bluuebunny/crossref_metadata_2025 huggingface dataset:
Retrieval of vertical ozone content using the Chappuis Band with high spectral resolution solar radiation measurements
Authors: Cachorro V. E. , Durán P. , de Frutos A. M.
Published: February 2004

A method based on the absorption of the Chappuis band in the visible region has been developed in order to retrieve column ozone content together with aerosol optical depth by means of direct solar irradiance measurements of high spectral resolution. Due to the great sensitivity in the visible Chappuis band region of ozone to aerosol contribution and viceversa a careful supervised fitting using adequate windows is carried out. The fit is according to the Ångström formula and the correct behaviour of the aerosol optical thickness in the 350–1000 nm spectral range supports the adequate retrieval of the ozone values. These values are obtained by means of a nonlinear least square fitting between experimental and modelled data of the aerosol optical thickness (without Rayleigh contribution). Analysing the retrieved ozone values for every wavelength in the Chappuis band a standard deviation of 0.035 atm‐cm in the retrieved ozone value must be assigned. Results during the year of 1995 are shown in a rural site (Boecillo), 25 km away from the city of Valladolid located in the center of the Castilla‐León Region (Spain). Our retrieved values of ozone content show a constant shift (around 0.03 atm‐cm) and less scatter compared with those of the Arenosillo (a Spanish station belonging to the Global Ozone Observing System of WMO). Seasonal variation during this measured period of a year are clearly manifested with a maximum around April and a minimum in October. Diurnal variations of 0.03 atm‐cm are easily observed and interdiurnal variations of 0.04 atm‐cm are also very frequent.
Apparent synfolding magnetization as a result of overlap of pre‐ and post‐folding magnetizations
Authors: Halim Nadir , Cogné Jean Pascal , Courtillot Vincent , Chen Yan
Published: February 2004

In most rocks, Natural Remanent Magnetization (NRM) can involve two or more superimposed components. Demagnetizations hopefully result in the separation of these components, which may then be resolved using least‐squares regression methods. In a study of Cretaceous redbed sites from Qaidam (China), thermal demagnetization of most specimens revealed 3 rectilinear segments in orthogonal plots, giving the appearance of a 3‐component magnetization. In order to help deciding if the intermediate straight segments could have resulted from overlap between post‐folding low temperature (LTC) and pre‐folding high temperature components (HTC), we modeled the overlap using a priori information on the directions and relative intensities of LTC and HTC derived from the actual observations. We constructed typical synthetic Zijderveld (1967) diagrams for each site, which could be compared with those obtained from the real samples. We conclude for the Qaidam samples that the intermediate temperature component (ITC) is most likely an artifact resulting from overlap between the LTC and HTC.
Shear‐wave velocity structure of the crust and upper mantle beneath the Kola Peninsula
Authors: Dricker I. G. , Roecker S. W. , Kosarev G. L. , Vinnik L. P.
Published: February 2004

We determined the shear‐wave velocity structure of the crust and upper mantle beneath the central part of the Kola peninsula from the analysis of P‐wave receiver functions and mantle P‐SV converted phases recorded at stations Apatity (APA) and Lovozero (LVZ). The times of P‐SV converted phases from the 410 and 660 km discontinuities are close to those predicted by the IASP91 model. Phase conversions at the crust‐mantle boundary beneath the Baltic shield northeast of LVZ and southwest of APA are consistent with a sharp transition from crust to mantle at a depth of 40 km, while conversions from the intervening Khibina plutonic region are consistent with a gradual transition between depths of 20 and 40 km. We infer that short (∼50 km) wavelength lateral variations in the crust‐mantle transition persist in this region, despite the inactivity of the Kola peninsula since Devonian times.
Further estimates of radiative forcing due to tropospheric ozone changes
Authors: de F. Forster P. M. , Johnson C. E. , Law K. S. , Pyle J. A. , Shine K. P.
Published: February 2004

Estimates from two 2‐D (latitude ‐ height) chemical transport models of changes in tropospheric ozone concentrations since pre‐industrial times were used to calculate the radiative forcing due to these changes. The ozone changes are from Cambridge (CAMB) and the U. K. Meteorological Office (UKMO) models. The global and annual mean forcing using the UKMO changes was 0.3 Wm−2 and was 0.5 Wm−2 using the CAMB changes. The UKMO ozone changes are typically 60% of those from CAMB and this entirely accounts for the different forcing. Although the uncertainty is large, the calculations continue to support the case that tropospheric ozone changes make a substantial contribution (about 15%) to the total greenhouse gas radiative forcing. It is also shown that ignoring cloud and the effects of stratospheric adjustment in the radiative forcing calculations leads to an over‐estimation of the forcing by about 66%, for the global average.
Dry deposition of peroxyacetyl nitrate (PAN): Determination of its deposition velocity at night from measurements of the atmospheric PAN and 222Radon concentration gradient
Authors: Schrimpf Wolfram , Lienaerts Karlheinz , Müller Klaus Peter , Rudolph Jochen , Neubert Rolf , Schüßler Wolfram , Levin Ingeborg
Published: February 2004

During the field campaign POPCORN (Photooxidant Formation by Plant Emitted Compounds and OH‐Radicals in North‐Eastern Germany) in August 1994 we measured the nighttime deposition velocities of PAN above a corn field. These are the first absolute measurements of PAN deposition velocities in the field. The deposition velocities were derived using a novel method, which uses measurements of the gradients of PAN and 222Rn and of the emission rates of 222Rn from the soil. A unique data set of about 250 field measurements of the PAN deposition velocity at night was thus obtained. The deposition velocity at night proved to be highly variable with an average of 0.54 cm/s and a standard deviation of 0.94 cm/s. Recent presumptions by Shepson et al. [1992] that the PAN deposition velocity is strongly reduced with increasing relative humidity could not be confirmed by our measurements.
A demonstration of precise calibration of tropospheric delay fluctuations with water vapor radiometers
Authors: Teitelbaum L. P. , Keihm S. J. , Linfield R. P. , Mahoney M. J. , Resch G. M.
Published: February 2004

The ability of water vapor radiometers (WVRs) to calibrate changes in tropospheric delay was demonstrated during very long baseline radio interferometer (VLBI) observations at Goldstone, California. WVR measurements reduced the observed VLBI delay variations over a 13 hr period by a factor of ≈2.5. When applied to shorter time scales, a ≈50% reduction in 100–700 s delay variations was achieved during conditions of high tropospheric activity. Thermal WVR noise precluded calibration of short time scale delay fluctuations during quiet tropospheric conditions.
Three‐dimensional numerical simulation of ion and electron accelerations by parametric decay of fast lower hybrid waves
Authors: Singh Nagendra , Al‐Sharaeh S. , Abdelrazek A. , Leung W. C. , Wells B. Earl
Published: February 2004

The transverse acceleration of ions by fast lower hybrid waves in the topside auroral ionosphere has remained elusive. The linear and nonlinear propagation and evolution of lower hybrid waves are essentially a three‐dimensional problem. Analytical treatment of this problem, including the essential kinetic effects for particle acceleration, is a formidable task if not impossible. We have treated this problem by a fully three‐dimensional particle‐in‐cell simulation. It is found that a fast lower hybrid pump wave undergoes a parametric decay generating secondary waves over a broad frequency range from the ion cyclotron frequency Ωi to just above the lower hybrid frequency ωℓh. These secondary waves are instrumental in the accelerations of both cold ions and electrons perpendicular and parallel to the ambient magnetic field B, respectively.
A new test of Earth tide models in central Europe
Authors: Baker T. F. , Curtis D. J. , Dodson A. H.
Published: February 2004

New improved ocean tide models allow a corresponding improvement in ocean tide loading computations. During recent years various groups have made more accurate tidal gravity observations in central Europe, using gravimeters carefully calibrated on various calibration lines. This progress with tidal gravity measurements and ocean loading corrections now allows a more comprehensive test of body tide models than was possible previously. It is shown that the O1 and M2 gravimetric factors, corrected using the latest ocean tide models, are very consistent between the sites, despite the different instruments and gravity calibration lines that have been used. These observations allow constraints to be put on the anelasticity of the Earth at tidal frequencies. The corrected gravimetric factors are consistent with recent model results for both the elastic PREM model and with α ≤ 0.09 for an anelastic PREM model with the Q for dissipation in shear varying as frequency to the power α.
The TDLAS instrument for the detection of total inorganic chlorine in the stratosphere
Authors: Johnson T. J. , Harris G. W. , Bonifer J. , Crowley J. N. , Zenker T. , Fischer H.
Published: February 2004

A novel method based on tunable diode laser absorption spectroscopy is proposed for the detection of the sum of the stratospheric chlorine components HCl, ClONO2, ClOOCl and ClO. All of the above species are detected as HCl following thermal decomposition (ClONO2 and ClOOCl) to ClO which is converted to Cl via reaction with NO; the Cl atom is then rapidly reacted with a hydrocarbon to form HCl. The method has been investigated in the laboratory for detection of ClONO2, for which the conversion efficiency was found to be 100% under suitable conditions.
Effect of aircraft exhaust sulfur emissions on near field plume aerosols
Authors: Brown R. C. , Miake‐Lye R. C. , Anderson M. R. , Kolb C. E.
Published: February 2004

A two dimensional, axisymmetric flowfield model with coupled gas phase oxidation kinetics and aerosol nucleation and growth dynamics is used to evaluate the effect of fuel sulfur oxidation in the Concorde engine on the formation and growth of volatile H2SO4/H2O aerosols in the near field plume. Rased on estimated exit plane sulfur speciation, results are shown for between 2% and 20% conversion of the fuel sulfur to S(VI) (SO3 and H2SO4) in engine. The primary motivation is to provide estimates for the changes in the number density and surface area density of sulfuric acid aerosols due to sulfur oxidation in the engine. This analysis indicates the need for experimental measurements of sulfur emissions at the exhaust exit, in addition to soot properties, to fully assess the atmospheric impact of aircraft emissions.
Hot mantle transition zone beneath Iceland and the adjacent Mid‐Atlantic Ridge inferred from P‐to‐S conversions at the 410‐ and 660‐km discontinuities
Authors: Shen Yang , Solomon Sean C. , Bjarnason Ingi Th. , Purdy G. M.
Published: February 2004

P‐to‐S conversions from the two primary discontinuities near 410 and 660 km depth (P410s and P660s phases) are evident in particle motions and receiver functions of teleseismic body waves recorded by broadband seismic stations in Iceland. The average arrival times of P410s and P660s are later by 7.0±0.5 s (standard error) and 4.7±0.2 s, respectively, than predicted by the iasp91 Earth model. Differential P660s–P410s travel times indicate that the upper mantle transition zone between the 410‐ and 660‐km discontinuities is 23±9 km thinner than in the iasp91 model. From estimates for the Clapeyron slopes of the phase transitions associated with these discontinuities, this lesser transition zone thickness is consistent with temperatures 180±70 K hotter than normal. We interpret this result as evidence for upwelling mantle flow at 400–700 km depth beneath Iceland and the adjacent Mid‐Atlantic Ridge.
The argon constraints on mantle structure
Authors: Allègre Claude J. , Hofmann Albrecht , O'Nions Keith
Published: February 2004

The 40Ar budget of the Earth offers a powerful and straightforward argument in favour of the existence of a layered structure to the mantle.

Whereas K‐Ar‐isotope systematics have been included as part of various discussions of mantle structure previously a simple and straightforward argument is presented here. Because 40Ar is produced by 40K the amount of K in the Earth may be constrained from 40Ar budgets. The atmospheric 40Ar budget implies that approximately half of all 40Ar produced within the Earth since its formation is retained within the solid Earth. It is argued that this additional 40Ar is located principally in the lower mantle.
Frequency spectra of wind velocity and temperature fluctuations observed with the MU Radar‐RASS
Authors: Yamamoto Y. , Tsuda T. , Adachi T.
Published: February 2004

MU radar‐RASS observations were conducted on July 28–31, 1994 at Shigaraki, Japan (35°N, 136°E), to determine the detailed time variations of all three wind components and virtual temperature in the lower troposphere. We discuss their frequency spectra in comparison with a theoretical model of gravity waves. Assuming hydrostatic equilibrium, we also estimated the vertical wind velocity from the time derivative of temperature variations at each altitude. We found that the frequency spectra of the derived vertical wind were more consistent with a gravity wave model than those spectra calculated from direct measurements of the vertical wind.
Influence of Okhotsk sea‐ice extent on atmospheric circulation
Authors: Honda Meiji , Yamazaki Koji , Tachibana Yoshihiro , Takeuchi Kensuke
Published: February 2004

An atmospheric general circulation model was used to investigate the influence of Okhotsk sea ice on large‐scale atmospheric circulation. Significant differences in the model responses between heavy and light ice cover are evident not only around the Sea of Okhotsk but also downstream towards North America in the form of a stationary wavetrain in the troposphere. Because the wave activity flux associated with the wavetrain emanates downstream from the Sea of Okhotsk, this remote response is regarded as a stationary Rossby wave excited thermally through an anomalous surface heat flux over the Sea of Okhotsk. The wavetrain is thermally reinforced downstream by the anomalous heat flux associated with wind anomalies over the ocean surface, which are induced by the wavetrain itself.
On the accuracy of arrival azimuth determination of sprite‐associated lightning flashes by Earth‐ionosphere cavity resonances
Authors: Füllekrug Martin , Reising Steven C. , Lyons Walter A.
Published: February 2004

Horizontal magnetic field variations in the frequency range of the Earth‐ionosphere cavity resonances are observed at Silberborn, Germany, simultaneous with 19 sprite‐associated lightning flashes in the midwestern United States, on July 15, 1995. The measured horizontal magnetic intensities are linearly related to the horizontal magnetic intensities of slow tails of radio atmospherics which were simultaneously recorded at Palmer Station, Antarctica. Enhancement of the Earth‐ionosphere cavity resonances is verified by spectral analysis, and the measured arrival azimuths are in agreement with the expected orientation of the Poynting vector along the great‐circle path. The estimated accuracy of the arrival azimuth determination is on the order of ±5°.
Long‐term ozone trends derived from the 16‐year combined Nimbus 7/Meteor 3 TOMS Version 7 record
Authors: McPeters R. D. , Hollandsworth S. M. , Flynn L. E. , Herman J. R. , Seftor C. J.
Published: February 2004

Ozone measurements from the Nimbus 7 TOMS instrument, which operated from November 1978 through early May 1993, have been extended through December 1994 using data from the TOMS instrument on‐board the Russian Meteor 3 satellite. Both TOMS data records have recently been recalibrated, and then reprocessed using the Version 7 retrieval algorithm. Long‐term trend estimates obtained from a multiple regression analysis show ozone losses in the extended data record similar to those reported in previous studies using Version 6 TOMS and SBUV data, and ground‐based Dobson data. Ozone continues to decline through the end of 1994, with the most significant ozone losses occurring in the high southern latitudes during October (−20% per decade) and in the northern mid‐ to high‐latitudes during March/April (−6 to −8% per decade). There is no significant ozone trend in the tropics. Annual‐average trends derived from the Nimbus 7 Version 7 data are 0–2.5% per decade less negative than those derived over the same time period using Version 6 data.
Accuracy of Arctic stratospheric temperature analyses and the implications for the prediction of polar stratospheric clouds
Authors: Knudsen Bjørn M.
Published: February 2004

Lower stratospheric temperature analyses from European Centre for Medium‐Range Weather Forecasts (ECMWF) are compared with radiosonde temperatures in the Arctic in the winters 1994/95 and 1995/96. In February and March 1996, the biases between mean layer radiosonde and ECMWF temperatures are −0.1±1.2, 0.0±1.1, +0.1±1.4, and −0.7±1.8 K at 100, 70, 50, and 30 mb, respectively. Individual measurements between the radiosonde standard levels, however, exhibit substantially larger standard deviations, and at temperatures low enough for the formation of polar stratospheric clouds (PSCs) the biases are −0.6±1.5, −1.1±1.8, −1.4±1.9, and −1.6±2.3 K, respectively. This means that ECMWF temperatures predict too small an extent of PSCs. The cause is the inability of models with limited resolution to catch small‐scale temperature variations, and UK Meteorological Office (UKMO) temperature analyses exhibit similar biases. Before January 30, 1996, when ECMWF changed to a 3‐dimensional variational analysis, substantial biases to mean layer radiosonde temperatures exist.
Foreword
Authors: Goldberg D.
Published: February 2004

Many scientific objectives nowadays are met with a interdisciplinary strategy, integrating measurements made on core samples with those made downhole and placing both in the context of regional geophysical and seismic studies. Vast improvements in downhole instrumentation technology and geophysical survey resolution, as well as laboratory measurements made on core samples, allow for the one‐to‐one correlation of similar measurements that together may be interpreted more effectively. The multiple scales of investigation used with seismic, downhole, and core data acquired in the same geological environment complement each other extremely well. Seismic sections are the basis for a regional description; downhole measurements are of an intermediate scale giving continuous information in the region surrounding the borehole; and core samples provide detailed information on physical properties and age.
Bio‐optical properties in relation to an algal bloom caused by iron enrichment in the equatorial Pacific
Authors: Kudela Raphael M. , Chavez Francisco P.
Published: February 2004

In 1995 a massive bloom of phytoplankton composed primarily of diatoms was created by adding small quantities of iron to the surface waters of the equatorial Pacific. As part of this experiment, several bio‐optical parameters were measured using drifter instruments and shipboard measurements. The iron‐induced bloom resulted in dramatic changes in the bio‐optical properties of the study site, potentially resulting in light limitation of the phytoplankton at the peak of the bloom. The results also provide evidence for a shift in photosynthetic energy partitioning by the phytoplankton assemblage upon initial exposure to iron. It is unclear, however, whether the physiological changes associated with the bloom were purely a result of the iron addition, or were also influenced by changes in species composition. We hypothesize that iron‐induced changes in bio‐optical properties of equatorial Pacific waters could have far‐reaching effects on the global climate system.
Comparisons of time series from two global models with tide‐gauge data
Authors: Tokmakian Robin
Published: February 2004

To understand climate change and to predict changes in climate, both atmospheric and ocean models need to be able to reproduce the responses of the system they are estimating. An evaluation of two related global ocean models, using a nine year time series of model sea surface heights and measurements of sea level from a global tide gauge set, shows that the ocean models satisfactorily reproduce the instantaneous local change in sea level. Sea level can be used as a tool to understand a model's ability to simulate sea surface temperature and thus influence atmospheric flows. The implication for climate modeling is that these ocean models can be used to simulate seasonal to decadal variations in global ocean circulation when coupled to high quality atmospheric models.
The cold stratospheric winters 1994/1995 and 1995/1996
Authors: Naujokat Barbara , Pawson Steven
Published: February 2004

The stratospheric winters 1994/1995 and 1995/1996 were both extremely cold. The temperature (T) fell below that necessary for type 1 polar stratospheric cloud formation (195 K at 50 hPa) on many consecutive days in each winter and occasionally reached values low enough for ice cloud formation. The meteorological situation each winter was quite different. Both seasons were cold throughout Dec and Jan. In 1995 a dynamical warming event increased T in the latter part of Feb but a further cold spell occurred in early Mar. In contrast, the polar vortex remained well developed until early Mar 1996, when T increased rapidly. While both winters were extremely cold at 50 hPa, the polar vortex was deeper in Feb 1996 when extremely low T occurred over large areas at 30 hPa.
On long‐term variations in the intensity of El Niño occurrences
Authors: Dovgalyuk Vladimir V. , Klimenko Vladimir V.
Published: February 2004

We consider a 396‐year record of the Southern Oscillation Index (SOI) the earlier portion of which (1601–1851) was obtained from a dendrochronological reconstruction of Lough and Fritts (1985). Associated with the SOI point processes of upcrossings the positive level and downcrossings the negative level can be interpreted as sequences of El Niño and La Niña occurrences. In the intensities of these processes we emphasize harmonics with periods of 74 and 320 years and prove their statistical significance.
Optical parameters of contrails from lidar measurements: Linear depolarization
Authors: Freudenthaler V. , Homburg F. , Jäger H.
Published: February 2004

By means of a scanning backscatter and depolarization lidar, contrails have been investigated for ages between a few seconds and 60 min. At environmental temperatures ≤−60°C the depolarization measured in the contrails rises from about 0.1 to 0.5 with time, whereas at higher temperatures around −50°C already the youngest contrails exhibit depolarization values of about 0.5. This indicates a strong dependence of the aerosol formation and growth on the environmental temperature which might influence their behavior in the atmosphere.
Temperature distribution in the laser‐heated diamond cell with external heating, and implications for the thermal conductivity of perovskite
Authors: Li Xiaoyuan , Manga Michael , Nguyen Jeffrey H. , Jeanloz Raymond
Published: February 2004

Applying external heating reduces temperature gradients in the laser‐heated diamond cell by roughly a factor of two as the external temperature is raised by 300 K. A heat conduction model is presented which can account for most of the reduction. In addition, the temperature distribution measured in a (Mg0.86Fe0.14)SiO3 perovskite sample at 56 GPa is compatible the expected inverse temperature‐dependence of thermal conductivity.
Trace chemical measurements from the northern midlatitude lowermost stratosphere in early spring: Distributions, correlations, and fate
Authors: Singh H. B. , Chen Y. , Gregory G. L. , Sachse G. W. , Talbot R. , Blake D. R. , Kondo Y. , Bradshaw J. D. , Heikes B. , Thornton D.
Published: February 2004

In situ measurements of a large number of trace chemicals from the midlatitude (37–57°N) lower stratosphere were performed with the NASA DC‐8 aircraft during March 1994. Deepest penetrations into the stratosphere (550 ppb O3, 279 ppb N2O, and 350 K potential temperature) corresponded to a region that has been defined as the “lowermost stratosphere” (LS) by Holton et al. [1995]. Analysis of data shows that the mixing ratios of long‐lived tracer species (e. g. CH4, HNO3, NOy, CFCs) are linearly correlated with those of O3 and N2O. A ΔNOy/ΔO3 of 0.0054 ppb/ppb and ΔNOy/ΔN2O of −0.081 ppb/ppb is in good agreement with other reported measurements from the DC‐8. These slopes are however, somewhat steeper than those reported from the ER‐2 airborne studies. We find that the reactive nitrogen budget in the LS is largely balanced with HNO3 accounting for 80% of NOy, and PAN and NOx together accounting for 5%. A number of oxygenated species (e. g. acetone, H2O2) were present and may provide an important in situ source of HOx in the LS. SO2 mixing ratios were found to increase in the stratosphere at a rate that was comparable to the decline in OCS levels. No evidence of particle formation could be observed. Ethane, propane, and acetylene mixing ratios declined rapidly in the LS with Cl atoms likely playing a key role in this process. A number of reactive hydrocarbons/halocarbons (e. g. C6H6, CH3I) were present at low but measurable concentrations.
Influence of solar irradiance on the Indian Monsoon‐ENSO relationship at decadal‐multidecadal time scales
Authors: Mehta Vikram M. , Lau K.‐M.
Published: February 2004

One hundred and nineteen years (1872–1990) of area‐averaged rainfall over India, and sea surface temperature (SST) averaged over the equatorial central and eastern Pacific Ocean (the so‐called Niño3 region) as an index of El Niño‐Southern Oscillation (ENSO) are analyzed to study their covariability. The monsoon rainfall, SST, and their covariability undergo decadal‐multidecadal variations without any dominant or distinct periodicities. The monsoon rainfall, SST, and their covariability are compared to solar irradiance at the top of the Earth's atmosphere for the same data period. At the 11 years time scale there is no consistent relationship between the monsoon rainfall and solar irradiance, and between the Niño3 SST and solar irradiance. The rainfall and irradiance appear to covary with nearly the same phase at multidecadal time scales. The Niño3 SST and irradiance appear to covary with nearly opposite phases at multidecadal time scales. The interannual covariability of the monsoon‐ENSO system is also apparently modulated by this multidecadal variability. Based on these results, a hypothesis about the possible influence of solar irradiance in the ENSO‐monsoon relationship at multidecadal time scales is described.
Fine structure effects in the O+‐O collision frequency
Authors: Hickman A. P. , Medikeri‐Naphade M. , Chapin C. D. , Huestis D. L.
Published: February 2004

Effects of the fine structure of atomic oxygen on the O+‐O ion‐neutral momentum transfer collision frequency νin are investigated to assess whether they could account for persistent discrepancies between theoretical calculations and values of νin determined from observations. Potential curves for the molecule have been calculated that include spin‐orbit effects and properly dissociate to the three asymptotic limits O+‐O(³PJ) (J=2,1,0). Coupled channel scattering calculations have been performed to determine νin. The results show that νin depends strongly on the initial fine structure level, and the average over a thermal distribution of initial states is a few percent smaller than recent theoretical calculations that neglect fine structure effects. The most recent analysis of observations leads to values of νin about 30% smaller than older values. The theoretical calculations are now consistent with the most recent experimental determinations of νin.
Seismic evidence against a mantle chemical discontinuity near 660 km depth beneath Izu‐Bonin
Authors: Castle John C. , Creager Kenneth C.
Published: February 2004

We analyze P‐wave codas from 12 deep Izu‐Bonin earthquakes recorded by hundreds of Western United States seismograph stations for evidence of small‐amplitude phases caused by near‐source mantle discontinuities. For nearly every event, the dominant phase in the coda is the result of an S‐to‐P conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km. This is interpreted as a thermally depressed spinel to perovskite and magnesiowüstite phase transition. If the 660‐km seismic discontinuity is also associated with a change in chemistry, it would be dynamically depressed by a subducting slab. We consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1%; this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles.
Numerical simulation of the interannual variability of the Mediterranean Sea upper ocean circulation
Authors: Pinardi N. , Korres G. , Lascaratos A. , Roussenov V. , Stanev E.
Published: February 2004

Numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the Mediterranean. From January 1980 to November 1988, the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents. The model prediction of the interannual fluctuations of the Sicily Strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region. The current anomalies persist for many months after a Winter atmospheric anomalous disturbance has occurred over the basin. The Eastern Mediterranean basin is the area where the interannual ocean response is most pronounced.
Upper mantle heterogeneities in the Indian Ocean from waveform inversion
Authors: Debayle E. , Lévêque J. J.
Published: February 2004

A waveform inversion method is applied to 156 Love and Rayleigh wave seismograms to build up a 3‐D model of the shear velocity in the upper mantle beneath the Indian Ocean. The first step of the method consists in finding, for each path, a radially stratified upper mantle model compatible with the Love and Rayleigh wave seismograms relative to that path. The fundamental mode and few higher modes are modelled in the waveform inversion. In a second step, the models related to the different paths are used in a tomographic inversion to map the 3‐D upper mantle structure. The 3‐D velocity model has a lateral resolution of 1000 km. No significant velocity anomalies are found below 300 km, although the resolution is still good. Continental roots are confined to the upper 300 km and low velocities below mid‐oceanic ridges to the upper 250 km, depending on their spreading rates. At shallow depths (<80 km), we find a positive velocity anomaly beneath the West Indian ridge near the Rodriguez triple junction, where gravimetric and bathymetric data indicate a reduced volcanic activity. At greater depth (around 200 km), a low velocity signature is found beneath the hot‐spot of Réunion‐Mauritius islands and the Central Indian ridge. It could reflect a real connection between the two structures.
Sensitivity of greenhouse summer dryness to changes in plant rooting characteristics
Authors: Milly P. C. D.
Published: February 2004

A possible consequence of increased concentrations of greenhouse gases in Earth's atmosphere is “summer dryness,” a decrease of summer plant‐available soil water in middle latitudes, caused by increased availability of energy to drive evapotranspiration. Results from a numerical climate model indicate that summer dryness and related changes of land‐surface water balances are highly sensitive to possible concomitant changes of plant‐available water‐holding capacity of soil, which depends on plant rooting depth and density. The model suggests that a 14% decrease of the soil volume whose water is accessible to plant roots would generate the same summer dryness, by one measure, as an equilibrium doubling of atmospheric carbon dioxide. Conversely, a 14% increase of that soil volume would be sufficient to offset the summer dryness associated with carbon‐dioxide doubling. Global and regional changes in rooting depth and density may result from (1) plant and plant‐community responses to greenhouse warming, to carbon‐dioxide fertilization, and to associated changes in the water balance and (2) anthropogenic deforestation and desertification. Given their apparently critical role, heretofore ignored, in global hydroclimatic change, such changes of rooting characteristics should be carefully evaluated using ecosystem observations, theory, and models.
on the source region of traveling convection vortices
Authors: Yahnin A. G. , Vorobjev V. G. , Bösinger T. , Rasinkangas R. , Sibeck D. G. , Newell P. T.
Published: February 2004

We reexamined two well‐documented traveling convection vortices (TCVs) which were observed in the pre‐noon sector to investigate their position relative to particle precipitation boundaries obtained from low‐altitude satellite measurements. DMSP observations of precipitating particles indicate that the TCVs' focii and trajectories lay inside of the plasma sheet near the CPS/BPS boundary. The TCVs occurred within the poleward part of the belt of trapped energetic (E > 30 keV) electrons. These findings and some auroral observations suggest that it is possible for TCVs to map to regions well within the magnetosphere, contrary to the general belief that TCV related currents originate at the magnetopause or LLBL.
Building reliable local models of the Venus gravity field from the cycles 5 and 6 of the Magellan LOS gravity data
Authors: Barriot J.‐P. , Balmino G. , Valès N.
Published: February 2004

We present a new way to build reliable local models of the Venus gravity field from the cycles 5 and 6 of the Magellan probe by directly considering the Doppler residual profiles instead of the derived residual acceleration profiles. We give an example over a test area.
Azimuthal velocity variations caused by borehole stress concentrations
Authors: Winkler Kenneth W.
Published: February 2004

In laboratory experiments, borehole stress concentrations caused by far‐field uniaxial stress produce measurable variations in compressional wave velocities near the borehole. Uniaxial stress was applied perpendicular to the axis of a borehole drilled through a sandstone block. Inside the fluid‐filled borehole, refracted compressional waves were propagated in the rock parallel to the borehole axis at discrete azimuths, producing a scan of velocity versus azimuth. As uniaxial stress increased, significant variations of compressional wave velocity with azimuth developed, of the order of 10% variation. The azimuth of velocity minima provides an excellent indicator of the direction of maximum far‐field uniaxial stress, accurate to 5°. The detailed variation of velocity with azimuth, while approximately sinusoidal, has sufficient character to provide new information about the rock. One interpretation yields the curvature of the velocity versus stress curve of the rock.
The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation
Authors: Slack Philip D. , Davis Paul M. , Baldridge W. Scott , Olsen Kenneth H. , Glahn Andreas , Achauer Ulrich , Spence William
Published: February 2004

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki‐Christofferson‐Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE‐SW to NNE‐SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north‐south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low‐velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW‐ESE), possibly heralding future extension across the Jemez lineament and transition zone.
Recent dueling propagation history at the fastest spreading center, the East Pacific Rise, 26°–32°S
Authors: Korenaga Jun , Hey Richard N.
Published: February 2004

The East Pacific Rise (EPR) 26°–32°S, located between the Easter microplate and the Juan Fernandez microplate, has the world's fastest spreading rate and tectonics characterized by dueling ridge propagation. A GLORI‐B and Sea Beam 2000 side scan/bathymetry survey, together with other geophysical data collection, was conducted aboard the R/V Melville covering an approximately 5° by 5° area centered on this part of EPR. The side scan and bathymetric data reveal the complete geometry of the propagator system. The western and eastern ridges both curve inwardly and are overlapped ∼120 km with an offset to overlap ratio of nearly one. Several abandoned ridges and rifts are clearly mapped on the east flank, and a few failed rifts are also mapped near the western ridge tip. A fracture zone discovered south of Easter Island indicates a stage of stable ridge‐transform intersection in the past. An inversion with magnetic anomaly vectors was carried out for the area. A resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map. The spreading and propagation rates were estimated from the magnetic isochrons. The net southward propagation rate of the western ridge is ∼120 km/m.y. for the last 1.9 m.y. The spreading rate of the eastern ridge has been ∼150 km/m.y., and its current propagation rate is estimated as ∼500 km/m.y. toward the north. Intense asymmetric spreading as high as 30% is observed at both ridges. Together with the side scan and bathymetric data, the magnetic lineations provide significant constraints on the evolution of the propagator system, and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m.y. A normal ridge‐transform intersection evolved into a nontransform overlapping offset ∼1.95 Ma, and the eastern and the western ridges have propagated alternately. The dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths, widths, and ridge curvatures in the transition zone.
Mapping the descent of Indian and Eurasian plates beneath the Tibetan Plateau from gravity anomalies
Authors: Jin Yu , McNutt Marcia K. , Zhu Yong‐sheng
Published: February 2004

The collision of India with Asia has produced a complicated continental‐continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the Tibetan Plateau. Numerous lines of evidence, including the development of two scales of folding in Tibet, suggest that the lowermost crust is behaving in a ductile fashion. This weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting. In this study, we use Bouguer gravity anomalies to map out the geometry of Indian and Eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the Moho and lateral variations in lithospheric strength determined from mechanical modeling. In our preferred model, the lithosphere beneath Tibet consists of two distinct units: (1) the underthrust (to the north) Indian plate, which sutures with the Eurasian plate in the upper mantle below the Yarlung‐Zangpo Suture or the Gangdese igneous belt, 200–400 km north of the Main Boundary Thrust, and (2) the underthrust (to the south) Eurasian plate. A subducting slab of Indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3.5 × 1017 N on the Indian plate. Thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates, while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity.
Age‐dependent variation in the magnetization of seamounts
Authors: Johnson H. Paul , Van Patten Darcy , Sager William W.
Published: February 2004

The magnetization of Pacific seamounts shows a surprisingly large variation when viewed as a function of time. Similar to normal upper oceanic crust, the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation. This high magnetization decays rapidly with time, resulting in a factor of 5 reduction in intensity in less than 500,000 years. Because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor, low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust. Some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years. These capping rocks can be locally abundant (i.e., Hawaii), are exposed to a different post formation environment, and can have a magnetic mineralogy that is different from submarine volcanoes. In spite of these differences, the magnetization of subaerially erupted rocks of the Hawaii/Emperor Chain also may show a similar rapid reduction in intensity with age. Finally, seamounts of Cretaceous age show a factor of 2 higher average magnetization when compared to Tertiary seamounts, similar to the previously observed increased magnetization of Cretaceous seafloor basalts recovered from Deep Sea Drilling Project/Ocean Drilling Project cores. This increase in Cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some, but not all of the ocean basins, prior and subsequent to the Cretaceous Normal Superchron. The processes potentially responsible for the elevated Cretaceous magnetization values include a high geomagnetic field intensity, large‐scale variations in whole rock major element geochemistry of the extrusive rocks, or a systematic acquisition of a secondary magnetization during the Cretaceous Normal Superchron.
Three‐dimensional magnetic and gravity studies of the Rodriguez Triple Junction in the Indian Ocean
Authors: Honsho Chie , Tamaki Kensaku , Fujimoto Hiromi
Published: February 2004

The Rodriguez Triple Junction (RTJ) is a ridge‐ridge‐ridge triple junction connecting three mid‐ocean ridges in the Indian Ocean: the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR), and Southwest Indian Ridge (SWIR). We carried out three‐dimensional analyses of Sea Beam bathymetry, magnetic, and gravity anomaly data covering an area of about 200 km × 140 km at the triple junction and obtained the magnetization distribution and the Bouguer anomaly field. In the magnetic inversion result some of the magnetic lineations of the CIR and SEIR are recognized within the SWIR valley, and the Bouguer anomaly field shows little variation over the SWIR valley in spite of its large topographic relief. These results indicate the tectonic rifting at the SWIR by crustal thinning without volcanic activity. A 4‐km‐wide graben in the western CIR flank was found to have extremely high magnetization and lower Bouguer anomaly than the eastern flank of the CIR. Because the CIR and SEIR have similar spreading directions and rates at the triple junction, we consider the graben represents a fossil propagating rift of the SEIR, which began to propagate toward the CIR at about 1 Ma and then became inactive when the SWIR grew to reach the CIR + SEIR spreading axis at about 0.5 Ma. We suggest the above process repeats several times and composes part of the triple junction evolution on a longer timescale. We also suggest that the whole process is closely related to formation of the CIR segments at the triple junction since 20 Ma.
Fault spacing in the El Teniente Mine, central Chile: Evidence for nonfractal fault geometry
Authors: Brooks Benjamin A. , Allmendinger Richard W. , de la Barra Iván Garrido
Published: February 2004

We utilize the excellent rock exposure provided by the network of mining tunnels in El Teniente Mine, Central Chile, to test the applicability of fractal fault spacing laws to a ∼2 km × 3 km × 1.3 km body of crystalline rock in an Andean shear zone. Fractal spacing laws can be particularly useful because they allow prediction of fault spacing at unsampled scales. Because of the restrictions of tunnel geometry in the mine we use only “scanline”, one‐dimensional, analyses. The large number of parallel tunnels allows us to assess the consistency of one‐dimensional scanline samples over a two‐dimensional area. We use two methods: 1) an interval counting technique and 2) standard cumulative statistics to test the hypothesis of fractal fault geometry in the shear zone. The interval counting method produces deceptively good fits to a fractal distribution. In fact, by analyzing the residuals and differential slopes of interval counting and cumulative statistical plots and by comparison with synthetic fractal, negative exponential, and log‐normal distribution data, we conclude that a negative exponential distribution best describes fault spatial distribution at the mine. We present three possible explanations for the negative exponential spacing distribution: 1) the rock body is at the earliest stage in a deformational path that will produce varying spacing distributions as deformation proceeds, 2) the spacing distribution is inherited through joint reactivation, and 3) the present spacing distribution is the result of a superposition of two spacing distributions each of which is not necessarily negative exponential. Additionally, we attempt to use the deviation of mean spacings in parallel scanlines (afterWu and Pollard(1995)) to further classify deformational maturity. We show that this method cannot be used to compare population development in faults which follow a negative exponential spacing distribution because of the negative exponential's equivalence of mean and standard deviation. Instead we use a relative measure of parallel scanline deviation to compare regions. The distinctly smaller standard deviation of mean spacings in one region (the T3 Isla region) may be a signature of increased local finite strain accumulation.