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1.
Young DA Wright AP Roberts JL Warner RC Young NW Greenbaum JS Schroeder DM Holt JW Sugden DE Blankenship DD van Ommen TD Siegert MJ 《Nature》2011,474(7349):72-75
The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ~34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5?km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1?km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes. 相似文献
2.
McConnell NJ Ma CP Gebhardt K Wright SA Murphy JD Lauer TR Graham JR Richstone DO 《Nature》2011,480(7376):215-218
Observational work conducted over the past few decades indicates that all massive galaxies have supermassive black holes at their centres. Although the luminosities and brightness fluctuations of quasars in the early Universe suggest that some were powered by black holes with masses greater than 10 billion solar masses, the remnants of these objects have not been found in the nearby Universe. The giant elliptical galaxy Messier 87 hosts the hitherto most massive known black hole, which has a mass of 6.3 billion solar masses. Here we report that NGC 3842, the brightest galaxy in a cluster at a distance from Earth of 98 megaparsecs, has a central black hole with a mass of 9.7 billion solar masses, and that a black hole of comparable or greater mass is present in NGC 4889, the brightest galaxy in the Coma cluster (at a distance of 103 megaparsecs). These two black holes are significantly more massive than predicted by linearly extrapolating the widely used correlations between black-hole mass and the stellar velocity dispersion or bulge luminosity of the host galaxy. Although these correlations remain useful for predicting black-hole masses in less massive elliptical galaxies, our measurements suggest that different evolutionary processes influence the growth of the largest galaxies and their black holes. 相似文献
3.
Using an Action Research Framework to Explore Assessment: A South African Case Study 总被引:1,自引:1,他引:0
Penny Singh 《Systemic Practice and Action Research》2006,19(2):179-187
This article is a reflection of my first experience with action research in trying to devise a system of assessment that would benefit both first and second language speakers of English in an assessment system that is plagued by its separatist past. In this paper, I discuss an action research study on oral assessment implemented at two tertiary institutions in South Africa. The actual details of each phase of the assessments are not reported on, but the methods used to facilitate the processes are. The findings and adaptations after each phase of the assessments are discussed to show the effectiveness of using an action research methodology.
相似文献
Penny SinghEmail: |
4.
This article is about structural realism, historical continuity, laws of nature, and ceteris paribus clauses. Fresnel's Laws of optics support Structural Realism because they are a scientific structure that has survived theory change. However, the history of Fresnel's Laws which has been depicted in debates over realism since the 1980s is badly distorted. Specifically, claims that J. C. Maxwell or his followers believed in an ontologically-subsistent electromagnetic field, and gave up the aether, before Einstein's annus mirabilis in 1905 are indefensible. Related claims that Maxwell himself did not believe in a luminiferous aether are also indefensible. This paper corrects the record. In order to trace Fresnel's Laws across significant ontological changes, they must be followed past Einstein into modern physics and nonlinear optics. I develop the philosophical implications of a more accurate history, and analyze Fresnel's Laws' historical trajectory in terms of dynamic ceteris paribus clauses. Structuralists have not embraced ceteris paribus laws, but they continue to point to Fresnel's Laws to resist anti-realist arguments from theory change. Fresnel's Laws fit the standard definition of a ceteris paribus law as a law applicable only in particular circumstances. Realists who appeal to the historical continuity of Fresnel's Laws to combat anti-realists must incorporate ceteris paribus laws into their metaphysics. 相似文献
5.
Wright FA Strug LJ Doshi VK Commander CW Blackman SM Sun L Berthiaume Y Cutler D Cojocaru A Collaco JM Corey M Dorfman R Goddard K Green D Kent JW Lange EM Lee S Li W Luo J Mayhew GM Naughton KM Pace RG Paré P Rommens JM Sandford A Stonebraker JR Sun W Taylor C Vanscoy LL Zou F Blangero J Zielenski J O'Neal WK Drumm ML Durie PR Knowles MR Cutting GR 《Nature genetics》2011,43(6):539-546
A combined genome-wide association and linkage study was used to identify loci causing variation in cystic fibrosis lung disease severity. We identified a significant association (P = 3.34 × 10(-8)) near EHF and APIP (chr11p13) in p.Phe508del homozygotes (n = 1,978). The association replicated in p.Phe508del homozygotes (P = 0.006) from a separate family based study (n = 557), with P = 1.49 × 10(-9) for the three-study joint meta-analysis. Linkage analysis of 486 sibling pairs from the family based study identified a significant quantitative trait locus on chromosome 20q13.2 (log(10) odds = 5.03). Our findings provide insight into the causes of variation in lung disease severity in cystic fibrosis and suggest new therapeutic targets for this life-limiting disorder. 相似文献
6.
We studied kit fox den characteristics of the northern McGregor Range of Fort Bliss Military Reservation, New Mexico, during 1994 to 1995. Twenty radio-collared kit foxes used 132 different dens, including 16 natal dens. Kit fox dens were located primarily in creosote-dominated habitat found in relatively flat, well-drained terrain. Natal dens were virtually indistinguishable from non-natal dens; however, natal den entrances were taller than non-natal den entrances. Entrances found at all dens were oriented more frequently toward the northwest and southwest. Kit foxes used more dens during the breeding (January-February) and pup-rearing season (May-July) than during gestation. 相似文献
7.
We compared the effects ofAlzheimer's peptide (Abeta1-42), a,-antichymotrypsin (ACT) and an ACT/Abeta1-42 mixture on human glioma DK-MG cells. The solution of Abeta (5 microM) formed by 2-h incubation at room temperature induced tumour necrosis factor-alpha (TNF-alpha) and interleukin (IL)-6 levels by 55 and 45%, respectively, and increased gelatinase B activity by 67%, while exposure of cells to the ACT/Abeta1-42 mixture (1:10 molar ratio ACT: Abeta1-42) under the same experimental conditions showed no effect on IL-6 levels or gelatinase B activity, but strongly induced TNF-alpha (by 190%), compared to the controls. Stimulation of the cells with Abeta1-42 alone, but not with ACT, increased by about 20% low-density lipoprotein (LDL) uptake and mRNA levels for LDL receptor and HMG-CoA reductase, while the ACT/Abeta1-42 mixture significantly increased LDL uptake (by 50%), up-regulated mRNA levels for LDL receptor and HMG-CoA reductase by 48 and 63%, respectively, and increased lipid accumulation by about 20-fold. These data suggest a possible new role for Abeta in Alzheimer's disease through its interaction with the inflammatory reactant, ACT. 相似文献
8.
9.
Spectral analysis of phylogenetic data 总被引:12,自引:0,他引:12
The spectral analysis of sequence and distance data is a new approach to phylogenetic analysis. For two-state character sequences,
the character values at a given site split the set of taxa into two subsets, a bipartition of the taxa set. The vector which
counts the relative numbers of each of these bipartitions over all sites is called a sequence spectrum. Applying a transformation
called a Hadamard conjugation, the sequence spectrum is transformed to the conjugate spectrum. This conjugation corrects for
unobserved changes in the data, independently from the choice of phylogenetic tree. For any given phylogenetic tree with edge
weights (probabilities of state change), we define a corresponding tree spectrum. The selection of a weighted phylogenetic
tree from the given sequence data is made by matching the conjugate spectrum with a tree spectrum. We develop an optimality
selection procedure using a least squares best fit, to find the phylogenetic tree whose tree spectrum most closely matches
the conjugate spectrum. An inferred sequence spectrum can be derived from the selected tree spectrum using the inverse Hadamard
conjugation to allow a comparison with the original sequence spectrum.
A possible adaptation for the analysis of four-state character sequences with unequal frequencies is considered. A corresponding
spectral analysis for distance data is also introduced. These analyses are illustrated with biological examples for both distance
and sequence data. Spectral analysis using the Fast Hadamard transform allows optimal trees to be found for at least 20 taxa
and perhaps for up to 30 taxa.
The development presented here is self contained, although some mathematical proofs available elsewhere have been omitted.
The analysis of sequence data is based on methods reported earlier, but the terminology and the application to distance data
are new. 相似文献
10.
Glucose/galactose malabsorption (GGM) is an autosomal recessive disease manifesting within the first weeks of life and characterized by a selective failure to absorb dietary glucose and galactose from the intestine. The consequent severe diarrhoea and dehydration are usually fatal unless these sugars are eliminated from the diet. Intestinal biopsies of GGM patients have revealed a specific defect in Na(+)-dependent absorption of glucose in the brush border. Normal glucose absorption is mediated by the Na+/glucose cotransporter in the brush border membrane of the intestinal epithelium. Cellular influx is driven by the transmembrane Na+ electrochemical potential gradient; thereafter the sugar moves to the blood across the basolateral membrane via the facilitated glucose carrier. We have previously cloned and sequenced a Na+/glucose cotransporter from normal human ileum and shown that this gene, SGLT1, resides on the distal q arm of chromosome 22. We have now amplified SGLT1 complementary DNA and genomic DNA from members of a family affected with GGM by the polymerase chain reaction. Sequence analysis of the amplified products has revealed a single missense mutation in SGLT1 which cosegregates with the GGM phenotype and results in a complete loss of Na(+)-dependent glucose transport in Xenopus oocytes injected with this complementary RNA. 相似文献