Using 87Sr/86Sr, δ18O and δ2H isotopes along with major chemical composition to assess groundwater salinization in lower Shire valley,Malawi

Groundwater resources in some parts of the lower section of Shire River valley, Malawi, are not useable for rural domestic water supply due to high salinity. In this study, a combined assessment of isotopic (⁸⁷Sr/⁸⁶Sr, δ¹⁸O and δ²H) and major ion composition was conducted in order to identify the hydro-geochemical evolution of the groundwater and thereby the causes of salinity. Three major end-members (representing fresh- and saline groundwater, and evaporated recharge) were identified based on major ion and isotopic composition. The saline groundwater is inferred to result from dissolution of evaporitic salts (halite) and the fresh groundwater shows influence of silicate weathering. Conservative mixing models show that brackish groundwater samples result from a three component mixture comprising the identified end-members. Hence their salinity is interpreted to result from mixing of fresh groundwater with evaporated recharge and saline groundwater. On the other hand, the groundwater with low TDS, found at some distance from areas of high salinity, is influenced by mixing of evaporated recharge and fresh groundwater only. Close to the Shire marshes, where there is shallow groundwater, composition of stable isotopes of water indicates that evaporation may also be an important factor.     

Ground-truthing 11- to 12-kHz side-scan sonar imagery in the Norwegia–Greenland Sea: Part I: Pockmarks on the Vestnesa Ridge and Storegga slide margin

 Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet bulk density: 1400–1600 kg m^-3; sound speed: 1480– 1505 m s^-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m^-1 deg C^-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local.     

Rhyodacites,andesites, ferro-basalts and ocean tholeiites from the galapagos spreading center

Volcanic rocks, dredged from depths greater than 1000 m on the Galapagos spreading center, show extreme chemical diversity, including rhyodacites, andesite, ferro-basalts, and low-K oceanic tholeiite. All samples have fresh glassy margins. The ferro-basalts contain up to 18.5% total iron as FeO and up to 3.75% TiO₂, while the oceanic tholeiites are as low as 0.02% K₂O. The ferro-basalts correlate with the previously proposed zone of high magnetic anomaly amplitudes which flank the Galapagos hot spot, and are consistent with a genesis by shallow fractional crystallization.     

La sismicité en corse: Revue historique et etude macrosismique de quelques evénéments récents

Résumé L'étude des documents (archives, presse et catalogues sismiques) que nous avons consultés, nous a permis de remonter jusqu'à la fin du XVIIIe siècle dans l'histoire sismologique corse. Le nombre des séismes ressentis (épicentres en mer ou sur l'île) recensés pendant ces deux siècles et demi est relativement faible. Les intensités maximales ont parfois atteint le degré VI, plus rarement VII. L'activité sismique touche principalement une région située au Nord-Est de la Corse, au voisinage de failles au rejeu récènt et séparent le domaine de la Corse schisteuse (alpin) du domaine de la Plaine Orientale (Quaternaire, Néogène). Une crise sismique récente (1978–1979) a de nouveau secoué cette même région où on a suggéré l'activité actuelle de certaines de ces failles. Les plus importants de ces séismes (I _max=VI et V) ontifait l'objet d'enquêtes macrosismiques qui permettent d'en préciser les épicentres. Deux d'entre-eux ont été étudiés en détail (3 avril 1978 et 27 mars 1979); en utilisant la relation de Kövesligethy, les hypocentres se situent respectivement à 4,5±0.5 km et vers 5 km, pour des coefficients d'absorption de 0,020 et 0,031. Ces profondeurs indiquent que ces deux séismes ont mis en jeu des éléments superficiels de la croûte corse (30 km).

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The documents that we have compiled (archives, newspaper reports and seismic catalogues) enabled us to go back to the end of the 18th century in the seismic history of Corsica. The number of felt earthquakes (epicentres off shore or on the island) which were documented during the two and a half centuries is relatively low. Maximum intensities have reached VI or rarely VII. The seismic activity is mainly located in the North-East of Corsica, near faults which have recently been active between the Corse schisteuse (Alpine domain) and the Plaine Orientale (Quaternary, Neogene). A seismic crisis happened recently (1978–1979) in this area where present activity of some of these faults has been suggested. Macroseismic studies have been conducted for the main shocks (I _max=VI and V) in order to locate them accurately. Two among them (April 3, 1978 and March 27, 1979) are studied here in detail. Using the Kövesligethy formula hypocenters are respectively found at 4.5±0.5 km and around 5 km, whereas the corresponding absorption factors are 0.020 and 0.031. Such depths show that the two events affected the upper crust of Corsica (30 km).

Contribution n^o 321 de l'Institut de Géophysique, Ecole Polytechnique Fédérale de Zürich.     

Volcano spacing,fractures, and thickness of the lithosphere

Both “hot-spot” type and possibly island-arc volcanoes may form at the intersections of fractures whose spacing is near the thickness of the lithosphere and increases with increasing thickness. An approximate equality between layer thickness and spacing of major fractures observed in some sedimentary rocks and clay cake models may thus extend to the “mega-joints” that have fractured the lithosphere and controlled volcano spacing on the earth, and possibly on Mars. If the hot-spot fractures are interpreted as due to shear, many hot-spot fracture systems suggest roughly north-south least principal stress, or, alternatively in some instances, a 90° rotation of this pattern.     

Pollutant dispersion close to an urban surface – the BUBBLE tracer experiment

Summary In this publication first results of an urban tracer experiment are reported. This experiment was realized in the framework of the Basel UrBan Boundary-Layer Experiment (BUBBLE) in an area with abundant information on turbulence and flow conditions available. Release height was close to roof level and so was the height of the concentration samplers. The meteorological conditions during the experiments were mainly convective, but due to the rough character of the underlying surface also the mechanical turbulence was substantial.The concentration distribution is found to be essentially Gaussian in the horizontal plane and some commonly used methods to estimate the plume widths in applied dispersion models are compared to the observations. From measurements at one site downwind of the source it is found that for a near-roof level source, only an insignificant vertical gradient in tracer concentration is present within a street canyon. Using a Lagrangian Particle Dispersion Model the tracer experiments are simulated. It is shown that the exact form of the parameterization for the flow and turbulence structure within the urban roughness sublayer is of great importance for the simulation results. Also the numerical simulation results underline the necessity (and difficulty) to describe the vertical profile of the dissipation rate of turbulent kinetic energy close to an urban surface.     

Palinspastic Reconstruction for Tectonics of Northeast China and Its Vicinity

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Genetic implications of geochemical factor analysis in a sediment-hosted Cu-Pb-Ba mineralization

Factor analysis has been applied to geochemical data obtained from Abra, a sediment-hosted Cu-Pb-Ba mineralization in the Late Proterozoic (1,100 Ma) Bange-mall Basin in Western Australia. From some 5,000 m of drill core 57 samples have been analyzed for 26 elements by ICP/AAS. Statistical analysis was performed on 19 elements (W, Zn, P, Pb, Co, Ni, Ba, Fe, Mn, Mg, V, Al, Ca, Cu, Ti, Sr, Na, K, Si). The data have been treated first by statistical standard methods, not considered here, then by representative graphic display of the spatial distributions of elements in a section through the mineralization, and finally by factor analysis. Three groups of elements can be distinguished according to their mode of spatial distribution in section, i.e., strata-bound, discordant, and equivocal. Eight factors have been found to explain 90% of the total variability, and the spatial distribution of these factors is discussed with respect to their geological plausibility.These findings were confirmed by previous geological and mineralogical studies. In addition, a second mineralizing fault system at the southern margin of the Abra Sub-Basin could be identified by means of the spatial distribution of factor scores.