bluuebunny/crossref_metadata_2025 huggingface dataset:
Backscatter model for the unusual radar properties of the Greenland Ice Sheet
Authors: Rignot Eric
Published: February 2004

A number of planetary objects exhibit large radar reflectivity and polarization ratios, and more recently, a similar behavior has been observed over a vast portion of the Earth's surface: the percolation facies of the Greenland Ice Sheet. Surface‐based ranging radar data and snow stratigraphy studies demonstrated that the radar properties of that portion of Greenland are caused by enhanced scattering from massive, large, solid‐ice bodies buried in the top few meters of the dry, cold, clean snowy surface of the ice sheet and created by seasonal melting and refreezing events. Here, we model the icy inclusions as randomly oriented, discrete, noninteracting, dielectric cylinders embedded in a transparent snow medium. An exact analytical solution is used to compute the scattered field from the cylinders. Using this model, we correctly predict the polarimetric radar observations gathered by an airborne imaging system at three wavelengths (5.6, 24, and 68 cm), between 19° and 65° incidence angle. The diameter and number density of the cylinders that are inferred from the radar data using the model are consistent with in situ observations of the icy inclusions. The large radar reflectivity and polarization ratios are interpreted as arising from internal reflections of the radar signals in the icy inclusions that first‐order external reflection models fail to predict. The results compare favorably with predictions from the coherent backscatter or weak localization theory and may provide a complementary framework for interpreting exotic radar echoes from other planetary objects.
Quantitative mineralogical characterization of lunar high‐Ti mare basalts and soils for oxygen production
Authors: Chambers J. G. , Taylor L. A. , Patchen A. , McKay D. S.
Published: February 2004

Efficient lunar resource utilization requires accurate and quantitative evaluation of mineral and glass abundances, distribution, and extraction feasibility, especially for ilmenite. With this in mind, true modal analyses were performed on high‐Ti mare basalts and soils with X ray/backscattered electron signal digital‐imaging techniques, and these data indicate that (1) ilmenite concentrations are similar for basalts and immature‐submature soils with similar TiO2 content; (2) ilmenite liberation of crushed mare basalts and immature‐submature mare soils are comparable (i.e., both contain similar amounts of free ilmenite); and (3) because of impact melting and agglutination of primary minerals, mature mare soils contain less ilmenite (both free and attached). Modal analyses of magnetic separates of high‐Ti mare basalts and soils show that (1) ilmenite was concentrated by a factor of ≥3.3 and (2) soil ilmenite was concentrated to factors of 1.7–2.3. The lower soil ilmenite separation efficiency is attributed to Fe°‐bearing agglutinitic glass and amorphous rinds adhered to soil particles. Mass yields of magnetically generated feedstocks were generally less than 5 wt % in most cases. Calculation of oxygen yield (as released by hydrogen gas reduction of ilmenite) show that (1) beneficiated basalt will provide the most oxygen (8–10%), because of higher ilmenite concentration; (2) reduction of raw immature‐submature mare soils and basalts will produce similar amounts of lunar liquid oxygen (LLOX) (2.1–3.1%); and (3) raw Fe‐rich pyroclastic soil, 74220, will provide more oxygen (5.4%) than beneficiated high‐Ti mare soils and half that of beneficiated high‐Ti mare basalts. High‐Ti mare soils are attractive resources for lunar liquid oxygen (LLOX) production because of their unconsolidated nature, high ilmenite abundance, and widespread occurrence. Energy‐intensive excavation and comminution likely prohibits the basalt mining during early lunar occupation. Orange soils are important resources for LLOX and various volatile elements, but slower reaction kinetics and glass sintering pose potential difficulties for large‐scale operations.
Processes involved in the formation of magnesian‐suite plutonic rocks from the highlands of the Earth's Moon
Authors: Snyder Gregory A. , Neal Clive R. , Taylor Lawrence A. , Halliday Alex N.
Published: February 2004

The earliest evolution of the Moon likely included the formation of a magma ocean and the subsequent development of anorthositic flotation cumulates. This primary anorthositic crust was then intruded by mafic magmas which crystallized to form the lunar highlands magnesian suite. The present study is a compilation of petrologic, mineral‐chemical, and geochemical information on all pristine magnesian‐suite plutonic rocks and the interpretation of this data in light of 18 “new” samples. Of these 18 clasts taken from Apollo 14 breccias, 12 are probably pristine and include four dunites, two norites, four troctolites, and two anorthosites. Radiogenic isotopic whole rock data also are reported for one of the “probably pristine” anorthositic troctolites, sample 14303,347. The relatively low Rb content and high Sm and Nd abundances of 14303,347 suggest that this cumulate rock was derived from a parental magma which had these chemical characteristics. Trace element, isotopic, and mineral‐chemical data are used to interpret the total highlands magnesian suite as crustal precipitates of a primitive KREEP (possessing a K‐, rare earth element (REE)‐, and P‐enriched chemical signature) basalt magma. This KREEP basalt was created by the mixing of ascending ultramafic melts from the lunar interior with urKREEP (the late, K‐, REE‐, and P‐enriched residuum of the lunar magma ocean). The trace and major element compositions of nearly all magnesian‐suite cumulates can be generated by 0–55% fractional crystallization of a primitive KREEP basalt combined with the trapping of varied proportions (generally ≤20%) of instantaneous, residual, KREEP‐basalt liquid. A few samples of the magnesian suite with extremely elevated large‐ion lithophile elements (5–10× other magnesian‐suite rocks) cannot be explained by this model or any other model of autometasomatism, equilibrium crystallization, or “local melt‐pocket equilibrium” without recourse to an extremely large‐ion lithophile element‐enriched parent liquid. It is difficult to generate parental liquids which are 2–4× higher in the REE than average lunar KREEP, unless the liquids are the basic complement of a liquid‐liquid pair, i.e., the so‐called “REEP‐fraction,” from the silicate liquid immiscibility of urKREEP. Scarce age information on lunar rocks suggests that magnesian‐suite magmatism was initiated at progressively more recent time from the northeast to the southwest on the lunar nearside from 4.45 to 4.25 Ga. This magmatic “event” could be due to melting of the lunar mantle beneath these regions and could have been generated either by latent heat during crystallization of the final, KREEP‐rich (and, thus, Th‐ and U‐rich), residual, lunar magma ocean liquid or heating due to radioactive decay of K, Th, and U.
How does Venus lose heat?
Authors: Turcotte Donald L.
Published: February 2004

The tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior. On the Earth, about 70% of the heat flow through the mantle is attributed to the subduction of cold lithosphere. In order to understand the tectonic and volcanic processes on Venus it is necessary to understand how heat is transported through its mantle. In this paper, three alternative end‐member hypotheses are considered. The first is the steady state loss of heat through the mantle to the surface in analogy to the Earth. However, without plate tectonics and subduction on Venus, a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination. The required plume flux would be equivalent to about 80 plumes with the strength of the Hawaiian plume. The required delamination flux implies a 50% delamination of the entire Venus lithosphere every 10 m.y. Neither appears possible, so that it is concluded that Venus cannot transport heat through its mantle to its surface on a steady state basis. The second hypothesis is that there has been a strong upward concentration of the heat‐producing elements into the crust of Venus; the heat generated is then lost by conduction. Surface measurements of the concentrations of the heat‐producing elements place constraints on this model. If everything is favorable this hypothesis might be marginally acceptable, but it is considered to be highly unlikely. The third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence. The near‐random distribution of craters suggests that the last subduction event occurred about 500 Ma. This model implies a thick thermal lithosphere (≈300 km) at the present time, which is consistent with a variety of surface observations. Lava lakes on the Earth are considered as analogies to plate tectonics; they also exhibit episodic subduction events.
Modeling of fluidized ejecta emplacement over digital topography on Venus
Authors: Johnson Jeffrey R. , Gaddis Lisa
Published: February 2004

The FLOW computer model of McEwen and Malin (1989) modified for application to the study of Venus fluidized ejecta blankets (FEBs) demonstrates that relatively low viscosities, yield strengths, and initial velocities are required to duplicate the observed flow paths of the outflow materials. The model calculates the velocities and simulated flow paths of gravity flows over Magellan topography. The model is formulated to determine flow movements from initial conditions, gravitational acceleration, and resistance to motion as described by Coulomb, viscous, and turbulent resistance forces. Successful duplication of observed FEB flow paths has been achieved for the FEB craters Addams, Isabella, and Cochran. When used as a simple energy‐line model, the model requires low coefficients of friction to extend FEBs to near their observed termini in the synthetic aperture radar (SAR) imagery, although the resulting straight flow lines do not follow the observed flow paths well. For Bingham flow, the model requires low values of viscosity and yield strength which are more similar to pyroclastic or debris flows than basaltic lavas. Flows of 100‐m depth require 1 to 2 orders of magnitude higher values of both viscosity and yield strength than 10‐m‐deep flows. The complicated nature of the flow lines for the low velocity model suggests that FEBs were probably emplaced under variably laminar and turbulent flow conditions, where underlying topography influenced both the direction and energy of flow materials.
Dating volcanism and rifting on Venus using impact crater densities
Authors: Price Maribeth H. , Watson Geoffrey , Suppe John , Brankman Charles
Published: February 2004

Although the distribution of impact craters on Venus is indistinguishable from a completely spatially random population on the basis of crater data alone, the addition of geologic information indicates that the craters are not random with respect to geology. Areas of low crater density correlate with concentrations of tectonic and volcanic features, as well as high proportions of faulted and embayed craters. High‐resolution mapping of large volcanoes, flood‐type lava flow fields, rifts, and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance. Because these features also appear young based on stratigraphic data, difference in age is the most likely reason for differences in crater density between geologically defined terrains. Crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters, as long as the mapping criteria are independent of impact crater locations. This result provides a tool to begin identifying and dating major geologic provinces, and to begin developing a geologic history of Venus. Using a simple data model, and calculating crater ages relative to an assumed global mean age of 300 Ma, we show that the ages of large volcanoes (72 ± 45 Ma), flow fields (128 ± 91 Ma), rifts (130 ± 145 Ma), and coronae (120 ± 115 Ma) are all substantially younger than the mean plains age, and probably represent ongoing volcanic and tectonic activity, rather than the end of a brief global resurfacing event with a mean age of 300 Ma.
Discrepant estimates of moments of inertia of the Moon
Authors: Bills Bruce G.
Published: February 2004

The moments of inertia of the Moon can be computed using various subsets of the degree two harmonics of the gravity field and the librational parameters. Recent estimates of these parameters allow four different solutions for the three principal moments of inertia. The solutions disagree by about 15 times the formal errors in the solutions. A possible resolution of these discrepancies involves nonrigid librational motion, either in the form of an elastic toroidal strain or with a fluid core which is only partially coupled to the rigid mantle. For this explanation to be viable, it must involve preferential strain in association with the longitudinal librations. Both gravitational and viscous coupling between a fluid core and a solid mantle are expected to have that behavior.
Ground‐based near‐infrared observations of the Venus nightside: 1.27‐μm O2(a1Δg) airglow from the upper atmosphere
Authors: Crisp D. , Meadows V. S. , Bézard B. , de Bergh C. , Maillard J.‐P. , Mills F. P.
Published: February 2004

Near‐infrared spectroscopic observations of Venus taken in 1975 revealed O2(a1Δg) airglow from both the dayside and nightside of the planet with emission rates exceeding 1 mega‐Rayleigh (1 MR = 1012 photons cm−2 s−1 into 4π sr). These large emission rates indicated that most of the atomic oxygen produced through the photolysis of CO2 on the dayside of Venus eventually recombined to produce O2 in the excited (a1Δg) state. This result was initially surprising because available laboratory measurements indicated O2(a1Δg) yields from atomic oxygen recombination reactions that were no larger than a few percent. More recent observations reveal even larger O2(a1Δg) airglow intensities as well as dramatic spatial and temporal variations in this airglow. High‐resolution (0.3 cm−1) spectra of the Venus nightside taken with the Canada France Hawaii Telescope/Fourier transform spectrometer in 1991 show spectrally integrated O2(a1Δg) intensities as large as 1.1 mW m−2 sr−1. Once these values are corrected for viewing angle and reflection from the underlying clouds, they indicate emission rates near 3 MR. These spectra also yield rotational temperatures of 186 ± 6 K in the emitting layer (90 to 115 km). Spectral image cubes taken with the Anglo‐Australian Telescope/infrared imaging spectrometer and the Canada France Hawaii Telescope/imaging Fourier transform spectrometer during 1991, 1993, and 1994 provide a more complete description of the spatial and temporal variability in this emission. Images extracted at wavelengths within the O2(a1Δg) Q‐branch (1.269 μm) often show contrasts larger than 10 to 1 across the nightside. Even though the disk‐averaged intensities are comparable to those seen in 1975, some localized regions have airglow emission rates larger than 5 MR. The brightest emission is often confined to 1000‐ to 2000‐km‐diameter regions. These bright regions have been detected over a broad range of latitudes and local times, but they are most often seen at low latitudes and at local times between midnight and 0300 on Venus. The intensity of the brightest spots can change by 20% in less than 1 hour, and they can vanish entirely in less than 1 day. These new observations are providing improved constraints on atmospheric chemical and dynamical models of the upper mesosphere and lower thermosphere of Venus.
A new axisymmetric MHD model of the interaction of the solar wind with Venus
Authors: De Zeeuw Darren L. , Nagy Andrew F. , Gombosi Tamas I. , Powell Kenneth G. , Luhmann Janet G.
Published: February 2004

A new two‐dimensional axisymmetric MHD model is used to study the interaction of the solar wind with Venus under conditions where the interplanetary field is approximately aligned with the solar wind velocity. This numerical model solves the MHD transport equations for density, velocity, pressure, and magnetic field on an adaptively refined, unstructured grid system. This use of an adaptive grid allows high spatial resolution in regions of large density/velocity gradients and yet can be run on a workstation. The actual grid sizes vary from about 0.06 Rv near the bowshock to 2 Rv in the unperturbed solar wind. The results of the calculations are compared with observed magnetic field values obtained from the magnetometer on the Pioneer Venus Orbiter, at a time when the angle between the solar wind velocity vector and the interplanetary magnetic field (IMF) was only 7.6°. Good qualitative agreement between the observed and calculated field behavior is found. The overall results suggest that the induced magnetotail disappears when the IMF is radial for an extended time period and implies that it weakens when the field rotated through a near‐radial orientation.
Analysis and interpretation of the surface topography and gravitational potential of Venus
Authors: Arkani‐Hamed Jafar
Published: February 2004

The relationship between the surface topography and gravitational potential of Venus and the Earth is investigated over the spherical harmonics of degree and order up to 75. The covarying harmonics of the topography and the potential, i.e. the harmonics having nonnegative degree/order correlation coefficients, are treated separately from the antivarying harmonics, those with negative degree/order correlation coefficients. The covarying harmonics provide reliable estimates of the compensation depth of the surface topography. The surface topography of the Earth specified by spherical harmonics of degree higher than 11 is essentially compensated isostatically at the Moho discontinuity, whereas the lower‐degree harmonics are largely supported by mantle dynamics. For Venus the harmonics of degree lower than 9 are probably supported by mantle dynamics, while those of degree higher than 35 are most likely supported isostatically at a depth of about 45 km. There is a gradual increase in the compensation depth of the surface topography as the degree of harmonics decreases from 35 to 9, implying a combination of isostatic and dynamic support of these harmonics. Part of the topography specified by antivarying harmonics of degree greater than about 20 cannot be maintained by an isostatic compensation or a steady state dynamic support; it is more likely time dependent. Assuming that this part of the topography arises from viscous deformation of mantle under surface loading, the gravity and topography relationship provides a first‐order estimate of 5 × 1024 Pa s for the viscosity of the Venusian mantle. This high viscosity results in a Rayleigh number of about 12000 for the mantle, which is only about 17 times greater than the critical Rayleigh number for convection, implying a very slow quasi‐steady convection in the mantle.
Mechanism of cyanoacetylene photochemistry at 185 and 254 nm
Authors: Clarke David W. , Ferris James P.
Published: February 2004

The role of cyanoacetylene (HC3N) in the atmospheric photochemistry of Titan and its relevance to polymer formation are discussed. Investigation of the relative light absorption of HC3N, acetylene (C2H2), and diacetylene (C4H2) revealed that HC3N is an important absorber of UV light in the 205‐ to 225‐nm wavelength region in Titan's polar regions. Laboratory studies established that photolysis of C2H2 initiates the polymerization of HC3N even though the HC3N is not absorbing the UV light. Quantum yield measurements establish that HC3N is 2–5 times as reactive as C2H2 for polymer formation. Photolysis of HC3N with 185‐nm light in the presence of N2, H2, Ar, or CF4 results in a decrease in the yield of 1,3,5‐tricyanobenzene (1,3,5‐tcb), while photolysis in the presence of CH4, C2H6, or n‐C4H10 results in an increase in 1,3,5‐tcb. The rate of loss of HC3N is increased by all gases except H2, where it is unchanged. It was not possible to detect 1,3,5‐tcb as a photoproduct when the partial pressure of HC3N was decreased to 1 torr. Photolysis of HC3N with 254‐nm light in the presence of H2 or N2 results in the formation of 1,2,4‐tcb, while photolysis in the presence of CH4, C2H6, or n‐C4H10 results in the formation of increasing amounts of 1,3,5‐tcb. Mechanisms for the formation of polymers are presented.
Monte Carlo calculations of plasma ion‐induced sputtering of an atmosphere: SO2 ejected from Io
Authors: Pospieszalska M. K. , Johnson R. E.
Published: February 2004

Monte Carlo calculations are used to describe the kinetics of a column of SO2 gas (1016 cm−2) on Io, which is bombarded by a flux of ions from the Io plasma torus, transferring 8 × 1011 eV cm−2 s−1 of kinetic energy to the atmospheric molecules. Both the vertical structure of the atmosphere and the atmospheric loss rate, referred to as sputtering, are calculated for a number of values of the momentum‐transfer cross section between the ions and the SO2 molecules. The calculations include UV heating and local thermodynamic equilibrium infrared cooling. It was found that the energy removed by sputtering becomes a significant fraction of the plasma energy deposited for the largest cross sections used, affecting the exobase temperature, but the exobase altitude and density change very little, ∼1.12–1.13 Io radii and 0.8–1.2 × 108 molecules cm−3. The calculated sputtering yields are compared to results from analytic expressions [Johnson, 1994a]. As the interaction cross section of the incident ion with an atmospheric molecule is decreased, so that the penetration depth increases, the dependence of the yield on the momentum‐transfer cross section approaches that calculated using the analytic model. The ejecta were found to have an energy spectra consistent with the analytic model, but the angular distribution of the ejecta and the incident angle dependence can differ significantly depending on cross‐section size. The results are used to reevaluate the multiple collision contribution to the sputter loss from Io.
A Heinrich‐like event, H‐0 (DC‐0): Source(s) for detrital carbonate in the North Atlantic during the Younger Dryas Chronozone
Authors: Andrews J. T. , Jennings Anne E. , Kerwin M. , Kirby M. , Manley W. , Miller G. H. , Bond G. , MacLean B.
Published: February 2004

In the North Atlantic we define H‐0 as a Heinrich‐like event which occurred during the Younger Dryas chron. On the SE Baffin shelf prior to 11 ka, surface water productivity was reasonably high, as measured by the numbers of diatom and planktic foraminifera per gram, but an abrupt increase in detrital carbonate (DC‐0 event) (from approximately 15% up to 50% carbonate by weight) occurred at 11 ± 14C ka and continued to circa 10 ka. These deposits, 2–6 m thick, are dominated by detrital calcite and silt‐ and clay‐sized sediments. During this event (DC‐0/H‐0), ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over Labrador‐Ungava. As a consequence, the pattern of ice‐rafted debris and sediment provenance shown by H‐O in the North Atlantic is different from that during H‐1 (14.5 ka) or H‐2 (20 ka) when the ice sheet extended along the axis of Hudson Strait and may have reached the shelf break; for example, there is no concrete evidence for DC‐O is cores on the floor of the Labrador Sea due east of Hudson Strait (HU75‐55,‐56), but H‐O has been noted in cores off Newfoundland and west of Ireland. A coeval carbonate event to DC‐0, but this one dominated by dolomite, occurs in HU82‐SU5 on the west side of Davis Strait with a source either from northern Baffin Bay or Cumberland Sound. Although other sources for North Atlantic detrital carbonate cannot be totally excluded, our evidence suggests that H‐0 represents the expression of glaciological instability of the Laurentide Ice Sheet within the general region of Hudson Strait and probably to the north (Cumberland Sound and northernmost Baffin Bay). There is one younger DC event, dated circa 8.4 ka, present in sediments along the Labrador margin and in Hudson Strait, which represents the final collapse of the ice sheet within Hudson Strait and Hudson Bay.
Evaluating the success of astronomical tuning: Pitfalls of using coherence as a criterion for assessing pre‐Pleistocene timescales
Authors: Shackleton N. J. , Hagelberg Teresa King , Crowhurst S. J.
Published: February 2004

The imprint of orbital variations on the geological record of climatic variability is well documented, especially for the Plio‐Pleistocene. There is considerable interest in developing very high resolution timescales through the Cenozoic and into the Mesozoic by tuning geological records to the assumed astronomical forcing. Since the precession signal is so highly amplitude modulated, it is widely believed that high coherence between record and assumed forcing in the precession band is an indication that the timescale is probably correct, because coherence is supposed to provide a measure of the degree of common amplitude modulation. We show that this is misleading; even a sinusoidal variation that has been “tuned” to an insolation record shows highly significant coherence at the 23‐kyr and 19‐kyr precession frequencies. Coherence is a good indication that the tuning has generated a consistent phase relationship, but complex demodulation is a better tool for assessing the relationship between amplitude modulation in the data and in the hypothetical forcing.
Environmental control of living symbiotic and asymbiotic foraminifera of the California Current
Authors: Ortiz J. D. , Mix A. C. , Collier R. W.
Published: February 2004

Plankton tows from the northern California Current constrain biological and physical influences on living planktonic foraminifera. In this region, the dominant factors controlling the size and distribution of symbiotic and asymbiotic species are light and food. Food decreases offshore. Light, needed for symbiont photosynthesis, increases offshore as water turbidity lessens. Asymbiotic foraminifera (e.g., right‐coiling Neogloboquadrina pachyderma, Globigerina quinqueloba, and Globigerina bulloides), which survive by grazing, dominate the coastal fauna. The most abundant of these species, right‐coiling Neogloboquadrina pachyderma, did not change in size in response to increasing food. Species that benefit from symbiont photosynthesis (Orbulina universa, Neogloboquadrina dutertrei, Globigerinoides ruber, and Globigerinita glutinata) dominate the offshore fauna. Individuals of these species are rare and have smaller shells in turbid waters where light is limited. G. ruber, which is near its thermal tolerance limit of ≈14°C, is the only species to demonstrate a clear temperature response. Although temperature may control a foraminiferal species' distribution near the limits of its thermal tolerance, food and light appear to provide the primary control under more favorable thermal conditions. We infer that gradients in food and light can result in quantifiable sedimentary patterns related to oceanic productivity through changes in plankton biomass and turbidity.
The marine record of deglaciation from the continental margin off Nova Scotia
Authors: Keigwin L. D. , Jones G. A.
Published: February 2004

Continental margin sediments off Nova Scotia accumulate at high rates (up to 360 cm kyr−1) and contain a history of millennial‐scale environmental changes which are dominated by the proximity of the Laurentide ice sheet during the latest Quaternary. Using stable isotope ratios of oxygen, accelerator mass spectrometer radiocarbon dating, micropaleontology, and sedimentology, we document these changes in six piston cores ranging in water depth from ∼450 to ∼4300 m. We find that maximum δ18O in N. pachyderma occurred about 15 ka and preceded the maximum abundance of this species in these cores by ∼1000 years. Between 13 and 14 ka we find a second peak in abundance of N. pachyderma, minimum δ18O, and two pulses of ice rafting. The sediment lithology supports terrestrial studies which indicate that there was a general withdrawal of ice beyond the upper Paleozoic and Mesozoic red beds by 14 ka in southeastern Canada, so the ice rafting events between 13 and 14 ka probably reflect ice stream activity in the St. Lawrence valley. The Younger Dry as event is recognized as a peak in abundance of N. pachyderma and ice rafting (dated as ∼11.3 ka), but meltwater discharge to the Gulf of St. Lawrence was either too small or occurred over too long a time to leave a distinct δ18O minimum off Nova Scotia. At 7.1 ka, in the middle of Holocene warming, we find a third peak in abundance of N. pachyderma and another δ18O minimum but no ice rafting. We interpret these data as evidence of a late‐occurring meltwater event which, if correct, could have originated in the Great Lakes, in the Labrador‐Ungava region, or in both. The final millennial‐scale phenomenon off Nova Scotia is the onset of “Neoglaciation,” marked by increased ice rafting and increased % N. pachyderma beginning about 5 kyr ago.
Geochemical tracers of mantle processes
Authors: White William M.
Published: February 2004

The mantle can be sampled directly only very rarely. Geochemists have thus come to rely heavily on mantle‐derived magmas to study the composition and evolution of the mantle. Only those compositional features that are unaffected by magmatic processes are useful as tracers of mantle processes. These include radiogenic isotope ratios such as those of He, Sr, Hf, and Os, stable isotope ratios, and ratios of highly incompatible elements or elements of similar incompatibility, such as Ba/Nb or Pb/Ce. The term “incompatible” denotes a preference of the element for a melt over mantle minerals. Highly incompatible elements will partition entirely into the melt under most circumstances, so that the ratio of two such elements in a basalt will be virtually identical to that ratio in its source. This is also true to a lesser degree of ratios such as La/Sm and Zr/Nb, as Zr and Sm are not highly incompatible elements.
Thermospheric connections
Authors: Hagan M. E.
Published: February 2004

Many people believe that the region of space familiar for visions of astronauts floating about their work outside a space shuttle cargo bay is empty, nothing, a vacuum. Contrary to popular opinion, the Earth's atmosphere extends even beyond that place where shuttle crews release, repair, or retrieve satellites. Despite its comparative anonymity, this remote layer of the atmosphere, known as the thermosphere, contributes to the protection of life on Earth. This is humankind's fundamental connection to the thermosphere. For that reason alone, it is deserving of our attention. The thermosphere is, in turn, affected by our activities. It is physically connected to the sun, Earth, and the remainder of Earth's atmosphere. Important insight into this complicated physical system frequently emerges when thermospheric scientists connect their individual research interests. This report is about all of these thermospheric connections.
Plate motions and crustal deformation
Authors: DeMets Charles
Published: February 2004

Over the past 30 years, the plate tectonic paradigm has revolutionized the earth sciences through its powerful yet elegant quantitative treatment of large‐scale crustal deformation, and through its conceptually integrative role within the earth sciences. This review summarizes several important developments in plate tectonic research for the period 1991 to early 1994, with emphasis placed on publications that have either significantly improved our understanding of plate motions and their relation to crustal deformation or have raised significant new questions. Revisions to the geomagnetic reversal time scale, Cenozoic plate motions, microplate kinematics, oblique convergence and deformation of forearcs, and kinematic information derived from detailed bathymetric and side‐scan sonar studies are featured.
Plate tectonics and the evolution of climate
Authors: Sleep Norman H.
Published: February 2004

Recent work has shown that climate, life, and tectonics interact in ways that are sometimes profound yet not obvious. I review two better understood terrestrial examples in this paper. First, the development of compressional orogens is strongly influenced by climate in that the extent of erosion determines whether high or low mountain ranges develop. Second, global geochemical processes are strongly influenced by tectonics and climate. For example, the build‐up of oxygen in the Earth's atmosphere has been aided by deposit of organic‐rich sediments in new ocean basins which prevented reaction of the oxygen with the reduced products of photosynthesis. The volume of the ocean depends on the extent to which water is subducted within the oceanic crust and sediments and on the extent which this water is returned to the surface at island arcs. Studies of island arc lavas erupted in deep water and 10Be indicate that water and sediments are actually subducted to great depths and are a significant component in island arc volcanics.
Advances in aerogeophysics and precise positioning: Gravity, topography, and high resolution applications
Authors: Bell Robin E.
Published: February 2004

Airborne magnetic surveys have long been a primary tool for geologic mapping, resource exploration, hazard evaluation and tectonic studies. Recent advances in precise positioning have enabled other geophysical tools such as laser and radar altimeters and the gravity meter to be mounted on airplanes and contribute to geologic and environmental studies. The principal advantages of airborne geophysical surveys over ground based studies result from enhanced experiment design including; improved access to remote and contaminated areas, optimized sampling, rapid sampling rate and tremendous potential for interdisciplinary studies.
Solar energetic particles: A paradigm shift
Authors: Reames Donald V.
Published: February 2004

Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MarylandThe first evidence of high‐energy particles from the Sun was obtained 50 years ago when Forbush [1946] used sea‐level ion chambers to study the large solar events of February and March of 1942. Over the next 20 years, observation of these solar energetic particle (SEP) events using neutron monitors and riometers (that measure radio opacity of the ionosphere) and, later, with detectors on balloons and satellites, led to an extensive body of knowledge on the time profiles, spectra and particle abundance in the large events. Meanwhile, there was already a rich history of the study of solar flares spanning 100 years since the first observations reported by Carrington [1860]. With no knowledge of the existence of coronal mass ejections (CMEs) [Kahler 1992], it was tempting to assume that the particle acceleration somehow occurred in spatial and temporal conjunction with the solar flare itself. Thus the “solar flare myth” [Gosling 1993] of particle acceleration began nearly 30 years ago.
Recent advances in flood frequency analysis
Authors: Bobée Bernard , Rasmussen Peter F.
Published: February 2004

Research on flood frequency analysis has taken place with varying intensity over the last couple of decades. The eighties proved to be important years with many significant contributions, reviewed for instance byGreis[1983],Potter[1987],Kirby and Moss[1987],Cunnane[1987],NRC[1988],WMO[1989], andBobée and Ashkar[1991]. Due to its large economical and environmental impact, flood frequency analysis remains a subject of great importance and interest, and the research on improved methods for obtaining reliable flood estimates has continued into the nineties, although with different emphasis. In the seventies and eighties much effort was spent on developing efficient at‐site flood frequency procedures. New distributions and estimation methods were introduced in the hydrologic journals, some of them developed specifically for flood frequency analysis. It seems that this tendency has decelerated somewhat in the beginning of the nineties. Researchers are increasingly realizing that the lack of sufficiently long data series imposes an upper limit on the degree of sophistication that can reasonably be justified in at‐site flood frequency analysis. It has been emphasized by many that instead of developing new methodologies for flood frequency analysis, effort should be spent on comparing existing ones and on looking for other sources of information [Potter, 1987;Bobée et al, 1993a]. Regionalization is probably the most viable avenue for improving flood estimates, and fortunately this seems to be the direction that the research in flood frequency analysis has taken in the nineties.
Recent advances in watershed modelling
Authors: Hornberger George M. , Boyer Elizabeth W.
Published: February 2004

Creation of, application of, and animated discussion of mathematical models of watershed processes have been favorite pastimes of hydrologists for many decades. The original impetus for the development of watershed models was engineering design—the need to estimate properties of the hydrograph to be expected from a given sequence of storms. In recent years, die main driving force for development and refinement of watershed models has shifted. In addition to long‐standing issues such as flood forecasting, the hydrological community now is involved very much in interdisciplinary scientific issues. Some of these critical and emerging areas in hydrological science were summarized by a committee of the National Research Council (NAS1991;Eagleson1991). Examples pertinent to watershed science include the need for improved understanding of: (1) how the flow of water through and storage of water within a catchment affects the chemical composition of water in streams, soils, and underlying rocks (e.g., seeTurner et al. 1990); (2) how the biophysical mechanisms that determine evapotranspiration from catchments can be parameterized (e.g.,Band1993); and (3) how space and time scales are linked in processes that convert rainfall and snowmelt into runoff (e.g.,Gupta et al. 1986). An ingredient common to these and other examples of “emerging areas” is that spatial and temporal variability in processes are of critical importance.
Mantle dynamics: Influence of the transition zone
Authors: Tackley Paul J.
Published: February 2004

The question of whether the mantle convects as a single layer, or whether the convection is split vertically into two or more layers, is perhaps the most fundamental unanswered question concerning the structure and dynamics of the mantle, and has ramifications throughout solid Earth geosciences. The most likely mechanism for enforcing layered convection is the action of the major phase transitions of the transition zone, and in particular, the strong endothermic phase transition between γ‐spinel and perovskite+magnesiowiistite that occurs at around 660 km depth. Because this transition is endothermic, it is deflected downwards in cold downwellings, and upwards in hot upwellings, resulting in a mass anomaly that impedes the penetration of the downwelling or upwelling. In the past few years, there has been a huge resurgence of interest and modeling activity devoted to studying the influence of the 400 and 660 km phase transitions, and the effects identified in these studies may provide the key to reconciling various seemingly contradictory geophysical observations.
Recent advances in statistical methods for the estimation of sediment and nutrient transport in rivers
Authors: Cohn T. A.
Published: February 2004

This paper reviews advances in methods for estimating fluvial transport of suspended sediment and nutrients. Research from the past four years, mostly dealing with estimating monthly and annual loads, is emphasized. However, because this topic has not appeared in previous IUGG reports, some research prior to 1990 is included. The motivation for studying sediment transport has shifted during the past few decades. In addition to its role in filling reservoirs and channels, sediment is increasingly recognized as an important part of fluvial ecosystems and estuarine wetlands. Many groups want information about sediment transport [Bollman, 1992]: Scientists trying to understand benthic biology and catchment hydrology; citizens and policy‐makers concerned about environmental impacts (e.g. impacts of logging [Beschta, 1978] or snow‐fences [Sturges, 1992]); government regulators considering the effectiveness of programs to protect in‐stream habitat and downstream waterbodies; and resource managers seeking to restore wetlands.
Convectively induced mesoscale weather systems in the tropical and warm‐season midlatitude atmosphere
Authors: Smull Bradley F.
Published: February 2004

As anticipated by Nelson [1991] in the last U.S. National Report, mesoscale meteorology has continued to be an area of vigorous research activity. Progress is evinced by a growing number of process‐oriented studies capitalizing on expanded observational capabilities, as well as more theoretical treatments employing numerical simulations of increasing sophistication. While the majority of papers within the scope of this review fall into the category of basic research, the field's maturation is evident in the emergence of a growing number of applications to operational weather forecasting.

Even as our ability to anticipate shifts in synoptic scale upper‐air patterns and associated baroclinic developments has steadily improved, lagging skill with regard to quantitative forecasts of precipitation—particularly in situations where deep moist convection is prevalent—has sustained research in warm‐season mesoscale meteorology. Each spring and summer midlatitude populations are exposed to life‐threatening natural weather phenomena in the form of lightning, tornadoes, straight‐line winds, hail, and flash floods. This point was driven home during the summer of 1993, when an extraordinarily persistent series of mesoscale convective systems (MCSs) led to unusually severe and widespread flooding throughout the Mississippi and Missouri river basins. In addition to this obvious impact on regional climate, the 1990's have brought an increased appreciation for the less direct yet potentially significant role that tropical convection may play in shaping global climate through phenomena such as the El Niño‐Southern Oscillation (ENSO).
Seismic wave theory in the US: 1991–1994
Authors: Park Jeffrey
Published: February 2004

The early 1990s saw many interesting contributions to the field of seismic wave propagation from a variety of US researchers, even as many wave‐propagation specialists shifted their research focus away from new methods for seismogram synthesis toward data acquisition and inversion. In part, this shift was a natural consequence of the explosive growth in the amount and availability of high‐quality digital seismic data, ready to be processed with interpretation tools developed in previous decades. As a result, observational seismology has recently made dramatic contributions toward imaging global geodynamics, revealing the great variety of crustal structures, and laying the foundation for a collaborative global network of broadband seismometers to monitor earthquakes and other seismic events. However, recent observational studies have brought to light new problems in wave propagation theory, and have made some older problems more difficult to ignore.
Recent advances in nonparametric function estimation: Hydrologic applications
Authors: Lall U.
Published: February 2004

Nonparametric function estimation refers to methods that strive to approximate a target function locally, i.e., using data from a “small” neighborhood of the point of estimate. “Weak” assumptions, such as continuity of the target function and its differentiability to some order in the neighborhood, rather than an a priori assumption of the global form (e.g., linear or quadratic) of the entire target function are used. Traditionally, parametric assumptions (e.g., hydraulic conductivity is log normally distributed, floods follow a log Pearson III (LP3) distribution, annual stream flow is either log normal or gamma distributed, daily rainfall amounts are exponentially distributed, and the variograms of spatial hydrologic data follow a power law) have dominated statistical hydrologic estimation. Applications of nonparametric methods to some classical problems (frequency analysis, classification, spatial surface fitting, trend analysis, time series forecasting and simulation) of stochastic hydrology are reviewed.
There is more to climate than carbon dioxide
Authors: Walker James C.G.
Published: February 2004

Discussion of climate change on a range of time scales has tended to focus on carbon dioxide and a changing greenhouse effect. Because carbon dioxide couples climate to ocean, land, and biota, it has appealed to scientists with an interest in the whole Earth system. Carbon dioxide has left a geological record in fossils, isotopes, and sediments, so we can reasonably expect to reconstruct its history. While important questions of detail remain to be resolved, many published applications of carbon cycle modelling suggest that we understand the biogeochemical cycles of carbon well enough to estimate carbon dioxide concentrations in the past and the future. Furthermore, we have an excellent instrumental record of recent changes in atmospheric carbon dioxide. While these considerations make carbon dioxide attractive to paleoclimatologists, they do not necessarily make it a major component of climate change. I shall argue in this paper that clouds deserve much more attention than they have been getting.
Ground‐based observations of Io
Authors: Matson Dennis L. , Johnson Torrence V. , Blaney Diana L. , Veeder Glenn J.
Published: February 2004

Ground‐based observations are providing new information about the volcanic phenomena at Io's surface. Thermal emission from lava can be seen routinely at infrared wavelengths. One result of recent work is the reinvigoration of a familiar theme — silicate volcanism. In the following paragraphs we will focus on such advances which have resulted from a better understanding of Io's thermal emission. This emission tells us about ongoing volcanic processes and heat flow. Io's total heat flow is especially important because of the tidal interactions among the bodies in the jovian system. The value of this heat flow not only constrains models for Io's interior but also those for Jupiter and for the long‐term orbital evolution of the whole system.
Global sea level change: Determination and interpretation
Authors: Douglas Bruce C.
Published: February 2004

The notion of sea level rise brings to the popular mind the specter of deep inundation of coastal regions. One pictures skyscrapers emerging from the waters like so many sleeping flamingos standing in the shallows of a lake. Of course if all of the world's ice sheets suddenly melted or collapsed, this vision would apply to New York City and its coastal counterparts. But there is a general consensus that such a calamity is not an immediate threat [Houghton et al, 1990]. The actual situation for the recent historical past and near future appears to be more benign, but with nonetheless extremely significant, even devastating impacts due to erosion and flooding of coastal areas.