Timing of hot spot--related volcanism and the breakup of madagascar and India   总被引：1，自引：0，他引：1  

Storey M  Mahoney JJ  Saunders AD  Duncan RA  Kelley SP  Coffin MF 《Science (New York, N.Y.)》1995,267(5199):852-855

Widespread basalts and rhyolites were erupted in Madagascar during the Late Cretaceous. These are considered to be related to the Marion hot spot and the breakup of Madagascar and Greater India. Seventeen argon-40/argon-39 age determinations reveal that volcanic rocks and dikes from the 1500-kilometer-long rifted eastern margin of Madagascar were emplaced rapidly (mean age = 87.6 +/- 0.6 million years ago) and that the entire duration of Cretaceous volcanism on the island was no more than 6 million years. The evidence suggests that the thick lava pile at Volcan de l'Androy in the south of the island marks the focal point of the Marion hot spot at approximately 88 million years ago and that this mantle plume was instrumental in causing continental breakup.  相似文献   

Welfare dependence within and across generations   总被引：2，自引：0，他引：2  

G J Duncan  M S Hill  S D Hoffman 《Science (New York, N.Y.)》1988,239(4839):467-471

A central question in the debate about the effects of welfare programs is whether their intended positive results--providing needed resources to recipients and their children--outweigh any unintended negative results--for example, the breaking up of families, reduction in work effort, or the fostering of a welfare "culture" that is passed on from parent to child. Recent research shows that although work effort is affected adversely by the generosity of welfare programs, effects on family structure appear quite weak, most welfare experiences are relatively short, and the majority of women who grew up in homes heavily dependent on welfare do not rely on those programs when they are young adults.  相似文献   

Deformation on nearby faults induced by the 1999 Hector Mine earthquake   总被引：3，自引：0，他引：3  

Fialko Y  Sandwell D  Agnew D  Simons M  Shearer P  Minster B 《Science (New York, N.Y.)》2002,297(5588):1858-1862

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.  相似文献   

Rational design of peptide-based HIV proteinase inhibitors   总被引：48，自引：0，他引：48  

N A Roberts  J A Martin  D Kinchington  A V Broadhurst  J C Craig  I B Duncan  S A Galpin  B K Handa  J Kay  A Kr?hn 《Science (New York, N.Y.)》1990,248(4953):358-361

A series of peptide derivatives based on the transition-state mimetic concept has been designed that inhibit the proteinase from the human immunodeficiency virus (HIV). The more active compounds inhibit both HIV-1 and HIV-2 proteinases in the nanomolar range with little effect at 10 micromolar against the structurally related human aspartic proteinases. Proteolytic cleavage of the HIV-1 gag polyprotein (p55) to the viral structural protein p24 was inhibited in chronically infected CEM cells. Antiviral activity was observed in the nanomolar range (with one compound active below 10 nanomolar) in three different cell systems, as assessed by p24 antigen and syncytium formation. Cytotoxicity was not detected at 10 and 5 micromolar in C8166 and JM cells, respectively, indicating a high therapeutic index for this new class of HIV proteinase inhibitors.  相似文献   

Artificial airglow excited by high-power radio waves   总被引：1，自引：0，他引：1  

Bernhardt PA  Duncan LM  Tepley CA 《Science (New York, N.Y.)》1988,242(4881):1022-1027

High-power electromagnetic waves beamed into the ionosphere from ground-based transmitters illuminate the night sky with enhanced airglow. The recent development of a new intensified, charge coupled-device imager made it possible to record optical emissions during ionospheric heating. Clouds of enhanced airglow are associated with large-scale plasma density cavities that are generated by the heater beam. Trapping and focusing of electromagnetic waves in these cavities produces accelerated electrons that collisionally excite oxygen atoms, which emit light at visible wavelengths. Convection of plasma across magnetic field lines is the primary source for horizontal motion of the cavities and the airglow enhancements. During ionospheric heating experiments, quasi-cyclic formation, convection, dissipation and reappearance of the cavites comprise a major source of long-term variability in plasma densities during ionospheric heating experiments.  相似文献   

Identity and search in social networks     

Watts DJ  Dodds PS  Newman ME 《Science (New York, N.Y.)》2002,296(5571):1302-1305

Social networks have the surprising property of being "searchable": Ordinary people are capable of directing messages through their network of acquaintances to reach a specific but distant target person in only a few steps. We present a model that offers an explanation of social network searchability in terms of recognizable personal identities: sets of characteristics measured along a number of social dimensions. Our model defines a class of searchable networks and a method for searching them that may be applicable to many network search problems, including the location of data files in peer-to-peer networks, pages on the World Wide Web, and information in distributed databases.  相似文献   

Mazama ash in the northeastern pacific     

Nelson CH  Kulm LD  Carlson PR  Duncan JR 《Science (New York, N.Y.)》1968,161(3836):47-49

Volcanic glass in marine sediments off Oregon and Washington correlates with continental deposits of Mount Mazama ash by stratigraphic position, refractive index, and radiocarbon dating. Ash deposited in the abyssal regions by turbidity currents is used for tracing of the dispersal routes of postglacial sediments and for evaluation of marine sedimentary processes.  相似文献   

Heterologous protein secretion from yeast   总被引：25，自引：0，他引：25  

R A Smith  M J Duncan  D T Moir 《Science (New York, N.Y.)》1985,229(4719):1219-1224

  相似文献   

Genome maps 1991 [wall chart   总被引：1，自引：0，他引：1  

M Chipperfield  M Maidek  P Pearson  M Ashburner  D M Glover  R D Saunders  I Duncan  D Hartl  J Merriam  G Lee 《Science (New York, N.Y.)》1991,254(5029):247-262

  相似文献   

Direct measurement of solar luminosity variation     

Willson RC  Duncan CH  Geist J 《Science (New York, N.Y.)》1980,207(4427):177-179

Two rocket flights of an absolute pyrheliometer, separated by 30 months, indicate an increase in solar luminosity (solar constant) of 0.4 percent. The significance of this result is considered in light of the instrument performance during the rocket flights and of pre- and postflight intercomparisons with independently maintained pyrheliometers. There is a high probability that the measured difference is real. Additional observations are required to determine whether the difference results from random fluctuations in solar luminosity, a nonrandom change of short duration, or a sustained change that has climatological significance.  相似文献