Mário E. C. Vieira 《Estuaries and Coasts》2000,23(2):199-207

Current meter data were acquired in Long Island Sound over a period of about 6 mo in 1988 at six different transects. The averages of the low-passed residuals represent the contributions from tidal steering and density forcing. It is found that the long-term residual circulation starts out with a classical estuarine pattern at the Race. The flows into and out of the Sound are vertically partitioned by Long Sand Shoal; they gradually revert back to classical vertically layered estuarine circulation as the Central Basin is traversed. Further west, the bottom oceanic water becomes a westward flowing swift jet close to the Connecticut shore, while the East River water is constrained to flow eastward along the Long Island coast. Counterclockwise gyres, identified in the Eastern Narrows and Western Basin, may increase residence times of polluted East River water in the western Long Island Sound.  相似文献   

    

D.O. Olago  F.A. Street-Perrott  R.A. Perrott  M. Ivanovich  D.D. Harkness 《Journal of African Earth Sciences》2000,30(4):1113

In this paper, a sequence of five Late Quaternary tephras occurring as discrete, well-preserved horizons in lake sediments on the northeastern flank of Mount Kenya are characterised and their ages determined by a combination of high-resolution indirect radiocarbon dating and direct dating. The grain size characteristics suggest that the tephras are of fairly local origin. These Na---K-rich alkali pyroclasts with a trachytic chemical composition have a highly correlated chemistry and mineralogy, suggesting that they were probably derived from the same genetic series and possibly erupted from a single source vent. Morphological differences are attributed to the peculiar characteristics of each eruption episode. The magma source was probably a small, highly differentiated magma chamber following the olivine basalt-trachyandesite-trachyte-phonolite series, which broadly reflects the Quaternary rock suite of Mount Kenya.  相似文献   

  总被引：1，自引：0，他引：1  

Michael A. Phedorin  Evgeny L. Goldberg  Vladislav A. Bobrov  Oleg M. Khlystov  Michael A. Grachev 《Geostandards and Geoanalytical Research》2000,24(2):217-226

A new technique for the determination of uranium and thorium in lacustrine sediments is based on non-destructive synchrotron radiation X-ray fluorescence analysis (SR-XRF) of sediment samples using monochromatic beams of different energies as excitation sources, with the recording and subsequent iterative processing of the X-ray fluorescence spectra. The technique has a multielement capability and enables fast, simultaneous analysis of a few tens of elements. The proposed SR-XRF technique was tested against INAA and ICP-MS methods and showed a number of advantages with a generally good correlation of results by the three methods. Uranium and Th profiles have been measured at a time resolution of 2 kyr in a drill core (BDP-96) from Lake Baikal bottom sediments deposited between 780 and 40 kyr B P. During this time span, peaks in the U and U/Th concentration marked warm climates. Oscillations of U and U/Th in Baikal bottom sediments during the Brunhes chron reflect climate-induced global change in the volume of polar ice, controlled by long-term cyclicity of the Earths orbital parameters. The response of these warm periods is similar to that observed earlier in shorter cores that span the two last interglacials (220-0 kyr BP).  相似文献   

    

Margaret M. Dekshenieks  Eileen E. Hofmann  John M. Klinck  Eric N. Powell 《Estuaries and Coasts》2000,23(5):593-610

Three models are combined to investigate the effects of changes in environmental conditions on the population structure of the Eastern oyster,Crassostrea virginica. The first model, a time-dependent model of the oyster population as described in Powell et al. (1992, 1994, 1995a,b, 1996, 1997) and Hofmann et al. (1992, 1994, 1995), tracks the distribution, development, spawning, and mortality of sessile oyster populations. The second model, a time-dependent larval growth model as described in Dekshenieks et al. (1993), simulates larval growth and mortality. The final model, a finite element hydrodynamic model, simulates the circulation in Galveston Bay, Texas. The coupled post-settlement-larval model (the oyster model) runs within the finite element grid at locations that include known oyster reef habitats. The oyster model was first forced with 5 yr of mean environmental conditions to provide a reference simulation for Galveston Bay. Additional simulations considered the effects of long-term increases and decreases in freshwater inflow and temperature, as well as decreases in food concentration and total seston on Galveston Bay oyster populations. In general, the simulations show that salinity is the primary environmental factor controling the spatial extent of oyster distribution within the estuary. Results also indicate a need to consider all environmental factors when attempting to predict the response of oyster populations; it is the superposition of a combination of these factors that determines the state of the population. The results from this study allow predictions to be made concerning the effects of environmental change on the status of oyster populations, both within Galveston Bay and within other estuarine systems supporting oyster populations.  相似文献   

    

L. D. Akulenko  S. A. Kumakshev  Yu. G. Markov  L. V. Rykhlova 《Astronomy Reports》2002,46(1):74-82

Refined analytical expressions for the frequencies corresponding to the Chandler motion of the pole and the diurnal rotation of the deformable Earth are derived. Numerical estimates of the period and amplitude of the polar oscillations are presented. The trajectory of the Chandler polar motion derived via numerical modeling is in qualitative and quantitative agreement with experimental data from the International Earth Rotation Service (IERS). An evolutionary model describing slow variations in the Earth’s rotation parameters under the action of the dissipative moments of the tidal gravitational forces on time scales considerably longer than the precession period of the Earth’s axis is constructed. The axis of the Earth’s figure tends to approach the angular momentum vector of the proper rotation.  相似文献   

  总被引：3，自引：1，他引：2  

T. Grove  S. Parman  S. Bowring  R. Price  M. Baker 《Contributions to Mineralogy and Petrology》2002,142(4):375-396

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D. Gournis  M. A. Karakassides  D. Petridis 《Physics and Chemistry of Minerals》2002,29(2):155-158

 The formation of superoxide and hydroxyl radicals at the surface of smectite clays due to oxygen reduction is demonstrated by electron paramagnetic resonance spectroscopy. The yield of hydroxyl radicals is mainly a function of particle size of the clays and depends, to a lesser extent, on the clay lattice iron. Synthetic laponite clay with small platelet size (∼20 nm) and without lattice iron is leading in the formation of hydroxyl radicals followed by montmorillonite (∼200 nm). Fluorohectorite (∼2000 nm) was inactive to hydroxyl radical formation by oxygen reduction. Received: 20 January 2001 / Accepted: 7 August 2001  相似文献   

    

F. Ouchterlony  M. Olsson  I. Bergqvist 《Fragblast: International Journal for Blasting and Fragmentation》2002,6(2):235-261

A Swedish table of blast damage depths has, in one form or other, been in use since the late-1970s. Its history and the underlying theory are described. New experimental and theoretical findings that point out a number of shortcomings in the table and the underlying theory are presented and discussed. A revised version of the blast damage table was recently introduced in conjunction with new but incomplete recommendations for cautious perimeter blasting. The new table leaves the difficult task of taking into account factory like decoupling, water in the borehole, the rock properties, type of initiation, charge length and the actual bit diameter to the user. This paper discusses different ways of doing this, based on the experimental findings and a recent formula for the prediction of the lengths of radial cracks behind the half-casts. The material presented in this paper is meant to supplement and extend the new Swedish recommendations for cautious perimeter blasting of tunnels, shafts, pits and road cuts.  相似文献   

    

O. M. Sæther  C. Reimann  B. O. Hilmo  E. Taushani 《Environmental Geology》1995,26(3):147-156

Groundwaters from crystalline and metamorphic rocks (hardrocks) and from Quaternary deposits, i.e., alluvial and glacial deposits (softrocks) from the counties of Nord-Trøndelag and Sør-Trøndelag were analyzed for major and minor elements and ions including fluoride. The median concentration of F^– in water from the hardrock aquifers is 0.28 mg/l (14.7 eq/l) in contrast to water from softrock aquifers in which it is found to be 0.05 mg/1 (2.6 eq/l). More importantly, ca. 15% of the locations where water was abstracted from hardrock wells contain 1.5 mg/l (78.9 eq/l) F^– or more. Thus, 15% of all hardrock wells returned F^– results that are at or above the maximum recommended value for drinking water. Of the softrock wells, none are above 1 mg/l. Geologists would normally expect higher F^–contents in groundwaters derived from acid rocks, e.g., in granitic or gneissic areas. When comparing the host lithology with the observed F^–contents, however, no clear relationship between F^– content and lithology is visible. The highest observed F^– values actually occur in gneissic host rocks. However, wells drilled in amphibolites/greenstones, mica schists, calcareous rocks, and sedimentary rocks all returned some analytical results above 1.5 mg/l F^–. These results suggest that all hardrock wells drilled should be tested for F^– and the users informed about the results and advised to take any necessary precautions. When applying the recently proposed Norwegian drinking water limits to our data, 51% of all softrock well waters and 56% of all hardrock well waters are unfit for consumption without prior treatment, although we analyzed only for about half of the proposed elements/parameters. This result seriously questions the concept of fixed action levels—many of them with totally unproven health implications—for so many parameters/elements for hardrock groundwaters.  相似文献   

    

Dr. W. R. Taylor  R. W. Page  G. Esslemont  N. M. S. Rock  D. I. Chalmers 《Mineralogy and Petrology》1995,52(3-4):209-230

Summary Rare-metals mineralization of the Brockman deposit (Halls Creek Mobile Zone, NW Australia) is hosted in a fluorite-bearing, rhyolitic volcaniclastic unit informally termed the Niobium Tuff. The Tuff, more correctly described as a tuffaceous volcaniclastic deposit, is the lowermost unit of a sequence of trachyte-to-rhyolite lavas, trachyandesite subvolcanic rocks, and volcaniclastic units of the Brockman volcanics located within the Halls Creek Group, a thick, early Proterozoic volcano-sedimentary sequence. High precision SHRIMP ion-microprobe zircon dating of the Niobium Tuff gives an eruption age of 1870 ± 4 Ma. Regional geochronological constraints indicate the Niobium Tuff was deposited about 15 Ma before major orogenic activity affected the area. Despite folding, faulting and low-grade metamorphism, the Brockman volcanics show excellent preservation of primary volcanological features, including pillow-lavas and pillow-breccias, that suggest a dominantly subaqueous, below-wave-base emplacement environment. The style of eruption products and magma volume constraints suggest the trachyte-dominated volcanics were erupted from a small shield volcanic complex probably in a rift-related basin in a shallow-marine setting. The tectonic setting was intraplate but differs from most Tertiary to recent trachyte volcanic complexes which are largely subaerial, are built on relatively thick continental crust, and show no post-eruptive orogenic history. Brockman-style rare-metal deposits are characterized by preservation of subaqueous volcanics beneath a thick sedimentary sequence, eruption of early incompatible-element enriched products followed by less differentiated magmas, and fine-grained mineralogy influenced by alteration processes. Prospects exist for discovery of analogous deposits, particularly in early Proterozoic mobile belts and Tertiary intraplate shield volcanic provinces.

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Geologie der an Vulkanite gebundenen Seltene-Metalle-Lagerstätte Brockman, Halls Creek Mobile Zone, Nordwest-Australien. I. Vulkanologie, Geochronologie und Petrographie der Brockman-Vulkanite

Zusammenfassung Selten-Metall-Vererzung der Brockman-Lagerstätten (Halls Creek Mobile Zone, NW Australien) sitzt in einer Fluorit-führenden, rhyolitischen vulkanoklastischen Einheit auf, die informell als der Niob-Tuff bezeichnet wird. Es handelt sich hier um ein vulkanoklastisches Tuff-Sediment; dieses ist die unterste Einheit einer Abfolge vor Trachyt bis Rhyolit-Laven, trachyandesitischen Subvulkaniten and vulkanoklastischen Einheiten der Brockman Vulkanite innerhalb der Halls Creek Gruppe, einer mächtigen frühproterozoischen vulkanosedimentdren Abfolge. Prüzisionsdatierungen des Niob Tuffs mit der SHRIMP Ionen-Mikrosonde ergeben ein Eruptionsalter von 1870 +-4 Millionen Jahren. Regionale geochronologische Zusammenhänge zeigen, daß der Niob Tuff etwa 15 Millionen Jahre vor einer größeren Orogenese, die das Gebiet betroffen hat, abgelagert wurde. Trotz Faltung, Bruchtektonik and niedriggradiger Metamorphose zeigen die Brockman-Vulkanite einen hervorragenden Erhaltungszustand primärer vulkanologischer Erscheinungen. Diese urnfassen auch Kissenlaven und KissenBreckzien, die eine vorwiegend subaquatische Ablagerung in ruhigem Wasser erkennen lassen. Die Art der Eruptionsprodukte and das Magmavolumen zeigen, daß die Trachytdominierten Vulkanite von einem kleinen Schildvulkan stammer, wahrscheinlich in einem Becken in einer Rift-Situation im seichten marinen Milieu. Die tektonische Situation war intraplate, aber unterscheidet sich von den meisten tertiären bis rezenten trachytischen Vulkan-Komplexen, die hauptsächlich subaerisch sind, auf einer relativ mächtigen kontinentalen Kruste aufsitzen, and keine post-eruptive orogene Entwicklung zeigen. Seltene-Metalle-Lagerstatten des Brockman-Typs rind durch die Erhaltung subaquatischer-Vulkanite unterhalb einer machtigen sedimentdren Abfolge gekennzeichnet; welters durch frühe Eruptionsprodukte, die an inkompatiblen Elementen angereichert sind, auf die dann weniger differenzierte Magmen folgten, und schließlich durch einen feinkörnigen Mineralbestand, der vor Umwandlungsprozessen betroffen war. Es besteht die Möglichkeit der Entdeckung analoger Lagerstätten, besonders in frühproterozoischen mobilen Gürteln und in tertidren intraplate Schildvulkan Provinzen.

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