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J.M. McArthur R.A. Benmore M.L. Coleman C. Soldi H.-W. Yeh G.W. O'Brien 《Earth and Planetary Science Letters》1986,77(1)
Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values ofδ13C andδ34S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in18O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H2S decreases theδ34S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature. 相似文献
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Distribution and significance of crystalline, perlitic and vesicular textures in the Ordovician Garth Tuff (Wales) 总被引：2，自引：0，他引：2
Diverse spherulitic and granular crystalline fabrics, perlitic textures and fabrics related to the growth and migration of
vesicles occur in the Garth Tuff, a largely welded Ordovician ignimbrite. Defining the distribution of such textures helps
to constrain the ignimbrite's cooling and degassing history. Suites of spherulitic and perlitic textures closely reflect variation
in cooling rates. Seven facies are defined based on the style and intensity of crystallisation: (1) a medium to coarsely crystalline,
equigranular facies; (2) an intensely spherulitic facies; (3) a sparsely spherulitic facies; (4) a pectinate facies; (5) a
microcrystalline to cryptocrystalline, equigranular facies; (6) a lithophysal facies; and (7) a transitional perlite–pectinate
facies. Textural changes from facies 1 to 5 reflect progressively higher cooling rates. Facies 1 occurs in proximal settings
in the ignimbrite's core. Facies 2 to 5 successively envelop facies 1, with facies 2 becoming the dominant fabric in the ignimbrite's
core in medial settings. Facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite's welded
margins. Crystallisation under hydrous conditions is reflected by second-boiling textures in the sporadically developed lithophysal
facies. The seventh facies reflects a subtle interplay between cooling, hydration and crystallisation which locally prevented
perlitic fracturing. The distribution of amygdales reflects patterns of volatile migration and entrapment. In the lower levels
of the ignimbrite, amygdales occur in irregular concentrations or rare subvertical pipe-like structures. Pipe-like structures
attest to fumarolic activity while the ignimbrite was in a rheomorphic state. Amygdales are widespread and evenly distributed
in the upper levels of the ignimbrite. However, the top of the welding profile is characterised by a thin, poorly vesiculated,
originally vitrophyric horizon that abruptly caps an intense concentration of amygdales. Ductile and brittle fabrics developed
during the upward migration of gas. Microscopic drag folds occur around some amygdales. Jigsaw-fit to clast-rotated breccias
originated through both late-stage pneumatic fracturing and autobrecciation. Vaporisation of water at the flow base provided
a significant source of volatiles in addition to gas released during cooling and crystallisation. Secondary alteration has
enhanced or modified some fabrics. Perlitic zones were susceptible to patchy chlorite–sericite–carbonate diagenetic alteration.
Diagenesis and metamorphism have contributed to the infilling of vesicles.
Received: 22 August 1997 / Accepted: 24 June 1998 相似文献
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Chemical and biological data were collected between 1947 and 1967 from Morton Dam and Upper Karori Reservoir, two artificial lakes which are part of the water supply of Wellington City, New Zealand. Chemical analyses included the major ions, nutrients, and physical and chemical properties of engineering importance. The emphasis in biological analyses was on the dominant phytoplankters. Bacteriological results are presented as tables of plate counts and “most probable numbers” of confirmed faecal coli‐forms; iron‐fixing bacteria were identified. Primary productivity has accelerated since 1940–42: annual maxima have increased 30‐fold at Morton Dam, and 10‐fold in Upper Karori Reservoir. In 1942, Morton Dam was believed to be oligotrophic, but the type and quantity of phytoplankton at present indicate a rapid progression to eutrophy. Despite the extent of biological changes, no dramatic changes in the physical and chemical characteristics of either reservoir can be detected. 相似文献
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W. H. Jefferys M. J. Fitzpatrick B. E. McArthur 《Celestial Mechanics and Dynamical Astronomy》1987,41(1-4):39-49
GaussFit is a new computer program for solving least squares and robust estimation problems. It has a number of unique features, including a complete programming language designed especially to formulate estimation problems, a built-in compiler and interpreter to support the programming language, and a built-in algebraic manipulator for calculating the required partial derivatives analytically. These features make GaussFit very easy to use, so that even complex problems can be set up and solved with minimal effort. GaussFit can correctly handle many cases of practical interest: nonlinear models, exact constraints, correlated observations, and models where the equations of condition contain more than one observed quantity. An experimental robust estimation capability is built into GaussFit so that data sets contaminated by outliers can be handled simply and efficiently.Invited Talk at the 18th meeting of the Division on Dynamical Astronomy of the American Astronomical Society held in Braintree, Massachussetts, May 7–9, 1987. 相似文献
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Perimeter shape of drainage basins is evaluated as a control of annual runoff depth from small Sierra Nevada basins underlain by granitic rocks. In contrast to three perimeter shape ratios, none of which contributes a statistical explanation of annual runoff depth, three terms of a Fourier expansion of the basin perimeter substantially improve the estimating model. When added to annual precipitation, the first, third, and seventh Fourier amplitudes increase statistical explanation by 15 percent and prediction accuracy by 10 percent. These improvements are much greater than corresponding gains atributable to other morphological variables. [Key words: basin runoff, basin shape, Sierra Nevada.] 相似文献
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Contourite drift systems form a significant component of the marine clastic sedimentary record. Although contourites form in all tectonic settings, few studies have described their development along convergent margins; such characterization is needed to underpin oceanographic and palaeoenvironmental studies in active settings. This study is the first to document contourite drift development along the Hikurangi subduction margin of New Zealand. Integration of bathymetric, seismic and well data enables five classes of drift to be recognized around the subduction wedge, occurring in three principal associations: (i) an upper slope drift association of giant elongate mounded (ca 150 km long, 50 km wide and up to 1100 m thick) and plastered drifts (ca 300 km long, 8 km wide and <600 m thick), which occurs upon and inboard of a major intrabasinal thrust‐cored high, whose long axis parallels the coast; shallow bottom currents disperse sub‐parallel to this axis; (ii) a spatiotemporally discontinuous association of confined and mounded hybrid drifts (ca 500 m long, <2 km wide and up to 500 m thick) that occurs along the mid‐to‐outer slope domain of the wedge, recording the interaction of along‐slope and downslope currents within trench‐slope basins; and (iii) a trench fill assemblage that implies the passage of abyssal bottom currents across a 40 km reach of the trench‐axial Hikurangi Channel‐levée, with associated modification of the channel form and of overbank sediment waves. The fundamental presence of contourites along this margin appears to depend on the orientation and strength of oceanographic bottom currents. However, drift type and evolution vary depending on the slope gradient and the presence of irregular seafloor topography created by tectonic structures. The documented drifts are generally smaller, less continuous, and develop more intermittently than similar styles of drifts documented on passive margins; this mode of occurrence may be characteristic of contourite development on convergent margins. 相似文献
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W. Alan Delamere Livio L. Tornabene Kris Becker Nathan T. Bridges Dennis Gallagher Laszlo Keszthelyi Guy K. McArthur Moses Milazzo Nicolas Thomas 《Icarus》2010,205(1):38-52
HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE. 相似文献
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Penetration of a layer of fibre by wind reduces its effectiveness as a barrier to heat flow. In the literature, the dependence of coat or clothing insulation I(u) on windspeed u is usually described by a relation of the form I(u) = I(0) – au
1/2, where a is a constant. Re-analysis reveals that it is more appropriate to treat coat conductance (proportional to 1/I) as a linear function of windspeed. Vapour conductance can also be treated as a linear function of windspeed.On leave from Washington State University, Department of Agronomy and Soils. 相似文献