The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U–Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed.The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt. 相似文献
The aim of this study is to determine the isotopic composition (δ18O and δ2H) of interstitial water in bentonites. The study had been carried out from adsorbed vapour in homoionic bentonites. We have worked with the fraction <20 μm of a bentonite from Serrata de Níjar (Almería, Spain). Homoionic sodium and calcium bentonites were prepared as well as variable quantities of exchangeable Na/Ca (75Ca/25Na; 50Ca/50Na; 25Ca/75Na), by mixing pure sodium and calcium suspensions in the appropriate quantities. To carry out the hydration of the samples, every one of them was previously dried at 300 °C overnight, they were subjected to controlled saturation conditions in an atmosphere of different relative humidity and at a constant temperature of 20 °C, until equilibrium was achieved. The different vapour pressures were: P/Po = 0.05; P/Po = 0.078; P/Po = 0.15; P/Po = 0.32; P/Po = 0.45; P/Po = 0.63; P/Po = 0.8; P/Po = 1, obtained from the saturated solutions of different salts or sulphuric acid.The saturation water was extracted for isotopic analysis from an aliquot of a saturated sample. The technique used was vacuum extraction based on a modification of the one described by Araguás-Araguás et al. (1995).The differentiation between the isotopes of 18O and 2H from interstitial water in the bentonite samples, depending on the different relative humidity, may affect the ions diffusion in the interstitial solutions, since in those from low relative humidities, there could be a higher diffusion velocity as they are formed by molecules of light isotopes, versus interstitial solutions from higher relative humidities or those near saturation. This fact should be taken into account in studies on cation and solute transport from these solutions inside bentonite, even though it is important to continue studying and corroborating it with a larger number of solutions isotopically marked. 相似文献
Nested Limited-Area Models require driving data to define their lateral boundary conditions (LBC). The optimal choice of domain size and the repercussions of LBC errors on Regional Climate Model (RCM) simulations are important issues in dynamical downscaling work. The main objective of this paper is to investigate the effect of domain size, particularly on the larger scales, and to question whether an RCM, when run over very large domains, can actually improve the large scales compared to those of the driving data. This study is performed with a detailed atmospheric model in its global and regional configurations, using the “Imperfect Big-Brother” (IBB) protocol. The ERA-Interim reanalyses and five global simulations are used to drive RCM simulations for five winter seasons, on four domain sizes centred over the North American continent. Three variables are investigated: precipitation, specific humidity and zonal wind component. The results following the IBB protocol show that, when an RCM is driven by perfect LBC, its skill at reproducing the large scales decreases with increasing the domain of integration, but the errors remain small even for very large domains. On the other hand, when driven by LBC that contain errors, RCMs can bring some reduction of errors in large scales when very large domains are used. The improvement is found especially in the amplitude of patterns of both the stationary and the intra-seasonal transient components. When large errors are present in the LBC, however, these are only partly corrected by the RCM. Although results showed that an RCM can have some skill at improving imperfect large scales supplied as driving LBC, the main added value of an RCM is provided by its small scales and its skill to simulate extreme events, particularly for precipitation. Under the IBB protocol all RCM simulations were fairly skilful at reproducing small scales statistics, although the skill decreased with increasing LBC errors. Coarse-resolution model simulations have difficulties in simulating heavy precipitation events, and as a result their precipitation distributions are systematically shifted toward smaller intensity. Under the IBB protocol, all RCM simulations have distributions very similar to the reference field, being little affected by LBC errors, and no significant differences were found between the small scales statistics and the precipitation distributions obtained over different RCM domains. 相似文献
The main focus of this study is the zonal contrast of the Sahel precipitation shown in the CMIP5 climate projections: precipitation decreases over the western Sahel (i.e., Senegal and western Mali) and increases over the central Sahel (i.e., eastern Mali, Burkina Faso and Niger). This zonal contrast in future precipitation change is a robust model response to climate change but suffers from a lack of an explanation. To this aim, we study the impact of current and future climate change on Sahel precipitation by using the Large Ensemble of the Community Earth System Model version 1 (CESM1). In CESM1, global warming leads to a strengthening of the zonal contrast, as shown by the difference between the 2060–2099 period (under a high emission scenario) and the 1960–1999 period (under the historical forcing). The zonal contrast is associated with dynamic shifts in the atmospheric circulation. We show that, in absence of a forced response, that is, when only accounting for internal climate variability, the zonal contrast is associated with the Pacific and the tropical Atlantic oceans variability. However, future patterns in sea surface temperature (SST) anomalies are not necessary to explaining the projected strengthening of the zonal contrast. The mechanisms underlying the simulated changes are elucidated by analysing a set of CMIP5 idealised simulations. We show the increase in precipitation over the central Sahel to be mostly associated with the surface warming over northern Africa, which favour the displacement of the monsoon cell northwards. Over the western Sahel, the decrease in Sahel precipitation is associated with a southward shift of the monsoon circulation, and is mostly due to the warming of the SST. These two mechanisms allow explaining the zonal contrast in precipitation change.
Snowfall changes in mountain areas in response to anthropogenic forcing could have widespread hydrological, ecological and economic impacts. In this paper, the robustness of snowfall changes over the French Alps projected during the 21st century and the associated uncertainties are studied. In particular, the role of temperature changes on snowfall changes is investigated. Those issues are tackled through the analysis of the results of a very large ensemble of high-resolution regional climate projections, obtained either through dynamical or statistical downscaling. We find that, at the beginning and at the end of the cold season extending from November to March (included), temperature change is an important source of spread in snowfall changes. However, no link is found between temperature and snowfall changes in January and February. At the beginning and at the end of the cold season, the rate of change in snowfall per Kelvin does not depend much on the bias correction step, the period or the greenhouse gas scenario but mostly on the downscaling method and the climate models, the latter uncertainty source being dominant. 相似文献
Fish school swimming speeds is essential for ecological and management studies. The multibeam sonar in horizontal beaming
provided dynamic echo traces of mobile fish schools. We used two school swimming speed indicators: the average of a series
of instantaneous speed values, and the exploratory speed. These swimming speeds were estimated for each fish school observed
on the basis of their Euclidian position within the sonar beams. The average ISS values per school ranged from 0.15 m s−1 to 4.46 m s−1, while the ESS values per school were lower, ranging from 0.04 m s−1 to 3.77 m s−1. Multibeam sonar technology makes it possible to measure fish school swimming speeds in their natural habitat at small spatio-temporal
scales. This methodology can therefore be used to analyse in situ their movements, and has a wide range of applications in
behavioural studies and management purposes. 相似文献
The increase in the amount of solar data provided by new satellites makes it necessary to develop methods to automate the detection of solar features. Here we present a method for automatically detecting active regions in solar extreme ultraviolet (EUV) images using a series of steps. Initially, the bright regions in the image are segmented using seeded region growing. In a second phase these bright regions are clustered into active regions. Partition-based clustering (both hard and fuzzy) and hierarchical clustering are compared in this work. The aim of the clustering phase is to associate a group to each segmented region in order to reduce the total number of active regions. This facilitates the documentation or subsequent monitoring of these regions. We use two indicators to validate the partitioning: i) the number of detected clusters approximates the number of active regions reported by the National Oceanic and Atmospheric Administration (NOAA) and ii) the area that defines each cluster overlaps with the area of an active region of NOAA. Experiments have been performed on over 6000 images from SOHO/EIT (195 Å). The best results were obtained using hierarchical clustering. The method detects a set of active regions in an image of the solar corona that successfully matches the number of NOAA regions. We will use these regions to perform real-time monitoring and flare detection. 相似文献
Volcán de Colima is currently the most active volcano in Mexico. Since 1998 intermittent activity has been observed with vulcanian eruptions, lava flows and growing domes that have collapsed producing several block-and-ash flow deposits. During the period of heightened activity since 1998 at Volcán de Colima, pyroclastic flows from dome or column collapse have not reached long distances, most of the time less than 6 km from the crater. In contrast, rain-induced lahars were more frequent and have reached relatively long distances, up to 15 km, causing damage to infrastructure and affecting small villages. In 2007 two rain gauge stations were installed on the southern flank of the volcano registering events from June through to October, the period when rains are intense and lahars frequent. By comparing lahar frequency with rainfall intensity and the rainfall accumulated during the previous 3 days, lahars more frequently occur at the beginning of the rainfall season, with low rain accumulation (< 10 mm) and triggered by low rain intensities (< 20 mm/h). During the months with more rainfall (July and August) lahars are less frequent and higher peak intensities (up to 70 mm/h) are needed to trigger an event. In both cases, lahars were initiated as dilute, sediment-laden streamflows, which transformed with entrainment of additional sediment into hyperconcentrated and debris flows, with alternations between these two flow types. A hydro-repellency mechanism in highly vegetated areas (i.e. evergreen tree types with considerable amount of resins and waxes such as pines) with sandy soils can probably explain the high frequency of lahars at the beginning of the rain season during low rainfall events. Under hydrophobic conditions, infiltration is inhibited and runoff is facilitated at more highly peaked discharges that are more likely to initiate lahars. 相似文献