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51.
Variability of mangrove ecosystems along the Brazilian coast 总被引:1,自引:0,他引:1
Yara Schaeffer-Novelli Gilberto Cintrón-Molero Raquel Rothleder Adaime Tânia Maria de Camargo 《Estuaries and Coasts》1990,13(2):204-218
Brazilian mangroves extend from 4°30′N to 28°30′S, varying greatly in growth form, species distribution patterns, and stand structure, in spite of a limited floristic diversity. We divided the Brazilian coastline into eight units, within which physiographic and climatic conditions are relatively uniform, and described mangrove occurrence, species distribution and structural attributes characteristic of each segment. In general, greates mangrove coverage and greatest forest stature are found in areas with a large surplus of rainfall over potential evapotranspiration and macrotidal regimes. An exception was the segment containing the mouth of the Amazon river, where freshwater systems dominate over brackish or marine associations. We believe that the variability in species associations and the dominance of each in a given environment is predominantly determined by the characteristics of the landforms that can be colonized by each species in a given region. The type, size, and frequency of occurrence of available landforms is a function of the particular mix of fluvial, tidal, and wave energies found in a region. Different species colonize these sites depending on their adaptations and edaphic preferences. Climate affects mangrove colonization and growth. We suggest that Brazilian mangroves play a minor role in modifying the geomorphic setting; the spatial arrangement of the various forest types is a response to the underlying topography and edaphic conditions, and to the constraints imposed by climatic and hydrologic factors. The spatial arrangement of species does not necessarily show successional processes, but may be the result of direct and differential colonization on appropriate substrates. 相似文献
52.
Jean-Pierre Pagès Jean-Pierre Frangi Pierre Durand Claude Estournel Aimé Druilhet 《Boundary-Layer Meteorology》1988,43(1-2):183-203
This paper presents data concerning the energy budget in the surface layer in the Sahel region (a semi-desert area). The results
are drawn from a measurement campaign made in the Niamey region in the Niger, in April–May 1984 (the Yantala Campaign). The
sensible heat flux is computed with the profile method, the ground heat flux is deduced from measurement of the temperature
field, and the radiative net flux is measured directly with a balancemeter. The latent heat flux, which is deduced from the
energy budget balance is very weak and within the accuracy limit of the method. The diurnal variation of the net flux is symmetrical,
with a maximum at noon. On the other hand, the sensible heat flux variation is asymmetrical, with an afternoon decrease much
slower than the morning increase. After 3.30 pm, it becomes higher than the net flux. This is compensated for by the sign
change of the ground heat flux, whose maximum is found in the morning at 11 am.
The second part of this paper shows the importance of one term in the surface-layer energy budget: the long-wave radiative
divergence between the ground and the top of the surface layer in high superadiabatic conditions. We show, with a radiative
model on the one hand and direct measurement of the radiative divergence on the other hand, that this term reaches several
tens of W m-2 in the superadiabatic conditions found in the Sahel region.
相似文献
53.
A. Piqué G. Bossière J. -P. Bouillin A. Chalouan Ch. Hoepffner 《International Journal of Earth Sciences》1993,82(3):432-439
Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic.
Correspondence to: A. Piqué 相似文献
54.
Dominique Bonnin Georges Calas Hélène Suquet Henri Pezerat 《Physics and Chemistry of Minerals》1985,12(1):55-64
New data on the structure of Garfield nontronite have been produced by the use of different spectroscopic techniques: Mössbauer spectroscopy, optical spectroscopy, X-ray absorption edges and EXAFS and NMR. The tetrahedral iron content is found to be no higher than 1 percent. All iron atoms belong to the octahedral sheet excluding the possibility of the presence of non crystallized phases. Some ambiguities remain about the coherence of the octahedral sheet because of the presence of two doublets in the Mössbauer spectrum and at least two lineshapes in NMR spectra of OH which correspond to different environments. 相似文献
55.
56.
A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations. 相似文献
57.
J Jouzel R Vaikmae J R Petit M Martin Y Duclos M Stievenard C Lorius M Toots M A Mélières L H Burckle N I Barkov V M Kotlyakov 《Climate Dynamics》1995,11(3):151-161
The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record. 相似文献
58.
Hélène Dupuis Alain Weill Kristina Katsaros Peter K. Taylor 《Annales Geophysicae》1995,13(10):1065-1074
Heat flux estimates obtained using the inertial dissipation method, and the profile method applied to radiosonde soundings, are assessed with emphasis on the parameterization of the roughness lengths for temperature and specific humidity. Results from the inertial dissipation method show a decrease of the temperature and humidity roughness lengths for increasing neutral wind speed, in agreement with previous studies. The sensible heat flux estimates were obtained using the temperature estimated from the speed of sound determined by a sonic anemometer. This method seems very attractive for estimating heat fluxes over the ocean. However allowance must be made in the inertial dissipation method for non-neutral stratification. The SOFIA/ASTEX and SEMAPHORE results show that, in unstable stratification, a term due to the transport terms in the turbulent kinetic energy budget, has to be included in order to determine the friction velocity with better accuracy. Using the profile method with radiosonde data, the roughness length values showed large scatter. A reliable estimate of the temperature roughness length could not be obtained. The humidity roughness length values were compatible with those found using the inertial dissipation method. 相似文献
59.
60.
Since Nb and Zr are only little fractionated during magmatic processes, the 92Nb-92Zr relative chronometer has the potential of dating the formation of the planetary bodies through their differentiation. Thus, we have analyzed the isotopic composition of zirconium in lunar, meteoritic and old and recent terrestrial samples.No isotopic variation has been found. However, the ratio of 3.8 Ga. old zircons from the Isua acid conglomerate is on the lower limit of the range of the standard measurements. If considered an anomaly, it would correspond to a +1.5 × 10?492Zr relative deficiency or to a 3.0 × 10?494Zr relative excess.Our data constrain the isotopic ratio at the time of formation of the solar system to be less than 0.007, so that the maximum sensitivity of the 92Nb-92Zr relative chronometer for the formation of planetary bodies is around 107 a. A discussion of some possible nuclear reactions indicates that zirconium isotopic variations in zircons are not easily produced, and that the 92Nb and 94Nb natural activities (Apt et al., 1974) cannot be explained by any single one of the processes proposed so far. 相似文献