Origin of extant domesticated sunflowers in eastern North America

Eastern North America is one of at least six regions of the world where agriculture is thought to have arisen wholly independently. The primary evidence for this hypothesis derives from morphological changes in the archaeobotanical record of three important crops--squash, goosefoot and sunflower--as well as an extinct minor cultigen, sumpweed. However, the geographical origins of two of the three primary domesticates--squash and goosefoot--are now debated, and until recently sunflower (Helianthus annuus L.) has been considered the only undisputed eastern North American domesticate. The discovery of 4,000-year-old domesticated sunflower remains from San Andrés, Tabasco, implies an earlier and possibly independent origin of domestication in Mexico and has stimulated a re-examination of the geographical origin of domesticated sunflower. Here we describe the genetic relationships and pattern of genetic drift between extant domesticated strains and wild populations collected from throughout the USA and Mexico. We show that extant domesticates arose in eastern North America, with a substantial genetic bottleneck occurring during domestication.     

Two new carnivores from an unusual late Tertiary forest biota in eastern North America

Late Cenozoic terrestrial fossil records of North America are biased by a predominance of mid-latitude deposits, mostly in the western half of the continent. Consequently, the biological history of eastern North America, including the eastern deciduous forest, remains largely hidden. Unfortunately, vertebrate fossil sites from this vast region are rare, and few pertain to the critically important late Tertiary period, during which intensified global climatic changes took place. Moreover, strong phylogenetic affinities between the flora of eastern North America and eastern Asia clearly demonstrate formerly contiguous connections, but disparity among shared genera (eastern Asia-eastern North America disjunction) implies significant periods of separation since at least the Miocene epoch. Lacustrine sediments deposited within a former sinkhole in the southern Appalachian Mountains provide a rare example of a late Miocene to early Pliocene terrestrial biota from a forested ecosystem. Here we show that the vertebrate remains contained within this deposit represent a unique combination of North American and Eurasian taxa. A new genus and species of the red (lesser) panda (Pristinailurus bristoli), the earliest and most primitive so far known, was recovered. Also among the fauna are a new species of Eurasian badger (Arctomeles dimolodontus) and the largest concentration of fossil tapirs ever recorded. Cladistical analyses of the two new carnivores strongly suggest immigration events that were earlier than and distinct from previous records, and that the close faunal affinities between eastern North America and eastern Asia in the late Tertiary period are consistent with the contemporaneous botanical record.     

A transitional snake from the Late Cretaceous period of North America

Snakes are the most diverse group of lizards, but their origins and early evolution remain poorly understood owing to a lack of transitional forms. Several major issues remain outstanding, such as whether snakes originated in a marine or terrestrial environment and how their unique feeding mechanism evolved. The Cretaceous Coniophis precedens was among the first Mesozoic snakes discovered, but until now only an isolated vertebra has been described and it has therefore been overlooked in discussions of snake evolution. Here we report on previously undescribed material from this ancient snake, including the maxilla, dentary and additional vertebrae. Coniophis is not an anilioid as previously thought a revised phylogenetic analysis of Ophidia shows that it instead represents the most primitive known snake. Accordingly, its morphology and ecology are critical to understanding snake evolution. Coniophis occurs in a continental floodplain environment, consistent with a terrestrial rather than a marine origin; furthermore, its small size and reduced neural spines indicate fossorial habits, suggesting that snakes evolved from burrowing lizards. The skull is intermediate between that of lizards and snakes. Hooked teeth and an intramandibular joint indicate that Coniophis fed on relatively large, soft-bodied prey. However, the maxilla is firmly united with the skull, indicating an akinetic rostrum. Coniophis therefore represents a transitional snake, combining a snake-like body and a lizard-like head. Subsequent to the evolution of a serpentine body and carnivory, snakes evolved a highly specialized, kinetic skull, which was followed by a major adaptive radiation in the Early Cretaceous period. This pattern suggests that the kinetic skull was a key innovation that permitted the diversification of snakes.     

North Pacific seasonality and the glaciation of North America 2.7 million years ago

In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitch's orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.     

Locating seismic scatterers at the base of the mantle beneath eastern Tibet with PKIKP precursors

Clear PKIKP precursors were observed from the Lanzhou CTBTO seismic array.We measured their incident angles,arriving azimuths and differential travel times with respect to the PKIKP arrivals using array analysis techniques.These measurements allowed us to locate the scatterers that generated the observed precursors.We found that the scatterers are located in the lowermost mantle beneath eastern Tibet,which is featured by a high-velocity anomaly based on previous tomographic studies.The high velocity anomaly...     

对北美文化多样性与教育的反思

通过对一群亚裔加拿大社会服务从业者与来自其同一族裔社区的妇女工作的经验的描述 ,就文化适切实践模式对文化的假设提出质疑 ,并论述了文化的复杂性和多变性 ,阐述了文化适切实践的话语如何把殖民和种族权力的关系简约为文化问题。     

Subcontinental-scale crustal velocity changes along the Pacific-North America plate boundary

Transient tectonic deformation has long been noted within approximately 100 km of plate boundary fault zones and within active volcanic regions, but it is unknown whether transient motions also occur at larger scales within plates. Relatively localized transients are known to occur as both seismic and episodic aseismic events, and are generally ascribed to motions of magma bodies, aseismic creep on faults, or elastic or viscoelastic effects associated with earthquakes. However, triggering phenomena and systematic patterns of seismic strain release at subcontinental (approximately 1,000 km) scale along diffuse plate boundaries have long suggested that energy transfer occurs at larger scale. Such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust, or from large-scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes. Here we report geodetic evidence for a coherent, subcontinental-scale change in tectonic velocity along a diffuse approximately 1,000-km-wide deformation zone. Our observations are derived from continuous GPS (Global Positioning System) data collected over the past decade across the Basin and Range province, which absorbs approximately 25 per cent of Pacific-North America relative plate motion. The observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north-northwest relative plate motion vector. We show that sites to the west of this boundary slowed relative to sites east of it by approximately 1 mm yr(-1) starting in late 1999.     

Molecular characterization of H1N1 influenza A viruses from human cases in North America

Subtypes of H1N1 influenza virus can be found in humans in North America, while they are also associated with the infection of swine. Characterization of the genotypes of viral strains in human populations is important to understand the source and distribution of viral strains. Genomic and protein sequences of 10 isolates of the 2009 outbreak of influenza A （H1N1） virus in North America were obtained from GenBank database. To characterize the genotypes of these viruses, phylogenetic trees of genes PB2, PB1, PA, HA, NP, NA, NS and M were constructed by Phylip3.67 program and N-Linked glycosylation sites of HA, NA, PB2, NS1 and M2 proteins were analyzed online by NetNGlyc1.0 program. Phylogenetic analysis indicated that these isolates are virtually identical but may be recombinant viruses because their genomic fragments come from different viruses. The isolates also contain a characteristic lowly pathogenic amino acid motif at their HA cleavage sites （IPSIQSR↓GL）, and an E residue at position 627 of the PB2 protein which shows its high affinity to humans. The homologous model of M proteins showed that the viruses had obtained the ability of anti-amantadine due to the mutation at the drug-sensitive site, while sequence analysis of NA proteins indicated that the viruses are still susceptible to the neuraminidase inhibitor drug （i.e. oseltamivir and zanamivir） because no mutations have been observed. Our results strongly suggested that the viruses responsible for the 2009 outbreaks of influenza A （H1N1） virus have the ability to cross species barriers to infect human and mammalian animals based on molecular analysis. These findings may further facilitate the therapy and prevention of possible transmission from North America to other countries.     

Large temperature drop across the Eocene-Oligocene transition in central North America

The Eocene-Oligocene transition towards a cool climate (approximately 33.5 million years ago) was one of the most pronounced climate events during the Cenozoic era. The marine record of this transition has been extensively studied. However, significantly less research has focused on continental climate change at the time, yielding partly inconsistent results on the magnitude and timing of the changes. Here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high-resolution (about 40,000 years) continental temperature record for the Eocene-Oligocene transition. We find a large drop in mean annual temperature of 8.2 +/- 3.1 degrees C over about 400,000 years, the possibility of a small increase in temperature seasonality, and no resolvable change in aridity across the transition. The large change in mean annual temperature, exceeding changes in sea surface temperatures at comparable latitudes and possibly delayed in time with respect to marine changes by up to 400,000 years, explains the faunal turnover for gastropods, amphibians and reptiles, whereas most mammals in the region were unaffected. Our results are in agreement with modelling studies that attribute the climate cooling at the Eocene-Oligocene transition to a significant drop in atmospheric carbon dioxide concentrations.     

美国西部开发中的城市化

美国西部开发中的城市化，与十叶当时的区域关系、采矿业、横贯大陆铁路铺设联系紧密。这种城市的崛起，推动了西部经济体系的形成和发展。     

An Arctic mammal fauna from the Early Pliocene of North America

A peat deposit on Ellesmere Island, Nunavut, Canada, allows a unique glimpse of the Early Pliocene terrestrial biota north of the Arctic Circle. The peat accumulated in a beaver pond surrounded by boreal larch forest near regional tree line in coastal hills close to the Arctic Ocean. The ecological affinities of the plant and beetle remains contained in the peat indicate that winter temperatures on Ellesmere Island were nearly 15 degrees C higher and summer temperatures 10 degrees C higher than they are today. Here we show that the mammalian remains buried in the peat represent mainly taxa of Eurasiatic zoogeographic and phyletic affinities, including the first North American occurrence of a meline badger (Arctomeles). This deposit contains direct evidence of the composition of an Early Pliocene (4-5 million years ago) arctic mammalian fauna during an active period of interchange between Asia and North America.     

北美超低渗致密油藏提高采收率技术现状

在过去的十年中,北美致密油气的开发技术日渐成熟,其中,最具代表性的技术是水力压裂和水平钻井技术的结合。然而,现有依靠天然能量的衰竭式开发只能采出超低渗储层中的少部分原油（采出程度低于10%）,导致致密储层中仍存在大量剩余油。因此,非常规超低渗油藏提高采收率技术的研究日趋重要。目前,全世界90%以上的致密油产量来自美国,其开采技术在非常规致密油的开发领域具有明显优势。并且,在过去的十年间,北美的石油公司已经对针对致密低渗油藏的提高采收率（EOR）技术进行了许多现场测试。鉴于美国在非常规致密油的勘探开发中具有领先地位,重点对北美致密油资源开发中提高采收率措施的现场经验进行了总结和评价,并指出了其目前面临的主要困难和挑战。此项研究对于中国未来超低渗致密油藏提高采收率技术的发展具有指导和借鉴意义。     

Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.     

北美典型气候区建筑围护结构HM模拟及分析

为明确气候参数、材料参数对建筑围护结构模拟结果的影响,以及热湿耦合传递模型(HM)和被普遍采用的Glaser模型之间的差异,在热湿过程模拟软件WUFI Plus中选择北美典型气候区的20个代表城市,设置15组外墙构件,进行空间单元HVAC需求、室内热湿环境和外墙热湿性能的全年模拟.模拟结果与室外气候参数进行相关性分析,表明室外气温、太阳辐射、相对湿度和风驱雨不同程度地影响相关结果.通过材料参数控制所得2组HM和3组Glaser模型的对比,表明液态水相关参数、含湿量引起的材料参数的变物性取值对模拟结果有显著影响.北美典型气候区HM模拟对我国建筑围护结构热湿过程相关的气候参数、材料参数和评估方法的研究具有借鉴意义.     

FAST——脉冲星观测研究的利器

 2016年9月25日落成的中国自行设计建造的500 m口径球面射电望远镜（FAST）是目前全世界最大口径的射电望远镜。在调试期间已成功发现了脉冲星,实现了国内设备发现脉冲星的零突破。脉冲星是宇宙中的一类奇妙天体,是验证强引力场、强磁场和高密度等极端物理环境下物理规律的"天然实验室"。作为脉冲星研究的利器,预计FAST大规模巡天能够大幅提高已知的脉冲星数目。FAST还将在低频引力波探测、脉冲星物理、星际和星系际介质探测、脉冲星时间尺度建立及脉冲星导航等领域取得突破性进展。在上述研究中,国内天文设备取长补短、相互补充,有望实现中国脉冲星观测研究跨越式发展。     

北美的物化技术溢出对中国产生的中性技术进步分析

采用GTAP模型实证分析中国通过从美国进口获得技术溢出的机制及其对中国经济增长的影响。模拟结果表明：地区之间的相对贸易量和地区之间结构相似性决定了物化技术溢出的大小，北美三大产业技术溢出均能促进中国产生的希克斯中性技术进步，其中只有第三产业对中国对应产生的产出有促进作用，对中国第一、二产业的产出作用增长的正负效应稍模糊，并提出了相关政策建议。     

北祁连山东段苏家山高Mg埃达克岩地球动力学意义

苏家山花岗闪长岩位于北祁连山东段,老虎山弧后盆地残留体北侧,侵入于原奥陶系阴沟群及前寒武纪变质岩系中,在空间分布上与米家山、扁强沟、清凹山、黄家凹山、银硐梁以及东侧南华山花岗岩共同构成长约220 km的花岗岩带,沿NW-NWW向展布.地球化学研究结果表明:苏家山花岗闪长岩富Na贫K,属钙碱性系列;高Sr低Y,Yb,高Mg#,Ni,Cr,可划归高Mg,Sr低Y型花岗闪长岩;微量元素蛛网图显示,岩石富集大离子亲石元素,亏损高场强元素及HREE,(w(La)/w(Yb))n>22,δEu轻微负异常;K-Ar同位素年龄为425 Ma.结果指示:苏家山花岗闪长岩具有高镁埃达克岩的特点,是在早古生代晚期北祁连东段造山作用过程中,滞留在地幔楔中的老虎山弧后盆地俯冲板片发生部分熔融,熔体与地幔橄榄岩发生明显的交代作用而形成.与此同时,阿拉善古陆南缘也卷入了造山作用,埃达克岩浆作用使陆壳在垂向上得以生长.     

借鉴北美英语教法的EDA技术教学探讨

探讨了北美英语教法对EDA技术教学的借鉴意义.针对EDA技术课程实践性强的特点,论述了将北美英语教法强调的原则贯穿于EDA技术教学过程的可行性,并提出了具体的教学实施过程.结果显示,与传统教学方法相比,这种借鉴可以更好地激发学生的学习兴趣,取得良好的教学效果.