bluuebunny/crossref_metadata_2025 huggingface dataset:
Is the Martian crust also the Martian elastic lithosphere?
Authors: Turcotte Donald L. , Shcherbakov Robert , Malamud Bruce D. , Kucinskas Algis B.
Published: December 2002

Correlations between topography, gravity, and areoid on Mars are used to constrain the crustal and lithospheric thicknesses on the planet. Assuming that the Hellas basin is in isostatic equilibrium with Airy compensation, point correlations between areoid anomalies and topography are used to obtain the mean crustal density and the crustal thickness. We find that the crustal thickness at the reference zero elevation is 90 ± 10 km. We also find that the mean crustal density is 2960 ± 50 kg m−3. We have also used several approaches to constrain the thickness of the elastic lithosphere. Using the spherical harmonic coefficients of the gravity potential and topography as a function of degrees, a relatively weak constraint on the globally averaged thickness of the elastic lithosphere is obtained. An improved constraint is obtained using wavelet transform analyses of great circle tracks of gravity and topography. The gravity‐topography admittance as a function of wavelet wavelength constrains the globally averaged thickness of the elastic lithosphere to be in the range 90 ± 10 km. The observation that the mean thicknesses of the crust and elastic lithosphere are likely to be equal suggests that a rheologically tougher crust is the elastic lithosphere.
Production of ethane and water in comet C/1996 B2 Hyakutake
Authors: Dello Russo Neil Dello , Mumma Michael J. , DiSanti Michael A. , Magee‐Sauer Karen
Published: December 2002

Ethane (C2H6) and water (H2O) were detected in Comet C/1996 B2 Hyakutake between UT 1996 March 23.4 (Rh = 1.08 AU preperihelion) and 1996 April 12.2 (Rh = 0.64 AU preperihelion). Our long‐slit infrared spectra featured both high spectral dispersion and high spatial resolution about the nucleus, permitting the extraction of rotational temperatures, production rates, and spatial distributions of species along the slit. Production rates were measured for water (on four dates) and ethane (on three dates). Their average relative abundance was C2H6/H2O = (6.2 ± 0.7) × 10−3. The spatial distributions of C2H6 and H2O molecules in the coma were consistent with both species being released directly from the nucleus on all dates, although asymmetries about the nucleus are seen for both gas and dust.
Biogeochemical changes within the Benguela Current upwelling system during the Matuyama Diatom Maximum: Nitrogen isotope evidence from Ocean Drilling Program Sites 1082 and 1084
Authors: Robinson Rebecca S. , Meyers Philip A.
Published: December 2002

The Matuyama Diatom Maximum (MDM) is a time of peak opal accumulation from 2.6 to ∼2.0 Ma within the Benguela Current upwelling system that was initiated by increased influence of Southern Ocean water on the eastern South Atlantic. We measured opal, total organic carbon (TOC), and CaCO3 fluxes and C and N stable isotopes in sediments deposited from 2.4 to 1.95 Ma at Sites 1082 and 1084 to explore the biogeochemical dynamics within the Benguela region. The infusion of Southern Ocean water delivered dissolved nutrients and Southern Ocean flora and fauna, resulting in local opal accumulation increasing up to 8 g/cm2ky and the production of diatom mats. Some δ15N measurements of diatom‐bound organic matter indicate that the mats grew within the Benguela region. The bulk sediment δ15N records are taken to reflect changes in the δ15N of nitrate in the incoming water, where lower values at 2.4 Ma reflect less nitrate utilization in the Antarctic. A long‐term increase in relative nitrate uptake in the Southern Ocean is evidenced by the gradual increase in δ15N toward 1.9 Ma.
Stochastic forcing of Pleistocene ice sheets: Implications for the origin of millennial‐scale climate oscillations
Authors: Hyde William T. , Crowley Thomas J.
Published: December 2002

Considerable evidence now exists for high‐frequency climate changes in the last ice age cycle (Dansgaard‐Oeschger and Bond cycles). Though large‐scale ice sheet flow has too long a time constant to play a significant role in these events, stochastically forced changes in the mass balance of the Pleistocene ice sheets may have resulted in significant variation in freshwater flux to the North Atlantic. In this paper we mimic the effects of “natural climate variability” with small (≤0.5W/m2) stochastic changes in insolation and examine the response in terms of ice volume change in an ensemble of coupled energy balance/ice sheet model runs. The total change in ice mass is comparable to a third of the volume of the current Greenland ice sheet, with meltwater discharge potentially sufficient to trigger Dansgaard‐Oeschger type events. In general, the Eurasian ice sheet is more sensitive to perturbations than the North American ice sheet. The distribution and occurrence of spectral peaks in the 1–9 kyr band is also comparable to the record of millennial oscillations over the last 500,000 years in a subpolar North Atlantic core. Our results suggest that considerable caution is necessary in interpreting the physical significance of sub‐Milankovitch spectral peaks in paleo time series.
Theoretical analysis on the penetration of power line harmonic radiation into the ionosphere
Authors: Ando Yoshiaki , Hayakawa Masashi , Molchanov Oleg A.
Published: December 2002

Radiation at the fundamental and higher harmonic frequencies from power lines affects the environment of Earth's ionosphere and magnetosphere, because such radiation may lead to the enhanced electron precipitation into the ionosphere due to wave‐particle interactions in the magnetosphere. In this paper we analyze theoretically the penetration of power line radiation, taking into account the presence of the anisotropic, homogeneous ionosphere. The electromagnetic field can be formulated by using the Fourier transform and field distribution obtained by proper deformation of the integral path with consideration of singularities. We calculate the field distribution as a function of altitude and distance from the source. From the numerical results, we can find several new findings: (1) the effect of the radiation from power line is limited in a relatively narrow region below the cutoff frequency of the guided mode in the Earth‐ionosphere waveguide, while the region affected by the power line radiation becomes much wider if the guided modes exist, (2) the radiated fields are directed almost horizontally, and (3) the altitude distribution of electromagnetic fields changes at the ion gyrofrequency. Comparison with the observed data shows a good agreement with respect to the region affected by power line radiation.
Transmitter‐receiver‐transmitter‐configured ground‐penetrating radars over randomly heterogeneous ground models
Authors: Gürel Levent , Oğuz Uğur
Published: December 2002

Ground‐penetrating radar (GPR) problems are simulated using the finite‐difference time‐domain (FDTD) method. The GPR model is configured with arbitrarily polarized three antennas, two of which are transmitting antennas fed 180° out of phase. The receiver is placed in the middle of two transmitters, where it receives no direct coupling from the transmitting antennas. The ground is modeled as a dielectric, lossy, and heterogeneous medium. The performances of the transmitter‐receiver‐transmitter‐configured GPRs above the heterogeneous ground models are investigated. The computational domain is terminated by perfectly matched layer (PML) absorbing boundaries. The PML is adapted to match both air and ground regions of the computation space.
Development of volcanic passive margins: Three‐dimensional laboratory models
Authors: Callot Jean‐Paul , Geoffroy Laurent , Brun Jean‐Pierre
Published: December 2002

Continental breakup above an anomalously hot mantle may lead to the development of volcanic margins. Volcanic margins are characterized by (1) thick seaward dipping lava flow sequences, (2) central intrusive complexes associated with dyke swarms parallel to the coast, and (3) high seismic velocity bodies in the lower crust attributable to magma underplating. A conceptual model for volcanic margins development has recently been proposed based on onshore studies of the Greenland margins and the British Tertiary Igneous Province. It is proposed that the long‐lived central intrusions are genetically linked to underlying persistent zones of mantle fusion. These localized melting domains (or soft spots), equivalent to small mantle diapirs, may locally soften the extending continental lithosphere. The low‐viscosity diapirs would (1) localize tectonic strain and (2) feed the volcanic margin with magma. Thus such soft spots can control the along‐strike magmatic and tectonic segmentation of volcanic margins. Recent geophysical investigations appear to show that the along‐strike structure of volcanic passive margins is compatible with such a segmentation process. Here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub‐Moho mantle may have a mechanical effect on continental breakup. Four‐layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle. The soft spots are simulated by low‐viscosity silicone putty emplaced within the brittle material. At the scale of the entire breakup zone, the soft spots display an oceanic‐type strength profile defining low‐strength zones where continental breakup is initiated. The rift orientation and segmentation are strongly controlled by the distribution of the low‐viscosity heterogeneities, rather than by the direction of regional extension. The experiments are compared with the geometry and segmentation of the onshore part of the Greenland volcanic margins.
Bed load at low Shields stress on arbitrarily sloping beds: Failure of the Bagnold hypothesis
Authors: Seminara Giovanni , Solari Luca , Parker Gary
Published: December 2002

The Bagnold hypothesis has been a major tool in the development of mechanistic models of bed load transport. According to the hypothesis, a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical, or threshold value associated with incipient motion of grains. The constraint determines a functional form for the areal concentration of bed load grains as a function of flow parameters. It is shown here, however, that the Bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose. In particular, under such conditions it is shown that no areal concentration, no matter how large, is sufficient to reduce the fluid shear stress at the bed to the critical value. This failure motivates the abandonment of the Bagnold constraint, even for nearly horizontal beds. The framework presented here, however, provides a natural basis for an entrainment‐based model of sediment transport that neither satisfies nor suffers from the drawbacks of the Bagnold constraint.
Kinematic dispersion in stream networks 2. Scale issues and self‐similar network organization
Authors: Saco Patricia M. , Kumar Praveen
Published: December 2002

When the flow parameters, such as celerity and hydrodynamic dispersion coefficient, are allowed to vary spatially within a basin, three mechanisms, namely, geomorphologic, kinematic, and hydrodynamic dispersion, contribute to the variance of the instantaneous response function. The relative contributions of the three dispersion mechanisms as a function of scale, or Strahler order of the basin, are studied. This analysis is performed for two study basins, the Vermilion and the Mackinaw river basins, in central Illinois. Log linear trends for all the dispersion coefficients as a function of scale are observed. These trends can be cast in the form of Horton law type of relations. The asymptotic behavior of the dispersion coefficients of basins with self‐similar network structure is consistent with the observations.
Kinematic dispersion in stream networks 1. Coupling hydraulic and network geometry
Authors: Saco Patricia M. , Kumar Praveen
Published: December 2002

We investigate the coupling of river network structure and hydraulic geometry thereby relaxing the assumption of spatially invariant celerities and hydrodynamic dispersion coefficients. The presence of spatially varying celerities induces a dispersion effect, referred to as kinematic dispersion, on the network travel time distribution. Its contribution to the total dispersion is comparable to that due to the heterogeneity of path lengths, that is, geomorphologic dispersion, and significantly larger than the hydrodynamic dispersion. If this contribution is ignored, the hydrograph shows a higher peak flow, a shorter time to peak, and shorter duration.
Infiltration in stratified, heterogeneous soils: Relative importance of parameters and model variations
Authors: Avanidou Theodora , Paleologos Evan K.
Published: December 2002

This study presents a framework that accounts for the uncertainty, relative importance, and relative contribution of uncertain and spatially variable parameters and the influence of statistical model assumptions in problems of infiltration in unsaturated, heterogeneous formations. The relative importance of the saturated hydraulic conductivity Ks, the van Genuchten α, and β parameters was quantified toward the mean pressure head profile and the fluctuations about it. The broad implications of our study for site characterizations efforts are the following. Statistical moments were seen not to be sufficient for producing reliable modeling estimates. Significant information on the detailed shape of the probability distribution is required in order to produce meaningful predictions. When limited data are available, optimum allocation of resources is accomplished by concentrating on Ks. When more extensive sampling can be done, both Ks and β appear to be critical. Total system performance analyses may not provide the most appropriate modeling estimates. The results of the relative importance of the parameters for a specific layer did not coincide with the conclusions drawn from an analysis of the system as a whole.
Conditional moments of the breakthrough curves of kinetically sorbing solute in heterogeneous porous media using multirate mass transfer models for sorption and desorption
Authors: Lawrence Alison E. , Sanchez‐Vila Xavier , Rubin Yoram
Published: December 2002

A methodology is presented for evaluating the temporal moments of solutes undergoing linear rate‐limited mass transfer processes based on a Lagrangian approach to solute transport in heterogeneous media. The temporal moments of sorbing solutes are written as a function of those of conservative tracers. The general continuous diffusion rate model that has recently appeared in the hydrologic literature is used to model the rate‐limited mass transfer processes. The methodology is also applied to desorption from an initially uniformly contaminated aquifer, and the concentration expected value and variance are found quasi‐analytically. The conditional temporal moments of sorbing solutes can be written as a function of the conditional moments of conservative tracers. Conditioning results in a reduction of the variance of travel time. The amount of reduction depends on the chemical model selected.
Sediment transport‐storage relations for degrading, gravel bed channels
Authors: Lisle Thomas E. , Church Michael
Published: December 2002

In a drainage network, sediment is transferred through a series of channel/valley segments (natural sediment storage reservoirs) that are distinguished from their neighbors by their particular capacity to store and transport sediment. We propose that the sediment transport capacity of each reservoir is a unique positive function of storage volume, which influences sediment mobility and availability through variations in bed surface texture, channel gradient, and availability of valley floor sediments for erosion. Examinations of the form of transport‐storage relations of degrading alluvial reservoirs using published field studies, flume experiments, and simulations support a conceptual model that includes two phases. In phase I, filled channels respond to variations in supply primarily by changes in stored sediment volume, with little change in transport rate. In phase II, channel mobility is responsive to supply through armoring and form roughness. Although these phases could represent idealized transport‐limited (phase 1) or supply‐limited (phase II) states, we propose that every alluvial reservoir responds to changes in sediment inputs by changing both storage and transport rate, the propensity for either depending on reservoir characteristics and the sediment exchange processes in the channel. Transport‐storage relations for phase II are approximately linear, but examination of numerical simulations and flume experiments indicates that armoring imparts positive curvature. Simulations of degradation of an alluvial reservoir with channel and valley floor surfaces indicate that interactions between channel lowering and lateral erosion are critical in the manifestation of a transport‐storage relation. Better knowledge of transport–storage relations could lead to improved sediment‐routing models for drainage basins wherein component sediment reservoirs dynamically adjust to varying sediment loads.
Regional estimation of rainfall intensity‐duration‐frequency curves using generalized least squares regression of partial duration series statistics
Authors: Madsen Henrik , Mikkelsen Peter Steen , Rosbjerg Dan , Harremoës Poul
Published: December 2002

A general framework for regional analysis and modeling of extreme rainfall characteristics is presented. The model is based on the partial duration series (PDS) method that includes in the analysis all events above a threshold level. In the PDS model the average annual number of exceedances, the mean value of the exceedance magnitudes, and the coefficient of L variation (LCV) are considered as regional variables. A generalized least squares (GLS) regression model that explicitly accounts for intersite correlation and sampling uncertainties is applied for evaluating the regional heterogenity of the PDS parameters. For the parameters that show a significant regional variability the GLS model is subsequently adopted for describing the variability from physiographic and climatic characteristics. For determination of a proper regional parent distribution L moment analysis is applied for discriminating between the exponential distribution and various two‐parameter distributions in the PDS model. The resulting model can be used for estimation of rainfall intensity‐duration‐frequency curves at an arbitrary location in a region. For illustration, the regional model is applied to rainfall data from a rain gauge network in Denmark.
Numerical evaluation of solute dispersion and dilution in unsaturated heterogeneous media
Authors: Cirpka Olaf A. , Kitanidis Peter K.
Published: December 2002

We investigate flow and solute transport in a heterogeneous unsaturated soil under gravity‐driven flow conditions using a direct simulation Monte Carlo method. We account for the cross correlation between the heterogeneous fields of unsaturated flow parameters representing a silty loam. We determine through particle tracking the statistical moments of the travel time distribution for nonpoint injection of a conservative tracer. The one‐particle mean and variance characterize the mean arrival time and spreading of solute integrated over an entire horizontal cross section of the domain, whereas the two‐particle semivariogram characterizes the spreading observed at individual points. From these quantities we derive macrodispersivities and effective dispersivities. The latter reflect solute dilution. Owing to the nonlinearity of unsaturated flow the dispersivity values at a given depth vary with the mean infiltration rate. The two‐particle semivariogram hardly differs from the one‐particle variance of travel time at low infiltration rates, whereas dilution and macrodispersion rates differ significantly at high infiltration rates. We compare the dispersivities determined through particle tracking with approximations from linear stochastic theories based on the covariance of the Eulerian velocity field [Dagan, 1988; Fiori and Dagan, 2000]. These linear stochastic theories slightly underestimate both dispersivity types. We present parametric models for the two types of dispersivities as function of the infiltration rate and the depth.
A mechanistic model of H218O and C18OO fluxes between ecosystems and the atmosphere: Model description and sensitivity analyses
Authors: Riley W. J. , Still C. J. , Torn M. S. , Berry J. A.
Published: December 2002

The concentration of 18O in atmospheric CO2 and H2O is a potentially powerful tracer of ecosystem carbon and water fluxes. In this paper we describe the development of an isotope model (ISOLSM) that simulates the 18O content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic 18OC16O (hereinafter C18OO) fluxes; CO2 oxygen isotope exchanges with soil and leaf water; soil CO2 and C18OO diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of H218O and C18OO with the atmosphere. The isotope model is integrated into the land surface model LSM, but coupling with other models should be straightforward. We describe ISOLSM and apply it to evaluate (1) simplified methods of predicting the C18OO soil‐surface flux; (2) the impacts on the C18OO soil‐surface flux of the soil‐gas diffusion coefficient formulation, soil CO2 source distribution, and rooting distribution; (3) the impacts on the C18OO fluxes of carbonic anhydrase (CA) activity in soil and leaves; and (4) the sensitivity of model predictions to the δ18O value of atmospheric water vapor and CO2. Previously published simplified models are unable to capture the seasonal and diurnal variations in the C18OO soil‐surface fluxes simulated by ISOLSM. Differences in the assumed soil CO2 production and rooting depth profiles, carbonic anhydrase activity in soil and leaves, and the δ18O value of atmospheric water vapor have substantial impacts on the ecosystem CO2 flux isotopic composition. We conclude that accurate prediction of C18OO ecosystem fluxes requires careful representation of H218O and C18OO exchanges and transport in soils and plants.
Influence of Black Sea and local biogenic activity on the seasonal variation of aerosol sulfur species in the eastern Mediterranean atmosphere
Authors: Kubilay Nilgün , Koçak Mustafa , Çokacar Tülay , Oguz Temel , Kouvarakis Giorgos , Mihalopoulos Nikos
Published: December 2002

Methanesulfonate (MSA) and non‐sea‐salt (nss) sulfate concentrations were measured in bulk aerosol samples collected during January 1996 to December 1999 at two stations along the Turkish and Cretan coastline of the eastern Mediterranean. The data set enabled the origin and variability of biogenically derived sulfate concentrations in the eastern Mediterranean atmosphere to be defined. Although similar seasonal patterns of MSA and nss‐sulfate concentrations for both stations were detected, the mean concentrations at Erdemli (Turkey) over the sampling period (42 ± 52 ng m−3 MSA, and 6.8 ± 5.2 μg m−3 nss sulfate) were found to be almost twice that of those measured at Finokalia, Crete (25.8 ± 14.9 ng m−3 MSA and 3.9 ± 1.7 μg m−3 nss sulfate). Analysis of the air mass back trajectory and the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) data suggests that the majority of the biogenic contribution at Erdemli is originated from summer coccolithophorid Emiliania huxleyi blooms developed in the Black Sea. The data further points to a significant relationship between Saharan dust transport events and local oceanic production of MSA through occasional fertilization of the eastern Mediterranean during wet deposition events. This process accounts for episodic, strong weekly changes in MSA concentrations during the spring months. Considering that Erdemli and Finokalia are approximately 1000 km apart from each other geographically, large differences in their MSA and nss‐sulfate concentrations indicates a considerable role of regional mesoscale atmospheric transport processes on the spatial structure of biogenically produced atmospheric sulfur aerosols.
Atmospheric deposition of inorganic and organic nitrogen and base cations in Hawaii
Authors: Carrillo Jacqueline H. , Hastings Meredith Galanter , Sigman Daniel M. , Huebert Barry J.
Published: December 2002

Atmospheric deposition of nitrogen (N) and base cations was measured for 5–7 years on the island of Hawaii and for 1.5 years on Kauai. On Hawaii, mean annual fluxes of K+, Mg2+, and Ca2+ were 15, 17, and 13 kg ha−1 yr−1, respectively. Fog interception was the largest deposition pathway. Sea salt contributed the majority of cations, although biomass burning and Asian dust were significant sources for some years. Total N deposition (inorganic and organic) averaged 17 kg N ha−1 yr−1. Fog interception was also the largest source of N, depositing 16 kg N ha−1 yr−1. Precipitation deposition was 1.0 and 0.2 kg N ha−1 yr−1, respectively on Hawaii and Kauai. Dry deposition on Hawaii was 0.1 kg N ha−1 yr−1. Organic N averaged 16 and 12% of total N in rain and fog, respectively. The δ15N values for NO3−‐N are consistent with long‐range transport of N from Asia in the spring/summer and from North America in the fall/winter as nonvolcanic sources. Atmospheric deposition on Hawaii may completely account for a previously identified soil N imbalance.
Origin of mafic magmas beneath northwestern Tibet: Constraints from 230Th‐238U disequilibria
Authors: Cooper Kari M. , Reid Mary R. , Dunbar N. W. , McIntosh W. C.
Published: December 2002

238U‐230Th disequilibria and Sr, Nd, and Pb isotopic and chemical data for young (<120 ka) trachyandesites from the Ashikule Basin (AKB) in northwestern Tibet provide constraints on the origin of magmas produced within this region of continental collision. Compared to lavas from both continental and oceanic settings, the AKB samples show large excesses of 230Th with respect to 238U (up to (230Th)/(238U) = 1.36). Partial melting of garnet‐bearing lithologies (garnet peridotite, garnet pyroxenite, or eclogite) could be responsible for these 230Th excesses and could plausibly occur either in the lithospheric mantle or in the lower crust. Small porosities (<0.4%) and slow melting rates (<10−4 kg m−3 yr−1) are required in the case of a garnet peridotite residue, although larger porosities (up to 18%) and melting rates (>10−3 kg m−3 yr−1) are permitted in the case of an eclogitic residue; hydrous metasomatic phases, if present, would lower these limits further. The source of the AKB magmas has probably been enriched in incompatible elements relative to bulk Earth since at least the middle Proterozoic (>1 Ga), likely via metasomatism by a relatively dry silicate melt, and, if the ultimate source of the lavas is mantle lithosphere, concentrations of incompatible trace elements in the source could be similar to those of xenoliths and massifs. The 230Th‐238U disequilibria provide additional information to evaluate the methods previously proposed to explain melt generation beneath northern Tibet. The measured 230Th enrichments are uncharacteristic of melts generated by subduction but could potentially be produced during shear heating of the uppermost lithospheric mantle, by convective removal of the lower lithosphere and heating of the remaining lithospheric mantle, or by decompression during extension across a releasing bend of a strike‐slip fault. The diversity of mechanisms that could be responsible for these relatively small‐degree melts suggests that the cause of melt production may have varied over time and/or over space in northwestern Tibet and therefore that timing of volcanism may not be directly related to any single tectonic event.
Seasonality of dissolved rare earth elements in the lower Mississippi River
Authors: Shiller Alan M.
Published: December 2002

Dissolved rare earth element (REE) concentrations were determined in a 27‐month time series of the lower Mississippi River. Overall, the results agree with limited previous investigations; that is, the river shows enrichment of heavy REEs relative to light REEs and also has a significant Ce anomaly. However, the previous investigations relied on only single samples from the river. This seasonal investigation reveals significant temporal variations in the river's REE chemistry. In particular, large (approximately fivefold) variations in light REE concentrations are observed. The light REEs follow a seasonality similar to particle‐reactive trace elements. Also, the Ce anomaly shows a corresponding seasonality with greatest fractionation when dissolved concentrations are lowest. The time series encompassed a period of extreme flooding in the U.S. Midwest. However, this event did not appear to affect dissolved REE concentrations. The results of this study are compatible with previous observations of dissolved trace elements in the lower Mississippi River which suggested the importance of redox processes within the river system in controlling seasonal concentration variability for many of these elements. Additionally, speciation modeling indicates that carbonate complexes likely dominate the solution chemistry of REEs in the lower Mississippi. The observed behavior of the dissolved REEs is likewise compatible with this speciation.
Si and O diffusion in olivine and implications for characterizing plastic flow in the mantle
Authors: Dohmen Ralf , Chakraborty Sumit , Becker Hans‐Werner
Published: December 2002

The relationship between diffusion of individual species and creep of silicates is unclear and has long been debated, factually anchored by the central observation that the activation energies for creep are higher than the activation energy for diffusion of any species. There have been numerous attempts to explain this difference. New advances in experimental technology enable us to demonstrate that this difference does not exist and was an artefact of the limited experimental resolution of earlier studies – the activation energies of creep and diffusion of Si in olivine are identical, 530 kJ/mol. This allows the creep mechanism in olivine to be understood as one of simple climb (consistent with microstructural observations) and opens possibilities of estimating, understanding and predicting the (much simplified) creep behavior of diverse silicates under a wide range of P‐T‐X conditions through the use of diffusion data–measured or computed.
Crustal structure and dynamics of the Tien Shan
Authors: Vinnik L. P. , Roecker S. , Kosarev G. L. , Oreshin S. I. , Koulakov I. Yu.
Published: December 2002

P receiver functions of many seismograph stations in the Tien Shan in central Asia show a Ps phase converted from the bottom of a fractured surficial layer with a reduction in S velocity of around 30%. In the northern Tien Shan the depth of the bottom of this layer increases sharply from a few kilometers beneath the foreland to 15 km beneath the Kyrgyz range. This low S velocity region extending to mid‐crustal depths is associated with a high level of ambient seismicity and is likely caused by fluids at high pore pressure that prevent closure of cracks. The low velocity layer is absent beneath some stations in the central Tien Shan, including a region of anomalously thin crust beneath the Naryn basin. The thin crust and reduced fracturing suggest a strong lithosphere that transfers compressive stress from the Aksay plateau in the south to the Kyrgyz and associated ranges in the north.
Precursory transient slip during the 2001 Mw = 8.4 Peru earthquake sequence from continuous GPS
Authors: Melbourne Timothy I. , Webb Frank H.
Published: December 2002

Two‐hour position estimates from a continuous GPS station located at Arequipa, Peru, document precursory deformation beginning 18 hours prior to an Mw = 7.6 aftershock of the June 23rd 2001 Mw = 8.4 earthquake. This preseismic signal appears on the north and east components as a slow displacement with an amplitude twice that of the subsequent coseismic. Analysis of three years of 18‐hour rate measurement shows this signal to be unprecedented and beyond four standard deviations from the mean rate. The best fitting centroid is directionally consistent with slow slip along the plate interface and suggests the preseismic deformation arises from creep near the aftershock rupture. This implies the Nazca‐South American plate interface slipped slowly prior to seismogenic faulting. These observations indicate the Mw = 7.6 earthquake grew out of slow slip along the plate interface and clearly demonstrate the breadth of slip rates accommodated by subduction zone plate interfaces.
Formation of volcanic rifted margins: Are temperature anomalies required?
Authors: Nielsen Thomas K. , Hopper John R.
Published: December 2002

A major controversy for understanding volcanic margin formation is whether a thermal anomaly in the mantle is required to generate the characteristic thick igneous crust. We use a fluid‐dynamical model of mantle flow that includes feedback from melting on the physical properties of the mantle to study the temporal evolution of volcanic margins during continental breakup. We test how weak the mantle must be to induce small‐scale convection and thus enhance melt production during breakup. We find that the mantle reference viscosity required to generate a breakup instability is sufficiently low that an unrealistic time dependence develops in the subsequent oceanic spreading system. The viscosity increase associated with dehydration melting can stabilize the system, but only at the expense of eliminating the breakup instability. An exhaustible reservoir of hot mantle resolves this inconsistency. Melt productivity increases through a deepening of the mantle solidus, while a concurrent increase in buoyancy and a decrease in viscosity in the melting region promote an initial transient phase of small‐scale convection.
Measurements of pernitric acid at the South Pole during ISCAT 2000
Authors: Slusher D. L. , Huey L. G. , Tanner D. J. , Chen G. , Davis D. D. , Buhr M. , Nowak J. B. , Eisele F. L. , Kosciuch E. , Mauldin R. L. , Lefer B. L. , Shetter R. E. , Dibb J. E.
Published: December 2002

The first measurements of pernitric acid at the South Pole were performed during the second Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT 2000). Observed HO2NO2 concentrations averaged 25 pptv. Simple steady‐state calculations constrained by measurements show that the lifetime of pernitric acid was largely controlled by dry deposition, with thermal decomposition becoming increasingly important at warmer temperatures. We determined that the pernitric acid equilibrium constant is less uncertain than indicated in the literature. One consequence of pernitric acid deposition to the snow surface is that it is an important sink for both NOx and HOx. Another is that the photochemistry of HO2NO2 in the Antarctic snowpack may be a NOx source in addition to nitrate photolysis. This might be one of the important differences in snow photochemistry between the South Pole and warmer polar sites.
The June 23, 2001 Peru earthquake and the southern Peru subduction zone
Authors: Giovanni Melissa K. , Beck Susan L. , Wagner Lara
Published: December 2002

The plate boundary between the South American and Nazca plate along the south‐central Peru coast has been the site of large destructive earthquakes for many centuries, including the June 23, 2001 (MW=8.4) event. This underthrusting event has a fault area of 320 km by 100 km based on relocated aftershocks during the first three weeks following the mainshock. Modeling of the teleseismic broadband P waves of the 2001 Peru earthquake indicates two pulses of moment release with the larger second pulse located 130 km southeast of the mainshock initiation, indicating a unilateral rupture to the southeast. Based on intensity and tsunami reports, previous earthquakes in 1868 and 1604 were larger than the 2001 earthquake, while an event in 1784 was smaller. This provides further evidence that the size of earthquakes along the Peru coast has changed between successive earthquake cycles.
Gravity wave interactions around the jet stream
Authors: Pavelin Edward , Whiteway James A.
Published: December 2002

VHF radar measurements have been applied to study the interaction between a mountain wave and an inertia‐gravity wave above the jet stream in the tropopause region. The mountain wave was observed to break and generate turbulence at a critical level that was induced by the inertia‐gravity wave. This indicates that wave‐wave interactions can be an important source of turbulence and, consequently, mixing in the tropopause region.
Prolonged post‐seismic deformation of the 1960 great Chile earthquake and implications for mantle rheology
Authors: Khazaradze G. , Wang K. , Klotz J. , Hu Y. , He J.
Published: December 2002

Contemporary crustal deformation of the southern Andean margin shows an interesting feature: While nearly all coastal GPS sites move landward, consistent with inter‐seismic deformation near a locked subduction fault, sites 300–400 km landward of the rupture region of the Mw9.5 1960 Chile earthquake are moving in the opposite direction. We attribute the seaward motion of these inland sites to a prolonged crustal deformation due to mantle stress relaxation following the 1960 great earthquake. In order to reproduce the observed seaward motion using a three‐dimensional finite element model we need to incorporate a mantle viscosity of about 3 × 1019 Pa s. The possibility that the seaward motion is caused by a silent slip event on the plate interface at large depths cannot be completely excluded, and our analysis provides a working model for future field tests.
Paleointensity study of Early Miocene lavas from Pingzhuang, Inner Mongolia, China
Authors: Shi Ruiping , Zhu Rixiang , Pan Yongxin , Shi Guanghai , Hoffman Kenneth A.
Published: December 2002

A paleomagnetic and geochronologic investigation has been conducted on 17 lava flows from Inner Mongolia, north China. Radiometric K‐Ar dating experiments give an age between 23 and 22 Ma for these volcanic units. Rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo‐single domain titanomagnetite. Stepwise thermal demagnetization isolated a well‐defined direction of characteristic remanent magnetization (ChRM) in all studied lava flows with a mean direction of D/I = 359.8°/62.2° (α95 = 3.8°). Fifty samples from 11 lavas were pre‐selected on the basis of thermomagnetic analyses to carry out Thellier‐Thellier paleointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks. Twenty‐three samples from 7 lavas yield reliable paleointensity estimates with a mean virtual dipole moment (VDM) of (3.9 ± 0.9) × 1022 Am2, about half the strength of the modern‐day field.
Broadband marine MT exploration of the East Pacific Rise at 9°50′N
Authors: Key Kerry , Constable Steven
Published: December 2002

We present the first use at a mid‐ocean ridge of a recently developed broadband marine magnetotelluric (MT) instrument. The extended high frequency performance of the instrument allows resolution of electrical resistivity structure at shallower depths than traditional marine MT sensors. Our two‐dimensional inversion model from data collected at four MT sites on the East Pacific Rise (EPR) at 9°50′N demonstrates the viability of the method to image electrical resistivity structure in both the crust and shallow mantle. While our pilot experiment falls far short of the coverage needed to provide rigorous constraints on structure, a low resistivity zone in the crustal portion of the inversion model agrees well with seismic tomography and seafloor compliance results. Resistivities beneath the ridge imply a crustal partial melt fraction of 1–20%. A total melt volume of about 0.75 km3 per kilometer of ridge implies an average melt residence time of about 1000 years.
Decreasing trends in sunshine duration over China for 1954–1998: Indication of increased haze pollution?
Authors: Kaiser Dale P. , Qian Yun
Published: December 2002

Analysis of meteorological data made available by the China Meteorological Administration (CMA) reveals significant decreases in sunshine duration over the latter half of the 20th century over much of China, especially in the eastern half of the country. Most of the decrease has occurred since about 1980, and appears to be related to large increases in atmospheric anthropogenic aerosol loading that have increased the aerosol extinction coefficient (AEC), as calculated from observations of (decreasing) visual range. A finding of particular interest is a significant decrease in summertime maximum temperature over this time period for a large portion of southeastern China near the Sichuan Basin (while the rest of China—and indeed most of the northern hemisphere—has warmed). This same region of southeastern China shows the largest decreases in sunshine duration and largest increases in AEC. The consistent spatial and temporal patterns of change in these variables seem to support the theory that aerosol loading has decreased the duration and intensity of sunshine in this region and thus lowered summertime maximum temperatures.
On ‘steady‐state’ heat flow and the rheology of oceanic mantle
Authors: Korenaga Jun , Jordan Thomas H.
Published: December 2002

A constraint on the rheology of oceanic mantle is derived on the basis of a recently established scaling law for the onset of convection with temperature‐dependent viscosity. Our approach is free from the assumption of statistically steady‐state convection, which has commonly been employed in previous studies. The estimated range of asthenospheric viscosity is 1–4 × 1019 Pa s if the activation energy is 300 kJ mol−1, and 4 × 1019– 1020 Pa s if the activation energy is 100 kJ mol−1. The former range is consistent with laboratory data as well as geodynamic inference based on the geoid. The latter is consistent with geodynamic inference based on seamount loading history. Different activation energies predict different temperature contrasts in convecting mantle. Seismic tomography has the potential to discriminate between these possibilities.
Whistler anisotropy instability: Wave‐particle scattering rate
Authors: Nishimura Kazumi , Gary S. Peter , Li Hui
Published: December 2002

An electron temperature anisotropy T⊥e > T∥e (where ⊥ and ∥ refer to directions relative to the background magnetic field) can excite the whistler anisotropy instability in a collisionless plasma. The resulting enhanced fluctuating magnetic fields scatter the electrons and reduce the anisotropy. We use particle‐in‐cell simulations in a spatially homogeneous plasma model to compute the maximum values of the wave‐particle scattering rate for electron anisotropy reduction. This rate scales as the electron cyclotron frequency but, for a fixed initial growth rate, is relatively independent of the electron temperature if kBTe ≤ 5 keV. For larger electron temperatures, the scattering rate decreases with increasing Te, suggesting that relativistic effects play a role.
Wind‐ACE solar wind correlations, 1999: An approach through spectral analysis
Authors: Matsui H. , Farrugia C. J. , Torbert R. B.
Published: December 2002

Solar wind correlations are investigated with a spectral analysis. We employed magnetic field and plasma data from Wind and ACE for a whole year (1999), when the inter‐spacecraft separation varied over a wide range in all directions. We derived the amplitude ratio, coherence, and phase lag for periods between 16 and 2048 min, which allowed us to compare the dependence of the correlation on periods. The amplitude ratio lies typically between and in the whole period range, while the coherence is only large for periods longer than ∼128 min. When the coherence decreases at short periods the phase lag becomes random, indicating that it is hard to predict the arrival time of fluctuations over such short periods at downstream points. The power spectral density of short‐period fluctuations follows a −5/3 power law variation with frequency, indicative of turbulence. The average values of the coherence depend on the spacecraft separation in the direction parallel to the Sun‐Earth line as well as in the direction perpendicular to it. The coherence scale length parallel to the Sun‐Earth line is estimated as 125 RE, while that perpendicular to it is 44 RE. One possible reason for this difference might be the convective effect of the solar wind.
A mixed magnetohydrodynamic‐kinetic theory of low‐frequency waves and instabilities in homogeneous, gyrotropic plasmas
Authors: Ferrière K. M. , André N.
Published: December 2002

We study low‐frequency waves and instabilities in homogeneous, collisionless plasmas whose thermal pressure is gyrotropic, i.e., characterized by two distinct components, P⊥ and P∥, perpendicular and parallel, respectively, to the magnetic field. In order to obtain the complete dispersion relation of low‐frequency waves, we need equations of state for P⊥ and P∥. After showing that these equations differ from the widely used double‐adiabatic, or CGL, magnetohydrodynamic (MHD) equations, we derive their correct expressions by resorting to the Vlasov equation. We thus obtain a dispersion relation, which is no longer strictly MHD: in addition to the three standard (fast, slow, and Alfvén) modes from double‐adiabatic MHD theory, our dispersion relation also contains the mirror mode from kinetic theory. We examine the physical characteristics and stability properties of each of these four modes and provide numerical solutions for their frequency and growth/decay rate, both for simple proton–electron plasmas and for multispecies plasmas.
Relation of substorm breakup arc to other growth‐phase auroral arcs
Authors: Lyons L. R. , Voronkov I. O. , Donovan E. F. , Zesta E.
Published: December 2002

High‐resolution CANOPUS meridian‐scanning photometer and all‐sky imager observations of preonset and expansion‐phase auroral arcs are analyzed for expansion‐phase onsets that evolve into full substorms and into pseudobreakups. One or more arcs are seen across the sky throughout the growth phase prior to onset. The onsets we have examined indicate that auroral breakup at onset does not generally occur along one of these arcs but instead often occurs along a thin breakup arc that forms equatorward of all growth phase arcs a few minutes prior to onset. The intensity of this breakup arc increases monotonically for the few minutes prior to the time normally identified as substorm onset and then typically increases dramatically. These results imply that the processes responsible for auroral breakup initiate a few minutes prior to the time normally identified as substorm expansion‐phase onset. We also find that arcs poleward of the arc that breaks up appear to be unaffected by substorm onset until expansion‐phase auroral activity moves poleward to the location of such arcs. Arcs poleward of the poleward‐most extent of pseudobreakup auroral activity show no apparent effects of a pseudobreakup. These results imply that the process that initiates the onset of substorms does not require the occurrence of plasma sheet changes, significant enough to affect magnetosphere‐ionosphere electrodynamics, along field lines that cross the equator tailward of the substorm onset region.