bluuebunny/crossref_metadata_2025 huggingface dataset:
Semidiurnal and diurnal tidal effects in the middle atmosphere as seen by Rayleigh lidar
Authors: Gille Sarah T. , Hauchecorne Alain , Chanin Marie‐Lise
Published: February 2008

The Rayleigh lidar at the Centre d'Essais des Landes has been adjusted to provide continuous day and nighttime measurements when clear weather permits with high resolution in both time and space. These measurements are processed to show tidal effects in the 30‐ to 80‐km height range. The observed temperature variations in two 11‐day series of data, from November 1988 and January 1989, are treated by spectral analysis and by least squares fitting sinusoids for 12‐ and 24‐hour periods. The observed tidal amplitudes correspond with experimental results from rocket studies and are generally consistent, though somewhat larger than predicted in theoretical models. Observed lidar phases are roughly corroborated by radar observations and also follow the same general trends as model predicitions.
Variations in boundary layer properties associated with clouds and transient weather disturbances at the South Pole during winter
Authors: Stone Robert S. , Kahl Jonathan D.
Published: February 2008

Both the increasing concentrations of greenhouse gases and potential changes in cloud distributions are likely to affect the surface energy budget of the polar regions. Changes in the polar atmosphere are linked to dynamical processes that control the transport of mass, heat, and moisture from lower latitudes and in turn, feed back into the global circulation. Radiation and meteorological data collected at the South Pole during the 1986 austral winter are analyzed to gain a better understanding of the relationships between cloud radiative effects, transport processes and the vertical distribution of temperature and wind. An algorithm is developed to characterize the quasi‐permanent surface‐based temperature inversion and the “warm” radiatively active layer above it. Mean winter temperature and wind profiles for clear and overcast conditions are combined with surface radiation measurements and synoptic circulation patterns to study the mechanisms that cause periodic weakening of the inversion. Results support previous studies that ascribe this weakening to (1) warm air advection, (2) downward vertical mixing of sensible and latent heat, and (3) longwave cloud radiative heating. The integrity of the inversion depends on the combined effects of all three mechanisms. Parameters representing the intensity of the inversion and the bulk wind shear through the lower troposphere are suggested as appropriate indices for the detection of climate change in the region of the Antarctic Plateau.
The reaction probabilities of ClONO2 and N2O5 on polar stratospheric cloud materials
Authors: Hanson David R. , Ravishankara A. R.
Published: February 2008

The reaction probabilities, γ, of ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) surfaces were determined, using reactant concentrations that are typical of the lower stratosphere, by measuring the first‐order reactant loss rate coefficients over the substrate located on the walls of a fast‐flow reactor. Reactants were detected using chemical ionization mass spectrometry, a sensitive technique which allows the use of low reactant concentrations. The reaction probabilities obtained for ClONO2 are: 0.3 (+0.7−0.1) on pure ice, 0.006 (±30%) over a NAT surface, and 0.3 (+0.7−0.1) on an HCl‐doped NAT surface. Those for N2O5 are: 0.024 (±30%) on pure ice, 0.0006 (±50%) on a NAT surface, and 0.0032 (±30%) on an HCl‐doped NAT surface. We observed that an ice surface will be converted into a less reactive HNO3‐doped ice surface in a relatively short time, and we present arguments that this surface consisted of a NAT layer. The large differences between our results and previous measurements for some of these γ can be attributed to the relatively large reactant concentrations used in the previous work. The major implications of this work for polar stratospheric chlorine activation are: an efficient loss of ClONO2 on pure ice surfaces, a very rapid rate for the reaction ClONO2 + HCl on NAT surfaces, and that pure ice surfaces will become “passivated” when coated with one monolayer of NAT crystal.
Simulations of the HDO and H218O atmospheric cycles using the NASA GISS general circulation model: Sensitivity experiments for present‐day conditions
Authors: Jouzel Jean , Koster R. D. , Suozzo R. J. , Russell G. L. , White J. W. C. , Broecker W. S.
Published: February 2008

Incorporating the full geochemical cycles of stable water isotopes (HDO and H218O) into an atmospheric general circulation model (GCM) allows an improved understanding of global δD and δ18O distributions and might even allow an analysis of the GCM's hydrological cycle. A detailed sensitivity analysis using the NASA/Goddard Institute for Space Studies (GISS) Model II GCM is presented that examines the nature of isotope modeling. The tests indicate that δD and δ18O values in nonpolar regions are not strongly sensitive to details in the model precipitation parameterizations. This result, while implying that isotope modeling has limited potential use in the calibration of GCM convection schemes, also suggests that certain necessarily arbitrary aspects of these schemes are adequate for many isotope studies. Deuterium excess, a second‐order variable, does show some sensitivity to precipitation parameterization and thus may be more useful for GCM calibration. Due to strong sensitivity over polar regions, GCM isotope modelers must choose carefully the numerical scheme for isotope transport and the formulation of kinetic fractionation processes at snow formation. The GCM results support the assumption that isotope fractionation does not occur during evaporation over continental areas.
Atmospheric fate of several hydrofluoroethanes and hydrochloroethanes: 2. UV absorption cross sections and atmospheric lifetimes
Authors: Orlando John J. , Burkholder James B. , McKeen Stuart A. , Ravishankara A. R.
Published: February 2008

The temperature dependent UV absorption cross‐sections of the hydrochlorofluoroethanes CF3CHFCl, CH3CF2Cl, and CF3CHCl2 are reported. The UV absorption cross‐sections were measured over the temperature range 203 to 295 K and the wavelength range 190 to 230 nm. The hydrofluorocarbons CH3CF2H and CF3CH2F were also studied at 295 K and upper limits for their UV absorption cross‐sections over the wavelength range 190 to 230 nm were obtained. The atmospheric lifetimes of these species were calculated using a one‐dimensional atmospheric model, using the kinetic data obtained in the previous paper (Gierczak et al. 1991) and the absorption cross‐section data obtained in the present study.
Miocene isotope reference section, Deep Sea Drilling Project Site 608: An evaluation of isotope and biostratigraphic resolution
Authors: Miller Kenneth G. , Feigenson Mark D. , Wright James D. , Clement Bradford M.
Published: February 2008

We developed an isotope (87Sr/86Sr, δ18O) reference section for the uppermost Oligocene to lower upper Miocene (ca. 25–8 Ma) at Site 608 in the northeastern North Atlantic. This site contains the least ambiguous magnetostratigraphic record of Miocene polarity changes available, providing direct correlations to the Geomagnetic Polarity Time Scale (GPTS). We integrate biostratigraphic, magnetostratigraphic, Sr isotope, and stable isotope data to provide a reference section for Miocene isotope fluctuations. The direct correlation of isotopes and biostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) provides relatively precise age estimates. We use these age estimates to evaluate the timing of first and last occurrences of planktonic foraminifera, and conclude that many of these are synchronous within a 0.5 m.y. resolution between subtropical Site 563 (33°N) and high‐latitude Site 608 (43°N). In addition, we use this chronology to estimate the ages of previously established Miocene oxygen isotope Zones Mi 1 through Mi 7 and to compare the Sr isotope record at Site 608 with previously published87Sr/86Sr records. We approximate latest Oligocene to early late Miocene (25–8 Ma) Sr isotope changes with two linear regressions. The rate of increase of87Sr/86Sr was high from the latest Oligocene (∼25 Ma) to earliest middle Miocene (∼15 Ma), with an estimated rate of 0.000059/m.y. Our ability to reproduce Sr isotope measurements is ±0.000030 or better, yielding a stratigraphic resolution of as good as ±0.5 m.y. for this interval. The rate of change was much lower from about 15 to 8 Ma (on average, 0.000013/m.y.), yielding Sr isotope stratigraphic resolution of worse than ±2.3 m.y. The causes of the late Eocene to Miocene87Sr/86Sr increases are not known. We speculate that a moderate87Sr/86Sr increase (0.000030/m.y) which occurred during the late Eocene‐latest Oligocene can be explained by intermittent glaciations and deglaciations of the Antarctic continent. These pulse‐like changes in the input of glacial weathering products yield what appears to be a monotonic, linear increase. The increase in the frequency of glaciations during the latest Oligocene‐early Miocene can explain the higher rate of change of87Sr/86Sr at this time. We speculate that by the middle Miocene, the development of a permanent east Antarctica ice sheet resulted in decreased input of glacial weathering products and a lower rate of87Sr/86Sr change.

Appendix 1 is available with entire article on microfiche.Order from American Geophysical Union, 2000 FloridaAvenue N.W., Washington, D.C. 20009. Document P90‐001;$2.50. Payment must accompany order.
Hydrologic science: A distinct geoscience
Authors: Eagleson Peter S.
Published: February 2008

Hydrologic science deals with the occurrence, distribution, circulation, and properties of water on Earth. It is clearly a multidisciplinary science, as through the hydrologic cycle water is important to and affected by physical, chemical, and biological processes within all the compartments of the Earth system: atmosphere, glaciers and ice sheets, solid earth, rivers, lakes, and oceans. Because of this geophysical ubiquity, and because water is necessary and limiting for life, concern for issues of hydrologic science has been distributed among the traditional geoscience and engineering disciplines. As a result, an infrastructure of hydrologic science (i.e., a clear identity, with supporting educational programs, research grant programs, and research institutions) has not developed, and a coherent understanding of water's role in the planetary‐scale behavior of the Earth system is missing. The supporting arguments, along with recommended scientific priorities are summarized here as proposed by the National Research Council Committee on Opportunities in the Hydrologic Sciences (see Table 1 for committee composition).
Incremental 40Ar/39Ar thermochronology of mylonitic rocks from the Northern Snake Range, Nevada
Authors: Lee Jeffrey , Sutter John F.
Published: February 2008

A 40Ar/39Ar thermochronologic study of muscovite, biotite, and potassium feldspar from footwall rocks of the Northern Snake Range Decollement, Nevada, was undertaken in order to constrain the timing and temperatures of mylonitic deformation during extension and the subsequent cooling (uplift) history of these mylonitic rocks. Mylonitic deformation has been bracketed between 37 Ma and about 24 Ma, and minimum temperatures during ductile thinning were about 300°C. Mineral cooling ages along the east flank of the range suggest a minimum TMAX gradient of about 55°C/Ma. This steep thermal gradient is the result of a localized heat source and/or shearing of colder rocks with older cooling histories over hotter rocks with younger cooling histories. Although mylonitic deformation may have spanned a 13 m.y. period (37 Ma to 24 Ma), deformation was not synchronous throughout the range. On the west flank of the range, age spectra from muscovite and microcline samples indicate that rocks were at temperatures well below 325°C and deformation had ceased by 30 Ma, while on the east flank of the range, lower plate rocks were at temperatures above 325°C and ductile deformation was still ongoing. This is also reflected in chrontours of published K/Ar muscovite ages, along with 40Ar/39Ar ages, which monotonically decrease in age from west to east. These data provide evidence for a temperature difference across the range within the same structural horizon from about 30 to 24 Ma. These relations suggest a lateral thermal gradient, the east flank of the range cooling more slowly than the west flank. Alternatively, the data suggest either an eastward dip of lower plate units prior to the onset of Tertiary mylonitic deformation or that layering dipped eastward due to deformation in an east dipping, top‐to‐the‐east shear zone. Both cases result in differential cooling of lower plate units as they pass through a series of subhorizontal isotherms. The 40Ar/39Ar data, along with published finite strain, structural, and quartz petrofabric data, provide constraints on the formation and geometric evolution of metamorphic core complex detachment faults.
The contact between the Higher Himalaya Crystallines and the Tibetan Sedimentary Series: Miocene large‐scale dextral shearing
Authors: Pêcher Arnaud
Published: February 2008

Space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the Himalayan belt (east and central Nepal, Garhwal, Zanskar). From one area to the other the evolution is very similar: (1) At the base of the pile, in the Main Central Thrust (MCT) shear zone, the stretching lineation, penetrative and regularly oriented N0°E to N30°E, indicates the MCT transport direction, very constant all along the belt, from the Eohimalayan main metamorphic development up to the late‐metamorphic movements. (2) At the top of the pile, at the contact between the crystalline unit and its sedimentary cover, gravity‐driven structures are confirmed (north‐vergent folds, ductile normal faulting). However, there are numerous local indications of late Miocene (syn‐ to late emplacement of the leucogranitic plutons) dextral shearing. (3) In between, across the medium part of the pile, the stretching lineation shows a conspicuous progressive regional clockwise rotation, clearly indicated by the strain trajectories mapped in Nepal and Garhwal. Points 1 and 2 show that the crystalline‐sedimentary boundary, despite its apparent structural and metamorphic continuity, is not only a normal fault but also an important dextral shear zone, which has acted since the upper Miocene as the main southern limit of the eastward extruding Tibetan block.
A case for neotectonic joints along the Niagara Escarpment
Authors: Gross Michael R. , Engelder Terry
Published: February 2008

In a search for proof that some joints on the Appalachian Plateau are neotectonic, orientation data were collected on more than 1800 fractures in exposures of the Lockport Dolomite in western New York and southern Ontario. Of greatest interest was a late‐forming ENE joint set present in the central and western portions of the field area. These joints increase in abundance with proximity to the Niagara Escarpment. The shape of the east–west trending Niagara Escarpment is defined by a series of asymmetric reentrants with a strong ENE linear trend, reflecting the bedrock joint pattern. The joints appear to have formed near the Earth's surface in response to low tensile stresses developed in bedrock adjacent to the retreating escarpment. Since the current shape and position of the escarpment were controlled by glacial activity during the last 3 m.y., it is possible that the ENE joints and reentrants are neotectonic features.
Methane emissions from the Amazon Floodplain: Characterization of production and transport
Authors: Wassmann R. , Thein U. G. , Whiticar M. J. , Rennenburg H. , Seiler W. , Junk W. J.
Published: February 2008

Methane production, transport and emission in a floodplain lake in central Amazonia were investigated by isotopic studies and gas exchange measurements. Samples of sediment free gas were depleted in δ13CCH4, δ13DCH4,and δ13CCO2 values. The isotopic composition of the sediment free methane clearly demonstrated a methane production by methyl fermentation. This finding was strengthened by the coexisting δ13CCO2 and δ13CCO2 values in the sediment free gas. The flux rates of methane ebullition and diffusion were measured during a complete annual cycle using the static chamber method. Significant differences were observed in the release of methane from individual vegetation types, i.e., phytoplankton, floating grass mats, and flooded forest. Each vegetation type showed a distinct seasonal pattern. The highest ebullition rates (mean value, 69 mg CH4 m−2d−1) were recorded in the flooded forest, covering the higher areas of the floodplains with a long subaerial period. Significantly lower averages of the gas bubble flux were recorded in the permanently aquatic areas of the lake (mean value, 29 mg CH4 m−2d−1) and in the intermediate area with floating grass mats (mean value, 23 mg CH44 m−2d−1. Ebullition was the predominant mechanism for the methane transport from the varzea sediment into the atmosphere with maximum values of up to 200 mg CH4 m−2d−1. The diffusive flux remained below 29 mg CH4 m−2d−1 at all sites throughout the entire annual cycle. The variation of the ebullutive flux was found to determine the spatial and temporal variation of the total methane flux in the varzea. We estimate that ebullition accounts for 80% of the total methane emission from the varzea.
Comments on the role of marine sediment burial as a repository for anthropogenic CO2
Authors: Berner Robert A.
Published: February 2008

In a recent letter to this journal, Smith and Mackenzie [1991] emphasized the role of near‐shore environments, such as bays and estuaries, as being the principal locus for the enhanced burial of organic matter due to anthropogenically derived nutrient inputs to the oceans. I commend them for this because of the relative lack of attention given to near‐shore environments by much of the biogeochemical and ocanographic community. It is along continental margins and not in the pelagic ocean where most organic matter, as well as most sediment, is buried. From an extensive review of the sediment literature I have shown [Berner, 1982] that principal organic carbonburial on a global basis occurs in deltas and other continental margin environments. (A large proportion of the organic C may be of terrestrial origin.) Although the average carbon content of the surficial portions of deltaic and continental shelf muds is only about 0.75%C, the large mass of sediment carrying this small carbon percentage adds up to a very large burial rate. The riverine flux of suspended sediment to the oceans has been estimated [e.g., Milliman and Meade, 1983; Milliman and Syvitski, 1992] to be between 12,000 and 25,000 Tg (102grams) per year, and most of this sediment is deposited at present on or near the continental shelves. At 0.75 % C this amounts to anannual carbon burial rate of 90–190 Tg C. Burial rates in other environments such as offshore upwelling zones of high productivity or the vast pelagic realms of the oceans do not come close to matching these rates. If one is to look for a locus for the burial of excess carbon from fossil fuel burning and deforestation, the place to look is near the continents.
Simulation of the North Atlantic Oscillation in a general circulation model
Authors: Pittalwala Iqbal I. , Hameed Sultan
Published: February 2008

The year to year variations of winter climate of the North Atlantic region including Northern Europe, Northwest Africa, and North America are largely controlled by the phases of the North Atlantic Oscillation (NAO). We have found that a coupled ocean‐atmosphere General Circulation Model simulates not only the NAO, but its observed influences on precipitation in northwest Africa, thereby showing that explanations of regional scale climatic variations may be found through diagnoses of such model simulations.
Vorticity and divergence in the high‐latitude upper thermosphere
Authors: Thayer J. P. , Killeen T. L.
Published: February 2008

Measurements made from the Dynamics Explorer‐2 satellite in November 1981 through January 1982 and November 1982 through January 1983 have been analyzed to determine the divergence and vertical component of vorticity of the high‐latitude neutral wind field in the upper thermosphere for quiet (Kp ≤3) and active (3+ ≤ Kp ≤6) geomagnetic conditions and for both northern (winter) and southern (summer) hemispheres. This analysis provides the first experimental determination of the large‐scale vorticity and divergence patterns in the polar thermosphere and provides insight into the relative strengths of the different sources of momentum and energy responsible for driving the winds. The principal findings from this work include the following: The mean neutral wind pattern is dominated by rotational flow rather than by divergent flow, with a typical vorticity: divergence ratio of ∼2:1 for active conditions and ∼4:1 for quiet conditions. Comparison of the divergence and vorticity patterns for quiet and active conditions indicates that the divergent component of the neutral flow intensifies more significantly with increasing geomagnetic activity than does the rotational component.
Relativistic electron enhancements; Simultaneous measurements from synchronous and low altitude satellites
Authors: Imhof W. L. , Voss H. D. , Mobilia J. , Datlowe D. W. , McGlennon J. P. , Baker D. N.
Published: February 2008

We present here for the first time simultaneous measurements of trapped relativistic electron enhancements at synchronous altitude and precipitating electrons in the bounce loss cone at low altitudes. The measurements show that the daily variations in the precipitation flux for L > 5 correlated well with the daily variations in the total flux at high altitude, both with respect to sudden enhancements as well as flux depletions. The daily averaged precipitating flux (E > 1 MeV) at L=6.1 to 7.1 was about 0.3 percent of the daily averaged directional flux (E ≥ 1.5 MeV) observed at synchronous altitude, whereas within narrow spikes the precipitating directional fluxes were often within a factor of 10 of the daily averaged trapped fluxes. Strong depletions in the synchronous altitude ≥1.5 MeV electron intensities are shown to be associated with low altitude measurements of the equatorward movement of precipitating spikes to lower L shells.
The effect of non‐volatile porous layers on temperature and vapor pressure of underlying ice
Authors: Kömle N. I. , Streiner G. , Baguhl M. , Kohl H. , Kochan H. , Thiel K.
Published: February 2008

The space missions to comet Halley have confirmed the view that on large parts of the surface of a cometary nucleus the ice is buried below a non‐volatile dust layer or even a sintered cohesive crust. In order to obtain a better understanding of such ice/crust systems, the thermal behavior of ice samples covered by non‐volatile porous material under illumination by an artificial ‘sun’ was studied by means of laboratory experiments. Both loose dust mantles and cohesive porous crusts were used as covers. For comparison an experiment with uncovered ice was also performed. We recorded temperature and gas pressure close to the surface of the porous ice and it was found that due to the suppression of gas outflow the ice temperatures in the covered ice became always higher than in the ‘free sublimation’ experiment. Furthermore, a pressure build‐up of several Pa was observed in the covered ice close to the surface. In some experiments the measured pressures remained significantly below the saturation vapor pressure which corresponds to the measured temperature at the same depth, indicating strong deviations from equilibrium conditions.
Characterization of quenched high pressure phases in CaSiO3 system by XRD and 29Si NMR
Authors: Kanzaki Masami , Stebbins Jonathan F. , Xue Xianyu
Published: February 2008

We have studied quenched high pressure phases in the CaSiO3 system by x‐ray diffraction (XRD) and 29Si MAS NMR, XRD study of the previously reported “ε‐CaSiO3 phase” synthesized at 12 GPa and 1500 °C reveals that it is actually a mixture of β‐Ca2SiO4 (larnite) and a previously unknown CaSi2O5 phase. This result is supported by the 29Si NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi2O5 phase may have a titanite (CaTiSiO5) structure in which Ti is replaced by an octahedral Si. Samples quenched from 15 GPa and 1500°C consist mostly of an amorphous phase, but a small amount of CaSiO3‐perovskite was identified by both XRD and NMR. The 29si NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.
The solar UV related changes in total ozone from a solar rotation to a solar cycle
Authors: Chandra S.
Published: February 2008

The Nimbus‐7 TOMS version 6 data, corrected for the instrument degradation, are analyzed to delineate the solar UV related changes in total ozone(TOZ) against background signals of dynamical origin. It is shown that the solar UV related change in TOZ over a solar cycle is about 1.5 percent that may be attributed to about 6 percent change in the solar UV flux near 200 nm. This estimate is also consistent with the solar UV related changes in TOZ over a time scale of a solar rotation. In the solar rotation case, ozone lags the solar UV by 3–4 days and its sensitivity to solar UV change is a factor of 2–3 less than for the solar cycle case. Both these effects are attributed to chemical time constants in the lower stratosphere that are comparable to the period of a solar rotation.
Simultaneous excitation of broadband electrostatic noise and electron cyclotron waves in the plasma sheet
Authors: Berchem Jean P. , Schriver David , Ashour‐Abdalla Maha
Published: February 2008

Electron cyclotron harmonics and broadband electrostatic noise (BEN) are often observed in the Earth's outer plasma sheet. While it is well known that ion beams in the plasma sheet boundary layer can generate BEN, new two dimensional electrostatic simulations show that field‐aligned ion beams with a small perpendicular ring distribution can drive not only BEN, but also electron cyclotron harmonic (ECH) waves simultaneously. Simulation results are presented here using detailed diagnostics of wave properties, including dispersion relations of all wave modes.
Long‐period S to P converted waves and the onset of partial melting beneath Oahu, Hawaii
Authors: Bock Günter
Published: February 2008

Strong S‐P converted waves are observed at the long‐period Global Digital Seismograph Network (GDSN) station Honolulu (HON), Oahu, Hawaii, preceding mantle S by 11±1 s. The lead times and polarities of precursors relative to S are consistent with a seismic velocity decrease at a depth between 70 and 80 km probably marking the asthenosphere‐lithosphere boundary (ALB) beneath HON. Comparison of the data with synthetic seismograms calculated for models containing a first order seismic discontinuity at 75 km depth suggests that the S wave velocity decreases by at least 15% across the ALB. The sharpness of the ALB cannot be resolved with long‐period data; the same results are obtained if the velocity decrease extends over a depth interval up to 40 km wide. Assuming aperidodite composition for the uppermost mantle, a lower limit of 8% for the volume percentage of partial melt and a minimum temperature of about 1450°C in the asthenosphere is consistent with the observations.
Fault steps and the dynamic rupture process: 2‐D numerical simulations of a spontaneously propagating shear fracture
Authors: Harris Ruth A. , Archuleta Ralph J. , Day Steven M.
Published: February 2008

Fault steps may have controlled the sizes of the 1966 Parkfield, 1968 Borrego Mountain, 1979 Imperial Valley, 1979 Coyote Lake and the 1987 Superstition Hills earthquakes. This project investigates the effect of fault steps of various geometries on the dynamic rupture process. We have used a finite difference code to simulate spontaneous rupture propagation in two dimensions. We employ a slip‐weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes. The geometry of the two parallel fault planes allows for stepover widths of 0.5 to 10.0 km and overlaps of −5 to 5 km. Our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps. A major difference between the compressional and dilational cases is, that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment. Therefore our dynamic study implies that a compressional step will be jumped quickly, whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity. We also find that the rupture is capable of jumping a wider dilational step than compressional step.
ELF radiation from the Tromsø “Super Heater” Facility
Authors: Barr R. , Stubbe P.
Published: February 2008

Direct comparisons have been made of the ionospheric ELF radiation produced by the new(1 GW ERP) and old (250 MW ERP) antennas of the Tromsø heater system, but no significant differences in the ELF signal strength have been detected. This initially surprising result is shown to require a value of unity for the index relating the received ELF signal strength to HF power input to the antenna. A series of experiments performed solely to derive more accurate values for this power index provided values ranging from 0.74 to 0.97, dependent on the ELF frequencies generated. It has been suggested that ELF radiation from the normal Tromsø heater facility should be limited by saturation effects, even when operating well below the maximum HF power density (3mW/m2 in the D‐region). No evidence for such saturation effects has been found even at power densities greater than 10mW/m2.
A jackknife for magnetostratigraphy
Authors: Tauxe Lisa , Gallet Yves
Published: February 2008

We investigate the problem of evaluating the reliability of a magnetostratigraphic section using a jackknife resampling scheme. We define a parameter, J as the slope relating the percentage of polarity zones of a sampled section remaining after random deletion of sites increasing in proportion to a total of 20% of the initial collection. So defined, J quantifies the dependence of a magnetostratigraphic record on the distribution of sampling sites. Based on simulated magnetostratigraphic sections, J was found to be an indicator of the approximate percentage of polarity zones present in the original reference sections that were recovered in the sampled section. J will perhaps be most helpful as a guide to investigators as to whether further sampling would provide useful information as well as to the non‐paleomagnetist user of black and white interpretive polarity logs as an estimate of the reliability of a given section.
U‐Pb dating of a remagnetized Paleozoic limestone
Authors: DeWolf Charles P. , Halliday Alex N.
Published: February 2008

The Upper Ordovician Trenton Group of New York State is one of a number of Appalachian limestones which yields paleomagnetic poles suggesting disturbance in the late Paleozoic. Calcite fractions dissolved from samples of the Trenton Limestone define a 238U‐206Pb age of 454±8 Ma (MSWD=12.8) in excellent agreement with 40Ar−39 Ar and U‐Pb ages for the deposition of the underlying Diecke Bentonite. This requires that the U‐Pb system in the calcite has been preserved since deposition and early diagenesis. If late Paleozoic remagnetization of this limestone was induced by fluids, these fluids did not interact with significant amounts of carbonate. The apparent robustness of the U‐Pb system, and the large spread in µ values (18.17–118.8), offers potential precision of ±1 Ma in determining the Paleozoic time scale.
Lava‐seawater interactions at shallow‐water submarine lava flows
Authors: Sansone Francis J. , Resing Joseph A. , Tribble Gordon W. , Sedwick Peter N. , Kelly Kevin M. , Hon Ken
Published: February 2008

Hydrothermal plumes associated with nearshore lava flows from Kilauea Volcano, Hawaii were studied on five occasions during 1989–1990 to address the current lack of data on direct lava‐seawater interactions. The following enrichments were found in the sea‐surface hydrothermal plumes above the active underwater lava flows: H2, 15,000x ambient seawater concentrations; Mn, 250x; and Si, 20x. Water temperatures reached 46°C. Lower concentrations and temperatures were observed in the plumes with increasing distance from shore, with H2, Si, and Mn concentrations linearly related to seawater temperature. Unlike deep sea spreading center hydrothermal plumes, no CH4 enrichment was observed. The elevated H2 is likely to be from water‐rock reactions, rather than from the release of magmatic gas. The plume mass/heat ratios presented here suggest that submarine flood basalts, although aerially large, should be relatively small immediate contributors to oceanic geochemical cycles compared to hydrothermal circulation through the crust.
Few millimeter precision for baselines in the California permanent GPS Geodetic Array
Authors: Lindqwister Ulf J. , Zumberge James F. , Webb Frank H. , Blewitt Geoffrey
Published: February 2008

Geodetic measurements with Rogue Global Positioning System (GPS) receivers from sites in the California Permanent GPS Geodetic Array (PGGA) have been analyzed using the GIPSY orbit‐determination and baseline‐estimation software. Based on an unbiased selection of 23 daily measurements spanning 8 months, the low‐frequency (≳ weeks) contributions to the long‐term repeatabilities of baseline measurements are approximately 5, 3, and 8 mm for the east, north, and vertical components. Short‐term contributions to the long‐term repeatabilities were evaluated by examining data from the week of October 21, 1990, which showed the lowest short‐term scatter. For this week, daily repeatabilities of 2–3 mm in the horizontal and 4 mm in the vertical have been achieved for the 172‐km JPL‐Pinyon baseline, consistent with carrier phase data noise of −6 mm. High quality (≲5 mm) repeatabilities have been achieved for all components of the other baselines as well.
3‐D modelling of seamount topography from satellite altimetry
Authors: Baudry Nicolas , Calmant Stéphane
Published: February 2008

We develop a complete set of algorithms to perform 3D modelling of seamount bathymery from satellite altimetry. The first stage of the data processing consists in gridding the geoid: to account for the long wavelength errors geoid heights are first bias‐adjusted at cross‐overs. Then a collocation on a regular grid is performed, accounting for the altimeter errors. In a second stage, geoid heights are converted into bathymetry. No simplifying assumption on the shape and location of the bathymetry highs is necessary. Bathymetric uncertainties due to the data sampling and the parameters of the mechanical and crustal models are evaluated.
Preliminary measurements of CO2 in melting snow
Authors: Sommerfeld R. A. , Musselman R. C. , Reuss J. O. , Mosier A. R.
Published: February 2008

Measurements of CO2 near the snow‐soil interface showed elevated concentrations up to 2120 ppmv. Concentrations greater than 1700 ppmv were observed 0.45 m above the snow‐soil interface. The increase in CO2 concentrations in the snow coincided with the beginning of melt. Measurements of the pH and alkalinity of the meltwater from the base of the snowpack were consistent with the measured CO2 levels. Decreases in pH at constant alkalinity of up to 0.8 units were associated with the excess CO2. The origin of the excess CO2 is uncertain but may be related to litter decomposition. Elevated levels of CO2 near the snow‐soil interface at the start of melt could have important effects on meltwater chemistry, especially since streams at this time flow under a covering of snow preventing equilibration with atmospheric CO2.
No‐net‐rotation model of current plate velocities incorporating plate motion model NUVEL‐1
Authors: Argus Donald F. , Gordon Richard G.
Published: February 2008

We present NNR‐NUVEL1, a model of plate velocities relative to the unique reference frame defined by requiring no‐net‐rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model NUVEL‐1 [DeMets et al., 1990]. Differences between NNR‐NUVEL1 and no‐net‐rotation plate motion model AM0‐2 [Minster and Jordan, 1978] are as large as 15 mm/yr. In NNR‐NUVEL1, the Pacific plate rotates in a right‐handed sense relative to the no‐net‐rotation reference frame at 0.67°/m.y. about 63°S, 107°E. This rotation nearly parallels, but is 32% slower than, the Pacific‐hotspot Euler vector of HS2‐NUVEL1, which is a global model of plate‐hotspot velocities constrained to consistency with NUVEL‐1 [Gripp and Gordon, 1990]. At Hawaii the Pacific plate moves relative to the no‐net‐rotation reference frame at 70 mm/yr, which is 25 mm/yr slower than the Pacific plate moves relative to the hotspots. Differences between NNR‐NUVEL1 and HS2‐NUVEL1 are described completely by a right‐handed rotation of 0.33°/m.y. about 49°S, 65°E. The 99% confidence ellipsoids for plate‐hotspot motion in model HS2‐NUVEL1 exclude the corresponding Euler vectors from model NNR‐NUVELl. Thus the no‐net‐rotation reference frame differs significantly from the hotspot reference frame. If the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame, then hotspots beneath the Pacific plate move with coherent motion towards the east‐southeast. Alternatively, the difference between reference frames may show that the uniform drag, no‐net‐torque reference frame, which is kinematically equivalent to the no‐net‐rotation reference frame, is based on a dynamically incorrect premise. Possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans.
Volcanic ash from the 1362 A.D. Oræfajokull Eruption (Iceland) in the Greenland Ice Sheet
Authors: Palais J. M. , Taylor K. , Mayewski P. A. , Grootes P.
Published: February 2008

A continuous record of electrical conductivity measurements (ECM) was made on site during the drilling of a 200 m ice core at Summit, Greenland and was used to identify horizons in the ice that might be linked to volcanic eruptions. In one detailed section that we studied a large peak in the number of particles, two orders of magnitude above the background, was measured. The particle peak was not associated with an ECM peak, however. The particles were identified as volcanic ash on the basis of both particle morphology and chemical composition. The ash composition suggests an explosive rhyolitic eruption and is believed to have originatedfrom Oræfajokull in Iceland in 1362 A.D.
Seismic imaging of subducted slabs: Trade‐offs with deep path and near‐receiver effects
Authors: Schwartz Susan Y. , Lay Thorne , Grand Stephen P.
Published: February 2008

The most promising procedures for determining detailed velocity structure of subducting lithosphere (seismic tomography and residual sphere analysis) require isolation of near‐source contributions to total travel‐time anomalies of seismic waves from earthquakes in the slab. Effects of heterogeneous structure along the raypaths through the deep mantle, lithosphere and crust beneath the recording stations must be removed from the data, using either empirical procedures or aspherical models. Comparisons of observed S wave travel‐time anomalies for earthquakes in the Kurile slab with both empirical path corrections and corrections computed for a high resolution tomographic shear velocity model beneath North America indicate that near‐source contributions are significantly smaller than suggested by recent slab modeling efforts that fail to fully remove the distant path effects. Imaging deep slab velocity structure and depth of penetration is likely to prove more difficult than originally expected, given that slab velocity heterogeneity may be relatively weak at depths greater than 300 km.
Tectonic framework of the Azores Triple Junction
Authors: Miranda J. M. , Luis J. Freire , Abreu I. , Victor L. A. Mendes , Galdeano A. , Rossignol J. C.
Published: February 2008

The lack of accurate and detailed magnetic information has, in the past, limited the development of well constrained models for the plate tectonic evolution of the Azores Triple Junction. An aeromagnetic survey, made possible by the existing airport facilities, has long been desired as it can provide high quality magnetic data, whose homogeneity and coherency is far better than those provided by classic marine surveys. The results presented in this paper concern only a part of the Aeromagnetic Survey conducted by the Portuguese Instituto Nacional de Meteorologia e Geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the Azores plateau.
Nitrogen stable isotope ratio of groundwater N2O
Authors: Ueda S. , Ogura N. , Wada E.
Published: February 2008

δ15N of N2O and NO3− were measured to determine the mechanisms producing N2O in nitrate contaminated groundwaters. These groundwaters contained N2O at concentrations between thirty to one hundred times higher than the atmospheric equilibrium. The N2O/NO3− molar ratios in groundwaters were from 0.2 × 10−3 to 4.4 × 10−3. The δ15N values of N2O, −5.8 to −21.3 permil, were much lower than those of coexistent NO3−, +3.6 to +7.3 permil. In the light of evidence that 15N in N2O from nitrification is highly depleted, the present findings suggest nitrification as a dominant mechanism for the production of N2O in the groundwaters. However, unlike other aquatic systems where nitrification is a major route of N2O production, no clear correlation was observed between ΔN2O and −ΔO2 in the groundwater. This poor correlation strongly suggests that N2O in the groundwater was predominantly produced in the soil column and/or unsaturated zone. Based on two‐year observation of N2O flux from a spring, a possible charging rate of N2O to the aquifer was estimated as 1.2–2.3 mg N2O‐N·m−2·y−1. The rate is much smaller than the reported soil emission rates, suggesting the groundwater N2O to be an insignificant source for atmospheric N2O. Large discrepancies in δ15N of N2O between the groundwaters and the surface airs support this view.
Major Late Miocene cooling of the middle crust associated with extensional orogenesis in the Funeral Mountains, California
Authors: Holm Daniel K. , Dokka Roy K.
Published: February 2008

The Funeral Mountains of the Death Valley region consist of a metamorphic core complex that underlies the Miocene Boundary Canyon detachment fault. Fissiontrack age determinations on sphene, zircon, and apatite from the highest grade portions of the Funeral Mountains (collected in Monarch Canyon) indicate rapid cooling from above ∼285°C at 9–10 Ma. The steep dT/dt slope implied by the cooling path envelope as well as its synchroneity with development of the Boundary Canyon detachment suggests tectonic unroofing of between 9.5 and 17 km since ∼10 Ma (assuming typical geothermal gradients prior to detachment development of between ∼20–30°C/km). Previous studies report muscovite 40Ar/39Ar plateau ages from the same area indicating that the Funeral Mountains core may have cooled below ∼350°C between 110‐55 Ma. These data, when combined with the fission track ages, suggest residence of the metamorphic terrain at 285–350°C from Late Cretaceous to late Miocene time. Recent studies, using petrologic constraints, have suggested that much of the unroofing of the Funeral Mountains, as well as development of tectonite fabrics, occurred during Mesozoic extension. Recognition of temperatures in the 300°C range during Miocene time suggests some of the ductile extensional fabrics may be late Miocene in age.
On the discontinuity in kinetic solutions of the collisionless polar wind
Authors: Khoyloo A. , Barakat A. R. , Schunk R. W.
Published: February 2008

Semi‐kinetic and kinetic models are powerful tools for attacking a range of flow conditions involving collisionless plasmas and they have been used for more than two decades to model the polar wind. With this approach, a Maxwellian ion velocity distribution is generally adopted at the lower boundary of the collisionless regime and the resulting distribution is then calculated at higher altitudes by using Liouville's theorem and conservation of energy and angular momentum. However, the resulting velocity distribution and associated moments (density, drift velocity, temperature, etc.) display sharp discontinuities at the boundary, which quantitatively affects the solution at higher altitudes. Analytical and numerical analyses for the polar wind indicate that the adoption of a Maxwellian for an expanding collisionless plasma in the diverging geomagnetic field is inconsistent and causes the discontinuity in the kinetic solutions. A non‐Maxwellian velocity distribution with a specific feature must be adopted at the lower boundary in order to get a continuous and consistent kinetic solution.
Potential coefficient recovery from the CSR set of simulated satellite gradiometry observations
Authors: Koop Radboud , Stelpstra David
Published: February 2008

An investigation was carried out to demonstrate the possibility of recovering spherical harmonic potential coefficients up to high degree and order from gradiometric measurements. Observations were taken from one month simulated GRM gradient data computed at the Center of Space Research at the University of Texas. For least squares adjustment a linear observation model was chosen from which the radial orbit uncertainties were eliminated. A solution up to degree and order 180 proved to be very promising, whereas a 360 solution showed degraded results due to the too short mission and the non‐homogeneous data coverage.
Seasonal and solar cycle variations in high‐ latitude thermospheric winds
Authors: Aruliah Anasuya L. , Rees David , Steen Ake
Published: February 2008

Thermospheric wind measurements have been collected systematically every winter for over nine years from a high‐latitude site at Kiruna, Sweden (67.8°N, 20.4°E). The database contains 1242 nights of data collected with a Fabry‐Perot Interferometer (FPI), perhaps the largest single‐site database of thermospheric winds. This analysis shows a marked seasonal and solar cycle variation. Particularly at high solar activity, sunward winds of the evening period (16–20 UT) are more than 50% stronger at Spring than at Autumn equinox. This large asymmetry in the behaviour of high‐latitude thermospheric winds at spring and autumn equinox has not yet been predicted by model simulations.
Relationship of springtime ozone depletion at arrival heights, Antarctica, to the 70 HPa temperatures
Authors: Nichol S. E. , Coulmann S. , Clarkson T. S.
Published: February 2008

The total ozone measurements from Arrival Heights (78°S) show severe depletion occurring in 1989 and 1990 with the minimum ozone values of 164 and 144 Dobson Units (DU) being measured in early October. These depletions represent the loss of about half of the total ozone column. Much milder ozone depletion was observed in the 1988 spring, with the minimum of 217 DU being observed on 23 September. The ozone depletion mechanism is linked to the occurrence of polar stratospheric clouds, and hence with low temperatures in the lower stratosphere in early spring. The Arrival Heights data show a shift in the ozone‐temperature relationship over the last three years, with the same total ozone values associated with progressively higher temperatures from year to year.