bluuebunny/crossref_metadata_2025 huggingface dataset:
Applications of spaceborne radar laboratory data to the study of aeolian processes
Authors: Greeley Ronald , Blumberg Dan G. , McHone John F. , Dobrovolskis Anthony , Iversen James D. , Lancaster Nicholas , Rasmussen Keld R. , Wall Stephen D. , White Bruce R.
Published: February 2004

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ=24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ=5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation‐free areas from orbit using radar systems.
The structure and compensation of the lunar highland crust
Authors: Wieczorek Mark A. , Phillips Roger J.
Published: February 2004

A new method of interpreting geoid to topography ratios (GTRs) on a sphere is presented, in which it is shown that the GTR is equivalent to a sum of spectrally weighted degree‐dependent admittances. Using this method combined with newly obtained gravity, topography, and near‐global surface iron concentrations from the Clementine mission, the structure and compensation of the lunar highland crust have been investigated. Geoid to topography ratios were tested against single‐layer Pratt and Airy compensation models, as well as dual‐layered Airy models. Regional lateral variations in crustal density are found to play an insignificant role in crustal compensation, and the single‐layer and dual‐layered Airy models both strongly suggest that the lunar crust is vertically stratified. The depth of the intracrustal interface obtained from these models is consistent with the existence of a 20‐km seismic discontinuity beneath the Apollo 12 and 14 sites. A uniform density crust with compensation occurring at the Moho is a viable interpretation of crustal structure only when the extreme limits of the observed GTR distribution are used.
Extensional tilt blocks on Miranda: Evidence for an upwelling origin of Arden Corona
Authors: Pappalardo Robert T. , Reynolds Stephen J. , Greeley Ronald
Published: February 2004

Subparallel ridges and troughs in the outer belt of Arden Corona, on the Uranian satellite Miranda, are interpreted as tilt blocks formed by extension and normal faulting. Fault scarps generally face outward from the corona, exposing dark material in the subsurface. Reconstruction of faults along a deep rift zone bounding the corona suggests initial dips of ∼50°. Local extension reaches ∼70%, extremely high in comparison to previous estimates of strain on icy satellites. A rise adjacent to the rift zone is modeled as flexural and indicates an effective elastic lithospheric thickness of ∼2 km at the time of flexure. The assumption that faulting has significantly weakened the lithosphere suggests a mechanical lithosphere thickness of ∼5 to 10 km. Corresponding thermal gradients in a frictionally controlled ice lithosphere are ∼8 to 20 K km‐1, and lithospheric tensional strength is ∼0.4 to 1.8 MPa. Normal faulting in Arden Corona indicates that internal upwelling likely formed the corona, and the outward facing direction of faults is consistent with such a model. An upwelling origin of Miranda's coronae eliminates the need to invoke catastrophic breakup and reaccretion of the satellite as an explanation for its surface geology.
Duration of tessera deformation on Venus
Authors: Gilmore Martha S. , Ivanov Mikhail A. , Head James W. , Basilevsky Alexander T.
Published: February 2004

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of craters fractured by late‐stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.
The persistence of equatorial ground ice on Mars
Authors: Mellon Michael T. , Jakosky Bruce M. , Postawko Susan E.
Published: February 2004

In the current Martian climate, ground ice is unstable in the equatorial regions and, if present, would undergo sublimation and diffusive loss to the atmosphere. Previous studies suggest that the ice table (the uppermost occurrence of ice in the regolith) would continuously recede throughout geologic history. We present new models of the behavior of ice in the Martian equatorial regolith which predict that porous interstitial ice will persist at relatively shallow depths for geologically long periods of time. The persistence of interstitial ice is due to recondensation of water vapor as it diffuses toward the surface, encountering colder temperatures. We discuss the implications for the formation of rampart craters and debris aprons.
The migration of volatiles on the surfaces of Mercury and the Moon
Authors: Butler Bryan J.
Published: February 2004

Radar observations of Mercury have provided the startling discovery of the probable existence of substantial deposits of ices in permanently shaded polar regions [Butler et al., 1993; Harmon et al., 1994]. This has renewed the old argument about the existence of such deposits in the polar regions of the Moon. It is most likely necessary for volatiles to be able to migrate to the polar regions of these bodies in order to build up such deposits. This paper presents the results of the extension of a Monte Carlo simulation of the migration of molecules on the surface of Mercury by Butler et al. [1993]. The results of the simulations show that for typical conditions on Mercury, ∼5–15% of all H2O which is placed randomly on the surface migrates to stable (permanently shaded) polar regions. For the Moon the numbers are ∼20–50%. The numbers for the migration of CO2 are ∼2% for Mercury and ∼13% for the Moon. These percentages are similar to those previously calculated and support the idea of ice deposits in the polar regions of both Mercury and the Moon.
A 70th degree lunar gravity model (GLGM‐2) from Clementine and other tracking data
Authors: Lemoine Frank G. R. , Smith David E. , Zuber Maria T. , Neumann Gregory A. , Rowlands David D.
Published: February 2004

A spherical harmonic model of the lunar gravity field complete to degree and order 70 has been developed from S band Doppler tracking data from the Clementine mission, as well as historical tracking data from Lunar Orbiters 1–5 and the Apollo 15 and 16 subsatellites. The model combines 361,000 Doppler observations from Clementine with 347,000 historical observations. The historical data consist of mostly 60‐s Doppler with a noise of 0.25 to several mm/s. The Clementine data consist of mostly 10‐s Doppler data, with a data noise of 0.25 mm/s for the observations from the Deep Space Network, and 2.5 mm/s for the data from a naval tracking station at Pomonkey, Maryland. Observations provided Clementine, provide the strongest satellite constraint on the Moon's low‐degree field. In contrast the historical data, collected by spacecraft that had lower periapsis altitudes, provide distributed regions of high‐resolution coverage within ±29° of the nearside lunar equator. To obtain the solution for a high‐degree field in the absence of a uniform distribution of observations, we applied an a priori power law constraint of the form 15×10−5/l2 which had the effect of limiting the gravitational power and noise at short wavelengths. Coefficients through degree and order 18 are not significantly affected by the constraint, and so the model permits geophysical analysis of effects of the major basins at degrees 10–12. The GLGM‐2 model confirms major features of the lunar gravity field shown in previous gravitational field models but also reveals significantly more detail, particularly at intermediate wavelengths (103 km). Free‐air gravity anomaly maps derived from the new model show the nearside and farside highlands to be gravitationally smooth, reflecting a state of isostatic compensation. Mascon basins (including Imbrium, Serenitatis, Crisium, Smythii, and Humorum) are denoted by gravity highs first recognized from Lunar Orbiter tracking. All of the major mascons are bounded by annuli of negative anomalies representing significant subsurface mass deficiencies. Mare Orientale appears as a minor mascon surrounded by a horseshoe‐shaped gravity low centered on the Inner and Outer Rook rings that is evidence of significant subsurface structural heterogeneity. Although direct tracking is not available over a significant part of the lunar farside, GLGM‐2 resolves negative anomalies that correlate with many farside basins, including South Pole‐Aitken, Hertzsprung, Korolev, Moscoviense, Tsiolkovsky, and Freundlich‐Sharonov.
Chemical components of the Martian soil: Melt degassing, hydrothermal alteration, and chondritic debris
Authors: Newsom Horton E. , Hagerty Justin J.
Published: February 2004

On the basis of measurements by the Viking and Soviet Phobos missions the fine grained surficial drift material on Mars is enriched in moderately volatile and mobile elements such as S, Cl, K, and Br. The compounds of these elements may amount to as much as 16 wt% of the soil, while the bulk of the soil appears to consist primarily of crustal material whose composition is similar to that of the basaltic Mars meteorites (Clark, 1993). Three possible geochemical sources (melt degassing, hydrothermal alteration, and accretion of chondritic material) may have contributed to the enrichments. We have evaluated possible chemical signatures for these sources by examining the relative abundances of chemical elements in the different components to the measured K abundance in the Mars soil and have identified characteristic elements for each component. The relative abundances of mobile elements in the different components are much less variable than their absolute abundances, which strongly depend on conditions such as temperature and water composition. The volatility controlled elemental signature of the melt degassing process includes enrichments of Zn, Mo, Cd, Ba, and W, relative to the elemental abundances in the other components and Martian meteorites. Hydrothermal alteration of minerals and glass, controlled by low‐temperature solubility, makes up a second component with a characteristic lithium enrichment signature. A chondritic component is enriched in siderophile and chalcophile elements including Ni, compared to material derived from the Martian mantle, which was depleted in siderophile elements due to core formation. Unfortunately, the existing data for the Mars soil do not include any of these key signature elements, and the uncertainties for the Viking analyses could allow large amounts of each of these elements. The Mars Pathfinder inorganic analysis experiment and the 2001 orbital gamma ray experiment have the potential to determine some of these characteristic elements and to reveal the possible contributions to the soil. The enriched components of the soil, especially Cd and Pb, could also be a potential hazard for future Martian explorers who will be exposed to the dust carried into their habitats on their space suits.
Raman spectroscopy for mineral identification and quantification for in situ planetary surface analysis: A point count method
Authors: Haskin Larry A. , Wang Alian , Rockow Kaylynn M. , Jolliff Bradley L. , Korotev Randy L. , Viskupic Karen M.
Published: February 2004

Quantification of mineral proportions in rocks and soils by Raman spectroscopy on a planetary surface is best done by taking many narrow‐beam spectra from different locations on the rock or soil, with each spectrum yielding peaks from only one or two minerals. The proportion of each mineral in the rock or soil can then be determined from the fraction of the spectra that contain its peaks, in analogy with the standard petrographic technique of point counting. The method can also be used for nondestructive laboratory characterization of rock samples. Although Raman peaks for different minerals seldom overlap each other, it is impractical to obtain proportions of constituent minerals by Raman spectroscopy through analysis of peak intensities in a spectrum obtained by broad‐beam sensing of a representative area of the target material. That is because the Raman signal strength produced by a mineral in a rock or soil is not related in a simple way through the Raman scattering cross section of that mineral to its proportion in the rock, and the signal‐to‐noise ratio of a Raman spectrum is poor when a sample is stimulated by a low‐power laser beam of broad diameter. Results obtained by the Raman point‐count method are demonstrated for a lunar thin section (14161,7062) and a rock fragment (15273,7039). Major minerals (plagioclase and pyroxene), minor minerals (cristobalite and K‐feldspar), and accessory minerals (whitlockite, apatite, and baddeleyite) were easily identified. Identification of the rock types, KREEP basalt or melt rock, from the 100‐location spectra was straightforward.
The infrared absorption spectrum of carbon dioxide ice from 1.8 to 333 μm
Authors: Hansen Gary B.
Published: February 2004

The optical constants of solid carbon dioxide are needed, for example, to accurately calculate the energy balance of the seasonal polar caps of Mars, which are composed primarily of CO2 ice. Only a few measurements of uncertain accuracy have been made in the broad wavelength regions of weak absorption between the isolated narrow infrared absorption bands of solid CO2, which determine the reflectance and emissivity of these polar caps. A laboratory experiment was therefore undertaken to improve on and extend the wavelength range of the previous data by measuring transmission through thick samples of high quality to determine the spectral absorption coefficient α in infrared wavelength regions of low absorption between 1.8 and 333 μm (30–5555 cm−1 in wavenumber). A technique was developed to grow clear, thick CO2 ice samples from the gas at a temperature of 150 K, with optical path lengths between 1.6 and 107.5 mm. The extremely fine spectral structure of this material was measured using resolutions finer than 0.5 cm−1. The transmission data for all thicknesses are combined in a minimizing routine that estimates both the scattering losses for each sample (typically <20% extinction) and α (between ∼0.1 to 4000 m−1) at each wavelength, on the assumption that the scattering varies slowly with wavelength. The accuracy of the derived absorption coefficients was also estimated and is best in regions where α is not near the limits of measurement. Absorption coefficients obtained from the new measurements are an order of magnitude or more lower than earlier estimates in the transparent wavelength regions between the strong infrared bands. Also, many weak bands between 1.8 and 5.2 μm, which were only poorly defined previously, have been more accurately measured.
Lunar crustal analysis of Mare Orientale from topographic and gravity correlations
Authors: von Frese Ralph R. B. , Tan Li , Potts Laramie V. , Kim Jeong Woo , Merry Carolyn J. , Bossler John D.
Published: February 2004

We investigated the use of spectral correlation analysis for modeling the crustal features of Mare Orientale from lunar 70th degree spherical harmonic topographic and gravity field models derived from Clementine satellite and earlier investigations. The analysis considered a 64°‐by‐64° region of the Moon centered roughly on Mare Orientale at an altitude of 100 km. The topography of the study region, which includes over 11 km of relief, was modeled for its gravity effects in lunar spherical coordinates by Gauss‐Legendre quadrature integration assuming a terrain density of 2.8 g/cm3. We observed substantial positive and negative correlations between terrain gravity effects and free‐air gravity anomalies that seriously limit the utility of simple Bouguer gravity anomalies for subsurface studies. Using the wavenumber correlation spectrum between the two data sets, we designed correlation filters to extract the common features. Possible interpretations for the terrain‐correlated free‐air gravity anomalies include isostatic crustal mass imbalances that may be equilibrated by radial adjustments of the Moho of up to 44 km, assuming Airy‐Heiskanen compensation and a mantle density contrast of 0.5 g/cm3 with the crust. These Moho adjustments define mass variations that account for most of the mascon and flanking negative free‐air gravity anomalies. Furthermore, their remarkable correlation with the topographic rings of Mare Orientale points to the possible influence of a strong local stress field of the crust in the development of the ring structures. Subtracting the terrain‐correlated free‐air anomalies from the free‐air gravity anomalies and terrain gravity effects yielded terrain‐decorrelated free‐air and isostatically compensated terrain gravity anomalies, respectively, that show zero correlation. This lack of correlation may be interpreted for a Moho that involves over 100 km of relief assuming Airy‐Heiskanen compensation of the crust. Beneath Mare Orientale, we observed a minimum crustal thickness of about 17 km. Corresponding terrain‐decorrelated free‐air gravity anomalies of Mare Orientale may be related to a central cone‐shaped body of 0.5 g/cm3 density contrast with apex extending nearly 5 km below the surface, which is surrounded by a ringed‐shaped body of −0.5 g/cm3 density contrast that may extend about 7 km below the surface. These bodies resulted possibly from meteorite impact that produced a roughly circular region of breccia and highly fractured crust with a higher density core where some remelting of the rocks about the impact site may have occurred.
Laser Raman spectroscopy of varnished basalt and implications for in situ measurements of Martian rocks
Authors: Israel Elisa J. , Arvidson Raymond E. , Wang Alian , Pasteris Jill D. , Jolliff Bradley L.
Published: February 2004

Earth‐based and orbital spectroscopy and Viking Lander data strongly suggest that rocks on Mars are coated with indurated dust deposits. Rock coatings will influence mineralogical measurements to be acquired in situ from Mars rovers and other surface missions, in that most techniques will detect the coatings and/or coatings plus underlying minerals. In this paper, we present results of experiments on varnished basalt cobbles from the Lunar Crater Volcanic Field, Nevada, using laser Raman spectroscopic microprobe measurements obtained with a 514.5 nm Ar+ laser and irradiation spot sizes from 1 to 20 μm. Measurements from thin sections and fresh, unprepared rock surfaces clearly allow identification of igneous minerals. Varnish spectra acquired from thin sections as well as from unprepared exterior rock surfaces typically show broad peaks consistent with the fine‐grained nature of varnish and the dominance of manganese oxide and clay minerals. Several areas also show sharp Raman peaks due to phosphates and anatase, indicative of the heterogeneity of varnish deposits. Raman spectra taken through varnished rock surfaces reveal the mineralogy of coatings (where varnish is >15 μm in thickness) and a combination of coatings and igneous minerals in areas with thinner varnish. Given the abundance of rock coatings observed at the Viking and Pathfinder landing sites, results indicate that techniques such as Raman spectroscopy (and any others with comparable penetration depths) will sample some combination of coatings and underlying rock materials. The ability to make many in situ measurements across samples, coupled with the ability to expose fresh rock surfaces, will be needed to unambiguously characterize both the coatings and the rocks on Mars from future landers and/or rovers.
Mars without Tharsis
Authors: Zuber Maria T. , Smith David E.
Published: February 2004

The significant power in the Martian gravitational field due to the Tharsis rise may mask or modify gravitational signatures that contain important information on Martian geophysical processes. In order to isolate gravity signals in regions where the field is significantly affected by Tharsis as well to investigate characteristics of the global field, we present a numerical technique to “remove” Tharsis from the Martian gravitational field. Our analysis will show that to first order Tharsis can be represented as a sixth‐degree spherical harmonic zonal gravitational feature in a reference frame in which the axis of symmetry runs through the center of the province. We produced the “Mars without Tharsis” (MWT) field by subtracting the gravitational signature of Tharsis from the full field and rotating the spherical harmonics back to the original coordinate system, in which the z‐axis is coincident with the planetary rotation axis. Our study yields limits on the moment of inertia factor of 0.361<C/MR2<0.366 for the range of allowable non‐hydrostatic contributions to the flattening. The analysis indicates that the formation of Tharsis may have resulted in re‐orientation of the spin axis, though if the planet retained rotational fossil bulge due to a thick elastic lithosphere then changes in the global figure could have been inhibited. The zonal and tesseral components of the degree 2 field in the GMM‐I and rotated configurations indicates that Tharsis is ∼5 times more axisymmetric about its “pole” than Mars as a whole is about the rotation axis. With the removal of Tharsis' long wavelength gravitational power, regional structures become better defined in a spatial sense while maintaining their local dynamic range. The Olympus Mons volcanic shield displays clear evidence of a gravitational flexural moat. Our method for removing Tharsis may also be applied to analyze other global‐scale geophysical features that may be considered axisymmetric to first order, such as mantle plumes, major volcanic constructs, or large impact basins.
Potential anomalies on a sphere: Applications to the thickness of the lunar crust
Authors: Wieczorek Mark A. , Phillips Roger J.
Published: February 2004

A new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed. We show that by raising the topography to the nth power and expanding this field into spherical harmonics, potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision. Using a filter for downward continuing the Bouguer anomaly, we have computed a variety of crustal thickness maps for the Moon, assuming both a homogeneous as well as a dual‐layered crust. The crustal thickness maps for the homogeneous model give plausible results, but this model is not consistent with the seismic data, petrologic evidence, and geoid to topography ratios, all of which suggest some form of crustal stratification. Several dual‐layered models were investigated, and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data. These models predict that the entire upper crust has been excavated beneath the major nearside multiring basins. Additionally, significant amounts of lower crustal material was excavated from these basins, especially beneath Crisium. This model also predicts that mantle material should not have been excavated during the South‐Pole Aitken basin forming event, and that lower crustal material should be exposed at the surface in this basin.
Role of orbital forcing in east Asian monsoon climates during the last 350 kyr: Evidence from terrestrial and marine climate proxies from core RC14‐99
Authors: Morley Joseph J. , Heusser Linda E.
Published: February 2004

Proxy records of northeast Asian precipitation (marine pollen) and northwest Pacific sea surface temperatures (marine fauna) contain concentrations of variance (frequency peaks) coincident with those of orbital variations (eccentricity, obliquity, and precession). Time series of these monitors of various components of the east Asian monsoon system are coherent with changing solar insolation and climate boundary conditions (ice volume, snow cover, albedo) at 41‐ and 23‐kyr frequencies. Winter sea surface temperature (WSST) maxima are in phase with global ice volume minima at all three Milankovitch frequencies (100‐, 41‐, and 23‐kyr periods). WSST maxima are also linearly related to summer insolation at 30°N, lagging radiation maxima by ∼55° (∼3.5 kyr) across the precessional frequency band (23‐kyr period). Our east Asian monsoon rainfall indicator (% Cryptomeria) reaches highest values immediately following summer insolation maxima during periods of low ice volume (interglacials). Across the eccentricity frequency band (100‐kyr period), % Cryptomeria maxima are in phase with ice volume minima. At the higher frequency of orbital precession, monsoon rainfall lags both summer radiation maxima (∼125°; ∼8 kyr) and ice volume minima (∼60°; ∼3.8 kyr). Whereas much of the variation in northwest Pacific WSST can be attributed to summer radiation forcing and changes in global climate, mechanisms driving east Asian monsoon precipitation are more complex, involving other (possibly internal) factors.
Changing atmospheric Δ14C and the record of deep water paleoventilation ages
Authors: Adkins Jess F. , Boyle Edward A.
Published: February 2004

We propose a new calculation method to better estimate the deep water ventilation age from benthic‐planktonic foraminifera 14C ages. Our study is motivated by the fact that changes in atmospheric Δ14C through time can cause contemporary benthic and planktonic foraminifera to have different initial Δ14C values. This effect can cause spurious ventilation age changes to be interpreted from the geologic data. Using a new calculation method, 14C projection ages, we recalculate the data from the Pacific Ocean. Contrary to previous results, we find that the Pacific intermediate and deep waters were about 600 years older than today at the last glacial maximum. In addition, there are possible signals of ventilation age change prior to ice sheet melting and at the Younger Dryas. However, the data are still too sparse to constrain these ventilation transients.
Bioturbational mixing depth and carbon flux at the seafloor
Authors: Trauth Martin H. , Sarnthein Michael , Arnold Maurice
Published: February 2004

Radiocarbon dating series, bulk sediment, and organic carbon flux from various Atlantic deep‐sea regions reveal that the thickness of the bioturbated zone increases by 2 cm if food supply increases by 1 gCm−2yr−1 (r = 0.8). Bulk sediment accumulation rates do not influence the depth of bioturbational mixing under normal pelagic sedimentary conditions. We believe that this relationship between nutrient supply and benthic mixing can be used for a quantitative and time‐variable unmixing procedure to improve high‐resolution stratigraphic correlations and paleoclimatic interpretations of deep‐sea records.
Barium accumulation in the Atlantic sector of the Southern Ocean: Results From 190,000‐year records
Authors: Nürnberg C. C. , Bohrmann G. , Schlüter M. , Frank M.
Published: February 2004

Extensive investigations of sedimentary barium were performed in the southern South Atlantic in order to assess the reliability of the barium signal in Antarctic sediments as a proxy for paleoproductivity. Maximum accumulation rates of excess barium were calculated for the Antarctic zone south of the polar front where silica accumulates at high rates. The correspondence between barium and opal supports the applicability of barium as a proxy for productivity. Within the Antarctic zone north of today's average sea ice maximum, interglacial vertical rain rates of excess barium are high, with a maximum occurring during the last deglaciation and early Holocene and during oxygen isotope chronozone 5.5. During these periods, the maximum silica accumulation was supposedly located south of the polar front. Glacial paleoproductivity, instead, was low within the Antarctic zone. North of the polar front, significantly higher barium accumulation occurs during glacial times. The vertical rain rates, however, are as high as in the glacial Antarctic zone. Therefore there was no evidence for an increased productivity in the glacial Southern Ocean.
Can the aluminum and titanium contents of ocean sediments be used to determine the paleoproductivity of the oceans?
Authors: Dymond Jack , Collier Robert , McManus James , Honjo Sus , Manganini Steve
Published: February 2004

In the central equatorial Pacific, settling particles collected with sediment traps have Al/Ti values greater than those of a typical crustal source. The Al/Ti values of the sediment trap material are not well correlated with either the flux of organic carbon or with total particle mass, suggesting that this ratio cannot be used as a simple index of paleoproductivity. However, the Al/Ti values of some trap samples are more than 3 times the ratio for typical crustal materials, indicating a noncrustal source of excess Al. Numerical analysis using linear programming indicates that the excess Al is associated with opal and that this biological component carries as much as 1200 ppm of Al. Because of the low particulate fluxes of lithogenic material in the equatorial Pacific and the relatively high opal content of equatorial sediment trap material, opal appears to be the predominant conveyor of Al to the equatorial sediments. Our sediment trap Al/Ti values are similar to ratios previously measured in equatorial sediments, suggesting that measurements of this element pair in ocean sediments may be an indicator of opal rain to the seafloor. The particle‐reactive nature of Al and Ti means their ratios in sediments may be relatively insensitive to opal preservation or other diagenetic processes. However, the Al/Ti values of raining particles are highly sensitive to the relative proportion of opal and lithogenic material. Where opal/lithogenic ratios are less than 20, the Al/Ti values are indistinguishable from a crustal source ratio. In addition, the amount of excess Al associated with opal in settling particles may be a function of the concentration of Al in the water column and of the processes that fractionate dissolved Al and Ti. These issues must be evaluated before the Al/Ti values of ocean sediments can be used as a reliable proxy of opal flux to the seafloor.
Glacial/interglacial changes in the equatorial part of the Somali Basin (NW Indian Ocean) during the last 355 kyr
Authors: Vénec‐Peyré Marie‐Thérèse , Caulet Jean Pierre , Grazzini Colette Vergnaud
Published: February 2004

Planktonic foraminiferal and radiolarian signatures are analyzed in the 355 kyr record of a core from the equatorial part of the Somali Basin in order to reconstruct the amplitude of oceanographic processes and to test the reproducibility of the biological response to the glacial/interglacial changes over four climatic cycles. The correspondence analysis applied to the faunal data set reveals that the dominance of mixed layer foraminifera observed during isotope transitions 10/9, 6/5, and 2/1 does not occur during transition 8/7 but appears later within isotope stage 7. This specific signature, interpreted as related to the overflow of water from the South China Sea, can be related to the magnitude of sea level rises in this part of the Indian Ocean. These deep oceanic records of high sea level stands are correlated with occurrences of emerged coral reef terraces in shallow environments.
Physical properties of Reykjanes Ridge sediments and their linkage to high‐resolution Greenland Ice Sheet Project 2 ice core data
Authors: Moros M. , Endler R. , Lackschewitz K. S. , Wallrabe‐Adams H.‐J. , Mienert J. , Lemke W.
Published: February 2004

Five gravity cores taken from the Reykjanes Ridge have been used to establish a link between sediment physical properties and atmospheric records documented by δ18O variations in Greenland ice cores over the last 45,000 calendar years. Marine Gamma Ray Attenuation Porosity Evaluator density and magnetic susceptibility variations could be linked with the ice core Dansgaard‐Oeschger and Bond cycles. This is supported by ice‐rafted detritus (IRD), grain size, the quartz/feldspar ratio, and carbonate, isotopic, and foraminiferal records. The covariation of the sediment physical properties and δ18O in Greenland ice indicates a coupling of atmospheric temperature and paleocirculation variations. Gradual reduced bottom currents (Iceland‐Scotland Overflow Water) and enhanced iceberg discharges have been reconstructed for cold atmospheric periods relative to interstadial times. In the study area the magnetic susceptibility signal is not related to the ice‐rafted detritus input but most probably reflects the variations of the Iceland‐Scotland Overflow Water intensity transporting titanomagnetite into the Reykjanes Ridge region.
Deglacial surface circulation changes in the northeastern Atlantic: Temperature and salinity records off NW Scotland on a century scale
Authors: Kroon D. , Austin W. E. N. , Chapman M. R. , Ganssen G. M.
Published: February 2004

Sea surface temperature and salinity estimates reconstructed from a core collected on the Barra Fan, northwest Scotland (56°43′N, 09°19′W; water depth 1320 m), show a series of rapid oscillations during the last deglacial period that are very similar to those observed in the δ18O records from Greenland ice cores. These records indicate that the transport of heat and salt toward the Nordic Seas was highest during the Bølling period. This “superconveyor” weakened after the Bølling, probably as a consequence of increased meltwater flux reducing the oceanic salt content, as suggested by the Barbados sea‐level record. Evidence for a phase of ice rafting during the Allerød is presented for the first time from this latitude in the northeast Atlantic. The Younger Dryas stadial, resolved here at a century/decadal scale, is characterized by very rapid oscillations in temperature and salinity, indicating that warm, relatively saline waters repeatedly displaced cool polar waters at this latitude. These observations attest to the inherent instability of the deglacial climate system.
Pliocene‐Pleistocene evolution of eastern tropical Pacific surface water circulation and thermocline depth
Authors: Cannariato K. G. , Ravelo A. C.
Published: February 2004

We evaluated the response of eastern equatorial Pacific upper water column hydrography to the Panama seaway closure and the initiation of large‐scale northern hemisphere glaciation, The δ18O gradient between Globorotalia tumida and Globigerinoides sacculifer indicates a general shoaling of the thermocline between 4.2 and 3.0 Ma which may be related to a recorded decrease in southeast trade wind strength. The δ13C gradients suggest that surface water nutrient concentrations have increased during the last 5 Myr and that upwelling intermediate waters began to change sources at ∼3.2 Ma. Changes in the trans‐Pacific east‐west temperature and nutrient gradients at 4.0 and 1.5 Ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation. Changes in the sensitivity of surface circulation to Milankovitch forcing, δ18O records linearly related to orbital variations only after 3.2 Ma, are associated with the seaway closure. Pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to Milankovitch forcing is enhanced by air‐sea interactions which maintain the relatively steep Pleistocene trans‐Pacific gradients. The δ13C records are dominated by the 41 kyr period and are usually coherent with high‐latitude climate, suggesting that associated changes in the global carbon reservoir dominate the tropical δ13C signals.
Wavelet analysis for geophysical applications
Authors: Kumar Praveen , Foufoula‐Georgiou Efi
Published: February 2004

Wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals. Since then, significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields. In geophysics the power of wavelets for analysis of nonstationary processes that contain multiscale features, detection of singularities, analysis of transient phenomena, fractal and multifractal processes, and signal compression is now being exploited for the study of several processes including space‐time precipitation, remotely sensed hydrologic fluxes, atmospheric turbulence, canopy cover, laud surface topography, seafloor bathymetry, and ocean wind waves. It is anticipated that in the near future, significant further advances in understanding and modeling geophysical processes will result from the use of wavelet analysis. In this paper we review the basic properties of wavelets that make them such an attractive and powerful tool for geophysical applications. We discuss continuous, discrete, orthogonal wavelets and wavelet packets and present applications to geophysical processes.
Imaging Doppler interferometry and the measurement of atmospheric turbulence
Authors: Roper R. G. , Brosnahan J. W.
Published: February 2004

Data taken by an imaging Doppler interferometer (IDI) 3.175‐MHz radar during the Arecibo Initiative on Dynamics of the Atmosphere in 1989 is interpreted in terms of turbulence parameters. The rms random velocity determined from scatterers within 2° of the zenith for the interval 1200–1416AST on April 10, 1989, is used to calculate the rate of dissipation of turbulent energy using two formulations, one based on an estimate of the lifetime (∼100 s) of the intermittent scatterers identified by the IDI technique, and another based on the turbulent structure function. Both techniques yield upper limit values of 0.01–0.06 W kg−1 for the rate of dissipation of turbulent energy, increasing with height over the range from 72 to 90 km. The energy‐bearing eddy scale of turbulence, neglecting buoyancy, is found to vary with height, being a minimum at 75 km (200 m) and a maximum at 90 km (400 m). The effects of buoyancy are discussed in detail, and the buoyancy, or outer scale, is found to vary from 300 m at 75 km to a maximum of 900 m at 90 km. The Kolmogorov microscale, or inner scale, is found to increase from 1 m at 72 km to 10 m at 93 km. Estimates of the vertical eddy diffusion coefficient are also made, and the values obtained are shown to be highly dependent on the assumptions underlying the calculations.
Studies on refractivity structure constant, eddy dissipation rate, and momentum flux at a tropical latitude
Authors: Rao D. Narayana , Kishore P. , Rao T. Narayana , Rao S. Vijaya Bhaskara , Reddy K. Krishna , Yarraiah M. , Hareesh M.
Published: February 2004

VHF and UHF Doppler radars provide a unique database to estimate the refractivity structure constant Cn2, eddy dissipation rate ∈, and vertical flux of horizontal momentum. Using the data collected from the Indian MST radar, these parameters are studied at a tropical latitude. The refractivity turbulence structure constant is estimated from the backscattered power of the received echoes. Cn2 (radar) and Cn2 (model), derived from radiosonde observations, are compared, and a fairly good agreement is seen. Diurnal and seasonal variations of Cn2 are also presented. The eddy dissipation rate is estimated from the radar echoes employing the power and spectral width methods. A fairly good agreement is seen between the two methods. Values of ε are found to vary from 10−6 to 10−3 m2 s−3 in a height range of 4–19 km. Cn2 and ε are observed to be minimum during a moderate jet stream wind of 50–60 m s−1. Vertical flux of horizonal momentum is computed using the symmetrical two‐beam method. Significant fluxes of westward and northward momentum are observed, and the values lie in the range of −1 to +1 m2 s−2. The implied accelarations are also estimated. The results presented are largely consistent with the results available in the literature.
Measurements and analysis of antenna impedance aboard the Geotail spacecraft
Authors: Tsutsui M. , Nagano I. , Kojima H. , Hashimoto K. , Matsumoto H. , Yagitani S. , Okada T.
Published: February 2004

A practical method of estimating the effective impedance of electric dipole antennas aboard the scientific spacecraft Geotail has been developed. The principle of the newly proposed method is to derive the complex impedance of the dipole antennas immersed in the space plasma by means of the analysis of signal waveforms captured when a calibration signal was imposed directly onto the antennas. The derived complex antenna impedance is expressed as frequency responses of its real and imaginary parts. During the flight of Geotail in the Earth's magnetosphere, the impedance measurements were conducted in various regions of the magnetosphere. It has been confirmed, from the analysis of the measured data, that the impedance of the Geotail antenna forms an equivalent electric circuit consisting of a resistance and a capacitance connected in parallel. It has been found that the resistance value is easily changed by the extremely rare ambient plasma density in the magnetosphere. The validity of the measured values have been examined along a theory of probes in a sheath region, taking into account antenna potentials which are lifted up due to the photoelectron emissions from the antenna surface. The analyzed result shows that the measured values of the antenna resistance are consistent with those within the sheath calculated from the theory. Correlation plots of the capacitance values obtained from both the measurements and the calculations have shown that the major parts of the measured capacitance represent the “sheath capacitance,” but other parts might result from a combination of the sheath impedance and the plasma impedance, in addition to the free‐space capacitance.
Studying the ionosphere with the Global Positioning System
Authors: Davies K. , Hartmann G. K.
Published: February 2004

Opportunities to use the Global Positioning System (GPS) for ionospheric total electron content (TEC) research are reviewed. The era of TEC measurements using very high frequency geosynchronous beacons is essentially over, and the new GPS TECs need to be treated in special ways if they are to augment the existing database of total electron content. Data taken at Boulder, Colorado, show that with appropriate smoothing, GPS TECs can be used to extend the existing database. The time delay data from the International GPS Geodynamics Service over central Europe are used to map the total electron content and reveal regional structures. Global electron‐density profiles can now be measured using the GPS/MET (meteorology) system. The measurement of precipitable water vapor in the troposphere to an accuracy of 10% requires that 99.9% of the ionospheric delay be removed.
An analysis of uniaxial bianisotropic two‐dimensional periodic and nonperiodic structures using a boundary integral equation method
Authors: Backelandt Bart , Olyslager Frank , De Zutter Daniël
Published: February 2004

A general boundary integral equation method has been implemented to calculate the mode propagation in and the plane wave scattering at periodic and nonperiodic two‐dimensional structures. The method includes the use of multiple‐region structures with arbitrary geometry, each consisting of uniaxial bianisotropic material and the calculation of relevant output parameters such as radar cross sections, diffraction efficiencies, and propagation constants. Numerical examples illustrate the generality and power of the applied method.
Quality control of automatically scaled vertical incidence ionogram data
Authors: Conkright R. O. , McNamara L. F.
Published: February 2004

Software procedures are described for the routine scanning of data sets from automatically scaled vertical incidence ionograms, with a view to identifying ionograms which have been incorrectly scaled. Such procedures have been made necessary by the increasing use of real‐time ionospheric data in operational systems and by the increasing flow of such data into the U.S. National Geophysical Data Center (NGDC), which have made the required data‐scanning functions overwhelming for a human operator. The procedures will detect pathological data sets and prevent them from being passed on to the operational system or to NGDC databases, as well as indicating unusual ionospheric conditions and possibly the need for modifications of the automated procedures. Particular reference is made to the SAO (standard ARTIST output) files created by the University of Massachusetts Lowell program ADEP (ARTIST display and editing program) and corresponding to ionograms obtained using a Digisonde and scaled in real time by ARTIST (automatic real‐time ionogram scaling with true‐height). These files were chosen because of their ready availability at NGDC and because they cover a wide range of latitudes, seasons, and ionospheric conditions. Similar procedures can readily be developed for other ionogram data sets. The basic change would be to the function which reads in the data, although experience will probably lead to the inclusion of procedures to counter idiosyncrasies of the ionogram scaling procedures, such as have been found with ARTIST.
Real‐time ionospheric science: The new reality
Authors: Klobuchar John A.
Published: February 2004

Until the early 1990s, near‐real‐time updates to monthly climatological ionospheric models were driven primarily by military requirements. Since then, civilian research and commercial requirements for corrections to precise satellite ranging systems have become the driving forces for real‐time ionospheric data, and they likely will continue to be major reasons for the availability of worldwide transionospheric observations in the future. This paper outlines the development of some of these requirements and describes the likely future availability of ionospheric observations for near‐real‐time updating of ionospheric monthly climatology.
Azimuth spread of two‐hop ionospheric modes
Authors: Sherrill W. M. , Brown Bradley C.
Published: February 2004

We use direction finding ionogram data to analyze the azimuth spread of two‐hop F region modes in which ground scatter occurs (1) in the Gulf of Mexico and coastal areas of southern Louisiana and (2) in rough terrain in the Painted Desert region of Arizona. Ocean surface/coastal ground scatter are observed to be quasi‐specular within the resolution of these measurements. Rough terrain ground scatter imposes a nominal 1.5° azimuth spread in addition to that introduced by the two traverses of the ionospheric F region.