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991.
A. K. Ambrosimov 《Physical Oceanography》1992,3(5):391-396
Algorithms for the compression and objective classification of low-volume samplings according to the test of their belonging to different statistical populations are given. Samplings are discriminated by thex
2 test. The distributions of the temperature patch horizontal scales in the upper ocean are analysed. It is shown that optimum empirical thresholds exist when the abstract classification mechanism goes over to the objective natural classification.Translated by Mikhail M. Trufanov. 相似文献
992.
Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control 总被引:5,自引:1,他引:5
Hydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO2 partial-pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO2 partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO3 as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits. 相似文献
993.
Patterns of Nd and Sr isotopic ratios produced by magmatic and post-magmatic processes in the Shiant Isles Main Sill, Scotland 总被引:3,自引:0,他引:3
K. A. Foland F. G. F. Gibb C. M. B. Henderson 《Contributions to Mineralogy and Petrology》2000,139(6):655-671
The Shiant Isles Main Sill of the British Tertiary Igneous Province is a classic example of a differentiated, alkaline basic
sill. Four separate intrusions, each emplaced internally in rapid succession, form a 165-m-thick sill hosted by Lower Jurassic
sedimentary rocks. Extensive Nd and Sr isotopic studies were conducted on samples from a vertical section through the sill
where the relationships of samples to one another are well defined. The results illuminate patterns of modification of isotopic
ratios and clarify the petrogenesis (magma sources, crustal contamination), magmatic processes (bulk mixing, interstitial
liquid mixing), and post-magmatic alteration (hydrothermal effects on Sr and Nd). Overall, the whole-rock initial 87Sr/86Sr ratios range from ∼0.7037 to 0.7061 while initial 143Nd/144Nd ratios vary from ∼0.51243 to 0.51286 (ɛNd∼−0.7 to +5.7) – values that contrast markedly with those of the country rock. Acid leaching (HCl) of the whole-rock samples
that removes analcime indicates that most of the scatter in the 87Sr/86Sr is caused by the ubiquitous sub-solidus, aqueous alteration during which more-radiogenic Sr was introduced into the sill,
especially along the margins, and also reveals magmatic isotopic ratios. In contrast, Nd was immobile during fluid interaction
so that the sill 143Nd/144Nd ratios were not affected, even <1 m from the country-rock contact. Using leached rock values, 87Sr/86Sr and 143Nd/144Nd ratios are inversely correlated from magmatic processes. Magmas with two distinct isotopic compositions were involved:
a more primitive one with 143Nd/144Nd ∼0.51285 and 87Sr/86Sr ∼0.7035 that produced the first two intrusions and a more evolved one (with 0.51252 and 0.7048) that produced the third
intrusion. Mixing of the two magmas was very limited, restricted to near contacts between units, and apparently occurred by
interstitial melt migration. The more evolved crinanitic magma was probably produced from a batch of the more primitive picritic
melt by a small degree of crustal contamination and crystal fractionation during a short crustal residence prior to ascent
and emplacement.
Received: 20 December 1999 / Accepted: 5 May 2000 相似文献
994.
Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism. 相似文献
995.
K. G. Miesserov 《Earth, Moon, and Planets》1982,27(1):13-25
The formation of the solar system is considered from the physico-chemical point of view. The main role in the process is ascribed to heavy metals and to the surface tension that had arisen as a result of appearance of a liquid layer of fused substance in the equatorial region of the protostar. The formation of the liquid layer was caused by the transfer of fused substance droplets under the action of centrifugal forces in the direction of the protostar surface. Due to the surface tension the prevalence of the centrifugal forces over the gravitational ones was able to reach the value when the density differentiation of the substance began to take place under the effect of the centrifugal forces, and accumulation of heavy metals proceeded in outermost equatorial region of the protostar. As a result the disk has been formed and a liquid ring was separated from the protostar. Later explosions on the young Sun sent parts of the hardened ring which possessed the first cosmic velocity to different distances away from the Sun. In such a way planets, their satellites, asteroids, meteorites and comets were formed. The physical characteristics of planets, the parameters of their orbits, and the data on the structure of meteorites are consistent with ideas developed in the paper. 相似文献
996.
The stability characteristics of parallel magnetic fields when fluid motions are present along the lines of force is studied. The stability criterion for both symmetric (m=0) and asymmetric (m=1) modes are discussed and the results obtained by Trehan and Singh (1978) are amended in the present study. The results obtained for the cylindrical geometry are shown to play an important role forka<4, wherek is the wave number,a is the radius of the cylinder, compared to the results obtained by Geronicolas (1977) for the slab geometry. 相似文献
997.
Using a quasi-two-dimensional model of the Venus ionosphere, we calculated the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80 to 100°), and compared them with data obtained from the Pioneer Venus Orbiter retarding potential analyzer. The calculated ion bulk velocity arises entirely from the solar EUV-induced plasma pressure gradient and has a magnitude consistent with observations; ionization by suprathermal electrons is neglected in those computations. We find that while photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92°, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95°. We also show that the main nightside plasma peak at approximately 140 km altitude is of the F2 type (i.e., is diffusion controlled). Its altitude and shape are thus quite insensitive to the altitude of the ion source. 相似文献
998.
S.R. Prabhakaran Nayar V. Sanalkumaran Nair V.N. Radhika K. Revathy 《Solar physics》2001,201(2):405-417
The solar wind plasma exhibits many features of the solar surface passed on to the interplanetary medium as temporal variations
due to the solar rotation. The yearly average values of solar wind velocity, and geomagnetic index A
p during 1965–1999 were found to exhibit long period evolution. They were found to peak around the declining phase of each
solar cycle. While the solar wind velocity peaks around the second half of the declining phase, the IMF field strength increases
around the first half of the declining phase of each solar cycle. The power spectrum of these parameters shows peaks around
37-day, 30-day, 27-day, 13.5-day, 9-day, and 7-day periods. The temporal evolution of the power spectrum of the solar wind
plasma parameters and the geomagnetic activity index A
p are also studied in detail and presented with the help of contour graphs. These studies indicate that the strength of the
quasi-periodicities in the interplanetary medium evolves with time. 相似文献
999.
Astrophysics and Space Science - In 1913 Henry N. Russell revealed to the astronomical community a diagram that would revolutionize stellar astronomy. He had plotted the absolute magnitude of field... 相似文献
1000.
Astrophysics - We used modified holographic Ricci dark energy to find anisotropic LRS Bianchi type I cosmological model in five-dimensions based on Lyra geometry. The exact solutions of the... 相似文献