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31.
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Catches of perch have strongly decreased in Lake of Geneva from 1975 to 1981. This decrease was attributed to the impact of
overfishing, enhanced by the accelerated growth of perch. Therefore, too many perches were captured before their first reproduction
and the stocks of reproducers were strongly reduced. Furthermore, adverse meteorological conditions occurred probably during
several periods of reproduction. The accelerated growth of perch was attributed to the combined effects of increasing eutrophication
(more food available) and of overfishing (decreased perch density in the shoals).
相似文献
33.
Using the standard map as a model problem and in the spirit of cluster analysis we have studied the invariance of the distributions of different indicators introduced to detect and measure weak chaos. We show that the problem is less straightforward than expected and that, except for very strong chaotic dynamical systems, all the complexities (islands, sticking phenomena, cantori) of mixed Hamiltonian systems are reflected into the indicators of convergence towards invariant distributions. 相似文献
34.
Claude T. Herzberg 《Geochimica et cosmochimica acta》1978,42(7):945-957
One petrogenetic grid for plagioclase-, spinel- and garnet-lherzolite analogues in the system CaO-MgO-Al2O3-SiO2 is presented from 1 bar to 30 kbar and 400 to 1500°C. Another grid for olivine-gabbro, spinel-gabbro and garnet-pyroxenite analogues in the same system is presented from 1 bar to 25 kbar and 500 to 1500°C. Both grids show the distribution of the mineral assemblages and the variations in the composition of clinopyroxene with temperature and pressure. They were developed by applying simple thermodynamic mixing models of clinopyroxene to experimentally determined clino-pyroxene compositions.Calcium tschermak's pyroxene (CaAl2SiO6) in complex CaMgSi2O6-CaAl2SiO6-Mg2Si2O6 clinopyroxenes is best represented by a local charge balance mixing model where Enthalpy and entropy changes of subsolidus reactions involving variations in the CaAl2SiO6 and Mg2Si2O6 content of clinopyroxene are interdependent due to nonideal mixing of these two end-members. CaAl2SiO6 can strongly reduce the mutual solubility of clinopyroxene and orthopyroxene at moderate pressures and high temperatures. Failure to take this into account can result in temperature underestimates (up to 150°C) of spinel-lherzolites, garnet-pyroxenites, low pressure garnet-lherzolites, spinel-gabbros, and high pressure plagioclase-lherzolites and olivine-gabbros. However, at temperatures and pressures where the Al2O3 content of clinopyroxene is low (e.g. garnet-lherzolite nodules in kimberlite), the mutual solubility is adequantely represented by experimental results in the system CaO-MgO-SiO2. 相似文献
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Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. εNd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a εNd-εSr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high87Sr/86Sr ratios and as such lie to the right of the correlation line towards seawater compositions.From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago. 相似文献
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Late-Proterozoic volcanic graywackes from the NW Hoggar (Algeria) have been investigated from geological and petrological points of view. Thirty-five samples have been analysed for major elements and the following trace elements: Li, Rb, Sr, Ba, Ni, Co, Cr, V, Zn, Cu.These graywackes constitutes a thick flysch-like formation deposited in a marine environment. They are composed of volcanic (mainly andesites) and plutonic detritus. The chemical analyses confirm this observation and demonstrate the very immature character of these rocks.The composition of the NW Hoggar graywackes differs chemically from other investigated graywackes in that they have a higher Ca, Na content and lower SiO2, Ni content.Several geological observations suggest that the average Hoggar graywacke composition approaches the composition of the W Hoggar Late Precambrian upper crust. The graywackes were formed as a result of erosion of penecontemporaneous calc-alkali volcanic suite, emplaced after widespread mafic intrusions in the Upper Proterozoic shelf deposits. 相似文献
40.
The WGS84 (World Geodetic System 1984) reference system is, originally, mathematically defined from the NSWC-9Z2 (Naval Surface Weapons Center — 9Z2) reference system. The WGS84 associated realization, called in this paper WGS84-D, is a 1 meter consistency NNSS (US Navy Navigation Satellite System) Doppler realized reference frame. In contrast, the ITRF (IERS Terrestrial Reference Frame) is a 1 centimeter consistency reference frame realized through the most accurate techniques of Space Geodesy. This work intends to improve the transformation parameters between the WGS84-D and the ITRF through the use of both a NSWC-9Z2/Doppler realization and an extension of the ITRF network. A strong linear correlation was also modeled between the Doppler determined scale factor and the mean smoothed sunspot number, due to uncompensated ionospheric effects. This correction improved NSWC-9Z2 (i.e. WGS84) Doppler realization consistency. The uncertainty of adjusted transformation parameters between the ITRF and the WGS84-D is improved by a factor 2 over previous determinations. 相似文献