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941.
For the purpose of detecting the effects of human activities on climate change, it is important to document natural change in past climate. In this context, it has proved particularly difficult to study the variability in the occurrence of extreme climate events, such as storms with exceptional rainfall. Previous investigations have established storm chronologies using sediment cores from single lakes, but such studies can be susceptible to local environmental bias. Here we date terrigenous inwash layers in cores from 13 lakes, which show that the frequency of storm-related floods in the northeastern United States has varied in regular cycles during the past 13,000 years (13 kyr), with a characteristic period of about 3 kyr. Our data show four peaks in storminess during the past 14 kyr, approximately 2.6, 5.8, 9.1 and 11.9 kyr ago. This pattern is consistent with long-term changes in the average sign of the Arctic Oscillation, suggesting that modulation of this dominant atmospheric mode may account for a significant fraction of Holocene climate variability in North America and Europe. 相似文献
942.
Ludwig A Debus L Lieckfeldt D Wirgin I Benecke N Jenneckens I Williot P Waldman JR Pitra C 《Nature》2002,419(6906):447-448
The two species of Atlantic sea sturgeon on either shore of the North Atlantic, Acipenser sturio in Europe and A. oxyrinchus in North America, probably diverged with the closure of the Tethys Sea and the onset of the North Atlantic Gyre 15-20 million years ago, and contact between them was then presumably precluded by geographic distance. Here we present genetic, morphological and archaeological evidence indicating that the North American sturgeon colonized the Baltic during the Middle Ages and replaced the native sturgeon there, before recently becoming extinct itself in Europe as a result of human activities. In addition to representing a unique transatlantic colonization event by a fish that swims upriver to spawn, our findings have important implications for projects aimed at restocking Baltic waters with the European sturgeon. 相似文献
943.
Tropical peatlands are one of the largest near-surface reserves of terrestrial organic carbon, and hence their stability has important implications for climate change. In their natural state, lowland tropical peatlands support a luxuriant growth of peat swamp forest overlying peat deposits up to 20 metres thick. Persistent environmental change-in particular, drainage and forest clearing-threatens their stability, and makes them susceptible to fire. This was demonstrated by the occurrence of widespread fires throughout the forested peatlands of Indonesia during the 1997 El Ni?o event. Here, using satellite images of a 2.5 million hectare study area in Central Kalimantan, Borneo, from before and after the 1997 fires, we calculate that 32% (0.79 Mha) of the area had burned, of which peatland accounted for 91.5% (0.73 Mha). Using ground measurements of the burn depth of peat, we estimate that 0.19-0.23 gigatonnes (Gt) of carbon were released to the atmosphere through peat combustion, with a further 0.05 Gt released from burning of the overlying vegetation. Extrapolating these estimates to Indonesia as a whole, we estimate that between 0.81 and 2.57 Gt of carbon were released to the atmosphere in 1997 as a result of burning peat and vegetation in Indonesia. This is equivalent to 13-40% of the mean annual global carbon emissions from fossil fuels, and contributed greatly to the largest annual increase in atmospheric CO(2) concentration detected since records began in 1957 (ref. 1). 相似文献
944.
945.
Zeolites crystallize in a variety of three-dimensional structures in which oxygen atoms are shared between tetrahedra containing silicon and/or aluminium, thus yielding negatively charged tetrahedral frameworks that enclose cavities and pores of molecular dimensions occupied by charge-balancing metal cations and water molecules. Cation migration in the pores and changes in water content associated with concomitant relaxation of the framework have been observed in numerous variable-temperature studies, whereas the effects of hydrostatic pressure on the structure and properties of zeolites are less well explored. The zeolite sodium aluminosilicate natrolite was recently shown to undergo a volume expansion at pressures above 1.2 GPa as a result of reversible pressure-induced hydration; in contrast, a synthetic analogue, potassium gallosilicate natrolite, exhibited irreversible pressure-induced hydration with retention of the high-pressure phase at ambient conditions. Here we report the structure of the high-pressure recovered phase and contrast it with the high-pressure phase of the sodium aluminosilicate natrolite. Our findings show that the irreversible hydration behaviour is associated with a pronounced rearrangement of the non-framework metal ions, thus emphasizing that they can clearly have an important role in mediating the overall properties of zeolites. 相似文献
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947.
948.
Rector Arya Ravindranath Duggirala Laura Almasy David L Rainwater Michael C Mahaney Shelley Cole Thomas D Dyer Ken Williams Robin J Leach James E Hixson Jean W MacCluer Peter O'Connell Michael P Stern John Blangero 《Nature genetics》2002,30(1):102-105
High-density lipoproteins (HDLs) are anti-atherogenic lipoproteins that have a major role in transporting cholesterol from peripheral tissues to the liver, where it is removed. Epidemiologic studies have shown that low levels of high-density lipoprotein-cholesterol (HDL-C) are associated with an increased incidence of coronary heart disease and an increased mortality rate, indicating a protective role of high concentrations of HDL-C against atherogenesis and the development of coronary heart disease. HDL-C level is influenced by several genetic and nongenetic factors. Nongenetic factors include smoking, which has been shown to decrease the HDL-C level. Exercise and alcohol have been shown to increase HDL-C levels. Decreased HDL-C is often associated with other coronary heart disease risk factors such as obesity, hyperinsulinemia and insulin resistance, hypertriglyceridemia and hypertension. Although several genes have been identified for rare forms of dyslipidemia, the genes accounting for major variation in HDL-C levels have yet to be identified. Using a multipoint variance components linkage approach, we found strong evidence of linkage (lod score=3.4; P=0.00004) of a quantitative trait locus (QTL) for HDL-C level to a genetic location between markers D9S925 and D9S741 on chromosome 9p in Mexican Americans. A replication study in an independent set of Mexican American families confirmed the existence of a QTL on chromosome 9p. 相似文献
949.
从材料性能的角度分析,不同材料对钢筋混凝土结构构件的影响机理与影响程度是不同的。钢材的影响主要与力学性能有关;混凝土的影响既与力学性能有关,也与热工性能有关;不可燃的面层主要与热工性能有关,而可燃面层可以加大火灾的危险性,不可燃面层是一层阻隔层,火灾时可以阻止热量向混凝土内部传播,因此混凝土构件火灾时的安全可靠性可以得到改善和提高。对于可以燃烧的装饰层而言,则会产生热荷载,结构构件在火灾时的损伤或破坏会加重。本利用欧洲规范采用的火灾时室内的温度-时间曲线、高温作用下混凝土和钢筋的本构关系,以及钢筋混凝土结构非线性分析方法,研究和分析了火灾时钢筋混凝土构件载面的温度分布和承载能力,并根据分析计算结果讨论了影响混凝土结构火灾安全性的因素。 相似文献
950.
陈立林 《玉林师范学院学报》2004,25(4):1-5,11
从对马克思主义关于人的全面发展思想的本质内涵分析入手,着重揭示了我国社会主义初级阶段促进人的全面发展的重大意义及现实途径。 相似文献