Bright Zone Research Articles

The San Gabriel Mountains bright reflective zone: possible evidence of young mid-crustal thrust faulting in southern California

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a line extending northeastward from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles basin and San Gabriel Mountains. Shots and receivers were spaced most densely through the San Gabriel Mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area. A stack of common-midpoint (CMP) data reveals a bright reflective zone, 1-s thick, that dominates the stack and extends throughout most of the mid-crust of the San Gabriel Mountains. The top of this zone ranges in depth from 6 s (∼18-km depth) in the southern San Gabriel Mountains to 7.5 s (∼23-km depth) in the northern San Gabriel Mountains. The zone bends downward beneath the surface traces of the San Gabriel and San Andreas faults. It is brightest between these two faults, where it is given the name San Gabriel Mountains ‘bright spot’ (SGMBS), and becomes more poorly defined south of the San Gabriel fault and north of the San Andreas fault. The polarity of the seismic signal at the top of this zone is clearly negative, and our analysis suggests it represents a negative velocity step. The magnitude of the velocity step is approximately 1.7 km/s. In at least one location, an event with positive polarity can be observed 0.2 s beneath the top of this zone, indicating a thickness of the order of 500 m for the low-velocity zone at this location. Several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone, possibly a ‘master’ decollement. (1) It represents a significant velocity reduction. If the rocks in this zone contain fluids, such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the SGMBS; near-lithostatic fluid pressure is required in the SGMBS. Such differential changes are believed to occur in the neighborhood of active fault zones, where ‘fault-value’ action has been postulated. Less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke. (2) It occurs at or near the brittle-ductile transition, at least in the southern San Gabriel Mountains, a possible zone of concentrated shear. (3) A thin reflection rising from its top in the southern San Gabriel Mountains projects to the hypocenter of the 1987 M 5.9 Whittier Narrows earthquake, a blind thrust-fault earthquake with one focal plane subparallel to the reflection. Alternatively, one could argue that the bends or disruptions in the reflective zone seen at the San Gabriel and San Andreas faults are actually offsets and that the reflective zone is therefore an older feature, possibly an older fault zone.

West margin of North America — a synthesis of recent seismic transects

A comparison of the deep structure along nine recent transects of the west margin of North America shows many important similarities and differences. Common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust, accreted oceanic/arc (or oceanic-like) lithosphere of Mesozoic through Cenozoic ages, Cenozoic accretionary prisms, Mesozoic accretionary prisms, backstops to the Mesozoic prisms, and undivided lower crust. Not all of these elements are present along all transects. In this study, nine transects, including four crossing subduction zones and five crossing transform faults, are plotted at the same scale and vertical exaggeration (V.E. 1 : 1), using the above scheme for identifying tectonic elements. The four subduction-zone transects contain actively subducting oceanic crust, Cenozoic accretionary prisms, and bodies of basaltic rocks accreted in the Cenozoic, including remnants of a large, oceanic plateau in the Oregon and Vancouver Island transects. Rocks of age and composition (Eocene basalt) similar to the oceanic plateau are currently subducting in southern Alaska, where they are doubled up on top of Pacific oceanic crust and have apparently created a giant asperity, or impediment to subduction. Most of the subduction-zone transects also contain Mesozoic accretionary prisms, and two of them, Vancouver Island and Alaska, also contain thick, tectonically underplated bodies of late Mesozoic/early Cenozoic oceanic lithosphere, interpreted as fragments of the extinct Kula plate. In the upper crust, most of the five transform-fault transects (all in California) reflect: (1) tectonic wedging of a Mesozoic accretionary prism into a backstop, which includes Mesozoic/early Cenozoic forearc rocks and Mesozoic ophiolitic/arc basement rocks; and (2) shuffling of the subduction margin of California by strike-slip faulting. In the lower crust, they may reflect migration of the Mendocino triple junction northward (seen in rocks east of the San Andreas fault) and cessation of Farallon-plate subduction (seen in rocks west of the San Andreas fault). In northern California, lower-crustal rocks east of the San Andreas fault have oceanic-crustal velocity and thickness and contain patches of high reflectivity. They may represent basaltic rocks magmatically underplated in the wake of the migration of the Mendocino triple junction, or they may represent stalled, subducted fragments of the Farallon/Gorda plate. The latter alternative does not fit the accepted ‘slabless window’ model for the migration of the triple junction. This lower-crustal layer and the Moho are offset at the San Andreas and Maacama faults. In central California, a similar lower-crustal layer is observed west of the San Andreas fault. West of the continental slope, it is Pacific oceanic crust, but beneath the continent it may represent either Pacific oceanic crust, stalled, subducted fragments (microplates) of the Farallon plate, or basaltic rocks magmatically underplated during subduction of the Pacific/Farallon ridge or during breakup of the subducted Farallon plate. The transect in southern California is only partly representative of regional structure, as the structure here is 3-dimensional. In the upper crust, a Mesozoic prism has been thrust beneath crystalline basement rocks of the San Gabriel Mountains and Mojave Desert. In the mid-crust, a bright reflective zone is interpreted as a possible ‘master’ decollement that can be traced from the fold-and-thrust belt of the Los Angeles basin northward to at least the San Andreas fault. A Moho depression beneath the San Gabriel Mountains is consistent with downwelling of lithospheric mantle beneath the Transverse Ranges that appears to be driving the compression across the Transverse Ranges and Los Angeles basin.

Influence of Severity of Myocardial Injury on Distribution of Macromolecules: Extravascular Versus Intravascular Gadolinium-Based Magnetic Resonance Contrast Agents

Objectives. This study sought to 1) compare the distribution of extravascular (573 Da) and intravascular (92 kDa) magnetic resonance (MR) contrast agents in reperfused infarcted myocardium, and 2) investigate the effect of injury severity on these distribution patterns.Background. Myocardial distribution of low and high molecular weight contrast agents depends on vascular permeability, diffusive/convective transport within the interstitium and accessibility of the intracellular compartment (cellular integrity).Methods. To vary the severity of myocardial injury, 72 rats were subjected to 20, 30, 45 or 75 min (n = 18, respectively) of coronary artery occlusion. After 2 h of reflow, the animals received either 0.05 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (Gd-DTPA-BMA) (n = 24), (Gd-DTPA)30-albumin (n = 24) or saline (control group, n = 24). Three minutes after injection, the hearts were excised and imaged (spin-echo imaging parameters: repetition time 300 ms, echo time 8 ms, 2-tesla system), followed by triphenyltetrazolium chloride staining for infarct detection and sizing.Results. Histomorphometric and MR infarct size (expressed as percent of slice surface) correlated well: r = 0.96 for Gd-DTPA-BMA; r = 0.95 for (Gd-DTPA)30-albumin. On Gd-DTPA-BMA–enhanced images, reperfused myocardial infarctions were homogeneously enhanced. The ratio of signal intensity of infarcted/normal myocardium increased with increasing duration of ischemia (overall p < 0.0001, analysis of variance [ANOVA]), indicating an increase in the distribution volume of Gd-DTPA-BMA in postischemic myocardium. On (Gd-DTPA)30-albumin–enhanced images, reperfused infarctions consisted of a bright border zone and a less enhanced central core. The extent of the core increased with increasing duration of ischemia (overall p value < 0.0001, ANOVA).Conclusions. At 2 h of reperfusion, the distribution of MR contrast agents in postischemic myocardium is 1) specific for extravascular and intravascular agents, and 2) modulated by the duration of ischemia.

An experimental investigation of the propagation of sound over a curved, rough, rigid surface

Small-scale laboratory experiments have been conducted to study the propagation of sound over a curved, rough, rigid, convex surface in the range kR=275–550, and kε=0.09–0.18, where k is the wave number, R the radius of curvature, and ε the characteristic roughness length scale of steep-sloped, densely packed scatterers. Experimental results are presented at 6, 8, and 12 kHz, with the receiver successively on the surface, along the line of sight behind the surface, and along a vertical axis in the shadow zone. At low frequencies (f=6 kHz, kε=0.09), a boundary wave caused by coherent multiple scattering develops near the surface, creeps into the deep shadow zone, and increases the sound-pressure levels measured in the deep shadow zone by up to 8 dB as compared to the levels measured in the absence of surface roughness. At high frequencies (f=12 kHz, kε=0.18), surface roughness causes incoherent scattering and decreases the measured sound-pressure levels by about 10 dB on the surface, in the shadow zone, and by 10 to 20 dB along the line of sight behind the apex. It is shown that incoherent scattering occurs mostly in the bright zone, before the apex, whereas coherent scattering (boundary wave growth) occurs all along the surface, even in the deep shadow.

Use of the fast field program for predicting diffraction of sound by curved surfaces

The fast field program (FFP) is a powerful tool for solving outdoor sound propagation problems in stratified atmospheres. It is shown here to be equally powerful for solving for the propagation of sound over curved surfaces of finite impedance. Results from scale model experiments are in excellent agreement with the FFP calculation in the deep shadow, penumbra, and bright zone behind the surface. The results are also compared with Berry and Daigle’s creeping wave series [J. Acoust. Soc. Am. 83, 2047–2058 (1988)]. The FFP calculation and creeping wave series yield identical results in the deep shadow. In the penumbra and bright zone behind the curved surface the FFP is in slightly better agreement with the data than the creeping wave series, although the differences are typically 1 dB or less which is comparable to the experimental error. The improvement is due to the more accurate sound speed profile and coordinate mapping used rather than to any differences in the computational method.

Radio continuum and long-slit optical spectroscopy of the Planetary Nebulae Cn 3-1 and M 3-27

Long-slit optical spectroscopy and VLA-B radio continuum (λ = 3.6 cm) observations toward the compact planetary nebulae Cn 3-1 and M 3-27 are presented. Optical spectra were taken at different position angles (PAs) with the 2.2 m telescope at Calar Alto (Spain), covering the range from 6549 to 6751 Å. The radio and optical data show that the ionized shell of Cn 3-1 is an ellipsoid (size ≃ 6″ × 5″, PA ≃ 72°) containing a bright ring-like equatorial zone (size ≃ 2″.6, expansion velocity ≃ 14 km s–1) and two bright point-symmetric arcs, extending from the equator towards the polar regions of the ellipsoid (Fig. 1). These arcs seem to be filamentary structures embedded in the ellipsoid. An ionized stellar wind has been detected through faint extended wings in the Hα (≃ 660 km s–1) and [N II] (≃ 460 km s–1) emission lines. M 3-27 is unresolved at 3.6 cm (size ≤ 0″6). The detected [N II] and [S II] emission lines arise in a compact (≤ 1″4) probably non-spherical region which is identified with the ionized shell of M 3-27. The Hα emission from M 3-27 is dominated by strong emission from an ionized stellar wind and exhibits a Type III P Cygni profile with very extended wings (≃ 3000 km s–1). The estimated kinematic age and ionized mass of Cn 3-1 (≃ 1300 yr, 4 × 10–2 M⊙) and M 3-27 (≤ 530 yr, ≃ 3 × 10–4 M⊙) indicate that both objects are young planetary nebulae. Extended halos (size ≃ 36″ in Cn 3-1, ≃ 24″ in M 3-27) have been spectroscopically detected in both objects. An analysis of the kinematic and emission properties shows that both halos are reflection nebulosities and suggests that the distribution of neutral material in them probably is largely isotropic. The results suggest that the halos correspond to isotropic mass ejections occurred in the last ≃ 2 × 104 yr of the AGB phase of the Cn 3-1 and M 3-27 progenitors.

Oxygen/sulfide regime and nematode fauna associated with Arenicola marina burrows: new insights in the thiobios case

Profiles of oxygen and sulfide around the burrows of the lugworm, Arenicola marina, from a North Sea tidal flat were examined with microelectrodes, and the steep gradients were related to the microdistribution of nematodes. Around the tail shaft free oxygen penetrated only 2 mm into the burrow wall, coinciding with a bright zone sharply limited by the ambient black sediment. Contrastingly, in normal bottoms of the tidal flat (“controls”) only the surface of the bright zone was supplied with free oxygen. Here, the dark colouration coincided with the presence of free hydrogen sulfide. Around the tail shaft the nearest free hydrogen sulfide was detected 6 mm from the burrow wall leaving several millimetres of black sediment without measurable free sulfide. We discuss how these divergencies may relate to the stability of the oxygen/sulfide gradients and the course of time involved in their formation. A total of 54 nematode species were identified. Based on non-metric Multidimensional Scaling Ordination, four nematode assemblages corresponded to four microhabitats of the A. marina burrow: the funnel, the feeding pocket, the tail shaft and the feacal cast. The tail shaft assemblage (oxic plus partly anoxic zones) was similar to that of the anoxic zone of the control sediment. It was dominated by the most abundant nematode in the present study, Metalinhomoeus biformis (mean abundance in tail shaft 202 indx10 cm-3). Adults of common nematode species from sulfidic microhabitats had a significantly higher length/diameter ratio than those inhabiting the oxic zone of the control sediment (p<0.001). The chemical recordings and metric analysis indicate that these slender nematodes around the A. marina tail shaft and in the reduced horizons of the reference sites represent thiobiotic assemblages, as compared to the shorter and stouter oxybiotic species characterising the assemblages from the surface zone and (partly) the funnel.

Bone changes after experimental osteotomies fixed with absorbable self-reinforced poly- l-lactide screws or metallic screws studied by plain radiographs, quantitative computed tomography and magnetic resonance imaging

The healing of the distal femoral osteotomy fixed with self-reinforced poly- l-lactide (SR-PLLA) or metallic screws in 16 rabbits was evaluated with plain radiographs, quantitative computed tomography and magnetic resonance imaging (MRI). At 36 weeks in the metallic fixation there was significantly more external callus than in the SR-PLLA fixation. On the metallic fixation side the cortical bone mineral density was significantly lower than on the non-operated side at 6 weeks as well as at 36 weeks. This decrease was not found in the SR-PLLA fixation. On the MRI on T1-weighted images a dark zone (signal void), and on the T2-weighted images a bright zone (increased signal intensity), was seen surrounding the screws, indicating oedema. At 36 weeks these oedematous zones were significantly smaller in the SR-PLLA fixation group than in the metallic one. The results give indirect evidence towards a more rapid and better osteotomy healing with the more physiologically elastic SRPLLA screws than with the metallic screws. Fixation with SR-PLLA screws may prevent stressprotection atrophy and weakening of the fixed bone usually caused by the rigid metallic fixation.

Identification of Myocardial Cell Death in Reperfused Myocardial Injury Using Dual Mechanisms of Contrast-Enhanced Magnetic Resonance Imaging

Because the magnitude of dysprosium-induced signal loss depends on the microheterogeneity of its distribution (exclusion from intracellular space), we proposed that loss of myocardial cell integrity would be reflected by decreased potency of dysprosium in the injured compared with normal myocardium. We measured the effect of dysprosium on magnetic resonance (MR) imaging signal intensity of reperfused infarcted and nonischemic myocardium and related it to tissue concentration of the contrast media. Rats were subjected to 1 hr coronary artery occlusion followed by 1 hr reperfusion. After 45 min of reflow, group 1 (n = 9) received 1.0 and 0.2 mmol/kg dysprosium diethylenetriamine pentaacetic acid-bismethylamide (Dy-DTPA-BMA) and gadodiamide (Gd-DTPA-BMA), respectively. Group 2 (n = 7) received no contrast agents. Excised hearts were imaged with spin-echo T1- and T2-weighted sequences. After imaging, hearts were stained (triphenyltetrazolium chloride) to define the injured zones. Concentrations of Dy-DTPA-BMA and Gd-DPTA-BMA in regional myocardial tissue were determined by induction coupled plasma-atomic emission spectrometry. Separate groups received one or the other contrast medium alone to control for potential error from the mixed effects of the two agents. Gd-DTPA-BMA delineated reperfused infarcted myocardium as a bright zone on T1-weighted images, thus indicating delivery of the agent and reperfusion at the tissue level. Dy-DTPA-BMA delineated the reperfused infarction as a bright region by decreasing the signal intensity of nonischemic myocardium significantly more than that of injured myocardium, despite being present in greater concentration (by 2.46-fold) in the injured myocardium. These findings are consistent with the hypothesis that the failure of myocardial cells to exclude the dysprosium compound is responsible for the diminished potency of dysprosium to cause MR imaging signal intensity loss in reperfused myocardial infarction. The combination of the two contrast media may define reperfusion of the myocardium at the tissue level (Gadolinium distribution) and the presence and extent of myocardial necrosis (diminished dysprosium effect) in reperfused myocardial infarctions.

Scale model experiments on the diffraction and scattering of sound by a partially rough curved surface

Scale model experiments have been conducted to study the propagation of sound over a curved surface partially covered with rigid random roughness elements in the high-frequency diffraction/low-frequency scattering regime. The effects of roughness are found by comparing the sound field with that obtained with a fully smooth curved surface. Experiments indicate that roughness in the shadow zone acts to tunnel more sound into the shadow zone of the partially rough surface, relative to the curved smooth surface, via a ‘‘creeping boundary wave.’’ This wave qualitatively follows the trends of both creeping waves due to diffraction and Tolstoy-type ‘‘boundary waves’’ due to coherent scattering. However, roughness in the bright zone causes less sound to propagate into the shadow zone of the partially rough surface, relative to the curved smooth surface, because of incoherent scattering. [This work is supported by a Fannie and John Hertz Fellowship.] a)Current address: National Center for Physical Acoustics, University, MS 38677.

Scale model experiments on the diffraction and scattering of sound by a curved rough surface

Scale model experiments have been conducted to study the propagation of sound over a curved rough surface with curvature much greater than the wavelength and roughness smaller than the wavelength. The MAE model [J. Acoust. Soc. Am. 92, 605–608 (1993)] is used to determine the sound field in the vicinity of the curved surface. The Tolstoy–Medwin roughness model [J. Acoust. Soc. Am. 79, 657–665 (1986)] is used to model the rough surface. The two models are combined to qualitatively describe the sound field. Experiments indicate that: (1) just above the surface, the roughness causes a boundary wave that is generated by the direct waves in the bright zone and by the creeping waves in the shadow zone; (2) there are two frequency regimes with the transition determined by the curvature and the roughness; (3) along the surface, more sound (compared with diffraction by the curved smooth surface) is tunneled into the shadow zone at low frequencies and less sound is tunneled at higher frequencies. Along the line of sight, the trend is similar but more complicated. [This work is supported by a Fannie and John Hertz Fellowship.]

A study of the reduction of explosive impulses by finite sized barriers

Two investigations have been carried out on the diffraction of high level (160-dB peak) impulses around barriers of finite size. One of the investigations, carried out with the barrier situated on hard ground, has shown peak sound levels to be reduced by as much as 12 dB by an effective barrier height of 1.8 m and source–receiver separation of 25 m. The results of this investigation have been compared with a number of linear diffraction theories for both thin and thick barriers. The major finding is that while most theories can reasonably predict the peak diffracted sound level and general wave shape they all fail to predict the sharp leading edge of the pulse which is observed in practice. For the second investigation, in which the barrier was situated on an acoustically soft surface, an effective barrier height of about 1.2 m gave a reduction of around 6 dB in the peak sound level for a source–receiver distance of 25 m compared to unobstructed propagation over the same surface. The results show reasonable agreement between prediction and theory for receiver positions well in the shadow zone. However, near the shadow boundary and in the bright zone agreement is not as good.

Computer vision profilometer: equipment and evaluation of measurements

This paper describes a new equipment that measures roughness values by a computer vision (CV) technique. Measurements carried out by a CV profilometer are also evaluated. A laser source (power 2 mW), a cylindrical lens and a charge coupled device (CCD) TV-camera with a suitable optical system form an image of the profile of the sample under inspection. This image is then transformed into a binary image by thresholding and the line that divides the bright zone from the dark zone is the sample profile. From this line the characteristic roughness values can be calculated. The roughness measurements are carried out both by the CV profilometer and a stylus profilometer on eight specimens with various degrees of finish. For each investigated parameter, a strong linear relationship exists between CV and stylus measurements.

Palynologic and petrographic variation in the Pond Creek coal bed, Pike County, Kentucky

Recent studies of the petrology and ash geochemistry of the Pond Creek coal bed (Middle Pennsylvanian Breathitt Formation) in Pike and Martin Countries, Kentucky, have defined vertical and lateral trends that are related to the development of the northeast-trending Belfry anticline contemporaneous with the deposition of the Pond Creek peat. To the northwest of the anticline the coal bed has a high-CaCO 3 middle zone and a high Fe 2O 3-high sulfur upper zone. To the east of the anticline, particularly in the Lick Creek and Jamboree 7 1 2 ′ quadrangles, the coal bed exhibits a distinctive petrographic zonation. In the latter area, the usual megascopic sequence is a basal durain followed by a bright clarain-durain/dull The palynology of the Pond Creek coal bed was investigated in six sets of bench/lithotype samples from the Lick Creek and Jamboree 7 1 2 ′ quadrangles. The low-vitrinite basal durain (zone 1) has a palynomorph assemblage which reflects the importance of tree ferns and herbaceous lycopods. The easternmost site has a bright clarain below the durain. The latter lithotype has a fair diversity of palynormph groups and also has the highest percentage of arborescent lycopod forms of any bench below the bone (zone 5). The bright zone 2 above the durain has the greatest maceral group range and the greatest floral diversity of any of the lithotypes. The zone 3 dull lithotype has increased amounts of herbaceous lycopods and decreased amounts of arborescent lycopods, sphenopsids and tree ferns. The zone 4 bright lithotypes are generally over 80% vitrinite and, in general, exhibit a dominance of arborescent lycopods although sphenopsid, tree fern and small fern forms exhibit varying importance. The high-ash bone (zone 5) has 71.6–77.0% vitrinite. This lithotype is marked by a dominance of herbaceous lycopods at the expense of arborescent lycopods and sphenopsids. The zone 6 bright clarian is marked by an increase in arborescent lycopods and a large decrease in herbaceous lycopods. The low-vitrinite terminal durain has an assemblage with high percentages of herbaceous lycopods and low percentages of arborescent lycopods as well as increase in tree fern spores over zone 6. In general, the petrology and palynology trends are predictable. Dull lithotypes have abundant herbaceous lycopod representation and bright lithotypes have abundant arborescent lycopod representation. The greatest deviation from this simple pattern is within the zone 4 bright lithotypes, where diverse palynomorph assemblages occur among the high-vitrinite benches. Overall, the greatest floral diversity appears to have occurred near the base of the coal bed.

Postnatal Development of the Pontine Projections from the Visual Cortex of the Mouse

We studied the postnatal development of the corticopontine tract in mice by the injection of the axon tracer DiI into the visual cortex. In the postnatal day (P) 0.5 mouse, labeled pyramidal tract fibers pass through the internal capsule and cerebral peduncle, grow over the basilar pontine gray, and enter into the medullary pyramid (in this study, P0 refers to the first 24 hours after birth). Small collateral branches arise from these pyramidal tract fibers on P0.5-1.0, and elongate quickly into the basilar pontine gray around P2-4. These collateral branches give off many secondary branches on P4 and form the bright terminal zone in the rostral portion of the lateral basilar pontine gray on P9. In the P16 mouse, this terminal zone is more restricted, suggesting, on the basis of the anterograde DiI labeling technique, that the visual corticopontine projection matures by P16. DiI-labeled pyramidal tract fibers distal to the branching point of the pontine collaterals are found during the postnatal two weeks, but disappear by the later stages. We conclude that the visual corticopontine tract develops as collateral branches of the transient pyramidal tract fibers arising from the visual cortex of the mouse, as just described in the rat (O'Leary and Terashima, Neuron 1:901-910, 1988).

Calcite Cements in the Modern Floridan Aquifer

Calcite cements in the Ocala (Eocene) and Suwannee (Oligocene) formations, southwestern Floridan aquifer have been studied to determine updip to downdip variations in cement chemistries and cathodoluminescence within a modern regional confined aquifer. Interparticle, intraparticle, and fracture-fill cements comprise 5-15% of the limestones. Five different calcite cement morphologies are distinguishable and occur throughout the aquifer: (1) circumgranular microspar, (2) fine- to medium-crystalline rhombs, (3) medium-crystalline syntaxial overgrowths on echinoderms, (4) fine-crystalline pore-filling mosaics, and (5) micrite. Type 5 occurs only below former exposure surfaces. Volumetrically, type 3 is the most important and type 4 is the least. Cathodoluminescence observations reveal only nonluminescent cements updip and an increase in luminescent zones and luminescent intensity downdip. Updip nonluminescent cements have very low Fe and Mn concentrations, but high Mg and Sr concentrations. These relations are interpreted to reflect oxidizing conditions and high rock/water interaction. Fe and Mn concentrations increase and Sr and Mg contents decrease downdip. These trends are interpreted to reflect reducing conditions, cross-formational flow, and slower rock/water interaction. Downdip cathodoluminescence zonations consist of a broad nonluminescent zone, followed by a thin bright orange zone, and then a dull luminescence zone. These geochemical and luminescent patterns along a regional flow linemore » in the confined Floridan aquifer have many similarities to those observed in calcite cements described from ancient aquifers.« less

Investigation of extinction in unsteady flames in turbulent combustion by 2D-LIF of OH radials and flamelet analysis

An experimental and theoretical investigation of premixed turbulent combustion in an engine simulator is presented. The distribution of hydroxyl radicals formed in the combustion of propane/air mixtures was visualized by 2D-LIF and used to monitor the progress of the combustion process. For stoichiometric mixtures, images showed a continuous wrinkled flame front, while in lean (λ=1.5) mixtures, local flame extinction was observed as discontinuities in the reaction zone. A bright active reaction zone was still observed in flame inlets and closed concave structures. The effects of self-absorption and of collisional quenching on the fluorescence signal are considered and appear to have only a minor net influence on the shape and width of the flame front. The images are evaluated and interpreted in terms of the Lewis number effect and the laminar flamelet model. Analysis was performed by determining the contour lines of the images (specifically, the ratios of average maximum to equilibrium OH concentration) and comparing with corresponding ratios from unstrained flame simulations. The results show that although the degree of turbulence is not high enough for straining effects to be important, flamelet curvature does play a significant role in the combustion of lean mixtures; this is manifested by a mean effective flame velocity that is less than the laminar burning velocity.

NITRIC OXIDE DISSOCIATION AND REDUCTION WITH HYDROGEN ON PLATINUM SURFACES

The surface structure dependence of the NO dissociation on Pt has been investigated using field electron microscopy (FEM). The first indication of dissociation is observed on rough kinked surfaces. This process starts around 300K, while at 370K a field emission pattern characteristic of an oxygen covered surface is found. If a NO covered field emitter is heated in the presence of hydrogen, the behavior until 350K is the same as observed for NO/Pt without H2. At 355K, however, a bright reaction zone is observed starting at (100), which shifts towards (110) and (111). Our interpretation of these results is that surfaces with a high concentration of kinks are most active for NO dissociation, followed by (100) and stepped (100) faces. These results are discussed in relation to literature data concerning NO dissociation on Pt.

The temporal evolution of a small auroral substorm as viewed from high altitudes with Dynamics Explorer 1

A small auroral substorm is investigated with auroral imaging photometers carried on the spacecraft Dynamics Explorer 1. Initial brightening along the auroral oval and the subsequent westward and poleward motions of intense, localized emission regions are associated with auroral surges. Following substorm onset, another region of less intense emissions is observed to develop at lower latitudes and adjacent to the bright region near local midnight. This second region expands towards the east. The bright zone of auroral emissions associated with the surges is interpreted as the signature of electron acceleration along magnetic field lines threading the boundary layer of the plasma sheet in the magneto‐tail. The more diffuse, less intense region is identified with eastward‐drifting electrons injected into the plasma sheet and ring current following substorm onset. No rapid poleward motion of the discrete aurora is detected during substorm recovery.

Early Diagenesis of a Phylloid Algal-Mound Complex, Laborcita Formation, Southeastern New Mexico: ABSTRACT

Marine carbonate cementation was the initial stage in the paragenesis of phylloid algal mounds in the Laborcita Formation (Wolfcampian), Sacramento Mountains, New Mexico, and the cements are almost identical to those in Holocene coral reefs of Belize. These cements include relicts of botryoids and crusts of needle crystals, in part defined by inclusion patterns and luminescent ghosts in mosaic calcite. Individual needle crystals are pseudohexagonal in cross section and range from less than 1 to 30 µm wide. These nonluminescent early cements line cavity walls, coat phylloid-algal blades and stromatolites, and are interlayered with marine sediment. Early cements also include bladed, fibrous, and rare radiaxial fibrous calcites, which are microdolomite-rich. They have a proximal nonluminescent zone, a central bright-luminescent zone, and a distal blotchy, moderate-luminescent zone. The bright zone may be time equivalent to bright-luminescent micritic coatings on botryoids and grains. Botryoids are encrusted by isopachous bladed cement, some of which has prismatic overgrowths containing an early inclusion-rich zone. This initial cementation was followed closely by: (1) dissolution of algal blades and mollusks, (2) in-situ brecciation, and (3) cementation by blocky calcite. Botryoidal and acicular cements are interpreted as originally marine aragonite precipitates, based on morphology, occurrence, susceptibility to diagenesis, and similarity to Holocene reef cements. The same criteria, plus the microdolomite inclusions, indicate that the bladed, fibrous, and radiaxial cements had a marine Mg-calcite precursor. This assemblage followed by prismatic overgrowths, dissolution, and blocky-calcite cementation indicates an evolution from marine to freshwater diagenesis. This early diagenesis was followed by, in order: (1) cementation by multiple stages of blocky calcite (interrupted by episodic fracturing), (2) local dolomitization, (3) stylolitization, (4) dedolomitization and blocky-calcite cementation, and (5) minor chertification. End_of_Article - Last_Page 315------------