排序方式： 共有23条查询结果，搜索用时 15 毫秒
11.
This article identifies the Pucarilla–Cerro Tipillas Volcanic Complex and its major eruptive source, the Luingo caldera (26° 10′S–66° 40′W). Detailed geological mapping, stratigraphic sections, facies analysis and correlations, including the identification of typical caldera components, allow us to infer the position of a collapse caldera, elongated at N65° and with a diameter of 19 km × 13 km, which is responsible for an eruption of 135 km3 (DRE) of magma. The high-crystal contents of the associated ignimbrites, combined with its tectonic setting, indicate that regional and local tectonic structures played a crucial role in the formation of the caldera. 相似文献
12.
13.
The water-table fluctuation (WTF) method is one of the most widely used means to estimate aquifer recharge. In the northeastern coast of the Buenos Aires province, Argentina, the geomorphological and climatic characteristics, as well as the presence of a shallow, homogeneous unconfined aquifer, make it possible to apply this methodology. The relationship between water-table fluctuations and precipitation in a humid climate, considering its seasonal variations, is assessed. Water tables were measured monthly between February 2008 and September 2010 in a monitoring network; rainfall data were analysed. The water table rises when the accumulated precipitation between measurements is more than 53?mm/month in the dry season and more than 97?mm/month in the rainy season. The index, relating water-table fluctuations and precipitation occurring between measurements, shows that values below 0 suggest no increase in the water reserves, whereas higher values entail an increase. In the study area, where there is a lack of historical data, finding out the relationship between water-table fluctuations and precipitation will constitute a tool for groundwater use and management, and set up an early warning system for dry periods. It could also be extrapolated to other regions with similar hydrological conditions lacking in data. 相似文献
14.
Palynologycal information was integrated with the aim to study vegetation history and environmental changes in Mar Chiquita coastal lagoon area (in the southeast of Buenos Aires province, Argentina) related to sea‐level fluctuations during the Holocene. Prior to 5000 yr BP a mosaic of psammophytic and halophytic communities developed on extensive sandy flats. Between 4000 and 2500 yr BP, during a period of sea‐level stabilisation, similar halophytic communities developed in the south and in the middle part of the present lagoon. Between 4500 and 3500 yr BP hydrophytic and halophytic communities developed in Laguna Hinojales. Beginning 2500 yr BP, when sea level was nearing its present position, modern plant communities became established. Pollen records suggest that littoral evolution of the area was much more complex than previous models have described, introducing new evidence of the geomorphological evolution of Mar Chiquita coastal lagoon. The estuarine environment was delimited by a system of barrier islands instead of a linear prograding barrier, as has been proposed. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
15.
Leticia Lescano Francisco Locati Jorge Sfragulla Silvina Marfil Aldo Bonalumi Pedro Maiza 《Environmental Earth Sciences》2017,76(18):631
In this work, a talc and vermiculite mine from the province of Córdoba (Argentina) was investigated with special emphasis on the occurrence of asbestiform and non-asbestiform phases. The meta-ultramafic rock was studied by a multimethodological approach, complementing field studies with petrographic-mineralogical, compositional and morphological analyses. Samples were examined by stereomicroscopy, polarizing light microscopy, SEM–EDS, XRD, DSC-TGA and FTIR. Complementary, compositional and textural analyses were performed with FE-SEM–EDS and EPMA. Talc-rich veins with a laminar and fibrous appearance were at first recognized. However, the fibrous morphology observed both in the field and by microscopy is due to an apparent habit because of the sample orientation. To avoid erroneous interpretations, studies by secondary electron images (SEM) are fundamental to carrying out this type of analysis. Tremolite was identified in different zones of the outcrop; however, only ~40% of the crystals located in the vermiculite zone have dimensions to be considered as asbestiform fibres in the range of respirable particles. In these types of complex deposits affected by superimposed metamorphic, igneous and deformational events, multimethodological approaches are necessary to develop models of occurrence of asbestiform morphologies that may be applicable to other with similar characteristics. 相似文献
16.
In this study the capability of the MM5 model in simulating the main mode of intraseasonal variability during the warm season over South America is evaluated through a series of sensitivity experiments. Several 3-month simulations nested into ERA40 reanalysis were carried out using different cumulus schemes and planetary boundary layer schemes in an attempt to define the optimal combination of physical parameterizations for simulating alternating wet and dry conditions over La Plata Basin (LPB) and the South Atlantic Convergence Zone regions, respectively. The results were compared with different observational datasets and model evaluation was performed taking into account the spatial distribution of monthly precipitation and daily statistics of precipitation over the target regions. Though every experiment was able to capture the contrasting behavior of the precipitation during the simulated period, precipitation was largely underestimated particularly over the LPB region, mainly due to a misrepresentation in the moisture flux convergence. Experiments using grid nudging of the winds above the planetary boundary layer showed a better performance compared with those in which no constrains were imposed to the regional circulation within the model domain. Overall, no single experiment was found to perform the best over the entire domain and during the two contrasting months. The experiment that outperforms depends on the area of interest, being the simulation using the Grell (Kain–Fritsch) cumulus scheme in combination with the MRF planetary boundary layer scheme more adequate for subtropical (tropical) latitudes. The ensemble of the sensitivity experiments showed a better performance compared with any individual experiment. 相似文献
17.
Regional climate change experiments over southern South America. II: Climate change scenarios in the late twenty-first century 总被引：1，自引：0，他引：1
We present an analysis of climate change over southern South America as simulated by a regional climate model. The regional model MM5 was nested within time-slice global atmospheric model experiments conducted by the HadAM3H model. The simulations cover a 10-year period representing present-day climate (1981–1990) and two future scenarios for the SRESA2 and B2 emission scenarios for the period 2081–2090. There are a few quantitative differences between the two regional scenarios. The simulated changes are larger for the A2 than the B2 scenario, although with few qualitative differences. For the two regional scenarios, the warming in southern Brazil, Paraguay, Bolivia and northeastern Argentina is particularly large in spring. Over the western coast of South America both scenarios project a general decrease in precipitation. Both the A2 and B2 simulations show a general increase in precipitation in northern and central Argentina especially in summer and fall and a general decrease in precipitation in winter and spring. In fall the simulations agree on a general decrease in precipitation in southern Brazil. This reflects changes in the atmospheric circulation during winter and spring. Changes in mean sea level pressure show a cell of increasing pressure centered somewhere in the southern Atlantic Ocean and southern Pacific Ocean, mainly during summer and fall in the Atlantic and in spring in the Pacific. In relation to the pressure distribution in the control run, this indicates a southward extension of the summer mean Atlantic and Pacific subtropical highs. 相似文献
18.
Silvina Stutz C. Marcela Borel Sonia L. Fontana Laura del Puerto Hugo Inda Felipe García-Rodriguez Marcela S. Tonello 《Journal of Paleolimnology》2010,44(3):761-775
We analyzed pollen, non-pollen palynomorphs (NPPs), calcareous microfossils, plant macrofossils, diatoms, chrysophyte cysts,
opal phytoliths and organic matter content in a 123-cm sediment sequence from Nahuel Rucá Lake, a shallow, freshwater system
in the southeastern Pampa grasslands, Argentina. Three stages in the lake evolution were identified. Before 3,680 cal year BP,
only pollen, NPPs (dinoflagellate cysts and acritarchs) and ostracods were recovered, suggesting brackish/saline conditions
in the lake and nearby areas. Freshwater conditions are, however, indicated by Myriophyllum, Pediastrum and Zygnemataceae. The brackish/saline conditions could have been caused by marine influence during a Holocene sea level
high stand that affected the area ca. 6,000 year BP. Between 3,680 and 390 cal year BP, macrophyte pollen and plant macrofossils
indicate increasingly freshwater conditions in the lake and the adjacent area. Diatom and ostracod assemblages, however, suggest
brackish and oligotrophic conditions, giving way to freshwater and meso-eutrophic conditions toward the end of this period.
The relationship between submersed macrophytes (Myriophyllum, Potamogeton, Ceratophyllum, Chara) and planktonic algae (Chlorophyta and diatoms), suggests a shift in the lake from a clear to a turbid state. This turbid
state is more evident after 390 cal year BP. High values of Pediastrum, Scenedesmus and diatoms (Cyclotella meneghiniana, Aulacoseira granulata, A. muzzanensis) observed during this stage could have reduced light penetration, with consequent loss of submersed plants. Pollen and plant
macrofossils in the uppermost 20 cm indicate a shallow, freshwater lake similar to present, though an increase in brackish/freshwater
diatoms suggests an increase in salinity, perhaps related to periodic droughts. Opal phytoliths yield a regional paleoclimatic
reconstruction that agrees closely with inferences made using pollen, mammals and sediment characteristics. 相似文献
19.
Remarkable variability in dyke features at the Vicuña Pampa Volcanic Complex,Southern Central Andes 
下载免费PDF全文
下载免费PDF全文 Silvina Guzmán Marco Neri Roberto Carniel Joan Martí Pablo Grosse Carolina Montero‐López Adelina Geyer 《地学学报》2017,29(4):224-232
Dykes at the Vicuña Pampa Volcanic Complex, which are mostly basaltic (trachy)‐andesite and (trachy)‐andesite, are exposed at the base and along the walls of a large depression resulting from intense degradation. Dykes intruding stiff layers (lavas, plugs and necks) are thin, mostly dip >60° and have coherent textures, whereas dykes intruding more compliant materials (breccias and conglomerates) tend to be thicker, have lower dips and have coherent, brecciated or mixed textures (coherent and brecciated textural domains in a single or compound dyke). Single dykes with brecciated and mixed textures are only found intruding near‐surface units. Dykes with mixed textures always have sharp contacts between domains. Dykes with sinuous domain contacts and enclaves of one domain inside the other are interpreted as resulting from dyke arrest, partial cooling and reinjection of new magma. Dykes with straight domain contacts are considered to be compound dykes, with a new dyke intruding along the margins of an older, solidified one. 相似文献
20.
Lescano Leticia Marfil Silvina Maiza Pedro Sfragulla Jorge Bonalumi Aldo 《Environmental Earth Sciences》2013,70(4):1809-1821
Vermiculites with impurities of amphibole, a dry fine residue and calcined materials from three deposits in the province of Córdoba, Argentina, were studied. From a commercial perspective, amphiboles are considered as “asbestos”, a group of silicate minerals with strong and flexible fibres that are heat resistant and chemically inert, and thus well suited for heat insulation. These fibrous particles have provoked controversies about the toxicity impact on human health and the development of diseases such as mesothelioma, asbestosis or lung cancer. Their commercialization and exploitation are currently prohibited. The sampled minerals were identified with a petrographic microscope, chemical analyses, SEM and XRD. It was concluded that amphibole minerals are present in all the deposits studied and in every phase of plant production, but not all of them have asbestiform characteristics. 相似文献