bluuebunny/crossref_metadata_2025 huggingface dataset:
Ozone depletion potential of CH3Br
Authors: Ko Malcolm K. W. , Sze Nien Dak , Scott Courtney , Rodríguez José M. , Weisenstein Debra K. , Sander Stanley P.
Published: February 2004

The ozone depletion potential (ODP) of methyl bromide (CH3Br) can be determined by combining the model‐calculated bromine efficiency factor (BEF) for CH3Br and its atmospheric lifetime. This paper examines how changes in several key kinetic data affect BEF. The key reactions highlighted in this study include the reaction of BrO + HO2, the absorption cross section of HOBr, the absorption cross section and the photolysis products of BrONO2, and the heterogeneous conversion of BrONO2 to HOBr and HNO3 on aerosol particles. By combining the calculated BEF with the latest estimate of 0.7 year for the atmospheric lifetime of CH3Br, the likely value of ODP for CH3Br is 0.39. The model‐calculated concentration of HBr (∼0.3 pptv) in the lower stratosphere is substantially smaller than the reported measured value of about 1 pptv. Recent publications suggested models can reproduce the measured value if one assumes a yield for HBr from the reaction of BrO + OH or from the reaction of BrO + HO2. Although the DeMore et al. [1997] evaluation concluded any substantial yield of HBr from BrO + HO2 is unlikely, for completeness, we calculate the effects of these assumed yields on BEF for CH3Br. Our calculations show that the effects are minimal: practically no impact for an assumed 1.3% yield of HBr from BrO + OH and 10% smaller for an assumed 0.6% yield from BrO + HO2.
Long‐term Stratospheric Aerosol and Gas Experiment I and II measurements of upper tropospheric aerosol extinction
Authors: Kent G. S. , Trepte C. R. , Lucker P. L.
Published: February 2004

A detailed analysis has been made of Stratospheric Aerosol and Gas Experiment I and II aerosol extinction data for the upper troposphere (6‐km altitude to the seasonally averaged tropopause) taken between 1979 and 1998. An improved method of separation of the volcanic and surface‐derived components of the aerosol optical depth has been used. The mean extinction, at a wavelength of 1.02 μm, of the nonvolcanic component of the upper tropospheric aerosol is found to increase from approximately 1×10−4 km−1 at 70°S to about 7 times that value at 70°N. Maximum downward transfer of volcanic material into the upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70°S or greater in the southern hemisphere and at about 50°N in the northern hemisphere. The almost 20‐year data sequence (1979–1981, 1984–1991, 1994–1998) has been examined for evidence of any long‐term trends in the aerosol optical depth of the upper troposphere. It is unlikely that any change in the upper tropospheric 1‐μm aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere.
Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil
Authors: Martins J. Vanderlei , Artaxo Paulo , Liousse Catherine , Reid Jeffrey S. , Hobbs Peter V. , Kaufman Yoram J.
Published: February 2004

Black carbon mass absorption efficiencies of smoke particles were measured for various types of biomass fires during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment using thermal evolution measurements for black carbon and optical absorption methods. The obtained results range between 5.2 and 19.3 m2 g−1 with an average value of 12.1±4.0 m2 g−1. Particle size distributions and optical properties were also measured to provide a full set of physical parameters for modeling calculations. Mie theory was used to model the optical properties of the particles assuming both internal and external mixtures coupling the modeling calculations with the experimental results obtained during the campaign. For internal mixing, a particle model with a layered structure consisting of an absorbing black carbon core, surrounded by a nonabsorbing shell, was assumed. Also, for internal mixing, a discrete dipole approximation code was used to simulate packed soot clusters commonly found in electron microscopy photographs of filters collected during the experiment. The modeled results for layered spheres and packed clusters explain black carbon mass absorption coefficients up to values of about 25 m2 g−1, but measurements show even higher values which were correlated with the chemical composition and characteristics of the structure of the particles. Unrealistic high values of black carbon absorption efficiencies were linked to high concentrations of K, which influence the volatilization of black carbon (BC) at lower temperatures than usual, possibly causing artifacts in the determination of BC by thermal technique. The modeling results are compared with nephelometer and light absorption measurements.
Spreading and growth of contrails in a sheared environment
Authors: Jensen Eric J. , Ackerman Andrew S. , Stevens David E. , Toon Owen. B. , Minnis Patrick
Published: February 2004

A case study of persistent contrail evolution in a sheared environment is simulated over time‐scales of 15–180 min using a large‐eddy simulation model with detailed microphysics. Model results are compared to satellite and in situ measurements of persistent contrails from the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) experiment. In simulations with large ambient supersaturations and moderate wind shear, crystals with lengths > 200 μm are generated within 45 min by depositional growth. These crystals fall rapidly, and the contrail horizontal extent increases due to the wind shear. Strong radiative heating (with rates up to 10 K d−1) drives a local updraft and lofts the contrail core several hundred meters. The observed rate of contrail spreading and maintenance of optical depths larger than 0.05 can be approximately explained simply by growth and precipitation of ice crystals nucleated during the initial contrail formation if the environmental humidity is very high (relative humidity with respect to ice >125%). This result is consistent with observed high humidities in regions where persistent contrails formed during SUCCESS.
Total global lightning inferred from Schumann resonance measurements
Authors: Heckman S. J. , Williams E. , Boldi B.
Published: February 2004

Radiation with frequencies of 5–30 Hz is ducted between Earth's surface and ionosphere with little attenuation; at the lowest frequencies, waves travel several times around the Earth before losing most of their energy. Much of this radiation is produced by lightning. Here we assume that all of this radiation is produced by lightning, and attempt to invert the observed electric and magnetic fields to infer the global lightning activity. We show 10 days of inversions. For these 10 days, the inferred average rate of vertical charge transfer squared is only 1.7 105 (C km)2/s. Other studies suggest that the root mean square moment change of a flash is about 166 Coulomb kilometers. If we naively assume that each of these flashes is composed of four equally sized strokes, then we conclude that our entire observed signal could be produced by only 22 flashes per second.
Ratio spectra as a quality control tool for solar spectral UV measurements
Authors: Bernhard G. , Seckmeyer G. , McKenzie R. L. , Johnston P. V.
Published: February 2004

Spectra sampled by a UV spectroradiometer throughout the day are ratioed against a reference spectrum from the same instrument, typically the spectrum obtained at noon. The resultant ratios are a useful quality control tool that allows the detection of errors in the data acquisition of the instrument and shifts in its wavelength calibration. The method was applied to measurements of two high‐quality spectroradiometers during the SUSPEN 1997 UV spectroradiometer intercomparison. Thus the wavelength calibration of both instruments was found to be stable to within ±0.01 nm. In both instruments, deficiencies were identified by this method, enabling remedial action to be taken.
Broadband lidar measurements of tropospheric water vapor profiles
Authors: South A. M. , Povey I. M. , Jones R. L.
Published: February 2004

A new technique has been developed for the measurement of vertical profiles of water vapor in the troposphere. The method, which is also appropriate for the measurement of gases such as ozone and nitrogen dioxide, is based on the application of differential optical absorption spectroscopy (DOAS) to backscattered photons from a broadband lidar (10–20 nm at full width half maximum) operating in the UV‐visible region of the electromagnetic spectrum. A Nd: YAG pumped broadband dye laser is used as the light source, and backscattered photons from the atmosphere are both time and spectrally resolved using an imaging spectrometer with a charge‐coupled device (CCD) detector. The new broadband lidar circumvents many of the difficulties and inaccuracies involved with traditional narrowband differential absorption lidar (DIAL) water vapor observations, for example, the laser bandwidth and wavelength‐positioning requirements demanded by the narrowness of the water vapor transitions. Water vapor mixing ratio profiles measured over Cambridge, England, in 1996 with the new instrument demonstrate good levels of agreement with quasi‐simultaneous radiosonde profiles from nearby weather stations.
Central Eistla Regio: Origin and relative age of topographic rise
Authors: McGill George E.
Published: February 2004

Large topographic rises on Venus pose a number of fundamental questions concerning the mantle processes responsible for them. Eistla Regio, a 7500 km‐long elevated region extending WNW from the west end of Aphrodite Terra, is a particularly interesting example. The Sappho Patera quadrangle, 0–25°N, 0–30°E, includes all of central Eistla Regio, a region dominated by two interesting, apparently hybrid structures (Irnini and Anala Montes) with characteristics of both shield volcanoes and coronae. Mapping and tectonic analysis within this region indicate that this part of the rise experienced progressive deformation extending from prior to emplacement of the regional plains to after emplacement of young lavas; that is, for most of the stratigraphically defined history of Venus. Estimates of the relative roles of uplift and volcanic construction in the evolution of Irnini and Anala Montes are critically dependent on whether they are primarily anomalous coronae that have been uplifted, or whether they are true shield volcanoes that have evolved into coronae. Studies elsewhere, as well as most models of corona evolution, favor the first alternative; the geology of central Eistla Regio favors the second, suggesting that local thinning of the lithosphere allowed shield volcanoes to evolve into coronae. Because these hybrid edifices are not typical of either most coronae or most shield volcanoes on Venus, they deserve detailed modeling. For now, the relative roles of uplift and volcanic construction in central Eistla Regio cannot be even approximately estimated.
Morphometry of wrinkle ridges on Venus: Comparison with other planets
Authors: Kreslavsky M. A. , Basilevsky A. T.
Published: February 2004

The widths of wrinkle ridges on Venus and the spacing of their network were measured at 30 randomly selected sites on the Venusian plains. At one of the sites, heights of ridges were also estimated. Results of our measurements indicate that (1) wrinkle ridges on Venus form a quasi‐periodic rather than chaotic network; (2) there are regional variations of the typical spacing within the network, with the mean for all 30 sites being 12.9 km, and the standard deviation of the means of all sites being 5.8 km; (3) there are regional variations of typical ridge width with the mean width being 1.25 km, and the standard deviation of the means of all sites being 0.66 km; (4) there is no site‐to‐site correlation between the mean ridge widths and spacings. Comparison with other terrestrial planets shows that wrinkle ridges on Venus are generally narrower than on the other planets, and the spacing of the ridge network on Venus is smaller than on Tharsis Plateau, Mars. Global surface shortening due to formation of the wrinkle ridge network is estimated to be of order of 0.1%.
Modeling the effect of sastrugi on snow reflectance
Authors: Leroux Catherine , Fily Michel
Published: February 2004

A snow reflectance model was developed to assess the bidirectional reflectance distribution function (BRDF) of snow over anisotropic rough surfaces such as fields of sastrugi commonly encountered in some areas of the ice sheets of Antarctica and Greenland. The sastrugi are modeled as regularly spaced identical rectangular protrusions with the same orientation. Interfacet multiple scattering is ignored. Each model sastruga has vertical walls and a small width compared to its length and to its height. The input data for the model are the Sun and the viewing directions, the number of sastrugi per pixel, their dimensions and the bidirectional reflectance for a flat snow surface. The latter is computed by a multiple scattering model, and snow particles are assumed to be spherical (Mie theory). In order to compare the model results with ground measurements acquired at the south pole by Grenfell et al. [1994] and Brandt et al. [1991], atmospheric effects were taken into account. The model reproduces the global influence of sastrugi on the shape of the BRDF. The influence of sastrugi is clearly visible for zenith viewing angles larger than 50°.
Water content of the Martian soil: Laboratory simulations of reflectance spectra
Authors: Yen Albert S. , Murray Bruce C. , Rossman George R.
Published: February 2004

Reflectance spectra from the surface of Mars collected by instruments such as the imaging spectrometer (ISM) onboard the 1988 Soviet Phobos 2 spacecraft exhibit strong 3 μm absorption features that have long been attributed to hydrated materials on the Martian surface. This interpretation is consistent with a series of chemical weathering models suggesting an abundance of palagonites, clays, and other hydrated mineral phases in the Martian fines. Little work, however, has been done to constrain the actual water content of the Martian surface materials. New laboratory data presented here show that the ISM spectra are consistent with up to 4% water by weight and that the deep hydration features observed in the spacecraft data could be due to less than 0.5% water if the hydrated phases are present in the form of grain coatings. These results are consistent with the somewhat uncertain in situ measurements obtained by the Viking landers which yielded approximately 2 wt % water from samples heated to 500°C. On the basis of this work, we expect the TEGA instrument on the Mars '98 lander to find less than 4% adsorbed or bound water in the upper few centimeters of the Martian soil.
Determining the composition of olivine from reflectance spectroscopy
Authors: Sunshine Jessica M. , Pieters Carlé M.
Published: February 2004

Reflectance spectra of olivines spanning the forsterite‐fayalite solid‐solution series have been analyzed with the modified Gaussian model (MGM). The compositional variability of the three primary absorption bands that constitute the diagnostic 10‐μm olivine feature has been quantified by examining the centers, widths, and relative strengths in 18 spectra ranging in composition from Mg‐rich forsterite to Fe‐rich fayalite. These analyses have also revealed several interrelationships among the three absorption bands that provide new insights into their crystallographic origins. The primary olivine absorptions near 1.0‐μm are well behaved and provide a means to remotely identify and estimate the composition of olivine from reflectance spectra. However, the spectral resolution and signal‐to‐noise of current remotely acquired data, combined with the significant overlap in the 1.0‐μm region, provide enough uncertainty to allow models based on simple least squares minimization to reach solutions that are mathematically satisfying yet physically unrealistic. More sophisticated models using inverse theory that incorporate constraints among the absorption bands as determined from these laboratory MGM analysis are shown to yield meaningful results which can be confidently used to estimate composition in remote data. The MGM and inverse theory are used to analyze the spectrum of the olivine‐rich asteroid 246 Asporina and to quantitatively show that the olivine component on Asporina is magnesium‐rich (i.e., forsteritic). In contrast to the systematic behavior of the three primary olivine absorptions, absorption features short of 0.7 μm are found to exhibit no obvious relationship to composition and as such are not recommended for use in remote applications.
Magmatic intrusions and a hydrothermal origin for fluvial valleys on Mars
Authors: Gulick Virginia C.
Published: February 2004

Numerical models of Martian hydrothermal systems demonstrate that systems associated with magmatic intrusions greater than several hundred cubic kilometers can provide sufficient groundwater outflow to form the observed fluvial valleys, if subsurface permeability exceeds about 1.0 darcy. Groundwater outflow increases with increasing intrusion volume and subsurface permeability and is relatively insensitive to intrusion depth and subsurface porosity within the range considered here. Hydrothermally‐derived fluids can melt through 1 to 2 km thick ice‐rich permafrost layers in several thousand years. Hydrothermal systems thus provide a viable alternative to rainfall for providing surface water for valley formation. This mechanism can form fluvial valleys not only during the postulated early warm, wet climatic epoch, but also during more recent epochs when atmospheric conditions did not favor atmospheric cycling of water. The clustered distribution of the valley networks on a given geologic surface or terrain unit of Mars may also be more compatible with localized, hydrothermally‐driven groundwater outflow than regional rainfall. Hydrothermal centers on Mars may have provided appropriate environments for the initiation of life or final oases for the long‐term persistence of life.
Laboratory studies of the optical properties and stability of oxygen on Ganymede
Authors: Baragiola Raúl A. , Bahr David A.
Published: February 2004

We present measurements of the optical properties of solid O2 and O2:H2O films and on the stability of oxygen in water ice at different temperatures. The experiments are meant to test hypotheses for the location of condensed molecular oxygen in Ganymede. We find that oxygen is not retained in water ice at Ganymede's recorded surface temperatures and analyze current questions associated with the existence of oxygen in this Jovian moon. The results suggest that solid oxygen and possibly other condensed gases exist on cold patches on the heterogeneous Ganymede's surface.
Surviving the limits to life at the surface of Mars
Authors: Clark Benton C.
Published: February 2004

The harshness of the environment at its surface severely limits the survival of unprotected terrestrial organisms on Mars. However, the Martian geologic record is consistent with an earlier history with much different climatic conditions, more conducive to life, followed by an extended climate change which led to the currently inhospitable state. Such a transition is ideal for evolutionary adaptation to new conditions because it provides both the forcing function and the extensive time for progressive specializations. It is shown that against each of the current limits to life (organics deprivation; restricted amounts and physical forms of H2O; limited availability of other key nutrients; ionizing radiation insult; and strongly oxidizing chemical species), there are plausible countermeasure solutions. Most, but not all, of these mechanisms already exist as known examples in terrestrial biota. A combination of these adaptations could conceivably allow highly tailored organisms, either active or dormant, to survive at or very near the surface of Mars. Even in carefully preserved regolith samples returned to Earth, microorganisms could be difficult to detect without specific biochemical probes and/or purposeful biological amplification by selective enhancement of the supply of nutrients, optimized incubation parameters, and highly sensitive detection of metabolic activity.
Phosphates and carbon on Mars: Exobiological implications and sample return considerations
Authors: Mojzsis Stephen J. , Arrhenius Gustaf
Published: February 2004

Much of the surface of Mars may preserve chemical information contained in rocks from the Noachian era, with ages that overlap the correspondingly earliest Archean geological history of the Earth, or from before around 3800 Ma (Ma = 1×106years). Metabolically sophisticated life, which utilized phosphate and carbon and was capable of fractionating carbon isotopes, was apparently present already on Earth by ∼3800 Ma, or within 600 Ma after the formation of the planet. An early appearance of life on Earth opens the strong possibility for a similarly early and rapid emergence of life on planet Mars. This hypothesis remains within the realm of plausibility so long as it can be established that liquid water and energy sources were available there for inchoate life, and that the life that emerged reached a level of complexity which could be recognized by its chemical, and perhaps morphological remains. Hypotheses to be used in the search for an ancient Martian biosphere from future sample return missions are testable by examining the record of life in ancient terrestrial sedimentary rocks, including those that contain rare and recognizable “physical” microfossils (“morphofossils” identified on the basis of their shape alone) and stable, authigenic biominerals which include carbonaceous matter having characteristically fractionated carbon isotope signatures (here termed “chemofossils”). Prior to sample return, these tests can be applied to the mineral associations of the SNC meteorites, a group of meteorites believed to have originated on Mars. Recent claims of a biological origin for secondary minerals and their features as well as for trace organic compounds in the Martian meteorite ALH84001, are derived in part from the interpretation of putative “nanofossil” shapes and the nature of the associated mineral assemblage in small carbonate deposits of an igneous rock. Such igneous samples would not normally be the best candidate to search for evidence of past life, even on Earth. Investigations of these mineral occurrences in the Martian meteorites and of the oldest geological records on Earth provide a useful framework for (1) using mineral phase relationships, (2) analytical data of stable carbon isotopic distributions, and (3) the problematic task of morphofossil interpretations, in the search for life via future sample return missions from the ancient surface of Mars.
Organic synthesis during fluid mixing in hydrothermal systems
Authors: Shock Everett L. , Schulte Mitchell D.
Published: February 2004

Hydrothermal circulation can lead to fluid mixing on any planet with liquid water and a source of heat. Aqueous fluids with differing compositions, especially different oxidation states, are likely to be far from thermodynamic equilibrium when they mix, and provide a source of free energy that can drive organic synthesis from CO2and H2, and/or supply a source of geochemical energy to chemolithoautotrophic organisms. Results are presented that quantify the potential for organic synthesis during unbuffered fluid mixing in present submarine hydrothermal systems, as well as hypothetical systems that may have existed on the early Earth and Mars. Dissolved hydrogen, present in submarine hydrothermal fluids owing to the high‐temperature reduction of H2O as seawater reacts with oceanic crustal rocks, provides the reduction potential and the thermodynamic drive for organic synthesis from CO2(or bicarbonate) as hydrothermal fluids mix with seawater. The potential for organic synthesis is a strong function of the H2content of the hydrothermal fluid, which is, in turn, a function of the prevailing oxidation state controlled by the composition of the rock that hosts the hydrothermal system. Hydrothermal fluids with initial oxidation states at or below those set by the fayalite‐magnetite‐quartz mineral assemblage show the greatest potential for driving organic synthesis. These calculations show that it is thermodynamically possible for 100% of the carbon in the mixed fluid to be reduced to a mixture of carboxylic acids, alcohols, and ketones in the range 250‐50°C as cold seawater mixes with the hydrothermal fluid. As the temperature drops, larger organic molecules are favored, which implies that fluid mixing could drive the geochemical equivalent of a metabolic system. This enormous reduction potential probably drives a large portion of the primary productivity around present seafloor hydrothermal vents and would have been present in hydrothermal systems on the early Earth or Mars. The single largest control on the potential for organic synthesis is the composition of the rock that hosts the hydrothermal system.
Crater ellipticity in hypervelocity impacts on metals
Authors: Burchell Mark J. , Mackay Neil G.
Published: February 2004

The change in crater shape of hypervelocity impacts as a function of impact angle has been studied for impacts at 2 and 5 km s−1 of millimeter‐sized spherical projectiles of cellulose acetate, aluminum and stainless steel impacting aluminum, and stainless steel, and lead targets. The variation in crater depth, length, and diameter with impact angle is presented and discussed. While the main dependence on impact angle is found to be of a cos θ nature, cases are found where the magnitude of the dependence changes at discrete angles of impact. This is interpreted as evidence of disruption of the projectile affecting the final size of the crater. Crater depth is found to depend on (cos θ)n, where n is strongly dependent on the projectile density and not the traditionally assumed constant value of two‐thirds. For aluminum impacting aluminum at 5 km s−1, n is found to be near unity. Reproducing the criteria used in analysis of crater shape in published work for the Long Duration Exposure Facility satellite yields a mean density for small particles in Low Earth Orbit of 3400 kg m−3. Normal incidence impacts of non‐spherical projectiles have also been studied and found to yield irregularly shaped craters, which can appear as oblique angle impacts of spheres. No sensible cuts on crater measurements permit clean separation of these two types of impacts.
Evidence for igneous activity and implications for the origin of a fretted channel in southern Ismenius Lacus, Mars
Authors: Carruthers Margaret W. , McGill George E.
Published: February 2004

Northern Arabia Terra, Mars is characterized by a complex landscape that includes fretted terrain, fretted channels, completely closed canyons, shallow troughs, and fracture‐bounded depressions. The importance of ground ice in the initiation and development of these features, and of large impact structures in controlling channel courses, has long been recognized. The development of the fretted channel systems, including the integration of closed canyons, craters, and surface depressions into throughgoing channels, has previously been attributed primarily to mass‐wasting processes. The two channel systems in southern Ismenius Lacus quadrangle, Mamers Valles and Ismeniae Fossae, incorporate craters of probable impact origin, but both also include irregular crater‐like features and scalloped channel reaches that have characteristics not consistent with an impact origin. Some of these are, by analogy with morphologically similar features on Earth and elsewhere on Mars, reasonably interpreted as of volcanic origin. The detailed morphologies of a few diagnostic features associated with Ismeniae Fossae suggest a hydrovolcanic origin, an inference supported by the probable presence of abundant ground ice or groundwater at the time of their formation. The evidence described here for hydrovolcanism supports the inference that, in addition to mass‐wasting processes, explosive excavation, intrusion‐driven sublimation, subsurface erosion, and erosion by running water may have played important roles in channel formation and evolution as well. All of these processes are consistent with magma interacting with groundwater or ground ice.
Obliquity variations due to climate friction on Mars: Darwin versus layered models
Authors: Spada Giorgio , Alfonsi Laura
Published: February 2004

In response to periodic variations in the obliquity, giant CO2 ice caps may form in the polar regions of Mars. The delayed response of the equatorial bulge of Mars to these periodic loads may introduce a secular term in the time evolution of the obliquity. Up to now, the variations in the flattening of Mars induced by the polar ice caps have been estimated using the Darwin model, which simply consists of a uniform sphere with Newtonian rheology. For Earth‐like viscosities of ∼3×1021 Pa s the Darwin model maximizes the friction and predicts a secular obliquity variation comparable to the currently observed value (∼24°). Our main purpose here is to address the role of two well‐recognized features of Mars (a thick lithosphere and a core) on the response of its equatorial bulge and thus on the secular changes of obliquity driven by climate friction. This is done using a layered model with Maxwell viscoelastic rheology. We show that for a wide range of admissible mantle viscosities the obliquity variation is sensibly smaller (by a factor of ∼2) than the one predicted by the Darwin model. On the basis of this finding, we can rule out the possibility that the present‐day obliquity of Mars totally results from climate friction.
Remanent ferromagnetism and the interior structure of Ganymede
Authors: Crary F. J. , Bagenal F.
Published: February 2004

The Galileo spacecraft discovered that Jupiter's moon, Ganymede, has both an internal magnetic field and a strongly differentiated interior. This has prompted new models of Ganymede's interior, featuring an iron core and dynamo action to produce the magnetic field. However, extant dynamo action is not necessary to produce Ganymede's magnetic fields. The rocky material within Ganymede is believed to be rich in iron‐bearing minerals, particularly magnetite, and its outer layer is capable of holding strong remanent magnetism. We investigate two sources of remanent ferromagnetism: Jupiter's magnetic field, especially in the past, when Ganymede was closer to Jupiter due to tidal evolution, and a paleomagnetic field from dynamo action which is no longer active. In this analysis we use a model of remanent magnetization similar to those which have been applied to the Moon, Mars, and Mercury, but we have generalized it to apply to strongly magnetic materials. We find that magnetization by the jovian background field can account for no more than 5.6% of Ganymede's internal field. However, a paleomagnetic field of roughly 10,000 nT or greater (at least 15 times the current field of Ganymede and a third that of the Earth's current field) is capable of producing sufficient remanent magnetization to account for the Galileo observations, given very favorable assumptions about the magnetic properties of the rock layer. For more probable values a paleomagnetic field equal to or greater than the Earth's current field is necessary.
Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations
Authors: Bemis Bryan E. , Spero Howard J. , Bijma Jelle , Lea David W.
Published: February 2004

Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature:δ18O relationships at 15°–25°C. Relationships for both species can be described by linear equations. Equations for O. universa grown under low light (LL) and high light (HL) share a slope of −4.80 (0.21‰ °C−1) with a HL‐LL offset of −0.33‰ due to symbiont photosynthetic activity. The effect of [CO32−] on O. universa is −0.002‰ µmol−1 kg−1 and is insensitive to temperature. For G. bulloides, ontogenetic effects produce size‐related trends in temperature:δ18O, whereby larger shells are enriched in 18O relative to smaller specimens. The O. universa temperature:δ18O relationships are more accurate than previously published equations for describing plankton tow data. Our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic Cibicidoides data below 10°C. Temperature:δ18O relationships for G. bulloides provide good agreement with field data for this species from the northeast Pacific.
Improving past sea surface temperature estimates based on planktonic fossil faunas
Authors: Waelbroeck C. , Labeyrie L. , Duplessy J.‐C. , Guiot J. , Labracherie M. , Leclaire H. , Duprat J.
Published: February 2004

A new method of past sea surface temperature (SST) reconstruction based on the modern analog technique (Prell, 1985) and on the indirect approach (Bartlein et al., 1986) has been developed: the revised analog method (RAM). Applied to planktonic foraminifera, this technique leads to significant improvements in modern SST reconstruction with respect to former methods: our estimates are characterized by much lower residuals and a better coverage of the observed SST range. Moreover, the error of RAM estimates of past SSTs is lower than that associated with former reconstructions, particularly at middle and high latitudes. In low latitudes, cold season SSTs reconstructed by RAM during glacials are 1°–3°C lower than previously estimated. Our results tend thus to reconcile paleoestimates of glacial temperatures based on planktonic microfossils and on continental data in the tropics.
Tropical Atlantic seasonal dynamics in the Early Middle Eocene from stable oxygen and carbon isotope profiles of mollusk shells
Authors: Andreasson Fredrik P. , Schmitz Birger
Published: February 2004

There are disagreements whether the tropical sea surface temperature during the warm early to early middle Eocene was equal to the present temperature or significantly lower. In order to determine the sea surface temperature of the eastern tropical Atlantic in the early middle Eocene we have established δ18O and δ13C profiles along the transect of growth of excellently preserved shells of Turritella gastropods and a Macoma bivalve from tropical West Africa. This method makes possible a distinction between the winter and the summer temperature. The δ18O and δ13C data indicate sea surface temperatures of 29°–31°C, which declined to 23°–25°C during seasonal upwelling. These conditions are similar to those of today and suggest that significant seasonal dynamics prevailed in the early middle Eocene tropics. The new data further contradict that the tropics cooled during the generally warm early to early middle Eocene, as inferred in studies on foraminifera δ18O.
Late Cenozoic Eolian deposition in the North Pacific: Asian drying, Tibetan uplift, and cooling of the northern hemisphere
Authors: Rea David K. , Snoeckx Hilde , Joseph Leah H.
Published: February 2004

A newly constructed record of eolian dust accumulation from the central North Pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3.6 Ma. We associate this sudden drying with the uplift of at least the northern portion of the Tibetan Plateau, shutting off the Indian Ocean moisture source to central and western China. This ten‐fold increase in atmospheric dust loading appears to be associated with the 1‐m.y.‐long shift toward heavy δ18O values that occurred at 3.6–2.6 Ma. The dust grain‐size record of wind intensity begins its late Cenozoic coarsening a million years before the drying event, at ∼4.5 Ma. The northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric CO2 in the latest Miocene and the early Pliocene closing of the Panamanian Seaway.
Benthic δ18O records in the North Atlantic over the Last Glacial Period (60–10 kyr): Evidence for brine formation
Authors: Vidal L. , Labeyrie L. , van Weering T. C. E.
Published: February 2004

Four high‐resolution benthic isotope records from North Atlantic and south Norwegian Sea cores are compared for the past 60 kyr. The benthic records show light δ18O peaks simultaneously to or following the Heinrich events. The amplitude of these light peaks varies between 1‰ for the most northern core ENAM93‐21 and 0.35‰ for the deep North Atlantic cores. The high δ18O gradient between the North Atlantic cores and the Pacific core V19‐30 suggests a regional influence of δ18O‐depleted meltwater to deeper depth during periods of iceberg discharges. The most likely process for transferring δ18O‐depleted meltwater to depth is sea ice formation induced by brine formation. During Heinrich events 4 and 3 this mecanism was of major importance (compared to open ocean convection) in the southern Norwegian Sea and at the northern boundaries of the Atlantic, leading to intermediate to deep water formation. Other δ18O excursions appearing in all the records point to the contribution of both brine formation and global changes in the δ18O of seawater associated with the Heinrich event periods.
Application of modern analog technique to marine Antarctic diatoms: Reconstruction of maximum sea‐ice extent at the Last Glacial Maximum
Authors: Crosta X. , Pichon J.‐J. , Burckle L. H.
Published: February 2004

Modern analog technique (MAT) applied to Antarctic diatoms is a new approach for quantitative sea‐ice paleoreconstructions in the Southern Ocean. In a first step we show that MAT is a better approach than the Imbrie and Kipp Method to reconstruct the modern sea‐ice pattern. We then use this approach to reconstruct sea‐ice presence in number of months per year during the last glacial maximum (LGM). At this time, sea‐ice presence was greater than today, leading to a shorter diatom growing season. The maximum sea‐ice extent, inferred from quantitative values of sea‐ice presence, was located 5–8° north of its actual position, leading to double the surface of modern winter sea ice. This greater sea‐ice extent may have played a significant role on atmospheric and surface oceanic circulations and therefore on southern mid‐latitude and high‐latitude climates. It may also have reduced the amount of heat, moisture, and CO2 from the ocean to the atmosphere, thus participating in the lowering of atmospheric CO2 during the LGM.
Abrupt cold spells in the northwest Mediterranean
Authors: Rohling E. J. , Hayes A. , De Rijk S. , Kroon D. , Zachariasse W. J. , Eisma D.
Published: February 2004

Hitherto unknown abundance peaks of left coiling (l.c.) Neogloboquadrina pachyderma from a Gulf of Lions piston core indicate that abrupt cold spells associated with Atlantic Heinrich events affected the Mediterranean. N. pachyderma (l.c.) is typical of (sub) polar waters in the open ocean. The southern edge of its glacial North Atlantic bioprovince reached south Portugal. Only trace abundances of N. pachyderma (l.c.) are known from Quaternary Mediterranean sediments, suggesting that no significant “invasions” occured via the Strait of Gibraltar. The Gulf of Lions abundance peaks therefore seem to reflect area‐specific thriving of a normally rare but indigenous taxon in the western Mediterranean through local favorable habitat development. The general planktonic foraminiferal record suggests that the basic hydrographic regime in the Gulf of Lions, with wintertime deep convective overturn, was relatively stable over the past 60 kyr. Under these conditions, high abundances of N. pachyderma (l.c.) would essentially imply temperature reductions of the order of 5°–8° relative to the present.
Impact of oceanic reorganizations on the ocean carbon cycle and atmospheric carbon dioxide content
Authors: Marchal Olivier , Stocker Thomas F. , Joos Fortunat
Published: February 2004

A zonally averaged, circulation‐biogeochemical ocean model is used to explore how the distribution of PO4 and δ13C in the major basins and the atmospheric pCO2 respond to rapid changes in the thermohaline circulation (THC). Different evolutions of the Atlantic THC are simulated by applying surface freshwater pulses typical, for example, of Heinrich events and the last deglaciation. In the model, when the THC completely collapses, PO4 increases (>0.5 mmol m−3) and δ13C decreases (<0.5‰) in Atlantic bottom waters because of a drop in ventilation by North Atlantic Deep Water (NADW). Although consistent with the traditional interpretation of sedimentary records of benthic foraminiferal Cd/Ca and δ13C, the relationship between the degree of PO4 enrichment and δ13C depletion and the degree of THC reduction is not linear. In the NADW formation area the preformed PO4 declines (<0.5 mmol m−3) because of an imbalance between biological uptake and PO4 supply from the deep, and the preformed δ13C rises (>1‰) because of a longer residence time of waters at the surface. These surface anomalies are propagated to the bottom North Atlantic when the THC resumes. When the thermohaline overturning is only partly reduced and at shallower depths, changes in bottom waters are accompanied by a PO4 decrease and δ13C increase at intermediate levels in the mid‐latitude Atlantic. This results in enhanced vertical gradients of these properties consistent with chemical and isotopic reconstructions for the last glacial maximum. Finally, the atmospheric pCO2 increases during the cold period in the North Atlantic when the THC is reduced with an amplitude (7–30 µatm) and timescale (∼10² to 1–2 × 10³ yr) depending on the intensity of the THC change. This is qualitatively consistent with recent data from an Antarctic ice core documenting a pCO2 increase during the Younger Dryas and after Heinrich events 4 and 5.
Millennial‐scale iceberg discharges in the Irminger Basin during the Last Glacial Period: Relationship with the Heinrich events and environmental settings
Authors: Elliot Mary , Labeyrie Laurent , Bond Gerard , Cortijo Elsa , Turon Jean‐Louis , Tisnerat Nadine , Duplessy Jean‐Claude
Published: February 2004

High‐resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the Irminger Basin. The last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from Iceland and corresponding to millennial‐scale instabilities of the coastal ice sheets and ice shelves in the Nordic area. A comparison with midlatitude sediment cores shows that ice‐rafted material corresponding to the Heinrich events was deposited synchronously from 40° to 60°N. There are thus two oscillating systems: every 5–10 kyr massive iceberg armadas are released from large continental ice caps, whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1.2–3.8 kyr. At the time of the Heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems.
Glacial/interglacial variations in production and nitrogen fixation in the Cariaco Basin during the last 580 kyr
Authors: Haug Gerald H. , Pedersen Thomas F. , Sigman Daniel M. , Calvert Stephen E. , Nielsen Bente , Peterson Larry C.
Published: February 2004

The effect of sea level change on nutrient supply to the anoxic Cariaco demonstrates the fundamental importance of nitrogen (N2) fixation and phosphate to oceanic production. As N2 fixation produces biomass of low δ15N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it contributes to the light interglacial δ15N (∼2‰–3‰) values observed in the Ocean Drilling Program (ODP) site 1002 sediment record. During the glacials the sediments are bioturbated (oxic conditions) with low total organic carbon (TOC) contents and sedimentary δ15N values of ∼5‰, suggesting that nitrogen (N2) fixation contributed little to the N nutrition of Cariaco surface waters. The most plausible explanation for the inferred glacial/interglacial changes in N2 fixation in the Cariaco is that they have occurred in response to variations in the N/P ratio of the nutrient supply, driven by changes in denitrification.
Early Pliocene climate: A perspective from the western equatorial Atlantic Warm Pool
Authors: Billups Katharina , Ravelo A. C. , Zachos J. C.
Published: February 2004

High‐resolution (∼3–4 kyr) planktonic stable isotope stratigraphies from Site 925 drilled on Ceara Rise (Ocean Drilling Program Leg 154) are used to investigate the role of the western equatorial Atlantic sea surface hydrography in early Pliocene (3.2–4.7 Ma) climate change. Oxygen isotope results from Globigerinoides sacculifer, a mixed layer dweller, suggest that equatorial sea surface temperatures were cooler than today by ∼2°–3°C, consistent with relatively strong northward advection of heat away from the equator. Oxygen isotope results from Neogloboquadrina dutertrei suggest that over the long term this thermocline dweller tracks global ice volume fluctuations. Stable isotope gradients between the two planktonic species throughout the entire interval imply a stable warm pool in the western equatorial Atlantic. We observe a rapid (3.4 kyr) decrease in G. sacculifer and the N. dutertrei δ18O values at 4.36 Ma that may reflect a freshening of the sea surface, a direct response to a southward displacement of the Intertropical Convergence Zone, and perhaps, an indirect response to restricted flow through the Central American Seaway.
Ice‐volume forcing of winter monsoon climate in the South China Sea
Authors: Chen Min‐Te , Huang Chi‐Yue
Published: February 2004

High‐resolution studies of a planktonic foraminifer core record from the South China Sea (SCS) (31KL: 18°45.4′N, 115°52.4′E, water depth 3360 m) reveal changes driven by ice‐volume forcings in the climate of the East Asian monsoon in the western Pacific marginal sea during the late Quaternary. The analyses of planktonic foraminifer faunal abundance data from the core indicate significant variations in the relative abundances of the dominant taxa over the past 100,000 years since the isotope stage 5. The transfer function estimates of faunal sea surface temperatures (SST) correlate well with a long‐term (104–105 years) trend of global glaciation. About 65,000 years ago, there was an observable change in the mode of SST variability as many low‐latitude records have shown. These findings suggest that the SCS surface circulation and the East Asian winter monsoon systems are mainly driven by variations in global glaciation levels. The association of surface ocean cooling in the SCS with global climatic events suggests that fluctuations in the strength of the East Asian winter monsoon may be linked to shifts in the latitudinal position of the westerly winds and the Siberian high‐pressure system.