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排序方式: 共有192条查询结果,搜索用时 15 毫秒
91.
Niels Otto Jensen 《Boundary-Layer Meteorology》1978,15(1):95-108
Turbulence and mean flow variables under unstable conditions are examined with special emphasis on the consequences of roughness and surface elevation change. An interpolation formula for
w
2, between neutral and free convection, is shown to bring order to the data. The spectral distribution of vertical wind variance is found to be in good agreement with results over horizontal homogeneous terrain, both with respect to form and position. In particular, the length scale m corresponding to the maximum of nS
w(n) is unchanged. Another turbulent length scale, (k/)z, is found to be substantially reduced in the disturbed zone of the internal boundary layer. To a first approximation, the flow-acceleration effect on the non-dimensional wind shear can be separated from the diabatic effect. 相似文献
92.
An instrusive dacite and a salic pumice, emplaced late in the evolution of the Miocene (c. 10 m.y.) Króksfjördur volcano, NW Iceland, contain a varied assemblage of xenolithic metaigneous rocks. Mineral and rock chemistry shows that the dacite is very similar to calc-alkaline salic rocks from the SW Pacific. It contains phenocrystic plagioclase, quartz, pyroxene, cummingtonite, hornblende, biotite, two oxides, apatite and zircon in a rhyolitic glass. The rock equilibrated at 700 to 750°C. P ~ 1.6 Kbar and PH2O ~ 1 Kbar. The xenoliths are layered gabbros, granophyres and various fine-grained hornfelses and show that the dacite magma was residing within a gabbro intrusion capped by granophyre prior to the eruption. The hornfelses are amphibole-plagioclase, amphibole-pyroxene-plagioclase and pyroxene-plagioclase rocks formed during high-temperature metamorphism of basic dykes cutting the gabbro intrusion. The gabbros and hornfelses mostly record higher metamorphic temperatures (850–940°C) than the dacite, and indicate that they were equilibrated during the ascent of a magma body into a hydrous high-level region within the volcano. During a following thermal decline, the hydrated magma cooled to form the first cummingtonite-bearing low-T magma to be recorded from the ocean ridge systems. 相似文献
93.
Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, Central Italy 总被引:2,自引:0,他引:2
This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both87Sr/86Sr andδ18O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ18O and87Sr/86Sr are thought to have mixed with mantle of “normal”δ18O and87Sr/86Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province. 相似文献
94.
Summary The design of a small pyranometer for measurements in field crops is described. Results of a series of laboratory tests with the instrument are given and discussed in relation to the use of the instrument in field crops.
With 8 Figures 相似文献
Zusammenfassung Es wird eine Konstruktion eines kleinen Pyranometers für Messungen in Getreidefeldern beschrieben. Ergebnisse einer Serie von Laboratoriumsprüfungen des Instruments werden wiedergegeben und hinsichtlich der Eignung des Geräts für die erwähnten Feldmessungen diskutiert.
Résumé On décrit ici la construction d'un petit pyranomètre destiné à la mesure du rayonnement dans des champs de céréales. On indique les résultats de séries d'observations comparatives faites en laboratoire au moyen du dit instrument. On les discute enfin en tenant compte des possibilités d'utiliser ce pyranomètre pour des mesures dans les champs.
With 8 Figures 相似文献
95.
This project examined concentrations, composition and turnover neutral sugars in the Delaware estuary to gain insights into dissolved inorganic nitrogen (DIN) use by heterotrophic bacteria and into the lability and diagenetic state of dissolved organic material (DOM) during passage through the estuary. Dissolved free monosaccharides were not measurable (<5 nM) in the estuary whereas concentrations of dissolved combined neutral sugars (DCNS) were high, much higher than observed in oceanic waters. DCNS made up a similar fraction of dissolved organic carbon (DOC) as in the oceanic waters examined to date, and the monosaccharide composition of the DCNS pool was similar to that of oceanic waters. The composition did not vary substantially within the estuary or seasonally, but it did vary among three size fractions of the organic matter pool. Glucose was enriched in the low molecular weight fraction of DOC and in particulate material, whereas the high molecular weight DOC fraction was slightly depleted in glucose. Depletion experiments indicated that DCNS is not used extensively on the day time scale in the Delaware estuary, although freshly-produced polysaccharides may still be important carbon sources for heterotrophic bacteria. The very low concentrations of free monosaccharides in the Delaware estuary help to explain why DIN use by heterotrophic bacteria is relatively low in this estuary. Although DOC-DIN interactions in the Delaware apparently differ from oceanic waters, the portion of DOM traced by DCNS, which is thought to be the labile fraction, appears to be similar to that of oceanic DOM, suggesting that organic material in the estuary is degraded extensively before being exported to the coastal ocean. 相似文献
96.
øystein Hov Asgeir Sorteberg Norbert Schmidbauer Sverre Solberg Frode Stordal David Simpson Anne Lindskog Hans Areskoug Pedro Oyola Heikki Lättilä Niels Z. Heidam 《Journal of Atmospheric Chemistry》1997,28(1-3):173-193
Observations and model calculations of the concentration of hydrocarbonsat five Scandinavian rural sites during March–June 1993are reported.Decreasing concentrations from March to June are observedat all sites. The highest concentrations of hydrocarbons were found in air massescoming in from the southwest to southeast, indicating that long rangetransport fromcontinental Europe and the U.K. is important in pollution episodes. An episode of elevated concentrations of hydrocarbons observed at three of the sites in the middle of Marchis described and discussed in relation to the synoptic situation and thepresenceof other chemical compounds (NO2, PAN, total nitrate andozone).A Lagrangian numerical model is used to calculate the concentrations of theindividual hydrocarbons at the fivesites and comparison with observations is made.The calculated concentrations for nonmethane hydrocarbons with quite longchemicallifetimes agree well with the observations.For the sum of observed and calculated hydrocarbons the correlationcoefficientsare in the range of 0.65–0.88 for the five sitesand the ratio between calculated and measured concentrations was0.72–0.97, indicating thatthe European VOC emission inventory is quite well estimated. 相似文献
97.
Analyses of environmental isotopes (18O, 2H, and 87Sr/86Sr) are applied to groundwater studies with emphasis on saline groundwater in aquifers in the Keta Basin, Ghana. The 87Sr/86Sr ratios of groundwater and surface water of the Keta Basin primarily reflect the geology and the mineralogical composition
of the formations in the catchments and recharge areas. The isotopic compositions of 18O and 2H of deep groundwater have small variations and plot close to the global meteoric water line. Shallow groundwater and surface
water have considerably larger variations in isotopic compositions, which reflect evaporation and preservation of seasonal
fluctuations. A significant excess of chloride in shallow groundwater in comparison to the calculated evaporation loss is
the result of a combination of evaporation and marine sources. Groundwaters from deep wells and dug wells in near-coastal
aquifers are characterized by relatively high chloride contents, and the significance of marine influence is evidenced by
well-defined mixing lines for strontium isotopes, and hydrogen and oxygen stable isotopes, with isotopic compositions of seawater
as one end member. The results derived from environmental isotopes in this study demonstrate that a multi-isotope approach
is a useful tool to identify the origin and sources of saline groundwater.
Electronic Publication 相似文献
98.
Niels Crosley Munksgaard 《Contributions to Mineralogy and Petrology》1984,87(4):351-358
O- and Sr-isotope data are reported for cordieritebearing dacites and andesites forming part of the Neogene volcanic province of SE Spain. The almandine-bearing biotite-cordierite-labradorite dacite from the Cerro del Hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail. A syngenetic derivation of the Hoyazo dacite magma and part of its inclusions (interpreted as restite) by means of anatexis of (semi-)pelitic rocks has previously been proposed.
18O values and 87Sr/86Sr ratios, thought to be closely representative of the original magmas, vary from +12.2 to +15.6 and 0.7095 to 0.7171, respectively. The metamorphic rock inclusions have
18O values (+13.0-+16.2) comparable to the range for the volcanics. These results support an anatectic origin for the entire suite of cordierite-bearing volcanics. The inclusions of basic igneous rocks in the Hoyazo dacite have, in comparison with the dacite, lower
18O values (+11.1-+13.1) and equal to lower 87Sr/86Sr ratios (0.7081–0.7112), confirming an independent origin.The six analysed samples of the Hoyazo dacite show a strong linear correlation of 87Sr/86Sr versus 87Rb/86Sr which, if interpreted as an isochron, yields an age of 210±17 Ma. Similar linear arrays for samples from the Mazarrón and Mar Menor areas yield nearly concordant ages. Samples for the Vera area define a 535±22 Ma line. These linear correlations may be interpreted either as (1) mixing lines, in which case the indicated ages have no geological meaning, or (2) true isochrons dating pre-eruptional events of the parent (meta-)sediments. 相似文献
99.
Michael R. Talbot Niels Bo Jensen Tine Lærdal Maria Letizia Filippi 《Journal of Paleolimnology》2006,35(3):467-489
Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression
that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand
with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content
(TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression
and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the
change in HI. Over the same time interval in all three lakes, δ13COM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ15N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline
waterbody, while high δ15N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong
impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression
upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency
to stable chemical stratification as the water column deepened. In the case of δ13C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic
carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by
mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent
in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading.
In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed
geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes.
IDEAL Contribution No. 152. 相似文献
100.
The most relevant controls on the water quality within the Cretaceous-Eocene limestone aquifer of the Keta Basin, Ghana, and the coastal sedimentary basin of Togo were assessed using Q-mode hierarchical cluster analysis (HCA) and mass-balance modelling. The pattern recognition technique of HCA was employed for partitioning hydrochemical data from a total of 65 surface and borehole samples from the study area into water groups. A spatial plot of the water groups consisting of samples from the limestone aquifer shows that the vast majority of samples belonging to the same group are located in close proximity to one another, suggesting the same processes and/or flow paths in the limestone aquifer system. Geochemical reaction models of selected water groups were constructed using PHREEQC-2. The hydrochemical compositions of the water groups and the mass-balance calculations indicate that the dominant processes and reactions responsible for the hydrochemical evolution in the system are: (1) carbonate equilibria, (2) silicate weathering reactions, (3) limited mixing with saline water, and (4) ion exchange. The combined use of HCA and mass-balance modelling has shown to be a useful approach in interpreting groundwater hydrochemistry in an area where large uncertainties exist in the understanding of the groundwater flow system. 相似文献