bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of shear load on frictional healing in simulated fault gouge
Authors: Karner Stephen L. , Marone Chris
Published: February 2004

We report on frictional strengthening (healing) in granular quartz gouge as a function of time of true stationary contact. To distinguish between the slip‐dependent [Ruina, 1983] and time‐dependent [Dieterich, 1979] friction constitutive laws, we designed tests similar to conventional slide‐hold‐slide (SHS) tests except that shear load was completely removed prior to holds. We find large healing values (0.033–0.054 for holds of 10¹–10² s) compared with quasi‐static SHS experiments, and our data indicate time‐dependent weakening in contrast with strengthening observed from SHS tests. Gouge layer compaction increases with increasing hold time, comparable to observations from conventional SHS tests. Our data indicate that purely time dependent processes have a minor influence on healing under the conditions studied and/or that such effects are efficiently erased by particle rearrangement during removal/reapplication of shear load. The data are not adequately described by either the time (Dieterich) or slip (Ruina) dependent state evolution laws.
A theory for AKR fine frequency structure
Authors: Yoon Peter H. , Weatherwax A. T.
Published: February 2004

An alternative explanation of the fine frequency structure (≤1 kHz) associated with auroral kilometric radiation (AKR) is presented. It is based on a realistic model of the electron distribution function inside the AKR source region. The resulting cyclotron maser instability growth rate is shown to possess an extremely narrow bandwidth, of the order Δω/ω ∼ 10−3, which may naturally account for the AKR fine frequency structure. The results differ from many previous calculations and simulations which relied on simpler electron distribution function models such as a loss‐cone, ring‐beam or DGH distribution.
The role of root distribution for climate simulation over land
Authors: Zeng Xubin , Dai Yong‐Jiu , Dickinson Robert E. , Shaikh Muhammad
Published: February 2004

A comprehensive global root database is used to derive vertical root distribution and rooting depth for various vegetation categories in one of the most widely‐used land models; i.e., the Biosphere—Atmosphere Transfer Scheme (BATS). Using a variety of observational datasets, observed root distribution is found to significantly improves the offline simulation of surface water and energy balance. Global climate modeling further demonstrates that observed root distribution primarily affects latent heat flux and soil wetness over tropical and midlatitude land, respectively.
Modification of the intermediate waters in the northeastern subpolar Pacific
Authors: Aydin Murat , Top Zafer , Fine Rana A. , Olson Donald B.
Published: February 2004

The modification of the intermediate layers in and around the Alaskan Gyre are investigated in relation to general circulation features of the region, using tracer data from the World Ocean Circulation Experiment P17N line. The Alaskan Gyre (AG) core lies between the North Pacific Current and the Alaskan Stream (AS) in the 52–54.5°N range and to the west of 144°W. The North Pacific Current feeds the Transition Zone (TZ) and the Western Subpolar Waters (WSW), which consequently constitute the inflow to the region from the northwestern Pacific. The Alaska Current (AC) is observed to the west of 144°W, between the North American coast and the AG. The AC halocline (σθ < 26.7) has distinct property distributions (low chlorofluorocarbon and oxygen saturations and high nutrient concentrations), which are correlated with eddies that are characterized by higher salt content and warmer temperatures (most significantly at 26.0 σθ). The AG is found to act as a reservoir where water mass modification takes place and the AS transports a mixture of waters from the AG and the AC west‐ward out of the region. The residence time estimate for the intermediate and upper layer water in the AG (σθ ≤ 27.2) is about 2 years, which corresponds to a volume exchange between the gyre and the surrounding circulation of 4.7 Sv in the 0–1000 m range. The layers above 26.9 σθ (∼0–200 m) are modified by vertical mixing mostly of WSW in the AG; the flux of modified waters out of the AG is approximately 0.9 Sv. Comparison of the present data with the 1984–1989 period suggests that mixing of water from different regimes has not changed dramatically.
Dynamics of transport of “Atlantic signature” in the Arctic Ocean
Authors: Nazarenko Larissa , Holloway Greg , Tausnev Nickolay
Published: February 2004

Understanding the circulation of the Arctic is the basis for considering questions relating to transport of contaminants from all sources. In this study a numerical ice‐ocean model has been used to address problems of the dispersion and transport pathways of contaminants in the water column. We investigate sensitivity of the contaminant distribution to parameterization of sub‐grid‐scale eddies and to numerical representation of tracer advection. An implementation of flux‐corrected transport helps preserve tracer properties along their advective paths. Eddy parameterization is found to be important insofar as eddy‐topography interactions contribute to propelling and sustaining narrow boundary currents. These boundary currents, cyclonic around the Arctic and each of its subbasins, are the main vehicle that transports Atlantic water properties. Consideration of ice and ocean models together does not lead to significant alterations in distribution of Atlantic properties. Although inclusion of sea ice affects circulation of the upper (halocline) layers, the main transport of Atlantic properties occurs in deeper layers, which are much less affected by ice processes.
Seasonal variation of the turbopause: One year of turbulence investigation at 69°N by the joint University of Tromsø/University of Saskatchewan MF radar
Authors: Hall C. M. , Manson A. H. , Meek C. E.
Published: February 2004

Following upgrades to the University of Tromsø/University of Saskatchewan MF radar, located in northern Norway at 69°N, 19°E, we have been able to complete a full calendar year of estimates of mesospheric turbulent intensity. The results represent the first such study using continuous measurements from this region, and temporal and height variations are satisfyingly in accordance with expectation. Since in the past there has been a degree of disagreement as to absolute intensities, we briefly compare our results with some from totally independent methods. The resulting dissipation rates, to be regarded as maxima for the turbulent dissipation, are used to identify an upper limit to the turbopause. The Arctic turbopause appears to exhibit an annual variation, being lower in the summer, in agreement with Danilov et al [1979] but refuting Blum and Schuchard [1918].
A numerical study on the coupling between sea surface temperature and surface evaporation
Authors: Tsintikidis Dimitris , Zhang Guang J.
Published: February 2004

The feedback between sea surface temperature (SST) and surface evaporation is an important issue in the study of climate change. To understand this feedback and its interaction with surface wind in the tropical Pacific Ocean (30°N – 30°S), and in particular over the warm pool region, a dynamic tropical atmospheric circulation model is used. The model consists of a two‐layer free troposphere and a well‐mixed boundary layer. It involves active interactions between the boundary layer flow, forced by an SST gradient, and the free atmospheric flow, forced by SST. Various SST fields (representing climatology, El Niño, and La Niña conditions) are used to drive the model. It is found that the binned averages of evaporation and wind speed increase with SST for up to about 300–301 K. From that point on they decrease with SST. In addition, negative SST anomalies correspond to excess latent heat flux and wind speed. These results are in agreement with relevant observations. To understand the thermodynamic versus dynamic effects of SST on surface evaporation, in one of the experiments we impose a sudden positive SST perturbation on the climatological SST field during the model integration. It is shown that while surface evaporation is initially enhanced in response to the SST change, as the atmospheric circulation gradually “feels” the SST perturbation, its dynamic effect through the circulation change becomes more apparent over the SST perturbation region. Overall, the results of our study show that in the low SST regime the behavior of evaporation is dictated by thermodynamics, whereas in the high SST regime it is dictated by atmospheric dynamic considerations.
Retrieval of upper stratospheric and mesospheric temperature profiles from Millimeter‐Wave Atmospheric Sounder data
Authors: von Engeln A. , Langen J. , Wehr T. , Bühler S. , Künzi K.
Published: February 2004

The Millimeter‐Wave Atmospheric Sounder (MAS) is a shuttle‐based instrument, observing the atmosphere in limb sounding geometry. Temperature information is derived from three oxygen lines near 60GHz. Temperature profiles in the altitude range from 30 to 90 km can be measured. The data evaluation is done with the aid of an atmospheric radiative transfer model. Usually, the radiation is assumed to be unpolarized, so that spectral power is a scalar quantity. This assumption is not valid for oxygen lines that show Zeeman splitting in the Earth's magnetic field. A more general form of the radiative transfer in which spectral power is represented by a vector has to be used in this case. The obtained information depends on the magnetic field. Using the MAS instrument characteristics, we investigate the possible temperature information that can be obtained from observation of polarized O2 emission lines with a spaceborne instrument. Synthetic retrievals show that the obtained accuracy of the retrieved temperature profile is below 3 K from 30 to 55 km and below 6 K for altitudes up to 90 km. The resolution is 4 km for altitudes of 30 to 50 km and 10 km above. The magnetic parameters influence the accuracy of the obtained profile by as much as 2 K. The obtained MAS temperature profiles were validated by a comparison to data from the Upper Atmosphere Research Satellite (UARS). Three UARS instruments were chosen: Microwave Limb Sounder, Improved Stratospheric and Mesospheric Sounder, and the Cryogenic Limb Array Etalon Spectrometer. All comparisons show similar features for the stratospheric and mesospheric temperatures.
Symmetries of volcanic distributions on Mars and Earth and their mantle plume dynamics
Authors: Matyska Ctirad , Yuen David A. , Breuer Doris , Spohn Tilman
Published: February 2004

The symmetries in the distribution of major volcanic centers on Mars have been analyzed by employing a technique involving the angular self‐similarity of point fields. The distribution of all central vent volcanoes exhibits an axisymmetry along with a reflecting symmetry about a plane whose normal is the axis of axisymmetry. The latter axis is skewed by about 30° from the rotational axis and is suggested to represent the early axis of symmetry of the interior dynamics. The high level of symmetry of Mars' volcanic distribution may be the result of a focusing of seismic energy from a major impact in the Hellas area that formed the Hellas basin and may have triggered the onset of volcanic activity in the greater Tharsis area. If the highland paterae near the Hellas region are not taken into account in the analysis, an asymmetry with respect to the reflecting plane appears. This is probably representative of the Late Hesperian and Amazonian, when the once global volcanic activity had retreated to the Tharsis province. These angular symmetries suggest a simple pattern of long‐wavelength axisymmetric martian mantle convection involving a large stationary plume, as shown by our three‐dimensional (3‐D) spherical‐shell simulations. On the basis of these calculations, we venture to propose that the volcanic history of Mars reflects the transformation of a bipolar mantle plume structure into a single megaplume by the process of plume‐plume collision acting in concert with the deep Martian phase transitions. This transformation might have overcome the original pattern induced by a major impact forming the Hellas basin. We have repeated the procedure for the Earth's hotspot distribution, which revealed both axisymmetry and a certain level of reflecting symmetry. This similarity between the two planets is striking. The symmetries of the Earth's hotspot distribution are probably due to the interaction of the lower mantle megaplumes with the endothermic perovskite to γ‐spinel phase transition.
Ice chemistry on the Galilean satellites
Authors: Delitsky Mona L. , Lane Arthur L.
Published: February 2004

Jupiter's icy satellites Europa, Ganymede, and Callisto are subject to energy fluxes from electrons, photons, and magnetospheric plasma ion bombardment. As water ice and CO2 ice are thought to be present on their surfaces, the radiolysis of these materials over time should produce more complicated CHO‐containing molecules. These may include CH3OH, H2CO, CH2CO, C3O2, HCOOH, CH3COOH, H2CO3, HCOOCH, (CH3)2CO, CH3CH2OH, HOCH2CH2OH, polymeric C3O2, and polymeric H2CO. The water fragmentation products OH, O2, HO2, H2O2, and O3 should also be produced. The molecules formed should be detectable with ground‐based intruments because of their many active infrared bands. Another product produced is CO, which will have a high vapor pressure over a Galilean satellite surface at typical temperatures. The vapor pressure of CO at the nighttime temperature of 70 K could be as high as 150 mbar. Ganymede's unique dipolar magnetic field should induce more chemistry in its polar regions due to the focusing of radiation to higher latitudes. The observed lack of leading/trailing asymmetry in its SO2 absorption correlates with this redirection of plasma ions toward the poles. The observance of O atom emissions at high latitudes by Hall et al. [1998] is also consistent with this picture. The ratio of plasma energies directed to the poles to those directed to the equatorial regions is ∼4.
Lithospheric controls on late Cenozoic construction of the Mongolian Altai
Authors: Cunningham W. Dickson
Published: February 2004

The Altai is one of the great Cenozoic intracontinental and intraplate mountain ranges of central Asia, but it is one of the less studied mountain belts on Earth from a modern structural geology standpoint, and few western scientists are familiar with its tectonic evolution. The range is located dominantly in Mongolia with important sectors in Russia, China, and Kazakhstan and structurally links with the Chinese Tien Shan and Russian Sayan ranges. The Altai is tectonically active and is best understood as two kinematically distinct mountain belts that intersect at 46°N, 96°E: the right‐lateral transpressive western Altai and the left‐lateral transpressive Gobi Altai. Transpressional deformation dominates the late Cenozoic deformation of the Altai and is manifested by throughgoing strike‐slip faults, restraining bends, thrust fault‐bounded ranges linked by strike‐slip faults, and possibly inverted Mesozoic graben. Regionally, there is strong correlation between Cenozoic fault trends and older basement strike trends. Cenozoic deformation regimes appear to be dictated by the angular relationship between preexisting basement structural trends and the prevailing NE directed maximum compressive stress. The Altai is constructed on Paleozoic terranes dominantly consisting of arc and subduction complex assemblages and passive margin sedimentary rocks juxtaposed to the west, southwest and south of the Precambrian block that underlies the Hangay Dome area of central Mongolia. Because the Hangay Dome is characterized by diffuse extension and not transpression, regionally upwarped topography, late Cenozoic volcanism, and elevated heat flow, it is kinematically separate from the Altai region and has previously been interpreted to overlie a mantle plume or asthenospheric diapir. It is proposed that anomalously hot mantle beneath the Hangay Dome coupled with the regionally domed surface topography creates a radial horizontal stress field that acts against the regional NE directed maximum horizontal stress within the Altai. These conditions may deflect postulated NE flowing lithospheric mantle/lower crust that is believed to be driving the shallow upper crustal deformation in the Altai.
A new multilayered model for intraplate stress‐induced differential subsidence of faulted lithosphere, applied to rifted basins
Authors: van Balen R. T. , Podladchikov Y. Y. , Cloetingh S. A. P. L.
Published: February 2004

In‐plane horizontal stresses acting on predeformed lithosphere induce differential flexural vertical motions. A high‐precision record of these motions can be found in the sedimentary record of rifted basins. Originally, it was proposed that rifted basins experience flank uplift and basin center subsidence in response to a compressive change of in‐plane stress, which agrees well with observed differential motions. Subsequently published models predicted that the vertical motions may be opposite because of the flexural state of the lithosphere induced by necking during extension. However, the total, flexural and permanent, geometry of the lithosphere underlying the rifted basin is the controlling parameter for the in‐plane stress‐caused vertical motions. The largest part of this preexisting geometry is caused by faulting in the uppermost brittle part of the crust and ductile deformation in the underlying parts of the lithosphere. We present a new multilayered model for stress‐induced differential subsidence, taking into account the technically induced preexisting geometry of the lithosphere, including faults in the upper crust. As continental lithosphere may exhibit flexural decoupling due to a weak lower crustal layer, the new multilayer in‐plane stress model discriminates the geometries of the separate competent layers. At a basin‐wide scale, the new model predicts that a compressive change of in‐plane force results in basin center subsidence and flank uplift, confirming the original hypothesis. Compared to all previous models, the new model requires a lower horizontal stress level change to explain observed differential vertical motions.
Strike‐slip movements and thrusting along a transpressive fault zone: The North Giudicarie line (Insubric line, northern Italy)
Authors: Prosser Giacomo
Published: February 2004

This paper analyzes the kinematic evolution and the deformation partitioning within an important transpressive fault zone located in the central part of the Alpine chain. The North Giudicarie line is a NNE trending fault which offsets the dextral Insubric line with an apparent left‐lateral displacement of about 70 km. The main fault plane of the North Giudicarie line dips about 35°–45° to the NW. The footwall is characterized by N‐S striking strike‐slip faults, which reactivate extensional faults of Early Jurassic to Late Cretaceous age. The early deformation history of the North Giudicarie line is revealed by basement‐and limestone‐mylonites. Shear sense of mylonites indicates on average top‐to‐the‐east thrusting. These movements took place during the late Oligocene‐early Miocene, when the Insubric line was active as a right‐lateral strike‐slip fault. Therefore, in this time span the North Giudicarie line can be interpreted as a dextral transpressive bend of the Insubric line. Mylonites have later been overprinted by brittle faults related to top‐to‐the‐SE thrusting of middle‐late Miocene age. During this event the shape of the Insubric line was strongly modified by left‐lateral transpression along the Giudicarie fault zone. Deformation was partitioned between prevailing compression along the Giudicarie line and left‐lateral strike‐slip movements along the N‐S striking faults. These faults transferred the strike‐slip component of the Giudicarie line into a wider area of the central southern Alps.
Accurate calculation of specific discharge in heterogeneous porous media
Authors: Zhou Quanlin , Bensabat Jacob , Bear Jacob
Published: February 2004

Existing continuous schemes for computing the specific discharge in heterogeneous porous media result in inaccuracy in simulating the local specific discharge field at and in the vicinity of an interface between subdomains of different material properties. The domain decomposition method used in multilayered systems and a consistent scheme in a randomly heterogeneous domain are applicable only to some particular cases, such as when the material interfaces are parallel to an axis. Along a material interface, the law of refraction of streamlines, related to both the tangential and the normal specific discharge components, should be satisfied. By incorporating these interface conditions into the weak formulation of Darcy's law, two consistent numerical schemes have been developed. These schemes can be applied to accurately and efficiently simulate the specific discharge field in a heterogeneous domain with different values of the ratio of hydraulic conductivities between adjacent elements. In particular, they provide an easy way for treating aquifers with semipervious layers as a single system. Numerical simulations have been used to demonstrate the capability and accuracy of the proposed schemes. Comparison between the specific discharge fields computed by the proposed schemes and by other techniques shows that the new schemes are superior to the traditional ones.
Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges
Authors: Schmidt Michael W. I. , Noack Angela G.
Published: February 2004

This review highlights the ubiquity of black carbon (BC) produced by incomplete combustion of plant material and fossil fuels in peats, soils, and lacustrine and marine sediments. We examine various definitions and analytical approaches and seek to provide a common language. BC represents a continuum from partly charred material to graphite and soot particles, with no general agreement on clear‐cut boundaries. Formation of BC can occur in two fundamentally different ways. Volatiles recondense to highly graphitized soot‐BC, whereas the solid residues form char‐BC. Both forms of BC are relatively inert and are distributed globally by water and wind via fluvial and atmospheric transport. We summarize, chronologically, the ubiquity of BC in soils and sediments since Devonian times, differentiating between BC from vegetation fires and from fossil fuel combustion. BC has important implications for various biological, geochemical and environmental processes. As examples, BC may represent a significant sink in the global carbon cycle, affect the Earth's radiative heat balance, be a useful tracer for Earth's fire history, build up a significant fraction of carbon buried in soils and sediments, and carry organic pollutants. On land, BC seems to be abundant in dark‐colored soils, affected by frequent vegetation burning and fossil fuel combustion, thus probably contributing to the highly stable aromatic components of soil organic matter. We discuss challenges for future research. Despite the great importance of BC, only limited progress has been made in calibrating analytical techniques. Progress in the quantification of BC is likely to come from systematic intercomparison using BCs from different sources and in different natural matrices. BC identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels. The key to estimating BC stocks in soils and sediments is an understanding of the processes involved in BC degradation on a molecular level. A promising approach would be the combination of short‐term laboratory experiments and long‐term field trials.
General CH4 oxidation model and comparisons of CH4 Oxidation in natural and managed systems
Authors: Grosso S. J. Del , Parton W. J. , Mosier A. R. , Ojima D. S. , Potter C. S. , Borken W. , Brumme R. , Butterbach‐Bahl K. , Crill P. M. , Dobbie K. , Smith K. A.
Published: February 2004

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (Wopt) increases with FC. The site specific maximum CH4 oxidation rate (CH4max) varies directly with soil gas diffusivity (Dopt) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH4 uptake, the potential for soil temperature to affect CH4 uptake rates increases as soils become less limited by gas diffusivity. Daily CH4 oxidation rate is calculated as the product of CH4max, the normalized (0–100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r2 = 0.73, mean error <6%).
Methane in the southern North Sea: Low‐salinity inputs, estuarine removal, and atmospheric flux
Authors: Upstill‐Goddard Robert C. , Barnes Jonathan , Frost Thomas , Punshon Steven , Owens Nicholas J. P.
Published: February 2004

Dissolved CH4was measured in coastal waters of the southern North Sea, in two adjacent U.K. estuaries with well‐defined turbidity maxima (Humber and Tyne) and in their associated river catchments, during a series of campaigns covering the period 1993–1999. In general, samples from all three environments were significantly to highly CH4 enriched relative to atmospheric air. Observed river water concentrations, ∼ 33–152 nmol L−1 (940–4305% saturation) for the Humber river catchment and ∼ 3–62 nmol L−1 (86–1754% saturation) in the river Tyne, were within but toward the low end of the range of CH4 concentrations in river waters world wide. In sea waters from the outer Wash estuary (U.K. coast) and adjacent to the Dutch coast, CH4 was highly but nonlinearly correlated with salinity, consistent with strong CH4 removal from river and/or estuarine CH4 sources influencing these locations. In transects along the Humber and Tyne estuaries, CH4 was highly negatively nonconservative, confirming the estuarine removal hypothesis. For both estuaries, highest CH4 concentrations, ∼190–670 nmol L−1 (6000–21,000% saturation) in the Humber and ∼650 nmol L−1(21,800% saturation) in the Tyne, were observed at very low salinity in the vicinity of the turbidity maximum. Importantly, these concentrations greatly exceeded measured river water values, implying for both situations the existence of a large in situ CH4 supply associated with high turbidity. Time series measurements at two locations in the upper Tyne subsequently confirmed the strong correspondence of dissolved CH4 and turbidity in the vicinity of the turbidity maximum. CH4removal estimated for the Humber, Tyne, Wash, and Rhine‐Scheldt estuaries was ∼ 90% of the low‐salinity CH4 input. On the basis of this and river discharge data, −7.I×108 mol CH4 may be removed annually in estuaries bordering the southern North Sea. Of this, ∼6.6×108mol may be lost by air‐sea exchange. This represents an additional atmospheric CH4 flux from the North Sea unaccounted for in previous work, which may have, consequently, underestimated this source by ∼50%. Upward scaling of this estimate based on the mean of reported river water CH4 concentrations implies a previously unaccounted for ∼6.3–24×109 mol (i.e., ∼ 0.1–0.4 × 1012 g) CH4 yr−1 which may be lost globally to gas exchange in estuaries, increasing previous such estimates by ∼8–50%. However, as it is based on data that exclude the possibility of elevated CH4 levels at estuarine turbidity maxima, even this revision is likely to be conservative. Detailed studies of CH4 distributions in major world estuaries would now be required in order to successfully reevaluate the CH4 budget of the coastal marine atmosphere.
Melting of a complete section of recycled oceanic crust: Trace element and Pb isotopic evidence from Iceland
Authors: Chauvel Catherine , Hémond Christophe
Published: February 2004

We present new trace element and isotopic data on a selection of least evolved lavas in Iceland. Pb isotopic compositions are variable: 206Pb/204Pb ranges from 18.05 in picrites to 19.15 in alkali basalts. Large excesses of Sr and Pb and a deficiency of Hf and Zr relative to rare earth elements are characteristic of the picritic lavas (Ce/Pb ≈ 10, Sr/Nd ≈ 35, and Sm/Hf ≈ 1.75), whereas more normal ratios are observed in the alkali basalts (Ce/Pb ≈ 40, Sr/Nd ≈ 10, and Sm/Hf ≈ 1.4). These variations are correlated to systematic changes in Sr, Nd, and Pb isotopic compositions. Using these new data, the relationship between the Iceland plume and the Mid‐Atlantic Ridge can be reexamined. The 87Sr/86Sr versus 206Pb/204Pb relationship demonstrates clearly that all isotopic variations in Iceland have their origin in the plume source. In addition, the composition of both Reykjanes and Kolbeinsey Ridges results from contamination of the depleted mantle by the Iceland plume. Trace element and isotopic heterogeneities within Iceland are related to rock type and not to location in the island. This is contrary to what would be expected if a uniform plume mixed with surrounding mid‐ocean ridge basalt (MORB), in which case the geochemical values would be more MORB‐like at the perimeter. Instead, it suggests that the Iceland plume itself was highly heterogeneous. We propose that the source of the Iceland plume was old recycled oceanic crust that was stored in the mantle for a long time (potentially since the end of the Archean). Melting of the basaltic portion of this crust led to the formation of the alkali basalts, and melting of the clinopyroxene‐plagioclase gabbroic portion of the same oceanic crust yielded the picritic basalts. In both cases, melting of the harzburgitic portion of the recycled lithosphere provided the necessary Mg and Ni. The tholeiites represent mixtures from the two sources. Iceland is therefore, together with Hawaii, a clear example of melting of a complete section of recycled oceanic lithosphere.
Temporal variability of Cd, Pb, and Pb isotope deposition in central Greenland snow
Authors: Sherrell R. M. , Boyle E. A. , Falkner K. K. , Harris N. R.
Published: February 2004

We present a decade‐long (1981–1990) high‐resolution (subseasonal) record of Pb and Cd concentrations and Pb isotopic composition in a series of 119 snow samples from a 6‐m snow pit at Summit, Greenland. Both metals show order of magnitude seasonal variability, with maxima in spring of every year, coinciding with sulfate peaks. These short‐term features complicate attempts to quantify secular decadal‐scale trends associated with anthropogenic source changes (e.g., phasing out of leaded gasoline). A small (<50%) decrease during the decade is estimated for Pb, but no significant trend is observed for Cd. Mean concentrations for the snow pit (Pb = 216, Cd = 11 pmol kg−1) are indistinguishable from mean values for nearly continuous samples of the 1–6 m section of a firn core drilled 1 km away, suggesting freedom from contamination artifact. An evaluation of potential sources confirms that Pb and Cd are dominated by anthropogenic inputs. Isotopic ratios (206Pb/207Pb and 208Pb/207Pb) determined on a subset of snow pit samples of varying ages within the decade indicate that springtime Pb concentration maxima are consistent with a mixture of eastern European, former Soviet Union, and western European sources, while seasonal Pb minima, especially from the early portion of the decade, plot along a different mixing line, suggesting a mixture of U.S. and European sources. The combination of Pb concentration and isotopic composition are consistent with an estimated decrease in U.S. Pb contributions of about twofold over the decade, which predicts a decadal concentration decrease in the snow of ∼30%. However, the secular trends in both concentration and isotopes are barely detectable against seasonal and interannual variability. The evidence for seasonally distinct source regions may be useful for interpretation of high‐resolution records of other chemical species in Greenland snow and ice. Analyses of two deep core sections dated at 1699–1700 and 1780–1788, compared to the snow pit data, indicate that both Pb and Cd deposition in central Greenland roughly doubled during the eighteenth century, then doubled again by the 1980s.
UserCalc: A Web‐based uranium series calculator for magma migration problems
Authors: Spiegelman M.
Published: February 2004

Measured departures from secular equilibrium in the uranium series decay chains provide important constraints on the rates of recent mantle and crustal processes. Nevertheless, the inferences drawn from these observations depend upon the models used to interpret them. While several useful models for U series exist, they are not all equally easy to use, and it is often the ease of use, rather than the utility of the model, that determines which models are applied. The purpose of this paper is to level the playing field by making some of the more general U series transport models accessible to a wider community through the UserCalc Web site available from Lamont‐Doherty Earth Observatory at www.ldeo.columbia.edu/~mspieg/UserCalc/. These models calculate the effects of both melting and melt transport on two U series chains (238U→230Th→226Ra and 235U→231Pa) and allow the user to input their own petrological insight and data to explore the behavior of this model. The purpose of the site is to provide a tool that can spur additional collaborations between geophysicists and geochemists as well as drive more rapid exploration/evolution of U series models.
Sr/Ca in multiple species of planktonic foraminifera: Implications for reconstructions of seawater Sr/Ca
Authors: Elderfield H. , Cooper M. , Ganssen G.
Published: February 2004

Sr/Ca ratios were measured on eight species of planktonic foraminifera from a core top transect of the North Atlantic. Five of the species (Globigerinoides ruber, Globigerina bulloides, Neogloboquadrina pachyderma, Globigerinoides sacculifer, Globigerinella siphonifera) show remarkably little within‐species variability in Sr/Ca (1.2 ± 0.1%). Three globorotaliid species (Globorotalia hirsuta, Globorotalia inflata, Globorotalia truncatulinoides) show somewhat greater variability (3–7%). Interspecies variations are of ∼10%. The variability in globorotaliid Sr/Ca is explained either by a greater temperature sensitivity than for other species or by depth‐dependent dissolution. Other than this effect, the overwhelming control on foraminiferal Sr/Ca appears to be seawater Sr/Ca. In contrast, time series Sr/Ca records of four species from a North Atlantic sediment core over a glacial‐interglacial cycle show very significant differences. N. pachyderma shows large‐amplitude changes in Sr/Ca (9.7%), in phase with δ18O, whereas G. ruber shows the smallest amplitude changes (2.6%) out of phase with δ18O and most similar to model predictions for temporal changes in seawater Sr/Ca. Records of G. inflata and G. bulloides are intermediate (amplitude changes of 3.3–4.7%) and out of phase with δ18O. The lack of coherence in the time series records between the species shows that factors in additional to changing seawater Sr/Ca must affect planktonic foraminiferal Sr/Ca.
Identification and implications of off‐axis lava flows around the East Pacific Rise
Authors: Reynolds Jennifer R. , Langmuir Charles H.
Published: February 2004

Off‐axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems. A detailed study of basaltic glass samples around the East Pacific Rise from 12°00′ to 12°30′N provides geological evidence for the existence of off‐axis eruptions with a distinctive chemical composition. This composition has not been found along the axis of the EPR from 8° to 14°N except at a ridge‐transform intersection but has been recovered in numerous locations that were farther than 1 km off axis. These off‐axis normal mid‐ocean ridge basalts, or OA‐NMORB, are distinguished by low Na2O, Sr, and Al2O3 and are unusually depleted in incompatible elements. Moderately enriched off‐axis transitional mid‐ocean ridge basalts (OA‐TMORB) with the same compositional tendencies can also be identified. Comparison of EPR axis lavas and the OA type suggests that they come from the same range of (unmelted) mantle source compositions but that the source of the OA magmas was depleted in incompatible trace elements by removal of a small‐ degree partial melt. This would be consistent with the OA type as an EPR pooled melt that is missing the low‐degree melt fraction from deep in the melting regime, which provides a reasonable physical model for their formation. In this case, off‐axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system. The OA‐NMORB are similar to depleted lavas from near‐EPR seamounts. Other seamount lavas with depleted trace elements have TMORB‐like major elements, and may be classified as OA‐TMORB. The similarity between seamount lavas and the lavas erupted off axis close to the EPR suggests that the two are manifestations of the same phenomenon. We suggest that seamount‐type volcanism effectively starts within 1–2 km of the axis. This is within the range where lavas derived from the axial plumbing system may also erupt. Therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off‐axis systems may overlap. Lavas erupted off axis may ultimately cover 20% of the seafloor around the EPR, which is substantially more than previous estimates that were based primarily on morphological studies.
Geologic mapping of the northern leading hemisphere of Europa from Galileo solid‐state imaging data
Authors: Figueredo Patricio H. , Greeley Ronald
Published: February 2004

The northern leading hemisphere of Europa was imaged at regional mapping resolution (∼230 m/pixel) by the Galileo spacecraft SSI camera. We produced geologic maps from a regional‐scale mosaic and a high resolution inset of this region. Twelve geologic units were sufficient to produce correlative geologic maps at both regional and local scales. Stratigraphic relationships indicate four major episodes in the geologic history of this area: The first episode includes background plains formation and modification, followed by a second period of extensive lineament formation. The third episode included extensive chaotic disruption of the surface at low and middle latitudes. The final episode includes the formation of new sets of ridges and bands, especially at high latitudes. The low crater density indicates that all these episodes in Europa's geologic history are geologically recent. The latitudinal distribution of chaos areas broadly matches that of areas of crustal thinning from models of tidal heating. We show that stress directions rotated clockwise with time, which is consistent with predictions from global stress models involving tidal deformation and nonsynchronous rotation of Europa's crust. On the basis of the change in lineament orientation with time, the reconstructed longitudinal positions of the studied area indicate that Europa's crust completed a full rotation relative to the tidally deformed interior.
The search for current geologic activity on Europa
Authors: Phillips Cynthia B. , McEwen Alfred S. , Hoppa Gregory V. , Fagents Sarah A. , Greeley Ronald , Klemaszewski James E. , Pappalardo Robert T. , Klaasen Kenneth P. , Breneman H. Herbert
Published: February 2004

Observational evidence and theoretical arguments suggest that Jupiter's satellite Europa could be geologically active and possess an “ocean” of liquid water beneath its surface at the present time. We have searched for evidence of current geologic activity on Europa in the form of active plumes venting material above the surface and by comparison of Voyager and Galileo images to look for any changes on the surface. So far, we have observed no plumes and have detected no definitive changes. The lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km2 y−1 in the regions analyzed, assuming alterations will cover contiguous areas of at least 4 km2 over a period of 20 years. Assuming this as a constant, globally uniform resurfacing rate leads to a minimum average surface age of 30 million years. We also suggest that the lack of obvious circular albedo patterns on the surface due to plumes, coupled with the presence of bright‐rayed craters such as Pwyll and the predicted sputtering erosion rate, implies that no large‐scale plume activity has taken place over at least the last few thousand years. We thus conclude that if Europa's surface is currently active, any changes must be relatively small in spatial scale or episodic in nature rather than continuous. To detect potential small‐scale surface changes, we need high‐resolution comparisons between the Galileo data and future Europa Orbiter images.
High‐energy neutrons from the Moon
Authors: Maurice S. , Feldman W. C. , Lawrence D. J. , Elphic R. C. , Gasnault O. , d'Uston C. , Genetay I. , Lucey P. G.
Published: February 2004

We present the first report dedicated to 0.6–8 MeV neutrons measured by the Lunar Prospector gamma‐ray spectrometer. The period of data collection extends from January 1998 to July 1999; the spacecraft altitude varied between 15 km and 140 km. Maps are at a 2° × 2° equal‐area resolution. They show high counts and high variability for the mare and, low counts and low variability for the highlands. Such variability is due to differences of Fe and Ti concentrations in the lunar regolith, plus other elements with lesser effect that have not yet been included in our calculations. Assuming that only FeO and TiO2 concentrations are responsible for the fast neutron structures, fluxes are 2.7 times more sensitive to FeO than to TiO2. The statistical resolutions are 1.3% in FeO and 3.5% in TiO2. No discrimination of FeO is possible below 6.5%. Results are supported by numerical simulations. We also present the first lunar neutron spectrum over 16 channels between 0.6 and 8 MeV. Ninety‐five percent of the counts are below ∼4.5 MeV. In flux units the spectrum peaks at ∼1.5 MeV. Its shape agrees with numerical simulations.
Lunar coordinates in the regions of the Apollo landers
Authors: Davies Merton E. , Colvin Tim R.
Published: February 2004

Laser retroreflectors were placed on the Moon by Apollo 11, Apollo 14, Apollo 15, and Lunakhod 2. Since then, lunar laser ranging retroreflector measurements of the true distances between retroreflectors and Earth‐based laser telescopes have been made by recording transit times. The accuracy of these measurements has evolved from 10 cm in the 1970s to 3 cm in the 1990s. Accurate lunar coordinates of the retroreflectors were determined from these measurements [Williams et al., 1996]. Apollo lunar surface experiment packages (ALSEP) were delivered to the lunar surface on missions 12, 14, 15, 16, and 17. Coordinates of these stations were determined from very long baseline interferometry (VLBI) measurements of their transmissions. The VLBI measurements were combined with the laser‐ranging coordinates available at that time [King et al., 1976]. These solutions for the ALSEP locations have been modified to be compatible with the current retroreflector solutions. Coordinates of lunar module locations and nearby craters are given.
Geology and mapping of dark terrain on Ganymede and implications for grooved terrain formation
Authors: Prockter Louise M. , Figueredo Patricio H. , Pappalardo Robert T. , Head James W. , Collins Geoffrey C.
Published: February 2004

Geological mapping of regional and high‐resolution Galileo images reveals a variety of units and structures within Ganymede's dark terrain. We have made a detailed study of areas within Galileo, Marius, and Nicholson Regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain. Dark terrain appears to become fractured as a precursor to grooved terrain formation; in places, dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths. Tectonic deformation may be, but is not always, focused through preexisting weaknesses caused by impact craters and furrows. A prominent groove lane, Anshar Sulcus, is inferred to have formed by the process of hanging wall rollover, accompanied by a small amount of right‐lateral horizontal shear offset resulting from NE‐SW extension.
Nomad Rover Field Experiment, Atacama Desert, Chile: 2. Identification of paleolife evidence using a robotic vehicle: Lessons and recommendations for a Mars sample return mission
Authors: Cabrol N. A. , Bettis E. A. , Glenister B. , Chong G. , Herrera C. , Jensen A. , Pereira M. , Stoker C. R. , Grin E. A. , Landheim R. , Thomas G. , Golden J. , Saville K. , Ludvigson G. , Witzke B.
Published: February 2004

During the Nomad Rover Field Experiment in the Atacama Desert (Chile), a potential fossil was identified in a boulder by the science team remotely located at NASA Ames Research Center, California. The science team requested the collecting of the boulder that was returned for laboratory analysis. This analysis confirmed the evidence of paleolife. As the first fossil identified and sampled by a remotely located science team using a rover, we use the case of sample I‐250697 to describe the process, both in the field and later in the laboratory during the rock analysis, which led to the identification, characterization, and confirmation of the evidence of paleolife evidence in I‐250697. We point out the lessons that this case provides for future Mars sample return missions.
North polar region of Mars: Topography of circumpolar deposits from Mars Orbiter Laser Altimeter (MOLA) data and evidence for asymmetric retreat of the polar cap
Authors: Fishbaugh Kathryn E. , Head James W.
Published: February 2004

We have used high‐resolution Mars Orbiter Laser Altimeter (MOLA) data to analyze the topography, morphology, stratigraphy, and geologic history of the Martian north circumpolar deposits. The present polar deposits are offset about toward 0°W from the rotational pole. An arc of irregular topography, concentric to Olympia Planitia and the cap, consists of polar material remnants, depressions which we interpret to be kettles, frost‐covered and residual ice‐filled craters, and frost patches. Olympia Planitia, originally thought to be a flat, sand‐covered plain, is characterized by a convex‐upward topography, contiguous with the polar cap. We interpret Olympia Planitia to represent a now dune‐covered extension of the polar materials. Together, Olympia Planitia and the outlying deposits delineate a former extent of the polar cap. Topographic data have clarified relationships among the circumpolar deposits. Contributors to these deposits include local volcanics, fluvial and aqueous sediments (from outflow channels and a possible standing body of water), pyroclastic ash, sublimation lag from the Olympia Lobe, and eolian‐reworked materials. Significant events in the history of the region include (1) formation of the northern lowlands; (2) emplacement of volcanic plains, fluvial and aqueous sedimentation, and subsequent desiccation, forming polygonal patterns which in part underlie the present polar layered deposits; (3) formation of the polar cap, composed primarily of layered deposits; (4) asymmetric retreat of the Olympia Lobe, resulting in sublimation lag deposits, polar remnants, and kettles; and (5) continued collection and reworking of sediments by eolian processes. The cause of the asymmetrical retreat of the Olympia Lobe is unknown.