37,514 research outputs found
Influence of Refractory Periods in the Hopfield model
We study both analytically and numerically the effects of including
refractory periods in the Hopfield model for associative memory. These periods
are introduced in the dynamics of the network as thresholds that depend on the
state of the neuron at the previous time. Both the retrieval properties and the
dynamical behaviour are analyzed.Comment: Revtex, 7 pages, 7 figure
Shelf-Ocean material exchange influencing the Atlantic chemical composition off NW Iberian margin since the last glaciation
Rivers are the main conduit of sediment to the shelf. The basin geology, the drainage area and the discharge rate are the major factors that determine their sediment load (Milliman and Syvistski, 1992). Besides suspended particles, dissolved components may also give some information on the eroded crust. Sr isotopes in carbonate shells of biological organisms have been used to study, in the geological record, the influence exerted by the chemical weathering of the continental crust on the seawater composition (Macdougall, 1991).
In this work, Sr isotope ratios obtained in tests of foraminifera representing the last 40 ka are presented and discussed in the scope of the palaeogeographical evolution of NW Iberia.
This work aims to present and discuss the results of Sr isotope analyses (performed, by TIMS, in the Isotope Geology Laboratory of the University of Aveiro) of tests of two species of foraminifera, from nine samples taken along the OMEX core KC 024-19 (181 cm; 42°08’98’’N, 10°29´96’’W, and 2765m), collected in the Galicia Bank area, off Galicia. Taking into account that Sr contained in the carbonate tests is usually considered as preserving the signature of the contemporaneous seawater, one planktonic species (Globigerina bulloides) and one benthic species (Cibicides wuellerstorfi) were selected in order to try to detect Sr isotope variations both through time and between two different levels of the water column.
The core age model, which records the last 40 ka, is based on a combination of oxygen isotope stratigraphy, eight AMS 14C datings and the synchronisation of the last four Heinrich Events in the Iberian Margin sedimentary records.
As a whole, the obtained 87Sr/86Sr ratios vary between 0.709209 and 0.709108, with a mean 2σ error of 0.000025. These values lie within the range of modern marine Sr isotope ratios (0.70910-0.70922), as previously defined using analyses of both seawater and marine carbonates (see compilation by Faure and Mensing, 2005). Despite their small variation, the 87Sr/86Sr ratios obtained in G. bulloides seem to indicate that Sr dissolved in seawater at the KC 024-19 core site became slightly less radiogenic after the Last Glacial Maximum (LGM). This decrease is concomitant with diminishing amounts of the detrital components in the sediments (Fig. 1). Therefore, both the composition of dissolved Sr, as revealed by results on tests of planktonic foraminifera, and the proportions of suspended terrigenous particulate material arriving at the KC 024-19 site point to a decreasing importance of the contribution of the erosion of the Iberian Variscan crust since the Last Glacial Maximum and in the Holocene.
The 87Sr/86Sr ratios measured in tests of benthic foraminifera (C. wuellerstorfi) are more erratic and no correlation can be established with palaeogeographical/palaeoclimatic constraints. The difference between the behaviour of Sr compositions in G. bulloides and C. wuellerstorfi may indicate that whilst the planktonic foraminifera should reproduce very closely the seawater composition, the benthic organisms should, in addition to the major role of seawater, also be affected by some sort of interaction with the sediments. As such, planktonic foraminifera are probably more reliable indicators of seawater composition in studies involving very small periods and corresponding very slight variations of the 87Sr/86Sr ratios.
Taking into account that G. bulloides is a common planktonic species (living mostly in the first 50m of the water column), whose tests seem to be in equilibrium with sea water composition, variations in its 87Sr/86Sr ratios can be related with changes in the chemical composition of the water of the Atlantic Ocean off NW Iberian Margin.
The highest 87Sr/86Sr values are contemporaneous with a period of low sea level (about -140 m; Dias et al., 2000) during the LGM. According to Dias et al. (2000) at 18 ka BP the shoreline was close to the shelf break. The summital parts of the Gerês and Estrela mountains were covered by local glaciers and close to the coast freezing occurred frequently. The river catchments, which extended far to the shelf, received more rainfall due to a longer, compared to present day conditions, wet season, which promoted both physical and chemical weathering. Higher pluviosity combined with the effect of spring ice melting maintained high river discharge and consequently caused very important sediment supply to the coastal zone. The extremely narrow shelf was a very energetic environment due to sea bottom inclination and very limited long wave refraction. Therefore, at that time, a long wet season and very competent rivers should have caused important erosion of the Variscan basement in NW Iberia. Additionally, then, the shoreline was much closer to the KC 024-19 site. The combination of all these factors favoured an important deposition of terrigenous sediments and the local slight enrichment in radiogenic Sr of the seawater.
With sea level rise, after the deglaciation and during the Holocene, the river estuaries became progressively far away from the shelf break. Their competence of transport also became progressively reduced and the offshore transport of detrital sediments became progressively lower. Conversely the biogenic carbonate proportion in the sediments increased, due to lower dilution by the terrigenous particles. Simultaneously, the values of 87Sr/86Sr in the seawater at the KC 024-19 site became lower, as a consequence of a complete homogenization with the ocean global composition, which was now more effective with the increasing distance towards shoreline
Eficiência do estimulo hormonal e bioestimulação na taxa de ovulação, indução e sincronização do estro em caprinos.
A bioestimulação como coordenador de comportamento sexual. Bioestimulação da puberdade em fêmeas e machos. Efeito bioestimulador de liberação do hormonio luteinico
On the finiteness of the noncommutative supersymmetric Maxwell-Chern-Simons theory
Within the superfield approach, we prove the absence of UV/IR mixing in the
three-dimensional noncommutative supersymmetric Maxwell-Chern-Simons theory at
any loop order and demonstrate its finiteness in one, three and higher loop
orders.Comment: 9 pages, 2 figures, revtex
Moving to the cloud: Estimating the internet connection bandwidth
IT Infrastructures (ITIs) have long been understood in terms of people and resources such as servers, routers, firewalls and operating systems, among other components, running and providing services inside the organization. The need to reduce the cost of ITI ownership, by offloading its capacity to third parties, has motivated organizations to consider the Cloud Computing alternative. The main drawback they face when opting for the cloud is the dependency on and requirements of the internet connection, since it must be fast and reliable. This paper addresses this concern by providing guidelines for estimating internet connection bandwidth requirements for a prototypical cloud-based organization represented by means of an IT infrastructure pattern. ITI patterns are reusable and proven solutions to support the ITI design process and to facilitate the communication among stakeholders.info:eu-repo/semantics/acceptedVersio
Aspectos anatômicos da folha de Senna obtusifolia (L.) H.S. Irwin & Barneby (Leguminosae - Caesalpinoideae) em microscopia eletrônica de varredura.
O feijão-comum no Brasil: passado, presente e futuro.
Introdução. Aspectos metodológicos. Resultados e discussões. Conjuntura da produção, área colhida e rendimento de feijão-comum no Brasil e nas cinco Regiões geográficas, nas três safras, de 2006 a 2011. Safra das "águas" ou 1ª safra. Safra da "seca" ou 2ª safra. Safra de inverno, ou safra de 3ª época ou safra irrigada ou 3ª safra. Panorama da produção, área colhida e rendimento de feijão-comum nos principais estados produtores da Federação, de 2006 a 2011: Paraná; Minas Gerais. Goiás. São Paulo. Bahia. Indicadores do agronegócio de feijão-comum no Brasil e nos cinco principais estados produtores da Federação. Classificação dos estabelecimentos agropecuários produtores de feijão-comum por grupo de área colhida. Tipo de cultivo. Tipo de agricultura. Principal mês de plantio no Brasil e principais estados produtores. Principal mês de colheita no Brasil e principais estados produtores. Mercado e consumo de feijão-comum no Brasil. Projeções de 2010/2011 a 2021/2022. Considerações finais. Referências.bitstream/item/89747/1/seriedocumentos-287.pd
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