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61.
Particulate dispersion from sources within a 10- to 13-m tall pine forest was studied experimentally at Brookhaven National Laboratory using stained ragweed pollen and other tracers ranging from 14 to 58 m in size. Forty-seven continuous point source releases lasting from 22 to 55 min were made at heights from 1.75 to 14.0 m from locations having a long fetch through the forest. In most experiments, differently colored ragweed pollen were emitted simultaneously from three locations. In other tests, several particle types were released from a single point. The sampling network consisted of 119 rotoslide samplers at heights from 0.5 to 21.0 m at 57 positions within and at the edge of the forest. Deposition to the ground was sampled by greased microscope slides at each position. Meteorological measurements were taken in and near the forest.Data were classified by particle characteristics, source height and meteorological parameters. Concentration patterns were illustrated on scale diagrams of the sampling grid. Changes in centerline and crosswind integrated concentrations, plume width and height, mass flux, deposition and deposition velocity were studied as a function of distance, particle size and wind speed. Results were compared to those obtained from similar releases over open terrain.In the forest, vertical predominates over lateral dispersion and considerable interchange occurs through the canopy. Flow is channelled somewhat by vegetation density differences but is generally in the direction of the mean wind above the forest. No systematic turning of the wind with height was observed. Most particles are lost to the foliage rather than to the ground and large particles are lost more rapidly than smaller ones. Rate of change in mass flux is similar to that over open terrain and is greater with light than with stronger wind speeds.This research was carried out under the auspices of the New York State Museum and Science Service and the U.S. Atomic Energy Commission (now Energy Research and Development Administration) and was partially supported by Research Grant No. R-800677 from the Division of Meteorology, U.S. Environmental Protection Agency. 相似文献
62.
G.R. Lewis N. Andr C.S. Arridge A.J. Coates L.K. Gilbert D.R. Linder A.M. Rymer 《Planetary and Space Science》2008,56(7):901-912
In this paper we present two methods to derive electron fluid parameters from the CAPS–ELS spectrometer on board the Cassini spacecraft currently in orbit around Saturn. In the first part of the paper we give a basic overview of the instrument and describe the challenges inherent in the derivation of density and temperature values using these techniques. We then describe a method to calculate electron moments by integrating the particle distribution function. We also describe a second technique in which we fit the electron energy spectrum with a Gaussian curve and use the peak energy of this curve to derive density and temperature values. We then compare the two methods with particular emphasis on their application to Cassini SOI observations in the saturnian environment and point out the limitations of the two techniques. We will show that results from the two very different methods are in agreement when the physical properties of the environment and of the observed electron populations have been inferred from inspection of the raw data. Finally we will suggest future developments that will remove these limitations. 相似文献
63.
A. D. Gilbert C. Froeschlé U. Frisch 《Celestial Mechanics and Dynamical Astronomy》1993,56(1-2):263-272
Wavelet analysis is applied to distributions of points generated by iterating the standard map. The initial condition is chosen so that the points fill the largest chaotic region. When the standard map parameterk=1.3, the distribution of points contains many voids corresponding to islands in the chaotic region. The wavelet transform is dominated by contributions from these islands. Fork=10 the chaos fills phase space and no structure is apparent; the wavelet transform reveals statistical fluctuations in the distribution of points. 相似文献
64.
Testate amoebae are informative about palaeoecological conditions, but the methods generally used for their analyses in lake
sediments differ from those used for their analyses in peats, making comparisons difficult. This study examines how filter
mesh size and total number of individuals counted affect species richness, Shannon diversity, equitability, density and assemblage
structure. We analysed the complete testate amoeba contents of six sediment samples from Lake Lautrey, France. The abundance
of testate amoebae was high (1,403–10,870 shells cm−3), and species smaller than 63 μm in both length and width represented up to 89% of total abundance and 43% of species richness.
A simulation showed that using 47- or 63-μm mesh-size filters reduced inter-sample differences and changed the patterns of
abundance, species richness and assemblage structure, causing loss of information and leading to potential erroneous palaeoecological
interpretation. Rarefaction analyses suggest that although 170 shells are sufficient to assess the general structure of assemblages,
such small sample sizes can underestimate species richness by overlooking taxa with relative abundances <4%. Total counts
of 400 shells yield better estimates of assemblage structure and recover at least 50% of total species richness, although
species with absolute frequencies below 2% may still be missed. Higher counts are required to obtain reliable estimates of
species richness and assemblage structure in samples that have high testate amoeba densities but are dominated by a few small
taxa. Further studies should determine the bioindicator value and functional roles of small and/or rare species in lakes and
thus to what extent overlooking them affects palaeoecological interpretations. 相似文献
65.
Gilbert Bellaiche 《Earth and Planetary Science Letters》1980,47(1):124-130
Direct observations from submersible prove that in the explored areas, the Izu-Ogasawara Trench is devoid of any significant sediment cover. The effects of the tensional forces affecting the oceanic crust have been observed in the outer wall of this trench, especially near its foot between 8200 and 8500 m depth. They lead to a staircase morphology (with rock exposures) corresponding probably with vertical faulting parallel to the trend of the trench axis. The inner wall, at about 5000 m depth, shows large rock exposures that have been sampled. These rocks are volcanic lavas of the tholeiitic series (icelandites). The staircase morphology observed in this part of the overriding plate seems to prove the occurrence of distensive forces induced probably by the underthrusting of the oceanic crust. No indication of compressive stress has been registered in the whole investigated area. 相似文献
66.
A general mixing equation with applications to Icelandic basalts 总被引:4,自引:0,他引:4
Charles H. Langmuir Robert D. Vocke Gilbert N. Hanson Stanley R. Hart 《Earth and Planetary Science Letters》1978,37(3):380-392
The mixing equation applied by Vollmer [1] to Pb and Sr isotope ratios is shown to be a general equation applicable to consideration of element and isotope ratios. The mixing equation is hyperbolic and has the form:Ax + Bxy + Cy + D = 0where the coefficients are dependent on the type of plot considered: i.e. ratio-ratio, ratio-element, or element-element. Careful use of this equation permits testing whether mixing is a viable process, places constraints on end member compositions, allows distinction between mixing of sources and mixing of magmas, and should allow distinction between recent mixing and long-term evolution of sources.The available chemical data for postglacial basalts from Iceland and along the Reykjanes Ridge are not consistent with either mixing of magmas or simple mixing of an enriched ocean island source with a depleted ocean ridge source. If the available analyses for basalts are representative of the source regions, the data are consistent with at least two models neither of which can be properly tested with the available data.(1) There are two separate mixing trends: one beneath Iceland with the alkali basalt source and a depleted Iceland source as end members; the second along the Reykjanes Ridge with a heterogeneous ocean ridge basalt source and a source similar to that for intermediate basalts on Iceland as end members. The depleted Iceland source and the depleted ocean ridge source are not the same.(2) The chemistry of the basalts is not determined by mixing. Instead the basalts are derived from a multiplicity of sources with a similar history which have been isolated for hundreds of millions of years. 相似文献
67.
Ship noise received on a horizontal array towed behind the ship is shown to be useful as a potentially diagnostic tool for estimating local acoustic bottom properties. In numerical simulations, tow-ship noise which bounces off the bottom is processed on a beamformer that shows the arrival angles; the beamformer output is readily interpreted by relating it to the Green's function of the acoustic wave equation. Simple signal processing is shown to be sufficient to extract the propagation angles of the "trapped" (i.e., propagating) modes of the acoustic waveguide. By relating the trapped modes to a basic geophysical model of the bottom, one can predict acoustic-propagation conditions for a particular bottom-interacting ocean acoustic environment. 相似文献
68.
69.
Gilbert Fielder 《Planetary and Space Science》1961,5(4):286-287
It is argued that a reliable theory of the stress history of the Moon should take into account several factors; and that direct observations of the Moon's surface can throw much light on this subject. 相似文献
70.
Tristan Garcia Gilbert Féraud Christophe Falguères Henry de Lumley Christian Perrenoud David Lordkipanidze 《Quaternary Geochronology》2010,5(4):443-451
Several hominid remains have been discovered in the open-air site of Dmanisi (Georgia), the oldest prehistoric site in Eurasia. Two major arguments prove that this site is close in age to the Plio-Pleistocene boundary: a Villafranchian fauna and the morphological characteristics of hominid remains recently ascribed to Homo georgicus. Direct dating of the lower hominid-bearing level was carried out on volcanic glass and minerals using the 40Ar/39Ar method. The concordant results from two different sampled locations allow the determination of the age of the earliest human presence in Eurasia. This radioisotopic result strengthens the argument that the first dispersal of hominids outside Africa occurred at least 1.8 Ma ago. 相似文献