40Ar/39Ar constraints on the temporal evolution of Graciosa Island, Azores (Portugal)

Lava flows spanning the eruptive record of Graciosa Island (Azores archipelago) and a gabbro xenolith were dated by ⁴⁰Ar/³⁹Ar in order to constrain the Pleistocene and Holocene volcanic evolution of the island. The results range from 1.05 Ma to 3.9 ka, whereas prior published K–Ar and ¹⁴C ages range from 620 to 2 ka. The formation of the Serra das Fontes shield volcano started at minimum 1.05 Ma, and the magmatic system was active for ca. 600 ky, as suggested by the formation of the gabbro xenolith by magmatic differentiation. Evolved magmas making up the Serra das Fontes–Serra Branca composite volcano were generated at ca. 450 ka. After a period of ca. 110 ky of volcanic inactivity and erosion of volcanic edifices, volcanism was reactivated with the formation of the Vitória Unit NW platform. Later, the development of the Vulcão Central Unit started with the formation of monogenetic cones located to the south of the Serra das Fontes–Serra Branca–Vitória Unit. This volcanism became progressively more evolved and was concentrated in a main eruptive center, forming the Vulcão Central stratovolcano with an age older than 50 ka. The caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera, resulting in the formation of a lava lake, which ultimately spilled over the caldera rim at ca. 11 ka. The most recent eruptions on Graciosa formed two small pyroclastic cones within the caldera and the Pico do Timão cone within the Vitória Unit at ca 3.9 ka.     

Diffuse and localized failure modes: Two competing mechanisms

The concept of failure is one of the most debated in soil mechanics, for two reasons essentially. First, this is a crucial issue in the engineering of structures and geotechnical project design. Second, this is still a challenging academic issue mobilizing significant scientific interest in the development of a unique framework to describe the different failure modes. In this respect, this paper revisits the localized failure mode, replacing the well‐known Rice criterion within the wider context of bifurcation. Considering a micro‐mechanical model, the main theoretical results are covered. In particular, it is established that localized failure is a particular case of failures observed within the so‐called bifurcation domain: the incremental strain within the localization band is associated with a vanishing value of the second‐order work. Copyright © 2010 John Wiley & Sons, Ltd.     

Biological pollution in the Mediterranean Sea: invasive versus introduced macrophytes

The authors have listed 85 species of macrophytes that have probably been introduced to the Mediterranean. Among them, nine species can be considered as invasive, i.e., playing a conspicuous role in the recipient ecosystems, taking the place of keystone species and/or being economically harmful: Acrothamnion preissii, Asparagopsis armata, Lophocladia lallemandii, Womersleyella setacea (Rhodophyta), Sargassum muticum, Stypopodium schimperi (Fucophyceae), Caulerpa racemosa, Caulerpa taxifolia and Halophila stipulacea (Plantae). These data fit well the Williamson and Fitter's "tens rule", which states that, on average, 1 out of 10 introduced species becomes invasive. Though some features (e.g. life traits, geographical origin) can increase the likelihood of a successful invasion, the success of invaders is far from being predictable. Since the beginning of the 20th century, the number of introduced species to the Mediterranean has nearly doubled every 20 years. Should these kinetics continue, and according to the tens rule, it can be expected that 5-10 newly introduced macrophytes shall become invasive in the next 20 years.     

Foreland and Hinterland basins: what controls their evolution?

Compressional systems are usually characterized by a positive topography above the sea level, which is continuously modified by the conjugate effects of tectonic contraction or post-orogenic collapse, thermo-mechanical processes in the deep lithosphere and asthenosphere, but also by climate and other surface processes influencing erosion rates.Different types of sedimentary basins can develop in close association with orogens, either in the foreland or in the hinterland. Being progressively filled by erosional products of adjacent uplifted domains, these basins provide a continuous sedimentary record of surficial, crustal and lithospheric deformation at and near plate boundaries.Selected integrated basin-scale studies in the Circum-Mediterranean thrust belts and basins, in Pakistan and the Americas, are used here to document the effects of structures inherited from former orogens, rifts and passive margins, active tectonics and mantle dynamics on the development and long term evolution of synorogenic basins.     

Testing spatial autocorrelation in weighted networks: the modes permutation test

In a weighted spatial network, as specified by an exchange matrix, the variances of the spatial values are inversely proportional to the size of the regions. Spatial values are no more exchangeable under independence, thus weakening the rationale for ordinary permutation and bootstrap tests of spatial autocorrelation. We propose an alternative permutation test for spatial autocorrelation, based upon exchangeable spatial modes, constructed as linear orthogonal combinations of spatial values. The coefficients obtain as eigenvectors of the standardized exchange matrix appearing in spectral clustering and generalize to the weighted case the concept of spatial filtering for connectivity matrices. Also, two proposals aimed at transforming an accessibility matrix into an exchange matrix with a priori fixed margins are presented. Two examples (inter-regional migratory flows and binary adjacency networks) illustrate the formalism, rooted in the theory of spectral decomposition for reversible Markov chains.     

Mapping land cover changes in Mexico, 1976–2000 and applications for guiding environmental management policy

Land use conversion typically implicates deforestation and fragmentation of primary land cover types, which invariably translates into impoverishment of both natural and cultural capital. Understanding where conversion is taking place crucially underpins sound environmental policy instruments to prevent these enormous social and economic costs. This paper examines 30 years of semi‐detailed (1:250 000) land cover mapping in Mexico. Pre‐existing analogue databases describing land cover patterns in the 1970s and 1990s were reviewed, corrected, reorganized and transformed into a digital format. Current land cover patterns were depicted by conducting updated reinterpretation on Landsat ETM+ imagery. Digital cartographic overlaying was performed and the results were used to construct a spatially explicit land use/land cover change (LULCC) database with an additional accuracy assessment procedure. The value of the results of this analysis is also seen in the light of their direct applications for identifying critical watershed trends, for guiding the allocation of financial funds for sound land use planning and for assessing the effectiveness of established protected areas. This effort highlights the importance of new and more effective geographical approaches to depict, understand and contribute to informed measures to mitigate ongoing negative trends in land cover and climatic changes.     

Using small spatial units to refine our perception of rural America

More than half of the US rural population lives inside metro or micropolitan areas and even at more disaggregated scales, such as the census tracts, most spatial units mix rural and urban population. At a national scale, only 30% of the country are inhabited by 100% urban or 100% rural population, implying that more than two third of the US territory are somewhere in between both situations. As the rural/urban dichotomy appears today to be blurred by the emergence of new phenomena like rurbanization or exurbanization, our perception of rural America may be somewhat twisted and the reality of rural areas underplayed. This paper focuses on using finer-grade spatial units such as the census blocks and block groups, in order to provide new elements about the extension, localization and characteristics of rural America as well as about its inner dynamics. To that end, we analyze and process geographical and social data at these two levels of information, and use population density as a main factor of analysis. This allows us not only to propose new measurement of the extent of rural space in the USA but also to propose a new vision of its spatial dynamics by studying how several social indicators such as income, median age or sex ratio reveal regional and micro-regional variations and situations in the rural part of the US.     

On the identification of Fe ix and Ni xi lines from coronal spectral

Identifications of coronal lines with transitions of Fe ix and Ni xi, as proposed by Wagner and House, and Svensson, Ekberg and Edlén, are checked by means of line intensity ratios, from observations obtained at the 1965 eclipse. All the suggestions of Svensson, Ekberg and Edlén are suitable.     

Trends of tropical deforestation in Southeast Mexico

We assessed deforestation in Southeast Mexico (a surface area of 29 000 km² in seven states) through the comparison of land use/land cover maps at a scale of 1:250 000. This facilitated mapping of the land use/land cover change (LULCC) processes and calculation of the rates of change and the change matrix for the period 1978–2000. An original method was used to assess the accuracy of the LULCC map. The verification sites were selected through a stratified random sampling and were corroborated with aerial photographs for 1978 and 2000. Error matrices were elaborated using both hard and fuzzy set approaches in order to take into account the errors related to generalization of the map in fragmented landscapes. The results showed an average annual deforestation rate of 1.1 per cent which represents an average annual loss of 190 000 ha of forest, or an estimated total reduction of 4.2 million ha over 22 years. Furthermore, deforestation processes are concentrated in some areas such as Yucatan and Chiapas states, which registered major forest conversions to grassland and slash‐burning. The overall accuracy of the LULCC map, assessed with hard and fuzzy set approaches, was 72 per cent and 88 per cent respectively.     

Tidal circulation and buoyancy effects in the St. Lawrence Estuary

Abstract

The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills.