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences, there is considerable agreement among the various records. When viewed over timescales of hundreds to thousands of years, relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another, suggesting that they are dominated by nondipole field behavior. When viewed over timescales of a few tens of thousands to hundreds of thousands of years, however, the records show coherence over large distances (at least thousands of kilometers) and may reflect changes in the dipole field. On the basis of a sequence spanning the Brunhes and Matuyama chrons, the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years, but these oscillations are not clear in the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence.
Modeling Sun‐aligned polar cap arcs
Authors: Crain D. J. , Sojka J. J. , Schunk R. W. , Zhu L.
Published: February 2004

We present results of a new model of the time‐dependent ionospheric response to a generalized steady state Sun‐aligned (SA) arc structure. The thermal and plasma structure of a “prototype” arc is compared to the general features of observed SA arcs. We find that the general features of electron density, Ne, electron temperature, Te, and ion temperature, Ti, are determined by the distribution of the particle precipitation and E×B convection associated with the SA arc. The model results are extended to predict the possible variation of Ne, Te, and Ti due to such arcs at Defense Meteorological Satellite Program altitude. An important parameter in determining the overall density enhancement produced by a SA arc is the speed of plasma flow across the arc. This is demonstrated by calculating the delta total electron content, ΔTEC, enhancement for various values of cross flow. Since cross‐flow is a relatively hard measurement to make, observations of ΔTEC associated with SA arcs can help place limits on the magnitude of the cross‐flow velocities for SA arcs. In conjunction with the National Science Foundation Coupling Energetic Dynamics of the Atmospheric Regions/High‐Latitude Plasma Structure initiative, we compare the results of the model with coordinated observations of several SA arcs which were observed at Qanaq, Greenland, on December 11, 1990. Comparison of our results and observations indicates a relatively low value for the cross‐flow speed (less than 25 m s−1) for these SA arcs.
Irregularity structures in the cusp/cleft and polar cap regions
Authors: Basu S. , Basu Su. , Chaturvedi P. K. , Bryant C. M.
Published: February 2004

A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Søndrestrom and Thule, Greenland, using the 243‐MHz transmissions from quasi‐stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high‐latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Søndrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Søndrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced‐antenna scintillation measurements at Søndrestrom detected considerable velocity shear, and the frequency spectra showed flat low‐frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high‐ and low‐ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low‐density regions imply the presence of small‐scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient‐drift instability mechanism generates the observed small‐scale irregularities associated with discrete density enhancements, whereas a shear‐driven instability, such as the nonlinear collisional Kelvin‐Helmholtz (K‐H) instability mechanism, may generate the irregularities in the intervening low‐density regions.
Experimental evidence for the formation and entry of patches into the polar cap
Authors: Valladares C. E. , Basu S. , Buchau J. , Friis‐Christensen E.
Published: February 2004

A mechanism for the formation of polar cap patches is proposed based on data collected by the Sondrestrom incoherent scatter radar, magnetometers located on the west and east coasts of Greenland and Digisondes, digital ionospheric sounders, operated at Qaanaaq and Sondrestrom. This instrumentation has served to identify the formation of patchlike structures and follow their temporal evolution and entry into the polar cap. Prior to the onset of the event, the background ionospheric plasma was moving in a poleward direction and formed part of a tongue of ionization directed toward and into the polar cap. The event starts with the appearance of a fast plasma jet containing eastward directed velocities in excess of 2 km s−1. This plasma jet consists of a channel extending 300 km in width, where the F region ion temperature reaches values in excess of 4000 K and the E layer Ti is enhanced above 5000 K. The existence of a region containing high electron temperatures associated with soft precipitation (likely the cusp) and located adjacent and equatorward of the fast plasma jet is very suggestive of the jet being the result of a tension force acting upon recently reconnected field lines. This view is supported by the fact that the fast plasma jet was seen in the prenoon sector under IMF By negative conditions. The elevated Ti values inside the plasma jet are exactly collocated with depleted F region densities. We suggest that the recombination loss of O+ is increased by a factor >10 due to the dependence of the O+ + N2 reaction on the ion temperature. The eroding action of the fast plasma jet proceeds until the poleward moving tongue of ionization is divided into regions containing high and low densities. Magnetic field perturbations associated with the fast plasma jet were observed by the different magnetometer stations. The large negative bays recorded at most of the sites have been used to trace the poleward motion of the jet. At the time that the fast plasma jet reaches Qaanaaq, the Digisonde there measured low ƒ0F2 values. A few minutes before and after this minimum, high ƒ0F2 values were observed. The ratio of the enhanced ƒ0F2 values on either side of the minimum, to the minimum ƒ0F2 is about 2 (or a density ratio of 4). This is the commonly accepted signature of a polar cap patch. The series of events leading to the formation of the patchlike density structure has been designated a density breakoff event.
The dark polar ionosphere: Progress and future challenges
Authors: Carlson Herbert C.
Published: February 2004

Since the end of the 1970s, we have seen enormous progress in our understanding of the polar ionosphere and its structuring. With this benchmark issue of Radio Science it is appropriate to reflect briefly on that passage and some key questions that lie ahead. The discussion here will concentrate on the winter hemisphere, in keeping with the conditions under which most of the data studied to date have been gathered. The polar ionosphere alternates between two states, depending on whether the interplanetary magnetic field (IMF) is southward or northward. The former state is characterized by ∼100–1000 km islands of enhanced F region plasma, originating in sunlit upper midlatitudes, entering and traversing the polar cap. They become highly structured and produce severe scintillation. Despite much progress on the source, evolution, and ultimate fate of this polar plasma, we remain challenged by the process(es) which chop entering plasma into such islands. For northward IMF we have learned much about the near‐Earth processes determining the character of polar cap arcs, velocity structure and electrodynamics, and energetics. A remaining challenge is to relate these structures to the topology and driving physical processes in the magnetosphere and solar wind. Here we sketch the principles behind the progress and the context of several key problem areas ahead.
The electromagnetic field of a vertical electric dipole in the presence of a three‐layered region
Authors: King Ronald W. P. , Sandler Sheldon S.
Published: February 2004

The electromagnetic field generated by a vertical electric dipole in the air over the surface of a two‐layered region is determined for continuous‐wave excitation. The region of interest consists of a conductor or dielectric with high permittivity, coated with an electrically thin layer of a dielectric under a half‐space of air. Simple explicit formulas are derived for the field at all points in all three regions including the surface wave. Typical applications are to microstrip circuits and antennas, communication over the earth when this is coated with a layer of asphalt or cement, and communication over the sea or a lake when this is under a layer of ice.
Modeling daytime F layer patches over Sondrestrom
Authors: Decker D. T. , Valladares C. E. , Sheehan R. , Basu Su. , Anderson D. N. , Heelis R. A.
Published: February 2004

A comprehensive, time‐dependent, high‐latitude, one‐species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N2 and O2; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time‐dependent global convection pattern and spatially localized regions of transient high‐speed flow. We find that the model is very sensitive to the details of the time‐dependent convection pattern, and both the time dependence and the high‐speed flows contribute to the F‐region structure. Further, when we use high‐speed flows based on specific radar observations the simulated density structure is in reasonable agreement with that day's digisonde observations.
Lateral wave contribution to the low–altitude radar propagation factor
Authors: Gilbert Gerald , Raiten Eric , Visser Matt
Published: February 2004

An explicit analytic expression for the lateral‐wave contribution to the low‐altitude radar propagation factor is calculated. The effect of lateral electromagnetic surface waves on low‐altitude radar propagation over both sea water and fresh water is discussed. It is shown that significant modifications to radar propagation can be attributed to lateral waves.
Two‐dimensional location and shape reconstruction
Authors: Kleinman R. E. , van den Berg P. M.
Published: February 2004

A method for reconstructing the location and the shape of a bounded impenetrable object from measured scattered field data is presented. The algorithm is, in principle, the same as that used for reconstructing the conductivity of a penetrable object and uses the fact that for high conductivity the skin depth of the scatterer is small, in which case the only meaningful information produced by the algorithm is the boundary of the scatterer. A striking increase in efficiency is achieved by incorporating into the algorithm the fact that for large conductivity the contrast is dominated by a large positive imaginary part. This fact, together with the knowledge that the scatterer is constrained in some test domain, constitute the only a priori information about the scatterer that is used. There are no other implicit assumptions about the location, connectivity, convexity, or boundary conditions. Some refinements of the algorithm which reduce the number of points at which the unknown function is updated are incorporated to further increase efficiency. Results of a number of numerical examples are presented which demonstrate the effectiveness of the location and shape reconstruction algorithm.
The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico)
Authors: Herrmann Uwe R. , Nelson Bruce K. , Ratschbacher Lothar
Published: February 2004

Terrane analysis has established differences in lithology, style of deformation, metamorphism, and radiometric ages among several basement complexes in southern Mexico. In this paper, we identify similarities in the geologic history of the adjacent Acatlán, Oaxaca, and Xolapa complexes through a U/Pb zircon study on crustal sources and timing of magmatic, metamorphic, and tectonic activity in the Xolapa complex. Our regional tectonic scenario is based on both new chronologic and previously established structural data. Inherited zircon in the Xolapa complex indicates the presence of a Proterozoic (1.0–1.3 Ga) crustal component with an age range that overlaps Grenville crystallization dates (1.0–1.2 Ga) from the Oaxaca basement, and an inherited Grenville crustal component (1.0–1.1 Ga) identified in zircons from the Acatlán complex. The Proterozoic component in the Xolapa complex indicates either that it received sediments from a continental region of Grenvillean age, for example the Oaxaca basement, which is the closest exposed Grenville crust in southern Mexico, or that the Xolapa complex has a Grenville basement. In the latter case, the Xolapa complex has been modified by widespread high‐grade metamorphism and large‐scale migmatization. Metamorphism and migmatization occurred from 66 to 46 Ma and locally continued into Oligocene time. Magmatism in the Xolapa complex terminated with crustal growth by plutonism, which is characterized by a systematic pattern of eastward‐younging crystallization ages, from 35 Ma in the west (west of Acapulco) to 27 Ma in the east (east of Puerto Angel). Metamorphism and migmatization in the Xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the Farallon and North American plates (e.g., rate and direction of subduction) and the early evolution of the Caribbean. This plate reorganization triggered sinistral transtension distributed across the southern Mexican continental margin and the eastward translation of the Chortis block. The eastward younging of the Xolapa plutons also is consistent with the motion of the Chortis block during eastward displacement of the Caribbean plate. This induced a shift of the magmatic arc from its Cretaceous to early Tertiary position along the Xolapa and Chortis blocks to its present mid‐Mexican position. We conclude that a single tectonic framework accounts for the Mesozoic and Cenozoic geologic history of the southern Mexican terranes.
Neogene transpressive evolution of the California Coast Ranges
Authors: Jones David L. , Graymer Russell , Wang Chi , McEvilly T. V. , Lomax A.
Published: February 2004

The San Andreas fault has long been viewed as a vertical structure that extends from the ground surface to the base of the lithosphere and separates the Pacific plate from the North American plate along a transform plate boundary. Analysis of Neogene structures in the central Coast Ranges indicates, however, that crustal blocks defined by strands of the San Andreas fault system have undergone pervasive compressive deformation that has shifted these blocks and their boundary faults east, relative to North America. This suggests that the blocks and their boundary structures are bounded below by an active decollement that appears to coincide with the midcrustal brittle to ductile transition at the base of the seismogenic zone. These compressive structures may be related to a currently seismically active fold and thrust belt that characterizes the eastern front of the Coast Ranges. Seismic profiles from the Central Valley, east of the range front, show that tectonic wedges (“blind thrusts”) developed within stratified rocks are present throughout this zone of deformation, and that crystalline basement that underlies these stratified rocks is not involved in the deformation. Crystalline basement may underlie most of the Coast Ranges, as indicated by the presence of metamorphic rocks of possible Sierran affinity preserved in Neogene thrust sheets as far west as Loma Prieta and by seismic velocities in the lower crust appropriate for mafic to intermediate granitoid rocks. On the basis of these relations, we suggest that the San Andreas fault system is confined to the brittle crust, above the decollement, and does not penetrate to the base of the lithosphere. Thus the San Andreas fault should not be considered as a “plate boundary.” Instead, the functional plate boundary within the Coast Ranges appears to be the inferred subhorizontal midcrustal decollement. This decollement corresponds approximately to the 350°C isotherm, the temperature at which quartz becomes ductile. If our hypothesis is proven to be correct by geologic and geophysical investigations currently underway, then extensive revisions in popular plate tectonic models applied to the Coast Ranges will be required. In addition, assessment of seismic hazards will be rendered more difficult, but more realistic, owing to realization of the greater likelihood for occurrence of thrust faulting on blind or buried faults or on faults currently deemed inactive.
Tertiary metamorphic core complexes in Sonora, northwestern Mexico
Authors: Nourse Jonathan A. , Anderson Thomas H. , Silver Leon T.
Published: February 2004

Several ranges encompassing more than 35,000 km2of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top‐to‐the‐southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U‐Pb and K‐Ar ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene‐early Miocene. Partitioning of extensional strain in Sonora was influenced by pre‐Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60°W striking Mojave‐Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic arc rocks and Upper Jurassic‐Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex‐related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic‐Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two‐mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre‐Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25–18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest‐British Columbia region but earlier than in the Mojave Desert‐Death Valley region. Middle Tertiary mylonitic fabrics of similar style and orientation have not been recognized farther south in Mexico. The southern terminus of the mid‐Tertiary Cordilleran core complex belt appears to be in Sonora.
Earthquake refraction profiles of the root of the Sierra Nevada
Authors: Savage Martha Kane , Li Li , Eaton Jerry P. , Jones Craig H. , Brune James N.
Published: February 2004

We examine the seismic structure of the Sierra Nevada using records of nine earthquakes and one explosion in and near the Sierra, recorded on stations in the Sierra. We first interpret travel times from these paths, which are confined to a single tectonic block, in terms of one‐dimensional structures. The most nearly reversed pair of earthquakes, the 1966 Truckee and 1983 Durrwood Meadows earthquakes, share refracted (Pn) arrival times (corrected to surface focus) along a line t=8.75±0.25+Δ/8.0, suggesting that a nearly flat layer of 8.0 km/s mantle material lies at depths of 46–48 km. First arrivals from these events do not constrain velocities from ≈30 to 45 km depth. Secondary arrivals and some first arrivals from other earthquakes suggest that velocities in part of this region range between 6.9 and 7.8 km/s. The presence of this “7.x‐km/s” layer can help to explain previous contradictory observations. The 7.x‐km/s layer could be interpreted as either the mafic bottom of a silicic, Mesozoic magmatic arc or as accreted mafic underplating or rejuvenated mantle related to Cenozoic arc volcanism or Basin and Range spreading. Arrivals at stations in the foothills and the crest of the Sierra cannot be fit with a single longitudinal structure, indicating a lateral variation of velocity structure. These variations support previously inferred variations of lithospheric structure, with higher‐velocity, thinner crust to the west beneath the Sierran foothills and slower‐velocity crust (or possibly upper mantle) beneath the high mountains in the eastern Sierra. Rapid changes in arrival times between stations separated by short distances in the eastern Sierra suggest that a sharp boundary exists between the Sierra and the Basin and Range at Moho depths. We also present fresh evidence of the asymmetry of the root of the Sierra, wherein arrivals from earthquakes on the west of the Sierra are delayed within the Sierra and return to original values in the Basin and Range, while arrivals from earthquakes and explosions from the Sierra into the Great Valley. We suggest that if the 7.x‐km/s material occurs in a wedge above the Moho, then the asymmetry can be explained by arrivals from the west being delayed by the dipping 8.0‐km/s Moho, while those from the east may be entering the root along a 7.x‐km/s layer that is near the depth of the Basin and Range Moho.
A screening model for nonaqueous phase liquid transport in the vadose zone using Green‐Ampt and kinematic wave theory
Authors: Weaver James W. , Charbeneau Randall J. , Lien Bob K.
Published: February 2004

In this paper, a screening model for flow of a nonaqueous phase liquid (NAPL) and associated chemical transport in the vadose zone is developed. The model is based on kinematic approximation of the governing equations for both the NAPL and a partitionable chemical constituent. The resulting governing equation is a first‐order, quasi‐linear hyperbolic equation to which the generalized method of characteristics can be applied. This approach generally neglects the contribution to the NAPL flux from capillary pressure gradients. During infiltration under ponded conditions, or when the NAPL flux exceeds the maximum effective conductivity of the soil, the effect of capillary suction is included in the model through the usage of the Green‐Ampt model. All of the resulting model equations are in the form of ordinary differential equations which are solved numerically by a variable time step Runge‐Kutta technique. Results from a simple column experiment were used to evaluate the vadose zone flow model assumptions. Independently measured parameters allow simulation without calibration of the model results. The match of the model to the data suggests that the model captures the qualitative behavior of the experimental system and is capable of an acceptable degree of quantitative agreement.
Fractal dimension to describe soil macropore structure using X ray computed tomography
Authors: Peyton R. Lee , Gantzer Clark J. , Anderson Stephen H. , Haeffner Brian A. , Pfeifer Peter
Published: February 2004

The size, shape, and arrangement of structured voids 1–10 mm in size play an important role in the transport of water and solutes through soil. However, these characteristics are complex and difficult to quantify. Improved methods are needed to quantify the characteristics of these voids to better understand and predict the behavior of water and solutes passing through them. This study applied fractal analysis to soil bulk density data measured by X ray computed tomography (CT), a relatively new tool for nondestructively measuring macropore‐scale density in soil cores. Studies were conducted using undisturbed soil cores (7.6 cm ID) from forested and cultivated sites in the A horizon of a Menfro silt loam soil containing macropores and using two groups of soil cores which were uniformly packed with Menfro aggregates from 1–2 mm in diameter for one group and <1 mm in diameter for the other group. Samples were probed using CT to produce a 512 by 512 digital matrix of CT pixel values corresponding to bulk density. Pixels above a specified CT “cutoff” value were designated as occupied. A box‐counting method was used to find the fractal dimension of the perimeters between occupied and unoccupied pixels and of the areas formed by the unoccupied pixels. For length scales from 1 to 10 mm, perimeters and areas of these regions appeared to be fractal systems. Single degree of freedom orthogonal contrast tests determined from analysis of variance showed significant differences between the fractal dimension for (1) forest and cultivated cores versus uniformly packed cores, (2) two groups of uniformly packed cores made of different aggregate sizes, and (3) forest versus cultivated cores.
Visualization of the role of the gas‐water interface on the fate and transport of colloids in porous media
Authors: Wan Jiamin , Wilson John L.
Published: February 2004

This paper exposes the significant role played by the gas‐water interface in the fate and transport of colloids in porous media and also introduces a micromodel method to allow direct visualization of colloid behavior in pore networks. The gas‐water interface was created by trapping air in the pore space. Various types of latex and clay particles, as well as bacteria, were studied. The results suggest that the gas‐water interface sorbs not only hydrophobic but also hydrophilic particles. The degree of sorption is controlled by particle surface hydrophobicity, solution ionic strength, and particle charge sign. Sorption increases with increasing particle hydrophobicity and solution ionic strength, while positively charged particles have a very strong affinity for the gas‐water interface. The sorption on the gas‐water interface is essentially irreversible, in that the capillary free energy provides a large attractive force to hold particles on the gas‐water interface after its rupture. These findings reveal a mechanism of vadose zone transport: A static gas‐water interface behaves as a sorbent phase retarding the transport of particulate contaminants. The visualization method developed in this research is very useful for the investigation of particulate contaminant behavior and interface‐related transport, especially in the context of bioremediation.
Predicting atrazine adsorption‐desorption in soils: A modified second‐order kinetic model
Authors: Ma Liwang , Selim H. M.
Published: February 2004

Atrazine retention on a Sharkey clay soil was quantified using a kinetic batch method for different soil to solution ratios. Time‐dependent adsorption‐desorption was also measured using a batch method with successive dilution steps. Adsorption was found to be highly kinetic in nature. In addition, adsorption‐desorption isotherms exhibited a strong hysteretic behavior. The extent of observed hysteresis increased with retention time. Attempts were made to describe atrazine retention based on a modified second‐order approach where heterogeneity of adsorption sites was assumed. Two retention sites were considered: type 1 (Se) represented that retained on noncatalytic sites with low binding energy, and type 2 (Sk) was that retained on catalytic sites and form strong interactions with matrix surfaces. A third type (Si) represented irreversible sites occupied by hydroxyatrazine following hydrolysis or other physical/chemical transformations. The rates of reactions were assumed as a function of vacant or available sites which were equally accessible to either Se or Sk. A direct method to quantify the adsorption capacity (ϕm) as a measure of the total retention sites by maintaining high atrazine concentrations in soil solution was not successful. However, the use of an indirectly estimated ϕm along with nonlinear least squares adequately described kinetic retention results. Based on least square best fit, model parameters were independent of initial concentrations (Ci) and an overall set of parameters was capable of describing an entire data set for all Ci. The model was also successful in predicting atrazine retention for a different data set and for all Ci using one set of model parameters which were obtained independently from another data set. Further application of the model was to test whether these independently estimated model parameters can predict the kinetics of adsorption‐desorption hysteresis. Unlike other approaches where adsorption and desorption isotherms were separately described, the second‐order model was capable of describing both adsorption and desorption kinetics. Improved hysteresis predictions were observed when model parameters were derived from experiments of simuar conditions.
Nonequilibrium mass transfer between the vapor, aqueous, and solid phases in unsaturated soils during vapor extraction
Authors: Armstrong J. E. , Frind E. O. , McClellan R. D.
Published: February 2004

Vapor extraction is a commonly used method for removing nonaqueous phase liquid volatile organic compounds (VOC) from the vadose zone. Experience indicates that in the absence of liquid VOC, the efficiency of vapor extraction systems decreases dramatically with time as effluent concentrations approach zero asymptotically. When such systems are restarted after a temporary shutdown, effluent concentrations are often found to recover for a short period before dropping back to preshutdown levels. This behavior is generally attributed to kinetic processes which limit the transfer of contaminant into the moving air. A numerical model is developed to simulate the rate‐limited extraction of volatile compounds governed by first‐order kinetic mass transfer processes. A sensitivity analysis is performed to identify model responses to various kinetic and equilibrium partitioning processes. The model is calibrated using experimental data collected from a pilot‐scale experiment involving vapor extraction of trichloroethylene from fine sand. An analysis of the relationships between airflow rates and the kinetic mass transfer coefficients under various pumping schemes shows that for a given condition, increasing the flow rate has little effect beyond a certain point. It is also shown that pulsed pumping is generally less efficient than continuous pumping at a low rate.
The use of simulation and multiple environmental tracers to quantify groundwater flow in a shallow aquifer
Authors: Reilly Thomas E. , Plummer L. Niel , Phillips Patrick J. , Busenberg Eurybiades
Published: February 2004

Measurements of the concentrations of chlorofluorocarbons (CFCs), tritium, and other environmental tracers can be used to calculate recharge ages of shallow groundwater and estimate rates of groundwater movement. Numerical simulation also provides quantitative estimates of flow rates, flow paths, and mixing properties of the groundwater system. The environmental tracer techniques and the hydraulic analyses each contribute to the understanding and quantification of the flow of shallow groundwater. However, when combined, the two methods provide feedback that improves the quantification of the flow system and provides insight into the processes that are the most uncertain. A case study near Locust Grove, Maryland, is used to investigate the utility of combining groundwater age dating, based on CFCs and tritium, and hydraulic analyses using numerical simulation techniques. The results of the feedback between an advective transport model and the estimates of groundwater ages determined by the CFCs improve a quantitative description of the system by refining the system conceptualization and estimating system parameters. The plausible system developed with this feedback between the advective flow model and the CFC ages is further tested using a solute transport simulation to reproduce the observed tritium distribution in the groundwater. The solute transport simulation corroborates the plausible system developed and also indicates that, for the system under investigation with the data obtained from 0.9‐m‐long (3‐foot‐long) well screens, the hydrodynamic dispersion is negligible. Together the two methods enable a coherent explanation of the flow paths and rates of movement while indicating weaknesses in the understanding of the system that will require future data collection and conceptual refinement of the groundwater system.