On the progenitors of core-collapse supernovae     

Douglas C. Leonard 《Astrophysics and Space Science》2011,336(1):117-122

Theory holds that a star born with an initial mass between about 8 and 140 times the mass of the Sun will end its life through the catastrophic gravitational collapse of its iron core to a neutron star or black hole. This core collapse process is thought to usually be accompanied by the ejection of the star’s envelope as a supernova. This established theory is now being tested observationally, with over three dozen core-collapse supernovae having had the properties of their progenitor stars directly measured through the examination of high-resolution images taken prior to the explosion. Here I review what has been learned from these studies and briefly examine the potential impact on stellar evolution theory, the existence of “failed supernovae”, and our understanding of the core-collapse explosion mechanism.  相似文献   

Ground water dependence of endangered ecosystems: Nebraska's eastern saline wetlands   总被引：1，自引：1，他引：0  

Harvey FE  Ayers JF  Gosselin DC 《Ground water》2007,45(6):736-752

Many endangered or threatened ecosystems depend on ground water for their survival. Nebraska's saline wetlands, home to a number of endangered species, are ecosystems whose development, sustenance, and survival depend on saline ground water discharge at the surface. This study demonstrates that the saline conditions present within the eastern Nebraska saline wetlands result from the upwelling of saline ground water from within the underlying Dakota Aquifer and deeper underlying formations of Pennsylvanian age. Over thousands to tens of thousands of years, saline ground water has migrated over regional scale flowpaths from recharge zones in the west to the present-day discharge zones along the saline streams of Rock, Little Salt, and Salt Creeks in Lancaster and Saunders counties. An endangered endemic species of tiger beetle living within the wetlands has evolved under a unique set of hydrologic conditions, is intolerant to recent anthropogenic changes in hydrology and salinity, and is therefore on the brink of extinction. As a result, the fragility of such systems demands an even greater understanding of the interrelationships among geology, hydrology, water chemistry, and biology than in less imperiled systems where adaptation is more likely. Results further indicate that when dealing with ground water discharge-dependent ecosystems, and particularly those dependent on dissolved constituents as well as the water, wetland management must be expanded outside of the immediate surface location of the visible ecosystem to include areas where recharge and lateral water movement might play a vital role in wetland hydrologic and chemical mixing dynamics.  相似文献   

Author' Reply     

Leonard F. Konikov  Rodney C. Ewing 《Ground water》2000,38(1):3-3

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Paleoecology and geochronology of glacial Lake Hind during the Pleistocene–Holocene transition: A context for Folsom surface finds on the Canadian Prairies     

Matthew Boyd  Garry Leonard Running  Karen Havholm 《Geoarchaeology》2003,18(6):583-607

Stratigraphic and paleoecologic (palynomorph, macrobotanical) data obtained from a cutbank of the Souris River in southwestern Manitoba establish some fundamental parameters of Folsom land‐use in association with a proglacial lake on the Canadian Prairies. By dating the regression of glacial Lake Hind, we observed that recorded Folsom sites are restricted to areas of the Hind basin drained shortly before 10,400 yr B.P. This pattern may therefore record the interception of seasonal resources on recently‐drained proglacial lake surfaces. Based on paleovegetation reconstructions, we note that these surfaces were rapidly colonized by emergent and aquatic vegetation following regression, generating a viable resource base for Folsom hunter‐gatherers. However, low plant productivity and diversity may have greatly limited the extent to which this locale was exploited, in contrast to nonperiglacial regions on the Plains. We also suggest that wetland plant succession during the Pleistocene‐Holocene transition was due, at least locally, to climate‐forced fluctuations in groundwater levels. © 2003 Wiley Periodicals, Inc.  相似文献   

美国内华达Goldstrike矿区卡林型贝茨金矿同变形的富矿分带   总被引：1，自引：0，他引：1  

Stephen G. Peters  Gregory C. Ferdock  Maria B. Woitsekhowskay  Robert Leonardson  Jerry Rahn 《地质找矿论丛》2000,15(1):1-16

贝茨金矿是卡林金矿带在的卡林型矿床,由多个高品位富矿体组成。金矿体产于不纯的碳酸盐岩角砾岩和侵入体中,明显受构造控制,矿体显示出同变形热液沉淀特征。在ＮＷＷ向的ＤＤＺ剪切褶皱构造带中,贝茨金矿的富矿构成空间上的分带形式,主要的富矿类型有含金红石的硅质富矿,伊利石－粘土－黄放富矿,含雄黄、雌黄的富矿,含辉锑矿的硅质富矿和多金属富矿。研究表明,成矿溶液为弱－中等盐度,在构造活动期上升并冷却。变形时水－  相似文献   

《中国西部生态系统综合评估》评述二   总被引：2，自引：0，他引：2  

JerryMelillo 《地理学报》2006,61(1):110-110

《中国西部生态系统综合评估》是由中国科学院地理科学与资源研究所所长刘纪远研究员主编、气象出版社2006年出版的一本专著（英文简写本于2005年已南气象出版社出版）。人类依赖于生态系统。地球上的各种生态系统在它们自然地发挥各自功能时,能够提供用于维持这个星球上的所有生命的各种材料、条件和过程,包括人类的生活。  相似文献   

为《中国西部生态系统综合评估》一书所作的序言     

Jerry Melillo 《地理学报(英文版)》2006,16(1):114-115

Human well-being is dependent on ecosystems. Earth＇s mosaic of ecosystems - forests, grasslands, wetlands, streams, estuaries, and oceans - when functioning naturally, provide materials, conditions, and processes that sustain all life on this planet, including human life. The benefits that all living things obtain from ecosystems are called ＂ecosystem services.＂ Some are very familiar to us, such as materials like food and timber that are essential for our lives and important parts of the global economy. What are equally important, but certainly less well recognized, are the array of services delivered by ecosystems that do not have easily assigned monetary values, but that make our lives possible. These include the purification of air and water, the decomposition of wastes, the recycling of nutrients on land and in the oceans, the pollination of crops, and the regulation of climate.  相似文献   

The Environment Act 1995 and quiet enjoyment: implications for countryside recreation in the national parks of England and Wales, UK   总被引：1，自引：0，他引：1  

Deborah J. Pearlman  Janet Dickinson  Linda Miller  Jerry Pearlman† 《Area》1999,31(1):59-66

Summary National parks in England and Wales have traditionally been seen as places of quiet, less hurried recreation. The phrase 'quiet enjoyment' was adopted to represent this notion, and this paper traces the policy-making process involved in attempting to conserve this view. The debate during the enactment of the Environment Act 1995 brought the use and meaning of the phrase to prominence; amongst other issues, the use of the phrase in tenancy law precluded its use in legislation. The loss of the phrase 'quiet enjoyment' could have prolonged effects on the way in which national parks are used for recreational purposes.  相似文献   

The effects of burrowing activity on archaeological sites: Ndondondwane,South Africa     

Kent D. Fowler  Haskel J. Greenfield  Leonard O. van Schalkwyk 《Geoarchaeology》2004,19(5):441-470

Burrowing activity is a widely recognized source of site modification. Most taphonomic studies of burrowers emphasize their destructive aspects on the archaeological record. Excavations at Ndondondwane, South Africa, suggest burrowing activity is destructive in some ways, but may also preserve cultural behavior. Drawing on both direct and indirect sources of evidence, we discuss how burrowing activity by rodents, earthworms, and termites can inform about pedogenic and depositional processes at archaeological sites and both preserve and destroy evidence of intra‐settlement patterns and early African cultigens. Specifically, we demonstrate the limited effect of earthworms on site stratigraphy, how the localized activity of termites have preserved casts of early African cultigens, and how the ability of archaeologists to distinguish the devastating effects of rodent burrowing from remains of architectural features have permitted important inferences about social and ritual life in early African farming communities. © 2004 Wiley Periodicals, Inc.  相似文献   

Connectivity as an emergent property of geomorphic systems     

Ellen Wohl  Gary Brierley  Daniel Cadol  Tom J. Coulthard  Tim Covino  Kirstie A. Fryirs  Gordon Grant  Robert G. Hilton  Stuart N. Lane  Francis J. Magilligan  Kimberly M. Meitzen  Paola Passalacqua  Ronald E. Poeppl  Sara L. Rathburn  Leonard S. Sklar 《地球表面变化过程与地形》2019,44(1):4-26

Connectivity describes the efficiency of material transfer between geomorphic system components such as hillslopes and rivers or longitudinal segments within a river network. Representations of geomorphic systems as networks should recognize that the compartments, links, and nodes exhibit connectivity at differing scales. The historical underpinnings of connectivity in geomorphology involve management of geomorphic systems and observations linking surface processes to landform dynamics. Current work in geomorphic connectivity emphasizes hydrological, sediment, or landscape connectivity. Signatures of connectivity can be detected using diverse indicators that vary from contemporary processes to stratigraphic records or a spatial metric such as sediment yield that encompasses geomorphic processes operating over diverse time and space scales. One approach to measuring connectivity is to determine the fundamental temporal and spatial scales for the phenomenon of interest and to make measurements at a sufficiently large multiple of the fundamental scales to capture reliably a representative sample. Another approach seeks to characterize how connectivity varies with scale, by applying the same metric over a wide range of scales or using statistical measures that characterize the frequency distributions of connectivity across scales. Identifying and measuring connectivity is useful in basic and applied geomorphic research and we explore the implications of connectivity for river management. Common themes and ideas that merit further research include; increased understanding of the importance of capturing landscape heterogeneity and connectivity patterns; the potential to use graph and network theory metrics in analyzing connectivity; the need to understand which metrics best represent the physical system and its connectivity pathways, and to apply these metrics to the validation of numerical models; and the need to recognize the importance of low levels of connectivity in some situations. We emphasize the value in evaluating boundaries between components of geomorphic systems as transition zones and examining the fluxes across them to understand landscape functioning. © 2018 John Wiley & Sons, Ltd.  相似文献