Constraints to Uranus' Great Collision III: The Origin of the Outer Satellites     

Adrián Brunini  Mirta Gabriela Parisi  Gonzalo Tancredi 《Icarus》2002,159(1):166-177

The theory of satellite loss resulting from a giant impact on Uranus (Parisi and Brunini 1997, Planet. Space Sci.45, 181-187) is revisited, in the light of the discovery of its five outer moons (Gladman et al. 1998, Nature392, 897-899; Gladman et al. 2000, Icarus147, 320-324; erratum 148, 320). Physical conditions and dynamical constraints in the great collision scenario and restrictions in the possible mechanisms for the origin of the outer uranian satellites are obtained from the knowledge of their actual orbital properties. We conclude that the existence of these moons implies that their origin must be connected to a breakup process. Other scenarios for their origin cast serious doubts on the occurrence of a giant collision at the end of Uranus' formation process to account for its large spin axis obliquity.  相似文献   

Within versus between-lake variability of sedimentary diatoms: the role of sampling effort in capturing assemblage composition in environmentally heterogeneous shallow lakes     

Gabriela S. Hassan 《Journal of Paleolimnology》2018,60(4):525-541

The effect of within-lake diatom assemblages variability on sample representativity and its subsequent impact on between-lake comparisons were addressed in three environmentally heterogeneous shallow lakes from the Argentinean Pampas. Surface sediment samples were collected from the open waters and the highly vegetated littoral areas on a seasonal basis and analyzed for diatom assemblages composition. Within-lake variability was assessed by comparing the Bray Curtis distances between original data and the Monte Carlo-simulated average assemblages composition through non-metric multidimensional scaling (NMDS). Diatom assemblages showed a high variability in composition, evidencing large dispersions of samples around the centroid in NMDS plots. Permutational multivariate analysis of variance tests signaled significant differences in average composition between the three lakes, related mainly to their differences in conductivity and depth. Representativity of original samples was assessed through principal coordinates analyses ordinations of the three lakes, being samples lying in the overlapping areas of the plot classified as poor representatives of between-lake differences. Several samples, both from littoral and open waters, were classified as poor representatives through this method. Simulation allowed us to evaluate the effect of sample replication on improving between-lake comparisons, and showed that collecting two littoral and two open-water samples allowed us to faithfully capture differences in average composition among the three lakes. Hence, the results suggest that using a single sample to estimate diatom assemblages composition in these lakes should be avoided, as it fails to capture between-lake differences, leading to biases in compositional comparisons among lakes and regions. Consequently, including multiple samples from each lake when constructing calibration sets would be the best option to obtain reliable paleoenvironmental reconstructions from single sediment cores in these environmentally heterogeneous shallow lakes.  相似文献   

Long term variation in rainfall erosivity in Uruguay: a preliminary Fournier approach     

Carolina Munka  Gabriela Cruz  Ruben Mario Caffera 《GeoJournal》2007,70(4):257-262

In order to explore the consequences of precipitation increase on soil erosion in Uruguay, the monthly Fournier Index (FI) and the annual Modified Fournier Index (MFI) were developed as a preliminary approach, covering consecutive decadal periods from 1931 to 2000. Rainfall data were used from 13 stations distributed all over the country. MFI shows a decrease in the 1940s, an increase during the 1950s, then a little decrease during the 1960s and 1970s and an increase in the 1980s, remaining near these last values during the 1990s. FI behavior in July and October in the last two decades shows a decrease in the Northeast region of the country—the region with highest annual precipitation—and a slight increase over the Southeast—the region with the lowest annual precipitation and the only region with a winter rainfall maximum. For the core months of winter and early spring there is a decline in erosivity in the Northeast, but an increase in the Southeast. January shows erosivity decreasing in the South during the last two decades. In April there is a peak in precipitation variability all across the country. FI shows an early increase during the 1940s in the Northwest, and generally very low values during the 1970s, with the return of high values in the 1990s, especially in the Northern corner. Results pose a challenge in order to improve research on the erosion problem, since the main source of freshwater nationwide remains surface river flow, which is prone to higher turbidity problems in areas of high soil erosion.  相似文献   

Teleconnection patterns and Rossby wave propagation associated to generalized frosts over southern South America     

Gabriela V. Müller  Tércio Ambrizzi 《Climate Dynamics》2007,29(6):633-645

Based on previous observational studies of the mean atmospheric circulation leading to generalized frosts (GF) in central Southern South America, it is possible to establish a hypothesis that specific large scale patterns are associated to the frequency of occurrence of these events through the propagation of Rossby waves remotely excited. This hypothesis is tested here through a teleconnection analysis for austral winters which present an extreme frequency of occurrence of GF in southeastern South America, particularly over the Wet Pampa area in Argentina. Rossby wave propagation regions are identified for two basic states given by the composition of winters with maximum and minimum frequency of GF occurrence, during the 1961–1990 period. The stationary wavenumber K _s indicates the regions where the Rossby wave propagation is permitted and those where it will be inhibited (K _s = 0), highlighting the importance of the jets as waveguides. Nevertheless, differences exist between both basic states analyzed. These differences indicate that the locations for wave generation and its later evolution are conditioned by the basic state. Results are validated through a baroclinic model, which simulates the Rossby wave patterns responsible for the teleconnection. Numerical experiments confirm that the principal wave activity takes place inside the subtropical and polar jets. In particular, for the basic state with maximum frequency of GF occurrence, the wave trains propagating inside the subtropical and polar waveguides merge just before entering the continent, as shown by the observations prior to the occurrence of GF events. This configuration favors the development of an intense south wind anomaly with large meridional extension which results in the intensification of anticyclonic circulation in southern South America. A conceptual model is presented to summarise all these results.  相似文献   

The effect of Southwestern Atlantic burrowing crabs on habitat use and foraging activity of migratory shorebirds     

Florencia Botto  Gabriela Palomo  Oscar Iribarne  Mariano M. Martinez 《Estuaries and Coasts》2000,23(2):208-215

Southwestern Atlantic estuaries (Southern Brazil to Northern Patagonia, Argentina) are characterized by the presence of an intertidal burrowing crab Chasmagnathus granulata. This crab species is an important bioturbator which lives in large assemblages and excavates semi-permanent burrows that affect sediment characteristics. Our observations showed that distribution of the crabs in the Mar Chiquita coastal lagoon, Argentina (37°45′S, 57°26′W) affected habitat use and feeding success of migratory shorebirds. During the migratory season the two-banded plover (Charadrius falklandicus) foraged more frequently inside crab beds, and yellowlegs (Tringa flavipes and T. melanoleuca) fed more freqeuntly outside crab beds. Focal observations on the feeding behavior of the white-rumped sandpiper (Calidris fuscicollis) and the two-banded plover inside and outside crab beds showed that the plover was a visual searcher and captured more prey inside crab beds, and the white-rumped sandpiper was a tactile feeder. Although consumption rates (prey min^?1) did not differ between sites, their efficiency (prey probe^?1) inside crab beds was less. These differences were probably related to changes in sediment characteristics and prey behavior, which vary with crab activity. Burrowing crabs alter the suitability of intertidal habitats used by shorebirds in southwestern Atlantic estuaries. We believe that the same process could be occurring with other burrowing curstaceans such as thalassinidean shrimps in other estuaries of the world and could have important implications for management of flats for shorebirds. *** DIRECT SUPPORT *** A01BY090 00007  相似文献   

Transgressive dunefield landforms and vegetation associations,Doña Juana,Veracruz, Mexico     

Patrick Hesp  Marisa Martinez  Graziela Miot da Silva  Natalia Rodríguez‐Revelo  Erika Gutierrez  Adriana Humanes  Daniela Laínez  Irene Montaño  Verónica Palacios  Agustín Quesada  Lorena Storero  Gabriela González Trilla  Carolina Trochine 《地球表面变化过程与地形》2011,36(3):285-295

Transgressive dune fields often comprise a multiplicity of landforms where vegetation processes largely affect landform dynamics, which in turn, also affect vegetation processes. These associations have seldom been studied in detail. This paper examines four separate landform types in a complex coastal transgressive dunefield located in the central Gulf of Mexico, in order to assess the relationships between dunefield habitat, local environmental factors, vegetation associations and landform evolution. Topographic surveys using tape and clinometer were conducted in conjunction with vegetation survey transects at four locations across the Doña Juana dunefield. Vegetation surveys allowed the estimation of relative plant cover of each plant species found along the transects. A large variety of landforms were found at the Doña Juana Dunefield: deflation plains, gegenwalle (counter) ridges, transverse dune trailing ridges, blowouts and parabolic dunes, aklé (fish‐scale shaped) dunefields and precipitation ridges, with plant species associations developing on these different landforms equally variable. Flood tolerant species were located in the lower parts (deflation plain and gegenwalle ridges) whereas the older and dryer parts were covered by coastal matorral shrubs. Burial‐tolerant species were dominant in the most mobile areas (blowouts and aklé dunefield and margin). The dune trailing ridge, with relatively milder conditions, showed the highest richness, with no dominant species. A dual interaction was found such that colonizing species both create and affect topography, and in turn, topography determines vegetation association and succession patterns. In coastal dunes, the vegetation and abiotic environment (namely the different landforms and the inherent micronevironmental variability) interact tightly and generate a complex and highly dynamic biogeomorphic system where substrate mobility and colonization processes reinforce one another in positive feedback. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Ancient Forests in European drylands: Holocene palaeoecological record of Mazarrón,south-eastern Spain     

José S. Carrión  Elena Fierro  Milagros Ros  Manuel Munuera  Santiago Fernández  Juan Ochando  Gabriela Amorós  Francisca Navarro  Tomás Rodríguez-Estrella  Saúl Manzano  Penélope González-Sampériz  Ana Moreno 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(4):512-525

This paper presents a new Holocene palaeoecological record from coastal south-eastern Spain, a region characterised by high plant species diversity, varied physiography, high risk of desertification, and a history of human pressure on the landscape that stretches to antiquity. The pollen sequence shows four main vegetation phases: the first characterised by mixed forests of Pinus and evergreen Quercus accompanied by broad-leaved mesophilous trees, and a diversity of Mediterranean scrub; the second phase is characterised by mesophytic decline and expansion of Artemisia; a third, mid-Holocene phase of thermo-mesophytic maxima with prevalence of forested landscapes; and, finally, the progressive opening of the landscape with sparse pines, halo-xerophytic grasslands and sclerophyllous brushwood. The current treeless situation of south-eastern Spain is a relatively recent feature resulting from a dramatic change in the ecological structure of the regional landscapes. This paper stresses the continued vulnerability of these arid systems in the face of a changing climate. This sequence adds to previous palaeobotanical records (pollen and charcoal) and archaeological reports to suggest that deforestation started earlier in low-elevation areas and river basins than in the inland mountains and platforms, a factor that appears in connection to human exploitation of the natural environment.  相似文献   

Hydrochemistry and nutrients dynamic in the Suquía River urban catchment’s,Córdoba,Argentina     

Andrea I. Pasquini  Stella M. Formica  Gabriela A. Sacchi 《Environmental Earth Sciences》2012,65(2):453-467

Suquía River is a medium-sized hydrological system (basin area of ~7,700 km²) that supplies fresh water to Córdoba city, a town of ~1,500,000 inhabitants in central Argentina. This paper examines the present-day hydrochemistry of Suquía River urban catchment analyzing its major and minor dissolved components, and the nutrients variability by means of QUAL-2K modeling software. The Suquía River has bicarbonate-type waters upstream the city and sulfate-type waters right downstream, whereas they exhibit a mixed-to-alkali-type cationic composition. The seasonal analysis of its major dissolved constituents clearly showed a dilution process during the wet season (i.e. austral summer). In the last 20 years, the Suquía River has modified its anionic composition, now showing higher relative concentrations of SO₄ ²⁻ as a consequence of urban activities. However, trace elements dissolved concentrations do not evidence a strong pollution effect. Nutrients [nitrogen species, total phosphorous (TP)] and related parameters, such as biochemical oxygen demand (BOD), and dissolved oxygen (DO), evidence a clear influence of human activities. The QUAL-2K model was used to evaluate the spatial behavior of selected nutrients and associated variables, (i.e. TP, N–NH₄ ⁺, N–NO₃ ⁻, DO, BOD). Nutrient concentrations are affected by point sources of contaminants, particularly domestic waste and sewage, as well as by diffuse agricultural pollution. A calibrated QUAL-2K modeling exercise clearly shows the impact of the Córdoba city’s municipal wastewater treatment plant on the Suquía River water quality.  相似文献   

Experimental constraints on the relative stabilities of the two systems monazite-(Ce) – allanite-(Ce) – fluorapatite and xenotime-(Y) – (Y,HREE)-rich epidote – (Y,HREE)-rich fluorapatite,in high Ca and Na-Ca environments under P-T conditions of 200–1000 MPa and 450–750 °C     

Budzyń  Bartosz  Harlov  Daniel E.  Kozub-Budzyń  Gabriela A.  Majka  Jarosław 《Mineralogy and Petrology》2017,111(2):183-217

Mineralogy and Petrology - The relative stabilities of phases within the two systems monazite-(Ce) – fluorapatite – allanite-(Ce) and xenotime-(Y) – (Y,HREE)-rich fluorapatite...  相似文献   

A model for CO<Subscript>2</Subscript> storage and seismic monitoring combining multiphase fluid flow and wave propagation simulators. The Sleipner-field case     

Gabriela B. Savioli  Juan E. Santos  José M. Carcione  Davide Gei 《Computational Geosciences》2017,21(2):223-239

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