Showing papers by "University of Lisbon published in 1993"

Mediterranean Outflow Mixing and Dynamics

Abstract: The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

Tectonics of the Beja-Acebuches Ophiolite: a major suture in the Iberian Variscan Foldbelt

Abstract: The Beja-Acebuches Ophiolitic Complex (BAOC) (south Portugal/Spain) corresponds to a high grade metamorphic belt along the boundary between Ossa-Morena and the South Portuguese Zones and comprises a lithostratigraphic sequence including (from top to bottom) metabasalts, (metamorphosed) multiple dyke intrusions in gabbro, flasergabbros and metaserpentinites. It is affected by three deformation phases. D1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north-north-east; this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the Serpa antiform, affecting Precambrian basement and Cambrian cover rocks. The obduction polarity ist north-eastwards, similar to the subduction polarity that generates the Beja Gabbroic Complex (BGC), implying a flake geometry. The second deformation phase, D2, is represented by an intense WNW-ESE sinistral shear event which is responsible for the shattered appearance of the suture; D2 is reactivated later by a more brittle D3 event involving thrusting to the south-west, again with a sinistral component. 40Ar/39Ar isotopic ages were obtained for (metamorphosed) multiple dyke intrusions in the BAOC's gabbro (342.6 ± 1.4 Ma), for metagabbroic cumulates (340.7 ± 1.9 Ma), and for the undeformed/unmetamorphosed BGC (341.1 ± 1.3 Ma) occurring to the north of the ophiolitic suture. These ages reflect a last regional cooling event in the area which post-dates the ophiolite emplacement and the intrusion of the BGC through this oceanic sequence.

Feeding of an Iberian stream cyprinid assemblage: seasonality of resource use in a highly variable environment.

Abstract: Food resource use by seven cyprinids from an Iberian stream was analysed over 9 months. Differences in food resource use were found both between species and within species between seasons. Plant material was a more important food for carp, nase, goldfish and barbel than for gudgeon, roach and chub, irrespective of the season. Chironomid larvae were the staple animal food for the former five species throughout the year. Roach and chub, especially the latter, displayed large seasonal variations in prey use, with chironomid larvae only being important during autumn. Ephemerellid nymphs and ephemeropteran imagos dominated the animal diet of chub during spring and summer, respectively. Dipteran adults and Formicidae were the most important prey for roach during spring and summer, with other common prey being ephemerellid nymphs and hydropsychid larvae. Food resource overlap among the three dominant species (roach, barbel and chub) displayed a large seasonal variation. High overlaps were observed during autumn when these species used the same resources. During summer overlaps were much lower with each species specialising on different prey. The remaining less abundant species had large diet overlaps amongst themselves and with barbel, over all seasons. It is suggested that morphological constraints, habitat partitioning and temporal changes in food resource limitation may be involved in producing these patterns of food resource use.

Fixed frequency sliding mode modulator for current mode PWM inverters

Abstract: A fixed frequency current mode PWM modulator that provides output current limiting in sliding mode voltage controlled DC-AC power converters is presented. The modulator is considerably simpler than previous solutions and well adapted for sliding mode control, which is advantageous when the inverter feeds nonlinear or wide ranging loads and performs as a switching power operational amplifier or power operational transconductance amplifier. >

Ion-beam channeling yields of host and impurity atoms in LiNbO3: Computer simulations.

Abstract: A Monte Carlo program for the simulation of channeling phenomena in ${\mathrm{LiNbO}}_{3}$ crystals is described. Results of the program are compared with experimental yields for Nb, Li, and O from angular scans through the 〈0001〉, 〈022\ifmmode\bar\else\textasciimacron\fi{}1〉, 〈04\ifmmode\bar\else\textasciimacron\fi{}41〉, and 〈112\ifmmode\bar\else\textasciimacron\fi{}0〉 axial directions and through the (0001) planar direction obtained with 1.6-MeV ${\mathrm{He}}^{+}$ and proton beams. A set of calculated angular scans for different axial and planar directions, and different impurity locations in ${\mathrm{LiNbO}}_{3}$, is presented. The program has been applied to identify the lattice location of Nd, Eu, and Hf in ${\mathrm{LiNbO}}_{3}$, and that of Lu and Hf in Mg-doped ${\mathrm{LiNbO}}_{3}$ by comparing the experimental data with the simulated angular scans. The results confirm that the Hf atoms occupy substitutional Li sites and Eu and Nd atoms lie at a position shifted by \ensuremath{\approxeq}0.4 \AA{} from the regular Li site along the c axis and towards the nearest oxygen plane in ${\mathrm{LiNbO}}_{3}$. The Hf and Lu atoms occupy the Nb and Li sites respectively if the ${\mathrm{LiNbO}}_{3}$ is codoped with Mg atoms.

Hyperfine quenching of the 1s22s2p 3P0 level in berylliumlike ions.

Abstract: In this paper, we used the multiconfiguration Dirac-Fock method to compute with high precision the 1${\mathit{s}}^{2}$2s2p $^{3}$${\mathit{P}}_{1}$--1${\mathit{s}}^{2}$2s2p $^{3}$${\mathit{P}}_{0}$ separation energy in berylliumlike ions, including the relativistic contribution to electron-electron correlations and radiative corrections The effect of the hyperfine interaction on both the energy and lifetimes of those levels has been evaluated In the absence of nuclear magnetic moment the $^{3}$${\mathit{P}}_{0}$ level can decay to the ground state only by an E1-M1 transition, with a very low probability We show that the hyperfine interaction increases dramatically the $^{3}$${\mathit{P}}_{0}$ transition probability

Aldehyde oxidoreductase activity in Desulfovibrio gigas: In vitro reconstitution of an electron-transfer chain from aldehydes to the production of molecular hydrogen

Abstract: The molybdenum [iron-sulfur] protein, first isolated from Desulfovibrio gigas by Moura et al. [Moura, J. J. G., Xavier, A. V., Bruschi, M., Le Gall, J., Hall, D. O., & Cammack, R. (1976) Biochem. Biophys. Res. Commun. 72, 782-789], was later shown to mediate the electronic flow from salicylaldehyde to a suitable electron acceptor, 2,6-dichlorophenolindophenol (DCPIP) [Turner, N., Barata, B., Bray, R. C., Deistung, J., LeGall, J., & Moura, J. J. G. (1987) Biochem. J. 243, 755-761]. The DCPIP-dependent aldehyde oxidoreductase activity was studied in detail using a wide range of aldehydes and analogues. Steady-state kinetic analysis (KM and Vmax) was performed for acetaldehyde, propionaldehyde, benzaldehyde, and salicylaldehyde in excess DCPIP concentration, and a simple Michaelis-Menten model was shown to be applicable as a first kinetic approach. Xanthine, purine, allopurinol, and N1-methylnicotinamide (NMN) could not be utilized as enzyme substrates. DCPIP and ferricyanide were shown to be capable of cycling the electronic flow, whereas other cation and anion dyes [O2 and NAD(P)+] were not active in this process. The enzyme showed an optimal pH activity profile around 7.8. This molybdenum hydroxylase was shown to be part of an electron-transfer chain comprising four different soluble proteins from D. gigas, with a total of 11 discrete redox centers, which is capable of linking the oxidation of aldehydes to the reduction of protons.

DNA strand breaks and chromosomal aberrations induced by H2O2 and 60Co gamma-radiation.

Abstract: DNA strand breaks and chromosomal aberrations (CAs) were studied in human cells treated with hydrogen peroxide or with ionizing radiation. DNA strand breaks could be produced at dose levels of H2O2 much lower than those which induced CAs. Doses as low as 0.5 mM of H2O2 produced about as many DNA strand breaks as 2 Gy of 60Co gamma-radiation. On the other hand, as much as 20 mM H2O2 produced only half as many CAs as 1 Gy of 60Co gamma-radiation. The different mechanisms involved in the production of human genetic damage by H2O2 and gamma-radiation are discussed.

Magnetic anisotropy and temperature dependence of the hyperfine fields of Cd 111 in single-crystalline cobalt

Abstract: The magnetic anisotropy of single-crystalline cobalt and the temperature dependence of the hyperfine fields of cadmium in cobalt were studied by the perturbed-angular-correlation technique without any applied magnetic field. A uniaxial basal-plane anisotropy was found instead of the expected threefold one, and was explained by lattice-deformation-induced anisotropy. The obtained temperature dependence of the angle between the magnetization and the c axis is in agreement with the uniaxial anisotropy constants ${\mathit{K}}_{1}$ and ${\mathit{K}}_{2}$ found by torque magnetometry. The room-temperature values of the hyperfine fields are ${\mathit{B}}_{\mathrm{hf}}$=288(3) kG and ${\mathit{V}}_{\mathit{Z}\mathit{Z}}$=32(5)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ V/${\mathrm{cm}}^{2}$.

Effects of season and body‐size on the distribution and diet of the Iberian chub Leuciscus pyrenaicus in a lowland catchment

Abstract: Seasonal and ontogenetic variations in space and food use by Leuciscus pyrenaicus were analysed in a Portuguese lowland catchment. Large fish occurred mostly in deep permanent-flowing sites whereas small fish occurred mostly in the shallowest intermittent-flowing site. No seasonal or size-related changes in feeding intensity were found, but the diet changed both across seasons and throughout ontogeny. The diet was dominated by aquatic prey over all seasons, but during winter and summer more plant material and terrestrial prey, respectively, were eaten. Throughout ontogeny fish shifted from soft-bodied to hard-shelled prey and decreased animal prey breadth. Mean prey size increased with fish size but the prey size spectrum was more variable for medium-sized fish than for either small or large fish. It is suggested that: (i) large fish avoid shallow drying areas owing to the risk of mortality, either by thermal and respiratory stresses or increased predation by mammals and birds; (ii) seasonal changes in diet are a response to differences in prey availability; and (iii) morphological constraints, prey handling costs and habitat partitioning are responsible for size-related changes in diet.

Direct embryo formation in leaves of Camillia japonica L.

Abstract: The culture conditions for direct embryo formation in leaves of Camellia japonica L. were established. An auxin treatment followed by incubation during 11 days in darkness on diluted Murashige and Skoog modified basal medium induced direct morphogenesis. The number of subcultures, subculture interval and leaf age affected in vitro leaf response. The results showed that the cells from a cultured leaf respond differently to the same culture conditions by forming embryos, roots, and non-morphogenic as well as organogenic callus. Direct embryo formation occurred only in the marginal leaf regions. Direct root formation only occurred in a well-defined region of the midrib whereas callus was preferentially formed on the leaf basis. The results suggest the existence of differences in morphogenic competence according to leaf regions. Plantlet regeneration was successfully achieved from somatic embryos and from leaf basisderived callus, via shoot bud induction.

A comparison of directed formulations for the capacitated minimal spanning tree problem

Abstract: In this paper, we compare linear integer programming directed formulations for the capacitated minimal spanning tree (CMST) problem. This problem is directly related to network design and consists of finding the cheapest way to link a set of terminals at different locations with a central node (a computer site, for instance). The present work focuses on a single commodity flow formulation presented in [3,4]. This formulation includes two sets of variables and, hence, it also must include coupling constraints between the two sets of variables. These coupling constraints can be defined in several ways. The main result of this work establishes that when the strongest form of the coupling constraints is used in the flow formulation (as in [4]), the "equivalent" formulation using only theX ij variables satisfies the so-called multistar constraints. These constraints are shown to be stronger than the weakest form of the generalized subtour breaking constraints and, hence, our result improves on the characterization given in [5] for the flow formulation presented in [4]. For certain parameters, the multistar constraints induce facets of the non-directed CMST polytope (see [2]). We also show that it is not possible to compare the linear relaxations of a formulation that involves the multistar constraints with one that involves the generalized subtour breaking constraints.

Mechanisms of shear failure in artificial fractures of sandstone and their implication for models of hydromechanical coupling

Abstract: Direct shear tests, in which the behaviour of the rock surfaces during shear could be continuously observed, were used to study the shear failure of a profiled and clean discontinuity artificially prepared from natural sandstone. Displacement transducers were used to measure the normal and shear displacements. A series of strain gauges glued on the sides of the upper block provided information on the change of the stress field occurring close to the discontinuities whilst shear displacement increased, and these changes were then compared with the behaviour of the profiled surface. The results of the laboratory tests, the sequence of photographs taken for most of them, and the results conducted with a sample of similar shape made from the same rock material and tested in a rotary shear machine, allowed several stages and mechanisms of failure to be defined: static friction and mobilization of initial shear stiffness; mobilization of sliding; mobilization of brittle fracture; post-peak failure of the teeth; descent of the teeth; gliding and ploughing; commencement of second cycle of shearing. In many respects these stages are similar to those occurring between sliding surfaces of metal and suggest that the analyses developed in tribology may be relevant to the development of constitutive models for predicting the hydromechanical coupled behaviour of a discontinuity with shear displacement. Such models will have to consider these different stages of shear, because the original discontinuity changes its geometry with displacement and is filled with gouge which changes its grain size with displacement.

Effects of plants on the accumulation of Zn, Pb, Cu and Cd in sediments of the Tagus estuary salt marshes, Portugal.

Abstract: Vertical distribution of Zn, Pb, Cu and Cd concentrations, organic matter content, pH and Eh in sediment between roots and in intertidal non-vegetated sediments from two sites on the Tagus salt marshes are compared. A sub-surface metal enrichment was observed at depths where roots are abundant. Increase of the non-available metals to plants in the salt marsh sediments indicates different sink capacities for metals in the Tagus salt marshes: Pb ≫ Zn > Cu > Cd.

Material properties, project design rules and performances of single and dual-axis a-Si:H large area position sensitive detectors

Abstract: We have developed large area (up to 80mm×80mm) Thin Film Position Sensitive Detectors (TFPSD) based on hydrogenated amorphous silicon (a-Si:H). Although crystalline silicon PSDs have been realized and applied to optical systems, their detection area is small (less than 10mm×10mm), which implies the need of optical magnification systems for supporting their field of applications towards large area inspection systems, which does not happen by using a-Si:H devices. The key factors for the TFPSDs resolution are the thickness uniformity of the constituting layers, the geometry and the position of the contacts. In this paper we present data on single and dual-axis rectangular TFPSDs correlating, their performances with the different underlying lateral effects. For the single axis-detector, with two opposite extended contacts, the output photocurrent difference to sum ratio is a linear function of the position of a narrow incident light beam, even for low illumination levels (below 20 lux). For the dual-axis detector with extended contacts, at all four sides (except for small gaps at the vertices due to edge effects) an almost linear relation has been found between the incident light spot position along both axis and the corresponding output photocurrents.

Hoops and their implicational reducts (abstract)

Abstract: 1. Preliminaries. A structure A = 〈A, ·, 1,≤〉 is a partially ordered monoid if 〈A, ·, 1〉 is a monoid, ≤ is a partial order on A, and for all x, y, z ∈ A, if x ≤ y, then x · z ≤ y · z and z · x ≤ z · y. A is integral if x ≤ 1, for all x ∈ A. Finally, A is residuated if for all x, y ∈ A the set {z : z · x ≤ y} contains a largest element, called the residual of x relative to y, and denoted by x → y. A partially ordered commutative, residuated and integral monoid 〈A, ·, 1,≤〉 can be treated as an algebra 〈A, ·,→, 1〉, since the partial order ≤ can be recovered via x ≤ y iff x → y = 1. We will refer to the algebras thus obtained by the acronym pocrim. The class M of all pocrims is a quasivariety definable by:

Control of Reproduction of the Shrimp Penaeus kerathurus Held in Captivity

Abstract: The control of the reproductive cycle of the shrimp Penueus kerathurus, held in captivity from early juvenile stages without eyestalk ablation, was attempted in two sets of experimental conditions during two successive years. In the first set of experiments, light regime, sex ratio and presence of sand substratum were assessed in relation to ovarian maturation and successful mating. Examination of thelyca obtained from cast exoskeletons showed that 40–82% successful mating had taken place. A sex ratio of 2:l female/male gave a higher percentage of fertilized females than a ratio of 1:l. The induction to full ovarian development was observed for every treatment except the tank without sand substrate, where no mated females were observed. In the second set of experiments, the role of diet, in particular the role played by a polychaete Nereis diversicolor, was assessed as a nutritive stimulator to induce maturation and spawning. With the same purpose, a parallel experiment in smaller aquaria was also conducted to assess the relative importance of mussel, squid, ragworm and crab and four composite diets consisting of two or three of the said food organisms. Nereis worm seemed to have a determinant role on the induction of shrimp ovarian maturation and spawning. The possible role of its fatty acids on shrimp reproduction is discussed. The experimental results suggest the possibility for extended control of the reproductive cycle of P. kerathurus in a controlled environment provided that proper food is available.

Preliminary note on the distribution and ecology of Culicoides imicola in Portugal.

Abstract: Data on Culicoides imicola were obtained during studies carried out during the recent outbreak of African horse sickness in Portugal. The previous most northerly published record of C. imicola in Portugal was 38 degrees 40'N (Pegoes). In the present work the geographical distribution of this species is extended to the parallel of 41 degrees 17'N. We have also confirmed the continuous presence of adult C. imicola in Southern Portugal (Alentejo and Algarve) throughout the year. In the laboratory we obtained this species from a sample of cattle faeces and from another of soil contaminated with animal excreta. In relation to host association 57.37% of C. imicola were trapped in the vicinity of pigsties. Finally, we collected 11,463 Culicoides of which 12.47% were C. imicola.

The Thermal Conductivity of Toluene and Water

Abstract: A new instrument is presented to measure the thermal conductivity of polar and electrically conducting liquids based on the transient coated hot-wire method. The performance of the apparatus has been assessed with toluene and water, which are recognized as standard reference materials for nonpolar and polar fluids, respectively. New results are reported fort the thermal conductivity of these liquids between 298 and 370 K and at pressures slightly above the saturation. The results show that the instrument is capable of an accuracy better than ±0.5%, while the precision and reproducibility are better than ±0.3%.

Subunit composition, crystallization and preliminary crystallographic studies of the Desulfovibrio gigas aldehyde oxidoreductase containing molybdenum and [2Fe-2S] centers

Abstract: The Desulfovibrio gigas aldehyde oxidoreductase contains molybdenum bound to a pterin cofactor and [2Fe-2S] centers. The enzyme was characterized by SDS/PAGE, gel-filtration and analytical ultracentrifugation experiments. It was crystallized at 4 degrees C, pH 7.2, using isopropanol and MgCl2 as precipitants. The crystals diffract beyond 0.3-nm (3.0-A) resolution and belong to space group P6(1)22 or its enantiomorph, with cell dimensions a = b = 14.45 nm and c = 16.32 nm. There is one subunit/asymmetric unit which gives a packing density of 2.5 x 10(-3) nm3/Da (2.5 A3/Da), consistent with the experimental crystal density, rho = 1.14 g/cm3. One dimer (approximately 2 x 100 kDa) is located on a crystallographic twofold axis.

Growth and nutrition of carob plants as affected by nitrogen sources

Abstract: Carob seedlings were grown at different proportions of nitrate NO3) and ammonium (NH4) supplied at a concentration of 3.5 mM nitrogen (N). The effect of five different combinations of NH4‐ and NO3‐N on growth rates, N and carbon (C) content of the plant and dry matter distribution were studied. The presence of NH4 in the nutrient solution enhanced plant growth rate, although net photosynthesis rates per unit area were lower than in NO3‐fed plants. Nitrate‐fed plants exhibited the highest rates of carbon dioxide (CO2) assimilation, water use, and N‐use efficiency. Nevertheless, plants growing only with NO3 were more fragile and susceptible to fungus infections. The role of cotyledons on the regulation of the supply of photosynthates from leaves to roots in the different combinations of NO3 and NH4 is described. Results suggest that the growth rates of carob may be improved using suitable combinations of NO3‐ and NH4‐N. Equal concentrations of these two N sources allows good development and adequat...

Sandstone weathering processes damaging prehistoric rock paintings at the Albarracin Cultural Park, NE Spain

Abstract: The rock paintings in cliff-foot caves of the Albarracin Cultural Park are known as some of the most important evidences of the Levantine prehistoric art of Spain (8000–3000 BP) The paintings are on sandstone (Buntsandstein facies) of Triasic age, which may develop intense weathering The analysis of the variables controlling the weathering indicate that salt and wetting-drying weathering are responsible for granular disintegration and flaking, which lead to rock painting deterioration