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291.
Surface sediment, speleothems, and bat guano from two adjacent, topographically different cave sites in eastern Spain have been studied palynologically to elucidate the potential of cave sediments for palaeoenvironmental reconstruction. A cave opening with a large entrance and constant width presented far fewer problems of alteration in the pollen assemblages than a sac‐like cave opening with a constricted entrance. Pollen concentration is linked primarily to the amount of pollen input rather than to the result of post‐depositional alteration. Sampling should be undertaken away from parietal and rear areas and avoiding moisture zones. Lateral differences in the pollen spectra indicate that sampling should be on the basis of a multiple‐profile approach and selection of dry rather than wet sediments. If these procedures are followed, within‐cave sediments can realistically reflect not only local but also regional vegetation of the site. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
292.
Well-exposed, vertically dipping, glacially polished outcrops of the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera include basin-floor deposits of the Upper Kaza Group overlain by slope channel complexes of the Isaac Formation. Within the 2·5 km thick Kaza and Isaac succession is an up to 360 m thick interval composed of diverse deep-water stratal elements including scour and interscour deposits, distributary channels, fine-grained turbidites, terminal splays, mass-transport deposits, erosional and levéed channels and avulsion splays, which collectively were formed during the development of an ancient passive-margin channel-lobe system. The proportion and vertical and lateral arrangement of stratal elements reveal three distinct complexes. The lower complex, consisting mostly of distributary channels and small and large scours, is interpreted to represent the detachment of lobes from an upflow levéed channel, wherein a well-developed channel-lobe transition zone was formed by efficient, siliciclastic flows during a period of sustained transport bypass and limited deposition coincident with the onset of falling relative sea level. The middle, comparatively thicker and more sandstone-rich complex, comprises distributary channel fills, fine-grained turbidites and lesser terminal splays that are interspersed with small scours, capped by a slope levéed channel filled with coarser-grained siliciclastic sediment. The abundance of basin-floor elements suggests negligible separation between the levéed channel and lobe, and therefore a poorly-developed channel-lobe transition zone, resulting from inefficient, siliciclastic-rich depositional flows that became dominant during lowstand and/or ensuing transgression. The stratal makeup of the upper complex resembles the lower detached complex, suggesting a return to efficient flows, and an abrupt change to mixed carbonate–siliciclastic sediments associated with highstand conditions. Accordingly, the stratigraphic architecture and stacking pattern of the Kaza–Isaac interval, which relate to the formation of multiple channel-lobe transition zones, were controlled by temporal changes in sediment supply and flow characteristics during the long-term progradation of the Laurentian continental margin. 相似文献
293.
Maia M. Moreira N. Vicente S. Mirão J. Noronha F. Nogueira P. 《Geology of Ore Deposits》2020,62(6):508-534
Geology of Ore Deposits - The Mociços Cu-deposit is part of a cluster of ancient copper mines in the Sousel-Barrancos metallogenic belt in the Ossa-Morena Zone at the SW Iberia. The orebodies... 相似文献
294.
R. O. Vicente 《Astronomische Nachrichten》1984,305(1):31-32
The campaigns of the programme of international cooperation, called MERIT, for the determination of the earth rotation parameters are reviewed, especially about the difficulties to infer results about polar motion from such short intervals of observations. It is pointed out the need to employ the same models and standards in order to facilitate the comparison of data obtained by classical and modern techniques. 相似文献
295.
This paper describes methods for combining independent observations of polar motion into a single time series, and applies these methods to satellite laser ranging, Doppler, and astrometric polar motion data taken over the period 1981--1983. The polar motion data are treated as complex valued time series, yielding one estimate of the noise level for each series, rather than a separate estimate for X and Y components. Estimates of the noise level which take advantage of the existence of simultaneous observations are preferred. Linear combination of the data may be performed in both the time and the frequency domain, and examples of each are present. The results of analyzing three years of observations show little difference between the Allan and conventional variance. Present day Doppler determinations do not show marked improvement compared to classical astrometric results. Thus, the astrometric observations are still useful. 相似文献
296.
297.
Giorgio Rivalenti Maurizio Mazzucchelli Vicente A.V. Girardi Riccardo Vannucci M. Adelaide Barbieri Alberto Zanetti Steve L. Goldstein 《Contributions to Mineralogy and Petrology》2000,138(4):308-325
Spinel–peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State,
Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information
they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects
of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cabuji are protogranular lherzolites
and porphyroclastic harzburgites recording average equilibrium temperatures of 825 ± 116 and 1248 ± 19 °C, respectively. Pressure
in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer). Both groups show major element chemical
variation trends in whole-rock and Ti and HREE (Er, Yb) variations in clinopyroxene consistent with fractional melting and
basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to
LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes.
HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de
Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature
(1035 ± 80 °C) and the pressure is 1–1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend
further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched
in La up to 30 × PM and are smoothly fractionated from LREE to HREE, with deep, negative, Zr and Ti anomalies. The geochemical
heterogeneities of the xenoliths from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic
xenoliths from Pico Cabuji represent the lower part of a mantle column (the head of a mantle diapir, at the transition conductive–adiabatic
mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column, chromatographically
enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different
geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes
of the Pico Cabuji porphyroclastic clinopyroxenes (143Nd/144Nd= 0.51339–0.51255, 87Sr/86Sr=0.70275–0.70319) and of Fernando de Noronha (143Nd/144Nd=0.51323–0.51285, 87Sr/86Sr=0.70323–0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to
low Nd–low Sr (EMI) and low Nd–high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters
indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type
at Fernando de Noronha. These different components enriched a lithosphere isotopically similar to DMM (depleted MORB mantle)
at both localities. At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the
∼ 8 Ma old lavas of the Remedios Formation, suggesting that this is the age of metasomatism. At Pico Cabuji, the mantle xenoliths
do not record the high 87Sr/86Sr component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was
not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando
de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the
two localities, which would be difficult to reconcile with their genetic relationship with the same plume.
Received: 12 June 1999 / Accepted: 13 December 1999 相似文献
298.
299.
Nota sobre la profundidad hipocentral del sismo del apenino toscano-romañol de 11 de febrero de 1939
Ing. Vicente Inglada 《Pure and Applied Geophysics》1948,12(5-6):181-188
Resumen En la presente Nota se determina por el método deKöveslighèty, valiéndose de fórmulas especiales y una variante que simplifica el cálculo extraordinariamente, la profundidad del sismo del Apenino toscano-romañol de 11 de febrero de 1939. Se obtiene para ella el valorh=6871 metros, que está en muy buen acuerdo con los dos de 7 y 10 km, obtenidos por el Prof.P. Caloi, valiéndose de los sismogramas registrados en las Estaciones próximas. Al final se hecen algunas consideraciones acerca de las ventajas, que, en ciertos casos, ofrece dicho metodo.Esta Nota va dedicada, como prueba de gran consideración y afecto, a nuestro querido amigo e ilustre Sismólogo Dr.G. Agamennone, con ocasión del homenaje que se le tributa en su 90° aniversario. 相似文献
300.
There was significant eruptive activity on Lanzarote (Canary Islands) in the 18th and 19th centuries, and future activity is a clear likelihood today. The fact that not all of the island is covered by geodetic monitoring prompted the authors to assess the possibility of using SAR interferometry for routine volcanic activity monitoring of the island. This paper presents the results obtained after testing this technique using six radar images acquired by the ERS-1 and ERS-2 satellites during the period 1992-2000. The analysis of these images has confirmed the existence of long-term stability coherence across most of the island: a prerequisite for using SAR interferometry in volcano monitoring. The analysis of 15 (redundant) interferograms allowed us to study both the impact of atmospheric artifacts on Lanzarote and any possible displacements, and to conclude that there has been no displacement greater than 3 cm on the island during the period in question. This result matches those obtained with the geodetic equipment installed on the island. Our results show that the inclusion of SAR interferometry in routine geodetic monitoring, supplementing terrestrial techniques, will clearly improve the chances of detecting any possible deformation, and its broad coverage will enhance the interpretation of any observed anomalies. 相似文献