bluuebunny/crossref_metadata_2025 huggingface dataset:
A synthesis of magnetospheric substorm models
Authors: Lui A. T. Y.
Published: February 2008

Three decades of research in magnetospheric substorms has not led to a general consensus view of the substorm process. Several substorm models, mostly phenomenological, are presently under consideration. These competing models, each being justifiable on the basis of certain features of a substorm, have major differences as well as similarities among them. A synthesis substorm model is desirable, as first suggested by Siscoe (1986). In this paper we construct a coherent description of substorm development by extracting some important components from these existing models. The scenario of the synthesis model includes the ionospheric influence on substorm expansion onset, current disruptions leading to convection surges and tailward propagating rarefaction waves, wave‐induced precipitation, local time expansion of the disturbance region via velocity‐shear‐related instabilities, plasma sheet heating by resonant absorption of hydromagnetic waves, and the formation of magnetic reconnection domains. This synthesis represents one possible way to integrate the different existing models coherently.
Solar cycle variations in the size and shape of the magnetopause
Authors: Petrinec S. P. , Song P. , Russell C. T.
Published: February 2008

The 10 years of the ISEE 1 and 2 mission covering much of solar cycle 21 and the beginning of solar cycle 22 allow us to study the position, shape, and motion of the magnetopause throughout the course of changing solar activity. We have determined the size and shape of the magnetopause for each observing season using the ISEE 1 and 2 magnetometer data. IMP 8 data were used to monitor the solar wind changes with the solar cycle. During the 1979–1980 season, at solar maximum the solar wind dynamic pressure was at its lowest values, and at solar minimum the solar wind pressure was at its largest values, more than double the value in the 1979–1980 season. During this solar cycle the magnetopause was about 0.5 RE farther when the interplanetary magnetic field (IMF) was strongly northward than when strongly southward. Both standoff distance values are found to be smaller than the value found by Fairfield [1971]. The standoff distance of the magnetopause for northward IMF is anticorrelated with the solar wind pressure as expected. However, the standoff distance for southward IMF seems relatively insensitive to solar wind dynamic pressure.
Geomorphology and stratigraphy of Alba Patera, Mars
Authors: Schneeberger Dale M. , Pieri David C.
Published: February 2008

Geomorphic and stratigraphic analysis of Alba Patera suggests a volcanic construct built by lavas with rheologic properties similar to basalts. A series of evolving eruptive styles is suggested by changes in morphology and inferred progressive reductions in flow volume with higher stratigraphic position. Alba Patera's volcanic history has been summarized into four main phases. The first is characterized by extensive flood like flows presumably erupted from fissures associated with the initial intrusion of magma into the region. The second phase is associated with the emplacement of pyroclastic rock, a more speculative interpretation. The third phase produced the voluminous tabular, crested, and undifferentiated flows, probably from a more centralized vent source. The fourth and last phase is marked the effusion of levee like flows and the collapse of the summit calderas and final graben formation.
The geometry of Aleutian subduction: Three‐dimensional seismic imaging
Authors: Boyd Thomas M. , Creager Kenneth C.
Published: February 2008

We constrain the subduction geometry of the Aleutian slab by the distribution of seismicity recorded at local seismic networks and by a teleseismic P wave residual‐sphere analysis of 11 earthquakes and one nuclear explosion. The source depth and mechanism of each earthquake are determined by modeling teleseismically recorded long‐period P and SH waveforms. We construct theoretical residual spheres by tracing rays through three‐dimensional thermal models of the subducting slab assuming a pressure‐independent thermal coefficient of velocity (∂ Vp/∂ T). Rays are initiated at the estimated source depths and at epicenters whose relative locations are consistent with those obtained from the observed travel times after correction for slab effects. For each event, we determine the values of slab strike and dip that provide the best match to the observations. Average values of 67% confidence intervals determined by an F test are ±5 ° in dip and ±10° in strike. Strike varies along the arc by 60°. For all but one of the modeled events, the estimated orientation of the slab's aseismic extension agrees to within two standard deviations of that derived from seismicity distributions. Assuming a constant dip, a slab penetration depth of 600 km fits the central Aleutian events best. This is at least 300 km below the deepest earthquakes. Slabs terminating above 400 km or below 1000 km provide significantly worse fits. Though there is a trade‐off between ∂Vp/∂ T and penetration depth, the best‐fitting value of ∂Vp/∂ T, using a 600‐km‐deep slab, is −0.5 ±0.1 ms−1°K−1. This is consistent with both ultrasonic laboratory measurements and values obtained for deeper earthquakes in several other subduction zones. ∂Vp/∂ T estimated from shallow thrust events is 20–50% larger than that for intermediate‐focus events suggesting that a simple thermal model with constant ∂Vp/∂ T under predicts the velocity anomalies shallower than 100 km depth. The fact that a 600‐km‐deep slab provides better fits than a 1000‐km‐deep slab implies that the downdip P rays exit the slab near a depth 600 km. A slab bend to either a shallower or steeper dip would satisfy the P times. Hand‐picked PcP travel times are most consistent with the slab bending to a 20° steeper dip before plunging into the lower mantle. In the western Aleutians, the relative plate motion becomes transform, seismicity ceases at less than 100 km depth and volcanism is no longer active. We image a slab‐like, high‐seismic velocity zone apparently associated with the along‐arc lateral transport of cold subducted lithosphere from the east. An earthquake whose location is known from local observations is analyzed in detail. The velocity structure inferred using this location is nearly identical to that obtained using our scheme of annihilating location information from both theoretical and observed times. A severe bias would result, however, from the commonly used procedure of analyzing travel times computed with respect to an incorrect event location, such as that of the ISC.
Seismicity and seismotectonics of Norway and nearby continental shelf areas
Authors: Bungum H. , Alsaker A. , Kvamme L. B. , Hansen R. A.
Published: February 2008

The earthquake activity of Norway and nearby offshore areas is low to intermediate, with few events above magnitude 5. Recent significant improvements in instrumental coverage in parallel with a better utilization of older (including historical) data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the Viking Graben, while from the northern North Sea to Svalbard the earthquakes in a broad sense follow the continental margin. Fifty‐one focal mechanism solutions from these areas, about half of them new, reveal stress directions that clearly indicate a connection to the plate tectonic “ridge push” force, at least for the areas at a minimum distance from the continental margin. Along the margin, stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects. A dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation. On a regional level, the seismicity in these areas correlate quite well with geologic features such as grabens, fault zones, fault complexes, fracture zones, and the margin itself, indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field. In the northern North Sea, however, an area with quite anomalous stress orientations, with strike‐slip faulting, is found in a region transitional between normal and reverse faulting. Most of the earthquake foci are confined to the presumably brittle parts of the crust, but many events are also located quite close to, and on both sides of, the Moho discontinuity.
The effect of small‐scale structure on normal mode frequencies and global inversions
Authors: Snieder Roel , Beckers Jos , Neele Filip
Published: February 2008

Presently very little is known about the characteristic length scales of the structures in the upper mantle. In generating synthetic seismograms and in large‐scale inversions, one usually assumes that the upper mantle is smooth, so that ray theory can be used. However, it is known that structures in the mantle exist on a scale of a few hundred kilometers, notably near subduction zones and other tectonic features. In this paper, a model of all the subduction zones and spreading ridges is used to investigate the effect of small‐scale structures on normal mode frequency shifts and on global inversions. The normal mode frequency shifts for this model have the same characteristics as observed normal mode frequency shifts. Furthermore, the global models obtained from inversions of synthetic data for the model of subduction zones and spreading ridges are similar in character to the models obtained from global inversions of real data. For a realistic set of wave paths, the model obtained from the synthetic inversions differs appreciably from a smoothed version of the true model. This is due to an uneven path coverage and to a lesser extent due to neglecting scattering effects. This means that the models obtained from global inversions of seismological data may contain appreciable artifacts.
Influence of glacial isostatic adjustment on tide gauge measurements of secular sea level change
Authors: Peltier W. R. , Tushingham A. M.
Published: February 2008

Modem tide gage records of secular sea level change are strongly contaminated by the ongoing influence of glacial isostatic adjustment. In this paper we employ an accurate high‐resolution model of the glacial isostatic adjustment process to filter this signal from time series consisting of annual mean heights observed on over 500 tide gages from which records of duration longer than 10 years are available. These records are all contained in the archive of the Permanent Service for Mean Sea Level at Bidston in the United Kingdom. Prior to removal of the isostatic adjustment contamination from these records the secular sea level change signal is seen to exhibit both rising and falling levels depending upon geographic location, with regions of falling level corresponding to those that were ice covered during the last glaciation event of the present ice age. After filtering, the geographic heterogeneity in rates is considerably reduced, so much so that a globally coherent component consisting of a rise of sea level at a rate of 2.4±0.9 mm yr−1 is revealed. This could conceivably be driven by the “greenhouse effect,” although no particular mechanism has been unambiguously identified by our analyses. We investigate the impact of a number of different variants of the analysis procedure upon the inferred global signal.
High‐temperature heat capacity and premelting of minerals in the system MgO‐CaO‐Al2O3‐SiO2
Authors: Richet Pascal , Fiquet Guillaume
Published: February 2008

Relative enthalpy measurements have been made from 800 to 1700 or 1800 K for periclase, lime, spinel, diopside, pseudowollastonite, and anorthite. Significant premelting effects have been observed for diopside. pseudowollastonite, and anorthite, showing that only part of the structural changes that take place near the melting point can be quenched to room temperature. With the heat capacities derived from these measurements and other data for SiO2polymorphs, corundum, forsterite and pyrope, the validity of proposedCpequations has been tested, particularly with respect to extrapolations at higher and lower temperatures. For high‐temperature extrapolations, the best results have been obtained with an equation of the formCp=k0+klnlnT+k1/T+k2/T2+ k3/T3. Comparisons of experimental heat capacities with values calculated from polyhedral modeling show that significant deviations from additivity can be found above 1000 K, likely as a result of anharmonic factors. This raises the possibility that enthalpies and entropies of formation are temperature dependent under mantle conditions. Finally, the difficulties of extrapolating differential scanning calorimetry data for mantle minerals are emphasized.
Diffusion domains determined by 39Ar released during step heating
Authors: Lovera Oscar M. , Richter Frank M. , Harrison T. Mark
Published: February 2008

The Arrhenius plot for 39Ar released during step heating of a sample containing a distribution of diffusion domains will depart from a straight line in ways that depend on the domain size distribution and the heating schedule used to extract the 39Ar. In a new series of experiments we make use of the heating schedule effect to confirm the multidomain nature of several alkali feldspar samples. We find that temperature cycling is particularly useful for determining the diffusion parameters (activation energy and frequency factor) of the different domains. We also use the log (r/r0) versus cumulative percent 39Ar released plot presented in an earlier paper to display better the sizes of the different domains and the proportion of argon that they contain. We conclude by showing that a particular choice of the geometry and number of domains is not critical to the determination of the thermal history provided the 39Ar data are satisfactorily fitted by the particular choice mode. Experiments on single crystals show that multiple diffusion domains are an intrinsic property of the feldspars and thus are not separable by a careful selection of the grains to be analyzed.
Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes
Authors: Sleep Norman H.
Published: February 2008

The thermal structure of the axial region of ridges spreading at 60 mm yr−1 was studied by comparing observations with numerical thermal models. Data from Deep Sea Drilling Project hole 504B constrain the temperature in the pillows and the upper part of the dike complex. Seismic reflection data constrain the depth to the magma chamber. Magnetic anomalies constrain the accumulation rate of pillows. The thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts. In the successful thermal models, hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0.8 km above the top of the magma chamber. The computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents. The computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern. Anhydrite was precipitated as seawater approached the hot axial zone. The anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater. The net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off‐axis circulation and returned to the ocean. This is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths. Alternatively, the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows. Anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean. The latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust.
Numerical simulations of brine migration by topographically driven recharge
Authors: Deming David , Nunn Jeffrey A.
Published: February 2008

The migration of abnormally warm, saline water through the Appalachian basin and North American midcontinent in Paleozoic time has been inferred from fluid inclusion studies, remagnetizations, and widespread potassic alteration. A time‐dependent numerical model of fluid, heat and solute transport is used to evaluate the viability of topographically driven recharge as a mechanism for brine migration. The model represents a wedge‐shaped sedimentary basin 400 km long by 6 km deep (maximum) with a basal aquifer 500 to 750 m thick overlain by a homogeneous aquitard. Temperature predicted by model simulations is found to be inconsistent with constraints inferred from fluid inclusion studies, unless average heat flow values greater than about 100 mW/m2 are used. Model simulations also lead to predictions of low heat flow and subsurface temperature in recharge zones that are generally not observed in modern orogenic zones. The initial solute content of pore waters in the model basin is flushed out by fresh water entering in the recharge zone before fluid velocities high enough to produce significant warming of the discharge zone can develop. Model simulations with source terms reveal that basin sediments can provide enough solute to maintain hot, hypersaline brine migration for about 1 m.y., at most. High fluid velocity in the basal aquifer is required to carry heat to the basin margins, but the higher the fluid velocity, the more quickly the basin's supply of solute is exhausted. Consideration of these constraints implies that topographically driven recharge may be an effective mechanism to explain regional brine migration only if flow is focused from regional scale recharge zones into more spatially restricted discharge zones.
Tectonic evolution of the Easter Microplate
Authors: Naar David F. , Hey R. N.
Published: February 2008

The plate tectonic history of the Easter microplate has been reconstructed by “closing” the microplate in a series of steps using the Pacific‐Nazca magnetic anomalies north and south of the microplate and the NUVEL 1 global plate motion model. After each step, the Easter microplate was rotated rigidly to match the Nazca and Pacific anomalies. Gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation, respectively, and show that rigid plate motions are insufficient to explain the complete tectonic evolution. Analysis of the magnetic anomaly data was guided by contoured SeaMARC II, Sea Beam, and 3.5‐kHz bathymetry data and a lineament map derived from SeaMARC II side scan and Sea Beam bathymetry data. The patterns of lineaments and bathymetric structures suggest that rotational deformation of the Nazca plate is the general mechanism that accommodates the space problems arising from transfer of the Nazca plate to the microplate and rapid rotation of the microplate against the Nazca plate. Similar but smaller amounts of deformation are predicted along the southern boundary of the microplate. Prior to the origin of the microplate, the East Pacific Rise (EPR) was offset in at least two places according to the older magnetic anomalies, yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets. The magnetic, bathymetry, and satellite altimetry data indicate that the microplate initially formed at (or perhaps southeast of) Easter Island near a left‐lateral offset of the EPR sometime between anomaly 3 and 3'. The East Rift started propagating north from the present location of Easter Island at ∼4.5 Ma, which is ∼1.5 m.y. earlier than previously proposed. However, the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of Easter Island. The geometry of the microplate changes very rapidly during its evolution. At the initial stages of development, the microplate resembles a large propagating rift system, suggesting that deformation may have been occurring throughout most of its interior up to about 2.47 Ma. At this time, the length to width of overlap ratio of the two rifts reaches a value of 3, the northward propagation slows down, the curved opening of the Southwest Rift becomes well established, and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate. At this time, the offset distance between the two overlapped rifts starts to increase. Some time after 2.47 Ma and before 1 Ma, the East Rift starts propagating northwestward, probably in response to the microplate rotation, and continues up until present. Also during this time period, the East Rift breaks into a series of northward propagating rifts, each propagating into the microplate interior, thereby transferring lithosphere from the microplate to the Nazca plate and reducing the total growth rate of the microplate.
Elasticity of germanate and silicate spinels at high pressure
Authors: Rigden Sally M. , Jackson Ian
Published: February 2008

Compressional and shear wave velocities have been measured as functions of pressure to 3 GPa for a suite of low‐porosity, isotropic, polycrystalline germanate and silicate spinels using the phase‐comparison variant of ultrasonic pulse interferometry. Curvature in velocity‐pressure trends below a threshold pressure of order 1.5 GPa is caused by closure of microcracks and imperfect acoustic coupling between the specimen and the acoustic buffer rod. Pressure derivatives of wave velocity are determined from linear velocity‐pressure trends above this threshold pressure. Previous work on sintered polycrystals of comparable acoustic quality has demonstrated that pressure derivatives determined in this way match those of the ideal zero‐porosity aggregate within 10%. The measured derivatives are used to calculate the pressure derivatives dK/dP and dG/dP of the bulk and shear moduli. Although systematic variation of dK/dP with composition is not evident, variation of dG/dP with K/G within the silicate and germanate series suggests a likely value of dG/dP ∼ 1.9 for γ‐Mg2SiO4.
Oligocene ash flow volcanism, northern Sierra Madre Occidental: Role of mafic and intermediate‐composition magmas in rhyolite genesis
Authors: Wark David A.
Published: February 2008

Field, geochemical, and isotopic data from the Tomochic volcanic center in Chihuahua, Mexico, are interpreted to indicate a genetic relationship between large‐volume rhyolite ash‐flow tuffs and associated more mafic lithologies. These lithologies include (1) porphyritic, two‐pyroxene andesite (>35 Ma) that was extruded mostly before ash‐flow volcanism, and (2) crystal‐poor basaltic andesite that was erupted mostly after ash‐flow activity (∼30 Ma) but which was also extruded earlier (∼34 Ma) with hybrid intracaldera lavas. Major silicic units at Tomochic include the Vista (∼34 Ma) and Rio Verde (∼32 Ma) rhyolite ash‐flow tuffs; also present are ash‐flow tuffs (∼38, 36, and 29 Ma) erupted from other sources. A model of rhyolite genesis by closed‐system crystal fractionation of andesite is consistent with geochemical and isotopic data. The least evolved Vista rhyolite was formed by fractionation of ∼65% the original mass of andesite; an additional ∼55% fractionation of plagioclase, alkali feldspar, quartz, biotite, hornblende, FeTi oxides, and sphene generated the most evolved Vista sample. Rio Verde rhyolites were generated from andesite by ∼50% mass fractionation of an assemblage dominated by plagioclase, pyroxene, and FeTi oxides. Initial Nd and Sr isotope ratios of andesite‐dacite lavas (εNd = −2.3 to −5.2; 87Sr/86Sr = 0.7060 to 0.7089) and of rhyolites (εNd = +0.5 to −2.7; 87Sr/86Sr = 0.7053 to 0.7066) partly overlap and extend from values near the mantle array toward values typical of old continental crust on an εNd‐87Sr/86Sr diagram. These isotope ratios, which do not correlate with indices of differentiation, are interpreted to indicate that parental andesite already contained a crustal component (possibly >20%) before fractionation to rhyolite. The isotopic and geochemical signatures of andesites apparently reflect the incorporation of crust by subduction‐related, mafic melts represented by (but more primitive than) exposed basaltic andesites, which have isotope ratios (εNd = +1.0 to −0.1; 87Sr/86Sr = 0.7044 to 0.7053) near “bulk earth”. The pattern of volcanic evolution at the Tomochic center, specifically the transition from andesitic to rhyolite dominated, with late extrusion of basaltic andesite, also occurred in other parts of the volcanic field, and roughly coincided with a sharp decrease in the rate of Farallon plate subduction. This change in subduction rate apparently resulted in a decreased flux of mafic melts into the crust from below, and was associated with the onset of crustal extension and hence, shorter residence times for mafic melts formerly ponded in the deep crust These, in turn, resulted in (1) the change from andesitic to rhyolite‐dominated volcanism as ascending intermediate‐composition magmas stalled, coalesced, and differentiated to produce rhyolite, (2) extrusion of basaltic andesite upon brittle failure of the shallow crust, and (3) subsequent termination of calc‐alkalic volcanism throughout the Sierra Madre Occidental.
Simplified moment tensor analysis and unified decomposition of acoustic emission source: Application to in situ hydrofracturing test
Authors: Ohtsu Masayasu
Published: February 2008

An application of a moment tensor analysis to acoustic emission (AE) is studied to elucidate crack types and orientations of AE sources. In the analysis, simplified treatment is desirable, because hundreds of AE records are obtained from just one experiment and thus sophisticated treatment is realistically cumbersome. Consequently, a moment tensor inversion based on P wave amplitude is employed to determine six independent tensor components. Selecting only P wave portion from the full‐space Green's function of homogeneous and isotropic material, a computer code named SiGMA (simplified Green's functions for the moment tensor analysis) is developed for the AE inversion analysis. To classify crack type and to determine crack orientation from moment tensor components, a unified decomposition of eigenvalues into a double‐couple (DC) part, a compensated linear vector dipole (CLVD) part, and an isotropic part is proposed. The aim of the decomposition is to determine the proportion of shear contribution (DC) and tensile contribution (CLVD + isotropic) on AE sources and to classify cracks into a crack type of the dominant motion. Crack orientations determined from eigenvectors are presented as crack‐opening vectors for tensile cracks and fault motion vectors for shear cracks, instead of stereonets. The SiGMA inversion and the unified decomposition are applied to synthetic data and AE waveforms detected during an in situ hydrofracturing test. To check the accuracy of the procedure, numerical experiments are performed on the synthetic waveforms, including cases with 10% random noise added. Results show reasonable agreement with assumed crack configurations. Although the maximum error is approximately 10% with respect to the ratios, the differences on crack orientations are less than 7°. AE waveforms detected by eight accelerometers deployed during the hydrofracturing test are analyzed. Crack types and orientations determined are in reasonable agreement with a predicted failure plane from borehole TV observation. The results suggest that tensile cracks are generated first at weak seams and then shear cracks follow on the opened joints.
Development of a two‐dimensional global tropospheric model: Model chemistry
Authors: Hough Adrian M.
Published: February 2008

A latitudinally averaged two‐dimensional model has been used to study the distributions, budgets, and trends of trace gases in the atmosphere from pole to pole and from the surface to 24 km. The chemical mechanism used contains 56 chemical species, including 12 hydrocarbons and 125 chemical and photochemical reactions, as well as wet removal processes and dry deposition. Apart from the stratospheric sources of ozone and nitrogen oxides the model chemistry is driven completely by the time‐dependent photolytic processes and the emission of 17 chemical species distributed according to 10 different source categories. Each source category is parameterized as a function of latitude and time of the year. The model results are generally in good agreement with observations, and the model reproduces the observed temporal and spatial variation in the mixing ratios of methane, carbon monoxide, hydrocarbons, and ozone. The global average concentration of the hydroxyl radical in the troposphere is 8.3×105 molecules cm−3, in agreement with recent calculations based on budgets and trends for CH3CCl3. Model budgets for NOx, CO, CH4, H2 and O3 are presented. These show that although the stratospheric source and dry deposition terms for ozone almost balance, the global annual turnover of ozone below 24 km, excluding the O3/NO/NO2 null cycle, is four times greater than the stratospheric source strength. These budgets also suggest that the observed increase in the mixing ratio of methane may be due not only to an increasing source strength but also to a downward perturbation in the abundance of the hydroxyl radical.
Regions where lightning flashes began
Authors: Proctor David E.
Published: February 2008

Regions where 773 flashes began during 13 thunderstorms were located by calculating centroids of the sources of the first six or 10 VHF pulses that were emitted by each flash. Sources were located by measuring differences in the times at which their pulses arrived at five widely spaced receivers stationed on the ground. We found that the distribution of origin heights was bimodal with peaks at 5.3 and 9.2 km above mean sea level (amsl). Standard errors in the coordinates of flash origins were estimated to be 20–80 m in X and Y and 160–240 m in the height coordinate Z. There were 431 flashes in the lower group and 342 in the higher. Flashes in the lower group were more numerous in 10 storms; in three storms, high flashes were in the majority. There was evidence to suggest that this condition depended partly on the phase of the host thunderstorm. Recorded E field changes produced by 165 flashes whose paths we had mapped convinced us that the vast majority of the 773 flashes, including the 342 high flashes, had also been negative. Flash origins tended to cluster in regions that were a few kilometers or less in horizontal diameter. Densities of flash origins in these regions ranged from 1.25 to 25 km−3. The origins of 658 flashes were mapped onto radar precipitation patterns of their host storms. We found that 66% of the flashes began within one picture element, approximately 270 m, of the contours for 20 dBZ; 27% began inside these contours, and most of these began at edges of high‐reflectivity cores. The remaining 7% began outside the 20‐dBZ contours. Similar results (73%: 19%: 8%) were obtained for 276 high flashes that began at heights above 7.4 km amsl, but 195 ground flashes scored 54%: 36%: 9% and showed a greater tendency to begin inside the 20‐dBZ contours. The distribution of distances between origins and their nearest 20‐dBZ contours showed a marked peak near zero. We concluded that charge‐density in thunderclouds was affected by the presence of heavy precipitation and that 20‐dBZ surfaces enclosed regions that carried excess negative charge.
Ozone depletion in the high latitude lower stratosphere: 1979–1990
Authors: Callis Linwood B. , Boughner Robert E. , Natarajan Murali , Lambeth James D. , Baker Daniel N. , Blake J. Bernard
Published: February 2008

Archived Stratospheric Aerosol and Gas Experiment (SAGE, SAGE II) and Solar and Backscattered Ultraviolet (SBUV) data are used to examine lower stratospheric O3 variations at 50° latitude in both hemispheres. These data indicate that from 1979 to 1985, 73–90% of the total O3 changes have occurred below approximately 25 km in altitude. Significant O3 depletions (up to 15%) have occurred in the partial column (127–15.8 mbar) in both hemispheres with indications of a recovery after 1985. Both the SAGE/SAGE II and SBUV observations show essentially the same changes for this partial column. Below 20 km and between 1979 and 1985, larger local O3 depletions are suggested by the SAGE/SAGE II data sets. Temporally, the largest 365‐day running mean O3 changes occur in 1982–1983 in the northern hemisphere and in late 1984 and early 1985 in both hemispheres. Possible explanations are discussed. Two‐dimensional model simulations of O3 changes from 1979 to 1990 have been carried out. Comparisons with O3 data are presented. Model results suggest that by 1985, significant declines in global O3 were caused by destruction by odd nitrogen associated with long‐term variations in the flux of precipitating relativistic electrons (2.6%); solar UV flux changes (1.8%); the dilution effect associated with the Antarctic O3 hole (1.2%); and atmospheric increases in CH4, N2O, and chlorofluorocarbons (0.4%). Analyses of drift‐corrected SBUV and Total Ozone Mapping Spectrometer (TOMS) data and model calculations indicate that between 1979 and 1985, reductions of 4.3 to 4.8% in total column O3 averaged between 65°S and 65°N have occurred. Calculations indicate a full global O3 decline of 5.2% (peak‐to‐peak) or 6% (annual average) between 1979 and 1985 with a partial recovery between 1985 and 1989.
The Development of a Long‐Range Foraminifer Transfer Function and Application to Late Pleistocene North Atlantic Climatic Extremes
Authors: Dowsett H. J.
Published: February 2008

North Atlantic transfer function GSF18 was developed from simplified core top planktic foraminifer census data. Simplifications are based upon similar environmental responses of some modern taxa and assumptions about the paleoecology of now extinct Pliocene taxa. GSF18 is evaluated through application to “time slice” data sets from the last glacial maximum (18 ka) and last interglacial (122 ka) of the North Atlantic Ocean. Quantitative sea‐surface temperature estimates from GSF18 at 106 glacial sites and 18 interglacial sites are approximately equivalent to Climate: Long Range Investigation, Mapping, and Prediction (CLIMAP) results at the same sites: a glacial North Atlantic considerably cooler than modern conditions and a last interglacial North Atlantic approximately the same as or slightly warmer than modern conditions. Minor differences are explained by the taxonomic structure of GSF18 and provide further insight into the environmental and temporal robustness of the transfer function.
Tectonic controls on the development of unconformities: The North Slope, Alaska
Authors: Coakley Bernard J. , Watts Anthony B.
Published: February 2008

The geology of northern Alaska is defined by two complementary, synchronous tectonic events. Opening of the Canada basin breached the south facing Ellesmerian passive margin in the Late Jurassic, cutting off northerly derived sediment and initiating thermal subsidence of the north facing Beaufort passive margin. Uplift of the Brooks Range on thrust faults loaded the lithosphere, creating the Colville trough, a foreland basin more than 10 km deep, and providing the bulk of the sediments that filled it. The northerly advance of the Brooks Range caused the two independent loads to flexurally interfere. The Beaufort passive margin is separated from the foreland by the Barrow Arch, which is, in part, the flexural bulge due to the interaction of these two loading events [Nunn et al., 1987]. A record of this interaction is preserved in the sediments north and south of the Barrow Arch. We have used this stratigraphy, coupled with simple basin modeling, to consider the flexural and stratigraphic effects of the evolving thrust wedge and basin filling. Using a simple sedimentation model and loading on an elastic lithosphere, we have produced two unconformity‐bounded sequences without sea level change. The unconformities are produced as a consequence of sedimentation and two increments in the thrust load that cause the flexural bulge to translate laterally, rise, and fall. The generation of unconformities over a flexural bulge by mass redistribution on a purely elastic lithosphere calls into question the use of these unconformities in other yoked systems to document viscoelastic lithospheric rheologies. Even if the lithospheric strength decays significantly on geologic time scales, we may not observe this decay in stratigraphy if the basin filling time is a significant fraction of the relaxation time. The Beaufort passive margin and the thrust belt strike at an oblique angle to each other, causing the independent bulges to constructively interfere in the west and destructively interfere in the east. The advance of the thrust‐related bulge beyond the eastern Beaufort shelf has, by flattening the basal decollement, exerted significant control on the thrust wedge of the Brooks Range, allowing the thrust wedge tip to advance beyond the coastline.
Development of the Late Proterozoic to Mid‐Paleozoic, intracratonic Amadeus Basin in central Australia: A key to understanding tectonic forces in plate interiors
Authors: Shaw R. D. , Etheridge M. A. , Lambeck K.
Published: February 2008

The intracratonic Amadeus Basin in central Australia is a complex, composite basin covering 17,000 km², with at least nine distinct episodes of evolution between 900 and 300 Ma. These have been identified in neighboring central Australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape. Several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early Australian plate. In the absence of direct geological evidence for thermal events in the Amadeus Basin after the initial period of subsidence ended, possibly at about 800 Ma, the correlations found between events in the Amadeus Basin, those in the neighboring interconnected basins, and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms. This suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide. Throughout the history of the Amadeus Basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified. There were also transitional periods when neither compression nor extension dominated, so that other processes masked the effects of weak horizontal stresses. The most recent compressional thrust belt concentrated at the northern margin between 300 and 400 Ma and has partly obscured the record of earlier basin shape and size.
Lateral extrusion in the eastern Alps, PArt 2: Structural analysis
Authors: Ratschbacher Lothar , Frisch Wolfgang , Linzer Hans‐Gert , Merle Olivier
Published: February 2008

The late Oligocene‐Miocene tectonic style of the Alps is variable along strike of the orogen. In the Western and Central Alps, foreland imbrication, backthrusting, and backfolding dominate. In the Eastern Alps, strike‐slip and normal faults prevail. These differences are due to lateral extrusion in the Eastern Alps. Lateral extrusion encompasses tectonic escape (plane strain horizontal motion of tectonic wedges driven by forces applied to their boundaries) and extensional collapse (gravitational spreading away from a topographic high in an orogenic belt). The following factors contributed to the establishment of lateral extrusion in the Eastern Alps: (1) a rigid foreland, (2) a thick crust created by indentation and earlier collision, (3) a decrease in strength in the crust due to thermal relaxation, (4) a crustal thickness gradient from the Eastern Alps to the Carpathians, and, possibly, (5) a disturbance of the lithospheric root. Northward indentation by the Southern Alps causes thickening in and in front of the indenter and tectonic escape. Gravitational spreading attenuates crustal thickness differences. Indentation structures occur in the western Eastern Alps and comprise folds, thrusts, and strike‐slip faults. These structures pass laterally into spreading structures, which encompass transtensional and normal faults in the eastern Eastern Alps. The overall structural pattern is dominated by escape structures, namely, sets of strike‐slip faults that bound serially extruding wedges. Structural complexity arises from (1) interference of major fault sets, (2) accommodation of displacement differences between the Eastern Alps and their fore‐ and hinterland, (3) displacement transfer from the Eastern Alps toward the Carpathians which act as a lateral unconstrained margin, and (4) crustal decoupling, which partitions extrusion into brittle upper plate and ductile lower plate deformation. The kinematics of lateral extrusion is approximated by an extrusion‐spreading model proposed for nappe tectonics.
Stress‐induced changes in plate density, vail sequences, epeirogeny, and short‐lived global sea level fluctuations
Authors: Cathles L. M. , Hallam A.
Published: February 2008

The stratigraphic record contains clear evidence that broad regions have experienced rapid changes in sea level unrelated to changes in glacial ice volume (e.g., third‐ and higher‐order Vail sea level variations, continental foundering). We propose that many of these changes can be caused by stress‐induced changes in plate density. Stress changes produce significant changes in the density of the crust and lithosphere (a point missed in previous investigations) and propagate across even the largest plates in less than 30,000 years. Lithospheric plates interacting at existing boundaries can produce stress‐related density changes sufficient to cause several meters change in plate elevation; these may account for many of the regressions and transgressions seen in the stratigraphic record. The creation of new rifts could increase plate compression enough to cause ∼50 m of plate subsidence. Plate elevation changes of up to −200 m could result from increased plate compression during continental collisions. A particularly enigmatic kind of rapid nonglacial global (NGG) sea level change is a coupled ∼50 m sea level fall (regression) and roughly equal rise (transgression) occurring in less than 106 years. These couplets, associated with black shales and marine extinctions, can be explained by the elastic snapback attending the rapid formation of a new rift. Isostatic disequilibria along the new rift depresses the seafloor sufficiently to cause a ∼50 m fall in sea level. Mantle flow restores isostatic equilibrium along the rift axis and erases the fall in ∼60,000 years. Unusually intense hydrothermal circulation along the new rift during the snapback promotes anoxic bottom water conditions and deposition of black shales. A rapid drop in global sea levels reduces the area of ocean bottom within the photic zone, causing overpopulation, food exhaustion, and extinctions. The connections between changes in plate stress and density have many implications. Perhaps the most far reaching is that tectonic events on the 3/4 of the globe that is covered by oceans, as well as the 1/4 that is subaerial, are recorded in sedimentary strata.
Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera: Ancient crust bounded by juvenile terranes
Authors: Samson Scott D. , Patchett P. Jonathan , McClelland William C. , Gehrels George E.
Published: February 2008

Nd isotopic data are reported for 52 samples from the crustal region between the Alexander‐Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic‐Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, a terrane of probable Early Permian to Late Triassic age, and four assemblages of metamorphic rocks that occur to the west of and within the Coast Mountains batholith. The Gravina belt has εNd(T) values that range from −1.1 to +8.3, similar to values of the underlying Alexander terrane, and consistent with the interpretation that it is a juvenile belt that formed in a back‐arc or intra‐arc basin within the Alexander terrane. Mid‐Cretaceous plutons that were emplaced into the Gravina belt have εNd(T) values of +4.4 to +5.7 and were probably produced by mantle‐derived melts that incorporated some Alexander terrane crust. The Taku terrane has εNd(0) values that range from −5.5 to +3.3, with corresponding depleted‐mantle model (TDM) ages of 440 to 1430 Ma. A mid‐Cretaceous pluton intruding the Taku terrane has an εNd(T) value of +5.1, a value indistinguishable from those determined for Cretaceous plutons intruding the Gravina belt. Metamorphic rocks east of and structurally overlying the Taku terrane are divided into the Tracy Arm assemblage, εNd(0)=−26 to 0, TDM=800–2450 Ma; the Endicott Arm assemblage, eNd(0)=−10 to −1.3, TDM=950–1500 Ma; the Port Houghton assemblage, εNd(0)=−9.4 to +1.1, TDM = 550–1500 Ma; and the Ruth assemblage, εNd(0) = −9.4 to +2.0, TDM=650–1300 Ma. These isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. The metamorphic rocks from these assemblages are lithologically very similar to rocks of the Yukon‐Tanana (YTT) terrane of eastern Alaska and Yukon Territory and have such similar U‐Pb detrital zircon ages and Nd isotopic compositions to YTT rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander‐Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander‐Wrangellia terrane.
Sulfur and nitrogen levels in the North Atlantic Ocean's atmosphere: A synthesis of field and modeling results
Authors: Galloway J. N. , Penner J. E. , Atherton C. S. , Prospero J. M. , Rodhe H. , Artz R. S. , Balkanski Y. J. , Bingemer H. G. , Brost R. A. , Burgermeister S. , Carmichael G. R. , Chang J. S. , Charlson R. J. , Cober S. , Ellis W. G. , Fischer C. J. , Hales J. M. , Hastie D. R. , Iversen T. , Jacob D. J. , John K. , Johnson J. E. , Kasibhatla P. S. , Langner J. , Lelieveld J. , Levy H. , Lipschultz F. , Merrill J. T. , Michaels A. F. , Miller J. M. , Moody J. L. , Pinto J. , Pszenny A. A. P. , Spiro P. A. , Tarrason L. , Turner S. M. , Whelpdale D. M.
Published: February 2008

In April 1990, forty‐two scientists from eight countries attended a workshop at the Bermuda Biological Station for Research to compare field measurements with model estimates of the distribution and cycling of sulfur and nitrogen species in the North Atlantic Ocean's atmosphere. Data sets on horizontal and vertical distributions of sulfur and nitrogen species and their rates of deposition were available from ships' tracks and island stations. These data were compared with estimates produced by several climatological and event models for two case studies: (1) sulfate surface distributions and deposition and (2) nitrate surface distributions and deposition. Highlights of the conclusions of the case studies were that the measured concentrations and model results of nitrate and non‐sea‐salt sulfate depositions appeared to be in good agreement at some locations but in poor agreement for some months at other locations. The case studies illustrated the need for the measurement and modeling communities to interact not only to compare results but also to cooperate in improving the designs of the models and the field experiments.