Geophysical investigation of the Colônia structure,Brazil

The origin of the nearly circular Colônia structure, located at the southwestern edge of the city of São Paulo, Brazil, has been the subject of a long‐standing debate, ever since the 1960s when the structure was first investigated by geophysical methods. The structure still raises interest for geological research, as its sedimentary infill holds important paleoclimatic information about the evolution of the tropical rainforest, as well as the interplay between the South American summer monsoon, the Intertropical Convergence Zone, and the southern Westerly wind belt—for possibly as long as several million years. In addition, the search for evidence to conclusively establish the origin of this structure continues, and the answer most likely lies in the lower portions of the basin's sedimentary infill, which also holds a significant potential for underground water resources. Here, we present the results from recent seismic (reflection and HVSR), gravimetric, and geoelectrical surveys. They have provided a reliable image of the sedimentary infill, and the maximum depth to basement within the structure has been constrained consistently by the different methods to approximately ?400 m. The geophysical data have also allowed to map the lateral contact between the crystalline basement rocks and the sedimentary infill, which indicates a diameter of approximately 2.8 km for the sedimentary basin, with 3.6 km being the diameter of the outermost limit of the structure. A total of six seismic stratigraphic boundaries were identified within the sedimentary infill, providing a framework for the planning of a deep drilling campaign and subsequent sampling program aimed at geological and paleoclimatology studies.     

Detailed seismic stratigraphy of Lago Puyehue: implications for the mode and timing of glacier retreat in the Chilean Lake District

Lago Puyehue is a glacigenic lake in the Chilean Lake District (40°S) with a complex deglaciation history. A detailed seismic–stratigraphic study of its sedimentary infill indicates a much earlier retreat of the glacier from the Lago Puyehue basin than the neighbouring glacier from the Lago Rupanco basin. Because of their close proximity, Rupanco meltwater streams played an important part in the depositional processes of Lago Puyehue. A timing discrepancy between the in‐lake ages of a sediment core and the outer‐lake ages of moraine deposits (re)opens the discussion on the timing of deglaciation in the Southern Hemisphere. Copyright © 2011 John Wiley & Sons, Ltd.     

Current Behaviour and Dynamics of the Lowermost Italian Glacier (Montasio Occidentale,Julian Alps)

Smaller glaciers (<0.5 km²) react quickly to environmental changes and typically show a large scatter in their individual response. Accounting for these ice bodies is essential for assessing regional glacier change, given their high number and contribution to the total loss of glacier area in mountain regions. However, studying small glaciers using traditional techniques may be difficult or not feasible, and assessing their current activity and dynamics may be problematic. In this paper, we present an integrated approach for characterizing the current behaviour of a small, avalanche‐fed glacier at low altitude in the Italian Alps, combining geomorphological, geophysical and high‐resolution geodetic surveying with a terrestrial laser scanner. The glacier is still active and shows a detectable mass transfer from the accumulation area to the lower ablation area, which is covered by a thick debris mantle. The glacier owes its existence to the local topo‐climatic conditions, ensured by high rock walls which enhance accumulation by delivering avalanche snow and reduce ablation by providing topographic shading and regulating the debris budget of the glacier catchment. In the last several years the glacier has displayed peculiar behaviour compared with most glaciers of the European Alps, being close to equilibrium conditions in spite of warm ablation seasons. Proportionally small relative changes have also occurred since the Little Ice Age maximum. Compared with the majority of other Alpine glaciers, we infer for this glacier a lower sensitivity to air temperature and a higher sensitivity to precipitation, associated with important feedback from increasing debris cover during unfavourable periods.     

The onset of extension during lithospheric shortening:a two-dimensional thermomechanical model for lithospheric unrooting

We model the evolution of the lithosphere during its shortening and consequent gravitational collapse with special emphasis on the induced variations in the surface stress regime and dynamic topography. In particular, we analyse the conditions leading, immediately after lithospheric failure, to local extension, eventually coeval with compression. Different crustal rheologies and kinematic conditions as well as thermally imposed mechanical rupture are considered. Numerical calculations have been performed by using a 2-D finite element code that couples the thermal and mechanical equations for a Newtonian rheology with a temperature-dependent viscosity. The results show that, after the failure of a gravitationally unstable lithospheric root, the replacement of lithospheric mantle by warmer asthenospheric material induces a considerable variation in the dynamic topography and in the surface stress regime. The occurrence of local extension, its intensity and its spatial distribution depend mainly on whether convergence continues throughout the process or ceases after or before the lithospheric failure. Similarly, uplift/subsidence and topographic inversion are controlled by kinematic conditions and crustal rheology. Mechanical rupture produces drastic changes in the surface stress regime and dynamic topography but only for a short time period, after which the system tends to evolve like a continuous model.     

GIS,multi‐criteria and multi‐factor spatial analysis for the probability assessment of the existence of illegal landfills

This work deals with the identification of potentially contaminated areas using remote sensing, geographic information systems (GIS) and multi‐criteria spatial analysis. The identification of unknown illegal landfills is a crucial environmental problem in all developed and developing countries, where a large number of illegal waste deposits exist as a result of fast, and relatively unregulated, industrial growth over the past century. The criteria used to perform the spatial analysis are here selected by considering the characteristics which are ‘desirable’ for an illegal waste disposal site, chiefly related to the existence of roads for easy access and to a low population density which facilitates unnoticed dumping of illegal waste materials. A large dataset describing known legal and illegal landfills and the context of their location (population, road network, etc.) was used to perform a spatial statistical analysis to select factors and criteria allowing for the identification of the known waste deposits. The final result is a map describing the likelihood of an illegal waste deposit to be located at any arbitrary location. Such a probability map is then used together with remote sensing techniques to narrow down the set of possibly contaminated sites (Silvestri and Omri, 2008 Silvestri, S. and Omri, M. 2008. A method for the remote sensing identification of uncontrolled landfills: formulation and validation.. International Journal of Remote Sensing, 29(4): 975–989. [Taylor & Francis Online] , [Google Scholar]), which are candidates for further analyses and field investigations. The importance of the integration of GIS and remote sensing is highlighted and represents a key instrument for environmental management and for the spatially‐distributed characterization of possible uncontrolled landfill sites.     

Coseismic rotation changes from the 2004 Sumatra earthquake: the effects of Earth's compressibility versus earthquake induced topography

Polar motion is modelled for the large 2004 Sumatra earthquake via dislocation theory for an incompressible elastic earth model, where inertia perturbations are due to earthquake-triggered topography of density–contrast interfaces, and for a compressible model, where inertia perturbation due to compression-dilatation of Earth's material is included; density and elastic parameters are based on a multilayered reference Earth. Both models are based on analytical Green's functions, propagated from the centre to the Earth's surface. Preliminary and updated seismological solutions are considered in elucidating the effects of improving earthquake parameters on polar motion. The large Sumatra thrust earthquake was particularly efficient in driving polar motion since it was responsible for large material displacements occurring orthogonally to the strike of the earthquake and to the Earth's surface, as imaged by GRACE gravity anomalies over the earthquake area. The effects of earthquake-induced topography are four times larger than the effects of Earth's compressibility, for l = 2 geopotential components. For varying compressional Earth properties and seismic solution, modelled polar motion ranges from 8.6 to 9.4 cm in amplitude and between 117° and 130° east longitude in direction. The close relationship between polar motion direction, earthquake longitude and thrust nature of the event, are established in terms of basic physical concepts.     

Calibrating biogeochemical and physical climate proxies from non-varved lake sediments with meteorological data: methods and case studies

Lake sediment records are underrepresented in comprehensive, quantitative, high-resolution (sub-decadal), multi-proxy climate reconstructions for the past millennium. This is largely a consequence of the difficulty of calibrating biogeochemical lake sediment proxies to meteorological time series (calibration-in-time). Thanks to recent methodological advances, it is now possible. This paper outlines a step-by-step, specifically tailored methodology, with practical suggestions for calibrating and validating biogeochemical proxies from lake sediments to meteorological data. This approach includes: (1) regional climate data; (2) site selection; (3) coring and core selection; (4) core chronology; (5) data acquisition; and (6) data analysis and statistical methods. We present three case studies that used non-varved lake sediments from remote areas in the Central Chilean Andes, where little a priori information was available on the local climate and lakes, or their responses to climate variability. These case studies illustrate the potential value and application of a calibration-in-time approach to non-varved lake sediments for developing quantitative, high-resolution climate reconstructions.     

Climate-mediated changes in small lakes inferred from midge assemblages: the influence of thermal regime and lake depth

Subfossil midge (Chironomidae and Chaoboridae) assemblages were examined in the surficial sediments (0?C1?cm) from small inland lakes in the Experimental Lakes Area (ELA) of northwestern Ontario, Canada. In these boreal lakes, maximum depth (Z_max), alkalinity, Secchi depth and chlorophyll-a concentrations explained significant amounts of variation in the subfossil assemblages. Constrained ordinations (redundancy analysis) indicated that the relationship between Z_max (as sqrt Z_max) and assemblage composition was strong enough to develop a paleolimnological inference model. Model statistics suggested that a robust model was generated (r ²?=?0.78, RMSEP?=?0.533, max bias?=?0.674); however, when the model was applied to a subfossil stratigraphy from an ELA lake sediment core, results suggested that the inference model had produced an unrealistically shallow Z_max inference. Further analyses indicated that thermal regime had a strong influence on assemblage composition; when the influence of thermal regime was partialled out, there was a much weaker relationship between Z_max and assemblage composition, particularly for stratified lakes. A thermal regime inference model was subsequently developed, which, when applied to the lake sediment core stratigraphy, indicated that the shallow Z_max inference may have been the result of a period of increased mixing or polymixis in this stratified lake. Water column mixing may increase due to hypolimnetic warming and increased water clarity resulting from declines in dissolved organic carbon. In a training set where there are strong correlations between lake depth and assemblage composition, this relationship is not necessarily a strict function of lake depth, but of some other highly correlated variable, likely thermal conditions.