Mass-Spectrometric 230Th-234U-238U Dating of the Devils Hole Calcite Vein     

Ludwig KR  Simmons KR  Szabo BJ  Winograd IJ  Landwehr JM  Riggs AC  Hoffman RJ 《Science (New York, N.Y.)》1992,258(5080):284-287

The Devils Hole calcite vein contains a long-term climatic record, but requires accurate chronologic control for its interpretation. Mass-spectrometric U-series ages for samples from core DH-11 yielded (230)Th ages with precisions ranging from less than 1,000 years (2sigma) for samples younger than approximately 140 ka (thousands of years ago) to less than 50,000 years for the oldest samples ( approximately 566 ka). The (234)U/(238)U ages could be determined to a precision of approximately 20,000 years for all ages. Calcite accumulated continuously from 566 ka until approximately 60 ka at an average rate of 0.7 millimeter per 10(3) years. The precise agreement between replicte analyses and the concordance of the (230)Th/(238)U (234)U/(238)U ages for the oldest samples indicate that the DH-11 samples were closed systems and validate the dating technique in general.  相似文献   

'Ain ghazal: a major neolithic settlement in central jordan     

Simmons AH  Köhler-Rollefson I  Rollefson GO  Mandel R  Kafafi Z 《Science (New York, N.Y.)》1988,240(4848):35-39

'Ain Ghazal, an archeological site located on the outskirts of Amman, Jordan, is one of the largest early villages known in the Near East. The site dates to the Neolithic period, during which mankind made one of its most significant advances, the adoption of domestic plants and animals as primary subsistence sources. Recent excavations at 'Ain Ghazal have augmented considerably current knowledge of several aspects of the Neolithic. Of particular interest has been the documentation of a continuous, or near continuous, occupation from early through late Neolithic components, and a concomitant dramatic economic shift. This shift was from a broad subsistence base relying on a variety of both wild and domestic plants and animals, to an economic strategy reflecting an apparent emphasis on pastoralism.  相似文献   

Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean     

Benitez-Nelson CR  Bidigare RR  Dickey TD  Landry MR  Leonard CL  Brown SL  Nencioli F  Rii YM  Maiti K  Becker JW  Bibby TS  Black W  Cai WJ  Carlson CA  Chen F  Kuwahara VS  Mahaffey C  McAndrew PM  Quay PD  Rappé MS  Selph KE  Simmons MP  Yang EJ 《Science (New York, N.Y.)》2007,316(5827):1017-1021

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.  相似文献   

Observation of charge transport by negatively charged excitons     

Sanvitto D  Pulizzi F  Shields AJ  Christianen PC  Holmes SN  Simmons MY  Ritchie DA  Maan JC  Pepper M 《Science (New York, N.Y.)》2001,294(5543):837-839

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.  相似文献   

Response     

Ludwig KR  Simmons KR  Winograd IJ  Szabo BJ  Riggs AC 《Science (New York, N.Y.)》1993,259(5101):1629-1627

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Photometric observations of jupiter at 2400 angstroms     

Hord CW  West RA  Simmons KE  Coffeen DL  Sato M  Lane AL  Bergstralh JT 《Science (New York, N.Y.)》1979,206(4421):956-959

The photopolarimeter instrument on Voyager 2 was used to obtain a map of Jupiter at an effective wavelength of 2400 angstroms. Analysis of a typical north-south swath used to make this map shows strong absorption at high latitudes by a molecular or particulate constituent in the Jovian atmosphere. At 65 degrees north latitude, the absorbing constituent extends to altitudes above the 50-millibar pressure level.  相似文献   

Belowground nematode herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems     

Edward Ayres  Diana H. Wall  Breana L. Simmons  Christopher B. Field  Daniel G. Milchunas  Jack A. Morgan  Jacques Roy 《Soil biology & biochemistry》2008,40(4):978-985

Grasslands are considered to be one of the most sensitive ecosystems to rising atmospheric CO₂ concentrations, since, in addition to direct effects of elevated CO₂ on plant growth, indirect increases in water availability as an effect of elevated CO₂ may enhance primary production and alter plant community composition in these typically dry ecosystems. Moreover, grasslands support large populations of belowground herbivores that consume a major portion of plant biomass. The direct trophic link between herbivores and plants suggests that primary consumers may be particularly sensitive to rising CO₂ concentrations; however, the responses of belowground grassland herbivores have rarely been investigated. Here, we report the response of a range of herbivorous nematode populations to elevated CO₂ concentrations from three distinct grassland experiments. The three studies each involved 5–6 years of CO₂ fumigation, utilized natural or representative plant and soil communities, and were sampled at the end of the growing season. In the vast majority of cases, elevated CO₂ did not affect the abundance of nematode families; only two nematode families were significantly influenced by CO₂ enrichment (Anguinidae increased in one case and Hoplolaimidae decreased in another). Similarly, elevated CO₂ did not influence the total abundance, family richness, diversity or plant parasitic index of the herbivorous nematode community. These neutral responses to CO₂ enrichment occurred despite increased root production in all three experiments, suggesting a simultaneous antagonistic mechanism may have operated, possibly decreased root quality and/or changes in the soil environment. Whatever the mechanism, our findings suggest that herbivorous nematodes in grassland ecosystems are resistant to rising atmospheric CO₂ concentrations.  相似文献   

Two invaders achieve higher densities in reserves     

Terrie Klinger  Dianna K. Padilla  Kevin Britton‐Simmons 《水产资源保护:海洋与淡水生态系统》2006,16(3):301-311

• 1. Invasive species threaten marine biodiversity on a global scale.
• 2. To test whether marine reserves provide resistance to invading species, the abundance of two conspicuous invaders, a seaweed and an oyster, were measured inside marine reserves and in comparable areas outside reserves in north‐western Washington State.
• 3. Densities of both invaders were significantly higher in marine reserves than in comparable unprotected areas outside reserves. Although the causal mechanisms have not yet been identified, differential rates of human harvest do not appear to be responsible for the patterns observed.
• 4. It is provisionally suggested that physical or biological aspects of the reserves themselves may directly or indirectly facilitate biological invasion.

Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

What is your diagnosis? Splenic torsion     

Jaeger GH  Maher E  Simmons T 《Journal of the American Veterinary Medical Association》2006,229(4):501-502

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