bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomorphic evolution of soil texture and organic matter in eroding landscapes
Authors: Rosenbloom Nan A. , Doney Scott C. , Schimel David S.
Published: February 2004

Geomorphic erosion on hillslopes creates a distribution of soil properties within the landscape that influences ecosystem, erosional, and hydrological processes. These soil properties typically reflect topography and define a template for plant productivity and consequent soil carbon accumulation. Erosion also redistributes soil carbon and, by burying or excavating carbon, changes turnover time and may figure prominently in the global carbon budget [Stallard, 1998]. In this paper, we present the Changing Relief and Evolving Ecosystems Project (CREEP), a theoretical, process‐response model that focuses on the redistribution of soil texture and soil carbon along a hillslope in response to geomorphic transport processes. The CREEP model simulates long‐term ecological and geomorphic landscape evolution by simulating general soil, vegetation, and hillslope transport relationships. In particular, the model allows for the removal and downslope transport of soil carbon, as well as for the burial and decomposition of carbon in the accumulation zone. CREEP model results suggest that sandy soils are more likely to differentiate downslope with respect to soil texture than are more clay‐rich soils and that this redistribution will lead to disproportionately broad areas of predominantly coarse‐grained particles on upper slopes. Gridded biogeochemical models, which may otherwise overlook landscape heterogeneity, may use CREEP estimates of the areal distribution of soil texture as a basis for parametrically capturing trace gas fluxes nonlinearly related to soil texture.
Methane emissions, groundwater levels and redox potentials of common wetland soils in a temperate‐humid climate
Authors: Fiedler Sabine , Sommer Michael
Published: February 2004

Typical groundwater soils in a temperate‐humid climate were investigated for the relationship between methane emissions and dynamic (groundwater level, redox status) as well as static soil properties over 2 years (July 1996 to July 1998, weekly probing, static chamber). High unit area emissions, like annual (5‐73 g CH4 m−2 yr−1) or mean daily fluxes (15–244 mg CH4 m−2 d−1) were observed at the wetter sites. Oxidation seemed to be the controlling subscale process for the emissions observed: Time with Eh above −75mV in the upper 20 cm was inversely correlated to net CH4 fluxes (nonlinear). Two factors, namely groundwater level and the quality of soil organic matter (SOM), built a factor hierarchy to control methane emission. Thereby SOM is strongly influenced by the land use practices, for example, erosional and depositional processes in a catchment. Soil morphology corresponded to static (e.g., Mnd, Fed) and dynamic soil properties like groundwater level or redox potentials. Because the linkage to Eh regional or global scale methane emissions may be estimated by coupling unit area emissions to soil maps based on soil morphology (e.g., soil taxonomy). Calculations of our own results exemplified (1) the significance of a representative set of unit areas for each climatic zone and (2) the potential importance of the temperate climatic zone for global methane budget.
Role of a Cl‐bearing flux in the origin of depleted ocean floor magmas
Authors: Nielsen Roger L. , Sours‐Page Rachel E. , Harpp Karen S.
Published: February 2004

Plagioclase‐hosted melt inclusions from ocean floor lavas are characterized by great diversity in their minor and trace element compositions. The incompatible element contents of the inclusions range from enriched to ultradepleted. Ultradepleted inclusions in lavas sampled from areas such as the Galapagos Platform and the Endeavour Segment of the Juan de Fuca Ridge have Ti/Zr values as high as 3000. Such high Ti/Zr (low Ti and much lower Zr, as little as >2 ppm) suggest a source material that was melted past the point of the exhaustion of clinopyroxene (harzburgite melting). Nevertheless, these same inclusions have (La/Sm)n > 0.25 (although with low La and Sm concentrations), low K2O (0.01–0.04 wt%), and high Cl (up to 1600 ppm). In addition, examination of melt inclusion data from over 30 locations worldwide shows that there is a correlation between the level of enrichment of the host lava and the trend in K2O versus Cl described by the inclusion population. Melt inclusions from enriched lavas are characterized by relatively low Cl contents at a given K2O and no high Cl inclusions. A wide range of Cl contents and uniformly low K2O characterizes inclusions from depleted lavas. Transitional lavas exhibit either intermediate slopes or two separate trends. In contrast, other incompatible elements, such as P, Ti, and the high field strength elements (HFSE), behave coherently. The paradox presented by high Cl and (La/Sm)n together with low HFSE and K2O in melt inclusion populations from depleted lavas, coupled with uniformly high incompatible element contents in enriched lava inclusions, is inconsistent with their derivation by variable degrees of melting. Correlation of Ti/Zr with Cl in the ultradepleted inclusions, plus the absence of high Cl contents in inclusions from enriched lavas supports the contention that this signal is a mantle phenomenon and not the result of alteration at or near the seafloor. The close association of ultradepleted and normal mid‐ocean ridge basalt (NMORB) melt inclusions, sometimes in the same phenocryst, together with the existence of a complete array of melt compositions (i.e., not two distinct populations), demonstrates that the magma types were part of the same magma production episode. This combination of characteristics exhibited by melt inclusions may be produced by the interaction of a harzburgite source with a Cl, light rare earth element–bearing fluid derived from deep hydrothermal circulation. Fluctuation of magma supply may allow the upper mantle to be periodically cooled and altered by hydrothermal fluids, then undergo melting during resurgence of the magma supply. The ultradepleted component produced by that fluxed harzburgite may comprise as much as 5–10% of the array of magmas that make up NMORB magmas. If this is true, it has important implications for the thermal and mass budget and perhaps the rheology of the upper mantle and lower crust.
Properties of the aerosol over the central Arctic Ocean
Authors: Bigg E. Keith , Leck Caroline
Published: February 2004

Transmission electron microscopy was used to examine the morphology and some physical and chemical properties of individual particles collected north of latitude 80°N by impaction or electrostatic precipitation during the period July 21 to September 18, 1996. Particles smaller than 50 nm in diameter showed no evidence of the presence of sulfuric acid, previously believed responsible for new particle formation. Many were crystalline or semicrystalline often with pentagonal or hexagonal habit and more heat resistant than ammonium salts. Most were unaffected by decane or xylene vapor, but some were wholly or partly dissolved, indicating that they were chemically different. Sulfuric acid, ammonia, and probably methane sulfonic acid appeared to be mainly responsible for growth of these particles to the sizes that could become involved in cloud drop formation, about 80–100 nm in diameter. Much greater diversity was present in larger particles, the most numerous class of which usually appeared to be very variable mixtures of sulfuric acid, methane sulfonic acid, and their ammonium salts. All contained significant organic material. Particles containing sea salt were generally larger than 250 nm in diameter and contained an amazing variety of other material, much of it organic. On days with sunshine unusual numbers of particles <5 nm and in the range 10–50 nm in diameter occurred. At the same time, concentrations of three different types of particle with diameters of the order of 100 nm were enhanced. One group was liquid when collected, had a large organic content, and wet the collecting surface. The other two were bacteria and flat insoluble plates some of which contained fragments of diatoms. It is proposed that they were ejected into the air by bubbles bursting on the open leads.
Central role of carbonyl compounds in atmospheric chemistry
Authors: Lary D. J. , Shallcross D. E.
Published: February 2004

With the exception of acetone it is not generally recognized how important atmospheric carbonyls and alkyl radicals are in the lower stratosphere and upper troposphere. Carbonyl compounds are the crucial intermediate species for the autocatalytic production of OH. For example, in the upper troposphere and lower stratosphere it is calculated based on data assimilation analysis of Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) data that CH3 production due to the degradation of carbonyls contributes around 40% to the overall production of CH3, a key initiation step for HOx production, with the contribution due to the photolysis of CH3CHO being comparable to that of acetone. So correctly modeling the alkyl radical concentrations is of central importance and has not be given the attention it deserves to date. The reactions of carbonyls with Br and Cl are also major sources of HBr and HCl. In short, carbonyl compounds play a central role in atmospheric chemistry close to the tropopause, and this is directly relevant to issues such as the assessment of the impact of air traffic, and ozone depletion.
Diurnal and seasonal variation of atmospheric dimethylsulfoxide at Amsterdam Island in the southern Indian Ocean
Authors: Sciare J. , Kanakidou M. , Mihalopoulos N.
Published: February 2004

Atmospheric dimethylsulfoxide (DMSO), an important dimethylsulfide (DMS) oxidation by‐product, has been monitored on a daily basis at Amsterdam Island in the southern Indian Ocean during three periods (January–September 1997, January–June 1998, and January–October, 1999). In addition, during an intensive experiment carried out in January 1998, DMSO diurnal variation has been studied in conjunction with parallel measurements of atmospheric DMS. Monthly average atmospheric DMSO mixing ratios range from 0.3 to 5.8 parts per trillion by volume and present a well‐distinguished seasonal cycle with a minimum in winter and a maximum in summer similar to that observed for atmospheric DMS simultaneously measured. DMSO presents also a well‐distinguished diurnal variation with maximum values around 0900 and minimum during night. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces quite well the diurnal variation of DMSO when assuming a 50% yield from the DMS+OH addition channel [Hynes and Wine, 1996]. However, in absolute terms the simulated DMSO concentrations were about a factor of 2 higher than those measured. Photochemically driven heterogeneous losses of DMSO on aerosol and/or clouds could account for this difference.
Thermal and Evolved Gas Analyzer: Part of the Mars Volatile and Climate Surveyor integrated payload
Authors: Boynton William V. , Bailey Samuel H. , Hamara David K. , Williams Michael S. , Bode Rolfe C. , Fitzgibbon Michael R. , Ko WenJeng , Ward Michael G. , Sridhar K. R. , Blanchard Jeff A. , Lorenz Ralph D. , May Randy D. , Paige David A. , Pathare Asmin V. , Kring David A. , Leshin Laurie A. , Ming Douglas W. , Zent Aaron P. , Golden D. C. , Kerry Kristopher E. , Lauer H. Vern , Quinn Richard C.
Published: February 2004

The Thermal and Evolved Gas Analyzer (TEGA) on the Mars Polar Lander spacecraft is composed of two separate components which are closely coupled: a Differential Scanning Calorimeter (DSC) and an Evolved Gas Analyzer (EGA). TEGA has the capability of performing differential scanning calorimetry on eight small (0.038 mL) soil samples selected in the vicinity of the lander. The samples will be heated in ovens to temperatures up to 950°C, and the volatile compounds water and carbon dioxide, which are released during the heating, will be analyzed in the EGA. The power required by the sample oven is continuously monitored during the heating and compared to that required to heat simultaneously a similar, but empty, oven. The power difference is the output of the DSC. Both endothermic and exothermic phase transitions can be detected, and the data can be used in the identification of the phases present. By correlating the gas release with the calorimetry, the abundance of the volatile compounds associated with the different phases can be determined. The EGA may also be able to detect the release of oxygen associated with any superoxide that may be on the surface of the soil grains. The instrument can detect the melting of ice in the DSC down to abundances on the order of 0.2% of the sample, and it can detect the decomposition of calcite, CaCO3, down to abundances of 0.5%. Using the EGA, TEGA can detect small amounts of water, down to 8 ppm in the sample, and it can detect the associated release of CO2 down to the equivalent abundances of 0.03%. The EGA also has the ability to determine the 13C/12C ratio in the evolved CO2, but it is not clear if the accuracy of this ratio will be sufficient to address the scientific issues.
Search for life on Mars in surface samples: Lessons from the 1999 Marsokhod rover field experiment
Authors: Newsom H. E. , Bishop Janice L. , Cockell Charles , Roush Ted L. , Johnson Jeffrey R.
Published: February 2004

The Marsokhod 1999 field experiment in the Mojave Desert included a simulation of a rover‐based sample selection mission. As part of this mission, a test was made of strategies and analytical techniques for identifying past or present life in environments expected to be present on Mars. A combination of visual clues from high‐resolution images and the detection of an important biomolecule (chlorophyll) with visible/near‐infrared (NIR) spectroscopy led to the successful identification of a rock with evidence of cryptoendolithic organisms. The sample was identified in high‐resolution images (3 times the resolution of the Imager for Mars Pathfinder camera) on the basis of a green tinge and textural information suggesting the presence of a thin, partially missing exfoliating layer revealing the organisms. The presence of chlorophyll bands in similar samples was observed in visible/NIR spectra of samples in the field and later confirmed in the laboratory using the same spectrometer. Raman spectroscopy in the laboratory, simulating a remote measurement technique, also detected evidence of carotenoids in samples from the same area. Laboratory analysis confirmed that the subsurface layer of the rock is inhabited by a community of coccoid Chroococcidioposis cyanobacteria. The identification of minerals in the field, including carbonates and serpentine, that are associated with aqueous processes was also demonstrated using the visible/NIR spectrometer. Other lessons learned that are applicable to future rover missions include the benefits of web‐based programs for target selection and for daily mission planning and the need for involvement of the science team in optimizing image compression schemes based on the retention of visual signature characteristics.
Nomad Rover Field Experiment, Atacama Desert, Chile: 1. Science results overview
Authors: Cabrol N. A. , Chong‐Diaz G. , Stoker C. R. , Gulick V. C. , Landheim R. , Lee P. , Roush T. L. , Zent A. P. , Lameli C. Herrera , Iglesia A. Jensen , Arrerondo M. Pereira , Dohm J. M. , Keaten R. , Wettergreen D. , Sims M. H. , Schwher K. , Bualat M. G. , Thomas H. J. , Zbinden E. , Christian D. , Pedersen L. , Bettis A. , Thomas G. , Witzke B.
Published: February 2004

Nomad was deployed for a 45 day traverse in the Atacama Desert, Chile, during the summer of 1997. During this traverse, 1 week was devoted to science experiments. The goal of the science experiments was to test different planetary surface exploration strategies that included (1) a Mars mission simulation, (2) a science on the fly experiment, where the rover was kept moving 75% of the operation time. (The goal of this operation was to determine whether or not successful interpretation of the environment is related to the time spent on a target. The role of mobility in helping the interpretation was also assessed.) (3) a meteorite search using visual and instrumental methods to remotely identify meteorites in extreme environments, and (4) a time‐delay experiment with and without using the panospheric camera. The results were as follow: the remote science team positively identified the main characteristics of the test site geological environment. The science on the fly experiment showed that the selection of appropriate targets might be even more critical than the time spent on a study area to reconstruct the history of a site. During the same operation the science team members identified and sampled a rock from a Jurassic outcrop that they proposed to be a fossil. The presence of paleolife indicators in this rock was confirmed later by laboratory analysis. Both visual and instrumental modes demonstrated the feasibility, in at least some conditions, of carrying out a field search for meteorites by using remote‐controlled vehicles. Finally, metrics collected from the observation of the science team operations, and the use team members made of mission data, provided critical information on what operation sequences could be automated on board rovers in future planetary surface explorations.
Emplacement and composition of steep‐sided domes on Venus
Authors: Stofan Ellen R. , Anderson Steven W. , Crown David A. , Plaut Jeffrey J.
Published: February 2004

Steep‐sided domes on Venus have surface characteristics that can provide information on their emplacement, including relatively smooth upper surfaces, radial and polygonal fracture patterns, and pits. These characteristics indicate that domes have surface crusts which are relatively unbroken, have mobile interiors after emplacement, and preserve fractures from only late in their history in response to endogenous growth or sagging of the dome surface. We have calculated the time necessary to form a 12‐cm‐thick crust for basalt and rhyolite under current terrestrial and Venusian ambient conditions. A 12‐cm‐thick crust will form in all cases in <10 hours. Although Venusian lava flows should develop a brittle carapace during emplacement, only late‐stage brittle fractures are preserved at steep‐sided domes. We favor an emplacement model where early‐formed surface crusts are entrained or continually annealed as they deform to accommodate dome growth. Entrainment and annealing of fractures are not mutually exclusive processes and thus may both be at work during steep‐sided dome emplacement. Our results are most consistent with basaltic compositions, as rhyolitic lavas would quickly form thick crusts which would break into large blocks that would be difficult to entrain or anneal. However, if Venus has undergone large temperature excursions in the past (producing ambient conditions of 800–1000 K [e.g., Bullock and Grinspoon, 1996, 1998]), rhyolitic lavas would be unable to form crusts at high surface temperatures and could produce domes with surface characteristics consistent with those of Venusian steep‐sided domes.
A pseudo three‐dimensional resonance line radiative transfer model with overlapping lines
Authors: Griffioen Erik
Published: February 2004

A description of a resonance line Feautrier radiative transfer (RT) code which allows for coupling between closely spaced emission lines is presented. This code has been used to model closely spaced H and D Lyman α 1215.67 Å and 1215.34 Å emission lines, respectively, in the Jovian thermosphrre. There is no direct interaction between these lines, but coupling and energy transfer may occur as the Voigt broadened lines overlap. The RT method is pseudo three‐dimensional in that a spherical shell atmosphere is accounted for in direct solar beam scattering but with one‐dimensional (1‐D) plane‐parallel multiple scattering. In addition, allowance is made for a varying solar zenith angle along the line of sight in calculating intensities. Using a model atmosphere for thermospheric D and H produced by a 1‐D photochemical‐diffusion model, we present results illustrating the relative importance of including coupling of H and D Lyman α emission lines in radiative transfer calculations for both the disk and limb of Jupiter. These results suggest that the spectral contrast between the H and D Lyman α features is reduced in long limb path geometries and that the disk measurement provides better detection possibilities.
Comparative terrestrial planet thermospheres: 3. Solar cycle variation of global structure and winds at solstices
Authors: Bougher S. W. , Engel S. , Roble R. G. , Foster B.
Published: February 2004

The comparison of planetary upper atmospheres using global databases has entered a new era with the advent of recent aerobraking measurements of the Mars thermosphere [e.g., Keating, et al., 1998a]. The present maturity of available modeling capabilities also permits us to contrast the Earth and Mars thermosphere structures, winds, and controlling processes using global three‐dimensional models [e.g., Bougher et al., 1999b]. This present effort focuses upon the comparison of the combined seasonal‐solar cycle responses of the thermospheres of Earth and Mars using the National Center for Atmospheric Research (NCAR) Thermospheric General Circulation Model (TGCM) utility to address the coupled energetics, dynamics, and neutral‐ion composition above ∼100 km. Extreme thermospheric conditions are expected at solstices, thereby revealing the changing importance of fundamental physical processes controlling the Earth and Mars thermospheric structures and winds. Seasonal‐solar cycle extremes in Mars exobase temperatures are calculated to range from 200 to 380 K, giving rise to maximum horizontal winds of nearly 215 to 400 m/s. Corresponding extremes in Earth exobase temperatures are 700 to 1600 K, with rather small variations in global winds. The orbital eccentricities of Earth and Mars are also shown to drive substantial variations in their thermospheric temperatures. For Mars, dayside exobase temperatures vary by ∼60 K (18%) from aphelion to perihelion during solar maximum conditions. Such large temperature variations strongly impact thermospheric densities and global winds. The corresponding Earth dayside temperatures also vary by 60–80 K between solstices. However, the percent temperature variation (5%) over the Earth's orbit and its overall impact on the thermospheric structure and winds are much smaller. Auroral activity may in fact obscure these orbital variations. Changing dust conditions throughout the Martian year modulate the aerosol heating of its lower atmosphere, yielding considerable variability in the height of the subsolar ionospheric peak about its observed seasonal trend (∼115–130 km). Significant further progress in the comparison of Earth and Mars thermospheric features and underlying processes must await expanded Mars global databases expected from Planet‐B and Mars Express (2004–2005).
Nitrogen isotope variations in Santa Barbara Basin sediments: Implications for denitrification in the eastern tropical North Pacific during the last 50,000 years
Authors: Emmer Edwin , Thunell Robert C.
Published: February 2004

Nitrogen isotope variations preserved in Santa Barbara Basin sediments are used to evaluate changes in denitrification in the eastern tropical North Pacific (ETNP) during the last 50,000 years. A significant component of the subsurface waters (∼100–400 m) that presently fill the Santa Barbara Basin is derived from the low‐oxygen, denitrifying zone in the ETNP, and the nitrate in these waters has a δ15N value of 8–9‰. During the last glacial, the δ15N values of Santa Barbara Basin sediments were typically 6–7‰, indicating decreased denitrification in the ETNP and a better oxygenated intermediate water mass in the Santa Barbara Basin at this time. This reduced denitrification during the last glacial would have increased the pool of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period. At the onset of deglaciation, sediment δ15N values increase by more than 2‰, indicating increased denitrification in the ETNP. During Younger Dryas time, δ15N values decreased by 3‰ and record a brief return to better ventilated conditions in the subsurface waters of the ETNP. This is followed by an increase in δ15N to over 9‰ at ∼10,000 years ago, indicating intense denitrification at the beginning of the Holocene.
Multiproxy reconstructions of the North Atlantic Oscillation
Authors: Cullen Heidi M. , D'Arrigo Rosanne D. , Cook Edward R. , Mann Michael E.
Published: February 2004

Multiproxy composite reconstructions of North Atlantic Oscillation (NAO) indices are presented spanning the last three centuries. These composite time series draw on four nearly independent reconstructions derived from different combinations of global and North Atlantic sector climate proxy data (e.g., tree rings and ice cores) as well as long instrumental records. A composite reconstruction based on all four series (R4) extends from 1750 to 1979, while another version based on only three series (R3) extends from 1701 to 1979. Both composite reconstructions outperform the individual series based on correlation and verification statistics. These results suggest that each individual series reflects different aspects of the NAO, such that when combined, they yield more robust reconstructions. As to be expected, these composite reconstructions, based on both proxy and long instrumental data, yield more optimal results than individual or composite reconstructions based on proxy data alone. The reconstructions presented here provide an improved means of addressing features evident in the instrumental record with respect to those of prior centuries. One such feature evident in the reconstructions is an apparent increase in variance in the instrumental period relative to the past record.
Orbitally induced climate and geochemical variability across the Oligocene/Miocene boundary
Authors: Paul Hilary A. , Zachos James C. , Flower Benjamin P. , Tripati Aradhna
Published: February 2004

To assess the influence of orbital‐scale variations on late Oligocene to early Miocene climate and ocean chemistry, high‐resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low‐ to high‐frequency variability across a 5‐Myr interval (20.5–25.4 Ma). The records also reveal several large‐scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi‐1 and Mi‐1a events of Miller et al. [1991]), and a long‐term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower‐resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice‐volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low‐frequency cycles (∼400 kyr) and long‐term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest‐resolution records of pre‐Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.
Millennial‐scale sea surface temperature variability in the western tropical North Atlantic from planktonic foraminiferal census counts
Authors: Hüls Matthias , Zahn Rainer
Published: February 2004

Planktonic foraminiferal census counts are used to construct high‐resolution sea surface temperature (SST) and subsurface (thermocline) temperature records at a core site in the Tobago Basin, Lesser Antilles. The record is used to document climatic variability at this tropical site in comparison to middle‐ and high‐latitude sites and to test current concepts of cross‐equatorial heat transports as a major player in interhemispheric climate variability. Temperatures are estimated using transfer function and modern analog techniques. Glacial‐maximum cooling of 2.5°'3°C is indicated; maximum cooling by 4°C is inferred for isotope stage 3. The SST record displays millennial‐scale variability with temperature jumps of up to 3°C and closely tracks the structure of ice‐core Dansgaard/Oeschger cycles. SST variations in part of the record run opposite to the SST evolution at high northern latitude sites, pointing to thermohaline circulation and marine heat transport as an important factor driving SST in the tropical and high‐latitude Atlantic, both on orbital and suborbital timescales.
Suppression of El Niño during the Mid‐Holocene by changes in the Earth's orbit
Authors: Clement Amy C. , Seager Richard , Cane Mark A.
Published: February 2004

A number of recent reports have interpreted paleoproxy data to describe the state of the tropical Pacific, especially changes in the behavior of the El Niño‐Southern Oscillation (ENSO), over the Holocene. These interpretations are often contradictory, especially for the eastern tropical Pacific and adjacent areas of South America. Here we suggest a picture of the mid‐Holocene tropical Pacific region which reconciles the data. ENSO variability was present throughout the Holocene but underwent a steady increase from the mid‐Holocene to the present. In the mid‐Holocene, extreme warm El Niño events were smaller in amplitude and occurred less frequently about a mean climate state with a cold eastern equatorial Pacific and largely arid coastal regions as in the present climate. This picture emerges from an experiment in which a simple numerical model of the coupled ocean‐atmosphere system in the tropical Pacific was driven by orbital forcing. We suggest that the observed behavior of the tropical Pacific climate over the mid‐ to late Holocene is largely the response to orbitally driven changes in the seasonal cycle of solar radiation in the tropics, which dominates extratropical influences.
Synchroneity of major Late Neogene sea level fluctuations and paleoceanographically controlled changes as recorded by two carbonate platforms
Authors: Betzler Christian , Kroon Dick , Reijmer John J. G.
Published: February 2004

Shallow‐water carbonate systems are reliable recorders of sea level fluctuations and changes in ambient seawater conditions. Drilling results from Ocean Drilling Program (ODP) Legs 133 and 166 indicate that the timing of late Neogene sedimentary breaks triggered by sea level lowerings is synchronous in the sedimentary successions of the Queensland Plateau and the Great Bahama Bank. This synchrony indicates that these sea level changes were eustatic in origin. The carbonate platforms were also affected by contemporary, paleoceanographically controlled fluctuations in carbonate production. Paleoceanographic changes are recorded at 10.7, 3.6, and 1.7–2.0 Ma. At the Queensland Plateau, sea surface temperature shifts are documented by shifts from tropical to temperate carbonates (10.7 Ma) and vice versa (3.6 Ma); the modern tropical platform was established at 2.0–1.8 Ma. At Great Bahama Bank, changes were registered in compositional variations of platform‐derived sediment, such as major occurrence of peloids (3.6 Ma) and higher rates of neritic carbonate input (1.7 Ma). The synchroneity of these changes attests to the far‐field effects of modifications in the oceanographic circulation on shallow‐water, low‐latitude carbonate production.
Shear instabilities of wave‐driven alongshore currents
Authors: Dodd Nick , Iranzo Vicente , Reniers Ad
Published: February 2004

The present state of research into and understanding of shear waves is appraised. In this paper these motions, which result from an instability of an alongshore, wave‐driven current, are described, their theoretical explanation is recounted, and the history of their discovery is related. The various investigations into their genesis, which attempted to develop an understanding of when and why they occur, are summarized, and more recent research into their finite amplitude development is discussed, focusing on the understanding gained. Attention is also given to observations of shear waves made in the field and attempts to observe them in the laboratory. Finally, work that still needs to be done is described.
Statistical perspectives on stratospheric transport
Authors: Sparling L. C.
Published: February 2004

Many long‐lived stratospheric chemical constituents enter the stratosphere through the tropical tropopause, are transported throughout the stratosphere by the Brewer‐Dobson circulation, and are photochemically destroyed in the upper stratosphere. These chemical constituents, or “tracers,” can be used to track mixing and transport by the stratospheric winds. Much of our understanding about the stratospheric circulation is based on large‐scale gradients and other spatial features in tracer fields constructed from satellite measurements. The point of view presented in this paper is different, but complementary, in that transport is described in terms of tracer probability distribution functions. The probability distribution function is computed from the measurements and is proportional to the area occupied by tracer values in a given range. The flavor of this paper is tutorial, and the ideas are illustrated with several examples of transport‐related phenomena, annotated with remarks that summarize the main point or suggest new directions. The examples illustrate how physically based statistical analysis can shed some light on aspects of stratospheric transport and dynamics that may not be obvious or quantifiable with other types of analyses. The dependence of the statistics on location and time is also shown to be important for practical problems related to statistical robustness and satellite sampling. An important motivation for the work presented here is the need for synthesis of the large and growing database of observations of the atmosphere and output generated by atmospheric models.
Sources of light in the deep ocean
Authors: Reynolds G. T. , Lutz R. A.
Published: February 2004

Studies during recent decades have shown that the deep ocean (depths below where solar luminance plays a direct environmental role) is far from a dark, cold, lifeless region. Evidence obtained by utilizing a variety of photo‐optical devices, providing spatial, temporal, and spectral information, has demonstrated that this portion of the Earth is a region rich in life and light. Findings to date have provided challenges for geologists, physicists, biologists, chemists, and oceanographers, and the sharing of techniques and expertise among these disciplines has demonstrated the rewards to be gained from interdisciplinary research. Bioluminescence has been found far below the depths at which it has received most attention historically. The study of this phenomenon is complicated by the fact that the measuring apparatus itself causes a stimulation of the luminescence so that a true background will be difficult to determine. Nuclear physics has played a role in that the electron resulting from the decay of an isotope of potassium (K40) provides an ubiquitous background of light through a process known as Cerenkov radiation. The possibility of light generated by cosmic rays must also be taken into account. An intriguing source of light at the very bottom of the sea is found at hydrothermal vents. Here are found not only light but also abundant life. At the vent orifice, temperatures are found to be as high as 250°–400°C. A large component of the light is due to thermal radiation. However, the light in the wavelengths 450–600 nm is significantly greater than the thermal flux at those wavelengths. Identifying the physical mechanisms that may account for this excess is important in providing insight into the processes occurring in the vents and plumes and in the accompanying ecosystem.
Hydrology of Yucca Mountain, Nevada
Authors: Flint Alan L. , Flint Lorraine E. , Kwicklis Edward M. , Bodvarsson Gudmundur S. , Fabryka‐Martin June M.
Published: February 2004

Yucca Mountain, located in southern Nevada in the Mojave Desert, is being considered as a geologic repository for high‐level radioactive waste. Although the site is arid, previous studies indicate net infiltration rates of 5–10 mm yr−1 under current climate conditions. Unsaturated flow of water through the mountain generally is vertical and rapid through the fractures of the welded tuffs and slow through the matrix of the nonwelded tuffs. The vitric‐zeolitic boundary of the nonwelded tuffs below the potential repository, where it exists, causes perching and substantial lateral flow that eventually flows through faults near the eastern edge of the potential repository and recharges the underlying groundwater system. Fast pathways are located where water flows relatively quickly through the unsaturated zone to the water table. For the bulk of the water a large part of the travel time from land surface to the potential repository horizon (∼300 m below land surface) is through the interlayered, low fracture density, nonwelded tuff where flow is predominately through the matrix. The unsaturated zone at Yucca Mountain is being modeled using a three‐dimensional, dual‐continuum numerical model to predict the results of measurements and observations in new boreholes and excavations. The interaction between experimentalists and modelers is providing confidence in the conceptual model and the numerical model and is providing researchers with the ability to plan further testing and to evaluate the usefulness or necessity of further data collection.
New inverse algorithm for focused radar image reconstruction of a rotating object
Authors: Kang Gum‐Sil , Kim Yong‐Hoon
Published: February 2004

A new inverse algorithm using the proposed method to obtain two‐dimensional (2‐D) high‐resolution inverse synthetic aperture radar (ISAR) images of rotating targets is described. Generally, to obtain images using ISAR, the reflected signals of an object are measured at a uniform frequency interval and rotation angle interval. If a 2‐D fast Fourier transform (FFT) is performed using the measured data, an unfocused image might be obtained. To obtain a focused image using a 2‐D FFT, the measured signal is resampled by interpolating the known samples disposed on a polar raster to obtain new samples disposed on a rectangular raster. To improve resolution and reduce computation time, an alternative to the resampling process is proposed in this paper. This proposed sampling method directly measures samples on a rectangular raster, thus avoiding the interpolation process at the cost of increased complexity in the sampling scheme. In this case it is assumed that rotation angle of object and step frequency can be controlled. In this paper, three types of image reconstruction methods are presented, and images obtained through simulation and experiment are compared. The methods are the unfocused imaging method, the focused imaging method with interpolation, and the focused imaging method with proposed scheme.
Noise nature investigation of sporadic Jovian decameter emission
Authors: Litvinenko Galina V. , Rucker Helmut O. , Vinogradov Vladimir V. , Ryabov Boris P. , Shaposhnikov Vladimir E.
Published: February 2004

The “superfine” time structure of the Jovian S burst decameter emission has been investigated with the aim to clarify the noise nature of the given radiation. The Jovian Io‐B noise storm taken for analysis in the paper has been observed on June 8, 1997, with an acousto‐optical spectrograph designed at the Observatory Nancay, and incorporated into the giant Kharkov, Ukraine, UTR‐2 radio telescope, operating from 10 to 25 MHz. The presence of the flicker noise component in the processed signals has been shown with an application of the wavelet transform and maximum likelihood techniques. The spectral parameters γ characterizing such processes have been calculated for different frequencies and different time intervals. The set of randomly distributed pulses has been considered as one of the possible models of S bursts adiation, capable for producing this noise process.
Inversion algorithm and measurement system for microwave tomography of buried object
Authors: Dourthe C. , Pichot C. , Dauvignac J. Y. , Cariou J.
Published: February 2004

The detection and identification of buried inhomogeneities using electromagnetic waves are of crucial importance for many applications. The paper deals with reconstructed images from measurements using an inversion qualitative algorithm for microwave tomography. The algorithm is based on diffraction tomography for detecting and locating buried objects. The backscattered field is measured at different frequencies over a probing line above or in contact with the soil at different receiver locations and for different positions of a transmitting antenna (multistatic configuration). The reconstruction algorithm processes the backscattered field generated by the buried inhomogeneities and incorporates the incident near‐field distribution transmitted in the soil by the broadband antenna. Tomographie reconstructions of buried objects are presented for a situation of practical interest using bow tie antennas in the frequency band [0.3–1.3] GHz.
On the radar estimation of turbulence parameters in a stably stratified atmosphere
Authors: Roper R. G.
Published: February 2004

Four estimators of the rate of dissipation of turbulent energy based on the velocity structure function, rms turbulent velocities, Brunt‐Väisälä frequencies, and ground diffraction pattern fading rates are discussed and compared using Arecibo Initiative on the Dynamics of the Atmosphere MF radar data in both the imaging Doppler interferometry (IDI) and spaced antenna modes. A consistent set of empirical equations is developed which define and describe the relationships between the turbulent velocity, the Brunt‐Väisälä frequency, the buoyancy length scale, the rate of dissipation of turbulent energy, the eddy diffusivity, and the time constant of the gravity wave generated, intermittent, decaying, coherent structures responsible for the scatterers detected by the IDI technique. It should be noted that the constants of proportionality are significantly different from the currently accepted values.
Determination of source thunderstorms for VHF emissions observed by the FORTE satellite
Authors: Tierney Heidi E. , Jacobson Abram R. , Beasley William H. , Argo Paul E.
Published: February 2004

This paper explains a method of locating groups of satellite‐observed VHF signals from lightning that emanate from isolated storm regions. LF/VLF signals recorded and located by the National Lightning Detection Network (NLDN) are often accompanied by broadband VHF emissions of sufficient intensity to trigger satellite‐based RF receivers. It has previously been shown that events recorded by the FORTE satellite and the NLDN, which have occurrence‐time differences of 0.3 ms or less, can be assumed to originate at the same approximate geolocation with 97.5% reliability. For such event pairs, the VHF events recorded by FORTE were assigned the NLDN locations. We use graphical analysis to identify groups of satellite data that can be assumed to originate from the same storms as the located events. To accomplish this, a value of total electron content (TEC) is derived from the frequency dispersion of each VHF signal recorded by FORTE. Scatterplots of TEC versus time often reveal curved clusters of satellite data. The variation of TEC with satellite elevation angle, and time, is consistent with a model in which the length of a signal's required propagation path through the ionosphere changes with satellite and storm position. We use the values of TEC for the located satellite data, a simple model of the ionosphere, and NLDN data to help us identify TEC clusters that can be assumed to originate from relatively compact, and unambiguous, geolocations. For the time period of April through September of 1998 we geolocate 65 groups of satellite data records. A total of 6131 satellite data records, which were not previously located, have been assigned the median NLDN coordinates of their corresponding storm.
A possible mechanism for echo striation generation of radar backscatter from midlatitude sporadic E
Authors: Maruyama Takashi , Fukao Shoichiro , Yamamoto Mamoru
Published: February 2004

The morphological resemblance is described between the quasi‐periodic scintillations of VHF radio wave transmissions from a satellite and the 46.5‐MHz radar backscatters of the middle and upper atmosphere (MU) radar radio waves, which are both associated with ionospheric irregularities in the sporadic E layer. Because these observation techniques are sensitive to different wavelengths of irregularities, the resemblance may be caused by coexisting irregularities. That is, radio wave scintillations reveal medium‐scale density‐enhanced regions with a specific scale length of several hundred meters in which small‐scale irregularities are embedded. The MU radar echoes may reveal backscatter signals from small‐scale irregularities. A periodic structure on a range‐time‐intensity (RTI) map reflected the spatial structure of the medium‐scale density‐enhanced regions. A model is proposed to explain the striations on the RTI map, which extend up to heights of several tens of kilometers even though sporadic E layers are usually thin.
Observations of auroral roar emissions at polar cap latitudes: Results from the Early Polar Cap Observatory
Authors: LaBelle J. , Hughes J. M.
Published: February 2004

Auroral roar, a radio emission occurring near the second and third electron cyclotron harmonics, has been monitored during a 51‐day interval in 1998 from three ground‐based high‐latitude observatories: Churchill (69° corrected geomagnetic latitude (cgm)), Baker Lake (74° cgm), and the Early Polar Cap Observatory (EPCO) at Resolute (84° cgm). These observations reveal a latitude dependence of the local time distribution of auroral roar events. In the auroral zone they occur predominantly in the premidnight‐midnight sector as previously reported, but in the polar cap they occur evenly throughout nighttime hours. Sorting the data by magnetic activity level confirms the effect; for example, the distribution of events at Baker Lake during times of extremely low activity (Kp < 1) is concentrated near magnetic midnight, consistent with Baker Lake lying in the auroral zone when the oval is contracted. The latitude dependence of the local time distribution may be explained by propagation effects, with inefficient propagation in the auroral zone causing only relatively locally generated emissions to be detected there, while stations well poleward of the auroral zone detect emissions from sources widely distributed in longitude and latitude. Statistics of the frequencies of auroral roar events detected at Resolute show evidence of an upper frequency cutoff near 2960 kHz. Ray‐tracing calculations suggest that it is plausible for auroral roar to reach Resolute from sources up to 1500 km distant in the auroral zone, but they do not provide an obvious explanation for the frequency cutoff.