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991.
I. M. Villa B. Grobéty S. P. Kelley R. Trigila R. Wieler 《Contributions to Mineralogy and Petrology》1996,126(1-2):67-80
We have investigated the mechanisms and pathways by which Ar diffuses through the McClure Mountains hornblende (ferroan pargasite),
selected as a good example of material normally dated during Ar-Ar studies. A coarse-grained hornblende separated from the
same hand specimen as the MMhb-1 age standard was subjected to a hydrothermal cold-seal bomb experiment and characterized
by TEM. Heated and unheated crystals were subjected to four different 39Ar/40Ar dating extraction techniques: conventional stepwise heating, infra-red laser spot, ultra-violet laser depth profiling,
and closed-system stepwise etching. The stepwise heating age spectrum reproduces the features often interpreted as resulting
from a concentric diffusive zonation, but the other three techniques yield results that are not compatible with such a simple
picture. The IR laser data indicate that the dependence of laboratory Ar loss on grain size, predicted by Fickian diffusion,
is at best poor and instead is related mainly to mineralogical variations. The depth profiles show the importance of planar
zones (spaced between <1 and >150 μm from TEM evidence) in providing fast pathways for inward diffusion of atmospheric Ar
from the capsule, but showed no evidence of diffusive profiles in the bulk of the hornblende lattice. The data from closed
system stepwise etching underscore the role of zones rich in planar defects both for Ar loss and for nucleation of etching.
The age spectra obtained by stepwise heating suffer from the differential breakdown of impurity phases, whose presence can
be diagnosed with several isotope correlation plots; particularly revealing are Cl-Ca-K trends. In addition to the problems
of mineral decomposition during in-vacuo laboratory degassing, an equally important decomposition occurs during many hydrothermal
experiments which, combined with problems of mineral purity, have led to an overestimation of the rate of argon diffusion
in hornblende. The response of hornblende to thermal disturbance in a hydrothermal environment can be every bit as complex
as breakdown in vacuo. Laboratory experiments on bulk samples have not succeeded in quantitatively constraining volume diffusion.
Received: 12 October 1995 / Accepted: 11 July 1996 相似文献
992.
Colllin Lee 《地理学报(英文版)》1996,(2)
I.IntroductionTheuseofquantitative111odeIstodevelopUnderstanding,prePareforecastsandexplorepolicyoptionsinthecontextofurbanandroponaldevelpl11entremainsapopularaCtivlty,althouglrathermoresointheacadel11iccol11n1unitythanintl1epIanningprofession.However,thehistoryoftheapplicationoffon11aln1odelstopolicyproblemsdoesnotproducegreatconfidence.objeCtiveevaluationoftheuseofl11odelsinpolicyn1akingiscomplicatedbythefaCtthat,asafonl1ercolleagueoncesaid,con1puter111odeIsarelikecl1ildrenfmostpeopleten… 相似文献
993.
Meromixis has several powerful effects on lakes, yet there is no single definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in Brownie Lake, Minneapolis, Minnesota, a small eutrophic lake that became meromictic around 1925. The results show that in Brownie the onset of laminations and changes in the iron to manganese ratio most likely occurred before the development of permanently anoxic bottom water and that changes in the diatom assemblage occurred later, most likely only when meromixis was well developed.This is the 11th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers. 相似文献
994.
995.
Wenjun Chen Michael D. Novak T. Andrew Black Xuhui Lee 《Boundary-Layer Meteorology》1997,84(1):99-124
Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field. 相似文献
996.
A. S. Naidu A. Blanchard J. J. Kelley J. J. Goering M. J. Hameedi M. Baskaran 《Marine pollution bulletin》1997,35(7-12):260-269
Surficial sediments from 31 stations on the northeastern Chukchi Sea, Alaskan Arctic were analysed by AAS and ICP-AES for Si, Al, Fe, Mn, Cu, Cr, Co, Zn, Ni and V. The metal concentrations are related to the sediment granulometry and show no correlations with organic carbon. Presumably all the heavy metals, except Cr and Co, are either adsorbed on clays and/or coprecipitated with ferrimanganic hydroxide. Cobalt is partitioned in an unknown mineral phase within the sand fraction.
Comparison of the heavy metal concentrations in muds of the Chukchi Sea with those of the high arctic shelves of Russia, East Greenland and the Beaufort Sea shows relatively lower levels of most metals in the Chukchi. Presumably these disparities are related to regional differences in sediment chemistries inherited from natural terrigenous sources rather than to any enhanced pollution in the non-Chukchi areas.
It is suggested that the concentrations and accumulation rates of the heavy metals reported here can serve as reliable baselines to monitor metal pollution in the Chukchi Sea. 相似文献
997.
998.
999.
George W. Luther III Zbigniew Wilk Robert A. Ryans A. Lee Meyerson 《Marine pollution bulletin》1986,17(12):535-542
The dissolved (<0.40 γm) fraction of water samples from Newark Bay, New Jersey was analysed for Zn, Cu and Pb content by differential pulse anodic stripping polarographic techniques. In the dissolved fraction, non-labile forms of Zn, Cu and Pb exist as shown by differential pulse anodic stripping voltammetry analysis on acidified samples and acidified-UV irradiated samples. The particulate fraction (>;0.40 μm) contains metals in the form of sulphides (Fe, Zn, Cu, Mn), oxides and oxyhydroxides (Si, Al, Fe, Cu, Ni, Sn), phosphate (Ca, Ce, La), clay minerals (Fe, Zn, Cu, Ti) and carbonaceous material (Fe, Cu, Zn) as demonstrated by X-ray microanalysis. The solid phases are likely present in colloidal form in the dissolved fraction of the water column as well. The forms of the metals in the water column are partially due to the resuspension of bottom sediments by dredging and natural processes, to sewage outfall and to natural geochemical processes. 相似文献
1000.
Vertical heat and momentum fluxes were measured by the eddy correlation method under near-neutral conditions during both day and night above a spruce forest canopy. Results show that 50% of the heat transported to the spruce canopy during night and away from the canopy during the day occurs in extreme magnitude events, the majority of less than one second duration. Extreme-magnitude events were more frequent and lasted longer during the day than during the night. The distributions of the duration of extreme events in the same direction as the net heat flux and the turbulence intensity for both day and night were similar.During the night, the mean horizontal windspeed was about 1 m s-1 and the measured coincident transport of momentum and heat accounted for 36% of the total heat flux. The predominant mechanism of forced convection during the extreme nightime heat transport events was excess heat sweeps in which the duration of the event is usually less than 1 s. During the day, the mean horizontal windspeed was about 2 m s -1 and the measured coincident transport of momentum and heat accounted for only 16% of the total heat flux. Local free convection was suggested to account for 27% of the total heat flux. The predominant mechanism of mixed convection during the extreme daytime heat transport events is deficit heat inward interactions. During both night and day, about 10% of the total heat transport occurred in extreme events working against the thermal gradient.Now atPurdue University, West Lafayette, IN, U.S.A. 相似文献