Dust Morphology Of Comet Hale-Bopp (C/1995 O1): I. Pre-Perihelion Coma Structures In

In 1996 comet Hale-Bopp exhibited a porcupine-like coma with straight jets of dust emission from several active regions on the nucleus. The multi-jet coma geometry developed during the first half of 1996. While the jet orientation remained almost constant over months, the relative intensity of the jets changed with time. By using the embedded fan model of Sekanina and Boehnhardt (1997a) the jet pattern of comet Hale-Bopp in 1996 can be interpreted as boundaries of dust emission cones (fans) from four — possibly five — active regions on the nucleus (for a numerical modelling see part II of the paper by Sekanina and Boehnhardt, 1997b). This revised version was published online in July 2006 with corrections to the Cover Date.     

Establishing the sediment stratigraphy of reservoirs in the southern Pennines,UK

Abstract

Reservoirs in the southern Pennines have been investigated to determine their potential for reconstructing a history of atmospheric pollution. A selection procedure has been used to identify sites with the best chance of obtaining a usable sedimentary record prior to sample collection. Five reservoirs were selected for core collection and sediment analysis using magnetic susceptibility (χlf), particle size, spheroidal carbonaceous particle (SCP) analysis and radiometric dating. Analysis reveals that largely undisturbed sedimentary records exist in the deepest zones of these reservoirs, thus reservoir sediments represent a valuable resource for investigating pollution histories in the southern Pennines.     

A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes     

Optimizing bioventing in shallow vadose zones and cold climates

Abstract

This paper describes a bioventing study design and initial activities applied to a JP-4 jet fuel spill at Eielson Air Force Base, Alaska. The primary objectives of the project were to investigate the feasibility of using bioventing technology to remediate JP-4 jet fuel contamination in a sub-arctic environment and to determine to what degree the biodegradation rate of JP-4 soil contaminants could be enhanced by increasing soil temperature, both actively by circulating heated groundwater and passively by utilizing solar energy. Biodegradation rates at the bioventing site remained relatively high during the winter months in the active-warming test plot and were consistently higher than those observed in the passive-warming and control test plots. These studies suggest that an active-warming system operated in conjunction with bioventing is a useful method for remediating fuel-contaminated areas in cold climates.     

Rate of oxygen removal in some South African groundwaters / Taux d'enlèvement d'oxygène dans de certaines eaux souterraines en Afrique du Sud

Abstract

Data are presented for the dissolved oxygen concentrations and radiometric ages of ‘normal’ (i.e. non-thermal and chemically not unusual) groundwaters in four aquifers in the Cape Province of South Africa. It is demonstrated that a significant amount of the atmospheric oxygen dissolved in the groundwater is consumed within a relatively short period of the order of 20 years. In some environments oxygen concentration measurements may provide a convenient method for locating areas of recharge in groundwater studies.     

Use of algal assays to determine the potential for phytoplankton growth / Utilisation des essais d'algues pour la détermination du potentiel pour ce qui regarde la croissance du phytoplancton

Abstract

Algal growth potential is defined as the maximum algal mass (dry weight) that can be produced in a natural water sample under standardized laboratory conditions. Algal growth potential measurements are designed to establish baseline data, growth limiting factors (nutrients), and the influence and source of various growth promoting nutrients and toxicants so as to provide improved means for predicting and controlling excessive algal growth in aquatic habitats. Data can be compared only when the variables which control algal growth are standardized. Algal growth potentials derived in the laboratory may not reflect natural conditions because of insufficient light or temperature, grazing by invertebrates or fish, or the presence of any toxic materials. An understanding of the principle of the test and the factors that affect the expression of algal growth potentials is critical to proper data interpretation.     

GRAPHICAL SOLUTION OF GROUND WATER FLOW PROBLEMS

Abstract

Graphical solutions to linear and radial ground water flow problems are described. These methods are based on finite difference approximations which have been widely used in heat flow problems. The case of the leaky aquiclude and vertical recharge to unconfined aquifers is discussed as well as constant head at a control and constant discharge or recharge. These solutions are useful only when the physical factors are known and an estimate of water level changes under certain conditions is desired. The methods are not intended to replace more rigorous mathematical solutions.     

Physicochemical and biological influences on sedimentary‐fabric formation in a salinity and oxygen‐restricted semi‐enclosed sea: Gotland Deep,Baltic Sea

Two ca 8000 year long sediment cores from the Gotland Deep, the central sub‐basin of the Baltic Sea, were studied by means of digital images, X‐radiographs and scanning electron microscopy–energy‐dispersive X‐ray mineralogical analysis to gain understanding of the physicochemical and biological influences on sedimentary‐fabric formation in modern and ancient seas with a high flux of organic carbon, and associated oxygen stress and depauperate ichnofauna. Four lithofacies were recognized: (i) sharply laminated mud; (ii) biodeformed mud; (iii) burrow‐mottled mud; and (iv) sedimentation‐event bed. The sharply laminated and burrow‐mottled facies dominate the cores as alternating long intervals, whereas the biodeformed and sedimentation‐event facies occur as thin interbeds within the sharply laminated intervals. The sharply laminated mud comprises alternating diatom‐rich and lithic laminae, with occasional Mn‐carbonate laminae. Lamination discontinuity horizons within the laminites, where the regular lamination is overlain sharply by gently inclined lamination, challenge the traditional view of mud accumulation by settling from suspension, but indicate localized accumulation by particle‐trapping microbial mats and, potentially, by the rapid lateral accretion of mud from bedload transport. The biodeformed interbeds record brief (few years to few decades) oxic–dysoxic conditions that punctuated the anoxic background conditions and permitted sediment‐surface grazing and feeding by a very immature benthic community restricted to the surface mixed tier. The likely biodeformers were meiofauna and nectobenthic pioneers passively imported with currents. The sedimentation‐event interbeds are distal mud turbidites deposited from turbidity currents probably triggered by severe storms on the adjacent coastal areas. The turbidite preservation was favoured by the anoxic background conditions. The long burrow‐mottled intervals are characterized by intensely bioturbated fabrics with discrete Planolites, rare Arenicolites/Polykladichnus and very rare Lockeia trace fossils, as well as bivalve biodeformational structures which represent shallowly penetrating endobenthic feeding and grazing strategies and permanent dwellings. These burrowed intervals represent longer periods (several years to few centuries) of oxic–dysoxic conditions that permitted maturation in the benthos by means of larval settling of opportunistic worm‐like macrofauna and bivalves, resulting in the development of a transition tier. These observations imply more dynamic and oxic depositional conditions in Gotland Deep than previously thought. Comparison to previous zoobenthic studies in the area allowed discussion of the benthic dynamics, and the identification of probable biodeforming and trace‐producing species. Implications for current biofacies and trace‐fossil models are discussed.     

Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P-T and Geochemical Studies on Xenoliths

Xenoliths hosted by Quaternary basanites and alkali basaltsfrom Marsabit (northern Kenya) represent fragments of Proterozoiclithospheric mantle thinned and chemically modified during riftingin the Mesozoic (Anza Graben) and in the Tertiary–Quaternary(Kenya rift). Four types of peridotite xenoliths were investigatedto constrain the thermal and chemical evolution of the lithosphericmantle. Group I, III and IV peridotites provide evidence ofa cold, highly deformed and heterogeneous upper mantle. Textures,thermobarometry and trace element characteristics of mineralsindicate that low temperatures in the spinel stability field(750–800°C at <1·5 GPa) were attained bydecompression and cooling from initially high pressures andtemperatures in the garnet stability field (970–1080°Cat 2·3–2·9 GPa). Cooling, decompressionand penetrative deformation are consistent with lithosphericthinning, probably related to the development of the Mesozoicto Paleogene Anza Graben. Re-equilibrated and recrystallizedperidotite xenoliths (Group II) record heating (from 800°Cto 1100°C). Mineral trace element signatures indicate enrichmentby mafic silicate melts, parental to the Quaternary host basanitesand alkali basalts. Relationships between mineral textures,P–T conditions of equilibration, and geochemistry canbe explained by metasomatism and heating of the lithosphererelated to the formation of the Kenya rift, above a zone ofhot upwelling mantle. KEY WORDS: East African Rift System; Anza Graben; in situ LA-ICPMS; peridotite xenoliths; thermobarometry     

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800{degrees}C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H₂O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H₂O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO₂, Na₂O, and K₂O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al₂O₃, Na₂O, andK₂O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al₂O₃/[Na₂O + K₂O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10^–15m²/s and (0·6–2·4) x 10^–14 m²/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10^–11m²/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H₂O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H₂O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis