Petrography, chemistry and origin of early diagenetic concretions in the Lower Carboniferous of the Isle of Man     

J. A. D. DICKSON  C. BARBER 《Sedimentology》1976,23(2):189-211

Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz. The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore-waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater. The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods. A four-fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions. Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth.  相似文献   

SURROGATE MEASURES OF URBAN SOCIAL INTERACTION     

Gary W. Shannon  John D. Nystuen 《The Professional geographer》1976,28(1):23-28

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Observation of temporal and spatial variations in the Fe/O charge composition of the solar particle event of 4 July, 1974     

T. P. Armstrong  S. M. Krimigis  D. Hovestadt  B. Klecker  G. Gloeckler 《Solar physics》1976,49(2):395-407

Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R _E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles.  相似文献   

The concept of limiting magnitude and the performance of the Isaac Newton telescope for prime focus photography     

D. E. Blackwell  P. A. Ibbetson  C. A. Murray  N. M. Pratt 《Astrophysics and Space Science》1976,43(2):469-473

A quantitative definition of limiting magnitude for stellar photographs is proposed, together with a method of measurement employing a Racine prism. Limiting magnitudes inU, B andV for the Isaac Newton telescope at Herstmonceux derived from a total of 72 prime focus photographs are presented and the results correlated with the size of the photographed image. The limiting magnitude for an extrapolated value of the seeing of 1 isB=22^m.0, in good agreement with a value obtained by scaling the limiting magnitudes of the 200 inch telescope.  相似文献   

Synchrotron radio emission from Uranus and Neptune     

Lawrence D. Kavanagh 《Icarus》1975,25(1):166-170

A number of charged-particle radiation belt models for Uranus and Neptune are postulated, and the synchrotron emission spectrum for each is calculated over the frequency range 18.75 to 2400 MHz. Although no radio observations of these planets exist at frequencies below 1400MHz, available observations are used in conjunction with the synchrotron calculations to establish a rough upper limit to the size and strength of the planets' magnetic field strength and radiation belt intensity. The magnitude of synchrotron radio emission expected in many of the models is within the detection range of several existing ground-based telescopes, at decimetric and metric wavelengths.  相似文献   

Pioneer 10 ultraviolet photometer observations of the Jovian hydrogen torus: The angular distribution     

R.W. Carlson  D.L. Judge 《Icarus》1975,24(4):395-399

The Pioneer 10 ultraviolet photometer observations of the Jovian hydrogen torus are analyzed to obtain the angular distribution. The cloud is asymmetric about Io, where the atoms presumably originate, with the greater density occurring in the trailing portion. A simple model which assumes Jeans escape from the atmosphere of Io is developed and compared to the observations. The results suggest that the exospheric temperature is high (～3000 K) and that the ionization lifetime of the cloud atoms is ～1 × 10⁵ sec.  相似文献   

The atmosphere of Io from Pioneer 10 radio occultation measurements     

A.J. Kliore  G. Fjeldbo  B.L. Seidel  D.N. Sweetnam  T.T. Sesplaukis  P.M. Woiceshyn  S.I. Rasool 《Icarus》1975,24(4):407-410

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.  相似文献   

Observations of Jupiter at 26.3 MHz using a large array     

Michael D. Desch  Thomas D. Carr  Jorge Levy 《Icarus》1975,25(1):12-17

A 640 element phase-steerable dipole array has been used to make highly sensitive observations of the planet Jupiter during the 1973 apparition. The satellite Io is found to have very little influence at the low flux levels, whereas the definition of sources A and B appears to be relatively flux independent. A two-dimensional analysis of the data in the Jupiter-Io plane has revealed considerable source B activity at low intensities which is not influenced by Io.  相似文献   

Interpretation of faraday rotation in the galactic magnetic field     

D. Nissen  K. O. Thielheim 《Astrophysics and Space Science》1975,33(2):441-451

The autocorrelation function of Faraday rotation measures is discussed in terms of different types of galactic field configurations. The autocorrelation function evaluated from published data of 139 radio galaxies and quasars is found to resemble a form typical for a quasi-longitudinal field, whereas the autocorrelation function of 38 pulsars turns out to be of the form expected for a longitudinal field. These observations are interpreted with respect to the position of the solar system relative to the neutral sheet in a quasi-longitudinal field configuration.Rotation measures calculated theoretically using a mathematical formulation of the quasi-longitudinal field model are adapted to experimental data. The resulting polarity of the global field structure is discussed in connection with the original dipole-like configuration the magnetic momentum vector of which is found to have been antiparallel to the angular momentum vector of the Galaxy. The relation between the field strength and the density of electrons is found to be consistent with earlier results.  相似文献   

Relict magnesian calcite oolite and subsidence of the Amazon shelf     

JOHN D. MILLIMAN  HENYO T. BARRETTO 《Sedimentology》1975,22(1):137-145

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon-14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO₃) and possess radial crystal microstructures. A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16-21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.  相似文献