bluuebunny/crossref_metadata_2025 huggingface dataset:
Bidirectional reflectance of selected BOREAS sites from multiangle airborne data
Authors: Russell Carol A. , Irons James R. , Dabney Philip W.
Published: February 2004

Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid‐state array spectroradiometer (ASAS)over Boreal Ecosystem‐Atmosphere Study (BOREAS) sites in Canada during four field campaigns in 1994. Atmospherically corrected bidirectional reflectance factors and estimates of spectral hemispherical reflectance for three canopies in the BOREAS southern study area (old aspen, old black spruce, and old jack pine) are presented. The multiangle spectral reflectance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 34°–37°) for the forested sites showed distributions of high backscatter and consistently low forward scatter due to shadowing. Position and shape of the retrosolar maximums for the three sites varied. Spectral distinction among the sites was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen, and in the near infrared (NIR), where the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26° backscatter in the principal plane would provide better discrimination among the major cover types than those obtained from a nadir view. Red reflectance was strongly dependent on view geometry for the spruce and jack pine sites due to the varying amounts of deep shadow and red‐reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less variation with changing view zenith and azimuth. Accordingly, angular effects on the normalized difference vegetation index were large for the spruce and jack pine canopies but small for the aspen site. Estimated spectral hemispherical reflectances for photosynthetically active radiation (PAR) (0.4–0.7 μm), red (0.63–0.69 μm), and NIR (0.83–0.87 μm) calculated using various combinations of azimuthal data sets as input to the Walthall et al. [1985] model showed the following trends: maximum estimates were generated using data from the solar principal plane (spp) only; minimum values were derived from perpendicular plane data and amounted to 50–83% (relative) of the corresponding spp value; and inclusion of data from three view azimuths together (spp + perpendicular + oblique) produced intermediate values totaling 73–91% (relative) of the spp result. A preliminary review of ASAS‐derived and independent ground‐based measures of PAR hemispherical reflectance revealed a sizable range in this parameter.
Stratospheric aerosol clouds due to very large volcanic eruptions of the early twentieth century: Effective particle sizes and conversion from pyrheliometric to visual optical depth
Authors: Stothers Richard B.
Published: February 2004

Spectral and pyrheliometric measurements of atmospheric transmission were made by staff members of the Astrophysical Observatory of the Smithsonian Institution at Washington, D.C., and at Mount Wilson, California, during the years 1901–1920. These valuable data are analyzed here with the help of a new inversion method to derive the effective, or area‐weighted, radii of stratospheric aerosols formed after three very large volcanic eruptions in this period. After the great eruptions of Katmai (1912) and Santa Maria (1902), reff remained close to 0.3 μm for at least two years. This near constancy of reff has been duplicated in modern times by the aerosols from El Chichón (1982). Following Ksudach's (1907) eruption, reff grew from 0.2–0.3 μm to 0.4–0.5 μm in about 1 year. Pinatubo's (1991) aerosols grew similarly. Generally speaking, the pyrheliometric optical depth perturbation, as measured for the three early eruptions, is not equal to the visual optical depth perturbation, but in fact the latter is larger by a factor of about 1.6, at least for reff = 0.25–0.45 μm. Wider physical implications of the present results are discussed.
Characterization of tropospheric aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product
Authors: Husar Rudolf B. , Prospero Joseph M. , Stowe Larry L.
Published: February 2004

The National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) is an instrument on a polar orbiting satellite that provides information on global aerosol distributions. The remote sensing algorithm is based on measurements of backscattered solar radiation which yield a measure of the “radiatively equivalent” aerosol optical thickness τAsat(EAOT) over the oceans. Seasonally composited EAOT data for the period July 1989 to June 1991 reveal many spatially coherent plume‐like patterns that can usually be interpreted in terms of known (or reasonably hypothesized) sources in association with climatological wind fields. The largest and most persistent areas of high EAOT values are associated with wind‐blown dust and biomass burning sources; especially prominent are sources in Africa, the middle East, and the Asian subcontinent. Prominent plumes over the midlatitude North Atlantic are attributed to pollution emissions from North America and Europe. Large plumes attributed to pollution aerosols and dust from sources in Asia are clearly visible over the western and central North Pacific. On a global scale the annually averaged northern hemisphere EAOT values are about 1.7 times greater than those in the southern hemisphere. Considering each hemisphere separately, EAOT values in summer are about twice those in winter. Within the midlatitude band 30°–60° (i.e., where anthropogenic emissions are greatest) the summer/winter ratio is about 3. The temporal variability of monthly mean EAOT in specific ocean regions often shows characteristic seasonal patterns that are usually consistent with aerosol measurements made in the marine boundary layer. Nonetheless, there are features in the EAOT distributions that can not be readily interpreted at this time. The AVHRR EAOT distributions demonstrate that satellite products can serve as a useful tool for the planning and implementation of focused aerosol research programs and that they will be especially important in studies of climate‐related processes.
Radar observations of a 3‐day Kelvin wave in the equatorial mesosphere
Authors: Riggin Dennis M. , Fritts David C. , Tsuda Toshitaka , Nakamura Takuji , Vincent Robert A.
Published: February 2004

Mesospheric radars are used to investigate the characteristics of a Kelvin wave from two equatorial sites: Jakarta, Indonesia, in the western Pacific and Christmas Island in the central Pacific. Our study focuses on the time span from mid‐January through mid‐October 1993. A Kelvin wave with a period near 3 days was detected throughout this 9‐month duration, although it underwent deep amplitude modulations on a ∼20‐day timescale. A fitting procedure is applied to study the phase/amplitude behavior of the wave. The vertical wavenumber was measured by the radars and found to be small, wandering around zero with only a weak bias toward downward phase progression. The long vertical wavelength suggests that the wave was predominantly zonal wavenumber 1. The amplitude of the wave measured by the Jakarta meteor scatter radar was much larger than the amplitude measured by the MF partial reflection radar at Christmas Island. The smaller wave amplitude at Christmas Island could at least partially be due to a measurement bias associated with MF radars. The radar at Jakarta is a VHF meteor scatter radar and is not susceptible to this bias. However, the mean velocities and the amplitudes of the tidal and quasi 2‐day wave components were in good agreement at the two sites. The estimated 9‐month averaged zonal acceleration was ∼0.67 ms−1 day−1 over Jakarta at 94–98 km and only about half as large over Christmas Island. The magnitude of the zonal acceleration occasionally showed large enhancements which suggest the importance of refractive effects associated with vertical and temporal variations in the mean winds. The larger 3‐day wave amplitudes and inferred acceleration at Jakarta may reflect its location in the western Pacific, a region of high convection, and hence an excitation region for equatorial waves. The relative phase of the wave between the two radar sites gradually shifted over a timescale of weeks. These smooth variations in relative phase are suggestive of a superposition of waves with different zonal wavenumbers, perhaps radiating preferentially from one longitude. The phase of the wave as a function of altitude and time was much more disordered at Jakarta than at Christmas Island. The conjecture can be made that the more chaotic phase structure observed over Jakarta is due to higher‐order zonal wavenumber components which weaken as they propagate eastward.
Kelvin waves in the equatorial lower stratosphere as revealed by cryogenic limb array etalon spectrometer temperature data
Authors: Shiotani M. , Gille J. G. , Roche A. E.
Published: February 2004

Space‐time variations of atmospheric temperature in the equatorial lower stratosphere are investigated using temperature data derived from the cryogenic limb array etalon spectrometer (CLAES) instrument onboard the Upper Atmosphere Research Satellite (UARS) for January 1992 to May 1993. During this period, clear evidence is observed for eastward progression of zonal wavenumber 1 temperature anomalies. This variation has a period of about 2 weeks and a maximum amplitude at the equator. The vertical scale of this variation is about 10 km and has an eastward phase tilt with increasing height. Consequently, the phase structure propagates downward with time. From these observed features it is concluded that the temperature variation seen in the equatorial lower stratosphere is due to so‐called “slow” Kelvin waves. This observational study using satellite measurements clearly shows much more detail on the global structure of “slow” Kelvin waves than previous studies. Space‐time spectral analysis of temperature variations for the entire CLAES observation period were also performed. During the period January 1992 to May 1993 the background zonal winds (derived from radiosonde observations at Singapore) change from easterlies to westerlies in agreement with the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere. The slow Kelvin wave activity (measured by the power spectral density averaged over a period of about 2 weeks) is vigorous during the easterly regime but rapidly decreases in the westerly regime of the QBO. This clearly indicates the different transmission characteristics of Kelvin waves in the two QBO wind regimes.
Voyager electron density measurements on Saturn: Analysis with a time dependent ionospheric model
Authors: Majeed Tariq , McConnell John C.
Published: February 2004

We have used a one‐dimensional chemical diffusive model of the ionosphere, in conjunction with the measured Voyager ultraviolet spectrometer (UVS) upper atmospheric temperature and composition structure, to analyze the Voyager measurements of Saturn's upper ionospheric electron densities. Electron density measurements are available from the analysis of the radio science (RSS) experiments. In addition, if interpreted as an atmospheric phenomenon the Saturn electrostatic discharges (SED), detected by the Voyager planetary radio astronomy instrument, the measurements also yield important information on electron densities at the peak. These latter data suggest a strong day‐to‐night variation in the peak electron densities with a maximum of ∼5 × 105 cm−3 near noon and a minimum of ∼103 cm−3 near midnight. The analysis of RSS data yields a peak of ∼104 cm−3 in the electron density profiles near the local terminators. As with simpler ionospheric models, we find serious difficulties in interpreting these electron density measurements because time constants in the model for the chemical sinks in the ionosphere, of mostly H+ ions, are >106 s much longer than a Saturnian day, suggesting little diurnal variation. We have investigated the effects of H2 vibrational excitation (characterized by a single vibrational temperature) and a gaseous H2O influx from Saturn's rings, as enhancers of plasma recombination, since they result in the conversion of slowly recombining H+ ions to more rapidly recombining molecular ions. This does introduce a larger diurnal variation in the peak electron densities, but not as strong as that deduced from the analysis of SED measurements. Furthermore, it also reduces the magnitude of the electron densities at the peak well below that inferred from the SED data. Based on the model results and extrapolation of calculations by Kim and Fox [1991] from Jupiter, we suggest that SEDs may not be associated with atmospheric storm systems. In our attempt to interpret the RSS measurements of electron density profile at sunrise, we have invoked an upward field‐aligned drift, H2 vibrational excitation and an influx of H2O to explain the results, and, in particular, maintain plasma peak at the measured altitude.
Evolution of Martian atmospheric argon: Implications for sources of volatiles
Authors: Hutchins Kevin S. , Jakosky Bruce M.
Published: February 2004

We have examined processes affecting isotopes of argon (36Ar, 38Ar, 40Ar) in order to determine important atmospheric sources and sinks. Our simple model for argon evolution incorporates production of radiogenic argon in the mantle, outgassing of all argon species by extrusive and intrusive volcanism, and loss to space by knock‐on sputtering above the exobase. Sputtering has been shown previously to be an important loss process for atmospheric species, especially isotopes of noble gases, which have few other mechanisms of escape. The integrated evolution of argon (36Ar, 38Ar, and 40Ar, respectively) is modeled in terms of these variables: (1) the planetary concentration of potassium, (2) the fraction of juvenile argon released catastrophically during the first 600 Myr., (3) potential variation in the time‐history of sputtering loss from that suggested by Luhmann et al. [1992], and (4) the volume of total outgassing to the surface as compared to outgassing contributed by volcanic release. Our results indicate that Mars has lost between 85–95% of 36Ar and 70–88% of outgassed 40Ar. Due to this substantial loss, the planet must have outgassed the equivalent of between 10 and 100 times the total volume of gases released by extrusive and intrusive volcanics. This indicates that volcanic outgassing, alone, is insufficient to explain the present‐day abundances of 36Ar and 40Ar in the Martian atmosphere. Similar calculations for 20Ne suggest outgassed volumes of between 100 and 1800 times in excess of that due to volcanism. This results in a distinct Ne/Ar elemental fractionation, with a preference for outgassing argon, of the order of 10 to 17. Although the results must be evaluated within the model uncertainties, the results are compelling in that they unequivocally show the existence of additional sources of atmospheric volatiles and helps define a means to identify them.
A model of radiative and conductive energy transfer in planetary regoliths
Authors: Hapke Bruce
Published: February 2004

The thermal regime in planetary regoliths involves three processes: propagation of visible radiation, propagation of thermal radiation, and thermal conduction. The equations of radiative transfer and heat conduction are formulated for particulate media composed of anisotropically scattering particles. Although the equations are time dependent, only steady state problems are considered in this paper. Using the two‐stream approximation, solutions are obtained for two cases: a layer of powder heated from below and an infinitely thick regolith illuminated by visible radiation. Radiative conductivity, subsurface temperature gradients, and the solid state greenhouse effect all appear intrinsically in the solutions without ad hoc additions. Although the equations are nonlinear, approximate analytic solutions that are accurate to a few percent are obtained. Analytic expressions are given for the temperature distribution, the optical and thermal radiance distributions, the hemispherical albedo, the hemispherical emissivity, and the directional emissivity. Additional applications of the new model to three problems of interest in planetary regoliths are presented by Hapke [this issue].
Modeling of albedo and thermal inertia induced mesoscale circulations in the midlatitude summertime Martian atmosphere
Authors: Siili Tero
Published: February 2004

Mesoscale circulations in the Martian atmosphere induced by variations in surface albedo (a) and soil thermal inertia (I) or both have been studied numerically with a two‐dimensional primitive equation model. The latitudinal and seasonal parameters correspond to northern midlatitudes and midsummer. The variations in the surface properties induce surface temperature variations, which, in turn, drive horizontal winds and vertical motions. A circulation cell not unlike a terrestrial seabreeze cell is formed at 1300–1400 local time (LT), the vertical motion peaks just prior to sunset and persists until 2100–2300 LT. The horizontal winds accelerate throughout the day and peak around 2100 LT. The acceleration picks up at 1600–1800 LT owing to the collapse of vertical momentum mixing. In the thermal inertia case the surface temperature gradient reverses around 1800 LT, leading to reversed forcing and nocturnal return flow. Owing to the Coriolis effect, the perpendicular wind component (ν) dominates the magnitude of the horizontal flow. The extrema of horizontal winds and vertical motions occurring for variations of a = 0.20–0.30 and I = 250–350 SI units and optical thickness τ = 0.4 can be up to 7–8 m/s and approximately 3–4 cm/s, respectively. The strength of the circulation is sensitive to the amount of suspended dust; the maximum wind speed is reduced by a factor of more than 2 when τ is changed from 0.1 to 1.0. The effects of superimposed thermal inertia and albedo variations depend on their respective magnitudes and signs: if a and I increase in the same direction, the circulations are amplified, and the phases are close to phases of circulations induced by thermal inertia variations alone. If the variations have opposite signs, the circulations are attenuated, and the time of largest forcing can shift from daytime to the night; circulation and surface stress patterns are also shifted in phase by up to several hours. Surface stress τ0 induced by the circulations is discernible as of a few hours after sunrise and peaks in the region of a and I variations. If the forcing is due to a or I variation alone or due to the amplifying combination of the two, τ0 peaks in the early afternoon (typically at 1200–1400 LT) and collapses 2–3 hours prior to sunset, primarily owing to the collapse of vertical momentum mixing. In case of opposing surface property variations, τ0 maximum is shifted in phase and can occur as early as 0800 LT owing to predominantly nocturnal forcing. The circulations studied here appear not to play a significant role in dust raising, as the magnitude of the stress generated is at least an order of magnitude below the estimated dust raising threshold.
Meteorite falls in June: Two sets of observations
Authors: Saul John M. , Lawniczak Anièce C.
Published: February 2004

Two types of meteoritic events are associated with the calendric period June 13–30. One is an excess of falls of certain basaltic meteorites. These comprise a new subgrouping of achondrites (designated “NUJ”) whose lithologies suggest possibly similar preterrestrial histories. Historical high‐energy impact‐like events on the Earth and Moon have also occurred in late June. In addition to much studied events in 1178, 1908 (Tunguska), and 1975, these include an impact‐like lunar event known to Islamic scholars whose occurrence in 617 A.D. may have been June 26–27.
SIR‐C/X‐SAR multifrequency multipass interferometry: A new tool for geological interpretation
Authors: Coltelli Mauro , Fornaro Gianfranco , Franceschetti Giorgio , Lanari Riccardo , Migliaccio Maurizio , Moreira João R. , Papathanassiou Konstantinos P. , Puglisi Giuseppe , Riccio Daniele , Schwäbisch Marcus
Published: February 2004

Radar remote sensing is a tool of increasing importance in the study of volcanic sites. Synthetic aperture radar (SAR) is a high‐resolution imaging tool used to survey areas which are not practical or safe to be directly inspected. With the introduction of the across‐track SAR interferometry (IFSAR) technique, digital elevation models (DEM) can be produced. The usefulness of such interferometric products depends on the ability to extract information that can be used for geological interpretation. We analyzed the shuttle imaging radar C (SIR‐C)/X‐SAR multifrequency multipass interferometry mission over Mount Etna, Sicily, and we performed a supervised geological interpretation of the coherence maps and a fractal‐based analysis of the IFSAR DEMs. The first permits us to recognize different volcanic terrain and to distinguish between vegetated and unvegetated areas, while the second allows us to validate the IFSAR DEMs and to detect large‐scale geological features. This latter analysis, performed over the photogrammetric DEM, enabled us to recognize artifacts caused by digitizing and resampling. Obviously, IFSAR DEMs are not affected by these problems. As a consequence, IFSAR products are a valuable aid in geological interpretation.
Gamma ray production and transport in Mars
Authors: Masarik Jozef , Reedy Robert C.
Published: February 2004

The elemental composition of the Martian surface can be determined by gamma ray spectroscopy from landers or orbiting spacecraft. We used the well‐tested Los Alamos High Energy Transport (LAHET) Code System (LCS) to numerically simulate the galactic‐cosmic‐ray‐induced processes producing secondary particles and γ rays in Mars. For γ ray lines, LCS was used only for the calculation of the particle fluxes in the Martian soil and atmosphere; these fluxes were then integrated with cross sections to calculate the production rates of γ rays as a function of depth. The fluxes of γ ray lines were then calculated both at the Martian surface and at an altitude of 378 km. The water content of the Martian soil and the atmospheric thickness were varied to establish the dependence of the particle distributions and γ ray fluxes on these parameters. Most attention is devoted to γ rays from neutron‐induced reactions in the Martian soil, but contributions from the atmosphere, natural radioactivity, and from proton interactions are also mentioned. Our results are compared with those from previous studies.
Buried stratigraphic relationships along the southwestern shores of Oceanus Procellarum: Implications for early lunar volcanism
Authors: Mustard John F. , Head James W.
Published: February 2004

The composition of mare‐highland boundaries is investigated in the region of southwestern Oceanus Procellarum using multispectral images from the Galileo solid‐state imaging system. The data are analyzed using a linear image‐based spectral mixture model to map the distribution and abundance of spectral end‐members representing mare basalt, mature highland soils, and fresh crater materials. The fraction images that result from the mixture modeling are then integrated into a geographic information system database to analyze the relationship between the compositional gradients and regional geologic and geomorphic units. Within this framework, three basic types of mixing gradients are recognized: narrow mixing zones, broad mixing zones, and complex mixing zones. The narrow zones appear to be the result of simple postformation lateral mass transport of surface materials due to the time‐averaged effects of impact cratering and regolith gardening. The broad or moderate mixing zones exhibit evidence for significant incorporation of basalt into the distal basin ejecta of Orientale, and thus are interpreted to indicate that pre‐Orientale volcanism and volcanic deposits were widespread in the Procellarum region. This is further corroborated by the mare‐rich ejecta blanket observed for the 100‐km‐diameter impact crater Letronne. The complex mixing zones show evidence for extensive pre‐Orientale, and perhaps pre‐Imbrium cryptomare, as well as some volcanism extending into the late Imbrian period. These results indicate that mare‐highland boundaries frequently contain a convolved geologic record that allows insight into the geologic events prior to the final one apparently recorded by surface deposits. For this region of the Moon, we now recognize that volcanism was well established and widespread in early Imbrian time but had become more focused to a few major volcanic regions (Humorum, Procellarum) by the late Imbrian.
Floor subsidence and rebound of large Venus craters
Authors: Brown C. David , Grimm Robert E.
Published: February 2004

The topography and geology of large craters on Venus reveal no evidence for floor rebound or relaxation; rather, the floors have subsided. Depressions of the floor centers relative to the margins are evident in topography, and floor faulting is interpreted as contractional failure. Of the likely processes responsible, only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas, assuming volcanism occurred within a few tens of millions of years after the impact. Thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere. Further, the predicted stresses are generally consistent with observed floor fracturing. We constrain the impact heat deposited in the lithosphere to be less than roughly 5 × 1023 J for diameters of ∼100 km. The absence of perceptible floor subsidence at craters this size on the Moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter, but also on gravity and target density. The unfractured melt sheets of three large, young, bright‐floored craters imply sufficient lithospheric rigidity to support the crater cavities. An elastic flexural rebound model restricts the elastic plate thickness to at least 10–15 km for the three craters, corresponding to maximum geotherms of ∼20–30 K km−1. Structural evidence for early rebound in older, dark‐floored craters may have been buried by subsequent volcanism.
The NEAR Radio Science investigations
Authors: Yeomans D. K. , Konopliv A. S. , Barriot J.‐P.
Published: February 2004

The science objectives for the Near‐Earth Asteroid Rendezvous (NEAR) Radio Science team include determining the masses of asteroids 253 Mathilde (to within 10%) and 433 Eros (<0.1%). While the NEAR spacecraft is in orbit about Eros, the radiometric spacecraft tracking data, the optical landmark tracking data, and the lidar altimetry measurements will be used to determine the gravity field of Eros. Comparisons of this gravity field with one determined from Eros shape models (assuming constant density) will provide constraints upon the internal structure of this asteroid. In addition, the spin state, principal axes, and moments of inertia will be determined as part of the Radio Science activities. A campaign to meet the radio science objectives will require the close cooperation of the NEAR Navigation, Imaging, and Lidar teams.
1995 observations of Martian dust storms using the Hubble Space Telescope
Authors: Wolff Michael J. , Lee Steven W. , Clancy R. Todd , Martin Leonard J. , Bell James F. , James Philip B.
Published: February 2004

A synoptic monitoring program of Mars using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope has resulted in the observation of discrete dust activity for two dates following the 1995 opposition. Data obtained on April 8 (Ls=82°) and August 21, 1995 (Ls=145°) reveal the presence of local dust storms in the northern polar region. In addition, the August images show activity over the Hellas basin. We present a multispectral analysis of the data from which we derive lower bounds to the atmospheric dust loading. Our technique directly models the absolute photometry, and consequently also provides estimates of the cloud opacity and the ozone columns for the regions analyzed.
Martian precession and rotation from Viking lander range data
Authors: Yoder C. F. , Standish E. M.
Published: February 2004

The spatial orientation of the Martian pole of rotation and axial rotation parameters have been determined at a midpoint in the Viking epoch (January 1, 1980). The mean obliquity ε and node ψ angles are ε=25.1894°±0.0001, ψ=35.4796°±0.0002(2σ). The mean precession rate is ψo=−7.83±0.30 arc sec yr−1. The corresponding moment estimate is C/MRe2=0.355±0.015 and covers two extreme theoretical estimates. Seasonal (annual and semiannual) variations in Mars' rotation angle have also been observed and are 279±100 and 311±100 milliarc‐second of angle (mas) respectively, after correcting for a general relativity effect. A simple model has been constructed for the main contribution involving seasonal mass exchange between the ice caps and atmosphere where the air pressure at the Viking 1 lander site is a proxy for the global ice cap/air pressure histories. The annual term is about 190 mas smaller and differs in phase by ∼30° compared to the model prediction. The semiannual term is 100 mas larger and also differs in phase by ∼30° compared to the model. A combination of tidally driven rotation changes, zonal winds, polar motion and ice cap model deficiencies may account for any discrepancy. The upcoming Pathfinder lander and Mars Global Surveyor missions carry ranging transponders which can dramatically improve our knowledge of Mars' interior through precise determination of the precession rate and detection of core and tidal effects. We advocate that plans be developed for a coordinated campaign of simultaneous ranging and Doppler tracking to each transponder.
Bismuth oxide nanoparticles in the stratosphere
Authors: Rietmeijer Frans J. M. , Mackinnon Ian D. R.
Published: February 2004

Platey grains of cubic Bi2O3, α‐Bi2O3, and Bi2O2.75 nanograins were associated with chondritic porous interplanetary dust particles W7029C1, W7029E5, and 2011C2 that were collected in the stratosphere at 17–19 km altitude. Similar Bi oxide nanograins were present in the upper stratosphere during May 1985. These grains are linked to the plumes of several major volcanic eruptions during the early 1980s that injected material into the stratosphere. The mass of sulfur from these eruptions is a proxy for the mass of stratospheric Bi from which we derive the particle number densities (p m−3) for “average Bi2O3 nanograins” due to this volcanic activity and those necessary to contaminate the extraterrestrial chondritic porous interplanetary dust particles via collisional sticking. The match between both values supports the idea that Bi2O3 nanograins of volcanic origin could contaminate interplanetary dust particles in the Earth's stratosphere.
The magnetic signature of rapidly deposited detrital layers from the Deep Labrador Sea: Relationship to North Atlantic Heinrich layers
Authors: Stoner Joseph S. , Channell James E. T. , Hillaire‐Marcel Claude
Published: February 2004

Rock magnetic parameters from deep Labrador Sea piston core (P‐094) are useful for recognizing rapidly deposited detrital layers, some of which correlate with North Atlantic Heinrich layers. Variations in magnetic properties and lithology distinguish two classes of rapidly deposited detrital layers during the last ice age: (1) seven detrital carbonate (DC) layers have high carbonate content and mean magnetite grain diameters about 4 times greater than the background sediments; (2) six low detrital carbonate (LDC) layers have very similar magnetic properties to DC layers but low carbonate content (similar to the background sediments). The magnetite associated with DC and LDC layers is well sorted and relatively uniform within and between layers. Ice‐rafted detritus (IRD), recognized by increased percentage of the > 125 micron grain‐size fraction and by high magnetic susceptibility, is associated with most DC and LDC layers but is not the dominant detrital constituent. The correspondence of DC and LDC layers with increased grain size of well‐sorted magnetite, but not with coarse fraction content, precludes ice rafting as the primary depositional mechanism. DC and LDC layers may have been deposited from suspended sediment derived from turbidite activity in the nearby Northwest Atlantic Mid‐Ocean Channel (NAMOC). This interpretation is supported by a piston core (P‐013) from a Greenland Rise site largely, but not completely, outside the influence of the NAMOC. The association of DC layers with IRD and the apparent age correlation of DC layers to North Atlantic Heinrich layers suggest that the ice advances which produced the IRD in P‐094 and in correlative North Atlantic Heinrich layers also triggered turbiditic flows down the NAMOC. Several DC and LDC layers, however, do not correlate with recognized Heinrich layers but appear to be coeval with recently discovered high‐frequency IRD‐rich layers in the North Atlantic.
Whither cyclostratigraphy? Testing the Gamma Method on Upper Pleistocene deep‐sea sediments, North Atlantic Deep Sea Drilling Project Site 609
Authors: Kominz Michelle A.
Published: February 2004

Gamma analysis, a procedure for tuning cyclic sediments in which gamma is defined as 1/sedimentation rate, was applied to the upper Pleistocene gray scale record of Deep Sea Drilling Project site 609 from the North Atlantic. The spectrum of the gamma‐tuned time series produces eccentricity, obliquity, and precession index signals when the primary periodicity is tuned to the precessional beat frequency. However, the timescales are significantly too long, suggesting that some of the primary cycles used in the gamma tuning procedure have higher frequency than the precessional beat. By filtering the SPECMAP‐calibrated time series at 10–12 kyr the spurious cycles are removed from the tuning procedure. In this case, the resulting gamma‐corrected spectra look much more like that of the SPECMAP‐tuned time series. However, the time series remains slightly too long. Forward modeling is consistent with the results from modeling the core. That is, the gamma method is quite robust at revealing orbital periodicity in an orbitally controlled record for which sedimentation rates are facies dependent. However, the method does not produce an accurate time series. Criteria for complete rejection of gamma results are clarified as either (1) least squares results include negative gamma values or (2) the resulting spectrum does not show an orbital signature. Variation on sedimentation rates greater than ±5% result in incorrect time series. However, variations on sedimentation rates as high as ±15% generally reproduce strong orbital spectra, and occasionally, the method can also generate an orbital spectrum with up to ±25% noise. The method is not as robust in predicting gamma values or sedimentation rates. Although it generally distinguishes facies with relatively high sedimentation rates from facies with relatively low sedimentation rates, the values predicted may be incorrect by a factor of 2. Thus while gamma tuning of the time series is an improvement over observed thicknesses, the resulting timescale is not correct. The implications of these results for cyclostratigraphy are significant. The generation of an orbital spectrum from a time series that is demonstrably incorrect casts doubt on cyclicity as a dating tool.
Calibration of stable oxygen isotope signatures in Galápagos corals
Authors: Wellington G. M. , Dunbar R. B. , Merlen G.
Published: February 2004

A 2‐year (1993‐1994) study was conducted in the Galápagos Islands (Ecuador) to determine the relationship between δ18O in skeletal carbonate and sea surface temperature (SST) in three species of reef‐building corals: Pavona clavus, Pavona gigantea, and Porites lobata. Coral samples were grown at 3, 10, and 3 m depth at Bartolomé Island, Champion iIsland, and Urvina Bay (Isabela Island), respectively. Hourly measurements of SST and sea surface salinity (SSS) were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth. In addition, surface waters were sampled periodically (bimonthly to monthly) at each site to determine variation in δ18O seawater. Results indicate the mean annual SSTs were similar between sites, varying from 22.9°C at Champion to 23.8°C at Urvina Bay. Comparisons of monthly SST averages between instrumental and remote sensing (satellite, 1° × 1° grid) data show a high correspondence (r2 ranging from 0.84 to 0.94), indicating that remote sensing data are useful for interpreting the δ18O record in corals when instrumental data are lacking. Here δ18Ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of SST in Galápagos. In general, higher‐resolution coral sampling/year resolved more of the monthly variation in SST, up to 97% at a sampling resolution of 1.4 samples per millimeter of linear skeletal growth. Comparisions of the δ18Ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking SST. In addition, comparisons between sites were highly concordant, with some differences reflecting local variation in SST. Seasonal patterns, however, were essentially the same over the entire region. Thus we conclude that the δ18Ocoral signal from coral skeletons in Galápagos can be used to interpret regional changes in SST variation.
Oceanic pH control on the boron isotopic composition of foraminifera: Evidence from culture experiments
Authors: Sanyal A. , Hemming N. G. , Broecker W. S. , Lea David W. , Spero Howard J. , Hanson Gilbert N.
Published: February 2004

Culture experiments were carried out with the species Orbulina universa at four different pH values (7.70±0.05, 8.15±0.05, 8.60±0.05, and 9.00±0.10) in order to establish the pH‐dependence of boron isotope fractionation between seawater and foraminifera. A clear relationship between the boron isotopic composition of the foraminifera and the pH of the seawater culture solutions was found, showing heavier boron isotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo‐pH tool. It is important to note that Orbulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic ratio than Globigerinoides sacculifer from coretop samples, suggesting that at least for this species, a vital effect is active.
Late Quaternary paleoceanography in the Fram Strait
Authors: Hebbeln Dierk , Wefer Gerold
Published: February 2004

Four sediment cores from the Fram Strait (78°N) have been studied to reconstruct the paleoceanography of this major connection of the Arctic Ocean to the world ocean. Back to oxygen isotope stage 5 (128 kyr ago) the stratigraphic interpretation of the cores is based on oxygen isotope and accelerator mass spectrometry data; the identification of oxygen isotope stage 6 is based on paleomagnetic and coccolith data. Parameters indicative for contrasting environmental conditions such as high input of ice‐rafted detritus (IRD) or seasonally open waters display characteristic sequences by which the paleoce anography of this region can be reconstructed for the last 180 kyr. The geographic setting of Fram Strait and the sediment data indicate a permanent meridional circulation pattern throughout the last 180 kyr, as it is typical, for example, for the present‐day circulation. Depending on the strength of advection, Atlantic waters appeared in Fram Strait as surface waters, resulting in seasonally ice‐free conditions, or as subsurface water masses, underlying polar waters perenially covered by ice. Sustained periods of seasonally ice‐free waters were largely restricted to interglacial stages 5.5, 5.1, and the Holocene. However, a few short events characterized by seasonally ice‐free conditions at 78°N are recorded during glacial stages as well, for example, stage 6, late stage 3, and stage 2 (“Nordway” events). These events provide for supply of moisture, and therefore they are closely related to the history of the European Arctic ice sheets. Strong input of IRD, reflecting severe glaciations on surrounding landmasses, occurred during most of stage 6, in early stage 3, and during stage 2. Variations in IRD input allow differentiation between three major source areas: (1) Svalbard/Barents Sea during major glacier advances in stages 2 to 5; (2) most likely Siberia during stage 6; and (3) Fennoscandia during the short events in glacial periods marked by seasonal open waters. Thus the circulation pattern in the Fram Strait remained rather stable throughout the last 180 kyr, while variations in the sediment composition are mainly due to the intensity of Atlantic water advection and to the prevailing IRD input pathway.
Hydrospheric transports, the oxygen isotope record, and tropical sea surface temperatures during the Last Glacial Maximum
Authors: Norton Fielding L. , Hausman Ezra D. , McElroy Michael B.
Published: February 2004

A model is developed to consider explicitly the effect of glacial‐to‐Holocene changes in transports of fresh and salt water on the distribution of oxygen isotopes in seawater and therefore on the calibration of the foraminiferal 18O paleothermometer. The published foraminiferal isotopic record is shown to be consistent with glacial surface temperatures at low latitudes cooler than today by about 4°C in the Atlantic and Indian Oceans and about 2°C in the Pacific, given glacial transport patterns supported by the geologic record. If, in addition, net vapor transport from the Indian and Atlantic Oceans to the Pacific was enhanced significantly during glacial times relative to today, the isotopic record would be consistent with a sea surface cooler by up to 5°C throughout the tropics at the last glacial maximum, as expected based on proxies for continental surface temperatures.
Comparison of Imbrie‐Kipp Transfer Function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas
Authors: Ortiz J. D. , Mix A. C.
Published: February 2004

We evaluate the reliability of statistical estimates of sea surface temperature (SST) derived from planktonic foraminiferal faunas using the modern analog method and the Imbrie‐Kipp method. Global core top faunas provide a calibration data set, while modern sediment trap faunas are used for validation. Linear regression of core top predicted SST against atlas SST generated slopes close to one for both methods. However, the Imbrie‐Kipp transfer function temperature estimates had an intercept 1.3°C warmer than modern analog estimates and 1.7°C warmer than recorded atlas SST. The RMS error for the core top data set using the modern analog method (1.5°C) was smaller than that of the Imbrie‐Kipp method (1.9°C). SST errors for the sediment trap faunas were not statistically different from those of the core top data set, regardless of method. Developing Imbrie‐Kipp transfer functions for limited regions reduced the RMS variability but introduced residual structure not present in the global Imbrie‐Kipp transfer function. Dissolution simulations with the sediment trap sample which generated the warmest SST residual for both methods suggests that the loss of delicate warm water foraminifera from midlatitude sediments may be the cause of this thermal error. We conclude that (1) the faunal structure of sediment trap and core top assemblages are similar; (2) both methods estimate SST reliably for modern foraminiferal flux assemblages, but the modern analog method exhibits less bias; and (3) both methods are relatively robust to samples with low communality but sensitive to selective faunal dissolution.
Marine core evidence for reduced deep water production during Termination II followed by a relatively stable substage 5e (Eemian)
Authors: Oppo Delia W. , Horowitz Michael , Lehman Scott J.
Published: February 2004

We present oxygen and carbon isotope records of benthic foraminifera from the glacial stage 6 through interglacial substage 5e (Eemian) sections of several cores from the subpolar North Atlantic. The cores range in water depth from 1451 to 2658 m. In one core, we generated estimates of sea surface temperature (SST) and of ice‐rafted detritus (IRD) delivery for all of stage 5. We reconstruct bathymeric profiles of δ13C for stage 6, Termination II, and three time slices of substage 5e. The δ13C profiles indicate that local deep water geometry during stage 6 was similar to that of the last glaciation, with glacial North Atlantic Intermediate Water (GNAIW) overlying deeper waters partially of southern origin. An anomalously large peak in IRD, coupled with planktonic δ18O evidence for iceberg melting immediately precedes Termination II and is otherwise similar to stage 2 and 3 Heinrich events. During the termination, low δ13C values are observed in cores near and above 2 km, providing evidence of reduced GNAIW production in association with deglacial melting. In the shallowest cores, low δ13C values persist into early substage 5e, indicating that the southward retreat of southern‐source waters was not completed until well after the beginning of the substage. In contrast to Termination I, SSTs remained cold until the end of the deglaciation; this may be why there is little evidence from marine and terrestrial sequences for a pronounced climate reversal on Termination II despite what is now clear evidence of a significant reduction in ocean ventilation. The faunal data suggest that SSTs during early 5e were about 7°C warmer than during the glaciation. SST rose several degrees from early to middle substage 5e, peaked in middle substage 5e at about 10°C above glacial values, and then gradually declined by about 5°C. These changes were linked to variations in water mass configuration, as interpreted from benthic δ13C evidence. Most of the evidence suggests that oscillations superimposed on the gradual SST trend were 1°–2°C, in contrast to the larger SST changes (3°–4°C) documented for substage 5d.
The California Current of the Last Glacial Maximum: Reconstruction at 42°N based on multiple proxies
Authors: Ortiz Joseph , Mix Alan , Hostetler Steve , Kashgarian Michaele
Published: February 2004

Multiple paleoceanographic proxies in a zonal transect across the California Current near 42°N record modern and last glacial maximum (LGM) thermal and nutrient gradients. The offshore thermal gradient, derived from foraminiferal species assemblages and oxygen isotope data, was similar at the LGM to that at present (warmer offshore), but average temperatures were 3.3° ±1.5°C colder. Observed gradients require that the sites remained under the southward flow of the California Current, and thus that the polar front remained north of 42°N during the LGM. Carbon isotopic and foraminiferal flux data suggests enhanced nutrients and productivity of foraminfera in the northern California Current up to 650 km offshore. In contrast, marine organic carbon and coastal diatom burial rates decreased during the LGM. These seemingly contradictory results are reconciled by model simulations of the LGM wind‐ field, which suggest that wind stress curl at 42°N (and thus open‐ocean upwelling) increased, while offshore Ekman transport (and thus coastal upwelling) decreased during the last ice age. The ecosystem of the northern California Current during the LGM approximated that of the modern Gulf of Alaska. Cooling and production in this region was thus driven by stronger open‐ocean upwelling and/or southward flow of high‐latitude water masses, rather than by coastal upwelling.