Cretaceous extinctions: evidence for wildfires and search for meteoritic material

Clay samples from three Cretaceous-Tertiary boundary sites contain 0.36 to 0.58 percent graphitic carbon, mainly as fluffy aggregates of 0.1 to 0.5 micrometers-apparently a worldwide layer of soot. It may have been produced by wildfires triggered by a giant meteorite. This carbon, corresponding to a global abundance of 0.021 +/- 0.006 gram per square centimeter, could have greatly enhanced the darkening and cooling of the earth by rock dust, which has been suggested as a cause of the extinctions. The surprisingly large amount of soot (10 percent of the present biomass of the earth) implies either that much of the earth's vegetation burned down or that substantial amounts of fossil fuels were ignited also. The particle-size distribution of the soot is similar to that assumed for the smoke cloud of "nuclear winter," but the global distribution is more uniform and the amounts are much greater, suggesting that soot production by large wildfires is about 10 times more efficient that has been assumed for a nuclear winter. Thus cooling would be more pervasive and lasting. No trace of meteoritic noble gases and no meteoritic spinel were found in these carbon fractions. Accordingly, limits can be set on the mass fraction of the meteorite that escaped degassing (相似文献   

辣椒新型质核互作雄性不育材料H9A不育机理研究

为研究辣椒雄性不育的机理,在花期,采用常规的石蜡切片法,对新型辣椒雄性不育材料H9A及其保持系H9B小孢子不同发育时期进行细胞形态学观察;采用DDRT-PCR技术对H9A与H9B小孢子发育过程进行mRNA差异显示,筛选差异表达的基因。切片观察发现:供试雄性不育材料的小孢子败育发生在单核前期,此时花蕾纵径0.526～0.572 cm,横径0.318～0.372 cm,花冠和花萼之比约为1∶4;通过DDRT-PCR,共获得27条差异表达的cDNA片段,其中在H9A中获得12条,在H9B中获得15条。结果说明新型辣椒雄性不育材料H9A的小孢子败育发生在单核前期,败育原因可能是绒毡层细胞程序性死亡异常并极度液泡化膨胀,四分体受挤压后破裂降解,不能形成正常的单核花粉粒。败育时期与花蕾外部形态相关。在败育期表达的27条差异cDNA片段为研究辣椒雄性不育机理的候选基因奠定了基础。     

A new astrophysical setting for chondrule formation

Chondrules in the metal-rich meteorites Hammadah al Hamra 237 and QUE 94411 have recorded highly energetic thermal events that resulted in complete vaporization of a dusty region of the solar nebula (dust/gas ratio of about 10 to 50 times solar). These chondrules formed under oxidizing conditions before condensation of iron-nickel metal, at temperatures greater than or equal to 1500 K, and were isolated from the cooling gas before condensation of moderately volatile elements such as manganese, sodium, potassium, and sulfur. This astrophysical environment is fundamentally different from conventional models for chondrule formation by localized, brief, repetitive heating events that resulted in incomplete melting of solid precursors initially residing at ambient temperatures below approximately 650 K.     

A formation mechanism for catalytically grown helix-shaped graphite nanotubes

The concept of a spatial-velocity hodograph is introduced to describe quantitatively the extrusion of a carbon tubule from a catalytic particle. The conditions under which a continuous tubular surface can be generated are discussed in terms of this hodograph, the shape of which determines the geometry of the initial nanotube. The model is consistent with all observed tubular shapes and explains why the formation process induces stresses that may lead to "spontaneous" plastic deformation of the tubule. This result is due to the violation of the continuity condition, that is, to the mismatch between the extrusion velocity by the catalytic particle, required to generate a continuous tubular surface, and the rate of carbon deposition.     

一种新型花烛切花保鲜剂的作用机理探析

花烛的很多品种极易衰败，它们是生产上的主要切花。通过近两年的研究，研制出一种新型环保型高效花烛切花保鲜剂，保鲜效果良好。其保鲜作用机理是增强花烛切花的抗逆性，降低切花的呼吸代谢，廷缓体内能源物质的消耗，促进切花细胞分裂素等的增加，延缓组织、细胞、亚细胞结构的破坏，减慢色素消失和廷缓切花变色，最终达到良好的保鲜效果。     

新农村文化建设机制创新的路径选择探析

机制创新是新农村文化建设自身有效运行的根本推动力。在对厦门市同安区、翔安区新农村文化建设情况进行调查访谈的基础上,深入分析了新农村文化建设中阻碍机制创新的制约因素,着力探讨了推进机制创新的现实路径选择,以期推进新农村文化建设的科学发展。     

Famine edema and the mechanism of its formation

Famine edema was produced experimentally in 34 normal men who lost a quarter of their body weight while subsisting for 6 months on a European type of semi-starvation diet. The ratio of extracellular water to cellular tissue was roughly doubled. Their clinical state closely resembled that seen in Europe in 1945. There were no signs of renal or cardiac failure. The plasma protein concentration fell only slightly and the A/G ratio remained within normal limits. The venous pressure was roughly 50 per cent below normal. Data from the field lend support to these indications that famine edema is not simply a result of hypoproteinemia or of renal or cardiac failure. It is concluded that there is a dynamic nonequilibrium state of the capillary wall and, accordingly, calculations from equilibrium equations are inadmissible.     

Rational design and synthesis of new polymeric material

Polymers constitute one of the four main areas of materials science. Modern research in polymers focuses on the ways in which tailored macromolecules organize themselves in the solid state and on materials design and synthesis at the interface between polymer science and the fields of ceramics, metals, and electroactive or electrooptical materials.     

现代英语新词的构成方法

现代科技的飞速发展,国际、国内政治斗争的日益激烈,从其他语言中借词,用自身的语言材料构词,是现代英语新词不断产生的主要原因。其构成方法可谓多种多样。     

微生物源活性物质的开发与化学新农药的创制

阐述了微生物农药的研究开发现状、开发利用微生物源活性物质尤其是微生物代谢产物的重要性、微生物源活性物质及其新农药的创制,探讨了从丰富的海洋微生物资源中寻找和发现新的拮抗微生物和有生物活性的、结构新颖的化学物质的前景。     

甘蓝型油菜雄性不育新材料RM3231A的遗传研究

以不育系RM3231A与Polima CMS和陕2A CMS的保持系、恢复系和核不育两型系以及116份国内外不同来源的甘蓝型油菜品种（系）的杂交遗传试验结果表明,RM5637A不育系是不同于Polima CMS和陕2A CMS及核不育两型系遗传背景的另一类细胞质雄性不育系。不育性除受胞质不育基因控制外,还受3对独立遗传并具有相同作用的重叠隐性基因控制,恢复源十分广泛,现有甘蓝型油菜中约有73．3％的品种（系)均为其恢复系,有1－3对显性纯合恢复基因共6类19种不同基因型,其中3对显性纯合基因型的约占6．4％,2对显性纯合基因型的约占14．3％,1对显性纯合基因型的约占65．1％。     

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Posttranslational modifications of proteins regulate many biological processes, including metabolism, signal transduction, and gene expression. The synthetic challenges associated with generating homogeneous populations of selectively modified proteins, however, have hindered detailed studies of the effects of these modifications on protein structure and function. Here, we report an approach to the cotranslational synthesis of selectively glycosylated proteins in which the modified amino acid is genetically encoded. We show that myoglobin containing beta-N-acetylglucosamine (GlcNAc)-serine at a defined position can be expressed in Escherichia coli in good yield and with high fidelity. The beta-GlcNAc moiety can be recognized by a saccharide-binding protein, or subsequently modified with a galactosyltransferase to build more complex carbohydrates. This approach should be generally applicable to other posttranslational modifications such as protein phosphorylation, acetylation, and methylation.     

Evolutionary formation of new centromeres in macaque

A systematic fluorescence in situ hybridization comparison of macaque and human synteny organization disclosed five additional macaque evolutionary new centromeres (ENCs) for a total of nine ENCs. To understand the dynamics of ENC formation and progression, we compared the ENC of macaque chromosome 4 with the human orthologous region, at 6q24.3, that conserves the ancestral genomic organization. A 250-kilobase segment was extensively duplicated around the macaque centromere. These duplications were strictly intrachromosomal. Our results suggest that novel centromeres may trigger only local duplication activity and that the absence of genes in the seeding region may have been important in ENC maintenance and progression.     

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网络菜市场作为农产品零售业的新形态,目前尚处于发展初期,网站数量年增长率达到92.46％,主要集中于经济发达地区.调查发现网络菜市场存在占有率低、农产品销售量小且网站失败多的现象.对国内已有网络菜市场的运营模式分析得到“农户+企业”、“农户直销”、“批发市场+个体”和“批发市场+企业”4种模式,对比分析得出“农户+企业...