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41.
Large glacial lakes modulated the return of meltwater to the ocean during deglaciation, and their drainage may have initiated global climate change. Yet few records of their drainage come from observations within their basins. Sediment cores from nine lakes along a 240-km transect from northwestern Quebec to northeastern Ontario cover a portion of former Lake Ojibway and provide a stratigraphy of the terminal phase of this large glacial lake. Magnetic susceptibility, density, grain size, X-ray fluorescence chemistry and X-ray diffraction data were used to characterize stratigraphic changes within the basin. The basal sequence consists of till and rhythmites, with ice-proximal debris flows overlain by varves. The varves thin up-section and become unrecognizable, which indicates decreased deposition rates. This fine-grained sediment forms the matrix of a clay-pebble conglomerate. The clay-pebbles are ice-rafted debris (IRD). The IRD flux was probably constant, whereas the sedimentation rate of the finer-grained matrix decreased. The end of IRD marks the cessation of icebergs in the lake and is the best indication for drainage of the glacial lake. The conglomerate is capped by laminated to massive gray silt deposited after lake drainage and marks the transition to organic-rich, post-glacial lakes. Such sequences place drainage into the broader context of deglaciation. 相似文献
42.
Mineralogical and geochemical diversity in cryoconite granules from Aldegondabreen glacier was investigated using FTIR spectroscopy. Results suggest that the technique is an effective tool for investigating mineralogy and identifying spatial differences in geochemistry, based upon characteristic spectral signatures. 相似文献
43.
Martin Fahey Matthew Helinski Andy Fourie 《Geotechnical and Geological Engineering》2011,29(5):709-723
Paste backfill used to provide ground support in underground mining is generated from full-stream tailings and is almost always
placed underground with cement. For the backfill, both the rate of strength development and the final strength are important
considerations for design, particularly when the backfill is subsequently exposed in the stope-mining sequence. There is strong
evidence that strengths measured on specimens obtained from coring the in situ cemented backfill are much greater than laboratory-cured
specimens with the same cement content. The paper reviews some of the experimental evidence showing that one of the major
reasons for the different strength is the difference in effective stress acting on the backfill during curing. Laboratory
specimens are (almost) always cured under zero total stress, so no effective stress develops. In contrast, backfill in a stope
may cure under high effective stress, which develops due to either “conventional” consolidation in free-draining backfills,
or to the so-called “self-desiccation” mechanism in fine-grained fills. Evidence is presented showing how the final strength
is affected by applying stress to specimens at different stages of curing and at different rates. It is shown that a fully-coupled
analysis of the filling behaviour is required to determine the appropriate effective stress regime to apply in curing laboratory
specimens, where “fully-coupled” in this context means taking account of the interaction of consolidation/drainage rate, filling
rate and cement hydration rate. Curing protocols for laboratory specimens are proposed, which would ensure that the strengths
obtained are representative of in situ conditions. 相似文献
44.
Gayle Hough Jennifer Green Paul Fish Andy Mills Roger Moore 《Marine Geophysical Researches》2011,32(1-2):151-162
Exploration and development of offshore hydrocarbon resources has advanced into remote deepwater regions over the last decade and poses significant technical challenges for the design and installation of wells and facilities at extreme water depths. Seafloor and shallow subsurface processes and conditions in these areas are complex and generally poorly understood, and the geohazards to development are larger scale and fundamentally different to those encountered onshore; consequently the geohazard risk to deepwater development projects is potentially significant and requires careful evaluation and mitigation during the front-end planning and engineering design stages of projects. There are no established industry standards or methods for the assessment of geohazards and engineering-quality geophysical data at the scale of development. The paper describes an integrated and systematic map-based approach for the assessment and mitigation of seabed geohazards and risk to proposed deepwater development. The approach employs a multi-disciplinary team working with engineering-quality field calibrated data to accurately map and assess seafloor ground conditions and ensure that development proposals are not exposed to intolerable geohazard risk. The approach taken is very similar to the practice of establishing geological models for land-based engineering projects, in which the complete geological history of the site is used to characterise and predict the performance of the ground. Such an approach is routine for major projects on land but so far does not seem to be common practice in the offshore industry. The paper illustrates the seafloor geomophological mapping approach developed. The products are being used to optimise development layouts to avoid geohazards where possible and to support site-specific engineering design of facilities based on a detailed understanding of the potential geohazard loadings and associated risk. 相似文献
45.
As the world's climate continues to change there is concern that this may cause malaria to spread to new areas. Here we examine whether past changes in temperature, in addition to social changes, may have affected malaria in England. Our analysis indicates that cold summers experienced during the 1800s may have contributed to the disappearance of this disease from the country. As our summers become progressively warner it is unlikely that malaria will become firmly re-established on these shores, although the possibility of future outbreaks should not be ignored. 相似文献
46.
Helen Valentine Will Saunders Andy Taylor 《Monthly notices of the Royal Astronomical Society》2000,319(2):L13-L17
We study the influence of different magnetic boundary conditions on the generation of magnetic fields by turbulent convection. It is found that the structure and strength of the generated field in the vicinity of the boundary is strongly dependent on the choice of boundary conditions. In the convective interior, however, the solutions remain largely insensitive to the boundary conditions. In all cases the overall efficiency of the dynamo process remains high with a steady state magnetic energy density between 12 and 25 per cent of the turbulent kinetic energy, and peak field values exceeding the equipartition level. These results support the idea that the solar granulation may constitute a dynamo source for magnetic fields in the quiet photosphere. 相似文献
47.
Gary Kocurek Nicholas Lancaster Mary Carr Andy Frank 《Journal of African Earth Sciences》1999,29(4):431
The Tertiary Tsondab Sandstone Formation, which underlies much of the present Namib Sand Sea, is a key element in understanding the Cenozoic evolution of the Namib Desert. Outcrops of the aeolian facies of the Tsondab Sandstone at Elim and Diep Rivier consist of two sequences of bioturbated cross-strata separated by likely formation-scale surfaces of stabilisation. Cross-strata consist of scalloped sets about 200 m in width and separated by southeast dipping bounding surfaces. Internally, sets contain reactivation surfaces of probable seasonal origin. The north to south-southeast dipping foresets define crescent shapes with a trough axis trending northeast. Although additional data are needed to define the Tsondab bedform, the outcrop data is best satisfied in computer simulations by north trending, east migrating main bedforms, which had relatively large and slow-moving dunes superimposed upon their eastern flanks and migrated to the north. Foresets dipping to the south to south-southwest at Elim suggest that superimposed dunes also occurred on the western flanks of the main bedform and migrated to the south, but that their record was largely lost with net eastward migration of the main bedform. This preliminary Tsondab model shares attributes such as trend, scale of cross-strata, and presence of scalloped sets with reactivation surfaces with computer models of the modern linear dunes in which large-scale sinuosity migrates alongcrest to the north. Differences emerge in the overall set architecture and the orientation of cross-strata and bounding surfaces, as well as the degree of vegetation that must have characterised Tsondab dunes. 相似文献
48.
Andy Louwyck Alexander Vandenbohede Griet Heuvelmans Marc Van Camp Kristine Walraevens 《Ground water》2023,61(1):100-110
The water budget myth, which is the idea that safe pumping must not exceed the initial recharge, gave rise to a controversy about the role of recharge in assessing the sustainability of groundwater development. To refute the concept of safe yield, a simplified water budget equation is used, which equals the total pumping rate to the sum of capture and storage change. Since initial recharge and discharge are canceled out from this equation, it is concluded that sustainable pumping has nothing to do with recharge. Investigating the assumptions underlying this equation, it is seen that it expresses the superposition principle, which implicitly assumes the groundwater reservoir can be depleted indefinitely and boundary conditions are an infinite source of water. To evaluate sustainability, however, the limits of the aquifer system must be examined accurately. Theoretically, this can only be accomplished applying nonlinear models, in which case setting up the simplified water budget equation is impossible without knowing the initial conditions. Hence, excluding recharge when assessing sustainable pumping may not be done inconsiderately, which is illustrated by two examples. An analytical solution, developed by Ernst in 1971 to simulate flow to a well in a polder area with a nonlinear function for drainage, even shows that it is not necessarily a misconception to assume the cone of depression stops expanding when the pumping rate is balanced by the infiltration rate. 相似文献
49.
The development of surface hydrological connectivity is a key determinant of flood magnitude in drylands. Thresholds in runoff response may be reached when isolated runoff-generating areas connect with each other to form continuous links to river channels, enabling these areas to contribute to flood hydrographs. Such threshold behaviour explains observed nonlinearities and scale dependencies of dryland rainfall–runoff relationships and complicates attempts at flood prediction. However, field methods for measuring the propensity of a surface to transmit water downslope are lacking, and conventional techniques of infiltration measurement are often inappropriate for use on non-agricultural drylands. Here, we argue for a reconceptualization of the dryland surface runoff process, suggesting that the downslope transfer of water should be considered alongside surface infiltration; that is, there is a need for the “aggregated” measurement of infiltration and overland flow hydraulics. Surface application of a set volume of water at a standardized rate generates runoff that travels downslope; the distance it travels downslope is determined by infiltration along the flow, integration of flow paths, and flow resistance. We demonstrate the potential of such a combined measurement system coupled with structure-from-motion photogrammetry to identify surface controls on runoff generation and transfer on dryland hillslopes, with vegetation, slope, surface stone cover, and surface roughness all having a significant effect. The measurement system has been used on slopes up to 37° compared with the flat surface typically required for infiltration methods. On average, the field workflow takes ~10–15 min, considerably quicker than rainfall simulation. A wider variety of surfaces can be sampled with relative ease, as the method is not restricted to stone and vegetation-free land. We argue that this aggregated measurement represents surface connectivity and dryland runoff response better than standard hydrological approaches and can be applied on a much greater variety of dryland surfaces. 相似文献
50.
Nicholas A. Cradock-Henry Bob Frame Benjamin L. Preston Andy Reisinger Dale S. Rothman 《Climatic change》2018,150(3-4):333-341
The parallel scenario process enables characterization of climate-related risks and response options to climate change under different socio-economic futures and development prospects. The process is based on representative concentration pathways, shared socio-economic pathways, and shared policy assumptions. Although this scenario architecture is a powerful tool for evaluating the intersection of climate and society at the regional and global level, more specific context is needed to explore and understand risks, drivers, and enablers of change at the national and local level. We discuss the need for a stronger recognition of such national-scale characteristics to make climate change scenarios more relevant at the national and local scale, and propose ways to enrich the scenario architecture with locally relevant details that enhance salience, legitimacy, and credibility for stakeholders. Dynamic adaptive pathways are introduced as useful tools to draw out which elements of a potentially infinite scenario space connect with decision-relevant aspects of particular climate-related and non-climate-related risks and response options. Reviewing adaptation pathways for New Zealand case studies, we demonstrate how this approach could bring the global-scale scenario architecture within reach of local-scale decision-making. Such a process would enhance the utility of scenarios for mapping climate-related risks and adaptation options at the local scale, involving appropriate stakeholder involvement. 相似文献