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121.
Investigating the ancient Martian magnetic field using microwaves   总被引:1,自引:0,他引:1  
The new microwave palaeointensity technique has been used to investigate samples from the Martian meteorite Nakhla. This technique is a promising new way to obtain absolute palaeointensity information regarding the ancient Martian magnetic field as recorded by the Martian meteorites. Assuming that a part of the magnetic remanence is of thermal origin and originating on Mars the two samples studied yield estimates of 4 μT for the Martian magnetic field at 1.35 Ga.  相似文献   
122.
Volcán Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km3 of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19-20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km3). Throughout medial-distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km3) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km3). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km3). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe-Ti oxides-apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO2), lower crystal contents (17-20%), lower density (1.0-1.3 g/cm3), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO2) compositions, higher crystal contents (25-35%), higher densities (up to 2.2 g/cm3), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24-32 km and a total column height of 34-46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4᎒8 kg/s and the volumetric discharge rate is ~3.6᎒5 m3/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km3 total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW-SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano.  相似文献   
123.
An integrated modelling approach (MIRSED) which utilizes the process‐based soil erosion model WEPP (Water Erosion Prediction Project) is presented for the assessment of hillslope‐scale soil erosion at five sites throughout England and Wales. The methodology draws upon previous uncertainty analysis of the WEPP hillslope soil erosion model by the authors to qualify model results within an uncertainty framework. A method for incorporating model uncertainty from a range of sources is discussed as a first step towards using and learning from results produced through the GLUE (Generalized Likelihood Uncertainty Estimation) technique. Results are presented and compared to available observed data, which illustrate that levels of uncertainty are significant and must be taken into account if a meaningful understanding of output from models such as WEPP is to be achieved. Furthermore, the collection of quality, observed data is underlined for two reasons: as an essential tool in the development of soil erosion modelling and also to allow further constraint of model uncertainty. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
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125.
The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m−2. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.  相似文献   
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128.
A case study was conducted on the potential impacts of climate change on fish habitat in a southeastern reservoir. A reservoir water quality model and one year of baseline meteorologic, hydrologic, and inflow water quality input were used to simulate current reservoir water quality. Total adult striped bass habitat, defined by specific quantitative temperature and dissolved oxygen criteria, was simulated. Daily reservoir volumes with optimal, suboptimal, and unsuitable temperature and DO were predicted for the year. Output from recent runs of atmospheric general circulation models (GCMs), in which atmospheric carbon dioxide concentrations have been doubled, was then used to adjust the baseline inputs to the water quality model. New sets of input data were created for two grid cells for each of three GCMs. All six climate scenarios are predicted to cause overall declines in the available summer striped bass habitat, mostly due to lake water temperatures exceeding striped bass tolerance levels. These predictions are believed to result from the consensus among GCM scenarios that air temperatures and humidity will rise, and the sensitivity of the reservoir model to these parameters. The reservoir model was found to be a promising tool for examining potential climate-change impacts. Some of the assumptions required to apply GCM output to the reservoir model, however, illustrate the problems in using large-scale gridcell output to assess small-scale impacts.  相似文献   
129.
The lack of correspondence between traditional economic development measures, such as GNP per capita, and human welfare measures, such as the PQLI, has been well documented in the literature. We devised and analyzed an economic development-welfare disparity measure, and found considerable geographic variability, with actual levels of human welfare much less than expected in many Middle Eastern and African countries, given the level of economic development. Exploratory correlation and regression analysis revealed that inequalities in the distribution of economic power and governmental expenditures contributed to the geographic variability. More importantly, each region improved its standing on the development-welfare disparity measure when relevant factors were controlled via multivariate regression.  相似文献   
130.
Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) BARNAMOL = barnacle/bivalve-dominated; (b) BIMOL = bivalve-dominated; (c) BRYOMOL = bryozoan/bivalve-dominated; (d) ECHINOFOR = echinoderm/benthic foraminiferal-dominated; (e) NANNOFOR = nannofossil/planktonic foraminiferal-dominated; (f) RHODALGAL = calcareous red algal-dominated; and (g) RHODECHFOR = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.  相似文献   
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