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71.
Udo Zimmermann Jennifer Tait Quentin G. Crowley Vanessa Pashley Gijs Straathof 《International Journal of Earth Sciences》2011,100(2-3):511-526
The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3?Ga, representing the local geology, with the youngest grain at 1030.2?±?10.9 (2??) (n?=?92?<10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be ??cap carbonates??, show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their ??13CVPDB isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C?CO isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8?±?10?Ma (2?? errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570?Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03?Ma with the detritus being purely locally derived. 相似文献
72.
Continuous heavy rainfall hit northern Peru in the second half of the 2008/2009 summer season. From the end of January to the beginning of March, the Cordillera Huayhuash experienced abnormally high precipitations that exceeded 270?mm. The antecedent rainfall, aggravated with a severe rainstorm of 20?mm on March 7 triggered a large debris flow in the upper Carhuacocha Valley early in the morning on March 8. The debris flow interrupted drainage from the upper part of the valley damming a lake in the narrow depression between the trough slope and the lateral moraine. As a result of the drainage interruption, water percolated through the moraine dam of Cangrajanca Lake where a secondary mass movement occurred in its inner slope. In September 2009, we mapped the debris flow and related landforms and estimated the total area and volume of both mass movements using geodetic measurements. About 104,000?m3 of sediments was moved from the trough slope towards the moraine from which 534,000?m3 flowed to Cangrajanca Lake subsequently. We analysed the rainfall conditions that triggered the debris flow using rainfall data from the nearby stations. We also compared the precipitation preceding the event with the rainfall thresholds for debris flow initiation. 相似文献
73.
Vanessa Cerqueira Koppe João Felipe Coimbra Leite Costa Rodrigo de Lemos Peroni Jair Carlos Koppe 《Natural Resources Research》2011,20(2):131-142
Data from a mineral deposit are commonly obtained by core drilling. This kind of sampling involves high costs, limiting the
number of drill holes. Additional holes should be located to bring the maximum benefit. The benefit can be evaluated by various
ways and must take into account the goals of sampling. This article presents a case study where the deposit has been sampled
and the new drillings must be added to reduce the uncertainty about a transfer function, Net Present Value (NPV) of the mining
project. There are basically two ways to choose locations where new drillings should be placed for cases where the aim of
sampling is to reduce uncertainty about a global function: the addition of new drillings outlining a quasi regular grid with
previously collected drillings or the addition of new drillings on the locations of high uncertainty about the attribute of
interest (or the attribute that is considered most influential in the transfer function). The performances of these patterns
on reducing the uncertainty measured by the function selected are compared. The results point out that the most efficient
pattern relates to the distribution (histogram) of the uncertainty about the attribute of interest. Thus, the choice of which
sampling pattern should be adopted varies depending on data distribution and its influence on the transfer function. 相似文献
74.
Journal of Geographical Systems - 相似文献
75.
Evaluating the genetic effects of the invasive Ocenebra inornata on the native oyster drill Ocenebra erinacea
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Eric Pante Pierre‐Yves Pascal Vanessa Becquet Amélia Viricel Benoit Simon‐Bouhet Pascale Garcia 《Marine Ecology》2015,36(4):1118-1128
Studies focusing on the effect of invasive species on the genetic diversity of native marine invertebrates remain scant. Here we report diversity among French populations of the intertidal gastropod Ocenebra erinacea (Linnaeus, 1758) sampled in the presence and absence of the invasive Ocenebra inornata (Recluz, 1851). Between 1999 and 2004, a total of 352 individuals of O. erinacea was collected from 15 sites (five of which had the invasive present) and was genotyped at the mitochondrial locus Cytochrome Oxidase 1 (cox1). No statistical difference was observed between polymorphism levels recorded within native populations exposed to the invasive, compared with populations sampled in the absence of O. inornata. No sign of native population decline was detected in response to the invader. While significant shifts in native O. erinacea population sizes have previously been reported in the literature, genetic effects may take longer to accumulate, or may be undetectable without a larger panel of genetic markers. By contrast, large genetic distances and significant population differentiation were recorded between Atlantic and Mediterranean O. erinacea samples, suggesting that these populations have distinct evolutionary histories. Comparison of genetic divergence within the closely related genus Nucella suggests that the Atlantic populations of O. erinacea and those from Thau Lagoon in the Mediterranean may belong to different species or subspecies. 相似文献
76.
Vanessa Thorn 《Geology Today》2007,23(4):153-157
With increasing interest in climate change and other issues associated with the history of the environment and anthropogenic evolution there is an ongoing requirement to investigate the Earth's natural systems. A key approach is for scientists to look back into geological time, perhaps millions of years, to see how the world has reacted to natural climatic changes in the past. Using evidence from the fossil record, fluctuations in palaeoclimate can be assessed with reference to changes in global floral and faunal patterns. Phytoliths, Greek for 'plant stones', is a fossil group that is being increasingly utilized in the understanding of past climates and environments. 相似文献
77.
The Fundamentals of Radionuclide Migration (FUNMIG) project was organised in some Research and Technological Development Components (RTDCs) integrated by a number of Work Packages (WP’s) attempting to cover the key migration processes at different scales (laboratory to repository) and in different geological media, mainly clay and granite, but also some salt rock related work was performed. The main RTDCs of interest for this review paper are numbers 1 and 2, which concerned laboratory studies of well-established and less-established migration processes. Some additional interesting outcomes arise also from RTDCs 3, 4 and 5 devoted to the upscaling and applicability of migration processes to clay, granite and salt rock environments, respectively. This review discusses the contribution of the various parts of the FUNMIG project in the process oriented context of understanding the mechanistic approximation to what can be seen as the final and thermodynamically stable stage of the long chain of processes which are covered under the general term of sorption and that culminate in the formation of solid solutions. 相似文献
78.
Christopher Potter Steven Klooster Seth Hiatt Matthew Fladeland Vanessa Genovese Peggy Gross 《Climatic change》2007,80(3-4):323-336
Afforestation of marginal agricultural lands represents a promising option for carbon sequestration in terrestrial ecosystems.
An ecosystem carbon model was used to generate new national maps of annual net primary production (NPP), one each for continuous
land covers of ‘forest’, ‘crop’, and ‘rangeland’ over the entire U. S. continental area. Direct inputs of satellite “greenness”
data from the Advanced Very High Resolution Radiometer (AVHRR) sensor into the NASA-CASA carbon model at 8-km spatial resolution
were used to estimate spatial variability in monthly NPP and potential biomass accumulation rates in a uniquely detailed manner.
The model predictions of regrowth forest production lead to a conservative national projection of 0.3 Pg C as potential carbon
stored each year on relatively low-production crop or rangeland areas. On a regional level, the top five states for total
crop afforestation potential were: Texas, Minnesota, Iowa, Illinois, and Missouri, whereas the top five states for total rangeland
afforestation potential are: Texas, California, Montana, New Mexico, and Colorado. Afforestation at this level of intensity
has the capacity to offset at least one-fifth of annual fossil fuel emission of carbon in the United States. These projected
afforestation carbon gains also match or exceed recent estimates of the annual sink for atmospheric CO2 in currently forested area of the country. 相似文献
79.
Vanessa R. Levesque Aram J. K. Calhoun Kathleen P. Bell Teresa R. Johnson 《社会与自然资源》2017,30(2):245-260
Given the complexity and multiplicity of goals in natural resource governance, it is not surprising that policy debates are often characterized by contention and competition. Yet at times adversaries join together to collaborate to find creative solutions not easily achieved in polarizing forums. We employed qualitative interviews and a quantitative network analysis to investigate a collaborative network that formed to develop a resolution to a challenging natural resource management problem, the conservation of vernal pools. We found that power had become distributed among members, trust had formed across core interests, and social learning had resulted in shared understanding and joint solutions. Furthermore, institutions such as who and when new members joined, norms of inclusion and openness, and the use of small working groups helped create the observed patterns of power, trust, and learning. 相似文献
80.