bluuebunny/crossref_metadata_2025 huggingface dataset:
Contrasting P‐T‐t paths: Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon Composite Terrane in the New England Appalachians
Authors: Wintsch R. P. , Sutter J. F. , Kunk M. J. , Aleinikoff J. N. , Dorais M. J.
Published: February 2008

Strongly contrasting pressure‐temperature‐time paths for the Avalon composite terrane and the structurally overlying Putnam‐Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U‐Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam‐Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One‐dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high‐grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high‐grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post‐Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.
Origin and tectonic significance of the Aldrich Mountains serpentinite matrix melange, northeastern Oregon
Authors: Carpenter Paul S. , Walker Nicholas W.
Published: February 2008

The Permian ‐ Triassic Aldrich Mountains serpentinite melange consists of a matrix of massive and foliated serpentinite (lizardite + chrysotile) that encases a variety of metamorphic, igneous, and sedimentary block types. Serpentinite matrix foliation trends NE‐SW, dips steeply (> 75°), and is parallel to long dimensions of blocks. At block‐matrix contacts, the foliation traces block margins. The volumetrically most abundant block types are greenstone, chert, and metagabbro; subordinate block types are amphibolite tectonites, epiclastic sediments (breccia, conglomerate, sandstone, and argillite), silicic volcanic rocks, silicic plutonic rocks, and ultramafic rocks. Three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix: (1) serpentinization and related hydrous alteration confined to matrix, ultramafic blocks, and margins of mafic blocks; (2) lower to middle greenschist and prehnite‐actinolite facies metamorphism characterized by static, non penetrative fabrics of some blocks, and; (3) dynamothermal epidote amphibolite to upper amphibolite/lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites. Mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures (550°–780°C) and low to moderate pressures (3–5 kbar). Most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix. The melange is interpreted to have formed within a Permian to Late Triassic sediment‐starved, tectonically active forearc. Fragments of abyssal oceanic crust and Early Permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc. Highly mobile serpentinite, generated beneath the forearc, was protrusively emplaced into the tectonized forearc and mixed with the blocks, forming the melange. Melange generation probably began in the late Early Permian and ceased by the Late Triassic.
Carbon fluxes and burial rates over the continental slope and rise off central California with implications for the global carbon cycle
Authors: Reimers Clare E. , Jahnke Richard A. , McCorkle Daniel C.
Published: February 2008

In situ microelectrode, box‐core pore water gradient, and in situ benthic chamber estimates of organic carbon degradation and CaCO3 dissolution are combined with organic‐C and carbonate‐C accumulation rates to approximate the total carbon flux to the seafloor along two transects of the continental slope and rise off central California. Microelectrode profiles of dissolved O2 demonstrate that sediments at 13 sites, ranging in water depth from 580 to 4080 m, become anoxic below the uppermost 0.4–3 cm of the sediment column. If a current‐swept area of nondeposition on the upper slope is excluded, we find total organic‐C and carbonate‐C fluxes to the seafloor vary from 40 to 100 μmol C cm−2yr−1 and from 32 to 91 μmol C cm−2yr−1, respectively. From the distribution of these fluxes there is no indication that total fluxes or remineralization rates of organic or carbonate carbon are influenced markedly by conditions in the oxygen minimum zone. Instead, the upper continental rise with its system of submarine valleys and fans stands out as the most important locus for carbon deposition and remineralization. When benthic fluxes and burial rates are extrapolated over the whole slope and rise of the region, aerobic respiration is the major mechanism of organic matter oxidation, and organic‐C and carbonate‐C recycling are on average 87% and 98% efficient, respectively. These results suggest that modern sediments on the outer regions of continental margins are important sources of CO2 that is injected directly into ocean deep water. However, if benthic carbon fluxes on the central California margin are typical of margins globally, this injection rate is less than 0.7 Gt C yr−1, which does not indicate a significant anthropogenic enhancement of carbon export to continental slopes and rises.
Interannual variations in tundra methane emission: A 4‐year time series at fixed sites
Authors: Whalen Stephen C. , Reeburgh William S.
Published: February 2008

Abstract. This paper summarizes 4 years (1987‐1990) of weekly net CH4 flux measurements at permanent sites representing important plant components of Arctic tundra. The data coincide with variations in precipitation and temperature of interest in regional and global modeling efforts and are useful in placing bounds on the role of tundra in the global CH4 budget. Precipitation in the study area during the summer emission period ranged from twice to half the long‐term mean, and air temperature anomalies were about +2 °C. This data set also permits consideration of temporal (seasonal to interannual) and spatial variability in CH4 flux. We studied the relationship between the net CH4 flux and subsurface properties (water table depth, thaw depth, soil temperature, /pCH4 distributions) at these permanent sites during the 1988 and 1989 emission periods. Net CH4 emission and subsurface properties are largely unrelated. Relationships between soil temperature (or any single variable) and emission are site specific and are of little value as flux predictors. Parameters that integrate conditions influencing flux appear to be the best flux predictors over the emission period. We estimate that Arctic wet meadow and tussock:shrub tundra presently emit about 42 ± 26 Tg CH4 yr−1 to the atmosphere. This estimate has a North American bias, but it is supported by measurements in a range of locations, transect studies, and model calculations.
Methane emission from rice fields: The effect of floodwater management
Authors: Sass R. L. , Fisher F. M. , Wang Y. B. , Turner F. T. , Jund M. F.
Published: February 2008

Rice fields emit methane and are important contributors to the increasing atmospheric CH4 concentration. Manipulation of rice floodwater may offer a means of mitigating methane emission from rice fields without reducing rice yields. To test methods for reducing methane emission, we applied four water management methods to rice fields planted on silty‐clay soils near Beaumont, Texas. The four water treatments investigated were: normal permanent flood (46 days post planting), normal flood with mid‐ season drainage aeration, normal flood with multiple drainage aeration, and late flood (76 days post planting). Methane emission rates varied markedly with water regime, showing the lowest seasonal total emission (1.2 g m−2) with a multiple‐aeration treatment and the highest (14.9 g m−2) with a late flood. Although the multiple‐ aeration water management treatment emitted 88% less methane than the normal irrigation treatment and did not reduce rice yields, the multiple‐aeration treatment did require 2.7 times more water than the 202 mm required by the normal floodwater treatment. A comparison of measured methane emission and production rates obtained from incubated soil cores indicated that, depending on time of season and flood condition, from zero to over 90% of the methane produced was oxidized. The average amount of methane which was oxidized during times of high emission was 73.1 ± 13.7 percent of that produced.
NOx And N2O Emissions From Soil
Authors: Williams E. J. , Hutchinson G. L. , Fehsenfeld F. C.
Published: February 2008

Emission Of NOx (principally NO) and N2O from soils is reviewed with particular emphasis placed on the atmospheric and ecological implications of this source. The photochemistry of these species in the atmosphere is summarized as well as the methods available for the determination of fluxes. Processes which produce and consume both NO and N2O in soils are principally microbiological in nature and are linked directly and indirectly with the chemical and physical factors that control gaseous transport through the soil medium. Linkages among these processes occur over many different temporal and spatial scales which makes interpretation of the available data difficult. A summary of results from laboratory and field studies shows that considerable spatial and temporal variability exists in the emissions. This variability can be related to factors such as temperature, water content, soil composition, nutrient availability, vegetation, disturbances (e.g., burning, agricultural practices), and others. Because NOx and N2O play central roles in many important environmental problems, there is a need for accurate estimates of the magnitude of the soil source, but the large degree of variability in the existing data makes extrapolation highly uncertain. To overcome this uncertainty, models are required which can simulate the processes responsible for production, consumption, and transport of these species at all relevant temporal and spatial scales. Integrated field studies will also be required to validate the model results.
A compilation of inventories of emissions to the atmosphere
Authors: Graedel T. E. , Bates T. S. , Bouwman A. F. , Cunnold D. , Dignon J. , Fung I. , Jacob D. J. , Lamb B. K. , Logan J. A. , Marland G. , Middleton P. , Pacyna J. M. , Placet M. , Veldt C.
Published: February 2008

Detailed and accurate emissions inventories are essential for reliable computer dispersion model simulation of the behavior of chemically and radiatively important atmospheric species. Currently, model simulations of the atmosphere are limited by the paucity of quality emissions data for input. As a first step in providing internationally recognized emissions inventories, we list here the inventories that are extant, together with their spatial and temporal characteristics and a few interpretive comments. The only global emissions inventory we regard as good is that for chlorofluorocarbons. Those for CO2, CH4, NOx, SO2, reduced sulfur, and radon we regard as fair. In selected regions, the spatial resolution of emissions is well determined for CO2, CO, NOx, and SO2. The temporal resolution of existing inventories is almost uniformly poor. Much remains to be done to generate emissions inventories adequate to fully support computer models of regional and global chemistry and climate.
A new Mesozoic isochron chart of the northwestern Pacific Ocean: Paleomagnetic and tectonic implications
Authors: Nakanishi Masao , Tamaki Kensaku , Kobayashi Kazuo
Published: February 2008

A new Mesozoic isochron chart of the northwestern Pacific Ocean has been completed by utilizing all the currently available data from 357 cruises. Our chart provides the first whole view of the Mesozoic magnetic anomaly lineations in the northwestern Pacific Ocean and yields essential information for understanding the problems of the Mesozoic magnetostratigraphic time scale and the evolutional history of the Pacific plate. The Mesozoic magnetostratigraphic time scale must be recalibrated using not only the Hawaiian lineation set but also other Mesozoic lineation sets. The lack of lineation M0 in the lineation sets other than the Hawaiian lineation set indicates that a reorganization of the ridges except the Pacific‐Farallon ridge occurred between chrons M1 and M0 (nearly 120 Ma). The eruption rate of the Shatsky hotspot seems to be comparable to that of the Reunion hotspot at their initial stage. The growth of the Pacific plate began in a small triangular region at about 190 Ma.
Latitudinal survey of spectral optical depths of the Pinatubo volcanic cloud‐derived particle sizes, columnar mass loadings, and effects on planetary albedo
Authors: Valero Francisco P. J. , Pilewskie Peter
Published: February 2008

We have made airborne measurements of spectral optical depths, radiative fluxes, and scattered radiation fields during the NASA Caribbean mission (July 7–14, 1991) to characterize the Pinatubo volcanic cloud. In this paper we report the latitudinal and spectral dependence of the volcanic cloud's optical depth. From these measurements we determine moments of the particle size distribution. The change in planetary albedo induced by the volcanic cloud is calculated for the range of measured optical depths with the objective of assessing the impact on the solar radiation budget. Mid‐visible optical depths higher than 0.4 were observed, placing the Pinatubo stratospheric cloud among the thickest ever measured. The latitudinal distribution of extinction shows that by July 7, 1991 the volcanic cloud had extended to 30°N. The effective particle radius was determined to be between 0.18 μm and 035 μm with a corresponding columnar mass loading between 35 and 80 mg.m−2.
Oceanic crustal thickness versus spreading rate
Authors: Chen Yongshun John
Published: February 2008

A compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness, away from plateaus, is 6 km; no systematic increase of crustal thickness with spreading rate is observed. Instead, the data show large variations in crustal thickness at slow spreading ridges (3 – 8 km for half rates < 20 mm/yr) and small variations in thickness at fast rates (5 – 7 km for half rates > 30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid‐ocean ridges. Both data sets support the speculation of a transition from a 3‐D structure of crustal accretion at slow ridges to a 2‐D accretion pattern at fast ridges.
Ion radial diffusion in an electrostatic impulse model for stormtime ring current formation
Authors: Chen Margaret W. , Schulz Michael , Lyons Larry R. , Gorney David J.
Published: February 2008

Guiding‐center simulations of stormtime transport of ring‐current and radiation‐belt ions having first adiabatic invariants μ ≳ 15 MeV/G (E ≳ 165 keV at L ∼ 3) are surprisingly well described (typically within a factor of ≲ 4) by the quasilinear theory of radial diffusion. This holds even for the case of an individual model storm characterized by substorm‐associated impulses in the convection electric field, provided that the actual spectrum of the electric field is incorporated in the quasilinear theory. Correction of the quasilinear diffusion coefficient
for drift‐resonance broadening (so as to define
) reduced the typical discrepancy with the diffusion coefficients
deduced from guiding‐center simulations of representative‐particle trajectories to a factor ∼3. The typical discrepancy was reduced to a factor ∼ 1.4 by averaging
,
, and
over an ensemble of model storms characterized by different (but statistically equivalent) sets of substorm‐onset times.
Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents
Authors: Halliday Alex N. , Davidson Jon P. , Holden Peter , Owen Robert M. , Olivarez Annette M.
Published: February 2008

The isotopic composition of Nd is uniform in metalliferous sediments formed at distances varying from >1000 km to within 10 km of the East Pacific Rise (EPR) palaeoridge. These data indicate that hydrothermal vent fluids, despite having concentrations more than 500 times greater, have no effect on the Nd isotopic composition of seawater. This implicates efficient scavenging of hydrothermal Nd by particulates, resulting in extremely short residence times (< 1 year) close to the hydrothermal vents. Therefore Nd isotopic studies of ancient seawater precipitates, particularly metalliferous sediments, cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates (or residence times) relative to those of modern oceans.
Milankovitch fluctuations on supercontinents
Authors: Crowley Thomas J. , Baum Steven K. , Hyde William T.
Published: February 2008

Observational and modeling studies support the concept of significant Milankovitch fluctuations in the pre‐Pleistocene. Many of these studies have focused on the mid‐Cretaceous, a time of high sea level and moderately dispersed land‐sea distribution. In this paper we utilize a two‐dimensional energy balance climate model to examine the potential effects of Milankovitch fluctuations for a supercontinent configuration (Early Jurassic, 195 Ma). We calculate that Milankovitch variations can modulate the magnitude of summer warming by as much as 14–16°C on Pangaea, with large changes occurring in both the northern and southern hemispheres. These values are comparable to or slightly larger than the calculated range for the present Eurasian landmass for the Pleistocene. In addition, mean monthly summer temperatures of 25°C reach 65° paleolatitude for “hot orbit” configurations. This latter result suggests that some organisms may have been able to migrate into higher latitudes during precession half‐cycles of 10,000 years duration, and a potential bias should be considered when evaluating evidence for past high‐latitude warmth. Our results indicate that general circulation model (GCM) runs for past time periods which use only one orbital configuration may be missing a substantial amount of the potential variance on large land masses.
Response of the dipole magnetosphere to pressure pulses
Authors: Lysak Robert L. , Lee Dong‐hun
Published: February 2008

The response of the magnetosphere to pressure pulses at the magnetopause has been studied using a three‐dimensional model of ULF waves in a dipole geometry. Pressure pulses at the magnetosphere directly excite congressional waves, which then convert to shear mode Alfvén waves due to inhomogeneity. The behavior of the system depends on the frequency of the source at the magnetopause, with vortex structures tending to form on field lines resonant with the source frequency. The perturbations between the vortices are skewed toward noon, in agreement with observations.
Solar wind Halo electrons from 1–4 AU
Authors: McComas D. J. , Bame S. J. , Feldman W. C. , Gosling J. T. , Phillips J. L.
Published: February 2008

In this study we use observations from the Ulysses solar wind electron spectrometer to make a first examination of the evolution of the solar wind suprathermal or halo electron population as a function of heliocentric distance beyond 1 AU. Solar wind electron analyzers previously flown to distances >1 AU did not resolve the halo electron distribution so that the 3‐D suprathermal solar wind electron observations from Ulysses represent a unique data set. As the core population cools with increasing heliocentric distance, no gap is formed between the core and halo populations. Rather, the halo electrons extend to increasingly lower energies. As predicted previously on theoretical grounds, the ratio of the core electron temperature to the low energy cutoff of the halo population appears to be roughly constant with a value of ∼7.5. The total integrated heat flux drops rapidly with increasing heliocentric distance; we find a best fit power law of R−2.36. In addition, we find that the ratio of the halo to core densities is roughly constant over heliocentric distance with the halo representing 4% of the total electron distribution. These results suggest that the halo population may not consist of truly non‐interactive test particles over the heliocentric range of 1–4 AU.
CO2transport at the air‐sea interface: Numerical calculations for a surface renewal model with coupled fluxes
Authors: Phillips L. F.
Published: February 2008

Air‐sea fluxes of CO2, H2O and heat have been calculated for a surface‐renewal model with turbulent mixing on the air side of the interface and coupling of heat and matter fluxes in the stagnant air layer adjacent to the surface of the liquid. The magnitude and direction of the CO2flux are strongly dependent on the temperature of the surface layer of liquid, as influenced by the air temperature and by the evaporation or condensation of water. Temperature effects arise both from coupling of heat and matter fluxes and from the temperature dependence of CO2solubility at the surface.
Earth potential over 4000 km between Hawaii and California
Authors: Lanzerotti L. J. , Sayres C. H. , Medford L. V. , Kraus J. S. , Maclennan C. G.
Published: February 2008

Measurements of the value of the large‐scale (4050 km) mean earth potential between California and Hawaii are reported as determined over a one year interval in 1990–91. The mean electric field measured during quiet geomagnetic intervals, 0.183 ± 0.056 mV/km, is about six times larger than that reported for a shorter interval over a slightly longer distance in the Atlantic [Lanzerotti et al., 1985], but of the same sign. The present results are also about three standard deviations from a null value, in contrast to the previous results, which were consistent with a null mean. If the mean potential is entirely attributable to leakage of a poloidal electric field from the core‐mantle boundary, then under some models of the mantle conductivity one can conclude that the deduced toroidal magnetic field intensity in the core would be of the same order as the poloidal magnetic field at the core‐mantle boundary. The measured potential gradient is smaller than that which might be expected from thermoelectric emfs at the core‐mantle boundary. The potential could also correspond to a south to north quasi‐steady water flow of ∼0.6 cm/sec.
Canali‐type channels on Venus: Some genetic constraints
Authors: Komatsu Goro , Kargel Jeffrey S. , Baker Victor R.
Published: February 2008

Canali‐type channels on Venus are unique because of their great lengths (up to 6800 km) and nearly constant channel cross sectional shapes along their paths. A simple model incorporating channel flow and radiative cooling suggests that common terrestrial‐type tholeiite lava cannot sustain a superheated and turbulent state for the long distances required for thermal erosion of canali within allowable discharge rates. If canali formed mainly by constructional processes, laminar tholeiitic flows of relatively high, sustained discharge rates might travel the observed distances, but the absence of levees would need to be explained. An exotic low temperature, low viscosity lava like carbonatite or sulfur seems to be required for the erosional genesis of canali.
Observations of reduced ozone concentrations in the tropical stratosphere after the eruption of Mt. Pinatubo
Authors: Grant W. B. , Fishman J. , Browell E. V. , Brackett V. G. , Nganga D. , Minga A. , Cros B. , Veiga R. E. , Butler C. F. , Fenn M. A. , Nowicki G. D.
Published: February 2008

The eruption of Mt. Pinatubo (15°N, 122°E) on June 15 and 16, 1991, placed a large amount of SO2 and crustal material in the stratosphere. Based on measurements of decreases of stratospheric ozone after previous volcanic eruptions, it was expected that the aerosols deposited into the stratosphere (both directly and as a result of SO2 conversion into particulate sulfate) by this eruption would give rise to significant ozone depletions. To check for such an effect, ozone profiles obtained from ECC sondes before and after the eruption at Brazzaville, Congo (4°S, 15°E), and Ascension Island (8°S, 14°W), are examined. Aerosol profiles determined from a lidar system in the western Pacific (4°–6°N, 125°E) show that most of the material injected into the stratosphere is located between 18 and 28 km with highest amounts at 24–25 km. For the period 3–6 months after the eruption, decreases in ozone are found at 16 to 29 km, with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13–20 Dobson units is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior to the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading determined by the lidar. In addition, a small increase in ozone density is found above about 28 km. Mechanisms that might explain the results such as heterogeneous chemistry, radiative effects, and dynamics are discussed.
Relative rate constants for the reactions of OH with methane and methyl chloroform
Authors: DeMore W. B.
Published: February 2008

Atmospheric lifetimes of methane and methyl chloroform are largely determined by the rates of their reactions with hydroxyl radical. The relative lifetimes for this loss path are inversely proportional to the ratio of the corresponding rate coefficients. In this work the relative rate constants were measured in a slow‐flow, temperature controlled photochemical reactor, and were based on rates of disappearance of the parent compounds as measured by FTIR spectroscopy. The temperature range was 277–356 K. Hydroxyl radicals were generated by 254 nm photolysis of O3 in the presence of water vapor. The preferred Arrhenius expression for the results is k(CH3 CCl3)/k(CH4 ) = 0.62exp(291/T), corresponding to a value of 1.65 at 298 K and 1.77 at 277 K. The respective uncertainties are 5% and 7%.
The effect of cement on the strength of granular rocks
Authors: Bernabé Yves , Fryer D. T. , Hayes J. A.
Published: February 2008

Relatively neglected by the oil industry in the past, the rock mechanics approach is now routinely applied to production problems such as subsidence or wellbore stability. As a result, there is a growing need for a better understanding of the strength of porous rocks. In addition to porosity, composition and amount of cement are two important parameters controlling the strength of granular rocks. In order to investigate their effect we prepared synthetic granular materials in which the amount and composition of cement could be varied. Synthetic materials are more reproducible and better characterized than natural rocks but do not always possess properties realistically imitating those of natural rocks. Here, we propose procedures to fabricate satisfactory materials that could also be fruitfully used in other experimental studies. We performed a series of triaxial tests on samples of cemented and un‐cemented material. We observed that cement, even in very small quantity, significantly increased the strength of granular materials. Our results suggest that the amount of cement was less important than the location where it was deposited. Strengthening was due to precise deposition of the cement at the grain‐to‐grain contacts, thus preventing sliding and rotation of grains.
The upper mantle discontinuities: Correlated or anticorrelated?
Authors: Stammler K. , Kind R. , Petersen N. , Kosarev G. , Vinnik L. , Qiyuan Liu
Published: February 2008

The discontinuities in the Earth's upper mantle at 410km and 660km are frequently considered as phase changes, which are exothermic and endothermic, respectively. From this follows that at laterally changing temperatures both discontinuities move in opposing vertical directions. That means they are negatively correlated. We attempt to test this hypothesis using travel times of seismic waves converted at both discontinuities underneath seismic stations. The broadband stations used are located in areas of high (e.g. western US) and low heat flow (e.g. Central Asia) [Sclater et al., 1980]. We observe a clear positive correlation of the conversion times of both discontinuities, which can however be explained with known lateral velocity changes above 410km. From the scatter of our data we conclude that the limit of a possibly existing and negatively correlated topography of the two discontinuities is about 28km. This is in contrast to recently claimed observations of about 35 km anticorrelated topography in the western Pacific [Revenaugh & Jordan, 1991; Stammler & Kind, 1992].
Sunlight penetration through the Martian polar caps: Effects on the thermal and frost budgets
Authors: Lindner Bernhard Lee
Published: February 2008

An energy balance model of the seasonal polar caps on Mars is modified to include penetration of solar radiation into and through the ice. Penetration of solar radiation has no effect on subsurface temperature or total frost sublimation if seasonal ice overlies a dust surface. An effect is noted for seasonal ice which overlies the residual polar caps. For the case of an exposed water‐ice residual polar cap, the temperature at depth is calculated to be up to several degrees warmer and the calculated lifetime of seasonal CO2 frost is slightly lower when penetration of sunlight is properly treated in the model. For the case of a residual polar cap which is perennially covered by CO2 frost, the calculated lifetime of seasonal CO2 frost is very slightly increased as a result of sunlight penetration through the ice. Hence, penetration of sunlight into the ice helps to stabilize the observed dichotomy in the residual polar caps on Mars, although it is a small effect.
40–60 day oscillations in the western tropical Pacific: Results from an eddy‐resolving global ocean model
Authors: Ponte Rui M. , Gutzler David S.
Published: February 2008

Preliminary analyses of results from the Semtner and Chervin [1992] eddy‐resolving general circulation model reveal the presence of energetic 40–60 day oscillations in the near‐surface layers of the western tropical Pacific. These oscillations have the strongest signal in meridional velocity (maximum amplitudes on the order of 10–20 cm s−1), in latitudinal bands west of the dateline centered at approximately 10°N and 10°S. The 40–60 day disturbances have zonal wavelengths of approximately 700 km at 10°N and 1000 km at 10°S, and show clear westward phase propagation at speeds of order 15–20 cm/s. Eastward (but slower) energy propagation is also apparent. The oscillations in the model are a result of oceanic instability processes and not a direct response to wind stress anomalies associated with the Madden‐Julian oscillation in the atmosphere. Enhanced energy at 40–60 days is also present in the Indian and Atlantic basins, suggesting that the 40–60 day time scale is a preferred scale for variability in the tropical oceans.
Fault stability under conditions of variable normal stress
Authors: Dieterich James H. , Linker M. F.
Published: February 2008

The stability of fault slip under conditions of varying normal stress is modeled as a spring and slider system with rate‐ and state‐dependent friction. Coupling of normal stress to shear stress is achieved by inclining the spring at an angle, ø, to the sliding surface. Linear analysis yields two conditions for unstable slip. The first, of a type previously identified for constant normal stress systems, results in instability if stiffness is below a critical value. Critical stiffness depends on normal stress, constitutive parameters, characteristic sliding distance and the spring angle. Instability of the first type is possible only for velocity‐weakening friction. The second condition yields instability if spring angle ø < ‐cot −1μss, where μss is steady‐state sliding friction. The second condition can arise under conditions of velocity strengthening or weakening. Stability fields for finite perturbations are investigated by numerical simulation.
Antipodal hotspot pairs on the Earth
Authors: Rampino Michael R. , Caldeira Ken
Published: February 2008

The results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution, or from their association with geoid highs and divergent plate margins. The observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality. At a maximum deviation of ≤700 km, 26% to 37% of hotspots form antipodal pairs in the published lists examined here, significantly more than would be expected from the general hotspot distribution. Two possible mechanisms that might create such a distribution include: (1) symmetry in the generation of mantle plumes, and (2) melting related to antipodal focusing of seismic energy from large‐body impacts.
Scattering of electromagnetic waves from a plasma: Enhanced ion acoustic fluctuations due to ion‐ion two‐stream instabilities
Authors: Wahlund J.‐E. , Forme F. R. E. , Opgenoorth H. J. , Persson M. A. L. , Mishin E. V. , Volokitin A. S.
Published: February 2008

Observations by the ElSCAT and Millstone Hill radars of strongly enhanced, often asymetric, ion acoustic line spectra in the topside auroral ionosphere have been reported recently by a number of researchers. Such strongly enhanced ion acoustic line spectra are shown to arise naturally in a plasma unstable to the ion‐ion two‐stream instability. A linear theory of density fluctuations is used for the calculations, which should be applicable to weakly unstable non‐magnetized plasmas near the onset threshold of the instability.
A mechanism for the Venus thermospheric superrotation
Authors: Alexander M. J.
Published: February 2008

A physical mechanism for in situ forcing of the Venus thermospheric superrotation is developed, which is in accord with the bulk of the constraining observations. Saturation of small‐ to medium‐scale internal gravity waves in the thermosphere can supply the necessary accelerations of the mean flow if the waves have predominantly large westward phase speeds, characteristic of the wind speeds in the upper clouds. Several scenarios for wave forcing are examined, and the Lindzen‐Holton parameterization is used to derive wave‐induced accelerations of the mean flow. The results are compared to the VTGCM, which uses a similar wave drag parameterization with an ad hoc (empirically based) asymmetry parameter to model the superrotation.
Inferences for the Socorro magma body from teleseismic receiver functions
Authors: Sheetz Kraig E. , Schlue John W.
Published: February 2008

Receiver functions constructed from teleseisms arriving from the northwest and recorded at station WTX in Socorro, NM show strong P‐to‐S conversions we interpret as coming from a mid‐crustal sill‐like magma body. Analysis of these conversions suggest that both the upper and lower surfaces of the magma body are distinct and provide sharp velocity contrasts with the surrounding host rock. Models that provide acceptable fits to these conversions all have magma body thicknesses less than 200 m. The absence of similar phases in data from the south and southeast suggests that the magma body is discontinuous near its edges.
The Role of Benthic Fluxes of Dissolved Organic Carbon in Oceanic and Sedimentary Carbon Cycling
Authors: Burdige David J. , Alperin Marc J. , Homstead Juliana , Martens Christopher S.
Published: February 2008

Benthic fluxes (sediment‐water exchange) of dissolved organic carbon (DOC) represent a poorly quantified component of sedimentary and oceanic carbon cycling. In this paper we use pore water DOC data and direct DOC benthic flux measurements to begin to quantitatively examine this problem. These results suggest that marine sediments represent a significant source of DOC to the oceans, as a lower limit of the globally‐integrated benthic DOC flux is comparable in magnitude to riverine inputs of organic carbon to the oceans. Benthic fluxes of DOC also appear to be similar in magnitude to other sedimentary processes such as organic carbon oxidation (remineralization) in surface sediments and organic carbon burial with depth.
Glacio‐isostatic crustal movements caused by historical volume change of the Vatnajökull Ice Cap, Iceland
Authors: Sigmundsson Freysteinn , Einarsson Páll
Published: February 2008

Measurements of the lake level of Lake Langisjór at the SW edge of the Vatnajökull ice cap indicate a tilt of 0.26 ± 0.06 μrad/year away from the ice cap in the years 1959–1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub‐lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajökull during the Little Ice Age. The inferred sub‐lithospheric viscosity is 1 × 1018 − 5 × 1019 Pa s.
Solar radiative forcing at selected locations and evidence for global lower tropospheric cooling following the eruptions of El Chichón and Pinatubo
Authors: Dutton Ellsworth G. , Christy John R.
Published: February 2008

As a result of the eruption of Mt. Pinatubo (June 1991), direct solar radiation was observed to decrease by as much as 25–30% at four remote locations widely distributed in latitude. The average total aerosol optical depth for the first 10 months after the Pinatubo eruption at those sites is 1.7 times greater than that observed following the 1982 eruption of El Chichón. Monthly‐mean clear‐sky total solar irradiance at Mauna Loa, Hawaii, decreased by as much as 5% and averaged 2.4% and 2.7% in the first 10 months after the El Chichón and Pinatubo eruptions, respectively. By September 1992 the global and northern hemispheric lower tropospheric temperatures had decreased 0.5°C and 0.7°C, respectively compared to pre‐Pinatubo levels. The temperature record examined consists of globally uniform observations from satellite microwave sounding units.
Discrete wave packets at the proton cyclotron frequency at comet P/Halley
Authors: Mazelle Christian , Neubauer Fritz Manfred
Published: February 2008

We present in this letter the first experimental evidence for wave packets near the local proton cyclotron frequency in the plasma environment of a comet. The observations have been made by the GIOTTO magnetometer experiment (MAG) around comet P/Halley on both sides of closest approach. The waves are always left‐handed in the spacecraft frame, elliptically or nearly circularly polarized and they propagate at small angles from the ambient magnetic field. Their period in the spacecraft frame always closely fits the local proton cyclotron period. The possible generation mechanisms of these waves are discussed. They can be consistently interpreted as waves generated by a resonant helical beam instability fed by the cometary pick‐up protons. These waves are intrinsically right‐handed waves in the plasma rest frame and are anomalously Doppler‐shifted from the plasma frame to the spacecraft frame because their phase velocity is small compared to the local solar wind speed. A generation by a resonance with heavy (water group) ions introducing a temperature effect is also discussed but is less satisfactory.
Evidence for upper stratospheric aerosols from balloon‐borne mass spectrometers
Authors: Krieger A. , Arnold F.
Published: February 2008

Strong evidence for the presence of aerosols well above the stratospheric H2SO4/H2O‐aerosol layer was obtained by balloon‐borne mass spectrometer measurements of ambient ions. The instrument was improved with respect to the detection of gaseous sodium compounds. From our measurements very low upper limits of gas phase sodium compounds (2×103 cm−3 around 40 km) could relieably be inferred. This supports the hypothesis that all gas phase metal compounds from meteor ablation are efficiently removed by heterogeneous nucleation involving preexistent aerosols which may be identified as meteoritic or spaceflight debris.
Moment tensor inversion from Mednet data (2) Regional earthquakes of the Mediterranean
Authors: Giardini Domenico , Boschi Enzo , Palombo Barbara
Published: February 2008

We present a method for the rapid determination of the moment tensor suitable for application to regional earthquakes up to 3000 km distance. The algorithm is based on the inversion of long‐period waveforms in the time‐domain and is routinely applied to significant events of the Mediterranean, from Iran to the Azores, using digital data recorded by the MedNet regional network. We show results for 20 significant earthquakes of 1990–1992, including the large shocks in Romania (May 30, 1990), Iran (June 20, 1990), Caucasus (April 29, 1991), Turkey (March 13, 1992) and Egypt (October 12, 1992), the damaging Italian earthquakes of Potenza (May 5, 1990) and Siracusa (December 13, 1990), unusual events in the Southern Mediterranean (November 11, 1990) and Holland (April 13, 1992) and the recent shocks of October 23, 1992 in Morocco and Caucasus. The comparison with published focal mechanisms and Centroid Moment Tensors show our results to be consistent with those retrieved using global data and local networks, proving that an advanced regional seismic network is capable of providing accurate, rapid control of regional seismicity.
The recent warming in eastern Minnesota shown by ground temperatures
Authors: Baker Donald G. , Ruschy David L.
Published: February 2008

Soil and ground (here defined as beyond the depth of weathering or soil) temperatures can be superior to surface temperatures as indicators of climatic trends, because the “noise” common to the latter has been effectively reduced or removed. As a result, the 12.8 m ground temperatures recorded routinely at the University of Minnesota St. Paul campus climatological observatory since 1963 can provide valuable information on climatic trends.

The observatory, located in the midst of land that has been in agricultural test plots since 1885, was instrumented with copper‐constantan thermocouples buried at various depths in the soil and ground. The two air temperature measurements, with which the 12.8 m ground temperature is compared, are made with maximum and minimum glass thermometers housed in standard temperature shelters.

The mean air temperature increases of 0.049°C and 0.035°C per year, 1963–1990, at the observatory and the nearby Eastern Minnesota station, respectively, compare favorably with the 12.8 m ground temperature mean increase of 0.041°C per year. Due to the brevity of the record and to its approximate coincidence with the most recent northern hemisphere warming period, the extrapolation of these rates beyond the record period must be done with care.
Empirical linkages between Arctic sea ice extents and Northern Hemisphere mid‐latitude column ozone levels
Authors: Marko John R. , Fissel David B.
Published: February 2008

Statistically significant correlations are demonstrated between annual mean column ozone data collected at mid‐latitude sites [Kerr, 1991] and mean annual and winter sea ice extents east of Greenland and in the Barents and Kara Seas. These results are discussed with reference to the locations of the correlated parameters relative to the “Basic Pattern” of stratosphere‐solar flux correlations [Labitzke and van Loon, 1992]. Possibilities for underlying linkage mechanisms are considered and related to recent decreasing hemispheric ozone level trends.
Changes in stratospheric ozone and temperature due to the eruptions of Mt. Pinatubo
Authors: Chandra S.
Published: February 2008

The impact of the Mt. Pinatubo eruptions on the total column ozone measured from the Nimbus 7 TOMS and the NOAA ‐11 SBUV/2 spectrometers has been studied. The ozone anomalies inferred from the two instruments agree within 1–2 % in the presence of large volcanic clouds produced by Pinatubo. The Pinatubo eruptions took place on June 15–16, 1991 during the easterly phase of the quasi biennial oscillations (QBO) and as such present a different dynamical scenario for the changes in stratospheric ozone compared to the El Chichon eruptions which took place during the westerly phase of the QBO. Within a few months after the eruptions, the total column ozone decreased by 5–6 % in the tropics, 3 to 4 % at mid‐latitudes and 6–9 % at high latitudes in the northern hemisphere. However, after the effects of QBO and interannual variability are taken into account, the decrease in the column ozone attributed to volcanic eruptions at these latitudes may not be more than 2–4 % – a conclusion in general agreement with a similar study of the El Chichon effects on the stratospheric ozone. The most noticeable effect of the Pinatubo eruptions, as observed during the El Chichon period, is the breakdown of the phase relation between ozone and temperature. This is attributed to additional heating in the lower stratosphere caused by volcanic aerosols.
Inner zone electron peaks observed by the “active” satellite
Authors: Kudela K. , Matisin J. , Shuiskaya F. K. , Akentieva O. S. , Romantsova T. V. , Venkatesan D.
Published: February 2008

Measurements with the SPE‐1 instrumentation on board the low‐altitude satellite, Active, provide energy spectra of electrons in three directions and in 7, 15, or 31 energy channels quasi‐logarithmically distributed approximately within the interval from 20 to 400 keV. The orbit allowed measurement of electrons in the altitude range from 500 to 2500 km and in all local time sectors within any 3‐month interval. In the inner zone, multiple peaks, with the lowest one at 30 keV, are found in the trapped population at L<1.30 and at altitudes above 800 km. The long drift period of such electrons indicates that the drift acceleration resonance model proposed to explain peaks at higher energies is inappropriate here.