Evolution of the Earth's atmosphere and its geological impact

Current problems with the ozone layer and rising carbon dioxide levels have highlighted the importance of the atmosphere, with its delicate balance of gases. In fact, the atmosphere has been changing since the Earth formed. From anoxic beginnings to its current oxygenated state, it has influenced depositional processes on the Earth's surface and helped the development of active, oxygen-breathing life forms that culminated in ourselves. Evidence for changes in the atmosphere is found in the sediments accumulated on the Earth's surface, where fossils are also preserved. In this article the major geological aspects of the atmosphere's evolution are considered; in particular, the role of oxygen is aired.     

Democratic input into the nuclear waste disposal problem: The influence of geographical data on decision making examined through a Web-based GIS

This paper elucidates the manner in which users of an online decision support system respond to spatially distributed data when assessing the solution to environmental risks, specifically, nuclear waste disposal. It presents tests for revealing whether users are responding to geographical data and whether they are influenced by their home location (Not in My Back Yard – style behavior). The tests specifically cope with problems associated with testing home-to-risk distances where both locations are constrained by the shape of the landmass available. In addition, we detail the users wider feelings towards such a system, and reflect upon the possibilities such systems offer for participatory democracy initiatives.The authors would like to thank the two anonymous referees for their helpful and generous comments.     

Origin of rhyolite and rhyodacite lavas and associated mafic inclusions of Cape Akrotiri,Santorini: the role of wet basalt in generating calcalkaline silicic magmas

Amphibole-bearing mafic inclusions (low to medium-K high-alumina basalt to basaltic andesite) comprise 4.1 vol% of calc-alkaline rhyolite and rhyodacite lavas on Akrotiri Peninsula, Santorini, Greece. Physical features indicate a magmatic origin for the inclusions, involving mingling with the host silicic magma and quenching. Water contents of the mafic magmas are estimated to have been above 4% at water pressures of 1.8 kbars or more at temperatures of approximately 950–1,000 °C. Three evolutionary stages are inferred in their petrogenesis. In the first stage infiltration of slab fluids promotes partial melting in the mantle to generate primitive wet basaltic magmas enriched in LREE, LILE, Th and U in comparison to N-type MORB. In the second stage storage and crystal differentiation of primitive magmas occurred in the lithospheric mantle or deep crust, involving olivine, spinel and clinopyroxene followed by amphibole and plagioclase. In the third stage differentiated mafic magma intrudes into porphyritic silicic magma at shallower crustal levels (estimated at 7–10 km). Mingling and quenching of the mafic magmas within the silicic host causes chemical or physical interactions between the inclusions and the host prior to and during eruption. The silicic lavas have geochemical affinities with the mafic inclusions, but are relatively depleted in MREE, HREE and Y and enriched in Rb relative to Ba and K. These observations are consistent with involvement of amphibole in magma genesis due either to crystal differentiation from wet basalt or to partial melting of mafic rocks with residual amphibole. Crystallization of wet basalt in the deep crust is preferred on the basis of physical considerations.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: I. Parsons     

On the dynamics of the jovian ionosphere and thermosphere.: IV. Ion-neutral coupling

We use the fully coupled, three-dimensional, global circulation Jovian Ionospheric Model (JIM) to calculate the coupling between ions in the jovian auroral ovals and the co-existing neutral atmosphere. The model shows that ions subject to drift motion around the auroral oval, as a result of the E×B coupling between a meridional, equatorward electric field and the jovian magnetic field, generate neutral winds in the planetary frame of reference. Unconstrained by the magnetic field, these neutral winds have a greater latitudinal extent than the corresponding ion drifts. Values of the coupling coefficient, k(h), are presented as a function of altitude and cross-auroral electric field strength, for different incoming electron fluxes and energies. The results show that, with ion velocities of several hundred metres per second to over 1 km s⁻¹, k(h) can attain values greater than 0.5 at the ion production peak. This parameter is key to calculating the effective conductivities required to model magnetosphere-ionosphere coupling correctly. The extent to which angular momentum (and therefore energy) is transported vertically in JIM is much more limited than earlier, one-dimensional, studies have predicted.     

Spatial variability of mercury emissions from soils in a southeastern US urban environment

We quantified gaseous mercury (Hg⁰) fluxes over soil surfaces in an urban setting during the winters of 2003 and 2004 across the metropolitan area of Tuscaloosa, AL. The objective was to provide a first inspection of the local spatial variability of mercury flux in an urban area. Flux sampling took place on bare, undisturbed, soil surfaces within four evenly spaced landuse areas of Tuscaloosa: industrial, commercial, residential, and mixed landuse. Median total gaseous mercury fluxes (ng/m² h) from each site were as follows: 4.45 (residential site), 1.40 (industrial site), 2.14 (commercial site), and 0.87 (mixed landuse site). Using non-parametric statistical analyses, the residential and mixed landuse sites were found to be statistically different from the overall median flux. Landuse and soil type are the suspected factors primarily controlling the observed spatially variable fluxes. The presence of statistically different fluxes over soil surfaces on a local scale in this preliminary study warrants additional investigation, particularly during the spring and summer seasons when terrestrial mercury emission is the highest. Providing such information will help develop better estimates of mercury emission from urban areas and, ultimately, lead to more accurate and useful spatially relevant inventories.     

Sedimentary and biotic evidence of a warm-water enclave in the cooler oceans of the latest Ordovician glacial phase, Yangtze Platform, South China block

Abstract   During the Hirnantian period, the Yangtze Platform was situated in the western part of the South China block (SCB) before its later rotation, in the middle–low paleolatitudes of the southern hemisphere in the northeast side of peri-Gondwana. It is part of the Kosov faunal province as indicated by the Hirnantia fauna. Sedimentary evidence shows the domination of cool ventilated marine water from its offshore ramp and shelf. Hirnantian shallow-water carbonate facies (Kuanyinchiao Bed) overlie earlier Ashgill graptolitic black shales (Wufeng Formation) as a result of marine regression. In the Yangtze Platform, however, we have found local areas of intertidal to nearshore facies that lack the typical highly diverse Hirnantia fauna. Some warm-water features (radial oolites, peloids, diverse solitary rugose corals and other benthic shelly fauna) occur commonly in some limited shallow areas, forming grainstones and packstones. Although interglacial episodes within the Hirnantian glaciation could be responsible for these features, their limited occurrence within the interior of the platform leads us to interpret the deposits as indicating that cold-water currents from the southeastern high latitudes were partly excluded from the nearshore area of the Yangtze Platform. The landmass of the eastern SCB in the Hirnantian epoch prevented access to some areas of the cold marine water masses that flowed from higher latitudes of Gondwana; the result was a persistence of warm-water shallow marine facies in some areas.     

Development of a GIS-based model for extrapolating mesoscale groundwater recharge estimates using integrated geospatial data sets

A geographic information systems (GIS)-based model was used to derive spatially explicit estimates of recharge using the elevation-dependent chloride mass-balance (EDCMB) approach in a 14-basin area in southwestern Nevada, USA. For each of the 14 basins, a non-linear regression equation was developed relating chloride enrichment to the elevation of spring watersheds. The ratio of chloride enrichment as a function of elevation was used in a GIS framework to transform continuous precipitation data to recharge. Spatial masks that represented two definitions of the lower limits of recharge—one definition based on alluvium/non-alluvium boundaries, the other based on both alluvium/non-alluvium boundaries and an elevation threshold—were applied to each basin. Resultant recharge estimates were then summed for each basin. When compared to summaries of previous recharge estimates for the study area, the EDCMB approach produced results that were within 14 and 3% of two studies, but were significantly greater (31%) than a third. GIS proved to be a very effective tool for combining large spatial data with widely different resolutions into an integrated data set, and also proved to be an efficient mechanism for implementing robust statistical models to estimate recharge.

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Résumé Un modèle basé sur les systèmes d’informations géographiques (SIG) a été utilisé pour obtenir des estimations explicites de la recharge dans l’espace, en utilisant l’approche du bilan des chlorures en fonction de l’altitude (EDCMB en anglais) dans une zone de 14 bassins, dans le Sud-Ouest du Nevada aux Etats-Unis. Pour chacun des 14 bassins, une régression non-linéaire a été établie, reliant l’enrichissement des chlorures à l’altitude des bassins d’alimentation des sources. Le rapport d’enrichissement des chlorures en fonction de l’altitude a été utilisé dans une approche SIG pour transformer des données pluviométriques continues en terme de recharge. Des masques spatiaux représentant deux définitions des limites de recharge les plus basses ont été appliquées à chaque bassin ; l’une basée sur les limites alluvions/autres formations, l’autre sur ces mêmes limites ainsi que sur un seuil altitudinal. Les recharges estimées résultantes ont ensuite été additionnées pour chaque bassin. Comparée aux précédents bilans de recharge estimés dans la zone d’étude, l’approche EDCMB conduit à des résultats se situant entre 14 et 3% de ceux de deux autres études et largement supérieurs (31%) à ceux d’une troisième étude. Le SIG s’est révélé être un outil très efficace pour combiner dans une base de données intégrée, des données spatiales étendues avec des résolutions largement différentes, ainsi que pour implémenter des modèles statistiques conséquents permettant d’estimer la recharge.

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Resumen Se utilizó un modelo basado en un Sistema de Información Geográfico (SIG) para derivar espacialmente estimados explícitos de recarga usando el enfoque de balance de masa de cloruro dependiente de elevación (EDCMB) en un área de 14 cuencas en el suroeste de Nevada, Estados Unidos. Para cada una de las 14 cuencas se desarrolló una ecuación de regresión no linear que relaciona el enriquecimiento de cloruro con la elevación de las cuencas con manantiales. La relación de enriquecimiento de cloruro en función de la elevación se usó en un marco SIG para transformar datos continuos de precipitación a recarga. Se aplicaron máscaras espaciales a cada cuenca las cuales representaron dos definiciones de los límites inferiores de recarga, una definición basada en límites de aluvión/sin aluvión, el otro basado en ambos límites aluvión/sin aluvión y una elevación normal de fondo. Los estimados resultantes de recarga fueron sumados para cada cuenca. Al compararse con estimados de recarga previos para el área de estudio se encontró que el enfoque EDCMB produjo resultados que estaban entre el 14 y 3% de dos estudios pero fueron significativamente mayores (31%) que un tercer estudio. El SIG ha demostrado ser una herramienta muy efectiva para combinar datos espaciales grandes con resoluciones muy distintas en un grupo integrado de datos y también ha mostrado ser un mecanismo eficiente para implementar modelos estadísticos robustos para estimar recarga.