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1.
This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies. 相似文献
2.
Doline probability map using logistic regression and GIS technology in the central Ebro Basin (Spain) 总被引:3,自引:0,他引:3
In the surroundings of Zaragoza, karstification processes are especially intense in covered karst areas where fluvial terraces
lie directly on Tertiary evaporites. Since the beginning of Quaternary, these processes have lead to the development of collapse
and subsidence dolines with a wide range of sizes, which have significant economic impacts. To reduce economic impact and
increase safety, a regional analysis of this phenomenon is needed for spatial management. Therefore, a probability map of
dolines was developed using logistic regression and geographic information system (GIS) techniques. This paper covers the
selection of input data, manipulation of data using the GIS technology, and the use of logistic regression to generate a doline
probability map. The primary variable in the doline development in this area is geomorphology, represented by the location
of endorheic areas and different terrace levels. Secondary variables are the presence of irrigation and the water table gradient. 相似文献
3.
4.
E. Cocheret de la Morinire B. J. A. Pollux I. Nagelkerken G. van der Velde 《Estuarine, Coastal and Shelf Science》2003,57(5-6):1079-1089
The spatial size distribution of grunts and snappers have previously indicated the separation of juveniles in nursery habitats from the adults on the coral reef. This implies life cycle migrations from nursery habitats (such as seagrass beds and mangroves) to the coral reef. If diet shifts are related to such migrations, then the diets of these fish must change before or around the fish size at which such migrations take place. A wide size range of juveniles of two grunt species (Haemulon sciurus and Haemulon flavolineatum) and of two snapper species (Lutjanus apodus and Ocyurus chrysurus) were caught in seagrass beds and mangroves, and their gut contents identified and quantified. Regression analysis between fish size and dietary importance of small crustaceans showed a negative relationship in all four species. Positive relations were found for H. sciurus, L. apodus and O. chrysurus between fish length and the dietary importance of decapods, and for L. apodusand O. chrysurus between fish length and prey fish importance. Critical changes in the fish diets with fish size were examined by application of a Canonical Correspondence Analysis (CCA). The CCA yielded three clusters of size-classes of fishes with similar diets, and application of a Mantel test showed that each of these clusters had significantly different diets, and that each cluster diet was significantly specialised. The size at which a fish species ‘switched’ from one cluster to another was compared with size-at-maturity data and with the typical size at which these species migrate from the nursery habitats to the coral reef. H. sciurus and H. flavolineatum may be prompted to migrate from the nursery habitats to coral reef habitats because of dietary changes, or because of the development of the gonads. For L. apodus and O. chrysurus, a dietary changeover forms a more likely explanation for nursery-to-reef migrations than does sexual maturation because these species reach maturity at sizes much larger than the maximum size of individuals found in nursery habitats. Although other factors may theoretically initiate or promote the migration patterns, the results of this study indicate that ontogenetic dietary changes may crucially influence the nursery-to-coral reef migrations of these reef fish species. 相似文献
5.
The gastropod limpet, Cellana grata, is common on moderately exposed to exposed Hong Kong rocky shores. On the basis of the existing literature on trace metal contamination in Hong Kong, four sites were identified, representative of highly polluted (Aberdeen and Lei Yue Mun) and clean (Cape d'Aguilar and Clear Water Bay) conditions. Limpets from the two polluted sites showed similar, but significantly higher body concentrations of copper than limpets from the two clean sites, which were also similar. Non-invasive measurements of cardiac activity of limpets from the four sites showed between site differences in baseline heart rates under standard seawater, irrespective of pollution level. When acutely exposed to water borne copper (2 h, 0.5+/-0.06 SD ppm), however, limpets from clean sites showed a significantly higher increase in inter-beating time (bradycardia) than those from polluted sites. These results highlight the potential use of cardiac activity to assess the exposure of natural populations of limpets to trace metal pollution. 相似文献
6.
A. A. De Almeida 《Earth, Moon, and Planets》1992,56(1):61-74
Emission fluxes of CN, C2 and C3 carbon-bearing molecular species observed in the coma of comets Bennett (1969i 1970II), West (1975n 1976VI), P Halley (1982i), Hartley-Good (19851) and Bradfield (1987s) are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C2 and C3 production rates on the heliocentric distance and the possible correlations among these radicals is studied and briefly discussed. 相似文献
7.
B. R. De 《Astrophysics and Space Science》1996,239(1):25-33
Maxwell's displacement current equation is interpreted in the light of recent work to show that static magnetic field in free space should have a colocated and contemporaneous mass that is neither embodied in, nor can be anticipated from, the mass-energy relation. Thus magnetostatic field in the universe represents an invisible mass. Some consequences are discussed. 相似文献
8.
H. A. R. De Bruin W. Kohsiek B. J. J. M. Van Den Hurk 《Boundary-Layer Meteorology》1993,63(3):231-257
A set of micro-meteorological data collected over a horizontal, uniform terrain (the plain of La Crau, France) in June 1987 is analysed. Conditions were predominantly sunny and arid, while due to the Mistral the wind speed could exceed 10 m/s. Verification of several methods to evaluate surface fluxes of heat, momentum and water vapour from the standard deviation of temperature, wind and specific humidity is presented. Also, a similar approach using the structure parameter of temperature is considered. These methods are all based on Monin-Obukhov (M-O) similarity theory. It is found that the standard deviation of temperature, vertical and horizontal wind speed as well as the structure parameter for temperature behave according to M-O similarity. It is shown that the sensible heat flux and friction velocity can be determined from a fast response thermometer and a cup anemometer. Also, it appears that the analytic solution of the set of governing equations as derived by the first author yields good results. M-O theory does not appear to work for the standard deviation of specific humidity. This may be due to the relative importance of large eddies. 相似文献
9.
De Lange N 《Erdkunde》1993,47(1):61-74
The reduction of the rate of growth of the Brazilian population from 3% in the 1960s to 2% by 1993 resulted from a significant reduction of fertility. According to the 1986 national maternal-child health and family planning survey (PNSMIPF) results, 43.3% of women were using some type of contraception: 65.6% of married or cohabiting women. Women relied most on sterilization (17.2%), followed by oral contraceptives (OCs) (17%), the rhythm method (2.8%), the condom (1.1%), and others (5.2%). Only in the south was OC use more prevalent (28.5%) than sterilization (12.2%). The proportion of sterilization reached 27.8% in the north/center-east urban regions. In urban areas sterilization averaged 18.7% vs. 12.7% in rural areas. 26.9% of currently married women in the age range of 15-44 years had been sterilized. A 1990 international estimation indicated that the proportion of sterilization amounted to 36.9% in China, 30.95 in India, 29.7% in Brazil, 47.6% in South Korea, 30.4% in Thailand, and 36.5% in the Dominican Republic. The 1986 PNSMIPF survey also indicated that sterilized women were better informed than other women about contraception. 75% of the former had used OCs, 5% had used the diaphragm, 3% the IUD, 30% the condom, and 35% coitus interruptus at one time or other. The sterilized women had an average of 1.7 more children than those who were using other methods of contraception. 48.6% of women were sterilized before the age of 30, when they had had an average of 3.6 children. 66.6% of the sterilized women who had given birth before the age of 20 had 4 or more live births as compared to 5.9% of sterilized women aged 30 with the same number of children. 47% of women without any schooling were using some kind of contraception vs. 77% of those who had completed secondary school. A multivariate analysis also showed that the age of the mother, the number of live births, the educational status of the mother, and family income were positively correlated with sterilization. 相似文献
10.
Models of giant gaseous protoplanets calculated by DeCampli and Cameron (1979) indicate that iron and probably other minerals in the interior of a planet would be in the liquid state during part of the protoplanet evolution. Liquid drops in a protoplanet would grow by coalescence much as cloud drops in the Earth's atmosphere grow to rain drops. We have modeled this process by using the stochastic collection equation (Slattery, 1978) for various initial conditions. In all of the cases considered, the growth time (to centimeter-sized droplets) is much shorter than the time, as estimated by detailed evolutionary calculations, that the drops are in the liquid state. Brownian collection is effective in quickly coalescing tiny liquid droplets to an average radius of about 0.005 cm with very few drops remaining with radii less than 0.001 cm. For radii larger than 0.005 cm gravitational collection is dominant. Since the particles are rapidly swept from interstellar grain sizes to much larger sizes, the opacity in the cloud layer is expected to drop sharply following melting of the grains. 相似文献