bluuebunny/crossref_metadata_2025 huggingface dataset:
The FIFE surface diurnal cycle climate
Authors: Betts Alan K. , Ball J. H.
Published: February 2004

We analyze the diurnal cycle of the 2‐m thermodynamic data averaged over the First International Land Surface Climatology Project (ISLSCP) Field Experiment site near Manhattan, Kansas, during 1987, using supporting soil moisture data, surface flux data, rainfall, and cloud information. Conserved variable plots are our primary analysis method. We present a summer mean, stratified into dry and wet days, and the monthly seasonal cycle. Further stratifications indicate the control of soil moisture on the surface evapotranspiration, vegetative conductance, and mean diurnal cycle for the boundary layer. We extract composite data sets for the daytime diurnal cycle over grassland in midsummer as a function of soil moisture and show that these are consistent with a mixed layer model for a rapidly entraining boundary layer.
ROCOZ‐A ozone measurements during the Stratospheric Ozone Intercomparison Campaign (STOIC)
Authors: Barnes Robert A. , Parsons Chester L. , Grothouse Arthur P.
Published: February 2004

We present a set of ROCOZ‐A (rocket ozonesonde) ozone measurements during the October/November 1988 (pre‐STOIC) and the July/August 1989 Stratospheric Ozone Intercomparison Campaign (STOIC) in southern California. ROCOZ‐A and its associated electrochemical concentration cell (ECC) ozonesondes participated in the comparisons as established techniques for the validation of lidar and microwave instruments that have been proposed for the Network for the Detection of Stratospheric Change (NDSC). For the proposed network instruments, STOIC has provided a picture of their performance characteristics in 1989 and has given an estimate of their future performance in the NDSC. For ROCOZ‐A, STOIC has added new information on its accuracy and precision. It is this continuing characterization that gives ROCOZ‐A its value in comparisons. The STOIC comparisons show a shift of 5–6% in ROCOZ‐A ozone densities (ROCOZ‐A higher) from October/November 1988 to July/August 1989. This shift has been seen in comparisons with the Stratospheric Aerosol and Gas Experiment II (SAGE II), ECC ozonesondes, and the Jet Propulsion Laboratory (JPL) lidar. The source of this shift has not been determined. Until this new error source is resolved, we recommend that the previously quoted accuracy estimate for ROCOZ‐A ozone measurements be increased from 5–7% to 8–10%. About 2% of the difference between ROCOZ‐A ozone measurements and those from the proposed network instruments in 1989 appears to be due to differences in atmospheric ozone between the two STOIC sites. A correction for these site‐to‐site differences brings the ROCOZ‐A ozone measurements within 10% of all of the other STOIC instruments, and the average agreement (ROCOZ‐A 6% higher) becomes consistent with the historical set of ROCOZ‐A comparisons. The STOIC comparisons have shown structures in stratospheric ozone that cannot be resolved by ROCOZ‐A with its 4‐km vertical resolution. In addition, comparisons with nighttime measurements from the Millitech microwave above 45 km show a divergence from the daytime ROCOZ‐A values that agrees with the general characteristics of the diurnal cycle in upper stratospheric ozone that is predicted by photochemical models. Evidence of this ozone cycle is also seen in ROCOZ‐A comparisons with the SAGE II zonal mean above 45 km but not in comparisons with the nighttime measurements from the JPL lidar. The effects of diurnal ozone change in the upper stratosphere and mesosphere will be an important consideration in future comparisons of NDSC instruments.
Investigation of OH absorption cross sections of rotational transitions in the band under atmospheric conditions: Implications for tropospheric long‐path absorption measurements
Authors: Dorn H.‐P. , Neuroth R. , Hofzumahaus A.
Published: February 2004

The accuracy of tropospheric hydroxyl radical measurements by long‐path absorption spectroscopy is ultimately limited by the uncertainty of the effective OH absorption cross sections. The latter were determined from calculated spectra for the Q1(2), Q1(3), and P1(1) rotational lines of the OH

transition at 308 nm. The calculations took into account Doppler broadening, measured data of the collisional broadening of OH by air molecules, and instrumental line broadening effects. The calculated spectra were compared with OH absorption spectra measured in a flow reactor near room temperature at 1013 hPa. Excellent agreement between calculated and measured OH spectra was achieved using the collision‐broadening parameters determined recently by Leonard (1990). The effective absorption cross sections at the line center (peak absorption cross sections) calculated for the Voigt line shape at 300 K and 1013hPa are:
,
,
. The full width at half maximum of the spectral lines is 2.5 pm, and the shape factor a is 1.41. Calculations of the effective absorption cross section under different atmospheric conditions demonstrate a strong pressure dependence for all lines. At 2800 m altitude (720 hPa) the peak absorption cross section of the OH Voigt lines is about 35% higher than at sea level. The temperature dependence in an interval of ±20 K around room temperature (at 1013 hPa) is only small (±3% for the Q lines and ±6% for the P1(1) line). The estimated total uncertainty of the effective absorption cross section is 8%.
A change of upper cloud structure in Jupiter's South Equatorial Belt during the 1989–1990 event
Authors: Satoh Takehiko , Kawabata Kiyoshi
Published: February 2004

We have analyzed two data sets of Jupiter's limb darkening taken before (March 1990) and after (March 1991) a South Equatorial Belt (SEB) disturbance. The vertical aerosol structure has been determined for the SEB, the Equatorial Zone (EZ), and the North Equatorial Belt (NEB). We find that all regions can be well represented with a two‐cloud layer model, with slightly higher NH3‐cloud top altitude in the EZ than in the two belts. The cloud single‐scattering albedos
(determined within ±0.003) show little difference in the EZ (from 0.9956 to 0.9952) and the NEB (from 0.9925 to 0.9926) during a 1‐year period, while a relatively larger decrease is found in the SEB from 0.9975 to 0.9957, indicating the recovery of the belt through the disturbance. The NH3‐cloud top altitude in the SEB shows the most prominent change, lower in 1990 and higher in 1991. We suggest that the brightening and the darkening of the SEB may originate not only from the changes of
but also from changes of the ammonia condensation level in the atmosphere, probably due to a decrease of the tropospheric temperature and/or an increase of the ammonia vapor.
Gravity survey of the Mount Toondina impact structure, South Australia
Authors: Plescia J. B. , Shoemaker E. M. , Shoemaker C. S.
Published: February 2004

Gravity and seismic reflection data, together with geologic mapping, indicate that the Mount Toondina feature in South Australia is best interpreted as an eroded 4‐km‐diameter impact structure consisting of a ring structural depression surrounding a pronounced central uplift. Beds at the center of the structure within the central uplift have been raised as much as 200 m from depth and deformed by convergent flow. Seismic reflection data indicate that deformation extends to depths of only ≈800 m; at greater depths the reflectors are nearly flat lying, indicating little or no deformation. Gravity data show residual anomalies of +1.0 mGal coincident with the central uplift and a −0.5 Mgal low associated with the ring structural depression. Modeling of the gravity data indicates that relatively high‐density material occurs within the central uplift, whereas the ring depression is filled with low‐density material. The deformation at Mount Toondina is typical of a complex impact crater; the 4‐km diameter is consistent with the expected threshold size for complex craters formed in weak to moderate strength sedimentary rocks.
Mid‐UV spectroscopy of propynenitrile at low temperature: Consequences on expected results from observations of Titan's atmosphere
Authors: Bénilan Yves , Bruston Paul , Raulin Francois , Cossart‐Magos Claudina , Guillemin Jean‐Claude
Published: February 2004

The interpretation of albedo spectra in the mid‐UV range is critically dependent on the knowledge of the absolute absorption coefficient, at specific temperatures and with high resolution for molecular compounds likely to absorb the scattered solar radiation in this wavelength range. Propynenitrile, a key molecule in prebiotic synthesis, already detected with IR and microwave sounding in Titan's atmosphere, exhibits intense absorption bands in the 185 to 230 nm interval. This paper deals with the absolute absorption coefficient k of propynenitrile at low temperature and high resolution and compares it to the room temperature values. The consequences of the observed k variations on the interpretation of Titan's atmosphere spectra, expected in the near future, are considered here. As an additional result, the pressure regulation system of the experiment allows us to assess the determination of the temperature dependence of the propynenitrile vapor pressure.
Belt‐zone variations in the Jovian cloud structure
Authors: Carlson Barbara E. , Lacis Andrew A. , Rossow William B.
Published: February 2004

Voyager IRIS observations of Jupiter in the far infrared (180–1200 cm−1) are sensitive to emission originating from pressures less than 2 bars, while the 5‐μm (1800–2300 cm−1) observations are primarily sensitive to emission originating from pressures greater than 2 bars. We use these differences in the location of the peak emission level to constrain the properties of the upper tropospheric cloud structure from the far‐infrared observations and then use the 5‐μm observations, with the upper tropospheric cloud structure fixed, to constrain the deep cloud structure. The relationship between observed 45‐ and 5‐μm brightness temperatures reveals three distinct regions between ±25° latitude: North Equatorial Belt hot spots, which are “hot” at both 45 and 5 μm; Equatorial Zone spectra, which are “warm” at 45 μm and “cold” at 5 μm; and North Tropical Zone spectra, which are “cold” at both 45 and 5 μm. We find that the hot extreme spectral ensemble is unique to belts, and the cold extreme spectral ensembles are unique to zones, but that all other intermediate spectral ensembles are common to both regions. Analyses of these spectra using an anisotropic multiple scattering radiative transfer model reveal that the primary difference between belts and zones is the increased opacity and vertical extent of clouds in zones relative to their belt counterparts. In addition, we find a shift in the location of the para hydrogen gradient toward lower pressures in zones, with an increase in the “equilibrated” cloud‐top para fraction. We suggest that all of the variations in gas abundance profiles, temperature, and cloud structure are consistent with the effects of a simple mean circulation and large‐scale wave motions.
Mid‐infrared transmission spectra of crystalline and nanophase iron oxides/oxyhydroxides and implications for remote sensing of Mars
Authors: Bell James F. , Roush Ted L. , Morris Richard V.
Published: February 2004

Ferric‐iron‐bearing materials play an important role in the interpretation of visible to near‐IR Mars spectra, and they may play a similarly important role in the analysis of new mid‐IR spacecraft spectral observations to be obtained over the next decade. We review existing data on mid‐IR transmission spectra of ferric oxides/oxyhydroxides and present new transmission spectra for ferric‐bearing materials spanning a wide range of mineralogy and crystallinity. These materials include 11 samples of well‐crystallized ferric oxides (hematite, maghemite, and magnetite) and ferric oxyhydroxides (goethite, lepidocrocite). We also report the first transmission spectra for purely nanophase ferric oxide samples that have been shown to exhibit spectral similarities to Mars in the visible to near‐IR and we compare these data to previous and new transmission spectra of terrestrial palagonites. Most of these samples show numerous, diagnostic absorption features in the mid‐IR due to Fe3+–O2‐ vibrational transitions, structural and/or bound OH, and/or silicates. These data indicate that high spatial resolution, moderate spectral resolution mid‐IR ground‐based and spacecraft observations of Mars may be able to detect and uniquely discriminate among different ferric‐iron‐bearing phases on the Martian surface or in the airborne dust.
Hematite, pyroxene, and phyllosilicates on Mars: Implications from oxidized impact melt rocks from Manicouagan Crater, Quebec, Canada
Authors: Morris Richard V. , Golden D. C. , Bell James F. , Lauer H. V.
Published: February 2004

Visible and near‐IR reflectivity, Mössbauer, and X ray diffraction data were obtained on powders of impact melt rock from the Manicouagan Impact Crater located in Quebec, Canada. The iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples. Phyllosilicate (smectite) contents up to ∼15 wt % were found in some heavily oxidized samples. Nanophase hematite and/or paramagnetic ferric iron is observed in all samples. No hydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250°C. Oxidative alteration is thought to have occurred predominantly during late‐stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near‐IR band minimum correlated with the extent of aleration (Fe3+/Fetot) and ranged from ∼1000 nm (high‐Ca pyroxene) to ∼850 nm (bulk, well‐crystalline hematite) for least and most oxidized samples, respectively. Intermediate band positions (900–920 nm) are attributed to low‐Ca pyroxene and/or a composite band from hematite‐pyroxene assemblages. Manicouagan data are consistent with previous assignments of hematite and pyroxene to the ∼850 and ∼1000 nm bands observed in Martian reflectivity spectra. Manicouagan data also show that possible assignments for intermediate band positions (900–920 nm) in Martian spectra are pyroxene and/or hematite‐pyroxene assemblages. By analogy with impact melt sheets and in agreement with observables for Mars, oxidative alteration of Martian impact melt sheets above 250°C and subsequent erosion could produce rocks and soils with variable proportions of hematite (both bulk and nanophase), pyroxene, and phyllosilicates as iron‐bearing mineralogies. If this process is dominant, these phases on Mars were formed rapidly at relatively high temperatures on a sporadic basis throughout the history of the planet. The Manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition, which is also indicated for Mars from analyses of soil at the two Viking landing sites.
The unified lunar control network: 1994 version
Authors: Davies Merton E. , Colvin Tim R. , Meyer Donald L. , Nelson Sandra
Published: February 2004

The objective of the unified lunar control network is to combine a series of control networks into one compatible network with its origin at the center of mass of the Moon and its coordinates referred to the mean Earth/polar axis system. The initial unified system contained 130 nearside points from Apollo data and 1026 from telescopic data. It also contained ten Mariner 10 points. The total number of points was 1166. The current network includes modifications to the past network and extends the coverage. Coordinates of points north of the Apollo region have been recomputed based on Galileo images from the second Earth‐Moon flyby. Coordinates of points in the Apollo region were held fixed; however, coordinates of points north of the Apollo region in the telescopic region and many Mariner 10 points were recomputed. All of the Mariner 10 points were remeasured and integrated into the network. Additional points in the Apollo region including the farside have been added. The unified network now contains 1478 points. Apollo, Mariner 10, and Galileo pictures all contained some farside points. The coordinates of the 1478 points are available only in the microfiche supplement to this paper.
Simultaneous adsorption of CO2 and H2O under Mars‐like conditions and application to the evolution of the Martian climate
Authors: Zent Aaron P. , Quinn Richard C.
Published: February 2004

The Martian regolith is the most substantial volatile reservoir on the planet; estimates of its adsorbed inventory have been based on simple measurements of the adsorption of either water or CO2 in isolation. Under some conditions, H2O can poison adsorbate surfaces, such that CO2 uptake is greatly reduced. We have made the first measurements of the simultaneous adsorption of CO2 and H2O under conditions appropriate to the Martian regolith and have found that at H2O monolayer coverage above about 0.5, CO2 begins to be displaced into the gas phase. We have developed an empirical expression that describes our co‐adsorption data and have applied it to standard models of the Martian regolith. We find that currently, H2O does not substantially displace CO2, implying that the adsorbate inventories previously derived may be accurate, not more than 3–4 kPa (30–40 mbar). No substantial increase in atmospheric pressure is predicted at higher obliquities because high‐latitude ground ice buffers the partial pressure of H2O in the pores, preventing high monolayer coverages of H2O from displacing CO2. The peak atmospheric pressure at high obliquity does increase as the total inventory of exchangeable CO2 increases.
Geology and landscape evolution of the Hellas region of Mars
Authors: Tanaka Kenneth L. , Leonard Gregory J.
Published: February 2004

Hellas basin on Mars has been the site of volcanism, tectonism, and modification by fluvial, mass‐wasting, and eolian processes over its more than 4‐b.y. existence. Our detailed geologic mapping and related studies have resulted in the following new interpretations. The asymmetric distribution of highland massifs and other structures that define the uplifted basin rim suggest a formation of the basin by the impact of a low‐angle bolide having a trajectory heading S60°E. During the Late Noachian, the basin was infilled, perhaps by lava flows, that were sufficiently thick (>1 km) to produce wrinkle ridges on the fill material and extensional faulting along the west rim of the basin. At about the same time, deposits buried northern Malea Planum, which are interpreted to be pyroclastic flows from Amphitrites and Peneus Paterae on the basis of their degraded morphology, topography, and the application of a previous model for pyroclastic volcanism on Mars. Peneus forms a distinctive caldera structure that indicates eruption of massive volumes of magma, whereas Amphitrites is a less distinct circular feature surrounded by a broad, low, dissected shield that suggests generally smaller volume eruptions. During the Early Hesperian, a ∼1‐ to 2‐km‐thick sequence of primarily fined‐grained, eolian material was deposited on the floor of Hellas basin. Subsequently, the deposit was deeply eroded, except where armored by crater ejecta, and it retreated as much as 200–300 km along its western margin, leaving behind pedestal craters and knobby outliers of the deposit. Local debris flows within the deposit attest to concentrations of groundwater, perhaps in part brought in by outflow floods along the east rim of the basin. These floods may have deposited ∼100–200 m of sediment, subduing wrinkle ridges in the eastern part of the basin floor. During the Late Hesperian and Amazonian, eolian mantles were emplaced on the basin rim and floor and surrounding highlands. Their subsequent erosion resulted in pitted and etched plains and crater fill, irregular mesas, and pedestal craters. Local evidence occurs for the possible former presence of ground ice or ice sheets ∼100 km across; however, we disagree with a hypothesis that suggests that the entire south rim and much of the floor of Hellas have been glaciated. Orientations of dune fields and yardangs in lower parts of Hellas basin follow directions of the strongest winds predicted by a recently published general circulation model (GCM). Transient frost and dust splotches in the region are, by contrast, related to the GCM prediction for the season in which the images they appear in were taken.
Auroral Lyman α and H2 bands from the giant planets: 2. Effect of the anisotropy of the precipitating particles on the interpretation of the “color ratio”
Authors: Prangé Renée , Rego Daniel , Gérard Jean‐Claude
Published: February 2004

Previous spectral analyses have given evidence of collisionally excited Jovian and (at times) Saturnian H2 Werner bands being absorbed by hydrocarbons at the shortest wavelengths along the auroral ovals, and of a longitudinal dependence of this absorption in the Jovian aurorae. This “color ratio” has been used to estimate the energy of the primary particles. In such estimates, particles are generally assumed to penetrate vertically into the atmosphere. However, the precipitating particle angular distribution is unknown, and a model developed for a diffuse aurora by Prangé and Elkhamsi (1991), for instance, predicts quite different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distributions peaking far from vertical may increase the energy derived from a given color ratio by as much as a factor of 3. We discuss previous interpretations of the color ratio longitudinal modulation (variation of the auroral atmosphere structure, or of the incident particle energy) in view of the subsequent increase in energy input. We argue that an interpretation in terms of energy variations only is not consistent with the energy available in the magnetosphere if the aurorae are diffuse, and we discuss this finding in the context of recent Hubble Space Telescope images.
Post‐Voyager brightness variations on Io
Authors: Sartoretti Paola , McGrath Melissa A. , McEwen Alfred S. , Spencer John R.
Published: February 2004

Imaging of Io with the faint object and planetary cameras of the Hubble space telescope in 1992 and 1993 at wavelengths of ∼3450, 3700, and 4100 Å shows two surface areas that have undergone significant, large‐scale change in reflectivity since the 1979 Voyager encounters. The first is located in Colchis Regio and covers ∼106 km2 between longitudes 150°–180° and latitudes −25° to +30°; the second is centered at longitude ∼130°, latitude ∼+30°, and extends for ∼105 km2. Both areas have darkened by ≥45% since 1979. In light of the active volcanism discovered on Io by the Voyager 1 encounter, it seems reasonable to infer that these large‐scale changes in surface morphology are due to some type of volcanic activity in the intervening 14 years. We hypothesize two possible causes for these darkenings: either large‐scale eruptions of the Pele type have covered areas of existing SO2 surface frost with new, much darker deposits, or previously active eruptions of the Prometheus type (or smaller‐scale venting), which apparently produce SO2 gas that is bright in the visible when it condenses as surface frost, have become inactive.
Uniqueness of a solution of a steady state photochemical problem: Applications to Mars
Authors: Krasnopolsky Vladimir A.
Published: February 2004

Based on the conservation of chemical elements in chemical reactions, a rule is proved that the number of boundary conditions given by densities and/or nonzero velocities should not be less than the number of chemical elements in the system, and the boundary conditions for species given by densities and velocities should include all elements in the system. Applications of this rule to Mars are considered. It is shown that the problem of the CO2‐H2O chemistry in the lower and middle atmosphere of Mars, say, in the range of 0–80 km does not have a unique solution, if only CO2 and H2O densities are given at the lower boundary, and the remaining boundary conditions are fluxes. Two examples of models of this type are discussed. These models fit the same boundary conditions, are balanced with a relative accuracy of 10−4 for H2 (and much better for other species), and predict O2, CO, and H2 mixing ratios which differ by orders of magnitude. One more species density, e.g., that of O2, should be specified at the boundary to obtain a unique solution. The situation is better if the upper boundary is extended to the exobase where thermal escape velocities of H and H2 can be specified. In this case, however, either the oxygen nonthermal escape rate (and hence the total hydrogen escape rate) or the O2 (or other species) density at the surface should be given as a boundary condition. Two models of the photochemistry of the Martian atmosphere, with and without nitrogen chemistry, are considered. The oxygen nonthermal escape rate of 1.2×108 cm−2 s−1 is given at 240 km and is balanced with the total hydrogen escape rate within an uncertainty of 1% for both models. Both models fit the measured O2 and CO mixing ratios, the O3 abundance, and the O2 1.27‐μm dayglow almost within the uncertainties of the measured values, though the model without nitrogen chemistry fits better. The importance of nitrogen chemistry in the lower and middle atmosphere of Mars depends on a fine balance between production of NO and N in the upper atmosphere which is not known within the required accuracy.
Spectrophotometry and organic matter on Iapetus: 1. Composition models
Authors: Wilson Peter D. , Sagan Carl
Published: February 2004

Iapetus shows a greater hemispheric albedo asymmetry than any other body in the solar system. Hapke scattering theory and optical constants measured in the laboratory are used to identify possible compositions for the dark material on the leading hemisphere of Iapetus. The materials considered are poly‐HCN, kerogen, Murchison organic residue, Titan tholin, ice tholin, and water ice. Three‐component mixtures of these materials are modeled in intraparticle, particle, and areal mixtures. In a computer grid search of ∼2 × 107 models, an intraparticle mixture of 25% poly‐HCN, 10% Murchison residue, and 65% water ice is found to best fit the spectrum, albedo, and phase behavior of the dark material. The Murchison residue and/or water ice can be replaced by kerogen and ice tholin, respectively, and still produce very good fits. Areal and particle mixtures of poly‐HCN, Titan tholin, and either ice tholin or Murchison residue are also possible models. Poly‐HCN is a necessary component in almost all good models. The presence of poly‐HCN can be further tested by high‐resolution observations near 4.5 μm.
Chemical studies of H chondrites: 6. Antarctic/non‐Antarctic compositional differences revisited
Authors: Wolf Stephen F. , Lipschutz Michael E.
Published: February 2004

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by putative volatility during nebular condensation and accretion) determined by radiochemical neutron activation analysis of 14 additional H5 and H6 chondrite falls. Data for the 10 most volatile elements (Rb → In) treated by the multivariate techniques of linear discriminant analysis and logistic regression in these and 44 other falls are compared with those for 59 H4–6 chondrites from Antarctica. Various populations are tested by the multivariate techniques, using the previously developed method of randomization‐simulation to assess significance levels. An earlier conclusion, based on fewer samples, that H4–6 chondrite falls are compositionally distinguishable from the Antarctic suite is verified by the additional data. This distinctiveness is higjily significant because of the presence of samples from Victoria Land in the Antarctic population, which differ compositionally from falls beyond any reasonable doubt. However, it cannot be proven unequivocally that falls and Antarctic samples from Queen Maud Land are compositionally distinguishable. Trivial causes (e.g., analyst bias, weathering) cannot explain the Victoria Land (Antarctic)/non‐Antarctic compositional difference for paradigmatic H4–6 chondrites. This seems to reflect a time‐dependent variation of near‐Earth meteoroid source regions differing in average thermal history.
Assessing the reliability of paleochemical tracers: Barium uptake in the shells of planktonic foraminifera
Authors: Lea David W. , Spero Howard J.
Published: February 2004

The trace metal composition of foraminiferal shells preserved in deep‐sea sediments is a critical source of data used in reconstructing the physical and chemical characteristics of ancient oceans. However, paleoceanographic information derived from these tracers is limited by the accuracy with which calcite shells record ambient seawater chemistry and the question of whether changing environmental conditions or interspecific differences affect shell chemistry. Experiments with living foraminifera are one of the most direct ways to quantitatively evaluate the relationship between shell and seawater chemistry and the influence, if any, of environmental variables. Here we present results from laboratory experiments on the living planktonic foraminifers Globigerina sacculifer and Orbulina universa that demonstrate that precipitated shells provide an accurate record of seawater barium, a tracer used in reconstructing nutrient and alkalinity distributions in past oceans. On the basis of culturing data at 29°C, 36.7‰ salinity, the best estimate of the partition coefficient D relating foraminifera shell Ba/Ca to seawater Ba/Ca is 0.147 ± 0.004. Data show that when environmental conditions are held constant, there is no significant variability in Ba uptake between individual shells of one or both species. Results also demonstrate that temperature varying over a 7°C range and salinity varying over a 3‰ range do not appreciably influence Ba uptake.
An ocean general circulation model view of the glacial Mediterranean thermohaline circulation
Authors: Bigg Grant R.
Published: February 2004

Reconstructions of the sea surface temperature and salinity of the Mediterranean Sea during the last glacial maximum (LGM) suggest that extremely cold winters occurred in the western basin and the Aegean, with highly saline conditions in the eastern basin. It is shown here that forcing the LGM Mediterranean Sea with a representative paleoclimate model results in significantly less extreme salinity conditions than are suggested by existing reconstructions. Modeled sea surface salinities are, at most, 0.1–0.2 ‰ higher than today at the LGM, rather than the previously suggested 2–3 ‰. Modeled sea surface temperatures over the western basin, however, show similar changes to these reconstructions, being 3°–4°C colder than present day. Another major difference between the LGM and control simulations is the location and spreading of subsurface water. The western basin suffers complete loss of stability in the LGM winter, leading to this basin becoming the source for the LGM deep water of the eastern basin. The ocean general circulation model results are shown to be qualitatively consistent with cooler, drier climate along the northern coast of the Mediterranean but not with higher surface salinity, thus refining existing paleoclimate reconstructions in the Mediterranean region.
A 600,000‐year record of Antarctic Bottom Water activity inferred from sediment textures and structures in a sediment core from the Southern Brazil Basin
Authors: Massé Laurent , Faugères Jean‐Claude , Bernat Michel , Pujos Annick , Mézerais Marie‐Laure
Published: February 2004

At the northern exit of the Vema Channel, in the Southern Brazil Basin, deep currents linked with Antarctic Bottom Water flow (AABW, below 4200 m) have formed contouritic accumulations along the continental rise. Lithologic and textural investigations were carried out on a Kullenberg core in order to establish a chronology of late Pleistocene‐Holocene fluctuations in AABW flow. The core, spanning the last 600,000 years, was recovered from a field of sediment waves. The deposits consist of fine grained muds. Carbonate contents are very low because deposition takes place near the present‐day carbonate compensation depth. The core stratigraphic framework is based on calcareous nannofossil and excess 230Th analyses. Two main types of facies can be identified: (1) yellowish brown muds, with frequent manganese enrichments forming dark laminae, and (2) homogeneous gray‐green muds. Two indicators of paleocurrent activity have been considered: (1) erosional sediment features that give evidence for high amplitude and short‐term current events, and (2) grain size fluctuations (percentage of panicles greater than 10 µm), indicating low amplitude and long‐term variations. Two periods can be defined. The first one (circa 600 to circa 350 kyr B.P.) is characterized by an instability in current activity, with strong flow events recorded as erosional surfaces. Long‐term fluctuations reveal the existence of several episodes of increased velocity occurring approximately every 50 kyr. The second period (350 kyr B.P. to present) is marked by globally weaker current activity and long‐term fluctuations of lower amplitude and longer duration. Maximum velocities occur preferentially during periods of climatic cooling. These fluctuations might be correlated with the 100 kyr eccentricity cycle of the Earth's orbit.
Relationship among submarine instabilities, sea level variations, and the presence of an ice sheet on the continental shelf: An example from the Verrill Canyon Area, Scotian Shelf
Authors: Mulder T. , Moran K.
Published: February 2004

Sea level low stands are associated with the growth of glacial ice sheets. During maximum stages of glaciation, ice sheets may extend to the outer continental shelf. These glacial maxima normally correspond to sea level low stands. Consequently, ice sheets may be supported directly by the seafloor inducing increased pore pressures in the sediment and increasing the overall bearing stresses on the margin. These stresses trigger undrained failure in upper slope sediment and bearing capacity failure in the middle to lower slope. After complete ice melting and dissipation of excess pore pressure, the inter‐glacial sediments become overconsolidated. Shear resistance is higher, and no gravity or earthquake‐induced failure occurs. The gravity events can only occur in underconsolidated and normally consolidated glacial and postglacial deposits. This explains the variation in volume and activity of mass wasting events during low and high sea level periods in areas where a glacial ice exists. Analyses of maximum ice loading events are presented for the Scotian slope. In contrast with traditional thought, these analyses suggest that ice loading, rather than high sedimentation rates, is the dominant mechanism for slope instability on glaciated margins.
Closing off the Southern Ocean surface
Authors: Heinze C. , Broecker W. S.
Published: February 2004

Equilibrium integrations with the three‐dimensional Hamburg ocean carbon cycle circulation model show the effect of a Southern Ocean surface outcrop that is closed off for gas exchange with the atmosphere and export production of particulate organic carbon and CaCO3 south of 32.5 °S. For such a close‐off, the northern Atlantic outcrop assumes a greater degree of control of atmospheric CO2 partial pressure. Contrary to expectations, the atmospheric pCO2 increases compared to a control run without close‐off. Maximizing the nutrient utilization in the North Atlantic can compensate for this increase. However, it does not draw down the pCO2 to glacial levels.
Chronology for climate change: Developing age models for the biogeochemical ocean flux study cores
Authors: Manighetti B. , McCave I. N. , Maslin M. , Shackleton N. J.
Published: February 2004

We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accelerator mass spectrometer dating of a key core, BOFS 5K, and correlation with the rest of the suite. The effects of bioturbation and foraminiferal species abundance gradients upon the age record are modeled using a simple equation. The degree of bioturbation is estimated by comparing modeled profiles with dispersal of the Vedde Ash layer in core 5K, and we find a mixing depth of roughly 8 cm for sand‐sized material. Using this value, we estimate that age offsets between unbioturbated sediment and some foraminifera species after mixing may be up to 2500 years, with lesser effect on fine carbonate (<10 µm) ages. The bioturbation model illustrates problems associated with the dating of “instantaneous” events such as ash layers and the “Heinrich” peaks of ice‐rafted detritus. Correlations between core 5K and the other cores from the BOFS suite are made on the basis of similarities in the downcore profiles of oxygen and carbon isotopes, magnetic susceptibility, water and carbonate content, and via marker horizons in X radiographs and ash beds.
Controls on the 87Sr/86Sr composition of seawater from the Middle Eocene to Oligocene: Hole 689B, Maud Rise, Antarctica
Authors: Mead Gregory A. , Hodell David A.
Published: February 2004

A 87Sr/86Sr isotope curve of the middle Eocene to Oligocene was produced from analysis of foraminifera in Ocean Drilling Program Hole 689B, Maud Rise, near the coast of Antarctica. Sediments from the hole are well preserved with no evidence of diagenetic alteration. The sequence is nearly complete from 46.3 to 24.8 Ma, with an average sampling interval of 166 kyr. Excellent magnetostratigraphy in Hole 689B allows calibration to the geomagnetic polarity time scale of Cande and Kent (1992). Marine strontium isotopic ratios were nearly stable from 46.3 to 35.5 Ma, averaging near 0.70773, after which they began to increase. A slow increase began after 40.4 Ma, rising at a rate of only about 8×10−6/m.y. from base values of 0.707707. From 35.5 Ma to 24.8 Ma the average slope increased to 40×10−6/m.y. The slope remained constant at least until 24.8 Ma, when the record becomes discontinuous owing to unconformities. We evaluate several possible controls on the marine strontium isotope curve that could have led to the observed growth in 87Sr/86Sr ratios near the Eocene/Oligocene boundary. Three mechanisms are considered, including the onset of Antarctic glaciation, increased mountain building in the Himalayan‐Tibetan region, and decreased hydrothermal activity. None of the mechanisms alone seems to adequately explain the increased 87Sr/86Sr ratios during the Oligocene. Glaciation as a weathering agent was too episodic and probably began too late to explain the upturn in marine 87Sr/86Sr ratios. There is evidence that uplift in the Himalayan‐Tibetan region began in the Miocene, much too late to control Oligocene strontium isotope ratios. Lastly, hydrothermal flux changes since the Eocene were apparently not great enough alone to account for the rise in marine 87Sr/86Sr ratios. We suggest that a combination of causes, such as decreased hydrothermal activity perhaps followed by increased glaciation and mountain building, might best explain the growth of the marine 87Sr/86Sr curve during the Oligocene.
Magnetic properties and origin of Upper Quaternary sediments in the Somali Basin, Indian Ocean
Authors: Meynadier Laure , Valet Jean‐Pierre , Grousset Francis E.
Published: February 2004

The magnetic concentrations and magnetic fluxes of marine sediments from the Somali Basin show positive correlations with insolation, using oxygen isotopes for calibration. Investigations of the rock magnetic properties indicate an increase in the magnetite/hematite ratio during warmer episodes and the presence of additional fine grains of magnetite. The magnetic susceptibility profiles of several other cores suggest that the same characteristics prevail over a large area within the basin. These features are opposite to the variations observed to the north of this area (DeMenocal et al., 1991) where eolian deposition is dominant. Magnetic measurements performed on the coarse and fine fractions of the sediment show that the magnetic signal is carried by the finer fraction. Analyses of the principal components of clay mineralogy show that river transported sediments are geographically very limited and confirm that there is no indication of significant eolian deposition. It is most likely that the Antarctic bottom currents were responsible for the transport of the magnetic particles and thus for the correlation between the magnetic and climatic records.
Early diagenesis of lipid biomarker compounds in North Atlantic sediments
Authors: Madureira Luiz A. S. , Conte Maureen H. , Eglinton Geoffrey
Published: February 2004

Near‐surface sediments (0–45 mm depth interval) from the Iceland Basin (59°N, 21°W, 3070‐m water depth) and Biscay Abyssal Plain (48°N, 17°W, 4105‐m water depth) were sectioned at millimeter‐scale resolution to assess alterations in key lipid biomarkers during early diagenesis. Inventories (µg cm−2in the topmost 45 mm) of biomarkers from presumed terrestrial sources (>C20linearn‐alkanes,n‐alkanoic acids,n‐alkanols) and from phytoplankton sources (phytosterols, long‐chain alkenones and alkenoates) were 1.5–4 times more abundant in the Iceland Basin core than in the Biscay Abyssal Plain core, indicating greater sedimentary inputs and/or preservation at the Iceland Basin site. Biomarker concentrations varied significantly over millimeter‐scale depth intervals. Pronounced subsurface concentration maxima were present at both sites. At the Biscay Abyssal Plain site, steep concentration gradients in the uppermost few mm of the sediment column indicate extensive diagenetic losses at or near the sediment/water interface. Strong covariance was seen in the concentrations of various compounds within each class (e.g., long‐chainn‐alkanes,n‐alkanols,n‐alkanoic acids and long‐chain alkenones and alkyl alkenoates), indicating that individual compounds within each class are associated with the same organic matrix and have similar degradation and mixing rates. Thus biomarker indices for assessment of land‐derived inputs (CPI, C29/C31, ACL) and sea surface temperature (IPT) remain largely unaltered by early diagenetic processes in oxic, abyssal sediments in spite of extensive bioturbation and degradative losses within the uppermost few centimeters of the sediment column.
Characteristics of small‐scale ionospheric irregularities as deduced from scintillation observations of radio signals from satellites ETS‐2 and Polar Bear 4 at Irkutsk
Authors: Afraimovich E. L. , Zherebtsov G. A. , Zvezdin V. N. , Franke S. J.
Published: February 2004

This paper presents some new results on the small‐scale inhomogeneous ionospheric structure obtained at a facility for spaced‐antenna reception of transionospheric signals from ETS‐2 and Polar Bear 4 near Irkutsk (Eastern Siberia, 52°N, 104°E). A technique based on transferring time spectra of scintillations to spatial spectra using measured horizontal irregularity drift velocities is used to obtain an estimate of the mean spatial spectrum of midlatitude scintillations. Two different methods were used to determine the inclination index of the scintillation spectrum, which was found to be equal to −2, in agreement with the value recently predicted for small‐scale F region irregularities generated through mapping of small‐scale, turbulent electric fields from the E region to the F region. Drift velocities of the diffraction pattern, and also the altitudes at which ionospheric irregularities are located, agree well with results obtained by other authors for midlatitudes. Using simultaneous measurements for a geostationary satellite and an orbiting satellite, the supposition about the existence of the southern boundary of the scintillation region has been confirmed. Finally, analysis of quasi‐periodic (QP) scintillations and simultaneously determined diffraction pattern velocities is used to show that the height of isolated irregularities giving rise to QP scintillations corresponds to the maximum of the ionospheric F2 region.
Exposure assessment at extremely low frequencies: Issues, instrumentation, modeling, and data
Authors: Sussman Stanley S.
Published: February 2004

Epidemiologic studies suggesting a possible link between exposure to ELF magnetic fields and increased cancer risk have spurred interest in the assessment of field levels and exposures in residences, the workplace, and other environments. Characteristics such as high degrees of spatial and temporal variability and the presence of multiple frequencies can complicate assessment strategies. Instrumentation development has proceeded rapidly, and a number of measurement systems of varying sophistication are now commercially available. For situations where measurement is not feasible or practical, computer modeling is expanding the scope of exposure estimation. Health‐oriented studies and measurement surveys are adding to a growing database of field and exposure levels.
Fast frequency domain electromagnetic modeling in axially symmetric layered media
Authors: Tamarchenko T. , Tabarovsky L.
Published: February 2004

We consider an axially symmetric model consisting of radially inhomogeneous horizontal layers. A fast hybrid numerical technique that efficiently combines the integral equations and finite difference methods is applied to simulate the electromagnetic field of an arbitrary source in such models. The boundary value problem is formulated in terms of the electric field components. First, the Green's tensor, which produces zero tangential components of electrical field on the plane boundaries, is calculated in each horizontal layer. Then Green's theorem is used to represent the solution in each layer as a surface integral. Matching the solution on the horizontal boundaries leads to a tridiagonal system of integral equations. The numerical realization of the method is based on a finite difference approximation to the initial boundary value problem in the radial direction and an analytical solution as a function of depth. An eigenvalue problem in the radial direction is solved within each horizontal layer. By matching the solutions at each interface, a tridiagonal system of linear equations is obtained and solved to produce reflection and propagation matrices, which are stored in the computer memory in order to eliminate repetitive calculations when simulating well‐logging responses. Several numerical examples demonstrate the high accuracy and efficiency of the method.
Full wave theories for rough surface scattering: An updated assessment
Authors: Collin R. E.
Published: February 2004

A numerical evaluation and analysis of Bahar's extended full wave theory for rough surface scattering is carried out. It is shown that the field correlation function in the extended full wave theory is dramatically different from that in the Kirchhoff theory. For this reason the approximation that the heights and slopes are uncorrelated is not valid. When the statistical analysis is carried out without any approximation no enhanced backscatter effects appear. Various artifacts in the theory, such as the pole in the scattering coefficient for vertical polarized waves incident on a rough surface, are also discussed. The revised version of the full wave theory, introduced by Bahar in 1992, is also examined. It is shown that this version of the theory reduces to the small perturbation theory in the small height small slope limit but only for a restricted range of surface parameters.This latest version of the theory eliminates the troublesome pole, agrees with the Kirchhoff theory in the high frequency small slope limit, but produces an unnatural enhancement of the cross sections in the quasi specular direction for surfaces with large rms slopes.