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When viewed from the air, Scottish ‘hummocky moraine’ can be resolved into a series of linear ridges that resemble those found at the margins of actively retreating glaciers today. Recent work has supported the interpretation of these linear ridges as ice-marginal landforms and the authors believe that the majority of ‘hummocky moraine’ deposits can be interpreted in this way. Consequently the pattern of deglaciation can be established fairly precisely from the pattern of linear ridges. This approach is applied to the landforms of the northern part of the Loch Lomond Stadial ice-field in order to reconstruct the regional pattern of deglaciation. This leads to important inferences about the significance of topographic control during deglaciation and more importantly it provides fresh insight into the environment of the British Isles during the Loch Lomond Stadial. 相似文献
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Matthew E. Hill Jr. Matthew G. Hill Christopher C. Widga 《Quaternary Science Reviews》2008,27(17-18):1752-1771
Body size changes of Bison and mortality age structure data document the effects of climate-driven environmental change and human hunting pressure on large mammals in North America. Morphometric and mortality data are drawn from 58 archaeological and 9 paleontological localities dating between 37,000 and 250 calBP. Proxy information on body size is based on measurements recorded on 901 adult calcanei (os calcis) and 1026 humeri. In addition, published mortality profiles from 24 archaeological faunal assemblages spanning the last 14,000 years were used to approximate the age structure of bison populations. These results suggest that dramatic diminution in bison body size occurred in several short bursts, rather than a continuous gradual decline. These periods of rapid size reduction correlate with times of ecological reorganization, when aridity-driven changes in grasslands decreased forage quality and availability. Mortality age data indicate that the decrease in body size occurred in a context where there was no evidence for a progressively severe juvenile bias in bison populations. Overall, it appears that the changes in body size were a reaction to environmental conditions rather than the result of human predation pressure. 相似文献
5.
Bruno Lanson Matthew A. Marcus Sirine Fakra Frdric Panfili Nicolas Geoffroy Alain Manceau 《Geochimica et cosmochimica acta》2008,72(10):2478-2490
It is now well established that a number of terrestrial and aquatic microorganisms have the capacity to oxidize and precipitate Mn as phyllomanganate. However, this biomineralization has never been shown to occur in plant tissues, nor has the structure of a natural Mn(IV) biooxide been characterized in detail. We show that the graminaceous plant Festuca rubra (red fescue) produces a Zn-rich phyllomanganate with constant Zn:Mn and Ca:Mn atomic ratios (0.46 and 0.38, respectively) when grown on a contaminated sediment. This new phase is so far the Zn-richest manganate known to form in nature (chalcophanite has a Zn:Mn ratio of 0.33) and has no synthetic equivalent. Visual examination of root fragments under a microscope shows black precipitates about ten to several tens of microns in size, and their imaging with backscattered and secondary electrons demonstrates that they are located in the root epidermis. In situ measurements by Mn and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) with a micro-focused beam can be quantitatively described by a single-phase model consisting of Mn(IV) octahedral layers with 22% vacant sites capped with tetrahedral and octahedral Zn in proportions of 3:1. The layer charge deficit is also partly balanced by interlayer Mn and Ca. Diffracting crystallites have a domain radius of 33 Å in the ab plane and contain only 1.2 layers (8.6 Å) on average. Since this biogenic Mn oxide consists mostly of isolated layers, basal 00l reflections are essentially absent despite its lamellar structure. Individual Mn layers are probably held together in the Mn–Zn precipitates by stabilizing organic molecules. Zinc biomineralization by plants likely is a defense mechanism against toxicity induced by excess concentrations of this metal in the rhizosphere. 相似文献
6.
Hayes RA Regondi S Winter MJ Butler PJ Agradi E Taylor EW Kevin Chipman J 《Marine environmental research》2004,58(2-5):665-669
Metallothionein has been assayed in a range of aquatic animal tissues as an indicator of metal exposure. We sequenced chub (Leuciscus cephalus) metallothionein cDNA which showed over 90% homology to common carp, goldfish and stone loach and 77% homology to rainbow trout sequences for metallothionein. We then used the extended primer method to develop an accurate quantitative competitive RT-PCR assay for metallothionein mRNA. RT-PCR was used to measure metallothionein mRNA in feral chub from a range of field sites, with different levels of heavy metal pollution, in the West Midlands, UK. Measurements were complemented by analysis of liver and gill metallothionein protein by capillary electrophoresis. There was no significant difference in the metallothionein protein levels between fish of different rivers and there was no evidence of elevation of mRNA at the sites of highest metal exposure. The level of metal exposure (e.g. zinc, nickel and cadmium each ranging between 15 and 28 microg/l ) at the pH (7.5-8.5) of these rivers appears insufficient to elevate hepatic or gill metallothionein in chub. A lack of elevation of hepatic metallothionein mRNA in chub exposed to zinc, copper and manganese for 24 h and 10 days in the laboratory also suggests a non-responsiveness of this species. 相似文献
7.
In this paper, we formulate a finite-element procedure for approximating the coupled fluid and mechanics in Biot’s consolidation
model of poroelasticity. We approximate the flow variables by a mixed finite-element space and the displacement by a family
of discontinuous Galerkin methods. Theoretical convergence error estimates are derived and, in particular, are shown to be
independent of the constrained specific storage coefficient, c
o
. This suggests that our proposed algorithm is a potentially effective way to combat locking, or the nonphysical pressure
oscillations, which sometimes arise in numerical algorithms for poroelasticity. 相似文献
8.
Peter H. Whetton Matthew H. England Siobhan P. O'Farrell Ian G. Watterson A. Barrie Pittock 《Climatic change》1996,33(4):497-519
The extent of agreement amongst current global climate models (GCMs) on the global pattern of rainfall change simulated under enhanced greenhouse conditions is assessed. We consider the results of five experiments which use a simple mixed layer ocean formulation and five which use a fully dynamic ocean model (coupled experiments). For many regions of the northern hemisphere there is strong agreement amongst both mixed layer and coupled experiments on the sign of simulated rainfall change. However, in the southern hemisphere there are large, and apparently systematic, differences between the coupled and mixed layer experiments. In particular, whereas the mixed layer experiments agree on simulated rainfall increase in summer in the tropics and subtropics of the Australian sector, the coupled experiments agree (although more weakly) on rainfall decreases. These differences appear to relate to the much reduced warming simulated by the coupled experiments in the high latitudes of the southern hemisphere. However, recent oceanographie evidence suggests that this suppressed warming may be considerably overestimated. We conclude therefore that despite the in-principle advantages of coupled models, it may be too soon to base some regionally specific climate change scenarios solely on the results of coupled experiments. 相似文献
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Matthew J. Spittal Frank H. Walkey John McClure Richard J. Siegert Kimberley E. Ballantyne 《Natural Hazards》2006,39(1):15-29
Preparedness is a key dependent variable in many studies examining people’s response to disasters such as earthquakes. A feature
of many studies on this issue, however, is the lack of attention given to psychometric issues when constructing measures of
preparedness. With regard to earthquake preparation, for example, many studies could be greatly improved by the use of a valid
and reliable measure of preparedness. This research developed such a measure that assessed both low-level preparedness, such
as having an emergency kit, and high-level preparedness, such as altering home structures to mitigate damage. Studies of Wellington
(New Zealand) residents using two samples totalling n=652 showed that 23 items measuring these different aspects of earthquake preparation could be combined into a reliable, valid,
unifactorial scale. This brief scale should have utility in multivariate studies of earthquake preparation, either as a dependent
variable, where preparation is the outcome variable of primary interest, or as one of several independent variables, where
preparation and other measures predict another outcome variable. 相似文献