bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal structure of the mesopause region at polar latitudes
Authors: Lübken F.‐J. , von Zahn U.
Published: February 2008

We present the results of temperature soundings performed on almost 180 days since 1980 in the 50‐to 120‐km altitude range at Andenes, Norway (69°N latitude). Most of the temperature profiles were obtained by ground‐based lidar; the others were derived from in situ density measurements. We present (1) monthly mean temperatures for 9 months of the year, including the mesopause temperature and altitude, and examine (2) seasonal effects, (3) interannual variability, (4) systematic differences to CIRA 1986, and (5) longer‐term effects related to the solar activity cycle. The main results are as follows: the mesopause is high (98 km) and warm (192 K) in a long period from October to March and low (88 km) and cold (129 K) in June and July. The transition between these two states in August is much faster than commonly anticipated. An intercomparison of our monthly mean temperature profiles with CIRA 1986 shows significant deviations. Above 80 km we find positive correlations between temperature and solar activity with regression coefficients in the order of 0.15 K/SFU.
Three‐dimensional simulation of 7Be in a global climate model
Authors: Brost Richard A. , Feichter Johann , Heimann Martin
Published: February 2008

In the upper troposphere and lower stratosphere, cosmic rays create Beryllium 7 atoms, which subsequently attach to submicron dust particles, so that wet deposition ultimately removes most 7Be from the troposphere. Because this source is weil known and because there is a large climatological data set for 7Be concentration in surface air and deposition on the surface, simulating 7Be provides a good test of the wet scavenging parameterization in a global climate simulation model, such as ECHAM2, which is the European Center for Medium Range Weather Forecasting (ECMWF) model with new physics introduced by the University of Hamburg. A simple parameterization in which the simulated condensation rate determines the scavenging frequency in each grid cell is used in the tracer transport model GLOMAC1, which is embedded in the meteorological model ECHAM2. In this paper we compare observed and model‐calculated values of monthly average and annual average surface concentration at a global network of 79 stations. The average absolute value of the error in simulated surface concentration is 1.4 mBq m−3 compared with an average observed concentration of 3.5 mBq m−3. At most stations and in most regions the simulated surface concentration has about the correct magnitude and seasonal cycle, although there is a bias so that the modeled concentration is high at mountain stations in the tropics and low at sea level in polar regions. There are less climatological deposition data than there are climatological concentration data, but the model basically simulates the correct latitudinal variation of the zonally averaged deposition, which has a peak at the polar front (30°–50°N), although the model also has a peak produced by convective precipitation in the intertropical convergence zone, a peak that is not observed. We think that 7Be, used in conjunction with other species such as 210Pb, provides an excellent test of wet scavenging in a global model.
Apparent tropospheric response to MeV‐GeV particle flux variations: A connection via electrofreezing of supercooled water in high‐level clouds?
Authors: Tinsley Brian A. , Deen Glen W.
Published: February 2008

The ionization production by MeV‐GeV particles (mostly galactic cosmic rays) in the lower atmosphere has‐well defined variations on a day‐to‐day time scale related to solar activity, and on the decadal time scale related to the sunspot cycle. New results based on an analysis of 33 years of northern hemisphere meteorological data show clear correlations of winter cyclone intensity (measured as the changes in the area in which vorticity is above a certain threshold) with day‐to‐day changes in the cosmic ray flux. Similar correlations are also present between winter cyclone intensity, the related storm track latitude shifts, and cosmic ray flux changes on the decadal time scale. These point to a mechanism in which atmospheric electrical processes affect tropospheric thermodynamics, with a requirement for energy amplification by a factor of about 107 and a time scale of hours. A process is hypothesized in which ionization affects the nucleation and/or growth rate of ice crystals in high‐level clouds by enhancing the rate of freezing of thermodynamically unstable supercooled water droplets which are known to be present at the tops of high clouds. The electrofreezing increases the flux of ice crystals that can glaciate midlevel clouds. In warm core winter cyclones the consequent release of latent heat intensifies convection and extracts energy from the baroclinic instability to further intensify the cyclone. As a result, the general circulation in winter is affected in a way consistent with observed variations on the interannual/decadal time scale. The effects on particle concentration and size distributions in high‐level clouds may also influence circulation via radiative forcing.
Cross sections for direct and dissociative ionization of NH3 and CS2 by electron impact
Authors: Rao M. V. V. S. , Srivastava S. K.
Published: February 2008

Cross sections for the production of positive ions by electron impact on NH3 and CS2 have been measured using a crossed electron beam‐molecular beam collision geometry. The present results on NH3 are considerably different from previous measurements which disagree among themselves. Except for the formation of CS2+, cross sections for other fragment ions resulting from CS2 have not been measured in the past. Threshold energies for the appearance of various positive ions are also reported. It is estimated that the present results are accurate to within 15%.
The particle size of Martian aeolian dunes
Authors: Edgett Kenneth S. , Christensen Philip R.
Published: February 2008

The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well‐sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. We use two independent approaches. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500±100 μm, or medium to coarse sand. Second, we determine expected dune particle sizes from grain trajectory calculations and the particle size transition from suspension to saltation. On Earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed (u*t) is near unity; for grains at u*t, this occurs at about 52 μm. Terrestrial dune sands have a mean of 250 μm and are composed entirely of grains >52 μm. The corresponding Martian transition grain size is about 210 μm, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that under Martian conditions, larger grains than on Earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. Thus, while terrestrial dune grains are in the fine to medium sand range, the average Martian dune sediments are probably medium to coarse sands. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface materials.
Does Sedimentary Organic δ13C Record Variations in Quaternary Ocean [CO2(aq)]?
Authors: Rau G. H. , Froelich P. N. , Takahashi T. , Des Marais D. J.
Published: February 2008

Ocean surface water [CO2(aq)] variations based on glacial/interglacial changes in sediment δ13Corg are shown to compare favorably with reconstructions based on ice core [CO2]. In particular, an approximate 80 uatm increase in atmospheric pCO2 during the last glacial‐interglacial transition is calculated to correspond to a 3–4 umolar increase in ocean surface water [CO2(aq)] at atmospheric equilibrium. A widespread marine δ13Corg decrease of 1–2 ‰ accompanied this event and was not preceded by an equivalent isotopic change in surface water total dissolved inorganic carbon. These observations support the hypothesis that [CO2(aq)] influences photosynthetic isotope fractionation between marine inorganic and organic carbon pools, and therefore that plankton/sediment δ13Corg may serve as a proxy for surface water [CO2(aq)].
Opening the carbon isotope "vital effect" black box, 2, Quantitative model for interpreting foraminiferal carbon isotope data
Authors: Spero Howard J. , Lerche Ian , Williams Douglas F.
Published: February 2008

Interpretation of carbon isotope records from late Quaternary planktonic foraminifers are confounded due to the presence of a significant physiological component in the carbon isotopic signal. A quantitative carbon isotope (QC) model is presented which relates the carbon isotopic composition of a foraminiferal shell to the physiological processes of respiration and symbiont photosynthesis and to the δ13C value of seawater ΣCO2. The QC model is calibrated with physiological and stable isotopic data from laboratory experiments with living planktonic foraminifers. Model simulations of chamber and shell δ13C values with the symbiont‐bearing foraminifers, Orbulina universa and Globigerinoides sacculifer, suggest (1) variations in symbiont density and photosynthetic rate (light level or habitat depth) are the primary physiological parameters controlling intraspecific carbon isotopic variability in these species, (2) respiration has little effect on the δ13C of O. universa, and (3) each chamber in a multichambered foraminiferal test will have a distinct δ13C value depending on its position in the test whorl. Size:δ13C value relationships reported for G. sacculifer from fossil assemblages can be explained as a function of increasing symbiont density during ontogenetic development.
Late Pleistocene Variability of the Carbon Isotopic Composition of Organic Matter in the Eastern Mediterranean: Monitor of Changes in Carbon Sources and Atmospheric CO2 Concentrations
Authors: Fontugne M. R. , Calvert S. E.
Published: February 2008

The organic carbon isotopic record of the sapropels (S1 and S3–S10) and intercalated marl oozes has been determined in a 12‐m piston core from the eastern Mediterranean. The δ13Corganic values are systematically lighter (mean=−21.0±0.82 ‰) in all sapropels and heavier (mean=−18.8±1.07‰) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the Corganic/N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in δ13Corganic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichment in 12C, implying that all values were even heavier originally, and there is no secular trend in the δ13Corganic record. The observed differences in δ13Corganic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the δ13Corganic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores.
Paleomagnetism and age determinations of the Deccan Traps (India): Results of a Nagpur‐Bombay Traverse and review of earlier work
Authors: Vandamme Didier , Courtillot Vincent , Besse Jean , Montigny Raymond
Published: February 2008

Review of available radiometric age determinations of the Deccan traps (India) shows a spectrum of K‐Ar ages that is highly polluted by argon loss. Stepwise40Ar‐39Ar age determinations include estimates of data quality and thus avoid contaminated results. The absolute age of the Deccan traps determined using 2240Ar‐39Ar plateau age spectra is 65.5 ± 2.5 Ma. Paleontological data on infratrappean and intertrappean sediments constrain Deccan age to between theA. mayaroensiszone, in the Upper Maestrichtian (about 67 Ma), and the P2 foraminifer zone, in the Lower Paleocene (about 60.5 Ma). Paleomagnetic study of a Nagpur‐Bombay traverse (preliminary results of which were used by Courtillot et al. (1986a, b) for a general discussion about Deccan volcanism and the Cretaceous‐Tertiary boundary) is presented in detail. All available paleomagnetic results from the Deccan traps (563 flows) are then compiled. Results considered to be transitional or to come from suspicious sites are removed leaving 485 flow results. This extensive data set from a single geological unit allowed us to look in some detail at its statistical distribution. The virtual geomagnetic poles (VGP) are approximately Fisher distributed but present a complex asymmetry. No regional variation can be seen (to within paleomagnetic uncertainties). Although the 3.5° angular difference between the separate normal (pole) and reversed (antipole) data is not statistically significant, it can be explained by either a 2.1 m.y. drift along the apparent polar wander path (APWP) of the Indian plate assuming a normal‐reverse‐normal (N‐R‐N) magnetostratigraphy, or a 3.5% contamination by a present field overprint, or a slight nondipole field component. A quality coefficient has been assigned to each result on the basis of existence and value of published 95% confidence angle. Because the normal and reversed mean poles become more precisely antipodal with higher‐quality data and with more recent publication date (as a consequence of the evolution of paleomagnetic techniques), we favor the overprint hypothesis. The angular standard deviation of our VGP set is 20% larger than the value predicted by the paleosecular variation model of McFadden and McElhinny (1984). Finally, our best estimate of the overall mean pole is located at 281.3°E, 36.9°N,A95= 2.4° (calculated from the 163 highest‐quality flow VGPs). It is in remarkable agreement with the reference APWP of Besse and Courtillot (1991), leading to an independent estimate of the age of Deccan volcanism at 67.2 ± 6.6 Ma. The same comparison following Acton and Gordon (1989) provides an estimated age at 67 ± 5 Ma. These estimates which are based on paleomagnetic data only are in perfect agreement with both radiometric and paleontological ages.
Coupled hydromechanical‐thermochemical processes in rock fractures
Authors: Tsang Chin‐Fu
Published: February 2008

The behavior of rock fractures under various coupled processes is reviewed under seven broad categories: (1) hydromechanical processes, (2) thermohydrologic processes, (3) hydrochemical processes, (4) thermohydromechanical processes, (5) thermohydrochemical processes, (6) hydromechanical‐chemical processes, and (7) thermomechanical‐hydrochemical processes. The state‐of‐the‐art and possible directions of further research in these coupled processes are discussed. Rock fracture behaviors considered include not only dilation, closure, shear, and joint propagation but also changes in flow permeability and chemical sorption and retardation capabilities. These properties are of critical concern in practical considerations of the rock mass as a medium to store and isolate chemical and radioactive wastes. Investigations of coupled processes in two‐fracture systems and multifracture systems are identified as important areas for future research. The need to consider coupled processes in borehole testing procedures involving rock fractures is emphasized.
A model for oblique spaced antenna technique for mesosphere‐stratosphere‐troposphere radars and its applications
Authors: Pan C. J. , Liu C. H.
Published: February 2008

The spaced antenna method for VHF/UHF radar applications is discussed for the general observational configurations including the off‐zenith observational mode. On the basis of the assumptions of “locally frozen‐in” and volume scattering a physical model is constructed. Cross‐correlation and autocorrelation functions are derived and applied for the discussion of the various aspects of the full correlation analysis. Spectral width, aspect sensitivity, and characteristic length are compared for different echo mechanisms in this paper. Some observational results based on the model predictions will be discussed.
Space‐based radar coherent processing during scintillation: VHF through L band
Authors: Knepp Dennis L. , Mokole Eric L.
Published: February 2008

Data taken during the Defense Nuclear Agency's Wideband Satellite experiment are analyzed to determine the impact of ionospheric scintillation on coherent integration performance of a space‐based radar (SBR). These analyses use a subset of the experimental data base from Kwajalein and Ancon in 1977 and from Kwajalein in 1979. The 1977 measurements are compared to those of 1979, which were closer to the peak of the solar cycle. Under assumptions that are appropriate for strong scintillation the one‐way propagation data from the experiment are transformed into data appropriate to a two‐way, SBR geometry; measured values of the coherent integration loss are calculated; and an analytical expression for the coherent integration loss is obtained. The analytical result is shown to provide a good fit to the experimentally generated, two‐way measurements for strong scintillation.
Alpine Corsica Metamorphic Core Complex
Authors: Fournier Marc , Jolivet Laurent , Goffé Bruno , Dubois Roland
Published: February 2008

Alpine Corsica is an example where superficial nonmetamorphic allochtonous units rest upon a highly strained metamorphic complex. Early ductile deformation under high pressure‐low temperature (HP‐LT) conditions is due to the westward thrusting of oceanic material onto a continental basement as shown by previous studies. New thermobarometric estimates yield minimal peak HP‐LT metamorphism conditions of 11 kbar at 400°C. The early deformation is overprinted by a ductile deformation with an eastward sense of shear postdating or contemporaneous with mineral recrystallizations in the greenschist facies conditions. Early compressive thrust contacts are reworked as east dipping ductile normal faults and the less competent units display only eastward shear criteria. The upper units are affected by an extensional brittle deformation, and east dipping brittle normal faults bound to the west the early to middle Miocene Saint‐Florent half‐graben. The greenschist metamorphic event lasted until 33 Ma, which is contemporaneous with the beginning of the extension in the Liguro‐Provençal basin. We interpret the second deformation stage as the result of a ductile extension following the overthickening of the crust due to the westward thrusting. Extension reduces the thickness of the crust so that upper units free from early P‐T conditions are brought into close contact with a HP‐LT metamorphic core complex. The geometry of the late extension is controlled by that of the early compressive thrust.
Thrust tectonics, crustal shortening, and the structure of the far‐eastern Nepal Himalaya
Authors: Schelling Daniel , Arita Kazunori
Published: February 2008

Balanced and restored structural sections across the far‐eastern Nepal Himalaya have been constructed in order to determine the structure and evolution of the Himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the Main Central Thrust (MCT). The far‐eastern Nepal Himalaya is comprised of three distinct, thrust‐bound tectonic packages; the Higher Himalayan (Crystalline) thrust sheet, the Lesser Himalayan (Metasediment) thrust package, and the Sub‐Himalayan imbricate fan. The Higher Himalayan Crystallines, consisting of kyanite‐ and sillimanite‐bearing gneisses intruded by the Miocene (?) Jannu leucogranites, have been thrust over the Lesser Himalayan Metasediments along the MCT for a distance of 140 km to 175 km. The Lesser Himalayan Metasediments are a 12 km thick unit consisting primarily of phyllites, metaquartzites, and mylonitic augen gneisses in which garnet, biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels. The Lesser Himalayan (Metasediment) thrust package is underlain by a decollement, the Main Detachment Fault (MDF), which lies at a calculated depth of between 6 and 10 km underneath the Mahabharat Lekh, and at a calculated depth of 20 to 25 km north of the Tamar Khola Dome. The Tamar Khola Dome overlies a footwall ramp along the MDF where the MDF cuts upsection through the Lesser Himalayan Metasediments. The Lesser Himalayan thrust package probably has an internal structure aproximating a hinterland‐dipping duplex, with the MCT and the MDF corresponding to the roof and floor thrusts, respectively. Both the Tamar Khola Thrust, an out‐of‐sequence breach thrust, and the Main Boundary Thrust (MBT) are splay thrusts off of the MDF. The Sub‐Himalaya, consisting of nonmetamorphosed sedimentary rocks, displays an emergent imbricate fan geometry and is underlain by the southern continuation of the MDF which lies at a depth of 5.5 km to 6 km beneath the Siwalik Hills. Folding and thrusting within the Lesser Himalayan thrust package and the Sub‐Himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening. Total north‐south shortening across the Higher, Lesser, and Sub‐Himalaya of far‐eastern Nepal, south of the Tibetan Plateau, has been of the order of 185 km to 245 km and has occurred at an average rate of 7.4 mm to 15.3 mm per year since the initiation of the MCT between 16 and 25 Ma.
Some thermomechanical aspects of the subduction of continental lithosphere
Authors: van den Beukel Jilles
Published: February 2008

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.
Transpression along the Southern San Andreas Fault, Durmid Hill, California
Authors: Bürgmann Roland
Published: February 2008

Transpression is expressed at Durmid Hill in the development and subsequent rotation of en échelon folds and ongoing vertical uplift The transpression‐related strain field can be quantified by an analysis of (1) orientation of rotated fold axes, which display an angle of about 20° between the San Andreas fault and the fold‐axial traces; (2) shortening perpendicular to the fold axes (40%–60%); (3) extension parallel to the fold axes (58%–68%), measured using a boudinaged ash bed; and (4) uplift determined from an antecedent stream (1.1–1.7 mm/yr), the topographic expression of Durmid Hill, and the exposed stratigraphy. The observed deformation may be approximated by a homogeneous, transpressional deformation. This model suggests that an estimated 9%–14% of the total displacement predicted by plate motion models across the San Andreas fault has been accommodated by the Durmid Hill deformation zone adjacent to the fault. This off‐fault deformation is not directly coupled to coseismic or interseismic movement on the fault itself.
Tectonic setting and U‐Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor‐Odin ‐ Pinnacles Area, Southern Omineca Belt, British Columbia
Authors: Carr Sharon D.
Published: February 2008

The Thor‐Odin ‐ Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite‐facies rocks that were deformed during Mesozoic‐Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep‐seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U‐Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee décollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal‐scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late‐synkinematic to posttectonic plutons. U‐Pb systematics reveal that zircons in high‐temperature shear zones may have suffered high‐temperature Pb loss, perhaps due to deformation‐ or fluid‐enhanced diffusion, and that monazite systematics from samples from high‐grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age.
Geology and geophysics of the Rukwa Rift, East Africa
Authors: Morley C. K. , Cunningham S. M. , Harper R. M. , Wescott William A.
Published: February 2008

Exploration in the Rukwa rift, using gravity and seismic reflection surveys, fieldwork, and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the Western rift. The stratigraphy comprises Precambrian basement, Karroo sandstones, shales and coals, upper Miocene red beds, and Miocene‐Recent lacustrine and fluvial sediments. During Miocene‐Recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast. The southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the Miocene‐Recent. Cenozoic age rift structures have a dominant NW‐SE and a subordinate N‐S trend. The NW‐SE trend tends to follow a Precambrian basement and later Karroo structural trend. NE‐SW seismic lines indicate up to 10 km extension of the Tertiary section in a direction oblique to the probable E‐W regional extension direction. In the southeastern portion of the basin both Karroo and Tertiary‐Quaternary sediments expand into the Lupa fault zone, reaching thicknesses of up to 3 and 7 km, respectively. Tertiary‐Quaternary sediment thicknesses decrease northwestward, accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the Lake Tanganyika rift.
Some thermal and paleomagnetic consequences of tilting a batholith
Authors: Beck Myrl E.
Published: February 2008

If a rock body becomes fully magnetized and then is folded, faulted, or otherwise tilted, its vector of remanent magnetization will tilt with it. However, if a rock body is tilted while it is very hot, and then cools and becomes magnetized, its vector of magnetization will show little if any of the effects of tilt. It is shown, for a wide variety of circumstances, that tilting a large batholith should not be expected to result in equal tilt of that batholith's magnetic direction. This follows from several elementary considerations: (1) Batholiths cool slowly, and slow cooling lowers the blocking‐temperature range for acquisition of remanent magnetization. (2) If a tilted block is large (of the order of 25 km in width) and has been tilted through even a moderate angle (25° will do), its “lower” end originally must have been very hot; quite possibly too hot to retain a remanent magnetization. (3) Tilt entails uplift, and uplift rearranges the geothermal gradient. Rapid uplift accompanied by erosion is particularly effective in raising the temperature of the upper levels of the crust, thereby delaying acquisition of magnetization until tilting is nearly complete. Other thermal constraints on the tilting of remanence vectors in batholiths are discussed; in general, it appears that quite special circumstances are required to tilt a large pluton without leaving a clear magnetic record. Few, if any, Cordilleran batholiths studied to date show this kind of evidence of tilt.
Late Gondwanide tectonic rotations within Gondwanaland
Authors: Dalziel Ian W. D. , Grunow Anne M.
Published: February 2008

Geologic and paleomagnetic evidence from the Ellsworth‐Whitmore mountains crustal block of West Antarctica, and from the Falkland Islands of South America, indicates that tectonic displacement of major portions of the “Samfrau geosyncline” occurred after the early Mesozoic Gondwanide folding but prior to seafloor spreading. Comparison with continental deformation in the collisional Alpine‐Himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the Gondwanian orogeny resulted from collision of a fore‐arc and magmatic arc terrane with the Panthalassic margin of the Gondwana craton during closure of a marginal basin. The displacements had major consequences for the ensuing evolution of the South Atlantic‐Weddell Sea‐Ross Sea region.
Proterozoic to Mesozoic East Gondwana: The juxtaposition of India, Sri Lanka, and Antarctica
Authors: Yoshida Masaru , Funaki Minoru , Vitanage Piyadasa W.
Published: February 2008

Jurassic and Cambro‐Ordovician paleomagnetic pole positions deduced from rocks of Sri Lanka and Antarctica generally conform when Sri Lanka is juxtaposed with the coast of Lützow‐Holm Bay, East Antarctica, in a manner similar to a current Gondwana reassembly. The Highland and Southwest groups (mostly Proterozoic) of Sri Lanka are correlated with the Ongul and Skallen groups of East Antarctica on the basis of similar lithology, structural characteristics, structural trend, tectonothermal history (ranging from early Proterozoic to Mesozoic), and the pressure‐temperature‐time path of the main metamorphism. Some neighboring lithotectonic units are also comparable between the two areas. Fracture‐lineament systems of Sri Lanka and Antarctica formed after the latest Silurian and during or before the Jurassic conform in this reconstruction. An improved fit of the 2000 m isobaths of India‐Sri Lanka‐Antarctica and continuation of the latest Archaean mobile belt from Enderby Land to peninsular India is obtained by fitting Sri Lanka into Lützow‐Holm Bay, and Enderby Land into the embayment in the east coast of peninsular India. This juxtaposition of peninsular India to East Antarctica involves about 80 km northward movement and about 30° clock‐wise rotation of Sri Lanka relative to India. The reassembly thus obtained accords well with data, supporting the idea that East Gondwana existed from the earliest Proterozoic to middle Mesozoic, although some disruption‐collision events of limited scale are considered possible.
Buckling of the oceanic lithosphere from geophysical data and experiments
Authors: Bull Jonathan M. , Martinod Joseph , Davy Philippe
Published: February 2008

Two major hypotheses have been advanced for the formation of the long wavelength (100–300 km) undulations of oceanic basement and overlying sediments developed in the central Indian Ocean basin: whole layer folding (buckling) and local thickening (inverse boudinage). Using appropriately scaled two‐layer analogue models for the oceanic lithosphere comprising a brittle layer above a ductile layer, we show that buckling of the entire brittle layer is likely to be the mode of deformation. However, the lithosphere‐asthenosphere boundary remains undisturbed. We find a relationship between the thickness of the brittle layer and the wavelength of folding such that the wavelength is 7 times the brittle layer thickness.
Chemistry of iron in marine aerosols
Authors: Zhuang Goushan , Yi Zhen , Duce Robert A. , Brown Phyllis R.
Published: February 2008

Abstract. Remote marine aerosols collected at four Pacific island stations (Midway, Oahu, Enewetak, and Fanning) and during four Pacific cruises were used in determining the chemical form of iron in marine atmospheric aerosols. Aerosols collected from an urban area, Xian, near the Chinese loess plateau were also studied using the same procedures. Marine and urban Chinese aerosol samples and Chinese loess samples were leached by acidic deionized water over a pH range of 2.0–5.6 to determine the extent of dissolution of iron in atmospheric waters. The results indicated that (1) more than 90% of the atmospheric iron in remote marine aerosols over the North Pacific is released in 2 N acidic solutions, and in the mean ~50% of the atmospheric iron in remote marine aerosols was in the form of Fe(II); and (2) the solubility of iron in the marine aerosols was 5–17 times greater than that of iron in Chinese loess at the same pH. This may be largely due to the photoreduction of Fe(III) to Fe(II) during its long‐range transport. At a total iron concentration of less than 20 nmol/kg, ∼55% of the atmospheric iron dissolves in solutions with a pH of 3.8 and 25–30% dissolves at a pH of 4.8. We postulate that (1) the mixing of mineral dust with sulfate/sulfuric acid aerosols as a result of coalescence processes within clouds might be a mechanism to increase the solubility of atmospheric iron in cloud water or rainwater; and (2) photochemical reduction of Fe(III) to Fe(II) may be an important mechanism increasing the solubility of iron, which would be rapidly released to rainwater, fog, or cloud droplets when the aerosol particles contact them during long‐range transport over the open ocean. The total atmospheric iron input was estimated to be ∼15 ‐ 25 × 1012 g/yr for the entire North Pacific Ocean.
Specifying land surface characteristics in general circulation models: Soil profile data set and derived water‐holding capacities
Authors: Webb Robert S. , Rosenzweig Cynthia E. , Levine Elissa R.
Published: February 2008

A standardized global data set of soil horizon thicknesses and textures (particle size distributions) has been compiled from the Food and Agriculture Organization of the United Nations/United Nations Educational, Scientific, and Cultural Organization (FAO/UNESCO) Soil Map of the World, Vols. 2–10 [1971–1981]. This data set was developed for use by the improved land‐surface hydrology parameterization designed by Abramopoulos et al. [1988] for the Goddard Institute for Space Studies General Circulation Model II (GISS GCM). The data set specifies the top and bottom depths and the percent abundance of sand, silt, and clay of individual soil horizons in each of the 106 soil types cataloged for nine continental divisions. When combined with the World Soil Data File [Zobler, 1986], the result is a l°×l° global data set of variations in physical properties throughout the soil profile. These properties are important in the determination of water storage in individual soil horizons and exchange of water with the lower atmosphere within global climate models. We have used these data sets, in conjunction with the Matthews [1983] global vegetation data set and texture‐based estimates of available soil moisture, to calculate the global distributions of soil profile thickness, potential storage of water in the soil profile, potential storage of water in the root zone, and potential storage of water derived from soil texture. Comparisons with the water‐holding capacities used in the GISS Model II show that our derived values for potential storage of water are consistently larger than those previously used in the GISS GCM. Preliminary analyses suggest that incorporation of this data set into the GISS GCM has improved the model's performance by including more realistic variability in land surface properties.
Emissions of volatile organic compounds from vegetation and the implications for atmospheric chemistry
Authors: Fehsenfeld Fred , Calvert Jack , Fall Ray , Goldan Paul , Guenther Alex B. , Hewitt C. Nicholas , Lamb Brian , Liu Shaw , Trainer Michael , Westberg Hal , Zimmerman Pat
Published: February 2008

Vegetation provides a major source of reactive carbon entering the atmosphere. These compounds play an important role in (1) shaping global tropospheric chemistry, (2) regional photochemical oxidant formation, (3) balancing the global carbon cycle, and (4) production of organic acids which contribute to acidic deposition in rural areas. Present estimates place the total annual global emission of these compounds between approximately 500 and 825 Tg yr−1. The volatile olefinic compounds, such as isoprene and the monoterpenes, are thought to constitute the bulk of these emissions. However, it is becoming increasingly clear that a variety of partially oxidized hydrocarbons, principally alcohols, are also emitted. The available information concerning the terrestrial vegetation as sources of volatile organic compounds is reviewed. The biochemical processes associated with these emissions of the compounds and the atmospheric chemistry of the emitted compounds are discussed.
Oxidation of atmospheric methane in soil: Measurements in the field, in soil cores and in soil samples
Authors: Koschorreck Matthias , Conrad Ralf
Published: February 2008

Methane fluxes and vertical profiles of CH4 mixing ratios were measured in different German soils both in situ and in soil cores. Atmospheric CH4 was oxidized in the soil by microorganisms resulting in an average CH4 flux of −1.39±1.5 μmol‐CH4 m−2 h−1. Methane deposition showed only a weak positive correlation (r2 = 0.38) with soil temperature but a relatively strong negative correlation (r2 = 0.61) with soil moisture indicating limitation of the CH4 flux by gas transport. Diffusion experiments in soil cores showed that gas transport between atmosphere and soil was faster than microbial CH4 oxidation. However, the diffusion from the gas‐filled soil pores to the CH4 oxidizing microorganisms may have been limiting. The main CH4− oxidizing activity was located in a few centimeter thick subsurface soil layer at the top of the Ah horizon, whereas no activity was found in the overlying O horizons and in deep soil below about 20‐cm depth. In contrast, the highest CO2 production was found in the topmost O horizon. The effective diffusion coefficient of CH4 in soil was determined using a method based on relaxation experiments with argon. The diffusion coefficient was used to model the CH4 oxidation in soil cores from the vertical profiles of CH4 mixing ratios. The thus calculated CH4 oxidation rates and their localization in the soil profile compared fairly well with those determined directly from incubated soil samples. Fluxes were similar within a factor of 2–4 whether derived from the model, calculated from the measured CH4 oxidation rates of soil samples, or measured directly.
Application of an eutrophic lake model to the origin of ancient organic‐carbon‐rich sediments
Authors: Hollander David J. , McKenzie Judith A. , Hsu Kenneth J. , Huc Alain Y.
Published: February 2008

In this paper we approach the problem of the origin of organic‐carbon‐rich sediments by using an integrated isotopic and organic geochemical study of the annual productivity/eutrophication cycle in the water column of Lake Greifen (Switzerland) and the historical expression of such cycles as found in the sediments. The Lake Greifen water column and sediment studies reveal that Δδ13C carbonate‐organic matter is correlated with the hydrogen indices (HI) of kerogens, explicable in terms of changing productivity and preservation of the organic matter, and the CO2budget of the water body. The lake model implies that, if high productivity in CO2limited surface waters (high nutrient/CO2ratio) was controlling the preservation of organic matter, a correlation between decreasing Δδ13C carbonate ‐organic matter and increasing HI values may be observed. In contrast, if low to moderate productivity in CO2unlimited surface waters (low nutrient/CO2ratio) where bottom water anoxia promotes the preservation of organic matter, a correlation between increasing Δδ13C carbonate‐organic matter and increasing HI values may be observed. Application of this model to two well known Jurassic sequences, gave a correlation of increasing HI values with decreasing Δδ13C carbonate‐organic matter for the Kimmeridge Clay Formation (United Kingdom), and increasing HI values with increasing Δδ13C carbonate organic matter for the early Toarcian shales (France). This suggests that the controlling factor of the former deposit was high primary bioproductivity and of the latter deposit anoxic bottom water conditions.
Observations of depleted stratospheric NO2 following the Pinatubo volcanic eruption
Authors: Johnston P. V. , McKenzie R. L. , Keys J. G. , Matthews W. A.
Published: February 2008

Following the eruption of Mount Pinatubo, and the subsequent transport of volcanic aerosol, significant depletions of stratospheric NO2 were observed at Lauder, New Zealand (45°S, 170°E). The effect was more pronounced in NO2 columns measured at sunrise than those measured at sunset, and observed column amounts were reduced by more than 40% in October 1991. Concurrent ozone measurements obtained using the same technique were compared with Dobson measurements which confirmed that in October, apparent reductions of only 10–15% can be explained by geometric changes in scattering caused by the aerosols. We therefore conclude that a real depletion of NO2 has occurred. NO2 levels at twilight are dependent on many factors, but these observations may provide supporting evidence for the hypothesis that heterogeneous processes can convert N2O5 to HNO3 on the sulfate aerosol surfaces [Rodriguez et al, 1991]. If this proves to be the case it could, in the presence of current high chlorine levels and volcanically enhanced aerosol densities, lead to accelerated global ozone depletion.
Inter‐hemispheric Transport of Volcanic Ash from a 1259 A.D. Volcanic Eruption to the Greenland and Antarctic Ice Sheets
Authors: Palais J. M. , Germani M. S. , Zielinski G. A.
Published: February 2008

A strong volcanic sulfuric acid signal corresponding to an age of 1259 A.D. has been reported in ice cores from Greenland, Antarctica, and Arctic Canada. Tiny (< 5 μm) volcanic glass shards were reported previously in samples from this layer in an ice core from the South Pole. Here we report the discovery of volcanic glass shards from a contemporaneous layer in an ice core from Summit, Greenland. The major element composition of the glass shards in the Greenland sample are identical to those from the South Pole, confirming the assumption that has been made previously that the sulfuric acid signal in the ice cores is an inter‐hemispheric time stratigraphic marker. The composition of these glass shards is similar to those produced by a 550–700 yrs. B.P. eruption of El Chichon volcano in Mexico, suggesting that it may be the source of the widely dispersed material.
Harmonic excitation of mantle Rayleigh waves by the 1991 eruption of Mount Pinatubo, Philippines
Authors: Kanamori Hiroo , Mori Jim
Published: February 2008

An unusually long (at least two hours) seismic wave train having periods of about 230 sec was recorded at many worldwide seismic stations during the major eruption of Mount Pinatubo in the Philippines on June 15, 1991. This wave train exhibits two sharp spectral peaks at 228 and 270 sec. The group velocity, phase velocity, and the particle motion of this wave train indicate that it is a Rayleigh wave. The most probable excitation mechanism is acoustic coupling of atmospheric oscillations that were set off by continuous thermal energy flux from the volcano. The two spectral peaks correspond to the characteristic periods of acoustic and gravity modes of the Earth's atmosphere. The magnitude of the vertical single force equivalent to the acoustic coupling is 1.6×1017 dynes over a frequency band of 0.003 to 0.01 Hz. The results suggest the possibility of using acoustically coupled Rayleigh waves for detection, characterization and quantification of volcanic eruptions. Acoustic coupling of the atmosphere and the solid Earth provides a unique seismic source with long duration.
Heat‐flow constraints on the West African lithosphere structure
Authors: Lesquer Alain , Vasseur Guy
Published: February 2008

Heat production measurements from the West‐African shield have been compiled using various samples of crustal rocks. There is no clear indication for a systematic variation of heat production with age which could explain the regional heat flow density (HFD) variations by corresponding variations of the crustal component. HFD values are positively correlated with the P‐wave travel time residuals deduced from teleseismic observations and negatively correlated with gravity isostatic anomalies. We propose that regional HFD variations are significative of change in the upper mantle structure.
Effective attenuation lengths of cosmic rays producing 10Be AND 26Al in quartz: Implications for exposure age dating
Authors: Brown Erik Thorson , Brook Edward J. , Raisbeck Grant M. , Yiou Françoise , Kurz Mark D.
Published: February 2008

We have measured cosmic ray produced 10Be (t½= 1.5 My) and 26Al (t½ = 0.72 My) as functions of depth in a core of quartz sandstone bedrock collected in South Victoria Land, Antarctica. These data were used to place limits on the exposure age and erosion rate of the exposed surface, and to calculate the effective attenuation lengths of cosmic rays producing these nuclides. These latter results indicate that the production attenuation length for 26Al

is comparable to that for 10Be
. This suggests that the production ratio 26Al:10Be does not vary substantially with depth in a rock, and that its value at the surface of exposed rock is not strongly affected by erosional losses, allowing wider use of the ratio in determining exposure ages.