bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic evolution of the Late Archaean to Early Proterozoic Mount Bruce Megasequence Set, western Australia
Authors: Blake Tim S. , Barley Mark E.
Published: February 2008

The Mount Bruce Megasequence Set (formerly the Mount Bruce Supergroup) was deposited on the Pilbara Craton during the late Archaean and early Proterozoic. It comprises two megasequences, the Chichester Range Megasequence and the overlying Hamersley Range Megasequence. Each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of Phanerozoic‐style tectonics. The basal Nullagine Supersequence, which comprises mostly subaerial basalt and terrigenous sedimentary rock, was deposited during an episode of WNW‐ESE directed crustal extension that was initiated at about 2770 Ma. The overlying Mount Jope Supersequence, which comprises mostly basalt and tuff, was deposited during the subsequent development of a WNW‐ESE trending rift before about 2690 Ma. Mudrock, carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 Ma Marra Mamba Supersequence Package were deposited on the ensuing divergent craton margin. An unconformity or condensed succession covering a time span of as much as 130 m.y. separates the top of the Chichester Range Megasequence from the Hamersley Range Megasequence during which time an oceanic island arc is interpreted to have collided with the southern Pilbara Craton margin. Subsequently, the subducting oceanic plate flipped from southwest dipping to northeast dipping, and convergence resulted in the formation of a marginal subduction‐related orogen with associated arc and backarc volcanism. Iron formation and distal arc‐derived tuffs of the Brockman Supersequence Package were deposited on a siliciclastic‐starved platform in a neutral continental backarc setting at around 2470 Ma. The overlying circa 2440 Ma Woongarra Supersequence was also deposited in a backarc continental setting and comprises iron formation, felsic tuffs, and probably coeval mafic rocks. The latter two rocks types were derived from a backarc magmatic belt. Shortening of the southern Pilbara Craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the Turee Creek Supersequence were deposited, and ultimately resulted in a continent‐continent collision. The Mount Bruce Megasequence Set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen.
Variscan extension in the Pyrenees
Authors: Vissers R. L. M.
Published: February 2008

Evidence for Variscan extension in the Pyrenees is summarized and used with other geological and geochronological data to reconstruct the Variscan extensional geometry and kinematics at a crustal scale. A regionally consistent pattern of ESE‐WNW trending synmetamorphic stretching directions indicates Variscan extension roughly parallel to the present alpine trend of the belt. The spatial distribution of Variscan low pressure ‐ high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping, crustal‐scale shear zones allowing heterogeneous ductile extension of the Variscan middle and lower crust. Extension occurred synchronous with uplift and emergence of large parts of the crust and deposition, during the Upper Carboniferous and Permian, of up to 2500 m of continental sediments and intercalated basaltic‐andesitic volcanics in fault‐bounded extensional halfgrabens. The extensional structures and related metamorphism postdate a Namurian to early Westphalian stage of crustal thickening, characterized by thrusting and steep folding. It is suggested that Variscan extension in the Pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere, enhanced by detachment of a cold and gravitationally unstable lithospheric root, which caused progressive uplift, tectonic denudation, and a highly anomalous thermal structure of the Variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the Early Permian.
Kinematics, topography, shortening, and extrusion in the India‐Eurasia collision
Authors: Le Pichon Xavier , Fournier Marc , Jolivet Laurent
Published: February 2008

We examine the problem of partitioning between shortening and extrusion in the India‐Asia collision since 45 Ma. We compute the amount of shortening expected from the kinematics of India's motion with respect to Eurasia, using the reconstruction at collision time to put bounds on the possible amounts of surface loss within Greater India and within Eurasia. We then compute the amounts of surface loss corresponding to the thickened crust of Tibet and of the Himalayas, assuming conservation of continental crust. The spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it. This distribution indicates an important eastward crustal mass transfer. However, the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis. The deficit may be accounted either by loss of lower crust into the mantle, for example through massive eclogitization, or by lateral extrusion of nonthickened crust. A mass budget of the crust of the Himalayas indicates that lower crust has not been conserved there, but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit. Alternatively, following Zhao and Morgan [1985], lower crust may have been transferred below the Tibetan crust. We conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between India and Asia since 45 Ma. This conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m.y. history of the collision.
Crustal heterogeneity and basement influence on the development of the Kenya Rift, East Africa
Authors: Smith Martin , Mosley Peter
Published: February 2008

Structural and metamorphic studies within the exposed Precambrian basement in Kenya indicate that a number of heterogeneities exist within the lithosphere beneath the line of the Kenya Rift. Of these, the most important is the mechanical and thermal contrast between thick, cold, and rigid Archean lithosphere and thinner anisotropic Proterozoic mobile belt crust/lithosphere. Structural, geophysical, and heat flow data indicate that during a late Proterozoic collisional event the margin of the Tanzanian Archean craton was reworked, overthrust, and effectively buried by tectonically emplaced “Mozambique Belt” rocks. Its position now lies some 100 km east and northeast of the exposed outcrop. These variations in crust/lithosphere type may be directly correlated with the morphotectonic and structural framework of the Kenya Rift and in particular are reflected in the spatial patterns of Cenozoic volcanism within the Gregory Rift zone. Along the craton margin and within the mobile belt a series of late Proterozoic continental‐scale NW‐SE and N‐S trending ductile/brittle shear zones are fundamental. The reactivation of these shear zones under varying stress field conditions is presented as a model to account for the patterns of rift subsidence, including the location and geometry of graben structures, and the intrusion of magmas since early Miocene times.
Refined estimation of marine and terrigenous contributions to sedimentary organic carbon
Authors: Jasper John P. , Hayes J. M.
Published: February 2008

Molecular carbon isotopic (13C/12C) ratios of a selected marine organic biomarker compound (a C37 alkadienone) from the Pigmy Basin (Deep Sea Drilling Program Hole 619) in the northern Gulf of Mexico varied by 5.3‰ in sychrony with the last ∼100 ka glacial‐interglacial cycle. This carbon isotopic (δ13C) variation is likely due to the effect of climatically linked variations in concentrations of CO2(aq) on photosynthetic fractionation of carbon isotopes by phytoplankton. Measurement of the δ13C of the alkadienone (δ37:2) to reconstruct the carbon isotopic composition of total marine organic carbon permits improved estimates of the time‐varying δ13C of preserved marine organic carbon end members. With the time series record of marine end members, the concentrations and net accumulation rates of terrigenous Corg and marine Corg are calculated and their paleoceanographic significance is considered. Although this method may be improved by consideration of additional biomarkers representative of diverse marine and terrigenous inputs, it is the first application of molecular isotopic analysis to quantifying marine and terrigenous Corg concentrations. Further application of such techniques will be become valuable in reconstructing regional and global budgets of marine and terrigenous Corg accumulation.
Constraints on the alkalinity and circulation of glacial circumpolar deep water from benthic foraminiferal barium
Authors: Lea David W.
Published: February 2008

The role of the ocean in driving the glacial reduction in atmospheric carbon dioxide remains an enigma. Several recent hypotheses have invoked a redistribution of oceanic alkalinity as a possible cause of the glacial‐to‐interglacial atmospheric pCO2 change [Boyle, 1988a, b; Broecker and Peng, 1989]. Here I attempt to use paleo‐barium data derived from Ba/Ca of benthic foraminifera to reconstruct chemical changes in glacial Circumpolar Deep Water (CPDW). These new data indicate that glacial CPDW was enhanced in Ba by up to 20%, presumably due to the elimination of the low‐Ba North Atlantic Deep Water (NADW) flux to the southern ocean. This change in Ba can be used to constrain changes in CPDW alkalinity due to circulation changes alone; the Ba data suggest glacial CPDW alkalinity was perhaps 20 to 25 μequiv/kg higher than in the modern ocean. Following the polar alkalinity hypothesis, this difference could account for about one‐third of the pCO2 drop [Broecker and Peng, 1989]. The new Ba data further confirm the homogeneous deepwater distribution of Ba during the last glacial maximum. Of the three paleochemical tracers of circulation recorded in benthic foraminifera (Cd/Ca, δ13C, Ba/Ca), the glacial Ba/Ca data appear to record the most marked reduction in the influence of nutrient depleted NADW in the deep oceans.
Alkalinity and carbon budgets in the Mediterranean Sea
Authors: Copin‐Montégut Claire
Published: February 2008

Alkalinity and salinity are linearly correlated in the waters of the most western part of the Mediterranean Sea. Nevertheless, the specific alkalinity of the saltier Mediterranean waters is higher than that of the less saline water of Atlantic origin. This may be explained by alkalinity input from rivers and the Black Sea. In the hypothesis of a steady state, the relationship between alkalinity and salinity permits calculation of the net flux of alkalinity between the Mediterranean Sea and the Atlantic Ocean, provided the water budget of the Mediterranean is known. The sources (rivers and the Black Sea) and sinks (seafloor) of alkalinity in the Mediterranean balance the net alkalinity transport into the Atlantic, if the water deficit of the Mediterranean is 2.2×1012 m3 yr−1 or 88 cm yr−1 per unit area. The budget of carbon in the Mediterranean Sea is more difficult to assess as the sources and sinks are numerous and not well‐documented. There is particularly a lack of data on the seasonal variations in the inorganic and organic carbon concentrations in the region of the strait of Gibraltar and on the partial pressure of CO2 in surface water of the Mediterranean Sea. It appears nevertheless that there is an important flux of inorganic carbon from the Mediterranean to the Atlantic due to the supply of carbon by the rivers and the Black Sea and to the transformation of 40% of the organic carbon entering the Mediterranean in inorganic carbon. Moreover, the Mediterranean is likely a sink for the atmospheric carbon.
Observations and modeling of biomass and soil organic matter dynamics for the grassland biome worldwide
Authors: Parton W. J. , Scurlock J. M. O. , Ojima D. S. , Gilmanov T. G. , Scholes R. J. , Schimel D. S. , Kirchner T. , Menaut J‐C. , Seastedt T. , Garcia Moya E. , Kamnalrut Apinan , Kinyamario J. I.
Published: February 2008

Century is a model of terrestrial biogeochemistry based on relationships between climate, human management (fire, grazing), soil properties, plant productivity, and decomposition. The grassland version of the Century model was tested using observed data from 11 temperate and tropical grasslands around the world. The results show that soil C and N levels can be simulated to within ±25% of the observed values (100 and 75% of the time, respectively) for a diverse set of soils. Peak live biomass and plant production can be simulated within ± 25% of the observed values (57 and 60% of the time, respectively) for burned, fertilized, and irrigated grassland sites where precipitation ranged from 22 to over 150 cm. Live biomass can be generally predicted to within ±50% of the observed values (57% of the time). The model underestimated the live biomass in extremely high plant production years at two of the Russian sites. A comparison of Century model results with statistical models showed that the Century model had slightly higher r2 values than the statistical models. Data and calibrated model results from this study are useful for analysis and description of grassland carbon dynamics, and as a reference point for testing more physiologically based models prediction's of net primary production and biomass. Results indicate that prediction of plant and soil organic matter (C and N) dynamics requires knowledge of climate, soil texture, and N inputs.
Radiative properties of Mount Pinatubo volcanic aerosols over the tropical Atlantic
Authors: Saunders Roger
Published: February 2008

Aircraft measurements of the broadband (0.3–3.0μm) and near infrared (0.7–3.0μm) optical depths of stratospheric aerosols over the south Atlantic five months after the eruption of Mount Pinatubo gave values in tropical latitudes up to 0.21 at 6°S. The measured downward shortwave irradiance was similar to the normal aerosol free case but 20% of the direct solar irradiance was scattered into the diffuse irradiance component. At 0.55μm wavelength an asymmetry factor of 0.75 decreasing to 0.5 at 2.2μm was inferred from narrowband radiances scattered by the aerosol. Model calculations using these measurements predict a decrease in tropospheric heating rate of less than 0.2°Cday−1 due to these aerosols.
A study of the impact of the space shuttle environment on faint far‐UV geophysical and astronomical phenomena
Authors: Lampton Michael , Sasseen Timothy P. , Wu Xiaoyi , Bowyer Stuart
Published: February 2008

FAUST is a far ultraviolet (1400–1800 Å) photon‐counting imaging telescope featuring a wide field of view (7.6°) and a high sensitivity to extended emission features. During its flight as part of the ATLAS‐1 payload aboard the STS‐45 mission in March 1992, nineteen deep‐space nighttime viewing opportunities were utilized by FAUST. Here we report the observed fluxes and their time and space variations, and identify the signatures of post‐sunset airglow phenomena and Orbiter vernier attitude‐control thruster firing events. We find that the Space Shuttle nighttime environment at 296 km altitude is often sufficiently dark to permit geophysical and astronomical UV observations down to levels on the order of 1000 photons/cm2 sr Å s, i.e., 0.01 Rayleighs/Å. We also find evidence for occasional geophysical fluxes of some tens or hundreds of Rayleighs in the upward‐looking direction.
Cirrus cloud detection from Airborne Imaging Spectrometer data using the 1.38 µm water vapor band
Authors: Gao Bo‐Cai , Goetz Alexander F. H. , Wiscombe Warren J.
Published: February 2008

Thin cirrus clouds are difficult to detect, particularly over land, in images taken from current satellite platforms. Using spectral images acquired by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) at 20 km altitude, we show that wavelengths close to the center of the strong 1.38 µm water vapor band are useful for detecting thin cirrus clouds. The detection makes use of the fact that cirrus clouds are located above almost all the atmospheric water vapor. Because of the strong water vapor absorption in the lower atmosphere, AVIRIS channels near 1.38 µm receive little scattered solar radiance from the surface or low level clouds. When cirrus clouds are present, however, these channels receive large amounts of scattered solar radiance from the cirrus clouds. Our ability to determine cirrus cloud cover using space‐based remote sensing will be improved if channels near the center of the 1.38 µm water vapor band are added to future satellites.
atmospheric helium isotopic ratio
Authors: Hoffman John H. , Nier Alfred O.
Published: February 2008

A recent study of the isotopic ratio of helium (3He/4He) in the terrestrial atmosphere led the individuals who made the investigation to suggest that this ratio is decreasing with time, probably due to anthropogenic release of low isotopic ratio crustal helium from oil and gas wells. The measurements made over an 11 year period from 1977 to 1988 led to the prediction that the rate of decrease was of the order of 1 × 10−9/yr. Several absolute isotopic ratio measurements made between 1969 and the present time also have been reported. These show a slight decrease in the helium isotopic ratio (much less than the 11 year data set shows). In the present paper, we report an unpublished, absolute helium isotopic ratio measurement the authors made in 1956. This value, when combined with the previously reported results, extends the time over which such measurements were made to a 32 year period and does not confirm that there is any decrease in the helium isotopic ratio in the atmosphere over this time.
Application of 2‐D travel‐time inversion of seismic refraction data to the Mid‐continent rift beneath Lake Superior
Authors: Lutter William J. , Trehu Anne M. , Nowack Robert L.
Published: February 2008

The inversion technique of Nowack and Lutter (1988a) and Lutter et al. (1990) has been applied to first arrival seismic refraction data collected along Line A of the 1986 Lake Superior GLIMPCE experiment, permitting comparison of the inversion image with an independently derived forward model (Trehu et al., 1991; Shay and Trehu, in press). For this study, the inversion method was expanded to allow variable grid spacing for the bicubic spline parameterization of velocity. The variable grid spacing improved model delineation and data fit by permitting model parameters to be clustered at features of interest. Over 800 first‐arrival travel‐times were fit with a final RMS error of 0.045 s. The inversion model images a low velocity central graben and smaller flanking half‐grabens of the Midcontinent Rift, and higher velocity regions (+0.5 to +0.75 km/s) associated with the Isle Royale and Keweenaw faults, which bound the central graben. Although the forward modeling interpretation gives finer details associated with the near surface expression of the two faults because of the inclusion of secondary reflections and refractions that were not included in the inversion, the inversion model reproduces the primary features of the forward model.
Radiative forcing due to ozone in the 1980s: Dependence on altitude of ozone change
Authors: Schwarzkopf M. D. , Ramaswamy V.
Published: February 2008

The radiative forcing of the surface‐troposphere system caused by the changes in ozone in the 1980s is sensitive to the altitude profile of these changes. In the tropics, inclusion of lower stratospheric ozone depletions observed by SAGE results in a substantial negative radiative ozone forcing. In mid‐latitudes, the magnitude of the negative stratospheric ozone forcing diminishes as the altitude of ozone depletion is raised above the tropopause. By contrast, the radiative forcing corresponding to the decadal tropospheric ozone increases observed at certain Northern Hemisphere mid‐latitude locations is strongly positive. The magnitude and sign of the total (tropospheric + stratospheric) ozone forcing in Northern Hemisphere mid‐latitudes is critically dependent on the vertical profile of the tropospheric ozone increases and the lower stratospheric losses near the tropopause.
What determines the cloud‐to‐ground lightning fraction in thunderstorms?
Authors: Price Colin , Rind David
Published: February 2008

The ratio of intracloud lightning (IC) to cloud‐to‐ground lightning (CG) in thunderstorms is observed to vary with latitude. The reason frequently given for this behaviour is that the height of the freezing level, and hence the negative charge center in thunderstorms, varies with latitude, resulting in higher ratios in the tropics compared with those in midlatitudes. This study shows that this hypothesis could well be incorrect. Analysis of cloud and lightning data indicates that the IC/CG ratio is linked to the thickness of the cold cloud region in thunderstorms (0°C to cloud top), rather than to the freezing level height.
Anisotropic magnetotail equilibrium and convection
Authors: Hau L. ‐N.
Published: February 2008

This paper reports on self‐consistent two‐dimensional equilibria with anisotropic plasma pressure for the Earth's magnetotail. These configurations are obtained by numerically solving the generalized Grad‐Shafranov equation, describing anisotropic plasmas with p∥ ≠ p⟂, including the Earth's dipolar field. Consistency between these new equilibria and the assumption of steady‐state, sunward convection, described by the double‐adiabatic laws, is examined. As for the case of isotropic pressure [Erickson and Wolf, 1980], there exists a discrepancy between typical quiet‐time magnetic field models and the assumption of steady‐state double‐adiabatic lossless plasma sheet convection. However, unlike that case, this inconsistency cannot be removed by the presence of a weak equatorial normal magnetic field strength in the near tail region: magnetic field configurations of this type produce unreasonably large pressure anisotropies, p∥ > p⟂, in the plasma sheet.
The role of density in the accumulation of basaltic melts at mid‐ocean ridges
Authors: Hooft E. E. , Detrick R. S.
Published: February 2008

It is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust. At the East Pacific Rise this neutral buoyancy level lies only 100–400 m below the sea floor, significantly shallower than the depths (> 1–2 km) of the magma bodies imaged in multichannel reflection data, suggesting that other factors must control the collection of melt in these reservoirs. The apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis, not the buoyancy of melt, is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid‐ocean ridges.
A new calibration point for the Late Miocene section of the geomagnetic polarity time scale: 40Ar/39Ar dating of lava flows from Akaroa Volcano, New Zealand
Authors: Baksi Ajoy K. , Hoffman Kenneth A. , Farrar Edward
Published: February 2008

Recent dating of geomagnetic field reversals over the past 2 Ma, by both the radiometric and astrochronologic techniques, indicates that the K‐Ar derived ages for these reversals are too young. We report herein on 40Ar/39Ar incremental heating study of two geomagnetic field reversals trapped in lavas on Akaroa Volcano, New Zealand, that demonstrate that the K‐Ar derived ages for field reversals at 10–9 Ma are too young by about 7%. The new age for the termination of chron 5, a critical tie point on the geomagnetic polarity time‐scale for late Miocene time, is 9.67±0.11 Ma.
Comparison of the mesospheric gravity waves observed with the MU Radar (35°N) and the Adelaide MF Radar (35°S)
Authors: Nakamura T. , Tsuda T. , Fukao S. , Kato S. , Vincent R. A.
Published: February 2008

Mesospheric gravity waves observed with the MU radar located near Kyoto (35°N, 136°E) and the Adelaide MF radar (35°S, 138°E) are compared for two campaigns in summer and winter. The frequency spectral densities of the horizontal wind motions were larger in summer than in winter at both locations, while the spectral density was almost identical if the summers or winters in both hemispheres are compared. The total variances, , of the waves with periods of 2 hours ‐ 30 minutes observed in the 70–80 km height range were almost the same at the two sites. This suggests that eddy dissipation processes due to turbulence caused by breaking gravity waves are almost the same magnitude between the hemispheres, which is different from the previous suggestion of the much larger dissipation in the northern hemisphere, derived from the tidal observations at Adelaide and Kyoto. The propagation directions of the different frequency components was also investigated. They seem to be similar at both locations in summer, but the directions were rather widely spread in winter at both locations. The MU radar results showed more preferential zonal directions for the gravity wave propagation compared to the results from Adelaide. These results indicate a difference in the propagation directions between the hemispheres, or the difference at the two locations of Adelaide and Kyoto, which might be caused by differences in topography around the sites.
Nonlinear wave‐wave interactions in the subauroral ionosphere on the basis of ISIS‐2 satellite observations of Siple Station VLF signals
Authors: Ohnami S. , Hayakawa M. , Bell T. F. , Ondoh T.
Published: February 2008

Nonlinear wave‐wave interaction between signals from a ground‐based VLF transmitter and narrow‐band ELF emissions in the subauroral ionosphere is studied by means of the bispectrum and bicoherence analysis. A bicoherence analysis has indicated that the sideband structures around the Siple transmitter signal received onbord the ISIS satellite are due to the nonlinear interaction between the Siple VLF signal and the pre‐existing ELF emission.
Venus methane and water
Authors: Donahue Thomas M. , Hodges R. Richard
Published: February 2008

The Pioneer Venus Large Probe Neutral Mass Spectrometer collected data that apparently, but almost surely misleadingly, indicate the presence of a large amount of methane (1000 – 6000 ppm) in the atmosphere of Venus from 60 km to the surface. The measured ratio of CH3D to CH4 was about 5 × 10−3. The ratio for Venus methane equilibrated with water would have been 9 × 10−2 and for terrestrial methane 6 × 10−4. Transfer of deuterium from atmospheric HDO to poorly deuterated methane can account for the strange gradient in the water vapor mixing ratio below 10 km previously reported. Similar deuterium transfer within the mass spectrometer causes reduction in the apparent ratio of HDO to H2O. A full accounting for the deuterium atoms increases the ratio from 100 to 157 times terrestrial. This, in turn, leads to a revised value of 28 ppm, constant with altitude below 25 km, for the mixing ratio of water vapor. Terrestrial methane mixed with 136Xe was introduced into the instrument for technical reasons and is the logical candidate for the source. But the methane detected, unlike 136Xe, closely mimics the behavior of an atmospheric gas. Despite this and the strange D/H ratio, the arguments against this methane being purely atmospheric are overwhelming. A preferable, but not provable, explanation is that it was generated by a reaction between an unidentified highly deuterated atmospheric constituent and a poorly deuterated instrumental contaminant.
The Gilbert/Gauss sedimentary geomagnetic reversal record from southern Sicily
Authors: van Hoof A. A. M.
Published: February 2008

A detailed paleomagnetic record of the Gilbert/Gauss reversal boundary in deep sea marls from southern Sicily shows that the remanence characterising the reversal is carried by a high‐temperature magnetite component. A low‐temperature component is intermediate between the high temperature component and a present‐day overprint, suggesting that it is obscured by weathering. The actual reversal record is clearly affected by filtering caused by early diagenetic acquisition of a chemical remanent magnetisation (CRM). Intermediate VGPs show a general confinement to a longitude over the Americas. More specifically, they are restricted to two clusters, a less distinct one off SE South America, and a very prominent one near NE North America The remarkable coincidence with two ‘Hoffmann clusters’ [Hoffmann, 1992] suggests that even a CRM acquisition filter cannot entirely obscure long‐lived transitional features.
Volcanic recycling of carbonates on Mars
Authors: Schaefer M. W.
Published: February 2008

Thermal erosion of carbonate deposits by turbulently‐flowing lava is investigated as a means of recycling carbon dioxide back into the atmosphere of Mars. Erosion rates of several meters/day are found, implying that up to hundreds of meters of carbonate could be removed over the lifetime of a flow. A large fraction of the northern plains and other parts of Mars were covered by lava during the Hesperian, and may have released the carbon dioxide trapped in carbonate deposits. This period of time, several times 108 years, is comparable to that for the re‐deposition of such carbonate deposits. Therefore, there could have existed a relatively dense atmosphere, and enhanced weathering and erosion, after the Noachian era. This may help explain the apparent observational evidence for late fluvial and lacustrine activity on Mars.
CCN formation from DMS oxidation without SO2 acting as an intermediate
Authors: Lin Xing , Chameides W. L.
Published: February 2008

An alternate pathway for generating cloud condensation nuclei (CCN) in the remote marine boundary layer (MBL) from the oxidation of biogenically‐derived dimethyl sulfide (DMS) is proposed. This pathway invokes the production of H2SO4 from DMS without SO2 serving as an intermediate in the oxidation sequence. The H2SO4 thus produced then undergoes bimolecular nucleation to form tiny sulfate condensation nuclei (CN). These CN rapidly grow to sizes sufficiently large to act as CCN. While CCN production from SO2 appears to be inefficient, model calculations indicate that the alternate pathway is capable of generating significant numbers of new CCN in the MBL within a two to three day time period. The alternate pathway also suggests a strong coupling between oceanic emissions of DMS and the number of CCN in the MBL.
Intercomparison of HALOE and ER‐2 aircraft H2O and CH4 Observations collected during the Second Airborne Arctic STratospheric Experiment (AASE‐II)
Authors: Tuck A. F. , Hovde S. J. , Kelly K. K. , Russell J. M. , Webster C. R. , May R. D.
Published: February 2008

HALOE observations of H2O and CH4 are compared with in situ techniques aboard the ER‐2 aircraft during the northern winter of 1991/92, in particular for the dates 911208, 920108, 920217, 920222 and 920320 when the spatial coincidences are close, within ±1° latitude and ±12° longitude. The temporal coincidence is within 4 hours. The version 9 retrievals of HALOE profiles are used, since they contain a self‐consistent aerosol correction. The results reveal the limitations of comparing high resolution in situ aircraft data with a remote sounding limb scanner. Of the 5 comparison dates, 3 had HALOE/ER‐2 coincidences which occurred near the edge of the Arctic vortex (911208, 920108, 920217); the vertical variability in the HALOE results and the horizontal variability in the ER‐2 observations on these days show that the vortex edge is not a region where exact agreement can be expected except by chance. On a 4th comparison date, there was substantial overlap away from the vortex edge, although for some species the aircraft data show considerable variability near the coincidence point. On the 5th comparison date, the ER‐2 had no overlap in altitude with the lowest HALOE observations; however, a short linear interpolation over ∼1 km altitude results in smooth composite profiles. Generally speaking the agreement between HALOE and the ER‐2 at overlap altitudes is about 12% in the case of water vapor, which shows low horizontal and vertical variability. The difference was shown to contain atmospheric variability in one case by using the HALOE viewpath superimposed on a potential vorticity map to to examine a pair of HALOE observations straddling the coincidence point in longitude. The agreement for methane, with limited data having altitude overlap, is better than 6%.
Characteristics of ion flow in the quiet state of the inner plasma sheet
Authors: Angelopoulos V. , Kennel C. F. , Coroniti F. V. , Pellat R. , Spence H. E. , Kivelson M. G. , Walker R. J. , Baumjohann W. , Feldman W. C. , Gosling J. T. , Russell C. T.
Published: February 2008

We use AMPTE/IRM and ISEE 2 data to study the properties of the high beta (βi > 0.5) plasma sheet, the inner plasma sheet (IPS). Bursty bulk flows (BBFs) are excised from the two databases, and the average flow pattern in the non‐BBF (quiet) IPS is constructed. At local midnight this ensemble‐average flow is predominantly duskward; closer to the flanks it is mostly earthward. The flow pattern agrees qualitatively with calculations based on the Tsyganenko [1987] model (T87), where the earthward flow is due to the ensemble‐average cross tail electric field and the duskward flow is the diamagnetic drift due to an inward pressure gradient. The IPS is on the average in pressure equilibrium with the lobes. Because of its large variance the average flow does not represent the instantaneous flow field. Case studies also show that the non‐BBF flow is highly irregular and inherently unsteady, a reason why earthward convection can avoid a pressure balance inconsistency with the lobes. The ensemble distribution of velocities is a fundamental observable of the quiet plasma sheet flow field.
The low‐latitude boundary layer at mid‐altitudes: Identification based on Viking hot plasma data
Authors: Woch Joachim , Lundin Rickard
Published: February 2008

On passes through the pre‐dawn to pre‐noon auroral oval the Viking spacecraft (apogee at 13,400 km) encounters often a generally well‐defined precipitation region of ions with characteristic energies of several hundred eV, with neither cusp nor plasma mantle characteristics. Furthermore, these ions are clearly separable from ions originating from the plasma sheet. Partial ion number densities in this plasma precipitation region strongly correlate with solar wind densities, supporting the view that it constitutes the mid‐altitude projection of the magnetospheric low‐latitude boundary layer (LLBL). The LLBL projection thus identified is observed with high probability in the MLT range from near‐noon to at least 6 MLT but occasionally extends toward 4 to 3 MLT. In the dawn to pre‐noon local time sector auroral ion precipitation pattern can be divided into precipitation originating from the plasma sheet and LLBL, i.e. generally no region was encountered with properties suggesting that it projects to the magnetospheric plasma sheet boundary layer (PSBL).
Flux transfer events: Spontaneous or driven?
Authors: Le G. , Russell C. T. , Kuo H.
Published: February 2008

Since flux transfer events (FTEs) occur predominantly under conditions of southward interplanetary magnetic field (IMF), one might expect that FTEs could be associated with the southward turning of the IMF which could lead to a transient increase in the merging rate. Lockwood and Wild [1993] recently suggested, in fact, that FTEs are driven by the IMF variations in which the Bz becomes more southward and rather than arising spontaneously during periods of steady IMF. To test this hypothesis, we have surveyed observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE spacecraft. The upstream solar wind data from IMP‐8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE to determine the time lag between them, which enables us to survey the simultaneous solar wind data during the FTEs. We find no evidence that FTEs are directly correlated with southward turning of the IMF. Rather, the FTEs more frequently occur when the IMF Bz is steady. A survey of 10‐minute intervals of simultaneous IMF data preceding each FTE show that the IMF fluctuates between northward and southward for less than 20% events and few of these turnings appear to be consistent with triggering the FTE. For majority of events, the IMF stays southward with fluctuations of clock angle less than 30° over the 10‐minute period. Thus we conclude that FTEs do occur spontaneously for steady southward IMF and are not simply directly driven by the observed fluctuations in the IMF Bz component.
Rapid descent of mesospheric air into the stratospheric polar vortex
Authors: Fisher Michael , O'Neill Alan , Sutton Rowan
Published: February 2008

Wind fields from a numerical simulation are used to give a detailed Lagrangian picture of air flow in the middle atmosphere of the southern hemisphere in winter and early spring 1991. Trajectories for many thousands of air particles exhibit rapid descent of mesospheric air into the stratospheric polar vortex, revealing its organizing and structure‐preserving properties. Results are used to account for measurements of low methane concentrations in the vortex made by the HALOE instrument on board the Upper Atmosphere Research Satellite.
Southwest Pacific temperatures: Diurnal and seasonal trends
Authors: Salinger M. James , Hay John , McGann Ron , Fitzharris Blair
Published: February 2008

Temperature trends are presented for a large part of the southwest Pacific. The trends differ from those documented for Northern Hemisphere land areas, where warming has occurred mainly through increases in minimum temperature. The New Zealand patterns are derived from recently completed analyses of monthly and annual mean maximum and minimum surface temperature records for a newly homogenised historical climate data series for New Zealand and outlying islands. They indicate that the warming in the New Zealand region over the past five decades can be attributed to increases in both mean maximum (mostly daytime) and mean minimum (mostly night time) temperature. All seasons show a temperature increase, with the largest occurring in summer (DJF).

Northern Hemisphere evidence suggests that changes in cloud cover and the presence of sulfate aerosols plays a direct role. The present results imply that, while the observed warming in a large portion of the Northern Hemisphere landmass may be significantly affected by both these factors, sulfate aerosol effects may be less important in the Southern Hemisphere.
Atmospheric attenuation length for relativistic solar protons
Authors: Ahluwalia H. S. , Xue S. S.
Published: February 2008

We compute the mean attenuation length (λf) in Earth's atmosphere for the protons produced in the solar flare of 29 September 1989. We use data obtained with three pairs of neutron monitors. The detectors in the pair have very similar values of the geomagnetic vertical cut‐off rigidity (R0) and overlapping asymptotic cones of acceptance but they differ in terms of the altitudes of the observing sites. We apply Wilson et al [1967] method to compute the values of λf for the three representative pairs which cover a range of solar proton rigidities. The computed mean values of λf are significantly larger than the consensus value of (100±5) g cm−2 reported in the literature [Duggal, 1979]. Also, they exhibit a linear correlation with R0.
Energy content of stormtime ring current from phase space mapping simulations
Authors: Chen Margaret W. , Schulz Michael , Lyons Larry R.
Published: February 2008

We perform a phase space mapping study to estimate the enhancement in energy content that results from stormtime particle transport in the equatorial magnetosphere. Our pre‐storm phase space distribution is based on a steady‐state transport model. Using results from guiding‐center simulations of ion transport during model storms having main phases of 3 hr, 6 hr, and 12 hr, we map phase space distributions of ring current protons from the pre‐storm distribution in accordance with Liouville's theorem. We find that transport can account for the entire ten to twenty‐fold increase in magnetospheric particle energy content typical of a major storm if a realistic stormtime enhancement of the phase space density f is imposed at the nightside tail plasma sheet (represented by an enhancement of f at the neutral line in our model)
Radially fractured domes: A comparison of Venus and the Earth
Authors: Janes Daniel M. , Squyres Steven W.
Published: February 2008

Radially fractured domes are large, tectonic and topographic features discovered on the surface of Venus by the Magellan spacecraft. They are thought to be due to uplift over mantle diapirism, and to date are known to occur only on Venus. Since Venus and the Earth are grossly similar in size, composition and structure, we seek to understand why these features have not been seen on the Earth. We model the uplift and fracturing over a mantle diapir as functions of lithospheric thickness and diapir size and depth. We find that lithospheres of the same thickness on the Earth and Venus should respond similarly to the same sized diapir, and that radially fractured domes should form most readily in thin oceanic lithospheres on Earth if diapiric activity is similar on the two planets. However, our current knowledge of the Earth's oceanic floors is insufficient to confirm or deny the presence of radially fractured domes. We compute the expected dimensions for these features on the Earth and suggest a search for them to determine whether mantle diapirism operates similarly on the Earth and Venus.
The non‐double‐couple component of deep earthquakes and the width of the seismogenic zone
Authors: Houston Heidi
Published: February 2008

It has recently been proposed that all deep earthquakes (> 400 km depth) are caused by a shear instability associated with the transformation of metastable olivine to its high‐pressure polymorphs within the cold core of subducting slabs (Green and Burnley, 1989; Kirby et al., 1991). If so, then the seismogenic zone would narrow with depth below 400 km following slab isotherms. Because the ratio of fault slip to length does not increase markedly with depth, and stress orientations indicate that deep fault planes are not parallel to the subducting slab, a fault plane large enough to generate a very large, deep earthquake would not fit within a narrow seismogenic zone and multiple fault planes would be needed. If the fault planes are not parallel due to spatial variations in stress, a deviatoric non‐double‐couple component (volume‐preserving, non‐shearing part) of the seismic moment tensor is likely to result. Thus, if the seismogenic zone narrows with depth, one would expect an increasing non‐double‐couple component (1) for large earthquakes as depth increases from 400 to 700 km, and (2) for very deep earthquakes (below 550 km depth) as seismic moment increases. Data from the Harvard CMT catalog, augmented by moment tensor inversions for the two largest deep earthquakes during the past thirty years, are examined for these features. Although significant scatter is present, the data roughly follow the trends described above, suggesting a relationship between the non‐double‐couple component of deep earthquakes and the geometry of a seismogenic zone narrowing with depth.
Record low total ozone during northern winters of 1992 and 1993
Authors: Bojkov Rumen D. , Zerefos Christos S. , Balis Dimitrios S. , Ziomas Ioannis C. , Bais Alkiviadis F.
Published: February 2008

The last two winter‐spring seasons (DJFM) distinguished themselves by being with the lowest ever total ozone over all three continental size regions between 45°N and 65°N of North America, Europe and Siberia. The total ozone deficiencies for the entire season over all of the above mentioned regions were about 11% and 13% below the long‐term normal during the two consecutive years (1991/92 and 1992/93 respectively ). This helped to pull down the cumulative ozone decline since the winter‐spring of 1969/70 to be about 14% in the latitude belt of the 45°N–65°N. Frequencies of days with ozone values deviating below the long‐term mean by more than 2σ have been ten times higher than their 35‐year average. There are evidences deduced from trajectories on potential temperature surfaces that transport of poor in ozone air masses forced in addition by vertical motions, could account for a number of the extreme cases. There is also evidence that cold air, known to have excess ClO content, has moved over the sun lighted latitudes on many occasions, when chemical ozone destruction could have been favored. These ozone deficiencies do not have similar rates of decline and did not reach even close to the extreme low values regularly observed during the Antarctic‐spring ozone hole phenomena.
Determination of product branching ratio of the ClO self‐reaction at 298 K
Authors: Horowitz A. , Bauer D. , Crowley J. N. , Moortgat G. K.
Published: February 2008

The Cl2‐sensitized continuous photolysis of O3 mixtures in O2 was investigated at 298 K using time resolved single wavelength and diode array spectroscopy to monitor the decay of O3 and the formation of OClO. Φ−O3 was found to be 4.1±0.4. This value, combined with the determination of the initial rates of OClO formation, allowed the evaluation of the following branching ratios into the three bimolecular disproportionation channels of ClO reactions; k3a/k3 = 0.40 ± 0.08, k3b/k3 = 0.42 ± 0.08 and k3c/k3 = 0.18 ± 0.02 where ClO + ClO → Cl2 + O2 (3a), ClO + ClO → Cl + Cl + O2 (3b) and ClO + ClO → Cl + OClO (3c).
Mechanisms of formation of stratospheric clouds observed during the Antarctic late winter of 1992
Authors: Gobbi Gian Paolo , Adriani Alberto
Published: February 2008

A periodicity was observed to drive the 1992 late winter formation of stratospheric clouds over McMurdo Station, Antarctica. Lidar and meteorological data show that intense stratospheric coolings, accompanied by generation of clouds, resulted from the transit of air parcels proceeding from latitudes near the edge of the polar vortex. Lidar depolarization measurements show that large cloud particles could survive for several days in undersaturated air. In the occurrence of further coolings, these particles would act as preferential growth nuclei. Depolarization measurements also indicate that most of the late winter inner vortex sulfuric acid aerosol was frozen. Periodically observed transit of vortex airmasses outside the terminator could be the cause of the wintertime ozone losses, occasionally observed at the very center of the continent.
Radar absorption due to impurities in Antarctic ice
Authors: Corr Hugh , Moore John C. , Nicholls Keith W.
Published: February 2008

Measurements of radar echo strength have been carried out on Ronne and George VI Ice Shelves, Antarctica. The differences in ice‐column absorption between the two sites cannot be resolved by considering ice temperatures alone. We present an absorption model that resolves the discrepancy by considering both the ice temperature and the ice impurity concentrations. In Antarctic coastal sites the concentrations of acid and sea salt impurities, both of which affect absorption, have a marked spatial variability and can dominate the absorption budget. Using the absorption model, impurity levels in the ice column at the George VI site are deduced from the radar strength measurements and found to be typical of levels measured in Antarctic ice at similar elevations and distances from the sea. In particular the measurements suggest that the concentrations of biogenically‐derived acids are much lower than the unusually high levels found on the east coast of the Antarctic Peninsula.