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901.
Mark A Clilverd Ellen Clarke Henry Rishbeth Toby D G Clark Thomas Ulich 《Astronomy& Geophysics》2003,44(5):5.20-5.22
Mark A Clilverd, Ellen Clarke, Henry Rishbeth, Toby D G Clark and Thomas Ulich look forward to a little less solar activity in 2100, using direct and proxy records of past solar and geomagnetic activity. 相似文献
902.
The inability to detect the presence of viable Helicobacter pylori bacteria in environmental waters has hindered the public health community in assessing the role water may play in the transmission of this pathogen. This work describes a cultural enrichment method coupled with an H. pylori‐specific PCR to identify these bacteria in water. While far from perfected at the present time, this represents an exciting new approach to studying the significance of water as a transmission mechanism for H. pylori. Evidence is presented that indicates culturable H. pylori bacteria were found using this enrichment/PCR method in a local groundwater source. 相似文献
903.
Hannington Mark D. Kjarsgaard Ingrid M. Galley Alan G. Taylor Bruce 《Mineralium Deposita》2003,38(4):423-442
Mineralium Deposita - The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The... 相似文献
904.
Brian Fry Donald M. Baltz Mark C. Benfield John W. Fleeger Arian Gace Heather L. Haas Zoraida J. Quiñones-Rivera 《Estuaries and Coasts》2003,26(1):82-97
Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To
study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations,
we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km2 marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m−2 in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m−2 in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small
harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical
approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations.
Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow
turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep
channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view
that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through
sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or
shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth
rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that
in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation
and restoration efforts should focus on these areas as important for sustaining shrimp fisheries. 相似文献
905.
Mark E Hodson 《Geochimica et cosmochimica acta》2002,66(5):819-828
A Soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the B horizon and then 1 cm of soil from the A horizon of a granitic podzol. Major and trace elements were leached from the column and accumulated in a reservoir at the base of the column. Total loss of elements from the soil over the course of the experiment ranged from 0.002 to 1 wt% with major elements and the light and heavy rare earth elements (REE) showing the largest percentage losses. Zirconium (0.002%) and then Al (0.008%) showed the lowest percentage loss. The light REE were leached out of the soil preferentially to the mid REE. All elements showed accumulation, by a factor of 2 to 11, in the quartz layers at the base of the column, particularly in the upper first 1 cm of the quartz. Major elements were leached from the column at a rate of 0.02 to 0.59 μmol h−1 whereas Zr, Nd, Sm, Gd, Dy, Rb, and Sr were leached at the rate of 0.5 to 30 × 10−6 μmol h−1. Concentrations of other REE in the reservoir increased over the duration of the experiment, but they were poorly correlated with time, so leaching rates were not calculated. Normalization of the major element leaching rates to take into account the constant flushing of water through the column, the average annual rainfall in the Allt a’Mharcaidh catchment in Scotland from where the soil was sampled, and the cross-sectional area of the soil in the column, together with the temperature of the soil in the column (70°C) compared with the average annual temperature of the Allt a’Mharcaidh catchment (5.7°C), gave major element release rates from the soil of 0.002 to 0.97 mEq m−2 yr−1 (depending on the choice of Ea, the dissolution activation energy), which are generally less than those measured in the field of 0.1 to 40.9 mEq m−2 yr−1.Calculations showed that despite the redistribution and loss of Zr from the column, assumptions of Zr mobility would have had a negligible effect on calculated element release rates of Na, Ca, Fe, and Mg. However, significant underestimates of the release of K (5%), Ti (57%), Al (5%), and Si (10%) as well as some trace elements (e.g., Nd, 23%; Rb, 54%; Sr, 24%) would have occurred. Concentrations of Ca and Sr leached from the column correlated well (RSQ = 0.93, p < 0.01), supporting the idea of the use of Sr release as a proxy for Ca release in weathering rate calculations. The release rates and percentage loss of REE from the soil varied between elements indicating that REE distribution patterns of rocks and soils may not be preserved in drainage waters. 相似文献
906.
We examine a database containing the results of 40Ar/39Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of 40Ar/39Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and 39Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield 40Ar/39Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar 40Ar/39Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms—a requirement implicitly met by those samples exhibiting high degrees of correlation. 相似文献
907.
Lateral thinking: 2-D interpretation of thermochronology in convergent orogenic settings 总被引:1,自引:0,他引:1
Lateral motion of material relative to the regional thermal and kinematic frameworks is important in the interpretation of thermochronology in convergent orogens. Although cooling ages in denuded settings are commonly linked to exhumation, such data are not related to instantaneous behavior but rather to an integration of the exhumation rates experienced between the thermochronological ‘closure’ at depth and subsequent exposure at the surface. The short spatial wavelength variation of thermal structure and denudation rate typical of orogenic regions thus renders thermochronometers sensitive to lateral motion during exhumation. The significance of this lateral motion varies in proportion with closure temperature, which controls the depth at which isotopic closure occurs, and hence, the range of time and length scales over which such data integrate sample histories. Different chronometers thus vary in the fundamental aspects of the orogenic character to which they are sensitive. Isotopic systems with high closure temperature are more sensitive to exhumation paths and the variation in denudation and thermal structure across a region, while those of lower closure temperature constrain shorter-term behaviour and more local conditions.Discounting lateral motion through an orogenic region and interpreting cooling ages purely in terms of vertical exhumation can produce ambiguous results because variation in the cooling rate can result from either change in kinematics over time or the translation of samples through spatially varying conditions. Resolving this ambiguity requires explicit consideration of the physical and thermal framework experienced by samples during their exhumation. This can be best achieved through numerical simulations coupling kinematic deformation to thermal evolution. Such an approach allows the thermochronological implications of different kinematic scenarios to be tested, and thus provides an important means of assessing the contribution of lateral motion to orogenic evolution. 相似文献
908.
Wendy M. Peace Malcom W. Wallace Mark P. Holdstock John H. Ashton 《Mineralium Deposita》2002,38(5):568-584
Recent exploration in the vicinity of the giant (>90 Mt) Navan orebody has resulted in the discovery of ore-grade mineralisation to the southwest of the deposit, much of which occurs within the Upper Pale Beds, a horizon that is only weakly mineralised above the main orebody. Within this new U lens, mineralisation preferentially occurs within bioclastic carbonate grainstones and calcareous quartz sandstones, and is dominated by sulphide replacement of the carbonate component of the host sequence. Much of the replacive mineralisation is spatially associated with hydrothermal cavities, which are filled by a variable mixture of brecciated replacement sulphide minerals, space-filling sulphide and gangue cements, and internal sediments. Mineralisation also occurs within veins and dissolution seams, and as disseminated sulphide minerals. Massive mineralisation is typically a complex, chaotic, combination of replacement, cavity, and fracture-filling sulphides. Fluid inclusion analyses of ore-stage saddle dolomite indicate temperatures at the time of sulphide precipitation of ~90–150 °C, with a maximum of 175 °C. These temperatures are lower than those typically proposed for Irish-type deposits. 相似文献
909.
Mark Purcell 《GeoJournal》2002,58(2-3):99-108
Much current activism and scholarship has raised concern that the various processes of neoliberal restructuring are threatening democracy. More specifically, researchers in geography and other social sciences have stressed that political and economic restructuring in cities is negatively affecting the enfranchisement of urban residents. Much recent research and writing has explored progressive responses to this perceived disenfranchisement in cities. One popular trend has been a fascination with the idea of the `right to the city' as a way to respond to neoliberal urbanism and better empower urban dwellers. I argue that the right to the city holds promise, but that in the literature the idea remains both theoretically and politically underdeveloped. It remains unclear (1) what the right to the city entails or (2) how it might address current problems of disenfranchisement. This paper examines the right to the city in greater depth. It does so by offering a close reading and analysis of the intellectual roots of the idea: the writings of Henri Lefebvre. I suggest that Lefebvre's right to the city is more radical, more problematic, and more indeterminate than the current literature makes it seem. The paper concludes by suggesting that the right to the city does offer distinct potential for resisting current threats to urban enfranchisement. However, the right to the city is not a panacea. It must be seen not as a completed solution to current problems, but as an opening to a new urban politics, what I call an urban politics of the inhabitant. 相似文献
910.
Mark L. Wilson 《Journal of Geographical Systems》2002,4(1):31-42
Many infectious diseases that are emerging or transmitted by arthropod vectors have a strong link to landscape features.
Depending on the source of infection or ecology of the transmitting vector, micro-habitat characteristics at the spatial scale
of square meters or less may be important. Recently, satellite images have been used to classify habitats in an attempt to
understand associations with infectious diseases. Whether high spatial resolution and hyperspectral (HSRH) images can be useful
in studies of such infectious diseases is addressed. The nature of questions that such studies address and the desired accuracy
and precision of answers will determine the utility of HSRH data. Need for such data should be based on the goals of the effort.
Examples of kinds of questions and applications are discussed. The research implications and public health applications may
depend on available analytic tools as well as epidemiological observations.
Received: 30 July 2001 / Accepted: 14 October 2001 相似文献