Micropaleontological record of Holocene estuarine stages in the Bahía Blanca estuary,Argentina     

《Journal of South American Earth Sciences》2013

Holocene paleoenvironmental changes have been interpreted on the basis of benthic foraminifera and calcareous nannofossils recovered in samples from Napostá Grande Stream, Bahía Blanca estuary, southern Buenos Aires Province. Samples are fine sands and clay sediments from a Holocene outcrop and were studied with quantitative techniques. The benthic foraminiferal assemblage is dominated by Ammonia parkinsoniana, Ammonia tepida, Bolivina pseudoplicata, Bolivina striatula, Bolivina sp., Buccella peruviana, and Elphidium spp. The calcareous nannofossil assemblage recovered is a typical cold-water association, dominated by Calcidiscus leptoporus, Coccolithus pelagicus, Emiliana huxleyi and Gephyrocapsa oceanica. A dendrogram classification by cluster analysis was made for each microfossil group. The results of these analyses were coincident, showing a liaison between changes in the assemblages of benthic foraminifera and calcareous nannofossils. Those results, jointly with the sedimentological information, lead to the identification of three different paleoenvironments along the Napostá N1 site. The lower part of the succession represents an estuarine environment with larger marine connection. The middle part represents a gradual passage to a more restricted estuarine environment, and the upper part represents the establishment of the modern continental fresh-water environment.  相似文献   

Polycyclic aromatic hydrocarbons in water,mussels (Brachidontes sp., Tagelus sp.) and fish (Odontesthes sp.) from Bahía Blanca Estuary,Argentina     

Andres H. Arias  Carla V. Spetter  Rubén H. Freije  Jorge E. Marcovecchio 《Estuarine, Coastal and Shelf Science》2009

Polycyclic aromatic hydrocarbons (PAHs), one of the major groups of anthropogenic environmental pollutants, were firstly identified and measured in coastal waters, native mussels and fish of an industrialized South American estuary.  相似文献   

Geochemical characterization, correlation, and optical dating of tephra in alluvial sequences of central western Argentina   总被引：1，自引：0，他引：1  

Phillip S Toms  Matthew King  Rob A Kemp 《Quaternary Research》2004,62(1):60-75

The synthesis of paleoclimatic archives provided by loess and alluvial sequences of central Argentina has been hindered by the lack of a cohesive lithostratigraphic framework extending across the Chaco-Pampean plains and catchments of the Rios Desaguadero, Colorado, and Negro. This condition originates in part from the dearth of absolute chronological controls. The occurrence of discrete tephra layers across this region may provide an opportunity to address this deficiency if a tephrochronological framework can be established. The potential of such a project is assessed within the context of a pilot study constrained within alluvial sequences of central western Argentina proximal to potential source vents in the Southern Volcanic Zone. The intersite discrimination and correlation of tephra layers on a geochemical basis is examined, with indirect chronological control for the eruption of each generated by optical dating. Alluvial sediments on either side of each of five tephra units at a type site were dated using the optically stimulated luminescence of fine-silt-sized quartz, thus providing an age control on each tephra (ca. 24,000, 30,000, 32,000, 39,000, and 48,000 yr). The geochemical composition of each tephra was derived. Using these data, tephra layers at other sites in the study area were geochemically analyzed and, in instances of statistical concordance in major oxide structure, correlated to the type site and therefore ascribed ages. This methodology identified a further sixth volcanic event between ca. 24,000 and 30,000 yr not registered by type-site tephras. The extension of this initial tephrochronological framework beyond the alluvial sequences of central western Argentina is encouraged by the occurrence of geochemically distinct tephra verified and dated in this study.  相似文献   

Microstructural analysis and P–T conditions of the Azul megashear zone, Tandilia, Buenos Aires province, Argentina     

M.C. Frisicale  F.J. Martinez  L.V. Dimieri  J.A. Dristas   《Journal of South American Earth Sciences》2005,19(4):433-444

A microstructural analysis was carried out on mylonitic rocks of the Azul megashear zone (AMSZ), Tandilia, which were formed in a range of metamorphic conditions from lower greenschist to amphibolite facies. Tailed porphyroclasts are common and mostly symmetric. Scarce asymmetric rotated porphyroclasts show both sinistral and dextral senses of shear. In sections parallel to the mylonitic foliation, porphyroclasts are round. The AMSZ is probably related to the late Transamazonian orogenic cycle and may be due to NNE–SSW-directed convergence. In weakly deformed protolith and protomylonites, quartz deforms by dynamic recrystallization, mainly subgrain rotation in dislocation creep Regime 2. K-feldspar porphyroclasts and plagioclase show scarce fracturation and deform by dynamic recrystallization along grain boundaries. Quartz microstructures in mylonites indicate predominantly Regime 3 grain boundary migration recrystallization. Feldspar structures indicate recrystallization through the nucleation and growth of new grains at grain boundaries. The temperatures of deformation from mineral assemblages in the CNKFMASH system in four bulk compositions are in the range of 400–450 °C, and the pressures are more than 6 kb.  相似文献   

Making autonomous geographies: Argentina’s popular uprising and the ‘Movimiento de Trabajadores Desocupados’ (Unemployed Workers Movement)     

Paul Chatterton 《Geoforum》2005,36(5):545-561

This paper addresses the idea of autonomy—the desire for freedom, self-organisation and mutual aid. Through challenging economic neoliberalism, state repression, a powerful transnational elite, and the commodification of nature and resources, many communities, especially in the global south, are trying to manage their own affairs. Using the example of the Movement of Unemployed Workers (El Movimiento de Trabajadores Desocupados) in Argentina, I explore the idea of autonomous geographies and how they are made and remade at three overlapping levels: the territorial, through the emergence of networked autonomous neighbourhoods which are selectively open and closed to translocal links; the material, through the development of a solidarity economy where immediate needs are met and work is redefined, and; the social, where collective action and daily practice helps constitute more collective, autonomous forms of social interactions. In their desire to manage connections with the outside world while at the same time inspire autonomous place projects elsewhere, the MTDs represent both a ‘militant localism’ and ‘militant pluriversalism’. Moreover, while such experiments in making and embedding ‘autonomous geographies’ face limits and have few widespread examples on which to draw, it is through constant questioning and collective struggle at the everyday level that autonomy is made real.  相似文献   

U–Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina): Correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane     

Cesar Casquet  Robert J. Pankhurst  C. Mark Fanning  Edgardo Baldo  Carmen Galindo  Carlos W. Rapela  Jose M. Gonzlez-Casado  Juan A. Dahlquist 《Gondwana Research》2006,9(4):524-529

Metamorphism of Grenvillian age (ca. 1.2 Ga; U–Pb zircon dating) is recognized for the first time in the Western Sierras Pampeanas (Sierra de Maz). Conditions reached granulite facies (ca. 780 °C and ca. 780 MPa). Comparing geochronological and petrological characteristics with other outcrops of Mesoproterozoic basement, particularly in the northern and central Arequipa-Antofalla craton, we suggest that these regions were part of a single continental crustal block from Mesoproterozoic times, and thus autochthonous or parautochthonous to Gondwana.  相似文献   

Evaluation of proposed waste rock dump designs using the SIBERIA erosion model     

G. R. Hancock  E. Turley 《Environmental Geology》2006,49(5):765-779

Computer-based landscape evolution models offer the ability to evaluate landscape stability over the short (annual), medium (decades to hundreds of years) and long-term (thousands of years). Modeling has advantages in that design ideas can be tested, different surface material properties can be evaluated and risk analysis carried out. Landscape evolution models allow landscape surface change through time. These models also offer the advantage that the landscape can be evaluated visually as it develops through time, which is not possible with other types of models. Landscape evolution models can be used for not only soil loss assessment (i.e. tonnes/hectare/year), but also to evaluate the method of soil loss (i.e. rill or interrill erosion). This study examines a range of waste rock dump designs for the Minera Alumbrera Ltd. copper mine, Argentina. An erosion assessment using the SIBERIA erosion model over a 1000-year simulation period demonstrates waste rock dump designs using a conventional stepped design of backsloping benches and caps with angle of repose slopes provide the lowest average erosion rates and depths of incision than do other designs. Caution should be applied in interpreting these results as the SIBERIA erosion model is sensitive to parameter input and in this case was calibrated and run using a generic set of parameters that are not site specific. Nevertheless, the results provide a guide as to the strengths and weaknesses of different rehabilitation designs and demonstrate the insights that modeling studies can provide.  相似文献   

Post-Wisconsinian paleoenvironments at Salinas del Bebedero basin, San Luis, Argentina     

Miguel A. González  Nora I. Maidana 《Journal of Paleolimnology》1998,20(4):353-368

We present a climatic reconstruction of Holocene lacustrine episodes in the Salinas del Bebedero basin (Argentina), based on geological and diatom information.Morphological, sedimentological and diatom evidence between 11600 ± 140 yr BP and 325 ± 95 yr BP, allowed us to interpret the paleoenvironments of the basin. Episodes of high energy (sandy levels) are linked to large inflow of meltwater through the Desaguadero River, related to development of glaciers on the Andes. This inflow is characterized by peaks of relative abundance of the brackish water diatom Cyclotella choctawatcheeana Prasad. The values of C. choctawatcheeana decrease in deposits of low energy (clay levels), where it co-dominates with oligohalobous Fragilaria and Epithemia spp.To the last two peaks of large inflow of meltwater, radiocarbon dates corrected to sidereal ages, are AD 1280/1420 and AD 1443/1656. These ages agree with two cold episodes clearly recorded in dendrological studies from the Patagonian Andes and were correlated to the Little Ice Age. Thus, older Holocene episodes of large inflow of water to the basin were correlated with the Neoglacial Advances defined by Mercer (1976) for the Andes.  相似文献   

Current channelling and three-dimensional effects detected from magnetotelluric data from a sedimentary basin in Sierras Pampeanas, Argentina     

C. Pomposiello  A. Osella  A. Favetto  C. Sainato  P. Martinelli  & C. Aprea 《Geophysical Journal International》1998,135(2):339-350

Aquifer-bearing intermontane sedimentary basins of the Sierras Pampeanas in the northwest of Argentina are in general very deep and narrow and contain economically important deposits of Tertiary sediments. This paper presents the results of a study to characterize the sedimentary basin bounded to the west by Sierra de Famatina and to the east by Sierra de Velasco, where an electromagnetic sensing technique, the magnetotelluric (MT) method, was applied. 12 MT sites were deployed along a 30  km E–W transect. Some of the data collected were used to derive a 2-D resistivity model of the basin. The model shows a subsurface trough with a thick (approximately 8  km) sedimentary sequence above it. Anomalous behaviour of the E–W electric-field component ( E _y ) was detected in the period range 1–100  s, where the amplitude of this component was below the instrumental noise level. The cause of this anomaly is not known, but it might be due to the presence of an embedded conducting body between 8 and 10  km, which would give rise to N–S current channelling.  相似文献   

Palaeomagnetism of the Silurian Lipeón formation, NW Argentina, and the Gondwana apparent polar wander path     

C. M. Conti  A. E. Rapalini  J. F. Vilas 《Geophysical Journal International》1995,121(3):848-862

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