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Susan J. Webb Lewis D. Ashwal R. Grant Cawthorn 《Contributions to Mineralogy and Petrology》2011,162(1):101-107
The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06?Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150?km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry. 相似文献
295.
Arthur W. Wells J. Rodney Diehl Grant Bromhal Brian R. Strazisar Thomas H. Wilson Curt M. White 《Applied Geochemistry》2007
Geological sequestration of CO2 in depleted oil reservoirs is a potentially useful strategy for greenhouse gas management and can be combined with enhanced oil recovery. Development of methods to estimate CO2 leakage rates is essential to assure that storage objectives are being met at sequestration facilities. Perfluorocarbon tracers (PFTs) were added as three 12 h slugs at about one week intervals during the injection of 2090 tons of CO2 into the West Pearl Queen (WPQ) depleted oil formation, sequestration pilot study site located in SE New Mexico. The CO2 was injected into the Permian Queen Formation. Leakage was monitored in soil–gas using a matrix of 40 capillary adsorbent tubes (CATs) left in the soil for periods ranging from days to months. The tracers, perfluoro-1,2-dimethylcyclohexane (PDCH), perfluorotrimethylcyclohexane (PTCH) and perfluorodimethylcyclobutane (PDCB), were analyzed using thermal desorption, and gas chromatography with electron capture detection. Monitoring was designed to look for immediate leakage, such as at the injection well bore and at nearby wells, and to develop the technology to estimate overall CO2 leak rates based on the use of PFTs. Tracers were detected in soil–gas at the monitoring sites 50 m from the injection well within days of injection. Tracers continued to escape over the following years. Leakage appears to have emanated from the vicinity of the injection well in a radial pattern to about 100 m and in directional patterns to 300 m. Leakage rates were estimated for the 3 tracers from each of the 4 sets of CATs in place following the start of CO2 injection. Leakage was fairly uniform during this period. As a first approximation, the CO2 leak rate was estimated at about 0.0085% of the total CO2 sequestered per annum. 相似文献
296.
Emerging infectious diseases (EIDs) of wildlife have characteristics that make them difficult to manage, leading to reactive and often ineffective management strategies. Currently, two fungal pathogens, Pseudogymnoascus destructans (Pd) and Batrachochytrium salamandrivorans (Bsal), are causing declines in novel host species. To improve the application of management strategies addressing the risk of these pathogens to North American wildlife, we queried wildlife managers about their concerns regarding managing populations of bats and amphibians potentially impacted by Pd and Bsal. Using these responses, we identified aspects of each decision problem that were shared across pathogens, regions and agencies – and found similarities in decision-problem elements for disease management. Reframing management problems as decisions can enable managers to identify similarities across EIDs, i.e. uncertainties within management actions, and improve reactive responses if proactive management is not possible. Such an approach recognizes context-specific constraints and identifies relevant uncertainties that must be reduced in developing a response. 相似文献
297.
Idealized forcing experiments with 1% per year CO2 increase to stabilized doubled and quadrupled CO2, twenty-first century transient scenario experiments (SRES scenarios A1B and B1), and stabilized twenty-second century A1B and B1 experiments with two global coupled climate models (PCM and CCSM3) are analyzed for possible future changes of El Niño events. With increased CO2 in the models, there is a reduction of amplitude of El Niño events. This is particularly apparent with larger forcing in the stabilized 4×CO2 experiment in PCM and the stabilized greenhouse gas A1B experiment in CCSM3, where the reduction of amplitude is outside the range of the inherent multi-century variability of El Niño in the control runs of the models and is statistically significant. With moderately increased forcing (stabilized 2×CO2 in PCM and the stabilized B1 experiment in CCSM3), the reduction in amplitude is evident, but it is not significant. The change in El Niño behavior with larger forcing is attributed to the change in base state temperature in the equatorial Pacific, which is similar with increased greenhouse gases (GHGs) in both models. Positive temperature anomalies in and below the thermocline, associated with a reduction of the trade winds, and weakened Pacific Ocean subtropical cells, produce a less intense thermocline, and consequently lower amplitude El Niño events. The previously noted intensification of El Niño tropical precipitation anomalies in a warmer mean base state that applied when there was no appreciable change in El Niño amplitude does not hold in the present study where the El Niño events decrease in magnitude in a future warmer climate. North American surface temperature anomalies associated with El Niño are reduced and become less significant in the future events, with the anomalously deepened Aleutian low in the North Pacific weakened and moved eastward with greater radiative forcing. Part of this is attributed to the smaller amplitude events and thus lower amplitude teleconnections as indicated by contrasting composites of medium and high amplitude El Niño events from the control runs. The change in midlatitude base state circulation also contributes to the change in El Niño teleconnections. The effects of this change in base state on the weakened El Niño teleconnections over North America are confirmed in sensitivity experiments with a version of the atmospheric model in which heating anomalies are specified to mimic El Niño events in a base state changed due to increased GHGs. 相似文献
298.
John A. Grant Matthew P. Golombek John P. Grotzinger Sharon A. Wilson Michael M. Watkins Ashwin R. Vasavada Jennifer L. Griffes Timothy J. Parker 《Planetary and Space Science》2011,59(11-12):1114-1127
The process of identifying the landing site for NASA’s 2011 Mars Science Laboratory (MSL) began in 2005 by defining science objectives, related to evaluating the potential habitability of a location on Mars, and engineering parameters, such as elevation, latitude, winds, and rock abundance, to determine acceptable surface and atmospheric characteristics. Nearly 60 candidate sites were considered at a series of open workshops in the years leading up to the launch. During that period, iteration between evolving engineering constraints and the relative science potential of candidate sites led to consensus on four final sites. The final site will be selected in the Spring of 2011 by NASA’s Associate Administrator for the Science Mission Directorate. This paper serves as a record of landing site selection activities related primarily to science, an inventory of the number and variety of sites proposed, and a summary of the science potential of the highest ranking sites. 相似文献
299.
Alastair Grant 《Marine Ecology》2000,21(2):97-112
Abstract. Much of the discussion of high deep‐sea diversity has assumed that asymmetric inter‐specific competition will rapidly lead to the elimination of many species unless restrained by extensive differentiation of niches, or the action of predation and/or environmental disturbance. This is true for some habitats, including rocky shores. However, experimental studies indicate that marine soft sediment communities do not function like this. In shallow‐water sediments, competition is usually symmetric and relatively weak. Asymmetric competition that leads to the elimination of one species by another on time scales shorter than one generation is rare, apart from interactions that involve large‐scale modification or disturbance of the sediment. Competition is therefore relatively unimportant as a process structuring communities and the impact of predation is usually to reduce rather than enhance diversity. These results have been largely ignored by the literature on deep‐sea diversity. If deep‐sea communities function in similar ways, coexistence of many species within small areas on short time scales does not need further explanation. We do , however, need to explain why abundances of all species remain bounded on large spatial scales and time scales of several generations. The model of diversity maintenance proposed by Huston (1979) and applied to the deep sea by Rex (1983) achieves this by implicit intra‐specific, density‐dependent processes that increase the population growth rate of species that have become locally rare. This would give robust maintenance of diversity, but there is no evidence for density dependence of this type in the deep sea, and no plausible mechanisms by which it could occur. Alternative models require either spatial heterogeneity on a scale much larger than that envisaged by the grain‐matching hypothesis or the placing of a cap on the abundance of common species, perhaps by frequency‐dependent predation. Arbitrating between these possibilities will require assessments of the population dynamics and spatial distribution of individual species on spatial and temporal scales much greater than those usually considered in the deep sea. 相似文献
300.
Unfinished Business in Geothermal Energy 总被引:1,自引:0,他引:1
by Grant Ferguson 《Ground water》2009,47(2):167-167