bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of methyl chloride in the northwest Atlantic
Authors: Tait V. K. , Moore R. M. , Tokarczyk R.
Published: February 2004

Methyl chloride was measured directly in seawater using a purge and trap system and gas chromatography with electron capture detection. The results indicate that surface waters of the northwest Atlantic are a source of methyl chloride to the atmosphere during late spring/early summer. The average surface concentration was 271 pM (σ = 68 pM), supersaturated with respect to an assumed tropospheric boundary layer mixing ratio of 0.7 ppbv. Elevated concentrations were observed throughout the region in waters above the seasonal thermocline. Near‐surface maxima of differing thickness and intensity were also seen within this upper layer. Broad maxima within the 200–800m depth range were associated with water masses more recently subducted from the surface than the surrounding main thermocline waters. Coastal inputs of methyl chloride appear not to be an important source in the area of study. Although there is some indication of elevated concentrations associated with higher phytoplankton activity in shelf edge regions, the picture concerning phytoplankton production of methyl chloride is still unclear. Further work is required to look at direct and indirect mechanisms by which phytoplankton may influence the distribution of methyl chloride in the oceans. The global ocean‐to‐atmosphere flux estimated from this data set, 3.3 ‐ 4.8 × 1010 mol CH3Cl yr−1 (1.7 ‐ 2.4 × 1012 g CH3Cl yr−1) suggests that the oceans may contribute a smaller flux of methyl chloride to the atmosphere than previously thought.
Energy budget measurements using eddy correlation and Bowen ratio techniques at the Kinosheo Lake tower site during the Northern Wetlands Study
Authors: den Hartog G. , Neumann H. H. , King K. M. , Chipanshi A. C.
Published: February 2004

Fluxes of heat and water vapor were measured on a 20‐m tower at Kinosheo Lake in the Hudson Bay lowlands using eddy correlation and Bowen ratio energy balance techniques. The study period was June 25 to July 28, 1990. Measurements were made over a peat bog consisting of a mixture of sphagnum moss and lichen hummocks and black pools. About 200 m west of the tower were several shallow ponds. The hummocks had a dry, insulating surface and were underlain by an ice layer near 50 cm depth until mid‐July. At the beginning of the period the black pools were covered with water, and although the free water gradually disappeared over the study period, they remained saturated to the end of July. The depth of peat near the tower was about 3 m. Despite the ice layer under the hummocks, their daytime surface temperatures were high, near 35°C, and after the middle of July, above 40°C. Inspection of temperature, precipitation, and radiation data showed that the midsummer period of 1990 was warmer, drier, and sunnier than usual at Moosonee and so by inference at Lake Kinosheo. When all the data were combined to yield average diurnal energy balance components, the eddy correlation fluxes accounted for 90% of the available energy. Latent heat flux averaged 46% of the total available energy and the sensible heat flux averaged 34%. Daytime Bowen ratios were near 1 for the experimental period, suggesting that the bog behaved more like a dryland than a wetland. Eddy correlation measurements of sensible heat and latent heat flux were less than those measured using the Bowen ratio energy balance technique, the average ratios being 0.81 and 0.86, respectively. These differences were possibly due to the difficulty in measuring energy balance components of net radiation and ground heat flux over the mosaic surface.
Enhancement of acidic gases in biomass burning impacted air masses over Canada
Authors: Lefer B. L. , Talbot R. W. , Harriss R. H. , Bradshaw J. D. , Sandholm S. T. , Olson J. O. , Sachse G. W. , Collins J. , Shipham M. A. , Blake D. R. , Klemm K. I. , Klemm O. , Gorzelska K. , Barrick J.
Published: February 2004

Biomass‐burning impacted air masses sampled over central and eastern Canada during the summer of 1990 as part of ABLE 3B contained enhanced mixing ratios of gaseous HNO3, HCOOH, CH3COOH, and what appears to be (COOH)2. These aircraft‐based samples were collected from a variety of fresh burning plumes and more aged haze layers from different source regions. Values of the enhancement factor, ΔX/ΔCO, where X represents an acidic gas, for combustion‐impacted air masses sampled both near and farther away from the fires, were relatively uniform. However, comparison of carboxylic acid emission ratios measured in laboratory fires to field plume enhancement factors indicates significant in‐plume production of HCOOH. Biomass‐burning appears to be an important source of HNO3, HCOOH, and CH3COOH to the troposphere over subarctic Canada.
Reactive nitrogen oxides and ozone above a taiga woodland
Authors: Bakwin Peter S. , Jacob Daniel J. , Wofsy Steven C. , Munger J. William , Daube Bruce C. , Bradshaw John D. , Sandholm Scott T. , Talbot Robert W. , Singh Hanwant B. , Gregory Gerald L. , Blake Donald R.
Published: February 2004

Measurements of reactive nitrogen oxides (NOx and NOy) and ozone (O3) were made in the planetary boundary layer (PBL) above a taiga woodland in northern Quebec, Canada, during June–August, 1990, as part of NASA Artie Boundary Layer Expedition (ABLE) 3B. Levels of nitrogen oxides and O3 were strongly modulated by the synoptic scale meteorology that brought air from various source regions to the site. Industrial pollution from the Great Lakes region of the U.S. and Canada appears to be a major source for periodic elevation of NOx, NOy and O3. We find that NO/NO2 ratios at this site at midday were approximately 50% those expected from a simple photochemical steady state between NOx and O3, in contrast to our earlier results from the ABLE 3A tundra site. The difference between the taiga and tundra sites is likely due to much larger emissions of biogenic hydrocarbons (particularly isoprene) from the taiga vegetation. Hydrocarbon photooxidation leads to relatively rapid production of peroxy radicals, which convert NO to NO2, at the taiga site. Ratios of NOx to NOy were typically 2–3 times higher in the PBL during ABLE 3B than during ABLE 3 A. This is probably the result of high PAN levels and suppressed formation of HNO3 from NO2 due to high levels of biogenic hydrocarbons at the ABLE 3B site.
A three‐dimensional transport model for the middle atmosphere
Authors: Rasch Philip J. , Tie XueXi , Boville Byron A. , Williamson David L.
Published: February 2004

In this paper we describe fundamental properties of an “off‐line” three‐dimensional transport model, that is, a model which uses prescribed rather than predicted winds. The model is currently used primarily for studying problems of the middle atmosphere because we have not (yet) incorporated a formulation for the convective transport of trace species, a prerequisite for many tropospheric problems. The off‐line model is simpler and less expensive than a model which predicts the wind and mass evolution (an “on‐line” model), but it is more complex than the two‐dimensional (2‐D) zonally averaged transport models often used in the study of chemistry and transport in the middle atmosphere. It thus serves as a model of intermediate complexity and can fill a useful niche for the study of transport and chemistry. We compare simulations of four tracers, released in the lower stratosphere, in both the on‐ and off‐line models to document the difference resulting from differences in modeling the same problem with this intermediate model. These differences identify the price to be paid in going to a cheaper and simpler calculation. The off‐line model transports a tracer in three dimensions. For this reason, it requires fewer approximations than 2‐D transport model, which must parameterize the effects of mixing by transient and zonally asymmetric wind features. We compare simulations of the off‐line model with simulations of a 2‐D model for two problems. First, we compare 2‐D and three‐dimensional (3‐D) models by simulating the emission of an NOx‐like tracer by a fleet of high‐speed aircraft. The off‐line model is then used to simulate the transport of 14C and to contrast its simulation properties to that of the host of 2‐D models which participated in an identical simulation in a recent NASA model intercomparison. The off‐line model is shown to be somewhat sensitive to the sampling strategy for off‐line winds. Simulations with daily averaged winds are in very good qualitative agreement but are less diffusive than when driven with instantaneous winds sampled at half‐hour intervals. Simulations with the off‐line and 2‐D models are quite similar in the middle and upper stratosphere but behave quite differently in the lower stratosphere, where the 3‐D model has a substantially more vigorous circulation. The off‐line model is quite realistic in its simulation of 14C. While there are still systematic differences between the 3‐D calculation and the observations, the differences seem to be substantially reduced when compared with the body of 2‐D simulations documented in the above mentioned NASA intercomparison, particularly at 31°N.
OH multipass absorption: Absolute and in situ method for local monitoring of tropospheric hydroxyl radicals
Authors: Armerding W. , Spiekermann M. , Comes F. J.
Published: February 2004

OH multipass absorption is an absolute and spatially resolving method for the in situ monitoring of tropospheric OH. The basis of the method is differential optical absorption spectroscopy. The absorption path in ambient air is folded within a 6 m White cell type multiple reflection system with an open path setup. SO2, CH2O, and naphthalene are measured at 308 nm simultaneously with OH by means of the first harmonic of the laser radiation. NO2 can be measured using the fundamental at 616 nm, respectively. The detection limit of the reported instrument is 2–5×105 OH cm−3 corresponding to a signal to noise ratio of better than one or two for a 1‐min monitoring time dependent on the boundary conditions. The performance of the method is demonstrated by tropospheric OH measurements achieved during a field campaign at the Taunus Observatory on the Kleiner Feldberg in the vicinity of Frankfurt/Main.
Ozone formation at a rural site in the southeastern United States
Authors: Kleinman Lawrence , Lee Yin‐Nan , Springston Stephen R. , Nunnermacker Linda , Zhou Xianliang , Brown Robert , Hallock Kristen , Klotz Paul , Leahy Daniel , Lee Jai H. , Newman Leonard
Published: February 2004

Trace gas measurements pertinent to understanding the transport and photochemical formation of O3 were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3 and the oxidation products of NOx: O3(ppb) = 27 + 11.4 (NOy(ppb) − NOx(ppb)), r2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3 level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to ≈50 ppb. The origin of the O3 above background was investigated by using a free radical budget equation to calculate an in situ O3 production rate in terms of measured concentrations of NO and free radical precursors (O3, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O3 indicates significant O3 production in the afternoon at a time when O3 concentration is either steady or decreasing. The afternoon near‐surface layer is thereby a source region for O3 which can be exported. In situ production accounts for approximately one half of the morning increase in O3 concentration on days with high O3; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour‐to‐hour changes in O3 is found in the diurnal cycles of SO2 and HNO3 which also have rapid increases in the morning. The day‐to‐day variability of O3 was investigated using a back trajectory model. NOy concentration at the measurement site could be reasonably accounted for by considering NOx emission sources located within 1‐day transport distance. In as much as there is a strong correlation between O3 and NOy, the coincidence between trajectory location and NOx emission sources appears to be an important factor influencing midday O3 concentration. Hydrocarbon measurements are consistent with NOx being the limiting factor for formation of O3.
Flux to the atmosphere of CH4 and CO2 from wetland ponds on the Hudson Bay lowlands (HBLs)
Authors: Hamilton J. David , Kelly Carol A. , Rudd John W. M. , Hesslein Raymond H. , Roulet Nigel T.
Published: February 2004

Ponds on peatlands of the Hudson Bay lowlands (HBLs) are complex ecosystems in which the fluxes to the atmosphere of CH4 and CC2 were controlled by interacting physical and biological factors. This resulted in strong diel variations of both dissolved gas concentrations and gas fluxes to the atmosphere, necessitating frequent sampling on a 24‐hour schedule to enable accurate estimates of daily fluxes. Ponds at three sites on the HBL were constant net sources of CH4 and CO2 to the atmosphere at mean rates of 110–180 mg CH4 m−2 d−1 and 3700–11,000 mg CO2 m−2 d−1. Rates peaked in August and September. For CH4 the pond fluxes were 3–30 times higher than adjacent vegetated surfaces. For CO2 the net pond fluxes were similar in magnitude to the vegetated fluxes but the direction of the flux was opposite, toward atmosphere. Even though ponds cover only 8–12% of the HBL area, they accounted for 30% of its total CH4 flux to the atmosphere. There is some circumstantial evidence that the ponds are being formed by decomposition of the underlying peat and that this decomposition is being stimulated by the activity of N2 fixing cyanobacteria that grow in mats at the peat‐water interface. The fact that the gas fluxes from the ponds were so different from the surrounding vegetated surfaces means that any change in the ratio of pond to vegetated area, as may occur in response to climate change, would affect the total HBL fluxes.
Age as a diagnostic of stratospheric transport
Authors: Hall Timothy M. , Plumb R. Alan
Published: February 2004

Estimates of stratospheric age from observations of long‐lived trace gases with increasing tropospheric concentrations invoke the implicit assumption that an air parcel has been transported intact from the tropopical tropopause. However, because of rapid and irreversible mixing in the stratosphere, a particular air parcel cannot be identified with one that left the troposphere at some prior time. The parcel contains a mix of air with a range of transit times, and the mean value over this range is the most appropriate definition of age. The measured tracer concentration is also a mean over the parcel, but its value depends both on the transit time distribution and the past history of the tracer in the troposphere. In principle, only if the tropospheric concentration is increasing linearly can the age be directly inferred. We illustrate these points by employing both a one‐dimensional diffusive analog of stratospheric transport, and the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS). Within the limits of the GCM, we estimate the time over which tropospheric tracer concentrations must be approximately linear in order to determine stratospheric age unambiguously; the concentration of an exponentially increasing tracer is a function only of age if the growth time constant is greater than about 7 years, which is true for all the chlorofluorocarbons. More rapid source variations (for example, the annual cycle in CO2) have no such direct relationship with age.
Atmospheric aerosol trace element chemistry at Mauna Loa Observatory: 1. 1979–1985
Authors: Zieman Jonathan J. , Holmes Jennifer L. , Connor Denise , Jensen Charlene R. , Zoller William H. , Hermann Diane M. , Parrington Josef R. , Gordon Glen E.
Published: February 2004

Aerosol samples were collected at the Mauna Loa Observatory in Hawaii from February 1979 to May 1985. The samples were analyzed via instrumental neutron activation analysis (INAA) for up to 47 elements and via ion chromatography for sulfate. The data are dominated by crustal dust that arrives via long‐range transport from Asia each spring, thus creating a “dust season.” Of the 47 elements detected, 37 have a notably higher mass average during the dust season. The data record is explored using enrichment factors, principal component analysis, and chemical mass balances (receptor modeling). The crustal material accounts for 60–70% of the overall aerosol mass during dust seasons, yet only 15–20% during nondust seasons. It is by far the largest contributor to the natural variation dominating the principal component analysis by describing greater than 60% of the overall variance. Particulate sulfate is another major component accounting for 10–40% of the aerosol mass during dust seasons and 60–75% of the mass during nondust seasons. Particulate sulfate can be derived from crustal material and sea salt. Anthropogenic activity also can produce particulate sulfate or its precursors that can adhere to the surface of crustal material that travels over a polluted area. Minor components in the downslope winds are marine sea‐salt aerosol contributing less than 3% of the aerosol mass during the dust season and 5–6% during the nondust season. Local basalt is considered to contribute less than 2% during the dust season and 3–4% during the nondust season. (Carbon mass is not determined and therefore no carbon‐based aerosols such as soot or organic aerosols are considered in the total aerosol mass.)
Measurement of tropospheric OH and HO2 by laser‐induced fluorescence at low pressure
Authors: Stevens P. S. , Mather J. H. , Brune W. H.
Published: February 2004

The hydroxyl radical (OH) is the primary oxidant in the atmosphere, responsible for many photochemical reactions that affect both regional air quality and global climate change. Because of its high reactivity, abundances of OH in the troposphere are less than 1 part per trillion by volume (pptv) and thus difficult to measure accurately. This paper describes an instrument for the sensitive detection of OH in the troposphere using low‐pressure laser‐induced fluorescence. Ambient air is expanded into a low‐pressure detection chamber, and OH is both excited and detected using the A2∑+(v′ = 0) → X2Π(v″ = 0) transition near 308 nm. An injector upstream of the detection axis allows for the addition of reagent NO to convert ambient HO2 to OH using the fast reaction HO2 + NO → OH + NO2. Using recent advances in laser and detector technologies, this prototype instrument is able to detect less than 1×105 molecules cm−3 (0.004 pptv) of OH with an integration time of 30 s with negligible interferences.
A two‐ion ice particle model of the polar summer mesopause region
Authors: Klostermeyer J.
Published: February 2004

A one‐dimensional model is used to compute height dependent concentrations of molecular ions, cluster ions, positively and negatively charged ice particles, and electrons in the high‐latitude summer mesopause region. In agreement with experimental results, the computed height profiles show a rapid transition from cluster to molecular ions between 85 and 90 km without ion depletion, whereas deep electron “bite‐outs” occur within a particle layer. The model can be forced by turbulent neutral density fluctuations. It is assumed that gradient mixing generates perturbations in the concentration of all particles which are considered as passive conservative constituents so that their energy‐wavenumber spectrum contains a viscous‐convective subrange due to a high Schmidt number. In particular at the boundaries of the particle layer, small fluctuations in the neutral gas give rise to strong fluctuations in the ionized constituents. Computed radar reflectivity profiles are characterized by a double‐peak structure and agree well with reflectivity profiles measured at frequencies of 53.5 and 224 MHz. Without assuming excessive water vapor mixing ratios, the model cannot explain measured reflectivities at frequencies near 1 GHz, supporting observational evidence that radar echoes in the VHF and UHF ranges are generated by distinct scatter mechanisms.
A spreading drop model for plumes on Venus
Authors: Koch Dorothy M.
Published: February 2004

Many of the large‐scale, plume‐related features on Venus can be modeled by a buoyant viscous drop, or plume head, as it rises and spreads laterally below a free fluid surface. The drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space. The boundary integral method is used to solve for the motion of the plume head and for the topography, geoid, and stress at the fluid surface. As the plume approaches the surface, stresses in the fluid above it cause it to spread and become thin below the surface. During the spreading, the surface swell above evolves through various stages whose morphologies resemble several different plume‐related features observed on Venus. When the plume head first approaches the surface, a high broad topographic dome develops, with a large geoid, and radial extensional deformation patterns. At later stages, the topography subsides and becomes plateau‐like, the geoid to topography ratio (GTR) decreases, and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression. We find that a low‐viscosity model plume head (viscosity that is 0.1 times the mantle viscosity) produces maximum topography that is 20% lower, and swell features which evolve faster, than for an isoviscous plume. We compare model results with both the large‐scale highland swells, and smaller‐scale features such as coronae and novae. The dome‐shaped highlands with large GTRs such as Beta, Atla, and Western Eistla Regiones may be the result of early stage plume motion, while the flatter highlands such as Ovda and Thetis Regiones which have lower GTRs may be later stage features. Comparison of model results with GTR data indicates that the highlands result from plume heads with initial diameters of about 1000 km. On a smaller scale, an evolutionary sequence may begin with novae (domes having radial extensional deformation), followed by features with radial and concentric deformation (such as arachnoids), and end with coronae (with mostly concentric deformation). The model predicts that the highlands evolve on a timescale of order 10 Ma, and the smaller‐scale features evolve in a 100 Ma timescale.
Venus O2 visible and IR nightglow: Implications for lower thermosphere dynamics and chemistry
Authors: Bougher S. W. , Borucki W. J.
Published: February 2004

The National Center for Atmospheric Research thermospheric general circulation model for the Venus thermosphere is modified to examine two observed night airglow features, both of which serve as sensitive tracers of the thermospheric circulation. New O2 nightglow data from the Pioneer Venus Orbiter (PVO) star tracker (O2 Herzberg II at 400–800 nm) and ground‐based telescopes (O2 IR at 1.27 μm) (Allen et al., 1992) yield additional model constraints for estimating Venus winds over 100–130 km. Atomic oxygen, produced by dayside CO2 photolysis peaking near 110 km, and transported to the nightside by the global wind system, is partially destroyed through three‐body recombination, yielding the O2 Herzberg II visible nightglow. This emission is very sensitive to horizontal winds at altitudes between 100 and 130 km. Other trace species catalytic reactions also contribute to the production of the very strong nightside infrared (1.27 μm) emission. This paper examines the dynamical and chemical implications of these new data using the Venus thermospheric general circulation model (VTGCM) as an analysis tool. Three‐dimensional calculations are presented for both solar maximum and solar medium conditions, corresponding to early PVO (1979–1981) and PVO entry (mid‐1992) time periods. Very distinct periods are identified in which zonal winds are alternately weak and strong in the Venus lower thermosphere. VTGCM sensitivity studies are conducted to assess the impacts of potential changes in thermospheric zonal and day‐to‐night winds, and eddy diffusion on the corresponding nightglow intensities. It appears that cyclostrophic balance extends above 80 km periodically, owing to a reversal of the upper mesosphere latitudinal temperature gradient, and thereby producing strong zonal winds and correspondingly modified O2 nightglow distributions that are observed.
Crustal structure of North Atlantic Fracture Zones
Authors: Detrick R. S. , White R. S. , Purdy G. M.
Published: February 2004

Seismic studies have established that large‐offset transforms along the slow spreading Mid‐Atlantic Ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extremely heterogeneous in both thickness and internal structure. It is frequently quite thin (<1–2 km thick) and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones. Anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the Mid‐Atlantic Ridge. The geological nature of the seismically anomalous crust found within Atlantic fracture zones, and how this crust forms, are still controversial. One interpretation consistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places. Variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three‐dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large‐scale detachment faults that form preferentially in the cold, brittle lithosphere near the ends of segments.
Polarimetric thermal emission from periodic water surfaces
Authors: Yueh S. H. , Nghiem S. V. , Kwok R. , Wilson W. J. , Li F. K. , Johnson J. T. , Kong J. A.
Published: February 2004

In this paper, experimental results and theoretical calculations are presented to study the polarimetric emission from water surfaces with directional features. It is observed that the measured Stokes parameters of corrugated fiberglass‐covered water surfaces are functions of azimuth angles and agree very well with theoretical calculations. The theory, after being verified by the experimental data, was then used to calculate the Stokes parameters of periodic surfaces without fiberglass surface layer and with rms height of the order of wind‐generated water ripples. The magnitudes of the azimuthal variation of the calculated emissivities at horizontal and vertical polarizations corresponding to the first two Stokes parameters are found to be comparable to the values measured by airborne radiometer and SSM/I. In addition, the third Stokes parameter not shown in the literature is seen to have approximately twice the magnitude of the azimuth variation of either Th or Tυ. The results of this paper indicate that passive polarimetry is a potential tool in the remote sensing of ocean wind vector.
Propagation effects caused by a rough ocean surface on the electromagnetic fields generated by lightning return strokes
Authors: Ming Ye , Cooray Vernon
Published: February 2004

In this paper we have analyzed the effects of propagation caused by a curved and rough ocean surface on the lightning generated radiation fields. In the analysis the electromagnetic fields and the return stroke spectrum was computed from a return stroke model recently introduced by Cooray [1993]. The results show that for frequencies higher than about 10 MHz the attenuation caused by the rough ocean surface is significant and, in the worst cases, the peak of the derivative of the radiation field can be attenuated by about 35% in propagating 50–100 km distance over a rough ocean surface. Comparison of the calculated spectrum with the experimentally observed spectrum show that some of the features in the measured spectrum can be accounted for by the propagation effects.
Fractal approach to lightning radiation on a tortuous channel
Authors: Vecchi G. , Labate D. , Canavero F.
Published: February 2004

In this paper the radiation from a geometrically fractal (i.e., arbitrarily irregular) discharge channel is investigated from the point of view of engineering applications of fractal geometry. Numerical results are presented for lightning return stroke radiation, which demonstrate that the time waveform of radiated field (in the Fraunhofer region) is a fractal, and within the framework of employed approximations, it has the same fractal dimension as the channel path. Some implications of this finding are discussed along with the limitations of the model.
Applications of the conjugate gradient fast Fourier Hankel transfer method with an improved fast Hankel transform algorithm
Authors: Liu Qing‐Huo , Chew Weng Cho
Published: February 2004

The conjugate gradient fast Fourier‐Hankel transforms (CG‐FFHT) method was recently proposed to solve the problems of electromagnetic wave propagation and scattering in axisymmetric inhomogeneous media. This new technique uses the CG method together with the FFHT to solve the wave equation iteratively. Each iteration of the CG method requires O(N log2 N) complex multiplications (N is the number of unknowns). For the application of low‐frequency induction logging, the number of iterations is very small (less than eight). Furthermore, the CG‐FFHT method only requires the storage of several vectors of dimension N. In this paper we present an improved fast Hankel transform (FHT) algorithm as well as some applications of the CG‐FFHT method. It is shown that the improved FHT algorithm results in better accuracy and is more efficient than the other FHT algorithms. Moreover, with this FHT algorithm there is no need to pad the function to be transformed with zeros. Several numerical examples will be shown to illustrate the use of the improved FHT algorithm as well as the applications of the CG‐FFHT method.
Structural and geophysical expression of the Solea graben, Troodos Ophiolite, Cyprus
Authors: Hurst Stephen D. , Moores Eldridge M. , Varga Robert J.
Published: February 2004

Field studies show that extensional structures in the north‐central part of the Troodos Ophiolite, including steep normal faults, grabens, dikes, and low‐angle detachment faults are related to E‐W (present coordinates) spreading. The Solea graben, directly north of the mafic‐ultramafic rock outcrops around Mount Olympus, consists of a 10‐ to 15‐km‐wide western portion that is underlain by a low‐angle detachment fault between the sheeted dike complex and the plutonic section. Structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40° or more about subhorizontal axes. The eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity. Most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross‐cutting ophiolitic dikes and hydrothermal mineralization. Timing of the uplift of the central part of the Troodos Ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low‐angle normal faulting. The Solea graben spreading center provides a model for the structure of modern mid‐ocean ridges that are spreading amagmatically.
A balanced model of the southern Central Andes
Authors: Schmitz Michael
Published: February 2004

To explain the actual crustal thickness of the southern Central Andes a balanced cross‐section at 21°S latitude was modeled. A forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations. In the model, 320 km of shortening since late Cretaceous, based on balanced geological cross sections, are assumed for the whole Andes from the trench to the foreland, and the initial crustal thickness varies between 30 and 40 km. It results that about 20% of the actual crustal volume must be explained by other sources than crustal shortening. Structures of high‐velocity material in the upper crust of the Eastern Cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc. Underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted Nazca plate may contribute to the lower crust of the forearc and arc regions. Crustal thickening of the Eastern Cordillera and parts of the Altiplano was undoubtedly produced by tectonic thickening, whereas the sources of thickening in the Western Cordillera and forearc region are uncertain.
Early to mid Tertiary crustal extension in the Central Alps: The Turba Mylonite Zone (Eastern Switzerland)
Authors: Nievergelt Peter , Liniger Markus , Froitzheim Nikolaus , Mählmann Rafael Ferreiro
Published: February 2004

In the Penninic‐Austroalpine boundary zone of Eastern Switzerland a clear distinction can be made between upper and lower stacks of nappes with contrasting tectonic evolution that are separated by the Turba mylonite zone (TMZ). The Austroalpine nappes, Platta ophiolites, and Malenco‐Forno‐Lizun ophiolites belong to the upper stack. These nappes were formed by predominantly west directed, late Cretaceous shortening and imbrication. The lower stack, comprising the Suretta nappe, Schams nappes, Avers schists, and Oberhalbstein flysch units, includes Tertiary sediments and was deformed and metamorphosed in early Tertiary time. Kinematic indicators in the mylonites of the TMZ, representing the boundary between the two stacks, show a top‐to‐the‐east directed movement, which is clearly distinct from the regionally prevalent top‐to‐the‐north direction of thrusting in the Tertiary. The main foliation in the shear zone is discordant with respect to the structures of the hanging wall, and concordant to the footwall. Vitrinite reflectance in shaly sediments above and below the TMZ in the northern part indicates that the TMZ coincides with a drop in peak metamorphic temperature from about 350°C in the top of the footwall to about 280°C at the base of the hanging wall. Therefore the TMZ is interpreted as an east dipping, low‐angle normal fault. The age of its activity is bracketed between about 45 and 30 Ma, based on overprinting relations with radiometrically dated features (45–30 Ma: main foliation in the footwall, older than TMZ activity; 30 Ma: Bergell granodiorite intrusion, younger than TMZ activity). Extension along the TMZ was followed by further north‐south shortening, as indicated by postmylonitic folding of the TMZ. Normal faulting along the TMZ accommodated relative uplift of the Lepontine structural dome, a large metamorphic complex in the Central Alps. Unroofing of this dome started before 30 Ma and continued in the Miocene with activity of further normal faults situated at deeper structural levels.
A tale of two continents: Some tectonic contrasts between the central Andes and the North American Cordillera, as illustrated by their paleomagnetic signatures
Authors: Beck Myrl E. , Burmester Russell R. , Drake Robert E. , Riley Paul D.
Published: February 2004

Comparison of patterns of paleomagnetic poles from orogenic belts with appropriate reference poles for the craton can help to delineate important large‐scale tectonic processes. Comparison of the paleomagnetic signatures of the western Cordillera of North America and the central Andes shows that the western edges of these belts have had profoundly different Mesozoic and Cenozoic histories. Specifically, the North American Cordilleran pattern shows strong evidence of post‐middle Cretaceous relative northward displacement of outboard crustal blocks, but there is almost no comparable evidence of margin‐parallel displacement in the Andes. We speculate that this may largely be a consequence of a simple difference in shape: the convex‐westward western margin of North America facilitates margin‐parallel displacement as a response to oblique subduction, whereas the concave‐westward margin of South America inhibits it. The patterns of block rotations found along the western edge of the two orogens also are quite different. Nearly everywhere within the western North American Cordillera crustal blocks have rotated clockwise since mid‐Cretaceous time, reflecting a pervading state of dextral shear. Within the western Andes of Peru and northern Chile, however, Mesozoic and Cenozoic rocks in Peru (and northernmost Chile) are rotated strongly counterclockwise, whereas rocks of the same age in the remainder of Chile (to about latitude 48°S) are rotated clockwise. A model combining oroclinal bending and block rotations driven by oblique subduction can account for the paleomagnetic observations.
Tectonic evolution of the boundary between the Central Iberian and Ossa‐Morena zones (Variscan belt, southwest Spain)
Authors: Azor Antonio , Lodeiro Francisco González , Simancas J. Fernando
Published: February 2004

Three tectonometamorphic units can be differentiated in the boundary between the Central Iberian and Ossa‐Morena zones of the Iberian Massif (Variscan belt). The three units have been named the Northern Unit, Central Unit, and Southern Unit. The Northern Unit corresponds to the border of the Central Iberian Zone; it evolved at low‐temperature and intermediate‐ or low‐pressure metamorphic conditions and was affected by top to the SE ductile shearing. The Central Unit, placed under the Northern Unit, preserves high‐pressure Silurian eclogitic assemblages retrograded by high‐ to medium‐temperature and, finally, low‐temperature ductile shearing with top to the NW sense of movement (oblique left‐lateral). The Central Unit is superposed on the Southern Unit. The latter corresponds to the border of the Ossa‐Morena Zone and underwent low‐pressure, intermediate‐temperature metamorphism synchronous with right‐lateral (top to the SE) ductile shearing. The envisaged tectonic evolution is as follows: after a stage of lower Paleozoic rifting, subduction of the Central Unit under the Northern Unit took place in Silurian times. As a result of the crustal thickening, a gravitational instability developed, giving way to left‐lateral extensional shearing that affected the entire Central Unit. The combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high‐pressure rocks of the Central Unit. This tectonic evolution reveals that the boundary between the Central Iberian and Ossa‐Morena zones is a suture of the Variscan belt.
Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska
Authors: Amato Jeffrey M. , Wright James E. , Gans Phillip B. , Miller Elizabeth L.
Published: February 2004

Field observations and U‐Pb isotopic data from plutonic and high‐grade metamorphic rocks within the Kigluaik gneiss dome on the Seward Peninsula, Alaska, document a Late Cretaceous age of peak metamorphism and shed light on the relationship between fundamentally mantle‐derived magmatism and the development of the gneiss dome. The dome consists of upper‐amphibolite‐facies to granulite‐facies metasedimentary rocks mantling the granitoid Kigluaik pluton. The main deformational fabric in the dome is a well‐defined, moderately dipping foliation and compositional layering that contains a pervasive, east‐west trending mineral‐elongation lineation defined by sillimanite and hornblende. Leucosomal segregations in migmatite are boudinaged and isoclinally folded, and late‐stage tension gashes are filled with melt, demonstrating that most deformation occurred at peak metamorphic temperatures. The large pluton in the core of the dome consists of a granitic cap overlying a mafic to intermediate root. Mafic pillows with crenulate margins and spectacular magma mingling textures indicate that the two magmas were coeval. The Kigluaik pluton is largely undeformed and discordant, although some dikes possess a weak deformational fabric. The lack of quench textures at the margins of the pluton and very limited alteration in adjacent wall rock suggest that the pluton was emplaced while country rocks were still at high temperatures. U‐Pb analyses of three fractions of zircon from a highly strained and concordant garnet‐bearing granite orthogneiss yield an intrusive age of 105 Ma; therefore significant deformation in the dome must have occurred after 105 Ma. Several U‐Pb analyses of monazite from metapelite and pegmatite (derived from partial melting of metapelite) yield an age of 91 ± 1 Ma for high‐temperature metamorphism and deformation. U‐Pb analysis of eight fractions of zircon from the Kigluaik pluton shows that it crystallized at 92 ± 2 Ma. Thus there is both field and geochronologic evidence for coeval magmatism, metamorphism, and deformation at about 92 Ma, which may represent the latest stages of a more protracted tectonic event. We present a model for gneiss dome development wherein a large, silicic to intermediate magmatic diapir with its mantling gneisses ascended from ∼35 km to ∼15 km during an event associated with crustal extension in northern Alaska. We emphasize the close link between plutonism and gneiss dome development and the fundamentally mafic character of this plutonism.
Field measurement of radial solute transport in fractured rock
Authors: Novakowski K. S. , Lapcevic P. A.
Published: February 2004

The results of a tracer experiment conducted in a single fracture are interpreted using a semianalytical model which accounts for advective dispersion, matrix diffusion, mixing in the test zones, and for tortuosity. The experiment was conducted using a conservative tracer which was injected into a steady divergent flow field. Arrival of tracer was monitored in an array of 13 boreholes intersecting the fracture over a square area of approximately 30 m on a side. The arrival of tracer was detected in 11 of the 13 boreholes. Simulation of the transport process suggests that matrix diffusion may play a significant role in the migration of solutes. Contrary to previously published results, the relative influence of advective dispersion was found to be increasingly diminished with increasing scale. Fracture apertures determined from the results of the tracer experiment were found to be in general agreement with apertures calculated from independent hydraulic tests. The individual advective processes that contribute to advective dispersion remain unresolved on the basis of this experiment.
Injection‐attachment of Methylosinus trichosporium OB3b in a two‐dimensional miniature sand‐filled aquifer simulator
Authors: Shonnard D. R. , Taylor R. T. , Hanna M. L. , Boro C. O. , Duba A. G.
Published: February 2004

For some potentially useful and emerging in situ bioremediation technologies it is important to control bacterial attachment to subsurface materials during the injection of microbial cell suspensions. In this study the attachment patterns of Methylosinus trichosporium OB3b were measured after horizontal injections into a two‐dimensional miniature aquifer simulator containing a wet homogeneous sand. In preliminary sand column assays, bacterial attachment to the sand was increased nearly 2 orders of magnitude compared to attachment in the presence of distilled water by raising the concentration of a pH 7.0 sodium‐potassium phosphate buffer to 10 mM. The maximal concentration of attached cells was ∼4×108 cells/g dry sand with both sand minicolumns and the wet sand aquifer simulator. For the latter this occurred on streamlines directly between the horizontal injector and withdrawal ports, where injection‐withdrawal velocities were the highest. The effects of a simulated groundwater cross flow during suspension injection on bacterial attachment to the aquifer simulator sands were also studied, and a peristaltic pumping method to counteract these groundwater flow effects resulted in a more localized pattern, i.e., without extensive downstream skewing of the bacterial attachment zone. Phenol red was utilized as a nonbinding, red‐colored tracer compound. It proved to be very convenient for quantitatively measuring the earlier breakthroughs of cells versus an inert tracer during the aquifer simulator and subsequent capillary tube cell injection experiments and also for visualizing the anticipated boundaries of cell attachment in the aquifer simulator. The effect of injection velocity on the observed bacterial attachment patterns in these experiments appears to be accounted for by colloid filtration theory.
Characterization of aquifer properties using artificial neural networks: Neural kriging
Authors: Rizzo Donna M. , Dougherty David E.
Published: February 2004

A method for pattern completion based on the application of artificial neural networks and possessing many operational objectives of the ordinary kriging approach, neural kriging, is developed. A neural kriging (NK) network is described, implemented in a parallelizing algorithm, and applied to develop maps of discrete spatially distributed fields (e.g., log hydraulic conductivity). NK is, in the case of two discrete field values, similar to indicator kriging. It uses a feed‐forward counterpropagation training approach because field observations are available and because fast yet reliable results are obtained. NK is data‐driven and requires no estimate of a covariance function. The optimal design of the NK network is found to depend on the number of hidden units in a more complex way than expected. The quality of the estimate of each pixel of the NK maps can be presented as well, as in kriging, to help identify areas in which additional information will be most beneficial. A comparison with a reference field shows that the NK network produces unbiased errors relative to sample bias and reproduces the variogram of a quantized random field with reasonable accuracy. Ordinary kriging (OK) followed by quantization can also perform well; however, estimation errors in the variogram selected for use in OK (in this case the range cofficient in particular) must be carefully examined and treated. The NK method can provide multiple realizations of the estimated field, all of which respect observations; hence conditional simulation is demonstrably possible. The combination of simplicity, interpolation, reasonably accurate prediction statistics, ability to provide conditional simulations, and computational speed suggest that artificial neural networks can be useful tools in geohydrology when applied to specific well‐defined problems for which they are well suited, such as aquifer characterization.