Analysis of geodetic satellite (Geos I) observations in North America

Geos I observations made at thirty optical and four Secor stations were analyzed in the geometric and short-arc modes for the purpose of detecting systematic distortions in the North American Datum. The results indicate that the Nad at its origin (Meades Ranch, Kansas) requires an easterly rotation in azimuth and a westerly rotation in the prime vertical plane, both in the magnitude of one second of arc to fit the satellite data. There is also evidence of the need for a reduction in scale of about 1:200.000.     

Wave diffraction and near-trapping by a multi-column gravity-based structure

This paper is concerned with the diffraction of water waves by offshore structures, with the ultimate aim of proposing tools for guiding airgap design. The diffraction of monochromatic waves by an array of four bottom mounted cylinders and a gravity-based structure is studied in detail using linear and second order theory. The phenomenon of near-trapping is investigated, allowing guidelines for airgap design to be established. When contemplating airgap design, however, it is crucially important that consideration is given to the largest waves in a sea state. Therefore, in this study a design wave, called NewWave, is proposed as a realistic model for large ocean waves and is used as the incident wave field in the wave-structure diffraction analysis.     

An inventory of factors that affect polysaccharide production by Phaeocystis globosa

Phaeocystis material contains polysaccharides that are built from at least eight different monosaccharides. Differences have been reported between the carbohydrate composition of different Phaeocystis species, and also between samples taken from Phaeocystis globosa blooms in different areas. In order to elucidate factors that could play a role in determining variation in carbohydrate composition and production, a number of Phaeocystis globosa strains were studied under laboratory conditions. Although there was a clear distinction of a northern and a southern cluster in the Phaeocystis globosa strains based on RAPD analysis, the differences in the composition of the mucopolysaccharides were relatively small. The contribution of glucose, however, ranged from 7–85% of total sugars. A strain that was cultured in seawaters of diverse origin produced polysaccharides of a different composition, suggesting the effect of environmental factors. The presence of bacteria affected neither the amount, nor the composition of the carbohydrates that were produced by Phaeocystis globosa. Glucose is part of both the intracellular polysaccharide pool and of the mucopolysaccharides in the colony matrix. Using specific digestion of the intracellular chrysolaminaran by laminarinase, the distribution of polysaccharides over different pools could be assessed. During growth of an axenic, mucus-producing strain, the portion of glucose present as chrysolaminaran appeared to increase. The polyglucose that was not digested by laminarinase remains unidentified. This study shows that environmental factors rather than strain differences determine differences in the sugar composition of Phaeocystis globosa, especially with respect to the glucose content of the material. A difference in the contribution of glucose could be correlated to the portion of cells in the culture that are not in the colonies. Our study emphasises that for studying polysaccharide dynamics in Phaeocystis globosa it is important to be able to discriminate between the different polysaccharide pools. Preliminary results of an enzymatic approach were promising     

Nonlinear wave conversion in electron-positron plasmas

Wave conversion mechanisms causing large-frequency shifts are considered for an electron-positron plasma in a strong magnetic field. In particular, we discuss the effects of the nonlinear erenkov as well as the cyclotron resonances in order to associate pulsar radio-emissions with our present model for nonlinear conversion of high-frequency radiation into the low-frequency region.     

Five WC9 stars discovered in the AAO/UKST Hα survey

We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated.     

Metamorphic history of the South Tibetan Detachment System,Mt. Everest region,revealed by RSCM thermometry and phase equilibria modelling

This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well‐exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ～340 °C. Temperatures increase down‐section, reaching ～650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ～310 °C km^?1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1‐km‐thick zone of distributed top‐down‐to‐the‐northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo‐isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up‐section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.     

The Petrology of Plutonic Blocks and Inclusions from the Lesser Antilles Island Arc

Three main groups of plutonic nodules are present in the LesserAntilles arc and are interpreted as (a) phenocryst clusters,(b) metamorphosed wallrock xenoliths and (c) cumulate texturedxenoliths. Large cumulate blocks display felsic-mafic layering,slump structures and auto-intrusive features. The majority ofspecimens are ad- and heteradcumulates with fewer ortho- andcrescumulates. Interstitial scoria and glass are present ina variety of samples. Plagioclase, amphibole, cino- and orthopyroxene,olivine, magnetite, biotite, ilmenite, quartz and apatite arepresent in various proportions in individual blocks. Plagioclaseand amphibole are modally predominant. Significant variationalong the arc is displayed in the rarity of orthopyroxene andabundance of amphibole in the southern islands compared withthe common presence of two pyroxenes in the northern islands. Plagioclase varies from An_100–36 with very low orthoclasecomponent, and usually precedes amphibole in a given crystallizationsequence. Only on Grenada are plagioclase-free blocks present.Olivine is restricted to assemblages where coexisting plagioclaseis more calcic than An₈₉ and its composition range is Fo_90–59.Clinopyroxene is predominantly calcic augite and cinopyroxene,olivine and plagioclase all coexist stably with amphibole. Ageneral trend of decreasing Ca content in clinopyroxene fromsouth to north in the arc is present. Orthopyroxene ranges from En_73–49 and is most common inassemblages where the coexisting plagioclase is more sodic thanAn₈₃ Coexisting pyroxenes define temperatures in the range 800–1050?C. Amphibole compositions include pargasite, magnesiohastingsite,magnesio-hornblende and tschermakitic hornblende. The K contentof the amphiboles increases from north to south in the samesense as the general tholeiitic-calcalkalic-alkalic variationof parental magmas. Magnetite is the dominant spinel phase but ferrian chromiteand chromian magnetite are present in some Grenada cumulatesand pleonaste is found in rare St. Kitts samples. Ilmenite ispresent in blocks from several islands; coexisting Fe-Ti oxidesdefine a temperature range of 710?-950 ?C at of 15.5 to 10.0 bars. Biotite, quartz and apatiteare restricted to evolve cumulate types. Some modification of interstitial scoria/glass compositionsfrom equilibrium melts has occurred in the majority of samples,but general similarity with erupted lava types is importantevidence for the cognate relationship of the cumulate assemblages.The role of H₂O is crucial in determining the calcic natureof island arc plutonic plagioclase when compared with relativelydry, layered tholeiitic plutons. Some modal and chemical featuresof cumulate-lava comparisons suggest plagioclase flotation maybe significant. A variety of thermodynamic calculations indicate temperaturesand pressures of crystallization in the range 850–1050?C, 4–10 kb. No evidence exists for systematic along-arcvariations in these parameters. Standard amphibole crystallinesolution models give unsatisfactory results for calculations. Some distinctive contrasts between cumulate and phenocryst modesare present. The abundance of amphibole in equilibrium withbasaltic melts in the plutonic situation compared with its rarityin lavas is striking. Plagioclase coexisting with a given meltis more anorthitic in the plutonic than the phenocryst mode. Least squares fractionation tests demonstrate the possibilityof relating basalt-andesite-dacite suites by fractional crystallizationof the cumulus phases. Trace element systematics of cumulate-lavasuites for individual islands also generally support this hypothesis.The suggestion of sole amphibole fractionation for the generationof andesite from basalt is discarded. ^* Present address: CRAE, Box 39598, Darwin, N.T. 5798, Australia.     

Mineralogy,paragenesis and genesis of the braunite deposits of the Mary Valley Manganese Belt,Queensland, Australia

The Mary Valley manganese deposits exhibit mineralogy and textures characteristic of at least four parageneses. The deposits consist mainly of isolated occurrences of braunite, together with a number of lower and higher valency manganese oxides, and manganese silicates, in bedded radiolarian cherts and jaspers of Permian age. The parageneses are: (a) Braunite — quartz (primary), (b) Braunite — hausmannite — spessartine — tephroite — quartz (metamorphic). (c) Hydrated manganese silicates — barite — braunite — hausmannite (hydrothermal veins), (d) Tetravalent manganese oxides (pyrolusite, cryptomelane, manjiroite, nsutite) (supergene). The primary mineralisation is interpreted as the result of the geochemical separation of Mn from Fe in a submarine exhalative system, and the precipitation of Mn as oxide within bedded radiolarian oozes and submarine lavas. During diagenesis this hydrothermal manganese oxide reacted with silica to produce primary braunite. The later geological of evolution of this volcanogenicsedimentary deposit involved metamorphism, hydrothermal veining by remobilised manganese, and supergene enrichment.     

Comparison of 4D tomographic mapping versus thin-shell approximation for ionospheric delay corrections for single-frequency GPS receivers over North America

The majority of navigation satellite receivers operate on a single frequency. They compensate for the ionospheric delay using either an ionospheric model which typically only corrects for 50% of the delay or a thin-shell map of the ionosphere. A 4D tomographic imaging technique is used to map the free electron density over the full-height of the ionosphere above North America during autumn 2003. The navigation solutions computed using correction based upon the thin-shell and the full-height maps are compared in this paper. The maps are used to calculate the excess propagation delay on the L1 frequency experienced by GPS receivers at selected locations across North America. The excess delay is applied to correct the single-frequency pseudorange observations at each location, and the improvements to the resulting positioning are calculated. It is shown that the thin-shell and full-height maps perform almost as well as a dual-frequency carrier-smoothed benchmark and for most receivers better than the unfiltered dual-frequency benchmark. The full-height corrections perform well and are considerably better than thin-shell corrections under extreme storm conditions.