bluuebunny/crossref_metadata_2025 huggingface dataset:
Model comparisons of methane oxidation across a management gradient: Wetlands, rice production systems, and landfill
Authors: Bogner Jean E. , Sass Ronald L. , Walter Bernadette P.
Published: February 2004

Through model experiments we quantitatively examined fractional methane oxidation and net methane emissions using three model systems which encompass a management gradient from pristine (wetlands) to managed (rice production) to highly engineered (landfills). Fractional methane oxidation is rarely determined in situ; thus our goal was to cross‐compare modeling results and major drivers to field and laboratory data for this important parameter in both pristine and managed systems. In the models, management factors are typically introduced as combinations of theoretical relationships, empirical functions, or scaling factors which drive net emissions through forcing of water table variations, availability of degradable organic carbon substrates, input and cycling of major nutrients, variability in plant communities, physical properties for gaseous transport, and indigenous capacity of soils for methane oxidation. The net methane emission and fractional oxidation vary by orders of magnitude within and among the three model systems, yet each model was quite consistent in its predictive ability. This study lays the groundwork for a more unified, modular approach to modeling methane emissions from soil sources where both natural (climatic and ecological) and anthropogenic factors are important drivers.
Estimating global land use change over the past 300 years: The HYDE Database
Authors: Goldewijk Kees Klein
Published: February 2004

Testing against historical data is an important step for validating integrated models of global environmental change. Owing to long time lags in the climate system, these models should aim the simulation of the land use dynamics for long periods, i.e., spanning decades up to a century. Developing such models requires understanding of past and current trends and is therefore strongly data dependent. For this purpose, a history database of the global environment has been developed: HYDE. This paper describes and analyzes parts of HYDE version 2.0, presenting historical population and land use patterns for the past 300 years. Results suggest, among other things, a global increase of cropland area from 265 million ha in 1700 to 1471 million ha in 1990, while the area of pasture has increased more than six fold from 524 to 3451 million ha. In general, the increase of man‐made agricultural land took place at the expense of natural grasslands and to a lesser extent of forests. There are differences between the several regions in the temporal pace of these land use conversions. The temperate/developed regions of Canada, United States, USSR, and Oceania appear to have had their strongest increase during the 19th century, while most of the tropical/developing regions witnessed the largest land use conversions at the end of the last century. Results of this analysis can be used to test integrated models of global change and are available at http://www.rivm.nl/env/int/hyde/.
Riverine‐driven interhemispheric transport of carbon
Authors: Aumont Olivier , Orr James C. , Monfray Patrick , Ludwig Wolfgang , Amiotte‐Suchet Philippe , Probst Jean‐Luc
Published: February 2004

Controversy surrounds the role of the ocean in interhemispheric transport of carbon. On one hand, observations in the atmosphere and in the ocean both seem to imply that the preindustrial ocean transported up to 1 Pg C yr−1 from the Northern to the Southern Hemisphere. On the other hand, three dimensional (3‐D) ocean models suggest that global interhemispheric transport of carbon is near zero. However, in this debate, there has been a general neglect of the river carbon loop. The river carbon loop includes (1) uptake of atmospheric carbon due to inorganic and organic erosion on land, (2) transport of carbon by rivers, (3) subsequent transport of riverine carbon by the ocean, and (4) loss of riverine carbon back to the atmosphere by air‐sea gas exchange. Although carbon fluxes from rivers are small compared to natural fluxes, they have the potential to contribute substantially to the net air‐sea fluxes of CO2. For insight into this dilemma, we coupled carbon fluxes from a global model of continental erosion to a 3‐D global carbon‐cycle model of the ocean. With rivers, total southward interhemispheric transport by the ocean increases from 0.1 to 0.35±0.08 Pg C yr−1, in agreement with oceanographic observations. Resulting air‐sea fluxes of riverine carbon and uptake of CO2 by land erosion were installed as boundary conditions in a 3‐D atmospheric model. The assymetry in these fluxes drives a preindustrial atmospheric gradient of CO2 at the surface of −0.6±0.1 μatm for the North Pole minus the South Pole and longitudinal variations that exceed 0.5 μatm. Conversely, the gradient for Mauna Loa minus South Pole is only −0.2±0.1 μatm, much less than the −0.8 μatm gradient extrapolated linearly from historical atmospheric CO2 measurements from the same two sites. The difference may be explained by the role of the terrestrial biosphere. Regardless, the river loop produces large gradients both meridionally and zonally. Accounting for the river carbon loop changes current estimates of the regional distribution of sources and sinks of CO2, particularly concerning partitioning between natural and anthropogenic processes.
Modeling distinct vertical biogeochemical structure of the Black Sea: Dynamical coupling of the oxic, suboxic, and anoxic layers
Authors: Oguz Temel , Ducklow Hugh W. , Malanotte‐Rizzoli Paola
Published: February 2004

A one‐dimensional, vertically resolved, physical‐biogeochemical model is used to provide a unified representation of the dynamically coupled oxic‐suboxic‐anoxic system for the interior Black Sea. The model relates the annual cycle of plankton production in the form of a series of successive phytoplankton, mesozooplankton, and higher consumer blooms to organic matter generation and to the remineralization‐ammonification‐nitrification‐denitrification chain of the nitrogen cycle as well as to anaerobic sulfide oxidation in the suboxic‐anoxic interface zone. The simulations indicate that oxygen consumption during remineralization and nitrification, together with a lack of ventilation of subsurface waters due to the presence of strong stratification, are the two main factors limiting aerobic biogeochemical activity to the upper ∼75 m of the water column, which approximately corresponds to the level of nitrate maximum. The position of the upper boundary and thus the thickness of the suboxic layer are controlled by upper layer biological processes. The quasi‐permanent character of this layer and the stability of the suboxic‐anoxic interface within the last several decades are maintained by a constant rate of nitrate supply from the nitrate maximum zone. Nitrate is consumed to oxidize sinking particulate organic matter as well as hydrogen sulfide and ammonium transported upward from deeper levels.
Potential impact of climate change on marine export production
Authors: Bopp Laurent , Monfray Patrick , Aumont Olivier , Dufresne Jean‐Louis , Le Treut Hervé , Madec Gurvan , Terray Laurent , Orr James C.
Published: February 2004

Future climate change will affect marine productivity, as well as other many components of Earth system. We have investigated the response of marine productivity to global warming with two different ocean biogeochemical schemes and two different atmosphere‐ocean coupled general circulation models (GCM). Both coupled GCMs were used without flux correction to simulate climate response to increased greenhouse gases (+1% CO2/yr for 80 years). At 2×CO2, increased stratification leads to both reduced nutrient supply and increased light efficiency. Both effects drive a reduction in marine export production (−6%), although regionally changes can be both negative and positive (from −15% zonal average in the tropics to +10% in the Southern Ocean). Both coupled models and both biogeochemical schemes simulate a poleward shift of marine production due mainly to a longer growing season at high latitudes. At low latitudes, the effect of reduced upwelling prevails. The resulting reduction in marine productivity, and other marine resources, could become detectable in the near future, if appropriate long‐term observing systems are implemented.
Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing?
Authors: Hanan Barry B. , Blichert‐Toft Janne , Kingsley Richard , Schilling J.‐G.
Published: February 2004

The geochemical variations in basalts from Iceland and the Reykjanes Ridge have for a long time been attributed to mixing between the depleted mid‐ocean ridge basalt (MORB) asthenosphere source and the enriched Iceland mantle plume (Schilling, 1973; Hart et al., 1973; Sun et al., 1975). In contrast, the occurrence of some Iceland basalts with relatively depleted incompatible trace element ratios and low Pb and Sr isotope ratios compared to normal MORB (NMORB) have been interpreted to indicate that the Iceland mantle plume transports both geochemically enriched and depleted material from the lower mantle into the upper mantle and that the depleted MORB asthenosphere source does not contribute significantly to the generation of the Iceland basalts (Thirlwall et al., 1994; Thirlwall, 1995; Hards et al., 1995; Kerr, 1995; Kerr et al., 1995; Fitton et al., 1997; Hardarson and Fitton, 1997; Hardarson et al., 1997; Kempton et al., 1998, 1999; Nowell et al., 1998). As a follow‐up to this controversy, we show on the basis of new Nb, Zr, Y, La, and Sm elemental abundances and Hf isotope ratios reported here, and published trace element and isotope data for Iceland and the North Atlantic MOR (50°–79°N), that the apparent distinction between Iceland basalts and MORB can readily be accounted for by a three‐component mixing model involving two incompatible trace element enriched components, one with relatively high, the other with low 206Pb/204Pb ratios, and the usual surrounding depleted MORB mantle source as the third component. The radiogenic 206Pb‐rich component represents the hot Iceland mantle plume source, while the enriched but low‐206Pb/204Pb EM1‐like component most likely represents entrained subcontinental lithospheric material embedded in the North Atlantic depleted MORB source. The incompatible element and isotope ratio variations do not require a depleted Iceland plume (DIP) component, nor do they exclude the presence of a MORB depleted asthenosphere source component for some of the Iceland basalts, particularly in recent time.
The 40Ar/39Ar age dating of the Madeira Archipelago and hotspot track (eastern North Atlantic)
Authors: Geldmacher Jörg , van den Bogaard Paul , Hoernle Kaj , Schmincke Hans‐Ulrich
Published: February 2004

The 40Ar/39Ar ages for 35 volcanic rocks and 14C ages for two charcoal samples from the Madeira Archipelago and Ampère Seamount (eastern North Atlantic) are presented. The volcanic evolution of Madeira can be divided into a voluminous shield stage (>4.6–0.7 Ma) and a subsequent low‐volume posterosional stage (<0.7–0 Ma). Volcanism during the shield stage originated from a two‐armed rift system, composed of the E–W oriented Madeira rift arm and the N–S oriented Desertas rift arm. Average growth rates for the submarine (5500 km3/Ma) and subaerial (100–150 km3/Ma) shield stages on Madeira are among the lowest found for ocean island volcanoes. It is proposed that Madeira represents the present location of a >70 Myr old hotspot which formed Porto Santo Island (11.1–14.3 Ma), Seine, Ampère (31 Ma), Corral Patch and Ormond (65–67 Ma [Féraud et al., 1982, 1986]) Seamounts, and the Serra de Monchique (70–72 Ma [McIntyre and Berger, 1982]) complex in southern Portugal. Age and spatial relationships result in a calculated absolute African plate motion above the hotspot of 1.2 cm/yr around a rotation pole located at 43°36′N/ 24°33′W.
Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume
Authors: Gurnis Michael , Mitrovica Jerry X. , Ritsema Jeroen , van Heijst Hendrik‐Jan
Published: February 2004

We explore the hypothesis that southern Africa is actively being uplifted by a large‐scale, positively buoyant structure within the mid‐lower mantle. Using a new formulation in which dynamic topography and uplift rate are jointly used, we place constraints on mantle density and viscosity. The solution of the momentum equation is coupled with the advection of the density field to solve for the surface uplift rate in both an axisymmetric and fully spherical geometry. We demonstrate how dynamic topography and its rate of change depend on density and lateral and radial variations in viscosity. In the full spherical models the geometry of mantle density is derived by scaling a tomographic shear velocity model. Using a variety of geologic observations, we estimate residual topography (i.e., the topography remaining after shallow sources of density are removed) and an average Cenozoic uplift rate to be 300–600 m and 5–30 m/Myr, respectively, for southern Africa. We are able to satisfy these constraints with a mantle model in which the mid‐lower mantle beneath southern Africa is 0.2% less dense and has a viscosity of ∼ 1022 Pa s. In addition, if the continental lithosphere is thick beneath southern Africa, as suspected from seismic inversions, and has a high effective viscosity, then we find that southern Africa can be further elevated owing to increased coupling between the deep mantle and surface. We show that recent estimates of mantle density, suggesting that the lowest parts of the African anomaly may be anomalously dense are compatible with geologic constraints. We conclude that uplift rate, when combined with estimates of present‐day dynamic topography, provides a powerful tool to constrain the properties of the deep mantle.
Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet‐visible images
Authors: Lucey Paul G. , Blewett David T. , Jolliff Bradley L.
Published: February 2004

The Clementine mission to the Moon returned global imaging data collected by the ultraviolet visible (UVVIS) camera. This data set is now in a final state of calibration, and a five‐band multispectral digital image model (DIM) of the lunar surface will soon be available to the science community. We have used observations of the lunar sample‐return sites and stations extracted from the final DIM in conjunction with compositional information for returned lunar soils to revise our previously published algorithms for the spectral determination of the FeO and TiO2 content of the lunar surface. The algorithms successfully normalize the effects of space weathering so that composition may be determined without regard to a surface's state of maturity. These algorithms permit anyone with access to the standard archived DIM to construct high spatial resolution maps of FeO and TiO2 abundance. Such maps will be of great utility in a variety of lunar geologic studies.
The materials of the lunar Procellarum KREEP Terrane: A synthesis of data from geomorphological mapping, remote sensing, and sample analyses
Authors: Haskin Larry A. , Gillis Jeffrey J. , Korotev Randy L. , Jolliff Bradley L.
Published: February 2004

Major features of the Moon's Procellarum KREEP Terrane include subdued relief and extensive resurfacing with mare basalt, consistent with high concentrations of Th and other heat‐producing elements at depth. We relate the chemistry of sampled materials to the geomorphology, Th surface concentrations determined by the Lunar Prospector (2° pixels), and FeO and TiO2 concentrations derived from Clementine ultraviolet‐visible spectral data. On the basis of geologic maps, each pixel was classified as mare, terra, or mixed. Near the periphery of the terrane, terra pixel compositions are relatively feldspathic; in the interior they mainly represent Imbrium basin rim or ejecta deposits and are mainly incompatible trace element rich norites and presumably represent materials from a thick section (tens of kilometers) of the pre‐Imbrium crust of the terrane excavated by the Imbrium event. (Although Imbrium ejecta are the principal source of surface terra materials, the Imbrium event did not create the Th‐rich Procellarum KREEP Terrane.) Broad, continuous expanses of mare pixels are observed, with little interruption from protruding terra or terra‐penetrating craters. The mare‐basalt‐dominated regoliths of these areas have a wide range of TiO2 concentrations (<1–15%) and higher Th concentrations (2 to 6+ ppm) than most sampled mare basalts. Traverse profiles show high Th over broad regions of highest FeO (>18%), leading to the conclusion that the high Th concentrations are in the mare basalts and are not present in the regoliths as terra‐derived materials. Volcanic glasses and impact glasses of mare basalt composition collected from the Procellarum KREEP Terrane support this conclusion.
Mars Color Imager (MARCI) on the Mars Climate Orbiter
Authors: Malin M. C. , Bell J. F. , Calvin W. , Clancy R. T. , Haberle R. M. , James P. B. , Lee S. W. , Thomas P. C. , Caplinger M. A.
Published: February 2004

The Mars Color Imager, or MARCI, experiment on the Mars Climate Orbiter (MCO) consists of two cameras with unique optics and identical focal plane assemblies (FPAs), Data Acquisition System (DAS) electronics, and power supplies. Each camera is characterized by small physical size and mass (∼6×6×12 cm, including baffle; <500 g), low power requirements (<2.5 W, including power supply losses), and high science performance (1000×1000 pixel, low noise). The Wide Angle (WA) camera will have the capability to map Mars in five visible and two ultraviolet spectral bands at a resolution of better than 8 km/pixel under the worst case downlink data rate. Under better downlink conditions the WA will provide kilometer‐scale global maps of atmospheric phenomena such as clouds, hazes, dust storms, and the polar hood. Limb observations will provide additional detail on atmospheric structure at 1/3 scale‐height resolution. The Medium Angle (MA) camera is designed to study selected areas of Mars at regional scale. From 400 km altitude its 6° FOV, which covers ∼40 km at 40 m/pixel, will permit all locations on the planet except the poles to be accessible for image acquisitions every two mapping cycles (roughly 52 sols). Eight spectral channels between 425 and 1000 nm provide the ability to discriminate both atmospheric and surface features on the basis of composition. The primary science objectives of MARCI are to (1) observe Martian atmospheric processes at synoptic scales and mesoscales, (2) study details of the interaction of the atmosphere with the surface at a variety of scales in both space and time, and (3) examine surface features characteristic of the evolution of the Martian climate over time. MARCI will directly address two of the three high‐level goals of the Mars Surveyor Program: Climate and Resources. Life, the third goal, will be addressed indirectly through the environmental factors associated with the other two goals.
Geological analysis of the Silver Lake Marsokhod field test from ground‐truth sampling and mapping
Authors: Grin Edmond A. , Reagan Mark K. , Cabrol Nathalie A. , Bettis E. Arthur , Foster C. Thomas , Stoker Carol R. , Moersch Jeffrey E.
Published: February 2004

We compare the results of the observations and conclusions obtained from a remote science team and field geologists during the 1999 Marsokhod Silver Lake field experiment. This comparison is used to determine methods that enhance the ability of a remote team using a rover to determine the geology and paleoclimate of a landing site analogous to sites that may be encountered during the Mars Surveyor Program. The mobility of the field geologists and their ability to rapidly gain visual and physical data from the field site give an immediate confidence to their interpretation of the geology. The finite time of teleoperation and the initial decisions and mechanical problems that caused the rover to linger and collect data in a small area of the test site for the first 5 days of the test limited the ability of the remote science team to gain a similar confidence. Nevertheless, by appropriate use of “orbital” and “descent” images as well as the rover's science payload, the remote science team interpreted the present and former climates, the geology and geological history, and present‐day biology relatively accurately. The few erroneous interpretations by the remote team resulted in part from biases brought to the test about the nature of the geology that was likely to be encountered. Data quality and variety also impacted the quality of the geological interpretations. Significant misinterpretations in the remotely determined geology resulted from the lack of detail in the optical imagery, a less than optimal understanding of the site's topography, and misinterpretation of some infrared data. All in all, this field test showed that the geology, paleoclimate, and biology of a site can be assessed to a reasonable level of confidence using a remotely controlled vehicle. It also showed that it is both feasible and desirable for a distributed science team to participate in long science missions from home institutions, as long as data flows efficiently to and from these institutions.
Acoustic environment of the Martian surface
Authors: Williams Jean‐Pierre
Published: February 2004

Prompted by the Mars Microphone aboard the 1998 Mars Polar Lander, a theoretical study of the acoustical environment of the Martian surface has been made to ascertain how the propagation of sound is attenuated under such conditions and to predict what sounds may be detectable by a microphone. Viscous and thermal relaxation (termed classical absorption), molecular relaxation, and geometric attenuation are considered. Classical absorption is stronger under Martian conditions resulting in sounds in the audible frequencies (20 Hz to 20 kHz) being more strongly attenuated than in the terrestrial environment. The higher frequencies (>3000 Hz) will be severely attenuated as the absorption is frequency dependent. At very low infrasound frequencies (i.e., <10 Hz), attenuation will be mostly due to geometric spreading of the propagating wave front and will therefore be more similar to the terrestrial surface environment. Probable sound sources in the landed environment include wind‐blown dust and sand from large dust storms, dust devils, and possible associated electrostatic discharge. The sounds most likely to be detected will be sounds generated by the lander itself and aeroacoustic noises generated by winds blowing against the lander and its instruments.
Lunar rare earth element distribution and ramifications for FeO and TiO2: Lunar Prospector neutron spectrometer observations
Authors: Elphic R. C. , Lawrence D. J. , Feldman W. C. , Barraclough B. L. , Maurice S. , Binder A. B. , Lucey P. G.
Published: February 2004

Lunar Prospector neutron spectrometer data have been used to map the global surface distribution of the incompatible rare earth elements gadolinium and samarium from the low‐altitude (30±15 km) mapping orbit. These results afford improved surface resolution and detailed views of the potassium, rare earth elements, and phosphorus (KREEP) distribution within and around Mare Imbrium and elsewhere. The Gd and Sm results serve as a complementary and independent check of the distribution of KREEP on the Moon, in contrast with the Lunar Prospector gamma ray spectrometer results for thorium. The neutron spectrometer observations reflect the presence of Fe and Ti as well as Gd and Sm. The contributions of Fe and Ti are removed using high spatial resolution Clementine spectral reflectance determinations of FeO and TiO2 abundances. Overall, the resulting Gd and Sm abundance map agrees with the Th abundance map determined using the Lunar Prospector gamma ray spectrometer. In general, the detailed features of the Procellarum/Imbrium KREEP terrane are found in both. For example, distinct highs in Gd, Sm, and Th abundances are resolved over the craters Mairan, Aristarchus, Kepler, Reinhold, Lalande, and Aristillus, over the Apennine Bench and Fra Mauro regions, and over the Montes Jura and Montes Carpatus, indicating an enhanced abundance of KREEP in these locations. The neutron observations also provide constraints on FeO and TiO2 abundances; for some high‐Ti locales, there is a significant disagreement with TiO2 abundances inferred from Clementine spectral reflectance.
Issues involved in a Martian sample return: Integrity preservation and the Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) position
Authors: Neal Clive R.
Published: February 2004

Returning geological samples from Mars is the next logical step as part of an integrated exploration program and is the critical subsequent stage in the search for evidence of life on the red planet. Analysis of such samples on Earth will generally produce data of superior accuracy and precision than can be accomplished in situ on the Martian surface, and such data will be invaluable in providing ground truth for remotely sensed observations. However, obtaining meaningful data requires that the samples returned to Earth suffer little (or preferably no) compositional or morphological changes during collection on Mars, transit to Earth, entry into Earth's atmosphere, impact on the surface, and long‐term curation. This paper presents recommendations from NASA's committee for Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) for preserving the integrity of Martian samples during a return mission. Potential contaminants include dead bacteria on the outbound spacecraft and materials that come into contact with the samples during collection and storage. Therefore it is recommended that the spacecraft be sterilized and cleaned at least to Pathfinder standards, with the components that interact with the samples having a higher degree of cleanliness. Furthermore, it is recommended that only pure, homogeneous materials should be used for components that come into contact with the samples. The adverse effects of temperature on the samples are explored, and it is demonstrated that heat sterilization or undue temperature increases in transit or reentry have the potential to destroy important mineralogical and geochemical information. Concomitant with this will be degassing of the samples and stable isotopic fractionation. Therefore it is recommended that samples be kept at 240 K or below during transit, reentry, and curation. It is further recommended that heat sterilization not be used, except in extreme circumstances, and other sterilization techniques, such as radiation, be explored. While the purpose of this paper is not to design a sample return mission, the recommendations presented here are intended to act as guidelines for returning Martian samples in their pristine state. In essence, any procedure or piece of equipment that has the potential to compromise the pristinity of Martian samples during the collection, transportation, or curation needs to be fully evaluated by a scientific committee with the requisite expertise prior to implementation. CAPTEM is willing to serve in such a capacity if requested by NASA.
Geological history of the Tyre region of Europa: A regional perspective on Europan surface features and ice thickness
Authors: Kadel Steven D. , Chuang Frank C. , Greeley Ronald , Moore Jeffrey M.
Published: February 2004

Galileo images of the Tyre Macula region of Europa at regional (170 m/pixel) and local (∼40 m/pixel) scales allow mapping and understanding of surface processes and landforms. Ridged plains, doublet and complex ridges, shallow pits, domes, “chaos” areas, impact structures, tilted blocks and massifs, and young fracture systems indicate a complex history of surface deformation on Europa. Regional and local morphologies of the Tyre region of Europa suggest that an impactor penetrated through several kilometers of water ice to a mobile layer below. The surface morphology was initially dominated by formation of ridged plains, followed by development of ridge bands and doublet ridges, with chaos and fracture formation dominating the latter part of the geologic history of the Tyre region. Two distinct types of chaos have been identified which, along with upwarped dome materials, appear to represent a continuum of features (domes‐platy chaos‐knobby chaos) resulting from increasing degrees of surface disruption associated with local lithospheric heating and thinning. Local and regional stratigraphic relationships, block heights, and the morphology of the Tyre impact structure suggest the presence of low‐viscosity ice or liquid water beneath a thin (several kilometers) surface ice shell at the time of the impact. The very low impact crater density on the surface of Europa suggests that this thin shell has either formed or been thoroughly resurfaced in the very recent past.
Observations of Martian ice clouds by the Mars Global Surveyor Thermal Emission Spectrometer: The first Martian year
Authors: Pearl John C. , Smith Michael D. , Conrath Barney J. , Bandfield Joshua L. , Christensen Philip R.
Published: February 2004

Successful operation of the Mars Global Surveyor spacecraft, beginning in September 1997 (Ls = 184°), has permitted extensive observations over more than a Martian year. Initially, thin (normal optical depth <0.06 at 825 cm−1) ice clouds and hazes were widespread, showing a distinct latitudinal gradient. With the onset of a regional dust storm at Ls = 224°, ice clouds vanished in the southern hemisphere, to reappear gradually after the decay of the storm. The zonally averaged cloud opacities show little difference between the beginning and end of the first Martian year. A broad low‐latitude cloud belt with considerable longitudinal structure was present in early northern summer. Apparently characteristic of the northern summer season, it vanished between Ls = 140° and 150°. The latitudinal extent of this feature is apparently controlled by the ascending branch of the Hadley circulation. The most opaque clouds (optical depth ∼0.6) were found above the summits of major volcanic features; these showed spatial structure possibly associated with wave activity. Variety among low‐lying late morning clouds suggests localized differences in circulation and microclimates. Limb observations showed extensive optically thin (optical depth <0.04) stratiform clouds at altitudes up to 55 km. Considerable latitude and altitude variations were evident in ice clouds in early northern spring (Ls = 25°); near 30 km, thin clouds extended from just north of the equator to ∼45°N, nearly to the north polar vortex. A water ice haze was present in the north polar night (Ls = 30°) at altitudes up to 40 km. Because little dust was present this probably provided heterogeneous nucleation sites for the formation of CO2 clouds and snowfall at altitudes below ∼20 km, where atmospheric temperatures dropped to the CO2 condensation point. The relatively invariant spectral shape of the water ice cloud feature over space and time indicates that ice particle radii are generally between 1 and 4 μm.
Potential links between surging ice sheets, circulation changes, and the Dansgaard‐Oeschger Cycles in the Irminger Sea, 60–18 Kyr
Authors: van Kreveld S. , Sarnthein M. , Erlenkeuser H. , Grootes P. , Jung S. , Nadeau M. J. , Pflaumann U. , Voelker A.
Published: February 2004

Surface and deepwater paleoclimate records in Irminger Sea core SO82‐5 (59°N, 31°W) and Icelandic Sea core PS2644 (68°N, 22°W) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard‐Oeschger (D‐O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record [Grootes and Stuiver, 1997]. During interstadials, summer sea surface temperatures (SSTsu) in the Irminger Sea averaged to 8°C, and sea surface salinities (SSS) averaged to ∼36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SSTsu dropped to 2°–4°C, in phase with SSS drops by ∼1–2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155–195 years. We also conclude that the abrupt stadial terminations, the D‐O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater‐covered northwestern North Atlantic. In the 1/1460‐year frequency band, benthic δ18O brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D‐O cycles.
Correlation of Late Miocene to Early Pliocene sequences between the Mediterranean and North Atlantic
Authors: Hodell David A. , Curtis Jason H. , Sierro Francisco J. , Raymo Maureen E.
Published: February 2004

Ocean Drilling Program (ODP) Site 982 in the North Atlantic contains a complete latest Miocene to early Pliocene section that was tuned to the astronomical timescale by correlating the record of gamma ray attenuation (GRA) bulk density to summer insolation at 65°N and the benthic δ18O signal to orbital obliquity for the interval from 4.6 to 7.5 Ma. The astronomical tuning of the Site 982 record permits a direct bed‐to‐bed correlation to the cyclostratigraphy of Messinian sections in the Mediterranean [Krijgsman et al., 1999a, 2001]. The benthic δ18O signal at Site 982 records a latest Miocene glacial period that lasted from ∼6.26 to 5.50 Ma and consisted of 18 glacial‐to‐interglacial oscillations that were controlled by the 41‐kyr cycle of obliquity. Although the intensification of glaciation at 6.26 Ma may have contributed to the restriction of the Mediterranean, it preceded the depositional onset of the lower evaporite unit at 5.96 Ma by some 300 kyr. The transition from Stage TG12 to TG11 at 5.5 Ma marks the end of the latest Miocene glacial period and precedes the Miocene/Pliocene boundary by 170 kyr. Although benthic δ18O values are relatively low and δ18O of bulk carbonate reaches a minimum at the Miocene/Pliocene boundary at 5.33 Ma, there is no single “event” that would indicate deglaciation and sea level rise as the cause of the reflooding of the Mediterranean. We conclude that glacioeustatic changes alone were not responsible for either the start or end of evaporite deposition during the Messinian, suggesting that tectonic or local climate changes in the Mediterranean region were the dominant cause(s).
The last 18 kyr fluctuations in Norwegian sea surface conditions and implications for the magnitude of climatic change: Evidence from the North Sea
Authors: Klitgaard‐Kristensen D. , Sejrup H. P. , Haflidason H.
Published: February 2004

A combined record of three cores spanning the last 18 kyr from the northern North Sea is investigated for content of benthic and planktonic foraminifera and stable oxygen isotopes. The paleoenvironmental development through this time period shows an early deglaciation (18–14.4 ka) and the Younger Dryas (12.7–11.5 ka) characterized by arctic/polar conditions and increased ice rafting in the Norwegian Channel. During the Bølling‐Allerød period, warm sea surface temperature (9°C) conditions similar to present conditions are inferred, while bottom waters stayed cold (0–1°C) with normal salinity. The Bølling‐Allerød period is interrupted twice at 13.9–13.6 ka (Older Dryas) and at 13.0–12.8 ka (Inter‐Allerød Cooling Period) by reductions in sea surface temperatures and increased sea ice cover. The beginning of the Holocene period is marked by increases in surface and bottom water temperature. Superimposed on the broad climatic changes through the Holocene, a series of short‐lived oscillations in the ocean circulation are recorded. The amplitude of these Holocene events appears larger in the early Holocene (prior to 8 ka) than compared with the remaining part of the Holocene. This amplification can possibly be attributed to a general increased freshwater budget in the North Atlantic at this time during the final stages of the deglaciation of the Laurentide and Scandinavian ice sheets.
Verification of a two‐dimensional/three‐dimensional hybrid ray‐tracing method for spatiotemporal channel modeling
Authors: Zhu Houtao , Takada Jun‐ichi , Araki Kiyomichi , Kobayashi Takehiko
Published: February 2004

Spatiotemporal channel modeling is fundamental to the design and implementation of future high‐speed wireless communication systems. This paper investigates a two‐dimensional‐three‐dimensional (2‐D/3‐D) hybrid ray‐tracing method for outdoor microcellular and picocellular urban environments. It also explores the spatiotemporal channel model derived from this ray‐tracing method. This 2‐D/3‐D ray‐tracing method launches rays in a horizontal plane and establishes ray paths in three‐dimensional (3‐D) space. Partition vector incorporates other acceleration techniques to produce a method that reduces tracing time. The conventional verification of path loss and delay profiles predicted by ray tracing was extended to include the verification of angle of arrival (AOA). The results prove the necessity of extension and verify that path loss and delay profiles cannot fully confirm the accuracy of ray tracing in a spatial domain. A good level of accuracy was observed in the comparison of three key areas: (1) path loss, (2) delay profile, and (3) AOA. Degradation of prediction accuracy caused by incorrectly predicted phases in ray tracing was investigated, and improvements were made by incorporating a “random‐phase” approach. It was found that measured path loss and delay profiles are almost fully confined within the 90% confidence interval, proving that the approach can account for the degradation effects caused by inaccurate antenna positions. The applicability of limited‐band and limited beam width channel parameters transformed from ray‐tracing results is also examined in the sense of an azimuth delay profile. The confirmation of the accuracy of the profile with measurement prepares ground for the use of ray‐tracing approaches to analyze system performance in real environments.
A discussion on the variations of MST/ST radar echo power with an atmospheric layer resolved by frequency domain interferometry technique
Authors: Chen J. S. , Chu Y. H.
Published: February 2004

The relationship between an atmospheric layer and its VHF backscatter resolved by the frequency domain interferometry (FDI) technique is investigated in this article. A theoretical expression connecting the radar echo power with the thickness and position of an atmospheric layer and the range weighting function of a matched filter is derived. It shows that the farther the layer locates from the central height of the radar volume, the weaker the echo power will be. This feature is attributed to the range weighting effect of the radar system. FDI observations also show that the echo power from the atmospheric layer located close to the central height of the radar volume is usually greater than that from the layer located at the edge of the volume, which is in good agreement with the theoretical prediction. Moreover, the correlation between the echo power and layer position is negative (positive) if the layer locates in the upper (lower) part of the radar volume. This behavior is consistent with the theoretical prediction and can also be interpreted by the range weighting effect. It is also suggested that the power‐position relation is capable of indicating the system bias causing the systematic error of FDI‐estimated layer position. In addition, numerical simulation is performed in this article to examine the difference between the range weighting functions resulting from rectangular and nonrectangular radar pulses for a given receiver impulse response.
Potential applications of 1–5 GHz radar backscatter measurements of seasonal land snow cover
Authors: West Richard D.
Published: February 2004

Lower frequency microwave radar observations can help to measure the properties of snow cover on land by providing information about the soil‐snow boundary condition. In this theoretical study, we examine the sensitivities of microwave radar measurements to soil and snow characteristics, and we compare a simple model with previously published data. Depending on the surface roughness, copolarized ratios or single polarization time ratios of radar backscattering may be affected only by the incidence angle and the dielectric contrast at the soil‐snow boundary. These measurements can reduce the number of unknowns in any corresponding higher‐frequency observations that are also affected by the lower boundary condition. The copolarized ratio is sensitive, first of all, to the snow density, which offers the possibility of measuring this parameter directly if a separate measurement of the soil temperature from a passive microwave system is also available. The thermal insulation provided by snow cover can have a powerful effect on the soil‐snow boundary by altering the soil temperature and therefore changing the dielectric contrast. Because the microwave response of snow cover is so sensitive to the conditions of the underlying frozen soil, it is important for future ground truth campaigns that measure snow conditions to also collect data on the temperature, water content, and texture of the soil.
Representation of a wave field in a randomly inhomogeneous medium in the form of the double‐weighted Fourier transform
Authors: Kravtsov Yuri A. , Tinin Mikhail V.
Published: February 2004

New integral representation of a wave field dn a continuously inhomogeneous random medium is suggested in the form of double‐weighted Fourier transformation, performed simultaneously with respect to coordinates of the source and the observer. The integral representation under consideration takes into account both the diffraction effects and the multiray effects. It incorporates many results of known techniques of wave propagation description in continuously inhomogeneous media: the methods of geometrical optics, smooth perturbations, phase screen, and two‐scale expansions. The method delivers new opportunities to retrieve small‐scale inhomogeneous structure of ionosphere plasma from radio‐sounding data and can serve as the basis for diffraction tomography of the ionosphere.
Estimating evaporation duct heights from radar sea echo
Authors: Rogers L. Ted , Hattan Claude P. , Stapleton Janet K.
Published: February 2004

The evaporation duct is a downward refracting layer that results from the rapid decrease in humidity with respect to altitude occurring in the atmospheric surface layer above bodies of water. The evaporation duct affects radar detection ranges at frequencies of approximately 1 GHz and above. Models based on Monin‐Obukhov similarity theory are usually used to calculate evaporation duct refractivity profiles from bulk measurements of air temperature, humidity, wind speed, and the sea surface temperature. Modeling results by Pappert et al. [1992] indicated that the falloff of radar sea echo as a function of range was an increasing function of the evaporation duct height. On the basis of those results, the authors proposed inferring the evaporation duct height by a slope fit to modeled clutter power, a nonlinear least squares inversion procedure. Data for testing the inversion procedure were obtained using the S band Space Range Radar at Wallops Island, Virginia. Evaporation duct heights were inferred from the radar data on the basis of the assumption of a range‐independent evaporation duct height and sea clutter radar cross section (σ°). Validation data consist of buoy and boat in situ bulk measurements. The result of comparing the radar‐inferred evaporation duct heights and those calculated from bulk measurements indicates that the radar‐inferred duct heights are strongly correlated with those from the in situ measurements, but there is some uncertainty as to whether they are biased or unbiased. That uncertainty arises from the assumed dependence of σ° on the grazing angle ψ. That dependence is currently a matter of debate in the open literature, with the lower and upper ends of modeling results being σ° ∝ ψ0; and σ° ∝ ψ4, respectively. We show results for both dependencies and note that the σ° ∝ ψ0; provides the best agreement with our measurements. It should be noted that inferring the evaporation duct height from radar sea echo is a problem that stresses the modeling of low‐grazing‐angle backscatter.
Generation of instantaneous maps of ionospheric characteristics
Authors: Stanisławska I. , Juchnikowski G. , Zbyszyński Z.
Published: February 2004

A way of producing limited‐area instantaneous maps of ionospheric characteristics is shown. An interpolation technique is applied for construction of the mapping model. The model combines monthly median maps of ionospheric characteristics and a set of measurements for a single moment of time that are exactly replicated during the mapping procedure. The accuracy of the mapping results is discussed, and samples of maps for different geophysical conditions for ƒoF2, ƒoF1,ƒoE and M(3000)F2 are presented.
Tracking tropospheric radio occultation signals from low Earth orbit
Authors: Sokolovskiy Sergey V.
Published: February 2004

Propagation of radio occultation signals through the tropical lower troposphere with severe refractivity gradients results in significant spreading of the signal spectrum. Under such conditions a signal acquisition technique which tracks large random troposphere‐induced phase accelerations more reliably than a generic phase‐locked loop has to be applied. This paper discusses the results of simulations of open loop tracking of radio occultation signals that were generated with data from high‐resolution tropical radiosondes. The signal has to be down‐converted in real time in the receiver on orbit to a low mean residual frequency by use of a phase (Doppler) model based on predicted orbits and refractivity climatology. The down‐converted complex signal is then low‐pass filtered and sampled. The phase in excess of the phase model must be reconstructed from the sampled and down‐linked signal in postprocessing. This may require an additional down‐conversion to eliminate (minimize) aliasing of harmonics in the spectrum. Then the accumulated phase can be reconstructed by resampling the signal at a higher rate to resolve the cycle ambiguities. A fast algorithm for prediction of the Doppler based on the refractivity climatology and an algorithm for the detection of Doppler mismodeling based on sliding window spectral analysis of the down‐converted signal are developed and tested. The accuracy of the Doppler modeling, ±(15‐20) Hz, the required filter bandwidth, 100 Hz, and the sampling rate, 50‐100 Hz, are estimated.