bluuebunny/crossref_metadata_2025 huggingface dataset:
Steady state toroidal magnetic field at Earth's core‐mantle boundary
Authors: Levy Eugene H. , Pearce Steven J.
Published: February 2008

Measurements of the DC electrical potential near the top of Earth's mantle have been extrapolated into the deep mantle in order to estimate the strength of the toroidal magnetic field component at the core‐mantle interface. Recent measurements have been interpreted as indicating that at the core‐mantle interface, the magnetic toroidal and poloidal field components are approximately equal in magnitude. A motivation for such measurements is to obtain an estimate of the strength of the toroidal magnetic field in the core, a quantity important to our understanding of the geomagnetic field's dynamo generation. Through the use of several simple and idealized calculations, this paper discusses the theoretical relationship between the amplitude of the toroidal magnetic field at the core‐mantle boundary and the actual amplitude within the core. Under physical conditions believed to characterize the core and mantle a low value of the field at the core‐mantle boundary is an inevitable consequence of the electrical conductivity contrast between core and lower mantle material; this conclusion is largely independent of details of the mantle's conductivity profile. Sample calculations, based on simplifying assumptions, are also used to explore the extrapolation of the toroidal field amplitude into the core. Even with a very low inferred value of the toroidal field amplitude at the core‐mantle boundary, (∼ few gauss), the toroidal field amplitude within the core could be consistent with a magnetohydrodynamic dynamo dominated by nonuniform rotation and having a strong toroidal magnetic field.
Thermal conductivity of Hawaiian basalt: A new interpretation of Robertson and Peck's data
Authors: Horai Ki‐iti
Published: February 2008

Robertson and Peck's data published in 1974 on the thermal conductivity of Hawaiian basalts are unique in the sense that the vesicularity of the samples used for the study covers a wide range of 2 to 98%. The data consist of the measurement results of thermal conductivity on 61 samples of basalt collected from Kilauea and other volcanoes in Hawaii. The thermal conductivity was measured, by the method of thermal stacking, on samples with their pores filled first with air and then with water. Thus the data are particularly suitable for evaluating the theoretical formula describing the thermal conductivity of fluid‐saturated porous rock. In the formula for the thermal conductivity of a two‐phase material the thermal conductivity of the solid matrix is an essential parameter, which Robertson and Peck determined by the wet cell method. The result of their determination was that they could not represent the data in the whole range of sample porosities with a single theoretical formula. This paper proposes that the thermal conductivity of the solid matrix can be estimated by extrapolating the data to zero porosity. For the extrapolation, either Fricke‐Zimmerman's or Schulz's formula of thermal conductivity for a two‐phase material having spheroidal inclusions can be used. In both models, for the results of extrapolation from the air‐saturated and the water‐saturated sample thermal conductivities to be identical, the spheroid's aspect ratio is assumed to be around 1/10. This is considered to be a combined effect of the spherical pores and the microcracks in the basalt samples. With the assumed aspect ratio of the spheroidal inclusions, either of the theoretical formulae agree with the data with sufficient precision.
The effects of noise due to random undetected tilts and paleosecular variation on regional paleomagnetic directions
Authors: Calderone Gary J. , Butler Robert F.
Published: February 2008

Random tilting of a single paleomagnetic vector produces a distribution of vectors which is not rotationally symmetric about the original vector and therefore not Fisherian. Monte Carlo simulations were performed on two types of vector distributions: (1) distributions of vectors formed by perturbing a single original vector with a Fisher distribution of bedding poles (each defining a tilt correction) and (2) standard Fisher distributions. These simulations demonstrate that inclinations of vectors drawn from both distributions are biased toward shallow inclinations. There is a greater likelihood of statistically “drawing” a vector shallower than the true mean vector than of drawing one that is steeper. The estimated probability increases as a function of angular dispersion and inclination of the true mean vector. Consequently, the interpretation of inclination‐only data from either type of distribution is not straightforward, especially when the expected paleolatitude is greater than about 50°. Because of the symmetry of the two distributions, declinations of vectors in each distribution are unbiased. The Fisher mean direction of the distribution of vectors formed by perturbing a single vector with random undetected tilts is biased toward shallow inclinations, but this bias is insignificant for angular dispersions of bedding poles less than 20°. This observation implies that the mean pole calculated from a large set of paleomagnetic directions obtained for coeval rocks over a region will be effectively unbiased by random undetected tilts of those rocks provided the angular dispersion of the undetected tilts is less than about 20°. However, the bias of the mean can be significant for large (>20°) angular dispersion of tilts. The amount of bias of the mean direction maximizes at about 10°–12° in mid‐latitude regions but is usually less than 8°. Consequently, large (>12°) inclination discordances are probably not the result of random undetected tilts, even if the angular dispersion of the tilts exceeds 20°.
Time intervals between characteristic earthquakes and correlations with smaller events: An analysis based on a mechanical model of a fault
Authors: Carlson J. M.
Published: February 2008

Certain correlations between characteristic events and smaller events are studied in the context of a spatially homogeneous, mechanical model for a fault. The model satisfies a one‐dimensional quasi‐linear wave equation in which the key nonlinearity is associated with the slipstick friction force at the interface between tectonic plates. We find that the model gives rise to slipping events of all sizes: the smaller events are consistent with the Gutenberg‐Richter statistical law, whereas the characteristic events have a separate statistical distribution. The crossover from smaller events to characteristic events coincides with a correlation length which sets the scale for clustering of smaller events before the characteristic rupture. Numerical results and scaling arguments relate the features of the distribution of characteristic events to the corresponding features of the distribution of smaller events as well as the length of the fault. The distribution of time intervals between characteristic events has the distinguishing features of asymmetry and a finite upper cutoff. The mean T and standard deviation σ of the time interval distribution are found to be related to features of the magnitude versus frequency distribution of smaller events, and the coefficient of variation σ/T is independent of the driving rate. Because this analysis relates properties of characteristic earthquakes to statistical features of smaller, more numerous events as well as geometric features of the fault, it may ultimately be useful for long‐term earthquake prediction.
Flow laws of polyphase aggregates from end‐member flow laws
Authors: Tullis Terry E. , Horowitz Franklin G. , Tullis Jan
Published: February 2008

An incompressible finite element model has been used to study the plane strain deformation of two‐phase aggregates deformed by dislocation creep. Input for the model includes the power law flow laws of the two end‐member phases and their volume fractions and configuration. The model calculates the overall flow law of the aggregate as well as the stress and strain rate variations within it. The input flow laws were experimentally determined for monomineralic aggregates of clinopyroxene and plagioclase. Results were calculated for a temperature of 1000°C, strain rates from 10−4 to 10−12S−1, and stresses of 1–1000 MPa. For these conditions, the end‐member flow laws intersect on a log stress versus log strain rate plot at 10−8S−1. Some runs were made on finite element grids fit to an actual diabase texture (∼64% pyroxene, ∼ 36% plagioclase.) Other runs were made on idealized geometries to test the effects of varying the volume fraction of two phases, shape of inclusions, and relative strengths of inclusion and matrix. Important results include the following: (1) The model results satisfy the requirement that the aggregate strength must lie between the bounds set by the end‐member flow laws and those set by assumptions of uniform stress and uniform strain rate. (2) The calculated diabase flow law matches well with that experimentally determined. (3) The aggregate strength within the uniform stress and uniform strain rate bounds is primarily affected by volume fraction, although certain phase geometries can also affect the strength. (4) Although the flow law for an aggregate of power law phases need not be a simple power law, we find it to be a good approximation. We have developed two simple methods of estimating the strength of an aggregate, given the end‐member flow law parameters and volume fractions; both give results that agree with the finite element model calculations. (1) One method takes into account the phase geometry and gives a strength for the aggregate at any strain rate. (2) The other method can be used even if the phase geometry is unknown and gives expressions for the aggregate flow law parameters.
Model for steady state friction
Authors: Lomnitz‐Adler Jorge
Published: February 2008

A model for dynamic steady state friction between two rough surfaces is developed in which the transfer of momentum from the horizontal to the vertical direction by collisions between asperities on opposing surfaces leads to a friction law which is independent of the detailed mechanism of energy dissipation. For nonfractal surfaces the model applies above a lower critical velocity which increases exponentially with smoothness. At high velocities there is velocity weakening and, as the smoothness of the surfaces increases, the velocity dependence rapidly approaches the experimentally observed logarithmic dependence, in agreement with phenomenological state variable friction laws. For fractallike surfaces the model applies over the whole velocity range above V = 0, showing velocity strengthening at low velocities and velocity weakening at high velocities, implying the existence of a stick‐sliplike instability.
Magnetism and magnetic mineralogy of ash flow tuffs from Yucca Mountain, Nevada
Authors: Schlinger Charles M. , Veblen David R. , Rosenbaum Joseph G.
Published: February 2008

The magnetic susceptibility (χ) and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement. Spatial variations in these properties, therefore, provide a potentially useful tool for geological studies. Maxima and minima in χ measured along profiles at Yucca Mountain, Nevada, identify persistent magnetic marker horizons within vitric portions of the Tiva Canyon and underlying Topopah Spring Members of the Paintbrush Tuff. The correlatable variations of χ with vertical position in these units exist over the 8 km of strike examined. The observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of Fe‐Ti oxide phenocrysts, and the presence, shape, size, and mineralogy of magnetic Fe‐oxide microcrystals that precipitated at high temperature after emplacement of each sheet. Vitrophyre near the top of the Topopah Spring Member has a distinctive maximum in χ, due to titanomagnetite phenocrysts. Susceptibility in the lower portion of the overlying Tiva Canyon Member reaches maximum values of about 1.3×10−2 (SI) near the base of the member, beneath the columnarly jointed vitrophyre, due to the presence of abundant superparamagnetic cubic Fe‐oxide. These elongate microcrystals are several tens of nm long, and are typically deformed and tapered. Up section in the Tiva Canyon Member, the susceptibility decreases, going to a minimum within the columnar vitrophyre. At this stratigraphic level the precipitated Fe‐oxides are larger and carry intense NRM of up to 30 A/m; χ is only 3×10−3. The whisker‐like microcrystals of Fe‐oxide, which typically nucleated on what appears to be chain silicate, grew as needles up to 25 μm in length. The size variations of the precipitated Fe‐oxides, which were established using transmission electron microscopy (TEM) and petrographic observation, are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization. Several interpretations of the shape anisotropy of the precipitated Feoxide are possible, including growth by a dislocation mechanism. Additionally, the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field. Susceptibility variations as established at the outcrop, as well as in the borehole, offer a potentially useful tool for stratigraphic correlation of ash flow sheets.
Paleomagnetism of Latest Cambrian‐Early Ordovician and Latest Cretaceous‐Early Tertiary rocks of the Florida Mountains, southwest New Mexico
Authors: Geissman John W. , Jackson Mike , Harlan Steve S. , Van Der Voo Rob
Published: February 2008

Paleomagnetic investigations have been carried out on latest Cambrian to Early Ordovician alkaline intrusions, their contact intermediate to mafic composition metaigneous rocks, and overlying lower Paleozoic and early Tertiary strata in the Florida Mountains, southwestern New Mexico. A well‐defined, predominantly normal polarity magnetization of high median destructive induction and high, usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late Cenozoic uplift. Combinations of stable endpoint and planar data are used in data analysis at the site level (D = 262°, I = −30°, α95 = 10°, k = 12; N = 22 site means). In addition to the presence of reverse polarity magnetizations at two sites, the dispersion of site mean directions in these rocks (VGP angular standard deviation of 19.6°) suggests that considerable variation of the geomagnetic field has been averaged. After correction for a combination of early Tertiary (Laramide) deformation and late Tertiary Basin‐Range block tilting, this magnetization gives a paleomagnetic pole at 6°S, 169°E and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503±10 Ma, based on published U‐Pb zircon isotopic age determinations. A lack of well‐defined, coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks, as interpreted with Rb‐Sr isotopic data. Hematitic sandstones of the earliest Ordovician Bliss Formation, which directly overlie the crystalline rocks in portions of the range, give dual‐polarity magnetizations whose directions suggest that these rocks have probably been remagnetized; this remagnetization possibly occurred during deformation in late Cretaceous/Tertiary time. Ordovician carbonate rocks of the El Paso and Montoya formations give only a Holocene magnetization. Paleocene to earliest Eocene(?) hematite‐cemented clastic rocks of the Lobo Formation possess dual‐polarity magnetizations. The origin of their magnetization is difficult to assess with the available data. In total, however, paleomagnetic data from the Bliss and Lobo formations argue against any significant vertical axis rotation of the area since Laramide deformation ending in Early Tertiary time. The intermediate to mafic composition metaigneous rocks from the Florida Mountains provide a latest Cambrian‐Early Ordovician paleomagnetic pole that compares favorably with other North American paleopoles satisfying several acceptance criteria. The Florida Mountains data further support recent constructions of apparent polar wander paths for North America and thus the hypothesis that significant counterclockwise rotation of North America occurred during the Ordovician.
Ground‐truthing 6.5‐kHz side scan sonographs: What are we really imaging?
Authors: Gardner James V. , Field Michael E. , Lee Homa , Edwards Brian E. , Masson Douglas G. , Kenyon Neil , Kidd Robert B.
Published: February 2008

A 1000‐km2 area on the distal lobe of Monterey Fan shows a digitate pattern of juxtaposed high and low backscatter on GLORIA side scan sonographs. This area was investigated using stereo photography, high‐resolution seismic profiles, and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6.5‐kHz side scan sonar. Stereo photography and bottom video were used to determine that the sediment‐water interface typically has a bed roughness less than 10 cm over the entire ground truth area; consequently, bed roughness is not a significant contributor to the sonar backscatter. Vertical‐incidence 3.5‐kHz profiles reveal that high‐backscatter areas allow less penetration and have slightly more relief than low‐backscatter areas. Closely spaced measurements of p wave velocity, density, and grain size made on transponder‐navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model. The model results demonstrate that the sediment‐water interface is, in most cases, acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface. The variability of the dominant lithofacies (sands with minor silty clays or silty clay with minor sands) accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers, but at the detailed scale of hundreds to a few square meters, we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence, surface lithology, measured acoustic impedance, or attenuation. However, there is a qualitative, but unexpected, correlation with lithostratigraphic variability. In this area, thick (up to 50 cm) sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand. This suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter. The ground‐truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel (∼70,000 m2) relative to the area of an individual core (0.006 m2), bottom photograph (∼5 m2), or seismic profile. Although 6.5‐kHz sonographs appear easy to interpret in a conventional and simplistic manner, caution should be used when interpreting lithofacies from backscatter intensities.
Ku band airborne radar altimeter observations of marginal sea ice during the 1984 Marginal Ice Zone Experiment
Authors: Drinkwater Mark R.
Published: February 2008

Pulse‐limited, airborne radar data taken in June and July 1984 with a 13.8‐GHz altimeter over the Fram Strait marginal ice zone are analyzed with the aid of large‐format aerial photography, airborne synthetic aperture radar data, and surface observations. Variations in the radar return pulse waveforms are quantified and correlated with ice properties recorded during the Marginal Ice Zone Experiment. Results indicate that the wide‐beam altimeter is a flexible instrument, capable of identifying the ice edge with a high degree of accuracy, calculating the ice concentration, and discriminating a number of different ice classes. This suggests that microwave radar altimeters have a sensitivity to sea ice which has not yet been fully exploited. When fused with SSM/I, AVHRR and ERS‐1 synthetic aperture radar imagery, future ERS‐1 altimeter data are expected to provide some missing pieces to the sea ice geophysics puzzle.
Laboratory experiments on the formation of salt fingers after the decay of turbulence
Authors: Taylor John
Published: February 2008

Previous laboratory experiments have shown that salt fingers are easily disrupted by turbulence. However, in the interior of the ocean, turbulence is both patchy and intermittent, so that there may be sufficient time for the fingers to form between turbulent events. The growth of salt fingers after disruption by turbulence was examined in an experiment in which an oceanic turbulent event was simulated by a grid falling through fingers set up in a laboratory tank. Both horizontal conductivity and temperature profiles showed that after the grid turbulence had decayed the fingers grew rapidly, and that the time scale for growth was of the same order as the e‐folding time for the fastest‐growing long salt finger modes that have been described previously by several authors. This time scale is consistent with the observation that the peak in the temperature gradient spectrum was close to the predicted wave number for fastest‐growing fingers. On the other hand, visualization of the fingers suggests that the period of exponential growth of the finger amplitudes was short, of the order of one e‐folding period. Finally, the results of these experiments suggest that with the typical intermittency of turbulence in the interior of the ocean, salt fingers should have sufficient time to grow between events.
A variational inverse method for the reconstruction of general circulation fields in the northern Bering Sea
Authors: Brasseur Pierre P.
Published: February 2008

The knowledge of the dynamical structures associated with the general circulation in the Northern Bering Sea and Bering Strait is improving as numerical models together with in situ observing systems become more and more sophisticated. The ultimate objective being the design of a realistic model, it is fundamental to confront the results stemming from numerical simulations with the extensive set of hydrographic data collected since the beginning of the century. A variational inverse model has been developed for the study of continental seas with the aim of reconstructing, on the basis of local measurements (such as CTD profiles carried out at isolated hydrographic stations), continuous fields of the state variables representative of the general circulation. A numerical inverse methodology based on a variational principle and a finite element discretization has been designed to generate dynamically consistent patterns, providing a practical tool for the visualization and the mathematical interpretation of observations. Horizontal distributions of temperature, salinity, nutrients, chlorophyll and other chemical tracers, as well as the horizontal transport of water are reconstructed by using synoptic data collected from the ISHTAR NSF Research Project. Among the three water masses that can be distinguished in the region, the Anadyr Stream is obviously the main source of nutrients for the whole basin. The trace of a permanent upwelling located close to the Siberian coast can be detected on a series of general circulation patterns representative of various tracers. The strong vertical motions resulting from this upwelling allow an intensive primary productivity and determine the particularly fertile conditions for the development of the ecosystem in the Chukchi Sea.
Atmospheric organochlorine pollutants and air‐sea exchange of hexachlorocyclohexane in the Bering and Chukchi seas
Authors: Hinckley Daniel A. , Bidleman Terry F. , Rice Clifford P.
Published: February 2008

Organochlorine pesticides have been found in Arctic fish, marine mammals, birds, and plankton for some time. The lack of local sources and remoteness of the region imply long‐range transport and deposition of contaminants into the Arctic from sources to the south. While on the third Soviet‐American Joint Ecological Expedition to the Bering and Chukchi Seas (August 1988), high‐volume air samples were taken and analyzed for Organochlorine pesticides. Hexachlorocyclohexane (HCH), hexachlorobenzene, polychlorinated camphenes, and chlordane (listed in order of abundance, highest to lowest) were quantified. The air‐sea gas exchange of HCH was estimated at 18 stations during the cruise. Average α‐HCH concentrations in concurrent atmosphere and surface water samples were 250 pg m−3 and 2.4 ng L−1, respectively, and average γ‐HCH concentrations were 68 pg m−3 in the atmosphere and 0.6 ng L−1 in surface water. Calculations based on experimentally derived Henry's law constants showed that the surface water was undersaturated with respect to the atmosphere at most stations (α‐HCH, average 79% saturation; γ‐HCH, average 28% saturation). The flux for α‐HCH ranged from −47 ng m−2 day−1 (sea to air) to 122 ng m−2 d−1 (air to sea) and averaged 25 ng m−2 d−1 air to sea. All fluxes of γ‐HCH were from air to sea, ranged from 17 to 54 ng m−2 d−1, and averaged 31 ng m−2 d−1.
Experiments on baroclinic vortex shedding from hydrothermal plumes
Authors: Helfrich Karl R. , Battisti Thomas M.
Published: February 2008

Laboratory experiments have been conducted to examine the effects of rotation on the structure and stability of hydrothermal plumes. It is shown that the plume forces a baroclinic vortex, consisting of an anticyclonic eddy at the neutral buoyancy level and cyclonic circulation in the ambient fluid around the rising plume. These features agree qualitatively with Speer's (1989) model. However, the experiments show that this state is unstable, leading to the breakup of the plume and the unsteady generation of baroclinic vortices (hetons) that propagate away from the source. Two nearby plumes interact repulsively for separations ≳NZs/ƒ, and initially coalesce for smaller separations. Here N is the buoyancy frequency, ƒ is the Coriolis frequency, and Zs is the height of the spreading level. It is suggested that oceanic hydrothermal plumes may shed isolated baroclinic vortices, which are capable of retaining anomalous properties (temperature, salinity, 3He, etc.) over large distances.
Internal waves and mixing in the upper equatorial Pacific Ocean
Authors: Wijesekera Hemantha W. , Dillon Thomas M.
Published: February 2008

Microstructure measurements in the equatorial Pacific at 140°W in late 1984 show a pronounced diurnal variation in both high‐frequency internal wave energy and kinetic energy dissipation rate. Observations indicated that after sunset, internal waves (presumably generated by convective overturns in the mixed layer) propagate downward and increase turbulence levels in the pycnocline. It is proposed that large mixed layer eddies in the South Equatorial Current interact with the large shear caused by the Equatorial Undercurrent to generate a westward going anisotropic wave field. The momentum transport in the radiated wave field results in a drag force on the equatorial mean flow field. The observed mean wind stress at 140°W during Tropic Heat I (which is twice as large as the annual mean wind) is closer to the estimated radiation stress (∼−10−4 m2 s−2) at the base of the mixed layer (≈30 m) than to the estimated turbulent stress (∼−10−5 m2 s−2). A wave dissipation model based on the observed turbulent kinetic energy dissipation rate is introduced in order to estimate the wave momentum flux divergence in the stratified region above the undercurrent core. The model predicts that most of the downward wave momentum flux penetrates through the undercurrent core. It is hypothesized that when the wind stress is strong, the equatorial Pacific Ocean responds by generating a westward traveling internal wave field which transports much of the surface wind stress below the actively mixing surface layer.
Gravity waves in the middle atmosphere observed by Rayleigh lidar: 1. Case studies
Authors: Wilson R. , Chanin M. L. , Hauchecorne A.
Published: February 2008

Density and temperature mesoscale fluctuations as observed in the stratosphere and mesosphere by means of two Rayleigh lidars with high resolution in time (15 min) and space (300 m), have been analyzed in some particular cases corresponding to different seasonal conditions. These case studies are characteristic of recurrently observed patterns and thus provide a description of the mesoscale fluctuation field in the middle atmosphere. The spatial, temporal, and spectral characteristics of the fluctuations are described and discussed in the framework of the gravity wave interpretation. Dominant wave modes with large period and large vertical wavelength (inertia‐gravity waves) are frequently observed in the stratosphere and lower mesosphere. These low‐frequency modes are not generally observed above 50‐ to 55‐km altitude, suggesting a strong damping of such waves in the mesosphere. The vertical growth of potential energy density indicates that the wave motions are generally not conservative in the middle atmosphere. The gravity waves amplitude appears too small to produce convective instabilities in the stratosphere. On the contrary, the amplitude of the fluctuations is close to the convective saturation limit deduced from the linear theory for wavelengths up to 3–5 km in the lower mesosphere, and up to 6–8 km above 60 km altitude. Furthermore, convectively instable layers, which can persist for periods longer than 1 hour, have been frequently observed in the mesosphere.
Deuterium excess in recent Antarctic snow
Authors: Petit J. R. , White J. W. C. , Young N. W. , Jouzel J. , Korotkevich Y. S.
Published: February 2008

Deuterium excess (d = δD ‐ 8 * δ18O) values in surface snow are presented for central and east Antarctica. The samples are primarily from Soviet, French, and Australian traverses. The d values exhibit a large change going from coastal sites to high‐altitude sites on the ice sheet. The d values are relatively constant at 3 to 6‰ from the coast to an altitude of 2500 m, and at higher elevations d increases steadily to values of 16 to 18‰ at Vostok and Plateau Station. The data is modeled as d versus δD using the kinetic Rayleigh model for isotopes in precipitation developed by Jouzel and Merlivat. The model accounts for kinetic fractionation during evaporation into undersaturated air over the ocean and during snow formation in <−10°C clouds where vapor is supersaturated with respect to snow. The overall pattern of d versus δD can be fit well with a supersaturation function which increases linearly with decreasing temperature and which predicts reasonable values of the supersaturation. Vapor originating from 20° to 60°S was tested with different supersaturation functions. The data could only be fit with moisture originating from 30° to 40°S, indicating that these latitudes are the main source of vapor for snow falling in Antarctica. The conclusion of a mid‐latitude vapor source for polar snow agrees with the analysis of d and δ18O seasonal cycles in Greenland snow performed by Johnsen and coworkers. The model was also tested with moisture simultaneously originating from all latitudes from 30°S to the Antarctic coast. The addition of up to 20% of moisture evaporated from latitudes south of 50°, and 5% from latitudes south of 60°, is compatible with low d values occasionally observed in snow near the coast. The conclusion of a “local moisture” effect for coastal and near coastal (<2000 m elevation) snowfall supports a similar conclusion by Saigne and Legrand from their analysis of methanesulphonic acid in Antarctic snow. Finally, the effects of changes in the sea surface temperature and changes in oceanic humidity on the d values observed in Antartic snow are greatly modified during the precipitation process. Hence the interpretation of d values in ice cores should be done in the context of a precipitation model.
Photochemical ozone production in tropical squall line convection during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A
Authors: Pickering Kenneth E. , Thompson Anne M. , Scala John R. , Tao Wei‐Kuo , Simpson Joanne , Garstang Michael
Published: February 2008

We have examined the role of convection in trace gas transport and ozone production in a tropical dry season squall line sampled on August 3, 1985, during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A (NASA GTE/ABLE 2A)in Amazonia, Brazil. Two types of analyses were performed. (1) Transient effects within the cloud are examined with a combination of two‐dimensional cloud and one‐dimensional photochemical modeling. Tracer analyses using the cloud model wind fields yield a series of cross sections of NOx, CO, and O3 distribution during the lifetime of the cloud; these fields are used in the photochemical model to compute the net rate of O3 production. At noon, when the cloud was mature, the instantaneous ozone production potential in the cloud is between 50 and 60% less than in no‐cloud conditions due to reduced photolysis and cloud scavenging of radicals. (2) Analysis of cloud inflows and outflows is used to differentiate between air that is undisturbed and air that has been modified by the storm. These profiles are used in the photochemical model to examine the aftereffects of convective redistribution in the 24‐hour period following the storm. Total tropospheric column O3 production changed little due to convection because so little NOx was available in the lower troposphere. However, the integrated O3 production potential in the 5‐ to 13‐km layer changed from net destruction to net production as a result of the convection. The conditions of the August 3, 1985, event may be typical of the early part of the dry season in Amazonia, when only minimal amounts of pollution from biomass burning have been transported into the region.
LIMS (Limb Infrared Monitor of the Stratosphere) observation of traveling planetary waves and potential vorticity advection in the stratosphere and mesosphere
Authors: Dunkerton Timothy J.
Published: February 2008

Eastward and westward traveling waves were observed by the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) during the northern winter 1978–1979. Eastward waves were prevalent in early winter and were involved in a minor Canadian warming in December 1978. A large westward traveling wave, as described by previous authors, was observed in January 1979 during a series of minor warmings. By comparing these two events, it is shown that in both cases the superposition of traveling and quasi‐stationary waves led to constructive interference that was responsible for the warmings. However, there was significant asymmetry between eastward and westward traveling components. Eastward disturbances were confined mostly within the polar vortex, whereas quasi‐stationary and westward traveling components propagated to the vortex periphery and beyond, into the tropics and mid‐latitude mesosphere. This behavior is consistent with Rossby wave propagation and indicates that the location and magnitude of planetary wave breaking is sensitive to the frequency spectrum entering the middle atmosphere. However, this asymmetry is also a signature of the nonlinear critical layer as it projects onto the frequency spectrum. Both interpretations are shown to be valid during wave events observed by LIMS. A local Eulerian analysis of potential vorticity (PV) transport indicates that adiabatic, geostrophic advection by the resolvable scales of motion explains qualitatively (but not quantitatively) the observed potential vorticity tendencies in the LIMS northern hemisphere winter. In particular, calculated advection explains the eastward rotation of the main vortex, intrusion of low PV air into the polar cap, and formation of high PV filaments at the vortex periphery.
On the explicit versus family solution of the fully diurnal photochemical equations of the stratosphere
Authors: Austin John
Published: February 2008

A hierarchy of models is developed to test the accuracy of the family solution of the photochemical equations of the stratosphere. Results of the explicit solution of the equations for a simplified mid‐winter disturbance near the 850 K isentropic surface are compared with the solutions for models containing 26, 20, 16, 14, 12, 10, and 8 families for a 12‐day integration. In general, it is found that the family approach can yield quite accurate solutions, providing enough families are retained. However, large errors, of order 20%, are produced in some radical concentrations such as OH for models with a small number of families. The minimum number of families consistent with economy in the solution of the equations while maintaining scientific accuracy is discussed. In particular, it is recommended that exact dynamical tracers be incorporated into family models to reduce computer memory requirements by parameterizing long time scale species such as H2O, CH4 and H2 in terms of the tracer. Other species such as N2O, total chlorine, and total nitrogen may also be so parameterized according to how the model is to be used. Even for models with an extensive set of families, the concentrations of radical species are significantly in error at dawn, suggesting the need for caution in comparisons with measurements made using occultation techniques. This may not lead to significant errors in the calculation of reservoir species such as O3 on time scales up to a few weeks but can seriously perturb the qualitative features of other important molecules such as ClONO2 and ClO, and this could lead to significant errors in the calculation of O3 in the longer term. The results have important implications for the development of three‐dimensional models of stratospheric chemistry. Some examples of the chemical schemes of those models which have been described in the literature are studied to determine their strengths and weaknesses. The schemes are shown to produce good results for species such as O3 and HNO3 but show signs of weaknesses, particularly in the radical species. Some changes to these schemes are suggested, with a view toward producing an improved three‐dimensional model in the future.
Interannual variations of ozone: Interpretation of 4 years of satellite observations of total ozone
Authors: Schneider Hans R. , Ko Malcolm K. W. , Peterson Charles A. , Nash Eric
Published: February 2008

Observations show that the monthly zonal mean total ozone between 1978 and 1982 varies by as much as 5% with respect to the 4‐year average. These variations have been analyzed with the aid of a two‐dimensional chemistry transport model. Observed temperatures and meridional stream functions, calculated from data, were used as inputs for the model. Deviations of the model‐calculated total ozone from the 4‐year mean were compared with the observed variations. The data sets used consisted of 4 years (November 1978 to October 1982) of solar backscattered ultraviolet (SBUV) and total ozone measurements and National Meteorological Center (NMC) temperature data. The diabatic circulation was obtained from radiative heating rates, which were calculated for each of the 48 months of the observations. It is shown that a substantial part of the observed variability of total ozone is reproduced by the model, despite uncertainties in the heating rates and without any special attempts to obtain consistent eddy diffusion or otherwise to tune the model. Our results indicate that the temperature dependence of the ozone gas phase chemistry plays a negligible role in explaining the observed interannual variations in total ozone. A large part of the variability can be explained by the year to year differences in the transport circulation. The variability of the diabatic circulation is shown to be largely independent of the ozone fluctuations and to be a consequence of the observed temperature variations.
A numerical study of thunderstorm electrification: Model development and case study
Authors: Norville K. , Baker M. , Latham J.
Published: February 2008

We have developed a numerical model for examining the thunderstorm electrification process in which we assume the electrification is entirely due to noninductive charge transfer between colliding ice crystals and hail. Since this ice‐hail charge mechanism is very dependent on particle sizes and distributions, we use an explicit microphysical framework. To maintain simplicity, the electrification model is kinematic; thus the temperature and velocity fields are input into the electrification model. These fields can be either calculated by a background model or retrieved from observations. For this study, we have used the cloud model of Taylor (1989) to generate the temperature and velocity fields to examine the July 19, 1981, CCOPE thundercloud. Using these fields, the electrification model produced time‐dependent ice particle concentrations, radar reflectivities, charge and vertical electric field distributions in good general agreement with those observed. The model produced a maximum electric field strength of 1.27 kV cm−1, which is on the order of that needed for lightning initiation, and this maximum occurred very close to the time of the observed discharge (as inferred by the sailplane measurements). Thus the ice‐hail charge mechanism appears to have played an important role in the electrical development of the July 19 cloud. The details of the electrification depended on the liquid water content and the glaciation processes, and particularly on the ice crystal characteristics. Rapid growth of the crystals to riming sizes (> 400 μ) yielded the most efficient charging. The electrification was also sensitive to the ice‐ice sticking efficiency but not to the characteristics of the large riming ice.
Anoxic basins of the eastern Mediterranean: An overview
Authors: Cita M. B.
Published: February 2008

It is well known that the Mediterranean Sea underwent several episodes of basin‐wide stagnation in the Plio/Pleistocene, but extant anoxic basins were first discovered between 1983 and 1984 in the Strabo Trench and in the southwestern slopes of the Mediterranean Ridge, facing the Sirte Abyssal Plain. Since then, several expeditions were carried out to investigate the geology and physiography of the basins as well as the physical and chemical characters of the water column. Although the geodynamic settings of the Tyro and Bannock basins are quite different, they are very deep, closed and filled by dense, salty, cold brines, whose presence is attributed to submarine dissolution of Messinian evaporites. Bannock Basin is interpreted as a collapse basin, with a complex configuration, central domes and several satellite basins most of which are anoxic. Unlike the Tyro Basin, it is characterized by precipitation of gypsum, occurring as euhedral crystals within the sediments, and dredged along the steep wall bordering the main basin to the East. The interface separating normal seawater from the brines corresponds to a well‐defined pycnocline with a density contrast of approximately 20%, where strong bacterial activity seems to occur and a sudden decrease in transmittance is recorded. Studies on the water column document drastic changes of all the various parameters measured (O2, H2S, Ca, PO4, Ba) across the interface. Sediments deposited beneath the brines are anoxic. This kind of geologically induced anoxia is persistent and diachronous, in contrast with the isochronous, episodic, repetitive and climatically induced anoxia recorded in the same area of the eastern Mediterranean.
Fluctuations in deep western North Atlantic circulation on the Blake Outer Ridge during the last deglaciation
Authors: Haskell Brian J. , Johnson Thomas C. , Showers William J.
Published: February 2008

Variations in the mean grain size of the detrital silt fraction from four piston cores along a depth transect on the Blake Outer Ridge were used to examine paleocirculation patterns and intensity between 2650 and 3818 m for the late Pleistocene and Holocene. Prior to 12 kyr, circulation was focused on the upper part of the ridge above 2700 m and may have corresponded to enhanced levels of intermediate water production in the North Atlantic. Deepwater circulation during deglaciation underwent a two‐step decrease to minimum speeds from 15.5 to 13.5 kyr, followed by a three‐step increase to maximum speeds at 10 kyr. Minima in circulation at 13.5 and 12 kyr can be directly associated with oxygen isotopic evidence for advection of reduced salinity surface water to the North Atlantic. Additional circulation minima at 15 and 10.5 kyr may also be the result of reduced salinity from high‐latitude meltwater discharge. Evidence for a drop to moderate circulation intensity during the Holocene is supported by isolated geochemical records but could also be explained by decoupling of ocean ventilation rates from bottom circulation or a change in sediment provenance at the end of deglaciation.
Evidence for Red Sea surface circulation from oxygen isotopes of modern surface waters and planktonic foraminiferal tests
Authors: Ganssen Gerald , Kroon Dick
Published: February 2008

Hydrographic data and oxygen isotopic analyses performed on surface waters and planktonic foraminiferal tests, collected during early summer from two succeeding years (1984, 1985) throughout the Red Sea, reveal two different hydrographic regimes. In 1984 the summer “normal” situation prevailed where surface waters from the Red Sea flowed out into the Gulf of Aden, while in 1985 a reversed inflow current occurred. The higher temperatures and salinities observed in 1985 indicate high evaporation rates and increased aridity in the northern Red Sea and caused this inflow of Indian Ocean Surface Water which origins from the active upwelling region in the Arabian Sea. Lower salinities and lower oxygen isotopes were observed up to 18°N. The occurrence of Globorotalia menardii during 1985 with its surprisingly constant isotope values up to the Gulf of Suez indicates northward flowing surface currents for the entire Red Sea. Isotope values from Neogloboquadrina dutertrei (1985) indicate subsurface shell formation to about 50 m water depth. Oxygen isotope analysis on Globigerinoides ruber and Globigerinoides trilobus from the 1984 and 1985 tracks suggests that both species calcify in isotopic equilibrium with the surrounding water in the Gulf of Aden and in the northern Red Sea, while an offset from equilibrium values of up to −0.4‰ is found in the Red Sea. Occurrences of G. menardii in Red Sea sediments may be useful as a tool for detecting unusual hydrographic situations preserved in the sediment record, when subsurface water was brought to the surface by upwelling in the Arabian Sea and flowed into the Red Sea. As this process is triggered by high evaporation rates in the northern Red Sea region, the appearance of this species in Red Sea sediments may also indicate periods with extreme arid conditions.
Upwelling signals at the northeastern Walvis Ridge during the past 500,000 years
Authors: Oberhänsli Hedi
Published: February 2008

Qualitative and quantitative analyses of planktonic foraminiferal assemblages from Deep Sea Drilling Project site 532 shed light on hydrographic changes over the Walvis Ridge during the past 500,000 years. From changes in distribution of foraminiferal assemblages, two major hydrographic regimes (coastal and geostrophic branches of the Benguela Current and the Angola Current) can be distinguished at site 532. It is suggested that the hydrographic situation on the northeastern Walvis Ridge was characterized by intensified upwelling and a westward expansion of the coastal upwelling cells during several global cooling pulses. During glacial stages 2–4, the middle part of stage 6, sporadically from the lower stage 8 through upper stage 10, and during stage 12, site 532 was located beneath the coastal branch of the Benguela Current because faunal distribution patterns indicate intensified upwelling. The Angola Current probably intruded the area of study during the lower stages 5, sporadically 6–8, and 11, as documented by the increased abundance of Neogloboquadrina dutertrei.