bluuebunny/crossref_metadata_2025 huggingface dataset:
Mars atmospheric dynamics as simulated by the NASA Ames general circulation model: 3. Winter quasi‐stationary eddies
Authors: Barnes Jeffrey R. , Haberle Robert M. , Pollack James B. , Lee Hilda , Schaeffer James
Published: February 2004

A set of simulations with the NASA Ames Mars general circulation model (GCM) has been analyzed to define the basic properties and dynamics of quasi‐stationary eddy circulations in the winter hemisphere. These circulations, differing substantially from those in low latitudes and the summer hemisphere, extend from the surface to the model top at ∼47 km; typically, the largest geopotential and wind perturbations are found at the highest model level. Near the surface the eddies are of largest amplitude in lower latitudes, but above ∼5–10 km the amplitudes are a maximum in middle and high latitudes. The vertical structure of the eddies is nearly equivalent barotropic; the phase variation in latitude is also relatively small. Zonal wavenumbers 1 and 2 dominate the stationary circulations, with wave 3 being of significance only at low levels (below ∼10 km). The quasi‐stationary eddies differ substantially in northern and southern winters, and undergo considerable changes with increasing dustiness in northern winter. The southern winter eddy circulation is dominated by zonal wavenumber 1, while the northern eddies have a strong wavenumber 2 component at low levels of dust loading; at high dust levels wavenumber 1 becomes dominant. This change may be at least partly the result of the dusty zonal‐mean state becoming more responsive to wave 1 forcing. The standing eddies in a GCM simulation for Ls ∼ 0° exhibit considerable similarity in basic structure and amplitude to those revealed by an recent analysis of Viking IRTM data [Banfield et al., 1996], though there are differences in the phases of the eddy patterns.
Mars ozone measurements near the 1995 aphelion: Hubble space telescope ultraviolet spectroscopy with the faint object spectrograph
Authors: Clancy R. Todd , Wolff Michael J. , James Philip B. , Smith Ed , Billawala Youssef N. , Lee Steven W. , Callan Michael
Published: February 2004

Ultraviolet (225–330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2–0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (Ls) of 63.5°, corresponding to the late northern spring season on Mars, and very near to Mars aphelion. The most important result of these observations is the measurement of low‐latitude ozone abundances (3.1−0.5+2.1), which are significantly (≥100%) elevated relative to the northern fall (Ls = 208°, pre‐perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion‐to‐aphelion increase in the global Mars ozone column (from 1.5−1.0+0.4 to 3.1−0.5+2.1 μm atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with Ls, which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3 ± 2.5 μm atm is retrieved for a high northern latitude region (71–75°N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18 ± 0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.
Fast nitrogen atoms from dissociative excitation of N2 by electron impact
Authors: Ajello Joseph M. , Ciocca Marco
Published: February 2004

The Doppler profiles of one of the fine structure lines of the N I (1200 Å) g 4S0‐4P multiplet and of the N II (1085 Å) g 3P0‐3D multiplet have been measured. Excitation of the multiplets is produced by electron impact dissociative excitation of N2. The experimental line profiles are evaluated by fast Fourier transform (FFT) techniques and analysis of the profiles yields the kinetic energy distribution of fragments. The full width at half maximum (FWHM) of N I (1200 Å) increases from 27 ± 6 mÅ at 30 eV to 37 ± 4 mÅ at 100 eV as the emission cross section of the dissociative ionization excitation process becomes more important relative to the dissociative excitation process. The FWHM of the N II (1085 Å) line is 36 ± 4 mÅ at 100 eV. For each multiplet the kinetic energy distribution function of each of the two fragment N atoms (ions) is much broader than thermal with a mean energy above 1.0 eV. The dissociation process with the largest cross section is predissociation and predominantly produces N atoms with kinetic energy distributions having mean energies above 0.5 eV. Dissociative processes can lead to a substantial escape flux of N I atoms from the satellites, Titan and Triton of the outer planets.
The impact of gravity waves on the Venus thermosphere and O2 IR nightglow
Authors: Zhang S. , Bougher S. W. , Alexander M. J.
Published: February 2004

The terrestrial Fritts and Lu [1993] gravity wave parameterization, based on the theory of wave saturation, has been adopted within the Venus thermospheric general circulation model (VTGCM) to examine the impact of upward propagating gravity waves on the Venus thermospheric subsolar‐to‐antisolar (SS‐AS) circulation and the resultant O2 IR nightglow. It is shown that the drag force generated by the breakdown of gravity waves can effectively slow down the winds, which results in reproducing the observed day/night contrasts in temperatures and densities. With stronger drag force on the dawn terminator than on the dusk terminator, the apparently superrotating zonal wind can be forced by the dissipation of gravity waves. The equivalent superrotating zonal wind varies both with local time and altitude. The simulated distribution of the O2 IR nightglow is strongly sensitive to small changes of the characteristic intrinsic phase speeds at the model lower boundary. This sensitivity implies that minor variations in gravity wave momentum fluxes to the thermosphere can give rise to large O2 IR emission variations with time and space, in agreement with ground‐based observations.
A three‐dimensional azimuthally symmetric model atmosphere for Io: 1. Photochemistry and the accumulation of a nightside atmosphere
Authors: Wong Mau C. , Johnson Robert E.
Published: February 2004

A three‐dimensional axisymmetric model is used to approximate the abundance of minor species in Io's atmosphere. This includes plasma and Joule heating, nonlocal thermodynamic equilibrium cooling, and photochemistry of SO2 with convective and diffusive transport, and extends our earlier work on a sublimation‐driven atmosphere [Wong and Johnson, 1995] in which we studied the effect of plasma heating on the dynamics of the atmosphere near the exobase. In the present model a more advanced numerical technique is employed and spherical geometry is used. The new results confirm that plasma heating determines the atmospheric temperature near the exobase. This heating also causes the average column density on the dayside trailing hemisphere to exceed that of the dayside leading hemisphere by ∼20%. We evaluated the suggestion that noncondensibles (O2 and, possibly, SO) would accumulate and dominate the atmospheric dynamics. We found that the buildup of a nightside atmosphere of noncondensible photochemical products does not overwhelm the dayside atmospheric flow, but it does raise the overall atmospheric pressure and reduce the wind speed. For a dayside hemispherically averaged column abundance of 5.5 × 1016 SO2 cm−2 the column abundance of O2 is found to be ∼1014 cm−2 on the dayside and ∼1015 cm−2 on the nightside, while SO (if assumed noncondensible) is ∼1015 cm−2 in most regions, ignoring plasma destruction processes. It is also found that the relative density of a photochemical product to that of SO2 and its spatial distribution depend strongly on the flow dynamics, a factor that cannot be accounted for in one dimensional models. The hemispherically averaged fraction of SO on the dayside is found to be 3% if SO is condensible and 6% if SO is noncondensible.
Argon‐40/argon‐39 age spectra of Apollo 17 highlands breccia samples by laser step heating and the age of the Serenitatis basin
Authors: Dalrymple G. Brent , Ryder Graham
Published: February 2004

We have obtained high‐resolution (21–63 steps) 40Ar/39Ar age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from Apollo 17 samples collected from the pre‐Imbrian highlands in the easternmost part of the Serenitatis basin. The samples include poikilitic melt rocks inferred to have been formed in the Serenitatis basin‐forming impact, aphanitic melt rock whose compositions vary and whose provenance is uncertain, and granulite, gabbro, and melt clasts. Three of the poikilitic melts have similar age spectrum plateau ages (72395,96, 3893 ± 16 Ma {2σ} 72535,7, 3887 ± 16 Ma; 76315,150, 3900 ± 16 Ma) with a weighted mean age of 3893 ± 9 Ma, which we interpret as the best age for the Serenitatis basin‐forming impact. Published 40Ar/39Ar age spectrum ages of Apollo 17 poikilitic melts are consistent with our new age but are much less precise. Two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of ≥3869 Ma (72558,7) and ≥3743 Ma (77135,178). Plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 ± 16 Ma to 3937 ± 16 Ma and have probably been affected by the ubiquitous (older?) clasts and by post‐formation heating (impact) events. Plateau ages from two of the aphanitic melt “blobs” and two granulites in sample 72255 fall in the narrow range of 3850 ± 16 Ma to 3869 ± 16 Ma with a weighted mean of 3862 ± 8 Ma. Two of the aphanitic melt blobs from 72255 have ages of 3883 ± 16 Ma and ≥3894 Ma, whereas a poikilitic melt clast (of different composition from the “Serenitatis” melts) has an age of 3835 ± 16 Ma, which is the upper limit for the accretion of 72255. These data suggest that either the aphanitic melts vary in age, as is also suggested by their varying chemical compositions, or they formed in the 72255 accretionary event about 3.84–3.85 Ga and older relict material is responsible for the dispersion of ages. In any case the aphanitic melts do not appear to be Serenitatis products. Our age for the Serenitatis impact shows, on the basis of the isotopic age evidence alone, that Serenitatis is >20–25 Ma and probably >55–60 Ma older than Imbrium (≤3870 Ma and probably ≤3836 Ma [Dalrymple and Ryder, 1993]). Noritic granulite sample 78527 has a plateau age of 4146 ± 17 Ma, representing a minimum age for cooling of this sample in the early lunar crust. So far there is no convincing evidence in the lunar melt rock record for basin‐forming impacts significantly older than 3.9 Ga.
Estimated detectability limits of iron‐substituted montmorillonite clay on Mars from thermal emission spectra of clay‐palagonite physical mixtures
Authors: Roush Ted L. , Orenberg James B.
Published: February 2004

The thermal emission properties of two Mars soil analogs, palagonite and iron‐substituted montmorillonite clay, and their physical mixtures are presented in the 5 to 25 μm (2000 to 400 cm−1) region. The clay spectrum has both a greater number of spectral features and higher emissivity contrast when compared to the palagonite spectrum. Emissivity peaks located near 8.9, 11.1, and 19.9 μm (1120, 905, and 505 cm−1) and troughs (restsrahlen features) located near 8.8, 9.7, 18.9, and 22.2 μm in the spectrum of the clay provide the ability to identify the presence of clay when it is physically mixed with the palagonite. In the mixtures studied, when 15 wt% of clay is mixed with 85 wt% palagonite, the 19.9 μm emissivity peak and the 9.7 μm reststrahlen trough are readily identifiable, but the 8.9 and 11.1 μm peaks and the 8.8, 9.3, and 18.9 μm troughs are only marginally identifiable. This indicates that identification of clays on Mars, using midinfrared data, should rely upon the sensitivity of the 19.9 and 9.7 μm features to the clays' presence.
Isotopic composition of carbonates in the SNC meteorites, Allan Hills 84001 and Zagami
Authors: Jull A. J. T. , Eastoe C. J. , Cloudt S.
Published: February 2004

We have measured the 13C/12C and 14C/12C ratios in CO2 released by acid etching of the meteorites Zagami and Allan Hills 84001. We use the 14C as a label to identify extraterrestrial carbonate phases, as they will have a low 14C/12C ratio (<∼4% modern terrestrial ratio) compared to recent terrestrial material. The new studies on Allan Hills 84001 confirm previous conclusions that the Fe, Mg‐rich carbonate grains in this meteorite contain carbon with δ13C as high as +45‰. In contrast, the carbon released from Zagami is depleted in 13C with δ13C as low as ∼20‰. We conclude that the isotopic composition of the carbon as carbonate released from acid etching of Zagami is different from the carbonates observed in both Allan Hills 84001 and Nakhla. With the assumption that all of these meteorites sample the surface of Mars, we propose that the Zagami carbonate samples a different carbon reservoir on this planet, such as a magmatic source. With this interpretation, the high δ13C values of carbonate observed in Allan Hills 84001 and Nakhla can be ascribed to a fractionated source compared with the originally light carbon. A likely origin for this 13C‐enriched component is an isotopically heavy Martian atmosphere; however, given the possibility of biological activity involving Allan Hills carbonates, we cannot exclude this as a source of the isotopic fractionation.
Cat Mountain: A meteoritic sample of an impact‐melted asteroid regolith
Authors: Kring David A. , Swindle Timothy D. , Britt Daniel T. , Grier Jennifer A.
Published: February 2004

Cat Mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule‐bearing clasts of L5 material. These clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years, probably within a melt breccia lens in the bottom of a large (>1 km diameter) crater on an L chondrite asteroid. Noble gas isotopes indicate that the sample was involved in at least two different impact events, approximately 880 and 20 Myr ago, following the 4.55 Ga accretion of primitive chondritic material. The 880 Ma event is responsible for the impact breccia texture of the sample, and the 20 Ma event reduced the sample to a meter‐sized object. We also infer that another impact occurred between 880 and 20 Ma (possibly the ∼500 Ma event recorded in many other L chondrites) to jettison the material from the asteroid belt into an orbit that evolved into an Earth‐crossing trajectory. The shock‐metamorphic processes that occurred at 880 Ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases. This reduced the reflectance of the L5 material and decreased the amplitude of its spectral absorption features. These characteristics are consistent with the spectral characteristics of some C class asteroids and suggest that some dark asteroids that appear to belong to the C class could be covered with shocked ordinary chondrite material. If one assumes that Cat Mountain came from the same asteroid as other L chondrites with the same cosmic ray exposure age, then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m. Furthermore, the structural integrity of Cat Mountain and other L chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere. However, in a regime where the asteroid is the target of impact fragmentation rather than the projectile, the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy. In this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material.
Provenance of icebergs during Heinrich Event 3 and the contrast to their sources during other Heinrich episodes
Authors: Gwiazda R. H. , Hemming S. R. , Broecker W. S.
Published: February 2004

The sources of ice‐rafted debris (IRD) from Heinrich layer 3 (H 3) in the North Atlantic were studied by means of Pb isotopic analyses of single‐ and multiple‐grain samples of ice‐rafted feldspars from core V28‐82 (49° 27′N, 22° 16′W). Pb ratios of H 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment. This similarity, as well as previous observations about the age and the Sr and Nd isotopic composition of the fine fraction of H 3 in cores of the same area, are all consistent with the hypothesis that in the eastern North Atlantic along the latitude band of maximum IRD deposition (40°–50°N), H 3 is a product of foraminifera dissolution and/or low productivity. In contrast, analyses of composite samples of H 1, H 2, H 4, and H 5 plot in 206Pb/204Pb versus 206Pb/204Pb coordinates along the Churchill province line, supporting the premise that the Hudson Strait was the major pathway of iceberg drainage during these four events. Since an advance of the Laurentide ice sheet during H 3 is demonstrated by increases in detrital carbonate at the level of H 3 in cores from the western North Atlantic, the results obtained here suggest that if indeed that advance took place, the iceberg discharge melted mostly in the western basin.
Panthalassa: paleotidal resonance and a global paleocean Seiche
Authors: Archer Allen W.
Published: February 2004

Various paleotidal periods can be extracted from analyses of cyclic tidal rhythmites, which are common in upper Carboniferous (∼300 Ma) rocks of the eastern and midcontinental United States. These periods provide details of the paleotidal regime that controlled sedimentation during deposition of the rhythmites. In addition to shorter‐term diurnal/semidiurnal periods and longer‐term neap‐spring tidal periods, some rhythmites have unusual intermediate periods of approximately 50 hours. These intermediate periods are not readily explainable by any known completely tidal period. It is hypothesized that this 50‐hour period could be the product of a basinal resonance, or seiche. Herein estimates are made regarding the required basin depths and widths necessary to produce a seiche of this period. These estimates are based upon paleogeographic reconstructions of the global paleocean, Panthalassa, that surrounded Pangea. These reconstructions provide basinal paleowidth estimates; subsequently, paleodepths can be extrapolated from the depth‐to‐width relationships in modern oceans. Combining such fine‐scale tidal‐rhythmite periodicities with large‐scale paleogeographic information provides many useful constraints for modeling of Pangean tidal systems. Forward modeling of global tides, based upon very large scale paleogeographic controls, when combined with reverse modeling, based upon fine‐scale, detailed rhythmite‐period analysis, allows detailed calibration of the paleotidal‐systems response. On the basis of available fine‐ and large‐scale information, the unusual 50‐hour period that occurs within cyclic rhythmites is most likely related to a global paleocean seiche.
Evaluation of sampling resolution in coral stable isotope records: A case study using records from New Caledonia and Tarawa
Authors: Quinn Terrence M. , Taylor Frederick W. , Crowley Thomas J. , Link Suzanne M.
Published: February 2004

We have generated a 40‐year‐long, monthly stable isotope record from a Porites lutea coral collected offshore of Amedee, New Caledonia (22°S, 167°E) to investigate the relation between sampling resolution in coral isotope studies and retrieval of sea‐surface environmental information. We interpret the high correlation between our oxygen isotope record and a twenty‐year long sea‐surface temperature record at the monthly timescale (r=0.88) to indicate that our coral isotope record is an accurate monitor of environmental conditions offshore of Amedee. The character of the signal and the percent variance explained in the record at the annual band, at the quasi‐biennial oscillation band ((QBO) 2.0‐2.4 years), and at the El Nino‐Southern Oscillation band ((ENSO) 3–8 years) changes little in response to a reduction in sampling density from monthly to bimonthly to quarterly. Similar results have been obtained in a reanalysis of a coral isotope record from Tarawa, Kiribati. Our results indicate that a significant amount of the information obtained from high‐density sampling can also be retrieved from lower‐density sampling. In particular, bimonthly sampling yields virtually no drop‐off in variance explained, and quarterly sampling is satisfactory for resolving interannual and decadal‐scale trends in time series. The proposed sampling approach may enable a more rapid filling in of numerous spatial holes in coral sampling sites needed for reconstruction of long‐term decadal‐scale variations in climate.
Ventilation changes in the northeast Pacific during the Last Deglaciation
Authors: van Geen A. , Fairbanks R. G. , Dartnell P. , McGann M. , Gardner J. V. , Kashgarian M.
Published: February 2004

Under present climate conditions, convection at high latitudes of the North Pacific is restricted to shallower depths than in the North Atlantic. To what extent this asymmetry between the two ocean basins was maintained over the past 20 kyr is poorly known because there are few unambiguous proxy records of ventilation from the North Pacific. We present new data for two sediment cores from the California margin at 800 and 1600 m depth to argue that the depth of ventilation shifted repeatedly in the northeast Pacific over the course of deglaciation. The evidence includes benthic foraminiferal Cd/Ca, 18O/16O, and 13C/12C data as well as radiocarbon age differences between benthic and planktonic foraminifera. A number of features in the shallower of the two cores, including an interval of laminated sediments, are consistent with changes in ventilation over the past 20 kyr suggested by alternations between laminated and bioturbated sediments in the Santa Barbara Basin and the Gulf of California [Keigwin and Jones, 1990; Kennett and Ingram, 1995; Behl and Kennett, 1996]. Data from the deeper of the two California margin cores suggest that during times of reduced ventilation at 800 m, ventilation was enhanced at 1600 m depth, and vice versa. This pronounced depth dependence of ventilation needs to be taken into account when exploring potential teleconnections between the North Pacific and the North Atlantic.
Vanadium in foraminiferal calcite as a tracer for changes in the areal extent of reducing sediments
Authors: Hastings David W. , Emerson Steven R. , Mix Alan C.
Published: February 2004

We have used the vanadium concentration in cleaned foraminiferal calcite as a tracer of seawater V changes in the past. Since the benthic flux of vanadium is sensitive to the redox potential of sediments, changes in the vanadium concentration of seawater should be a reflection of changes that control the redox state of sediments. V/Ca in G. sacculifer from an eastern equatorial Atlantic core (ENO66‐17GGC) is 21.6 (±2.8) nmol V/mol Ca. This value does not change over 35 kyr, indicating that there was no measurable change in seawater vanadium levels over this period. Potential artifacts from partial dissolution are not significant based on low, constant values for foraminiferal fragmentation (6–7%) in the top 50 cm of the core. A minor correction to account for vanadium associated with Mn carbonate overgrowths, estimated from two Caribbean cores where this mixed phase dominates the deeper V/Ca values, has been applied. Changes in the areal extent of anoxic and suboxic sediments are thus constrained by this constant value and the standard deviation of the measurement, ±12%. Based on a mass balance for vanadium where suboxic sediments are a source and anoxic sediments are a sink to the ocean, suboxic sediments are predicted to have changed by no more than 0.5–1.5 times the current value, assuming no change in the areal extent of anoxic sediments. This corresponds to 1.3–3.5% of total ocean sediments. Given a constant area of suboxic sediments, the areal extent of anoxic sediments did not increase by more than fivefold, or 1.5% of the ocean floor over the past 35 kyr. The significant reductions in deep water oxygen levels and consequent changes in sediment redox conditions required by polar nutrient depletion scenarios are not reflected in the foraminiferal vanadium data over the past 35 kyr. This suggests that models which call on less or no changes in deep water oxygen are more likely alternatives.
The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content
Authors: Russell Ann D. , Emerson Steven , Mix Alan C. , Peterson Larry C.
Published: February 2004

We examine the utility of the uranium (U) content of planktonic foraminifera tests as an indicator of past changes in seawater U content. The U/Ca ratio in foraminifera from Atlantic and Caribbean cores is constant in the Holocene and decreases by ∼25% during the last glacial period. Magnesium/calcium (Mg/Ca) ratios of the same samples show similar trends. While the timing of the U/Ca changes appears to be associated with glacial‐interglacial changes, the magnitude of the change is too large to be caused by variations in the extent of anoxic or suboxic sediments or by changes in riverine input. We assume that the same process produced changes in both U/Ca and Mg/Ca ratios because of a strong correlation between the two ratios. Partial dissolution of the calcite is ruled out, because we observe the same changes in well‐preserved cores from basins with opposite dissolution histories. We also reject exchange between foraminiferal and pore water U because of the oxic depositional environment of both cores and because of the consistency in the U/Ca trends from cores in different parts of the ocean. We suggest that the observed foraminiferal U/Ca and Mg/Ca trends may be the result of a temperature effect on the incorporation of these metals. If this is true, it introduces the possibility of a new paleotemperature indicator but complicates the use of U/Ca ratios in planktonic foraminifera tests as an indicator of past seawater U changes.
Decoupling of carbonate preservation, carbonate concentration, and biogenic accumulation: A 400‐kyr record from the central equatorial Pacific Ocean
Authors: LaMontagne R. W. , Murray R. W. , Wei K. ‐Y. , Leinen M. , Wang C. ‐H.
Published: February 2004

In order to investigate the paleoceanographic record of dissolution of calcium carbonate (CaCO3) in the central equatorial Pacific Ocean, we have studied the relationship between three indices of foraminiferal dissolution and the concentration and accumulation of CaCO3, opal, and Corg in Core WEC8803B‐GC51 (1.3°N, 133.6°W; 4410 m). This core spans the past 413 kyr of deposition and moved in and out of the lysoclinal transition zone during glacial‐interglacial cycles of CaCO3 production and dissolution. The record of dissolution intensity provided by foraminiferal fragmentation, the proportion of benthic foraminifera, and the foraminiferal dissolution index consistently indicates that the past corrosion of pelagic CaCO3 in the central equatorial Pacific does not vary with the observed sedimentary concentration of CaCO3. Although there is a weak low‐frequency variation (∼100 kyr) in dissolution intensity, it is unrelated to sedimentary CaCO3 concentration. There are many shorter‐lived episodes where high CaCO3 concentration is coincident with poor foraminiferal preservation, and where, conversely, low CaCO3 concentration is coincident with superb foraminiferal preservation. Spectral analyses indicate that dissolution maxima consistently lagged glacial maxima (manifest by the SPECMAP δ18O stack) in the 100‐kyr orbital band. Additionally, there is no relationship between dissolution and the accumulation of biogenic opal or Corg or between dissolution and the burial ratio of Corg/CINorg (calculated from Corg and CaCO3). Because previous studies of this core strongly suggest that surface water productivity varied closely with CaCO3 accumulation, both the mechanistic decoupling of carbonate dissolution from CaCO3 concentration (and from biogenic accumulation) and the substantial phase shift between dissolution and global glacial periodicity effectively obscure any simple link between export production, CaCO3 concentration, and dissolution of sedimentary CaCO3.
An iceberg drift and decay model to compute the ice‐rafted debris and iceberg meltwater flux: Application to the interglacial North Atlantic
Authors: Matsumoto Katsumi
Published: February 2004

An iceberg drift and decay model that computes the long‐term ice‐rafted debris (IRD) and iceberg meltwater flux over an entire ocean basin is presented. The model requires atmospheric and oceanic flow fields and has three main operations: iceberg drift, decay, and debris release. Using present atmospheric and oceanic flow fields in the North Atlantic, the model is able to reproduce modern iceberg drift paths and seasonal iceberg occurrences. Using the same flow fields, IRD and iceberg meltwater flux to the North Atlantic are computed. Core‐top data do not preserve an adequate record of present‐day IRD distribution on the ocean floor; thus modeled IRD results are compared with IRD results from marine isotopic stage 5e (the last interglacial), a period most similar to present interglacial conditions. Similarity between the modeled and observed IRD patterns confirms that present ocean surface conditions affecting iceberg drift and decay are similar to those of stage 5e. Detailed comparison reveals icebergs from stage 5e reaching as far east as 20°W, which is not reproduced by the model under existing oceanographic conditions. This discordance suggests that the 5e IRD data set includes deposits from times colder than today, either because of truly colder intervals in 5e or because of dating uncertainties in the data. Modeled meltwater flux to the North Atlantic exhibits large seasonal and spatial variations. Using results from a recent study of North Atlantic Deep Water (NADW) circulation sensitivity to freshwater forcing and assuming a steady Greenland ice volume, iceberg meltwater forcing is insufficient during interglacial conditions to produce even a partial NADW collapse.
Environmental rock‐magnetism of pelagic clay: Implications for Asian eolian input to the North Pacific since the Pliocene
Authors: Yamazaki Toshitsugu , Ioka Noboru
Published: February 2004

We conducted a rock‐magnetic study of pelagic sediments in order to document variations of Asian eolian input to the North Pacific since the Pliocene. The materials studied consist of five pelagic‐clay (red‐clay) cores of several meters long taken from the central North Pacific and surface sediments of box cores obtained at 40 sites along two lines, a N‐S line along 175°E and an E‐W line along 20°N. The magnetic susceptibility (concentration of magnetic minerals) and S ratio (relative abundance of low‐coercivity magnetic minerals) of the five pelagic‐clay cores have decreased synchronously since about 2.5 Ma. This age coincides with the onset of the northern hemisphere glaciation and Chinese loess deposition. Geographical distribution of S ratios measured on the surface sediments revealed that they are remarkably low in the central North Pacific. These facts suggest that S ratio can be used as a proxy for Asian eolian dust. Mössbauer spectroscopy showed that hematite dominates the high‐coercivity minerals. Observation with a transmission electron microscope suggests two sources of magnetic minerals in the pelagic clay: biogenic magnetite produced in situ and detrital magnetite and hematite. The variations of S ratio and magnetic susceptibility with time and region can be explained by a model that Asian eolian dust has higher (lower) hematite (magnetite) concentration than other sources of magnetic minerals and that its input to the central North Pacific has increased since 2.5 Ma. We recognized subtle S‐ratio fluctuations of about 400 kyr in recurring intervals superimposed on the decreasing trend since 2.5 Ma. Rock‐magnetic parameters diagnostic of magnetic grain size, on the other hand, showed small or no variations with age, although eolian grain size is generally thought to have increased significantly since the Pliocene based on sedimentological studies.
Late Quaternary high‐resolution biomarker and other sedimentary climate proxies in a Northeast Atlantic Core
Authors: Madureira Luiz A. S. , van Kreveld Shirley A. , Eglinton Geoffrey , Conte Maureen H. , Ganssen Gerald , van Hinte Jan E. , Ottens Janneke J.
Published: February 2004

We studied variations in terrigenous input and sea surface temperature over the last 208 ka using a multiproxy approach, involving direct comparison of terrigenous and marine molecular biomarker, foraminiferal abundance, and other sedimentological data for the same horizons in a single northeast Atlantic core (core T88‐9P; 48°23′N, 25°05′W, 3193 m water depth, 790 cm core length) with a well‐resolved δ18O stratigraphy. The abundances of three land‐derived, long‐chain compound classes, then‐alkanes (C27, C29, and C31), then‐alkanoic acids (C24, C26, and C28), and then‐alkanols (C24, C26, and C28) show glacial/interglacial related changes in the past 208 ka which generally parallel the weight percent noncarbonate. The abundances of these three compound classes are higher in glacial than in interglacial sediments. Such changes are consistent with the expected higher dust inputs during glaciations as well as enhanced advection of particulate matter in current systems. Ice‐rafted debris (Heinrich events) may also contribute to the terrigenous biomarker record. The sea surface temperature (SST) signals derived from the alkenone unsaturation index (UK′37) give an average SST during interglacial periods of 13°C, which is about 3–4°C higher than the average glacial SST estimate. The UK′37−derived SST values dropped markedly in some Heinrich layers, while sometimes leading the onset of the layer; the percentNeogloboquadrina pachyderma(s), which is another frequently used qualitative temperature proxy, generally shows maxima coinciding with the layers. Our results show that most Heinrich layers were produced during times of intensely cold waters in the eastern North Atlantic. They demonstrate that sufficient alkenones are present in the sediments for alkenone stratigraphy to be used as a tool for the assessment of short‐term variations in sea surface temperatures even in these oligotrophic North Atlantic waters.
Three‐dimensional electromagnetic field computation by the multiscalar boundary element method
Authors: Liu Eric H. , Lamontagne Yves
Published: February 2004

A new frequency domain iterative three‐dimensional surface integral equation method (SIE) is proposed and tested in a number of electromagnetic computation problems. In contrast to the usual surface current SIE formulation, it is formulated by applying the scalar Green's theorem to the three Cartesian components of the magnetic field independently. The boundary conditions are implemented iteratively. Numerical results are given in three areas of electromagnetic computation: eddy current induction, EM wave scattering off dielectric targets, and the EM response of a conductor in a conductive host. Some of our numerical results involve two boundaries. Comparisons are made with previously published results where available, and close quantitative agreement is found. The numerical results show that the method can handle a very wide range in conductivity (up to 1015) and in dielectric constant (up to 150) without encountering numerical instability. Whether the method will always yield a convergent solution for different kinds of EM problems is yet to be studied, but for problems for which convergence can be achieved, we found the method has advantages in terms of computer operations and storage over existing techniques.
Ionospheric tomography using the residual correction method
Authors: Sutton Eric , Na Helen
Published: February 2004

Ionospheric tomography systems provide data that can be used to reconstruct images of ionospheric electron density. Since ionospheric tomography systems have fundamentally poor vertical resolution, a priori information on the vertical distribution of ionospheric electron density must be used in the reconstruction algorithm. This can be accomplished using an existing technique called orthogonal decomposition. However, the poor vertical resolution of the imaging system makes the reconstruction problem numerically ill conditioned. This paper presents a new ionospheric reconstruction algorithm called the residual correction method (RCM). The RCM is a fast, efficient, and numerically stable ionospheric tomography algorithm. This paper will present results demonstrating the performance of the RCM algorithm using a realistic example.
Examination of the oxygen red line signature of a polar cap ionization patch as seen from the Dynamics Explorer 2 satellite
Authors: Garner T. W. , Killeen T. L. , Burns A. G. , Winningham J. D. , Coley W. R.
Published: February 2004

Ground‐based observations of polar cap ionization patches have been made for many years. Optical measurements indicate that enhancements occurring in the 630.0‐nm emission line are correlated with these ionization patches. The present study examines data from the Fabry‐Perot interferometer (FPI) on the Dynamics Explorer 2 spacecraft to determine if patches can be observed optically from space. Since the FPI scanned the forward horizon, it was able to observe the vertical profile of airglow emissions associated with patches, thus giving information about the vertical structure. FPI measurements have been correlated with data from several in situ instruments on the spacecraft. These in situ measurements include electron density, atomic oxygen density, electron temperature, neutral temperature, and electron energy flux. Three polar cap patches have been detected both by the FPI and the in situ instruments. The red line emission patches are enhancements in the 630.0‐nm red line in a specific altitude region which create an irregular altitude profile. The red line emission region associated with polar cap ionization patches varies from patch to patch and occurs in the altitude range from below 300 km to 400 km.
Three‐dimensional quasi‐linear electromagnetic inversion
Authors: Zhdanov Michael S. , Fang Sheng
Published: February 2004

One of the most challenging problems of electromagnetic (EM) geophysical methods is developing three‐dimensional (3‐D) EM inversion techniques. This problem is of utmost importance in practical applications because of the 3‐D nature of the geological structures. The main difficulties in 3‐D inversion are related to (1) limitations of 3‐D forward modeling codes available and (2) ill‐posedness of the inversion procedures in general. The multidimensional EM inversion techniques existing today can handle only simple models and typically are very time consuming. We developed a new approach to a rapid 3‐D EM inversion. The forward scattering problem is solved using a new quasi linear (QL) approximation of the existing integral equation algorithms, developed for various sources of excitation. The QL approximation for forward modeling is based on the assumption that the anomalous field is linearly related to the normal field in the inhomogeneous domain by an electrical reflectivity tensor. We introduce also a modified material property tensor which is linearly proportional to the reflectivity tensor and the complex anomalous conductivity. The QL approximation generates a linear equation with respect to the modified material property tensor. The solution of this equation is called “a quasi‐Born inversion”. We apply the Tikhonov regularization for the stable solution of this problem. The next step of the inversion includes correction of the results of the quasi Born inversion: after determining a modified material property tensor, we use the electrical reflectivity tensor to evaluate the anomalous conductivity. Thus the developed inversion scheme reduces the original nonlinear inverse problem to a set of linear inverse problems, which is why we call this approach “a QL inversion”. Synthetic examples (with and without random noise) of inversion demonstrate that the algorithm for inverting 3‐D EM data is fast and stable.
An exact formulation for the electromagnetic fields of a cylindrical antenna with a triangular current distribution
Authors: Werner D. H. , Huffman J. A. , Werner P. L.
Published: February 2004

A mathematically exact formulation for the vector potential and corresponding electromagnetic fields of a triangular current cylindrical dipole are presented for the first time in this paper. These exact expressions converge rapidly in the near‐field region of the antenna allowing them to be used for the efficient and accurate computational modeling of electrically short cylindrical antennas. The exact series expansion of the triangular current vector potential is shown to contain two fundamental exponential integrals and their higher‐order associated integrals. Numerically stable forward recurrence relations have been derived which may be used for the efficient evaluation of these higher‐order integrals in addition to the cylindrical wire kernel. These recursions may also be employed in the computation of the electric and magnetic fields. It is demonstrated that the classical thin wire forms of the vector potential and electromagnetic fields are actually special cases of the more general exact expansions. Finally, the exact formulation was used to investigate the near‐field behavior of traditional thin wire as well as moderately thick wire dipoles. Several near‐field plots are presented including a vector plot of the total electric field in the vicinity of a moderately thick quarter‐wavelength dipole.
New improper real and complex solutions for printed‐circuit transmission lines and their influence on physical effects
Authors: Shigesawa Hiroshi , Tsuji Mikio , Oliner Arthur A.
Published: February 2004

For most printed‐circuit transmission lines under most conditions of operation, the dominant mode on the line is purely bound at lower frequencies and becomes leaky at higher frequencies. We reported recently that by changing only the relative line dimensions (such as a strip width or a slot width) the bound and leaky modes can propagate simultaneously over a frequency range instead of being separate. More recently, we showed that this effect is general, occurring for many different line structures. The explanation for this initially unexpected behavior remained obscure, however. We report here the discovery of two new improper (or nonspectral) modal solutions, one real and one complex, which are in themselves completely nonphysical but whose evolution as a function of dimensional change serves to explain how the simultaneous propagation effect can occur. In this paper we present this evolution of behavior and also describe the unusual field properties of these two new improper modes.
Nonlinear hybrid vector finite element method for modeling wave propagation in nonlinear media
Authors: Polstyanko Sergey V. , Lee Jin‐Fa
Published: February 2004

In this paper a unified computer‐aided approach based on the vector finite element method is proposed to study the propagation of optical waves in dielectric nonlinear waveguides. Most of the existing methods are based on the scalar Helmholtz wave equation and cannot adequately describe the vectorial properties of fields in nonlinear optical structures. To take into account vectorial field properties and properties of the medium without limitations, a new nonlinear hybrid vector finite element method based on the vectorial wave equation is developed. Several numerical examples are given to demonstrate the applicability of this method.
A hybrid finite element integral equation approach for the eigenmode analysis of complex anisotropic dielectric waveguides
Authors: Rogier Hendrik , Olyslager Frank , De Zutter Daniël
Published: February 2004

This paper presents a full‐wave hybrid approach to analyze complex (lossy) dielectric open waveguides. The configuration consists of inhomogeneous and homogeneous regions and is bounded by open space. The inhomogeneous regions are analyzed by a finite element method and the homogeneous ones by a boundary integral equation technique. The presented method makes efficient use of the matrix sparsity originating from the finite elements. The technique is applied to obtain the dispersion characterics and eigenfield patterns of modes propagating in anisotropic waveguides.
Finite element solution of dielectric‐ferrite resonators
Authors: Dillon Bernice M. , Gibson Andrew A. P.
Published: February 2004

Planar ferrite resonators are widely used in microwave and millimeter‐wave junction circulators. Recent trends toward broadbanding and miniaturization of devices have led to interest in novel inhomogeneous structures. To simplify design procedures, a finite element package for the calculation of modal resonant frequencies in terms of a specified applied bias field and material magnetization has been developed. The frequency dependence of the ferrite tensor components is included in the analysis, and so the resulting eigenvalue problem is nonlinear. An iterative finite element scheme is used to solve this problem for resonators with arbitrary geometries, making full use of the symmetry of the modes. For the special case of homogeneous resonators of arbitrary shape the calculations may be simplified through the use of normalized parameters. To illustrate the technique and the limitations of the use of normalized parameters, a homogeneous ferrite ring resonator and an inhomogeneous dielectric‐ferrite ring resonator have been analyzed. In both cases, good agreement with the exact solution is obtained. Calculations are also presented for a practical circulator geometry with threefold symmetry: a dielectric triangle inserted within a ferrite disk.
Resonance characteristics on a dielectric disk resonator coupled with a straight waveguide
Authors: Tomabechi Y. , Hwang J. , Matsumura K.
Published: February 2004

Resonance characteristics of whispering gallery modes on a dielectric disk coupled with a straight waveguide are investigated theoretically and experimentally. Taking account of a phase perturbation caused by a coupling phenomenon, we analyze the resonance characteristics of the disk resonator with the straight waveguide. The straight waveguide is placed near the disk to excite the whispering gallery modes or to detect an output power from the disk. In a region where a gap between the straight waveguide and the disk is small, a phase distribution of an electromagnetic field is perturbed by the coupling phenomenon. The coupling phenomenon is analyzed by a coupled mode equation for nonparallel dielectric waveguides. As the result, a discrepancy of resonance frequency intervals (free spectral range) between the analytical results and experimental ones is several megahertz in a 24‐GHz band. It is also found that a relative error between the analytical and experimental resonance frequencies is about 1.3%. The analytical method and the results will be applied to the precise design of a high Q resonator with an external circuit in millimeter to optical range.