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61.
Palaeomagnetic pole positions have been determined for a collection of igneous rocks, comprising nearly five hundred samples, from the Cape Verde Islands of Santa Antao, Sao Vicente, Sao Nicolao and Sao Tiago. Limited data from the islands of Sal, Maio and Fogo are also presented. Stratigraphic control suggusts that the lavas are overwhelmingly Miocene in age on Sao Tiago and Sao Nicolao. Similarity in the palaeomagnetic pole positions indicates that Miocene lavas are also dominant on Santa Antao and Sao Vicente.
Substantial areas within two of the islands are of reversed polarity only, suggesting either a rapid extrusion rate, or the existence of a long reversed polarity epoch during the Miocene period. The palaeomagnetic pole positions for each island are close to the present geographic pole, excluding the possibility of Post-Miocene differential crustal spreading (or rotation about a vertical axis) in this part of the Atlantic. The palaeomagnetic pole position for the entire survey is consistent with the Miocene geographic pole being removed from, but close to, the present geographic pole; and is in harmony with the European polar wandering curve. 相似文献
Substantial areas within two of the islands are of reversed polarity only, suggesting either a rapid extrusion rate, or the existence of a long reversed polarity epoch during the Miocene period. The palaeomagnetic pole positions for each island are close to the present geographic pole, excluding the possibility of Post-Miocene differential crustal spreading (or rotation about a vertical axis) in this part of the Atlantic. The palaeomagnetic pole position for the entire survey is consistent with the Miocene geographic pole being removed from, but close to, the present geographic pole; and is in harmony with the European polar wandering curve. 相似文献
62.
An energetic solar proton and electron event was observed by particle detectors aboard Explorer 33 (AIMP-1) and OGO-3 during the period July 16–19, 1966. Optical and radio observations of the sun suggest that these particles were produced by a flare which may have occurred on July 16 near the central meridian of the invisible hemisphere. The active region to which the flare is assigned is known to have produced the energetic particle events of July 7 and 28, 1966. The propagation of the particles in the July 16–19 event over the 180° extent of solar longitude from the flare to the earth is discussed, and it is concluded that there must exist a means of rapidly distributing energetic particles over a large area of the sun. Several possible mechanisms are suggested. 相似文献
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66.
Differential survival of coral transplants on various substrates under elevated water temperatures 总被引:1,自引:0,他引:1
Yap HT 《Marine pollution bulletin》2004,49(4):306-312
Closely related scleractinian coral species that exhibited similar survival patterns under relatively normal field conditions responded very differently to the occurrence of an environmental disturbance. The two species studied were Porites cylindrica and Porites rus which occur in the same reef zones in shallow reef flats. Transplants of both species were evenly distributed and attached to three different types of substrate: live coral colonies of P. cylindrica, dead coral colonies (also of P. cylindrica), and epoxy coated metal grids that were raised above the sandy substrate. With the onset of above-normal water temperatures due to the El Ni?o episode of 1998, P. cylindrica transplants immediately showed signs of bleaching stress and tissue necrosis, followed by algal overgrowth and mortality soon afterwards. In contrast, transplants of P. rus bleached more slowly and suffered less mortality, with a few actually showing signs of recovery at the end of the experimental period which covered a total of 14 weeks. These differences in responses could be attributed to properties of the symbiotic zooxanthellae, of the host coral tissue itself, or both. Over-all, survival was good on the metal grids (average of 35%), and on the live coral (average of 22%). It was poor on the dead coral (average of 6%). The metal grids as well as live coral tissue apparently provided a favorable substrate for the attached coral fragments, even for those of a different species. Under the conditions of this particular study, attachment of live coral fragments on already dead colonies for the purpose of increasing live coral cover on the reef did not yield favorable results. This is an area that requires further investigation. 相似文献
67.
Assessing uncertainties in climate change impact analyses on the river flow regimes in the UK. Part 2: future climate 总被引:3,自引:0,他引:3
The first part of this paper demonstrated the existence of bias in GCM-derived precipitation series, downscaled using either a statistical technique (here the Statistical Downscaling Model) or dynamical method (here high resolution Regional Climate Model HadRM3) propagating to river flow estimated by a lumped hydrological model. This paper uses the same models and methods for a future time horizon (2080s) and analyses how significant these projected changes are compared to baseline natural variability in four British catchments. The UKCIP02 scenarios, which are widely used in the UK for climate change impact, are also considered. Results show that GCMs are the largest source of uncertainty in future flows. Uncertainties from downscaling techniques and emission scenarios are of similar magnitude, and generally smaller than GCM uncertainty. For catchments where hydrological modelling uncertainty is smaller than GCM variability for baseline flow, this uncertainty can be ignored for future projections, but might be significant otherwise. Predicted changes are not always significant compared to baseline variability, less than 50% of projections suggesting a significant change in monthly flow. Insignificant changes could occur due to climate variability alone and thus cannot be attributed to climate change, but are often ignored in climate change studies and could lead to misleading conclusions. Existing systematic bias in reproducing current climate does impact future projections and must, therefore, be considered when interpreting results. Changes in river flow variability, important for water management planning, can be easily assessed from simple resampling techniques applied to both baseline and future time horizons. Assessing future climate and its potential implication for river flows is a key challenge facing water resource planners. This two-part paper demonstrates that uncertainty due to hydrological and climate modelling must and can be accounted for to provide sound, scientifically-based advice to decision makers. 相似文献
68.
Sune G. Nielsen Helen M. Williams William L. Griffin Norman Pearson 《Geochimica et cosmochimica acta》2009,73(24):7387-6704
Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots of continents. These models have different implications for the growth of continents, but it is currently difficult to determine the origin of individual eclogite suites.Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations and strongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first examination of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tl isotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in South Africa. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios. These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried the eclogites to the surface.The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl is identical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kimberlitic material that was not removed by the cleaning procedure.Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotope signatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sample is consistent with containing a minor component (<3%) of ocean crust altered at low temperatures.Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because of the stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no other chemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin. However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish the origin of cratonic eclogites. 相似文献
69.
Globalization would appear to be a subject that easily could be addressed in geography classrooms, yet this is not always the case. In terms of pedagogy, many geographers are concerned whether the field has been adequately engaging various components of this topic (e.g., connectivity, core-periphery), especially in terms of making the subject matter relevant to students. This article presents the results of a recent course project situated at the intersection of city-level resilience to hazards and connectivity with the global economy, utilizing SWOT analysis. The student projects demonstrated that this comparatively simple analytical tool was a useful means for exploring and integrating key topics in globalization and urban-economic geography, while also facilitating a problem-based learning environment. 相似文献
70.
G. Nishani Musafer M. Helen Thompson E. Kozan R. C. Wolff 《Natural Resources Research》2017,26(2):223-236
A real-world mining application of pair-copulas is presented to model the spatial distribution of metal grade in an ore body. Inaccurate estimation of metal grade in an ore reserve can lead to failure of a mining project. Conventional kriged models are the most commonly used models for estimating grade and other spatial variables. However, kriged models use the variogram or covariance function, which produces a single average value to represent the spatial dependence for a given distance. Kriged models also assume linear spatial dependence. In the application, spatial pair-copulas are used to appropriately model the non-linear spatial dependence present in the data. The spatial pair-copula model is adopted over other copula-based spatial models since it is better able to capture complex spatial dependence structures. The performance of the pair-copula model is shown to be favorable compared to a conventional lognormal kriged model. 相似文献