bluuebunny/crossref_metadata_2025 huggingface dataset:
Emissions of N2O from the burning of biomass in an experimental system
Authors: Hao W. M. , Scharffe D. , Lobert J. M. , Grutzen P. J.
Published: February 2008

Fifteen experiments were conducted in an open burning system to determine the amounts of N2O produced from burning savanna grass (Venezuelan Trachypogon, Australian Sorghum intrans), straw, hay, oak, pine needles and litter of pine forest. Samples in stainless steel canisters were analyzed within one day after sampling in order to avoid artifact N2O formation during storage. Emissions of N2O were observed to be nearly coincident with the emissions' of CO2 during the burning period for all the experiments. The amount of N2O produced was dependent on the nitrogen content of the biomass. About 0.7% of the fuel nitrogen was oxidized to N2O and most of the N2O was formed during the flaming stage. Using our experimental data and the amounts of different types of vegetation burned globally per year, we estimate that about 2.7 × 1011 g of N2O‐N are produced per year from burning biomass, contributing only 2% to the global source of N2O.
Radar interferometric imaging of field‐aligned plasma irregularities in the equatorial electrojet
Authors: Kudeki Erhan , Sürücü Fahri
Published: February 2008

A multiple‐receiver radar technique for imaging the spatial distribution of ionospheric plasma irregularities is introduced and demonstrated with equatorial electrojet data obtained at the Jicamarca Radio Observatory. The images obtained with a few seconds time resolution enable the monitoring of the temporal evolution of the irregularity structures within the radar field of view. Daytime electrojet images contain signatures of localized irregularity patches which drift in the east‐west direction at about the ion‐acoustic velocity.
Generation of broadband electrostatic noise by electron acoustic solitons
Authors: Dubouloz N. , Pottelette R. , Malingre M. , Treumann R. A.
Published: February 2008

Broadband electrostatic noise (BEN) bursts whose amplitude sometimes reaches about 100 m V m−1 have been observed by the Viking satellite in the dayside auroral zone. These emissions have been shown to be greatly influenced by nonlinear effects and to occur simultaneously with the observation of particle distributions favouring the destabilization of the electron acoustic mode. It is shown that electron acoustic solitons passing by the satellite would generate spectra that can explain the high‐frequency part of BEN, above the electron plasma frequency.
Helium‐3 inside Atoll Barrier Reef interstitial water: A clue for geothermal endo‐upwelling
Authors: Rougerie Francis , Andrié Chantai , Jean‐Baptiste Philippe
Published: February 2008

Interstitial waters from boreholes in the reef conglomerate of Tikehau atoll (S.W. Pacific) contain positive anomalous concentrations of dissolved inorganic nutrients compared to adjacent oceanic and lagoonal waters. These anomalies have been interpreted by geothermal circulation of deep oceanic waters penetrating the porous reef carbonates and ascending through the atoll flanks by thermo‐convective advection as already proposed for other atolls. We present here a new strong evidence of this geothermal circulation inside atoll reefs from the record of helium‐3 anomalies in borehole waters of Tikehau atoll. These results bear directly on three controversial aspects of reef history: the efficiency of thermal energy for circulation of reef pore waters, the sources of nutrients to support the net productivity of reef ecosystems, the early diagenesis of reef foundation carbonates.
On the cavity mode nature of low‐latitude Pi 2 pulsations
Authors: Sutcliffe Peter R. , Yumoto Kiyohumi
Published: February 2008

In this paper confirmatory and new observational evidence is presented in an attempt to clarify the source mechanism of low‐latitude Pi 2 pulsations. We commence by extending our recent investigation on the occurrence of dayside Pi 2 pulsations at low and mid latitudes. Using the technique of data adaptive filtering further evidence is presented to confirm that the pulsations occur almost simultaneously in both the nightside and dayside hemispheres at low latitudes. We then demonstrate that the spectra of the dayside and nightside pulsations are similar. The spectra of Pi 2 pulsations are furthermore compared with those of Pc 3 pulsations and shown to be different in a number of respects. We believe that these various observed characteristics are a consequence of the cavity mode nature of low‐latitude Pi 2 pulsations.
Electric field measurements during supercharging events on the MAIMIK Rocket Experiment
Authors: Denig W. F. , Maynard N. C. , Burke W. J. , Maehlum B. N.
Published: February 2008

We report on the observation and analysis of quasi‐DC electric fields measured during active electron emission from the MAIMIK sounding rocket. The MAIMIK mother‐daughter tethered rocket consisted of two separately instrumented payload sections which were detached in‐flight. The daughter payload included a variable current, 8‐keV electron accelerator and instrumentation to measure the vehicle potential relative to the ionospheric plasma. The electric field detector was part of the quiescent mother payload which separated from the daughter at a constant speed. Of particular interest to the present study were the observations of impulsive electric fields measured at the initiation of the beam for emission currents which drove the daughter payload into an overcharged state. At locations well removed from the active payload the electric field signature consisted of a 1‐ to 2‐ms, monopolar impulse field that had a rapid risetime and a more gradual decay. The components of the vector field were examined using a cylindrical coordinate system that placed the active payload at the origin and fixed the azimuthal injection angle of the beam. In general, the radial components of the field were directed out from the positively charged central body and presumed to be of electrostatic origin. The tangential components had consistent positive deflections indicating that these field components were mostly electromagnetic in nature. We suggest that the measured impulse fields were due to a shielded ion blast wave propagating away from the highly charged central body with azimuthal asymmetries created by the distribution of space charge associated with a virtual cathode and escaping beam electrons. The time‐varying electrostatic fields drove transient Hall currents that were carried by cold electrons E×B drifting near the head of the blast wave.
Solar wind dynamic pressure variations and possible ground signatures of flux transfer events
Authors: Sibeck D. G. , Croley D. J.
Published: February 2008

We follow the effects of a series of solar wind dynamic pressure pulses through the solar wind, across the bow shock and magnetosheath, into the magnetosphere, and down to the dayside ionosphere. Although magnetic field variations associated with the pulses were observed at both ISEE 2 and IMP 8, less significant dynamic pressure pulses were observed by IMP 8. We demonstrate that each pressure pulse had an intrinsic solar wind origin, i.e., was not generated by processes occurring at the Earth's bow shock. Both upstream and downstream of the Earth's bow shock, the pulses were identified by depressed magnetic field strengths and enhanced plasma densities. The pulses compressed the magnetosheath and magnetosphere, causing satellites in the magnetosphere to briefly observe enhanced magnetic field strengths, and/or enter the low‐latitude boundary layer or magnetosheath. Satellites already in the magnetosheath observed a brief burst of enhanced magnetosheath flow caused by the relative inward motion of magnetosheath flow patterns. Alfvén and fast mode compressional waves propagated rapidly down to the polar ionosphere, where they produced transient ionospheric flows at latitudes equatorward of the convection reversal boundary. The flow patterns moved westward around the auroral oval.
Adhesion and abrasion of surface materials in the Venusian aeolian environment
Authors: Marshall John R. , Fogleman Guy , Greeley Ronald , Hixon Raymond , Tucker David
Published: February 2008

In laboratory simulations of the Venusian environment, rock and mineral “target” surfaces struck by aeolian particles develop a thin layer of accretionary material derived from the particles' attrition debris. Accretion may be (in part) a manifestation of “cold welding,” a process well known in engineering, where bonding occurs between metals at a tribological interface. Accretion on geological materials was found to occur at all Venusian surface temperatures and for all types of materials tested. First‐order variations in the amount deposited by particles are related to relative attrition susceptibilities. Second‐order variations relate to properties of the particle‐target interface. Variations in accretion volume are apparently independent of mineral chemistry and are only weakly dependent on crystallography. The results suggest that accretion should be a fairly universal phenomenon in areas of Venus subject to aeolian activity.
The tectonic factor in sea level change: A countervailing view
Authors: Sloss L. L.
Published: February 2008

It is now demonstrable that many unconformity‐bounded assemblages of strata, the sequences and groupings of sequences of seismic stratigraphy, can be correlated subglobally. The facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths, and thus to sea levels. Lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis. Students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments. Proof of this control is buried hi the sedimentary fills of cratonic basins, but it has been difficult to produce such proof in a form digestible by Neo‐Neptunist doubters. Digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study. For example, the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution, the hallmark of a bivariate‐normal distribution. This revelation makes available a number of Gaussian statistical measures as descriptors of time‐variable basin geometry. Extraction and analysis of geometric parameters lead to the realization that basin evolution, necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments, has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons. Secular variation in radial heat flow, leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism.
Large inversions of altimeter and tied tide gauge data for the Mediterranean Sea mean surface
Authors: Houry S. , Mazzega P.
Published: February 2008

Satellite altimeter measurements and tide gauge data tied (TTG data) to a global geodetic reference frame by space techniques of positioning will provide in a near future consistent and complementary information to map the mean sea surface (MSS) with a subdecimetric absolute accuracy. Information is consistent because, after standard data processing, both kinds of sea level measurements are given relative to the same geocentric reference ellipsoid. Information is also complementary because: first, satellite altimetry draws a dense sampling pattern of the ocean while TTG measurements are local; second, the properties of the data errors are very dissimilar in the two techniques. Both kind of sea level measurements are already available in the Mediterranean basin. This study is the first attempt to compute an accurate MSS model with mixed sets of such data. A total inverse method, previously developed to analyze satellite altimetry, is greatly improved in order to recover a full spectrum of the MSS (down to wavelengths λ ≃ 100 km) and to invert rigorously large data sets including consecutive satellite tracks. The MSS solution derived solely from altimetry in the Mediterranean Sea has a formal error of 12 cm under the repeat satellite tracks, up to 30 cm near the coasts, with a further truncation error of 10 cm. Then an analysis of the discrepancy between this solution and TTG data allows us to point out unsuspected inconsistencies in the two data sets which are clearly identified. Nevertheless the usefulness of the TTG data is demonstrated by an estimate of their impact on the error spectrum of the MSS.
Ice‐3G: A new global model of Late Pleistocene deglaciation based upon geophysical predictions of post‐glacial relative sea level change
Authors: Tushingham A. M. , Peltier W. R.
Published: February 2008

A new high resolution global model of late Pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice‐ocean‐solid Earth interaction is treated in a gravitationally self‐consistent fashion. For the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem. Only radiocarbon controlled relative sea level histories from sites that were actually ice covered (with one or two additions) are employed to constrain the model, leaving relative sea level (RSL) data from sites that were not ice covered to be employed to confirm its consistency. Results for these confirmatory analyses are reported elsewhere. Here the new deglaciation model, referred to as ICE‐3G, is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories, including geologically controlled retreat isochrones, oxygen‐isotope data from deep‐sea sedimentary cores, and coral terrace elevations. The latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial‐interglacial transition.
Numerical simulation of the crystallization of multicomponent melts in thin dikes or sills: 2. Effects of heterocatalytic nucleation and composition
Authors: Hort Matthias , Spohn Tilman
Published: February 2008

A kinetic model is presented for the crystallization of two‐component melts in thin dikes or sills specifically providing for the nucleation of one crystalline phase on the surface of another crystalline phase during simultaneous crystallization. This process is termed heterocatalytic nucleation. It is assumed that the activation energy for heterocatalytic nucleation is reduced by a factor F from the activation energy for ordinary nonheterocatalytic nucleation. The impediment of crystal growth of crystals growing on each others' surfaces is specifically accounted for. The resulting nucleation and growth rate functions are temperature and concentration dependent. The crystallization time was found to be almost independent of the strength of the heterocatalytic nucleation, but the crystallization paths deviate increasingly less from thermodynamic equilibrium with increasing rate of heterocatalytic nucleation. Crystal sizes for heterocatalytic nucleation are typically found to be about an order of magnitude smaller than for nonheterocatalytic nucleation. For initial compositions significantly different from the eutectic composition, two modes are observed for the crystal size distributions of the liquidus phase. The primary modes of the liquidus phase distributions are attributed to nonheterocatalytic nucleation, while the secondary modes are caused by heterocatalytic nucleation. The secondary modes may become the prominent modes in terms of amplitude for sufficiently small values of F. The bimodal crystal size frequency distributions offer an explanation for porphyritic textures of igneous rock in small‐scale intrusions by a one‐stage cooling history. The fine‐grained groundmass is the result of heterocatalytic nucleation, while the phenocrysts nucleated nonheterocatalytically. A strong coupling via a thermostat effect between undercooling and crystallization rate causes the longest part of the melt volume to crystallize at small and nearly constant rates of undercooling. The degree of crystallinity of the liquidus phase close to the margin of the intrusion may vary between 10% and 100% as a consequence of small changes of the initial composition of the melt. The latter two results are almost independent of the value of F. The suppression of the crystallization of the liquidus phase can be explained by the strong metastable crystallization of the solidus phase. This result shows that the degree of crystallinity close to the margin cannot be taken as a simple indicator of the cooling rate of an intrusion but is a complex function of kinetic parameters, cooling rate, and melt composition.
Revised and synthetic apparent polar wander paths of the African, Eurasian, North American and Indian Plates, and true polar wander since 200 Ma
Authors: Besse Jean , Courtillot Vincent
Published: February 2008

We have reviewed paleomagnetic data available for the Eurasian, African, North American and Indian plates over the last 200 Ma. Selection criteria are those generally accepted, with an emphasis on evidence for lack of remagnetization, accurate dating and proper structural analysis. This results in 23, 35, 51 and 2 poles for Eurasia, Africa, North America and India, respectively. We believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander (APW) paths than averaging of larger data sets involving less stringent selection criteria. We propose and describe revised APW paths, but more importantly, we next use relative motion models to transfer all data in a common reference frame. We find good agreement between transferred data, when they are averaged separately for each plate in independent 20‐Ma windows. This is a check on consistency of paleomagnetic data, kinematic models, and of the geocentric dipole hypothesis. Transferred data from all plates are averaged in 20‐Ma windows to generate synthetic APW paths for all plates studied. These synthetic paths are in agreement with the original (revised) APW paths that use only data from a single plate. Moreover, both geographic and time resolution are improved (spatial confidence intervals are of the order of 5°). The synthetic paths display interesting features, such as a previously ill‐recognized APW loop for Eurasia. Paleomagnetic and hotspot APW are next compared, and a determination of true polar wander (TPW) is derived. We find significant TPW, amounting to over 20° in the last 200 Ma. TPW appears to be episodic, with a standstill between 180 and 110 Ma. There is general agreement with a previous study of Livermore et al. (1984). The TPW standstill appears to correlate with a time of decreasing reversal frequency, ending with the Cretaceous Long Normal Superchron. Other periods of fast TPW would seem to correspond to increasing reversal frequency. However, it is suggested that the major TPW hairpin at 50 Ma might correspond to the collision of India with Eurasia. Tentative correlations with core or mantle indicators can be understood as a result of couplings between the core, mantle and lithosphere, we believe primarily related to episodic evolution of the D″ layer.
First epoch geodetic measurements with the Global Positioning System across the Northern Caribbean Plate Boundary Zone
Authors: Dixon T. H. , Gonzalez G. , Lichten S. M. , Katsigris E.
Published: February 2008

The first geodetic survey across the northern Caribbean plate boundary zone with the Global Positioning System (GPS) was conducted in June 1986. Baseline vectors defined by the six station regional GPS network ranged from 170 to 1260 km in length. Repeatability of independent daily baseline estimates was better than 8 mm plus 1.3 parts in 108of baseline length for horizontal components. The wet tropospheric path delay during the experiment was both high, sometimes exceeding 30 cm at zenith, and variable, sometimes exceeding 5 cm variation over several hours. Successful carrier phase cycle ambiguity resolution (“bias fixing”) could not be achieved prior to construction of a regional troposphere model. Tropospheric calibration was achieved with water vapor radiometers at selected sites, and with stochastic troposphere models and estimation techniques at remaining sites. With optimum troposphere treatment and single‐day orbital arcs, we resolved most biases on baselines up to about 550 km in length. With multiday orbital arcs we resolved most biases in the network regardless of baseline length. Our results suggest that constraints on plate boundary zone deformation in the Greater Antilles, and on the North America‐Caribbean relative plate motion vector, can be obtained with a series of GPS experiments spanning less than 10 and 15 years, respectively.
Unlocking the Ice House: Oligocene‐Miocene oxygen isotopes, eustasy, and margin erosion
Authors: Miller Kenneth G. , Wright James D. , Fairbanks Richard G.
Published: February 2008

Oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on Antarctica since the earliest Oligocene (circa 35 Ma). The growth and decay of ice sheets during the Oligocene to modern “ice house world” caused glacioeustatic sea level changes. The early Eocene was an ice‐free “greenhouse world,” but it is not clear if ice sheets existed during the middle to late Eocene “doubt house world.” Benthic foraminiferal δ18O records place limits on the history of glaciation, suggesting the presence of ice sheets at least intermittently since the earliest Oligocene. The best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic δ18O records. Although planktonic isotope records from the western equatorial regions are limited, subtropical planktonic foraminifera may also record such ice volume changes. It is difficult to apply these established principles to the Cenozoic δ18O record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events. We improved Oligocene to Miocene correlations of δ18O records and erected eight oxygen isotope zones (Oi1‐Oi2, Mi1‐Mi6). Benthic foraminiferal δ18O increases which are associated with the bases of Zones Oil (circa 35.8 Ma), Oi2 (circa 32.5 Ma), and Mil (circa 23.5 Ma) can be linked with δ18O increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near Antarctica. Our new correlations of middle Miocene benthic and western equatorial planktonic δ18O records show remarkable agreement in timing and amplitude. We interpret benthic‐planktonic covariance to reflect substantial ice volume increases near the bases of Zones Mi2 (circa 16.1 Ma), Mi3 (circa 13.6 Ma), and possibly Mi5 (circa 11.3 Ma). Possible glacioeustatic lowerings are associated with the δ18O increases which culminated with the bases of Zone Mi4 (circa 12.6 Ma) and Mi6 (circa 9.6 Ma), although low‐latitude planktonic δ18O records are required to test this. These inferred glacioeustatic lowerings can be linked to seismic and rock disconformities. For example, we link 12 Oligocene‐early late Miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries (= inferred eustatic lowerings) of Haq et al. (1987).
Forced nutations of the Earth: Influence of inner core dynamics: 3. Very long interferometry data analysis
Authors: Herring Thomas A. , Buffett Bruce A. , Mathews P. M. , Shapiro Irwin I.
Published: February 2008

We discuss the analysis of 798 very long baseline interferometry (VLBI) experiments carded out between July 1980 and February 1989, and the determination from this analysis of corrections to selected coefficients in the International Astronomical Union (IAU) 1980 theory of the nutations of the Earth. Our analysis confirms earlier VLBI results and indicates that most of these corrections can be explained by carefully accounting for (1) corrections to the IAU 1980 rigid‐Earth nutation series, (2) the presence of the Earth's inner core, (3) the difference between the dynamic flattening of the Earth inferred from the precession constant and that inferred from seismic models of the internal density structure of the Earth, and (4) the effects of mantle anelasticity and ocean tides. The standard deviations of the corrections to the coefficients are 0.04 milliarcseconds (mas) for terms with periods under 430 days, and 1.0 mas for the terms with a period of 18.6 years. The unresolved issues raised by our analysis are the origins of corrections to the out‐of‐phase retrograde annual(0.39 mas) and the in‐phase prograde 13.66 day (− 0.25 mas) nutations. Our analysis also yields a correction to the IAU 1976 value for the luni‐solar precession constant of −0.32±0.13 arc sec/century (cy).
The continental crustal age distribution: Methods of determining mantle separation ages from Sm‐Nd isotopic data and application to the southwestern United States
Authors: DePaolo D. J. , Linn A. M. , Schubert G.
Published: February 2008

Samarium‐neodymium isotope systematics provide a means of determining the age of the continental crust, where ”age“ refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm‐Nd model age or the mantle separation age. Systematic analysis of rocks from the continents can be used to determine the age distribution by mass in the continents; this is a unique record of the thermal and chemical evolution of Earth over almost 4 b.y. The establishment of mantle separation ages for crustal rock reservoirs requires precise geochronological information typically provided by U‐Pb isotopic measurements of zircon, plus models for (1) Nd isotopic evolution in the mantle source of continental crust, (2) Nd isotopic evolution of crustal reservoirs subsequent to formation, (3) petrogenesis of certain types of granitic rocks in the crust, (4) the mechanisms of formation of hybrid crust (mixtures of material newly differentiated from the mantle with material derived from older crust), and (5) the evolution of the lower parts of the continental crust during continental margin magmatism and intracontinental extension. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The greatest uncertainties come from a lack of adequate information on lower crustal processes and composition. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass‐age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. Almost all of the Phanerozoic additions occur in areas where there was no preexisting continental basement; Mesozoic magmatic additions account for only about 2% of the crustal mass in the area underlain by Precambrian basement. In the region of post‐Precambrian continental growth, the regions farthest from the Precambrian continental edge are composed of mostly (>85%) of material newly differentiated from the mantle. Adjacent to the Precambrian continental edge the new crust is typically composed about 50% of material newly differentiated from the mantle and 50% of old crustal material. There is no correlation between crustal thickness and the crust formation age in the area studied; the age‐area and age‐mass curves are nearly identical.
PKP(BC) versus PKP(DF) differential travel times and aspherical structure in the Earth's inner core
Authors: Shearer Peter M. , Toy Kenneth M.
Published: February 2008

PKP(BC) versus PKP(DF) differential travel times show coherent patterns of residuals indicating aspherical structure within the Earth's inner core. Using a waveform correlation method, we measure 318 differential PKP(BC) versus PKP(DF) travel times from 5 years of Global Digital Seismograph Network (GDSN) short‐period data at ranges between 145° and 155°. In addition, we obtain 19,470 probable BC versus DF travel times at these ranges by searching through 23 years of the International Seismological Centre (ISC) catalog. Plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths. This pattern is robust with respect to range and source depth. These anomalies are almost certainly due to aspherical inner‐core structure since PKP(BC) and PKP(DF) have nearly coincident ray paths in the mantle and outer core. The GDSN and ISC residual patterns are in general agreement, but the ISC residuals underpredict the GDSN residuals by about a factor of 2. The ISC data have better spatial coverage, particularly for N‐S ray paths within the inner core. These N‐S ray paths show positive residuals of about 0.5 s, indicating faster inner‐core P velocities in this direction, a result consistent with previous studies of absolute PKP(DF) travel times but derived from an independent data set. Due to the sparse ray coverage of our data, either heterogeneity or anisotropy within the inner core with about 1% velocity variations could explain these residual patterns. However, inversions of the ISC data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two‐parameter anisotropy model.
Rapid inversion of two‐ and three‐dimensional magnetotelluric data
Authors: Smith J. Torquil , Booker John R.
Published: February 2008

We have developed an efficient iterative inversion method applicable to both two‐dimensional (2D) and three‐dimensional magnetotelluric data. The method approximates horizontal derivative terms with their values calculated from the fields of the previous iteration. The equations at each horizontal coordinate then become uncoupled. At each iteration this allows separate inversions for the improved conductivity profile beneath each measurement site. Resultant profiles are interpolated to form a new multidimensional model for which the fields for the next iteration are calculated. The method is extremely fast, and tests with 2D data show very promising results.
Variations in permafrost thickness in response to changes in paleoclimate
Authors: Osterkamp T. E. , Gosink J. P.
Published: February 2008

Solutions to the Stefan problem for the motion of the base of ice‐bearing permafrost in response to changes in paleoclimate were obtained using perturbation, finite difference, and finite element methods. Paleotemperature models were used to investigate the thickness response, to compare solution methods, to determine the current state of the permafrost, and to determine constraints on the models. The perturbation and finite difference methods used the approximation of linear temperature profiles, while the finite element method did not. There was a transient thickness response of about 41 kyr, implying that paleotemperature records of greater length are needed for models and that the permafrost loses its “memory” of past conditions for much longer times. Faster thawing rates, slower freezing rates, and greater variations in thickness were found for the perturbation and finite difference solutions compared to the finite element solution. These results appear to be caused by the simplifying assumptions in the former solutions. A lag (about 20 kyr) exists between changes in surface temperature and thickness response, and a small thermal offset is apparent in the finite element solution. Small asymmetries exist in the freezing and thawing rates and the thickness response. Paleotemperature models based on ice cores predict current permafrost thicknesses that are too large. Models with the long‐term mean surface temperature of permafrost within a few degrees of the current value of −11°C and with the last glacial period temperatures no more than 6°–8°C lower are compatible with current Prudhoe Bay conditions. These include models developed for East Siberia, from isotopic profiles in deep‐sea sediments, and for Barrow, Alaska, modified for Prudhoe Bay. These models predict that the permafrost thickness at Prudhoe Bay varied by about ±10% or less (±60 m) over the last glacial cycle. Freezing and thawing rates were less than 6 mm yr−1. At present this permafrost should be near its long‐term equilibrium thickness and thawing at <1 mm yr−1.
Integration of east African Paleostress and present‐day stress data: Implications for continental stress field dynamics
Authors: Bosworth William , Strecker M. R. , Blisniuk P. M.
Published: February 2008

Borehole breakout studies, aligned Quaternary volcanic vents, and kinematic analysis of Quaternary faults indicate that the present‐day least horizontal stress direction (Shmin) in Kenya is aligned approximately NW‐SE and in central Sudan is nearly N–S. Limited data in eastern Ethiopia suggest a NE‐SW Shmin orientation. The regional pattern for Shmin is therefore roughly radially disposed about the Afar plate junction. Geologic structures in the Gregory Rift, western Kenya, suggest that up to 0.6–0.4 Ma, Shmin was oriented E‐W, normal to the main rift trend. Similar arguments for the Ethiopian Rift indicate that during its main phases of extension, Shmin was oriented NW‐SE. These data indicate that the central east African stress field underwent a significant realignment during the Quaternary, rotating about 45° in a clockwise sense in the Kenyan Rift. Similar rotations may have occurred in eastern Sudan and eastern Ethiopia on the periphery of the Ethiopian Rift. In Kenya, this stress reorientation resulted in dextral oblique reactivation of older normal faults and formation of new normal faults with dextrally oblique slip components. The large‐scale structural framework of the Kenyan rift is no longer suitably oriented to accommodate large extensional strains. East African extension may now be transferred to the western branch of the rift system (Tanganyika‐Malawi rifts) and rejuvenated basins south of the Gregory Rift in Tanzania (Eyasi‐Manyara rifts). Rapid rotations of near‐surface stress tensors (of the order of 7.5°/105 years in the case of East Africa) are now documented in several continental extensional systems. It is unlikely that large‐scale mantle circulation patterns can fluctuate at such rapid rates. This suggests that drag at the base of the lithosphere, at least in continental extensional areas, is not the dominant force controlling the orientation of the near‐surface stress field. Rather, intraplate forces, produced and propagated from distant plate boundaries (ridge‐push, subducting‐slab‐pull), may play a greater role in configuring the continental crustal stress regime.
Reformulation of Stokes's theory for higher than second‐degree reference field and modification of integration kernels
Authors: Vaníček Petr , Sjöberg Lars E.
Published: February 2008

An argument is put forward in favor of using a model gravity field of a degree and order higher than 2 as a reference in gravity field studies. Stokes's approach to the evaluation of the geoid from gravity anomalies is then generalized to be applicable to a higher than second‐order reference spheroid. The effects of truncating Stokes's integration and of modifying the integration kernels are investigated in the context of the generalized approach. Several different modification schemes, starting with a Molodenskij‐like modification and ending with the least squares modification, are studied. Particular attention is devoted to looking at both global and local biases and mean square errors of the individual schemes.
Oxygen isotope correlation of cetacean bone phosphate with environmental water
Authors: Yoshida Naohiro , Miyazaki Nobuyuki
Published: February 2008

The variation with time in the oxygen isotope ratio of the oceans is of prime interest in a variety of research fields. An excellent correlation between oxygen isotope ratios of cetacean (whales, dolphins, and porpoises) bone phosphate and their environmental water is found in this study. Bone phosphate samples of dolphins living in fresh waters are more depleted in oxygen 18 than those of cetaceans living in the oceans, reflecting the clear difference in the isotope composition of water. Cetaceans distributed in the higher latitudes in the oceans are more depleted in oxygen 18 than those distributed in the lower latitudes where seawater is slightly enriched in oxygen 18 relative to that in the higher latitudes. The present results provide a promising tool for estimating the oxygen isotope ratio of the oceanic water of the past without assuming water temperature.
Overview of the Frontal Air‐Sea Interaction Experiment (FASINEX): A study of air‐sea interaction in a region of strong oceanic gradients
Authors: Weller Robert A.
Published: February 2008

From 1984 to 1986 the cooperative Frontal Air‐Sea Interaction Experiment (FASINEX) was conducted in the subtropical convergence zone southwest of Bermuda. The overall objective of the experiment was to study air‐sea interaction on 1‐ to 100‐km horizontal scales in a region of the open ocean characterized by strong horizontal gradients in upper ocean and sea surface properties. Ocean fronts provided both large spatial gradients in sea surface temperature and strong jetlike flows in the upper ocean. The motivation for and detailed objectives of FASINEX are reviewed. Then the components of the field program are summarized. Finally, selected results are presented in order to provide an overview of the outcome of FASINEX.
Ice‐ocean interaction on Ronne Ice Shelf, Antarctica
Authors: Jenkins A. , Doake C. S. M.
Published: February 2008

Detailed glaciological studies have been completed at 28 sites lying on an approximate flow line, extending 760 km across Ronne Ice Shelf. Parameters measured at each location include ice velocity, thickness, principal strain rates, surface elevation, temperature, and accumulation rate. The data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site, from the principles of mass and energy conservation. These calculations indicate basal melting in excess of 1 m yr−1 over the first 100 km of the flow line downstream of the grounding line, where the ice shelf is between 1200 m and 1600 m thick. The maximum melt rates in this region occur near the inland margin and exceed 4 m yr−1. Melting continues at a lesser rate over the next 200 km before freezing commences. Freezing then dominates up to the final 100 km before the ice front, resulting in the accumulation of a layer of basal sea ice up to 50 m thick. This is rapidly removed as melt rates increase to over 6 m yr−1 at the ice front. The cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85% of the mass discharged across the grounding line before it reaches the ice front. Supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections, observed during radio echo sounding of ice thickness. In the region where mass balance calculations suggest an accretion of basal sea ice, reflection coefficients are consistently low, ranging from −6 dB to −38 dB. Most of these weak reflections are believed to originate from the true base of the ice shelf, the additional energy loss being the result of increased attenuation of the radar signal within the saline layer. The derived pattern of basal melting and freezing is consistent with a simple model of sub‐ice‐shelf oceanic circulation, involving a deep thermohaline convection cell. Dense, saline water, which is formed over the continental shelf during winter when the sea surface freezes, drains into the deepest parts of the sub‐ice‐shelf cavity. At the inland margin, where this water mass comes into contact with the ice shelf, its temperature is 1°C above the local pressure freezing point. Melting of ice results, producing a buoyant outflow of cold, relatively fresh water, along the ice shelf base. Basal freezing occurs towards the ice front, where the ascending water becomes supercooled. This circulation has important implications for the production of Antarctic Bottom Water and for the response of the ice shelf to driving stresses, through the temperature dependent viscosity of ice.
Evidence of a critical Richardson number in moored measurements during the upwelling season off northern California
Authors: Kundu Pijush K. , Beardsley Robert C.
Published: February 2008

Evidence is found for a cutoff gradient Richardson number ofRi∼ 0.25 in the moored measurements of temperature and current obtained during the second Coastal Ocean Dynamics Experiment (CODE 2), conducted off northern California during the upwelling season of March–August 1982. The cutoff atRi∼ 0.25 was noticed even when the instruments were separated by depth intervals as large as 40 m, or when the time series (having a sampling interval of 7.5 min) were averaged over an hour. During several strong wind events which tended to increase the mean shear,Ri∼ 0.25 occurred in the main thermocline beneath the surface mixed layer over the shelf. On the other hand, several instances were found when a lowRiwas not directly related to any wind event, suggesting that superposition of various waves can locally generate unstable conditions. The deeper water over the shelf and slope was generally stable withRi» 0.25. These results suggest that in coastal areas with strong currents, the intensification of mean vertical shear seems to be very important in bringing about Kelvin‐Helmholtz‐type instabilities, as compared with the open ocean without strong currents where superposition of internal waves is believed to be primarily responsible for causing the instabilities. Because the density structure almost everywhere in the ocean is in the form of steps, and because the overall Richardson number across vertical scales as large as 20 m are frequently near critical, it follows that appreciably thick regions in the ocean frequently remain on the brink of Kelvin‐Helmholtz instability, even though the actual mixing may take place within thin interfaces between relatively homogeneous layers. A small increase of the overall shear then greatly increases the frequency of billow events and vertical mixing, and a small decrease of the overall shear greatly decreases such activity. It is suggested that in numerical circulation models the Richardson number be constantly checked and vertical mixing be used to maintain the Richardson number above a minimum value of 0.25.
Aircraft observations of the mean and turbulent structure of the atmospheric boundary layer during spring in the central Arctic
Authors: Walter Bernard , Overland James E.
Published: February 2008

The NOAA P‐3 research aircraft carried out measurements of the mean and turbulence structure of the planetary boundary layer on March 27 and 30, 1989, at 83°N, 10°E in the central Arctic. The ice motion was strongly decoupled from the atmosphere; surface winds were 2–3 m s−1 and winds at 100 m were 10 to 12 m s−1 on March 27 and 8 m s−1 on March 30. Geostrophic drag coefficients were 0.014 and 0.015. The mean vertical structure on both days is characterized by a shallow slightly stable layer ∼50 m deep at the surface with a 300‐m‐deep strong inversion above. Speed shear was 0.1 s−1 in the slightly stable layer and direction shear at the top of the layer was 15°–20° on March 27 and 35°–40° on March 30. Profiles of the Richardson number for each day showed that Ri was less than 0.25 in the slightly stable layer and rapidly increased above. A low level jet was located at the top of the slightly stable layer just below the level where the Richardson number became very large. Shear‐induced turbulence at the top of the slightly stable layer resulted in a maximum of turbulence kinetic energy just below the wind maximum on March 27 and at the wind maximum on March 30. Heat flux profiles from the gust probe measurements showed a maximum of upward flux at the top of the slightly stable layer of 9 W m−2, implying that the flux was countergradient. This upward flux was responsible for maintaining the strength of the inversion above this level. Spectra of turbulent vertical velocity showed a peak at a wavelength of 350–400 m. The source of this peak is gravity waves from an upper level shear layer. It is speculated that the observed countergradient flux was a result of gravity waves and/or wave‐turbulence eddy flux. Use of an E‐ε turbulence closure model allowed an estimation of the sensible heat flux from the air to the snow surface of 4 W m−2. This surface flux played only a secondary role in cooling the surface layer and establishing the structure of the boundary layer compared to inversion layer processes.
Measurement of soil nitrogen oxide emissions at three North American ecosystems
Authors: Williams E. J. , Fehsenfeld F. C.
Published: February 2008

Results from measurements of emission of nitrogen oxides from soils at three North American ecosystems are presented. These measurements were conducted during the summer and fall of 1988 at (1) a grassland site near Nunn, Colorado, (2) a coastal marine environment at North Inlet, South Carolina and (3) a deciduous forest near Oak Ridge, Tennessee. Emission of NO was highest from the grassland soil (mean: 10.0 ng N m−2 s−1), intermediate at the forest area (mean: 0.28 ng N m−2 s−1) and lowest at the coastal site (mean: 0.034 ng N m−2 s−1). A comparison of the results from the present study with previous measurements indicates that NOx (NO + NO2) emission from grasslands and temperate forests are similar within each ecosystem independent of location. This suggests that simple approaches may be used to estimate soil emissions over wide areas.
Stratospheric aerosol optical depth observed by the Stratospheric Aerosol and Gas Experiment II: Decay of the El Chichon and Ruiz volcanic perturbations
Authors: Yue G. K. , McCormick M. P. , Chiou E. W.
Published: February 2008

The decay of the El Chichon perturbation to the optical depth of stratospheric aerosols at 1.02 μm, 0.525 μm, and 0.453 μm is calculated from the Stratospheric Aerosol and Gas Experiment II (SAGE II) data set for the period December 1984 to December 1988. It is found that the perturbed optical depths at middle and higher latitudes of both hemispheres exhibited an exponential decay superimposed by a seasonal oscillation with maximum and minimum occurring in local winter and local summer, respectively. Microphysical processes and variation of the tropopause height alone cannot explain this seasonal change of optical depth. The magnitudes of the exponential component at higher latitudes were, in general, larger than those at lower latitudes. For optical depths in tropical regions, the seasonal oscillations were small and were disturbed by the eruption of Nevado del Ruiz on November 13, 1985. The increase in ratio of optical depth at 0.525 μm to that at 1.02 μm from about 2.0 at the beginning of 1985 to about 3.5 at the end of 1988 indicates the average size of aerosol particles in the stratosphere is diminishing since the eruption of El Chichon. The l/efolding time for El Chichon decay derived from the SAGE II data set is in reasonably good agreement with those derived by other methods.