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91.
Disposal of radioactive waste in the sea floor of fracture zones associated with the flanks of the Mid-Atlantic Ridge may
be a satisfactory alternative to land disposal. Effective physiographic, sedimentary, chemical, and oceanographic barriers
exist in these aseismic deep canyons, especially in the eastern Atlantic. In addition, the major producers of radioactive
wastes are likely to be near the Atlantic Ocean. If such a disposal strategy is adopted, intensive study of the sedimentologic
and oceanographic properties of oceanic fracture zones will be necessary. 相似文献
92.
93.
Knut Kaiser Lars Opgenoorth Werner H. Schoch Georg Miehe 《Quaternary Science Reviews》2009,28(15-16):1539-1554
Charcoal and fossil wood taken from palaeosols, sediments and artificial structures were analysed in order to evaluate the regional pedoanthracological potential and to obtain information on Holocene environmental changes, particularly on possible past tree occurrences in southern Tibet. This research was initiated by the question to what extent this area is influenced by past human impact. Even recent evaluations have perceived the present treeless desertic environment of southern Tibet as natural, and the previous Holocene palaeoenvironmental changes detected were predominantly interpreted to be climate-determined. The material analysed – comprising a total of 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites (c. 3500–4700 m a.s.l.) – represents the largest systematically obtained data set of charcoal available from Tibet so far. 27 taxa were determined comprising trees, (dwarf-) shrubs and herbs as well as grasses. The predominant tree taxa were Juniperus, Hippophae, Salix and Betula. According to their present-day occurrence in the region, the genera Juniperus and Hippophae can be explicitly attributed to tree species. Further, less frequently detected tree taxa were Populus, Pinus, Quercus, Taxus and Pseudotsuga. Charcoal of Juniperus mainly occurred on southern exposures, whereas Betula was associated with northern exposures. In contrast, the (partly) phreatophytic taxa Hippophae and Salix showed no prevalent orientation. The distribution of radiocarbon ages on charcoal revealed a discontinuous record of burning events cumulating in the Late Holocene (c. 5700–0 cal BP). For southern Tibet, these results indicated a Late Holocene vegetation change from woodlands to the present desertic pastures. As agrarian economies in southern and south-eastern Tibet date back to c. 3700 and 5700 cal BP, respectively, and the present-day climate is suitable for tree growth up to c. 4600 m a.s.l., we concluded that the Late Holocene loss or thinning out of woodlands had been primarily caused by humans. 相似文献
94.
Robert M. Moore Stephen Punshon Claire Mahaffey David Karl 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(9):1449-1458
Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N2) fixation is estimated to supply 8×1012 moles N y?1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H2), but the extent to which it leads to supersaturation of H2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H2 concentration (nmol L?1), and rates of N2 fixation (μmol N m?3 d?1), determined using 15N2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation (r=0.98) between dissolved H2 and rates of N2 fixation, with the greatest supersaturation of H2 and highest rates of N2 fixation being observed in the subtropical gyres at the southern (~18°S) and northern (18°N) reaches of the transect. The lowest H2 saturation and N2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H2 supersaturations and N2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements. 相似文献
95.
Karl Krejci 《International Journal of Earth Sciences》1925,16(2):99-127
Ohne Zusammenfassung 相似文献
96.
Based on GIS and statistical methods, with the help of searching historical literatures and calculating the landscape indices, the land use changes of Qian'an County in both spatial and temporal aspects from 1945 to 1996 has been analyzed in this paper. And the driving forces of land use changes and their ecological effects are discussed too. The main findings of this study are as follows: (1) Land use changed greatly in Qian'an during 1945-1996, characterized by a decrease in grassland, wetland and water bodies, and an increase in cultivated land, saline-alkali land, and the land for housing and other construction purposes. Grassland decreased by 175,828.66 ha, and cultivated land increased by 102,137.23 ha over the half century. Accordingly, the main landscape type changed from a steppe landscape to a managed agricultural ecosystem. (2) Results of correlation analysis show that the land use change in the study area was mainly driven by the socioeconomic factors. (3) The ecological effects of land use change in the area are characterized by serious salinization, degression of soil fertility and the weakening of landscape suitability. 相似文献
97.
Hubert Engelbrecht Karl B. Föllmi Ursula Baumer Johann Koller 《Swiss Journal of Geoscience》2010,103(1):3-13
A resin nodule was found in glauconite-rich detrital sediments of the Cretaceous Garschella Formation (Aptian to Albian) outcropping at Langer Köchel (Bavaria, S Germany). Gas chromatographic and mass spectrometric analyses of the fossil resin revealed dealkylation and the total defunctionalisation of its polycyclic constituents. Besides many unspecific components a specific one, agathalene, has survived. Agathalene also presents a strongly degraded product, but may have been derived from its natural precursor agathic acid, which is a very specific constituent (biomarker) of recent and fossil kauri resin. Although agathalene is a far less specific secondary biomarker, it indicates the botanic origin of the fossil resin nodule. Besides other potential producers of agathic acid, precursors of the present-day conifer species Agathis dammara and A. australis were distributed in a wider palaeophytogeographic range than today and might have been the botanical source of kauri resin. In view of the east–west directed Early Cretaceous surface current system of the Tethys ocean, the palaeogeographic provenance of the Werdenfels resin nodule probably was a mainland positioned further to the east or southeast of the Helvetic shelf, to where it was transported probably by driftwood of the resin-producing Agathis sp. 相似文献
98.
This study investigates the physical conditions (water depth, current speed, salinity, temperature) in Lianzhou Bay, a shallow coastal bay in southern China, during two expeditions in the dry and wet seasons of 2011. Based on these expedition data, basic hydrodynamic parameters like Brunt-Väisälä Frequency, Richardson Number, Rossby radius, and Resonance Period are calculated. The results show that Lianzhou Bay is characterized by comparatively small quantity of freshwater input and weak stratification. Strong tides, which are spatially uniform within the bay, cause turbulent mixing. Residence time of the water is shorter in winter due to a stronger coastal current in that season. Consideration of the water movement may help to reduce the harmful ecological impact of aquaculture waste water discharge. 相似文献
99.
R. P. Bewick P. K. Kaiser W. F. Bawden N. Bahrani 《Rock Mechanics and Rock Engineering》2014,47(5):1647-1671
A particle-based distinct element method and its grain-based method are used to generate and simulate a synthetic specimen calibrated to the rupture characteristics of an intact (non-jointed) low-porosity brittle rock deformed in direct shear. The simulations are compared to the laboratory-generated ruptures and used to investigate rupture at various normal stress magnitudes. The fracturing processes leading to shear rupture zone creation and the rupture mechanism are found to be normal stress dependent (progressing from tensile splitting to shear rupture) and show partial confirmation of rupture zone creation in nature and in experiments from other materials. The normal stress dependent change is found to be due to the orientation of the major principal stress and local stress concentrations internal to the synthetic specimens being deformed. The normal stress dependent rupture creation process results in a change to the rupture zone’s geometry, shear stress versus horizontal displacement response, and thus ultimate strength. 相似文献
100.
Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely. 相似文献