bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagnosing the imbalance of Yanamarey Glacier in the Cordillera Blanca of Peru
Authors: Hastenrath Stefan , Ames Alcides
Published: February 2004

A detailed glaciological observation program was conducted on the Yanamarey Glacier in the Cordillera Blanca of Peru, including the monitoring of net balance and ice flow velocity during 1977–1988 and mappings of the surface topography in 1973, 1982, and 1988. These observations are here evaluated to combine net balance, surface lowering, and ice flow into a consistent picture of the mode of operation of a tropical glacier on the scale of a decade. The glacier extends between 5100 and 4500 m with a total area of 9×105 m2 and length of about 1.3 km. Maximum flow velocity is 17.4 m yr−1 and maximum volume flux 336×103 m3 yr−1. In the ablation area, net balance is about −6 m yr−1 and surface lowering 3 m yr−1. About half of the mean annual water discharge from the glacier of 80 L s−1 is not renewed by precipitation but supplied by the ice thinning. The rate of surface lowering of 1.5 m yr−1 liquid water equivalent translates to a glacier average departure heat supply for melting of 16 W m−2. Sensitivity analyses indicate that this could be produced by a cloudiness increase of less than one tenth, an air temperature decrease of 2°C, an increment in specific humidity of less than 1 g kg−1, or some combination of heat budget processes. Such changes in the atmospheric environment would be required to stabilize the glacier at its recent volume. As another indication of the recent imbalance, the maximum volume flux is found some 100 m below the equilibrium line altitude. Under continuation of the recent climatic conditions, the glacier may survive for more than half a century.
Asian influence over the western North Pacific during the fall season: Inferences from lead 210, soluble ionic species and ozone
Authors: Dibb J. E. , Talbot R. W. , Klemm K. I. , Gregory G. L. , Singh H. B. , Bradshaw J. D. , Sandholm S. T.
Published: February 2004

Aerosol samples collected over the western Pacific during the NASA/Global Tropospheric Experiment Pacific Exploratory Mission (PEM‐West A) expedition (September–October 1991) revealed mean 210Pb concentrations in the free troposphere in the 5–10 fCi m−3 STP range. Most soluble ionic aerosol‐associated species were near detection limits [≪40 parts per trillion by volume (pptv)] in these same samples. The altitude distribution of O3 near Asia closely resembled that of 210Pb, while no relationship was found between the concentrations of O3 and 7Be. Free tropospheric air over the western Pacific was depleted in soluble aerosol‐associated species but enriched in 210Pb and O3, indicative of deep wet convection over the Asian continent. The influence of Asian air on the composition of the free troposphere over the western Pacific was evident on most of the PEM‐West A flights. However, evidence of continental influence was largely restricted to those species that are relatively insoluble (or have insoluble precursors), hence escape scavenging during vertical transport from the boundary layer into the free troposphere by wet convective activity.
Possible secular change and land‐to‐ocean extension of air pollution from measurements of atmospheric electrical conductivity over the Bay of Bengal
Authors: Kamra A. K. , Deshpande C. G.
Published: February 2004

Diurnal variations of the atmospheric electrical conductivity and electric field at a fixed point in the Bay of Bengal, where an oceanographic ship, ORV Sagarkanya, remained stationary for a total of 28 days, are reported. From observations, one can speculate a secular decrease in total conductivity in the Bay of Bengal by at least 40% since the Cobb and Wells (1970) measurements during 1967 global expedition. Problem of the land‐to‐ocean extension of air pollution has been studied from the conductivity measurements made in the monsoon season when surface winds are persistently southwesterly. Values of conductivity near the eastern coastline of India where windflow is from land to sea are about half of those near to the western coastline where windflow is from sea to land. It is concluded that to know the air conductivity at a point over sea, the age of air mass over sea is a better determining factor than the distance from the coastline.
A preliminary study of the effect of ammonia on particle nucleation in the marine boundary layer
Authors: Coffman Derek J. , Hegg Dean. A.
Published: February 2004

A ternary nucleation model for the H2SO4‐NH3‐H2O system is presented in an effort to examine the effect of NH3 on heteromolecular homogeneous nucleation in the marine boundary layer (MBL). The results from this nucleation model suggest that ammonia could, in fact, enhance the nucleation rate over that of the binary system, H2SO4‐H2O. The magnitude of this enhancement is introduced as an enhancement ratio, which, in principle, is applicable to any binary nucleation rate for H2SO4‐H2O. Also presented are preliminary results from a simple aerosol model using this enhancement ratio. These results suggest that under conditions typical of the marine environment it may be possible to produce enough particles to balance the various particle sinks characteristic of the MBL.
Tunable diode laser measurements of methane fluxes from an irrigated rice paddy field in the Philippines
Authors: Simpson I. J. , Thurtell G. W. , Kidd G. E. , Lin M. , Demetriades‐Shah T. H. , Flitcroft I. D. , Kanemasu E. T. , Nie D. , Bronson K. F. , Neue H. U.
Published: February 2004

This paper presents the first micrometeorological‐based measurements of methane (CH4) emissions from Asian rice paddies of which we are aware. The research features the tunable diode laser trace gas analyzer system (TGAS) recently developed at the University of Guelph. CH4 fluxes were measured between March 9 and 24, 1992, from an irrigated rice paddy field at the International Rice Research Institute (IRRI), the Philippines. The daytime CH4 flux averaged 6.0 μg m−2 s−1. The CH4 fluxes displayed a diurnal trend similar to daily soil temperature curves, with peak emissions of about 8 μg m−2 s−1 in the early afternoon. A tenfold increase in CH4 emissions (to about 70μg m−2 s−1) during a brief weeding experiment resulted from soil disturbance. Up to 25 μg m−2 s−1 of CH4 were released during a drying of the field, after which unsuitable soil redox potentials apparently suppressed methanogenesis. The CH4 flux was also arrested when the field was flooded with oxygen‐rich water during a heavy rainstorm.
Comparison of the magnetic properties and Mössbauer analysis of glass from the Cretaceous‐Tertiary boundary, Beloc, Haiti, with tektites
Authors: Thorpe A. N. , Senftle F. E. , May L. , Barkatt A. , Adel‐Hadadi M. A. , Marbury G. S. , Izett G. A. , Maurrasse F. R.
Published: February 2004

The magnetic properties of black Beloc glass have been measured. The Curie constant, the magnetization, and the magnetic susceptibility of the Beloc glass fall within the known ranges observed for tektites. However, the temperature‐independent component of the magnetic susceptibility is slightly higher than that found for tektites. Moreover, it is not possible to match the experimental magnetic data for the Beloc glass with the calculated values using the previously reported Fe3+/Fe2+ ratio of 0.7. The oxidation state of Fe was therefore redetermined by Mössbauer measurements, and the Fe3+/Fe2+ ratio was found to be 0.024±0.015. Using the redetermined value of the ratio, the magnetic parameters were again calculated using formulas that are applicable to tektites, and good agreement was found between the calculated and experimental values. The experimental magnetic measurements and the redetermined Fe3+/Fe2+ ratio of the Beloc glass specimens are essentially the same as those found for tektite glass.
Geographic variation in Australasian microtektite concentrations: Implications concerning the location and size of the source crater
Authors: Glass B. P. , Pizzuto J. E.
Published: February 2004

Geographic variations in the concentration of Australasian microtektites in 42 cores from the Indian Ocean, western equatorial Pacific Ocean, and the Philippine, Celebes, and Sulo Seas were used to predict the location of the Australasian tektite source crater and to estimate its size. The location that seems to best explain the geographic variations in microtektite concentrations is located in central Cambodia at about 12°N latitude and 106°E longitude. The diameter of the source crater is estimated to be between 32 and 114 km based on thickness of the microtektite layer at each site and distance from the predicted source area in Cambodia. The large range in estimated size of the source crater is due to a lack of knowledge about the value of the exponent that describes the radial decrease in ejecta thickness. Additional microtektite/ejecta‐bearing sites closer to the source region could help resolve this problem.
Collisional evolution of the Earth's orbital debris cloud
Authors: Rossi A. , Cordelli A. , Farinella P. , Anselmo L.
Published: February 2004

We have developed a numerical algorithm to model the future collisional evolution of the low‐orbiting Earth debris population, accounting for both the wide spectrum of masses (or sizes) of the orbiting objects, and their different altitudes, which result in a variable efficiency of the drag‐induced decay. The evolution process has been assumed to be caused by a number of source and sink mechanisms, such as launches, explosions, atmospheric drag, and mutual collisions. The collisional outcomes have been described through a semiempirical model for the fragment mass distributions, consistent with the available experimental evidence. A runaway exponential growth of collision fragments is always found in our model. Although its timing and pace are sensitive to some poorly known parameters, fairly plausible parameter choices predict that the runaway growth will occur within the next century, starting in the crowded shells between 700 and 1000 km of altitude and, somewhat later, between 1400 and 1500 km. The runaway growth is delayed until a few centuries in the future only if the catastrophic breakup threshold in specific impact energy for orbiting objects exceeds that for natural rocky bodies by at least a factor of 10. Our simulations show that the sensitivity of the results to future launch and/or deorbiting and removal policies is rather weak, so that drastic measures will need to be taken soon in order to significantly avoid or delay a catastrophic outcome.
Experimental reduction of lunar mare soil and volcanic glass
Authors: Allen Carlton C. , Morris Richard V. , McKay David S.
Published: February 2004

We have reduced high‐titanium lunar mare soil and iron‐rich lunar volcanic glass with hydrogen at temperatures of 900–1100°C. Ilmenite is the most reactive phase in the soil, exhibiting rapid and complete reduction at all temperatures. Ferrous iron in the glass is extensively reduced concurrent with partial crystallization. In both samples pyroxene and olivine undergo partial reduction along with chemical and mineralogical modifications. High‐temperature reduction provides insight into the optical and chemical effects of lunar soil maturation, and places constraints on models of that process. Mare soil and volcanic glass are attractive feedstocks for lunar oxygen production, with achievable yields of 2–5 wt%.
Thermal emission measurements 2000–400 cm−1 (5–25 μm) of Hawaiian palagonitic soils and their implications for Mars
Authors: Roush Ted L. , Bell James F.
Published: February 2004

The thermal emission of two palagonitic soils, common visible and near infrared spectral analogs for bright soils on Mars, was measured over the wavelength range of 5 to 25 μm (2000 to 400 cm−1) for several particle size separates. All spectra exhibit emissivity features due to vibrations associated with H2O and SiO. The maximum variability of emissivity is ≈20% in the short wavelength region (5 to 6.5 μm, 2000 to 1500 cm−1), and is more subdued, <4%, at longer wavelengths. The strengths of features present in infrared spectra of Mars cannot be solely provided by emissivity variations of palagonite; some other material or mechanism must provide additional absorption(s).
Structural history of Maxwell Montes, Venus: Implications for Venusian mountain belt formation
Authors: Keep Myra , Hansen Vicki L.
Published: February 2004

Models for Venusian mountain belt formation are important for understanding planetary geodynamic mechanisms. A range of data sets at various scales must be considered in geodynamic modelling. Long wavelength data, such as gravity and geoid to topography ratios, need constraints from smaller‐scale observations of the surface. Pre‐Magellan images of the Venusian surface were not of high enough resolution to observe details of surface deformation. High‐resolution Magellan images of Maxwell Montes and the other deformation belts allow us to determine the nature of surface deformation. With these images we can begin to understand the constraints that surface deformation places on planetary dynamic models. Maxwell Montes and three other deformation belts (Akna, Freyja, and Danu montes) surround the highland plateau Lakshmi Planum in Venus' northern hemisphere. Maxwell, the highest of these belts, stands 11 km above mean planetary radius. We present a detailed structural and kinematic study of Maxwell Montes. Key observations include (1) dominant structural fabrics are broadly distributed and show little change in spacing relative to elevation changes of several kilometers; (2) the spacing, wavelength and inferred amplitude of mapped structures are small; (3) interpreted extensional structures occur only in areas of steep slope, with no extension at the highest topographic levels; and (4) deformation terminates abruptly at the base of steep slopes. One implication of these observations is that topography is independent of thin‐skinned, broadly distributed, Maxwell deformation. Maxwell is apparently stable, with no observed extensional collapse. We propose a “deformation‐from‐below” model for Maxwell, in which the crust deforms passively over structurally imbricated and thickened lower crust. This model may have implications for the other deformation belts.
Discrimination of poorly exposed lithologies in imaging spectrometer data
Authors: Farrand William H. , Harsanyi Joseph C.
Published: February 2004

High spectral resolution imagery produced by imaging spectrometers enables the discrimination of geologic materials whose surface expression is subpixel in scale. Moreover, the use of such data makes it possible to distinguish materials which are characterized only by subtle differences in the spectral continuum. We define the “continuum” as the reflectance or radiance spanning the space between spectral features. The capability to distinguish subpixel targets will prove invaluable in studies of the geology of the Earth and planets from airborne and spaceborne imaging spectrometers. However, subpixel targets can only be uniquely identified in a truly optimal sense through the application of data reduction techniques that model the spectral contribution of both target and background materials. Two such techniques are utilized herein. They are a spectral mixture analysis approach and a low probability detection routine based on orthogonal subspace projection. These techniques were applied to the problem of detecting two different volcanic tuff units, one basaltic and one rhyolitic, in two different scenes of data measured by the airborne visible/infrared imaging spectrometer (AVIRIS). These tuff units have limited exposures from an overhead perspective and have spectral signatures which differ from those of background materials only in terms of subtle slope changes in the reflectance continuum. Of the two approaches, it was found that the low probability detection algorithm was more effective in highlighting those pixels that contained the target tuff units while suppressing the response of undesired background materials.
A review of the origins of subparallel ridges and troughs: Generalized morphological predictions from terrestrial models
Authors: Pappalardo Robert T. , Greeley Ronald
Published: February 2004

The morphologies of ridges and troughs on Earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites. We consider tension fracturing, normal faulting, thrust faulting, buckle folding, strike‐slip tectonism, fissure eruption, wall diapirism, surface folding of a flow, and near‐surface (laccolithic) intrusion. The landform characteristics and geologic associations predicted of each process are generalized to be independent of planet‐specific parameters. We find that the combinations of landforms derived from each process are unique, making each distinguishable in the idealized case. Morphological parameters most diagnostic of formational process are ridge and trough cross‐sectional shape, termination style, and incipient character. The characteristics described are useful in constraining the origin of ridge and trough terrains observed in Voyager images and will aid in analysis of high‐resolution images from the Galileo and Cassini spacecraft.
Dust torus around Mars
Authors: Juhász Antal , Horányi Mihály
Published: February 2004

We investigate the orbital dynamics of small dust particles generated via the continuous micrometeoroid bombardment of the Martian moons. In addition to Mars's oblateness, we also consider the radiation pressure perturbation that is complicated by the planet's eccentric orbit and tilted rotational axis. Considering the production rates and the lifetimes of dust grains, we show that particles from Deimos with radii of about 15 μm are expected to dominate the population of a permanently present and tilted dust torus. This torus has an estimated peak number density of ≃5×10−12 cm−3 and an optical depth of ≃4 × 10−8.
Abyssal bedforms as indicators of changing bottom current flow: Examples from the U.S. East Coast continental rise
Authors: Flood Roger D.
Published: February 2004

Direct indications of circulation patterns in the deep sea (especially along continental rises and sediment drifts) can be obtained through the analysis of abyssal bedforms such as furrows and mud waves. Such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments. Water that sinks in the northern North Atlantic forms part of the Western Boundary Undercurrent along the continental rise of eastern North America and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time. Studies of furrows and mud waves along the Blake‐Bahama Outer Ridges show that they are active sedimentary features today, and that bottom flow speeds near the time of the Younger Dryas were increased over later intervals. A more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern.
Magnetostratigraphic, biostratigraphic, and stable isotope stratigraphy of an Upper Miocene drill core from the Salé Briqueterie (northwestern Morocco): A high‐resolution chronology for the Messinian stage
Authors: Hodell David A. , Benson Richard H. , Kent Dennis V. , Boersma Anne , Rakic‐El Bied Kruna
Published: February 2004

We report a high‐resolution stable isotope, carbonate, magnetostratigraphic, and biostratigraphic record from a 175‐m drill core from the Salé Briqueterie, which is part of the Bou Regreg section in northwestern Morocco. The Salé drill core spans the interval from paleomagnetic Chron C4n partim to C3r (earliest Gilbert), which represents the time leading up to and including the isolation and desiccation of the Mediterranean (i.e., the Messinian salinity crisis). During Chrons C3An and C3Ar (6.935 to 5.894 Ma) the isotope and carbonate signals display quasi‐periodic variations with estimated periods of 40 and 100 kyr, respectively. We interpret the 40‐kyr δ18O variations as reflecting changes in global ice volume caused by obliquity‐induced changes (41 kyr) in solar insolation in polar regions. The 100‐kyr carbonate variations probably represent long‐term modulation of the amplitude of the precessional cycle (∼21 kyr), which is not resolved by our sampling frequency. The cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of Shackleton et al. (1994a) from stage TG24 in Chron C3r (earliest Gilbert) to stage C3Ar.δ18O.18 at the base of Chron C3Ar (6.935 Ma). A major change in paleoceanographic conditions is recorded across the Tortonian/Messinian boundary, which we correlate to Chron C3Bn at 7.04 Ma. Benthic foraminiferal δ18O values increased by an average of 0.4‰ in two steps at 7.17 Ma and 6.8 Ma and δ13C values decreased by 0.7–0.8‰ between 7.1 and 6.8 Ma, representing the late Miocene carbon shift. The first step in δ18O values coincides with an inferred reversal in deep water circulation through the Rifian Corridor, and the second correlates with the base of the Tripoli Formation and onset of “crisis conditions” in the Mediterranean. We suggest that the increase in δ18O values represents, at least in part, an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the Mediterranean. Average δ18O values remained high throughout most of Chrons C3Ar and C3An, reaching maximum δ18O values during isotope stages TG20 and 22 in Chron C3r (earliest Gilbert). The glacio‐eustatic falls associated with these events may have resulted in the complete isolation of the Mediterranean from the world ocean (Shackleton et al., 1994a). Following stage TG12 in the Salé record, there exists a trend toward progressively lower δ18O values that may represent a series of marine transgressions that eventually reflooded the Mediterranean and ended the Salinity Crisis.
Research perspectives of sediment waves and drifts: Monitors of global change in deepwater circulation
Authors: Mienert J. , Flood R. D. , Dullo W. C.
Published: February 2004

The purpose of this special section in Paleoceanography is to present interdisciplinary approaches for contributing to the reconstruction of ocean circulation and its response to climate changes. A high‐priority objective for understanding the causes and mechanisms of climate change is the monitoring of past ocean circulation and oceanic heat and nutrient transport. Lehman and Keigwin [1992] have shown that cooling, for example, during the younger Dryas event, may have culminated in a cessation of the oceans conveyor circulation. The cooling in the North Atlantic was apparently the result of reduced northward heat transport in the upper water masses of the North Atlantic conveyor belt. In contrast, intervals with a strong surface and deepwater circulation were marked by a high northward heat transport. For the understanding of the causes and the timing of such rapid,highfrequency events, marine records of high deposition ratecores are needed. These cores should provide evidence for changes in abyssal circulation and heat transport, as well as arecord of surface and deepwater characteristics. The sediment drifts of the North Atlantic and in other ocean basins are one of the major targets for the recovery of sediments with high deposition rates (>10 cm/kyr) and for reconstructing the role of both intermediate and deepwater production in the conveyor belt, that is drawing low‐latitude heat northward. We stress the need for international programs targeting high deposition rate areas on sediment drifts and sediment waves in order to understand (1) the evolution of the conveyor belt and (2) its dynamics and variability. The North Atlantic, where sediment drifts are concentrated, will provide ideal study areas with time resolutions comparable to those of ice core records but with records linked directly to the record of changing bottom water flow. Therefore one can address the changes in circulation, heat and carbon budget on high and ultrahigh resolution records.
Late Pleistocene climatic trajectory in the phase space of global ice, ocean state, and CO2: Observations and theory
Authors: Saltzman Barry , Verbitsky Mikhail
Published: February 2004

We consider the evolution of three main slow‐response variables describing the state of the climate system during the late Pleistocene (global ice mass, atmospheric carbon dioxide, and the ocean state) as measured, respectively, by the SPECMAP δ18O record, the Vostok CO2 record, and the North Atlantic sea surface temperature record at 50°N (core K708‐1), which is coherent with other ocean state properties. Their coevolution is portrayed in the form of a trajectory in the phase space of the three variables and its projections on its three phase planes. The oscillatory nature and phase lags of the variables are clearly illustrated, suggesting a “physical process” scenario that can account for the observations. The basic element of this scenario is a free, near‐100 kyr period, oscillation driven by internal instability, involving feedbacks between all three variables under the influence of Earth‐orbital (Milankovitch) forcing and long‐term tectonic CO2 forcing. It is shown that a phenomenological theory advanced by the authors, emphasizing the role of CO2, provides a credible account of the phase‐space trajectory.
Influence of variable rates of neritic carbonate deposition on atmospheric carbon dioxide and pelagic sediments
Authors: Walker James C. G. , Opdyke Bradley C.
Published: February 2004

Short‐term imbalances in the global cycle of shallow water calcium carbonate deposition and dissolution may be responsible for much of the observed Pleistocene change in atmospheric carbon dioxide content. However, any proposed changes in the alkalinity balance of the ocean must be reconciled with the sedimentary record of deep‐sea carbonates. The possible magnitude of the effect of shallow water carbonate deposition on the dissolution of pelagic carbonate can be tested using numerical simulations of the global carbon cycle. Boundary conditions can be defined by using extant shallow water carbonate accumulation data and pelagic carbonate deposition/dissolution data. On timescales of thousands of years carbonate deposition versus dissolution is rarely out of equilibrium by more than 1.5 × 1013 mole yr−1. Results indicate that the carbonate chemistry of the ocean is rarely at equilibrium on timescales less than 10 ka. This disequilibrium is probably due to sea level‐induced changes in shallow water calcium carbonate deposition/dissolution, an interpretation that does not conflict with pelagic sedimentary data from the central Pacific.
Depositional fluxes, palaeoproductivity, and ice rafting in the NE Atlantic over the past 30 ka
Authors: Manighetti B. , McCave I. N.
Published: February 2004

Over the last 30 ka, sediment flux to the northeast Atlantic Ocean has been strongly influenced by the growth and decay of northern hemisphere ice sheets, input of ice‐rafted detritus, the migration of the polar front, and associated changes in patterns of biological productivity. We examined cores from 47°N to 60°N along 20°W to determine the flux of components including carbonate, organic carbon and terrigenous material and divided into size fractions. During the glacial period, fine carbonate flux was low and ice‐rafted input high. Burial flux during the Holocene became dominated by coccolith and foraminiferal carbonate, with minor organic matter and biogenic silica. Estimates of palaeoproductivity are ambiguous: a method which corrects for water depth and sedimentation rate suggests no clear glacial to Holocene change, whereas a method based on percentage of organic carbon suggests increased productivity from glacial to Holocene of around 60%. Sites of sediment focusing saw a change from enhanced fine terrrigenous flux in the glacial to enhanced fine carbonate in the Holocene. After compensating for sediment focusing, glacial ice‐rafted flux distribution shows a decrease from south to north across the area, reflecting cyclonic surface water circulation. Deposition of ice‐rafted detritus during Heinrich events H1 and H2 led to enhanced preservation of organic matter immediately beneath the layers, indicating a rapid accumulation rate.
Late Quaternary surface circulation in the east equatorial South Atlantic: Evidence from Alkenone sea surface temperatures
Authors: Schneider Ralph R. , Müller Peter J. , Ruhland Götz
Published: February 2004

Angola Basin and Walvis Ridge records of past sea surface temperatures (SST) derived from the alkenone Uk37 index are used to reconstruct the surface circulation in the east equatorial South Atlantic for the last 200,000 years. Comparison of SST estimates from surface sediments between 5° and 20°S with modern SST data suggests that the alkenone temperatures represent annual mean values of the surface mixed layer. Alkenone‐derived temperatures for the warm climatic maxima of the Holocene and the penultimate interglacial are 1 to 4°C higher than latest Holocene values. All records show glacial to interglacial differences of about 3.5°C in annual mean SST, which is about 1.5°C greater than the difference estimated by CLIMAP (1981) for the eastern Angola Basin. At the Walvis Ridge, significant SST variance is observed at all of the Earth's orbital periodicities. SST records from the Angola Basin vary predominantly at 23‐ and 100‐kyr periodicities. For the precessional cycle, SST changes at the Walvis Ridge correspond to variations of boreal summer insolation over Africa and lead ice volume changes, suggesting that the east equatorial South Atlantic is sensitive to African monsoon intensity via trade‐wind zonality. Angola Basin SST records lag those from the Walvis Ridge and the equatorial Atlantic by about 3 kyr. The comparison of Angola Basin and Walvis Ridge SST records implies that the Angola‐Benguela Front (ABF) (currently at about 14–16°S) has remained fairly stationary between 12° and 20°S (the limits of our cores) during the last two glacial‐interglacial cycles. The temperature contrast associated with the ABF exhibits a periodic 23‐kyr variability which is coherent with changes in boreal summer insolation over Africa. These observations suggest that surface waters north of the present ABF have not directly responded to monsoon‐modulated changes in the trade‐wind vector, that the central field of zonally directed trades in the southern hemisphere was not shifted or extended northward by several degrees of latitude during glacials, and that a cyclonic gyre circulation has existed in the east equatorial South Atlantic over the last 200,000 years. This scenario contradicts former assumptions of glacial intensification of the Benguela Current into the eastern Angola Basin and increased coastal upwelling off Angola.
A 300 kyr high‐resolution aridity record of the North African continent
Authors: Matthewson A. P. , Shimmield G. B. , Kroon D. , Fallick A. E.
Published: February 2004

New oxygen isotope data and a high‐resolution carbonate record of a core from the northwest African margin are presented, in combination with geochemical (Zr/Rb ratio, Al, Ba, organic carbon, and biogenic opal fluxes) and sedimentological (median grain size) parameters, to show variations in African continental aridity, wind strength, and upwelling productivity. The carbonate record is diluted by the supply of aeolian dust from the arid Saharan region of north Africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines. Spectral analysis of the carbonate profile and comparison with SPECMAP (stacked oxygen isotope) and ETP (solar radiation) profiles shows that the precessional (23 kyr) insolation‐induced African climate system is decoupled from global (ice volume) climate changes operating on a 100 kyr frequency. Detailed studies of glacial terminations I, II, and III confirm the presence of rapid, short timescale climatic variations (in aridity).
The role of hydrological processes in ocean‐atmosphere interactions
Authors: Webster Peter J.
Published: February 2004

Earth is unique among the planets of the solar system in possessing a full hydrological cycle. The role of water in the evolution of planetary atmospheres is discussed. As the atmospheres of the planets developed and modified the early climates of the planets, only the climate trajectory of Earth intercepted the water phase transitions near the triple point of water, thus allowing the full gamut of water forms to coexist. As a result, transitions between the water phases pervade the entire system and probably are responsible for the creation of a unique climate state. The interactions between the components of the climate system are enriched by the nonlinearity of the water phase transitions. The nonlinear character of the phase transitions of water suggests that the climate should be particularly sensitive to hydrological processes, especially in the tropics. Signatures of the nonlinearity are found in both the structure of the oceans and the atmosphere. Specific processes that determine the character of ocean‐atmosphere interaction, including the role of ambient water vapor and clouds, the selective absorption of radiation by the ocean, the distribution of total heating in the ocean‐atmosphere system, and the role of the flux of freshwater, are discussed in detail. Models of the ocean and atmospheric and oceanic data and models of the coupled system are used to perform systematic analyses of hydrological processes and their role in system interaction. The analysis is extended to consider the role of hydrological processes in the basic dynamics and thermodynamics of oceanic and atmospheric systems. The role hydrological processes play in determining the scale of the major atmospheric circulation patterns is investigated. Explanations are offered as to why large‐scale convection in the tropical atmosphere is constrained to lie within the 28°C sea surface temperature contour and how hydrological processes are involved in interannual climate variability. The relative roles of thermal and haline forcing of the oceanic thermohaline circulation are discussed. Hydrological processes are considered in a global context by the development of a conceptual model of a simple planetary system. The constancy and maintenance of the very warm tropical sea surface temperatures are seen to be critical for the stability of climate. However, within the confines of the simplicity of the theory, the climate system, dominated by hydrological processes, conspires to maintain the temperature of the tropical warm pools.
Noble gas state in the mantle
Authors: Ozima Minoru
Published: February 2004

Noble gas elemental and isotopic data on mantle‐derived materials such as mid‐ocean ridge basalt (MORB), hotspot volcanics, diamonds, and mantle xenoliths published up to August 1993 are reviewed critically. Characteristic features of the mantle‐derived materials, which are important in evaluating the significance of the data, are discussed. We choose MORB glasses, hotspot volcanics, mantle xenoliths, and diamonds as the sources to infer the noble gas state in the mantle: MORB and hotspot volcanics to represent the modern depleted and the less degassed mantle, and diamonds to represent the ancient mantle. Because of various fractionation processes, the elemental abundance data obtained from the mantle‐derived materials is unlikely to constrain the noble gas composition of the mantle, except for the systematic enrichment of the heavier noble gases relative to air. On the other hand, apart from the secondary addition of radiogenic components such as 4He, 21Ne, 40Ar, 129Xe, and 131–136Xe, the noble gas isotopic compositions deduced from the mantle‐derived materials can provide useful information as to the values in the mantle; a best estimate of the mantle noble gas state is presented.
Three‐dimensional seismic models of the Earth's mantle
Authors: Ritzwoller Michael H. , Lavely Eugene M.
Published: February 2004

Accurate models of the distribution of elastic heterogeneity in the Earth's mantle are important in many areas of geophysics. The purpose of this paper is to characterize and compare quantitatively a set of recent three‐dimensional models of the elastic structure of the Earth, to assess their similarities and differences, and to analyze their fit to one class of data in order to highlight fruitful directions for future research. The aspherical models considered are the following: M84C (Woodhouse and Dziewonski, 1984), LO2.56 (Dziewonski, 1984), MDLSH (Tanimoto, 1990a), SH.10c.17 (Masters et al., 1992), and S12_WM13 (Su et al., 1994). Through much of the discussion, M84C and LO2.56 are combined into a single whole mantle model, M84C + LO2.56. The fit of each model to previously tabulated even degree normal mode structure coefficients taken from Smith and Masters (1989a) and Ritzwoller et al. (1988) for multiplets along the normal mode fundamental and first, second, and fifth overtone branches is also presented. Rather than concentrating on detailed comparisons of specific features of the models, analyses of these models are general and statistical in nature. In particular, we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations. In general, the results of the comparisons between the models are encouraging, especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle. There remain, however, significant discrepancies in amplitude and in the radial distribution of heterogeneity, especially near the top of the upper mantle and near the top of the lower mantle. The confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies. The factors that may be responsible for the differences in the models and/or for the misfit between the observed and predicted normal mode data are divided into two types: intrinsic (or procedural) and extrinsic (or structural). We discuss only three extrinsic factors at length here, including errors in the reference crustal models, unmodeled topography on discontinuities in the interior of the mantle, and errors in the assumed relationships between shear (υs) and compressional (υp) heterogeneity.
A comparative study of two‐dimensional multiple scattering techniques
Authors: Elsherbeni Atef Z.
Published: February 2004

The scattering of an electromagnetic plane wave from an arbitrary configuration of parallel circular cylinders is investigated using four different techniques. The cylinders are made of perfectly conducting or homogeneous dielectric material. These techniques are a boundary value type of solution, an iterative scattering procedure, a hybrid approach based on a combination of exact and method of moments solution, and a high‐frequency asymptotic approximation. The analysis is given in detail for the transverse magnetic (TM) polarization, and that for the transverse electric (TE) polarization is outlined. Numerical results are provided to show the major differences between these techniques and the validity of using circular cylinders in modeling composite two‐dimensional scatterers.
Advanced directional sea spectrum studies with the Bribie Island MF/lower HF surface‐wave radar
Authors: Parkinson M. L.
Published: February 2004

The directional factor of sea spectra is an important oceanographic quantity which has been measured infrequently by a limited number of techniques. Here I report the use of a large‐phased antenna array transmitting at 1.98, 3.84, and 5.80 MHz for measuring the directional factor from the observed first‐order surface‐wave sea echoes (i.e., those due to Bragg resonance) measured simultaneously at many azimuths through rapid, digital control of beam direction. This measurement process compares with the extraction of directional factors from second‐order radar cross sections in that both techniques rely on the statistical quality of observations and the stability of integral inversion algorithms employed. Preliminary observations show that, in principle, this new technique can be used to measure the directional factor automatically, including dominant sea direction and angular spread with high resolution (±2°) in near‐real time. The strategy required to achieve high accuracy is outlined.
Electromagnetic field generated in model of human head by simplified telephone transceiver
Authors: King Ronold W. P.
Published: February 2004

Possible adverse effects of electromagnetic fields on the human body and especially on the nervous system and the brain are of increasing concern, particularly with reference to cellular telephone transceivers held close to the head. An essential step in the study of this problem is the accurate determination of the complete electromagnetic field penetrating through the skull into the brain. Simple analytical formulas are derived from the theory of the horizontal electric dipole over a layered region. These give the components of the electric and magnetic fields on the air‐head surface, in the skin‐skull layer, and throughout the brain in terms of a planar model with the dimensions and average electrical properties of the human head. The specific absorption rate (SAR) is also determined.
Profile inversion in a cylindrically stratified lossy medium
Authors: Xia Jake J. , Habashy Tarek M. , Kong Jin Au
Published: February 2004

We present an algorithm for simultaneously inverting an unknown cylindrically stratified permittivity and conductivity profile. The algorithm is based on a recently developed inversion method referred to as the renormalized source type integral equation approach. This method provides an explicit dependence on the unknowns to be inverted for and hence allows one to compute the derivatives of the fields with respect to these unknowns (the Jacobian and Hessian matrices) in an explicit form. Furthermore, by prestoring the entries of the inversion equation, which depend only on the background medium and are independent of the unknown profile, the method reported here does not require solving the full forward problem repeatedly as in the case of the distorted Born approach and is therefore faster in implementation.
Magnetic fields and cancer development in animal models
Authors: Holmberg Bo , Rannug Agneta
Published: February 2004

Several laboratory animal studies have been up to July 1993 published with the aim to investigate possible carcinogenic or cocarcinogenic effects of magnetic fields. No large‐scale study of complete carcinogenicity, considering tumor as an endpoint, has been performed. Nor has any study on tumor initiation only been performed. In studies designed for tumor promotion, small parallel series for complete carcinogenicity have also been included in some experiments. These small series do not permit an evaluation of the possible complete carcinogenicity or noncarcinogenicity of magnetic fields. Studies designed to illuminate the possibility of a tumor‐promotive effect of magnetic fields seem not to support the hypothesis that continuous magnetic fields act as promoters in rats or mice. Observations on skin tumors in mice seem to indicate a possible copromotional effect of a 2‐mT 60‐Hz magnetic field and a simultaneous treatment with a known skin tumor promoter in the sensitive Sencar mouse strain.
Mid‐America Computerized Ionospheric Tomography Experiment (MACE '93)
Authors: Kronschnabl G. R. , Bust G. S. , Cook J. A. , Vasicek C. J.
Published: February 2004

A computerized ionospheric tomography (CIT) experiment utilizing an array of nine Navy Navigation Satellite System (NNSS) receivers deployed along a north‐south line from South Dakota to south Texas (spanning over 2000 km) is currently underway. The Mid‐America Computerized Ionospheric Tomography Experiment (MACE '93) began collecting data from three receivers deployed in Texas on June 1, 1993. This “short communiqué” presents preliminary results from the experiment.
Comparison of geometries for ionospheric tomography
Authors: Sutton Eric , Na Helen
Published: February 2004

The distribution of electron density in the ionosphere can be imaged using computerized tomography techniques. Because of the limited view angle, resolution in the vertical direction is poor but can be improved by using a priori information in the reconstruction algorithm. The orthogonal decomposition algorithm provides a theoretical framework that unifies several traditional tomographic reconstruction algorithms, depending upon the choice of basis functions for the image domain. In this algorithm, the image is reconstructed as the weighted sum of a set of orthonormal basis functions. For ionospheric tomography, a priori information on the radial distribution of electron density can be used to guide the selection of basis functions. The resolution performance of the orthogonal decomposition algorithm depends upon the geometry used for the reconstruction. This paper analyzes three possible geometries. The factors that influence the selection of a particular geometry are (1) the ability to incorporate a priori information into the algorithm and (2) the ability to solve for the radial distribution. An analysis of the radial resolution in ionospheric tomography is also presented.
Two‐frequency correlation function of the single‐path HF channel: Theory and comparison with the experiment
Authors: Fridman S. V. , Fridman O. V. , Lin K. H. , Yeh K. C. , Franke S. J.
Published: February 2004

The two‐frequency mutual coherence of HF electromagnetic field fluctuations caused by ionospheric irregularities in an oblique radio path is studied. The single scattering approximation is first developed and then extended to stronger field fluctuations by applying Rytov's approximation. Of considerable interest for wideband HF cpmmunication applications is the two‐frequency, two‐time correlation function of the channel 〈u* (ω,t1)u(ω + Ω,t1 + t)〉, where u(ω,t) is the complex amplitude of the radio wave transmitted at a frequency ω, measured by a receiver at time t. Our results show that this particular quantity behaves as though there were no diffraction effects (no Fresnel‐filtering effects). Thus the correlation time τ0 is close to Λ /ν regardless of the ratio between the irregularity size and the Fresnel length. Here Λ is the irregularity size and ν is the component of the drift velocity perpendicular to the ray path. Typical scales for the two‐frequency mutual correlation function are studied, and simple physical interpretations are developed. For example, the correlation bandwidth Ωc is determined by the condition that the rays connecting the transmitter and receiver at ω and ω + Ωc are separated from each other (in the vicinity of the reflection point) by a distance of the order of Λ. A procedure is described which allows the size of irregularities and components of plasma drift to be estimated from one of the mutual correlation functions and from the autocorrelation function. This procedure was applied to measurements from the University of Illinois sounder (transmitter is located in Platteville, Colorado, receiver in Urbana, Illinois). In the examples considered, irregularities with a size of the order of a few hundred meters drifting with a velocity 20–100 m s−1 can explain the fading. The observed magnitude of fading is consistent with an rms irregularity fluctuation of the order of a few tenths of one percent.