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  • 1
    ISSN: 0016-7835
    Keywords: Key words Manganese nodules ; Thorium dating ; Uranium isotopes ; Peru basin ; TIMS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Thorium- and uranium isotopes were measured in a diagenetic manganese nodule from the Peru basin applying alpha- and thermal ionization mass spectrometry (TIMS). Alpha-counting of 62 samples was carried out with a depth resolution of 0.4 mm to gain a high-resolution 230Thexcess profile. In addition, 17 samples were measured with TIMS to obtain precise isotope concentrations and isotope ratios. We got values of 0.06–0.59 ppb (230Th), 0.43-1.40 ppm (232Th), 0.09–0.49 ppb (234U) and 1.66–8.24 ppm (238U). The uranium activity ratio in the uppermost samples (1–6 mm) and in two further sections in the nodule at 12.5±1.0 mm and 27.3–33.5 mm comes close to the present ocean water value of 1.144±0.004. In two other sections of the nodule, this ratio is significantly higher, probably reflecting incorporation of diagenetic uranium. The upper 25 mm section of the Mn nodule shows a relatively smooth exponential decrease in the 230Thexcess concentration (TIMS). The slope of the best fit yields a growth rate of 110 mm/Ma up to 24.5 mm depth. The section from 25 to 30.3 mm depth shows constant 230Thexcess concentrations probably due to growth rates even faster than those in the top section of the nodule. From 33 to 50 mm depth, the growth rate is approximately 60 mm/Ma. Two layers in the nodule with distinct laminations (11–15 and 28–33 mm depth) probably formed during the transition from isotopic stage 8 to 7 and in stage 5e, respectively. The Mn/Fe ratio shows higher values during interglacials 5 and 7, and lower ones during glacials 4 and 6. A comparison of our data with data from adjacent sediment cores suggests (a) a variable supply of hydrothermal Mn to sediments and Mn nodules of the Peru basin or (b) suboxic conditions at the water sediment interface during periods with lower Mn/Fe ratios.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2013-08-31
    Description: High resolution digitized images of M81 in the radio continuum, H alpha, H I, and I band are used to see how well various density wave models agree in detail with observations. It was found that the observed width of the nonthermal radio arms favors a cloudy version of a density wave model. The radial distribution of the set of giant radio H II regions disagrees with the simple expression of Shu and Visser for star formation by a density wave. The observed displacements of the giant radio H II regions from the spiral velocity shock indicate that some revisions in the details of the ballistic particle model of Leisawitz and Bash are necessary.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington Star Formation in Galaxies; p 227-233
    Format: application/pdf
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  • 3
    ISSN: 1437-3262
    Keywords: Manganese nodules ; Thorium dating ; Uranium isotopes ; Peru basin ; TIMS
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Thorium- and uranium isotopes were measured in a diagenetic manganese nodule from the Peru basin applying alpha- and thermal ionization mass spectrometry (TIMS). Alpha-counting of 62 samples was carried out with a depth resolution of 0.4 mm to gain a high-resolution230Thexcess profile. In addition, 17 samples were measured with TIMS to obtain precise isotope concentrations and isotope ratios. We got values of 0.06–0.59 ppb (230Th), 0.43–1.40 ppm (232Th), 0.09–0.49 ppb (234U) and 1.66–8.24 ppm (238U). The uranium activity ratio in the uppermost samples (1–6 mm) and in two further sections in the nodule at 12.5±1.0 mm and 27.3–33.5 mm comes close to the present ocean water value of 1.144±0.004. In two other sections of the nodule, this ratio is significantly higher, probably reflecting incorporation of diagenetic uranium. The upper 25 mm section of the Mn nodule shows a relatively smooth exponential decrease in the230Thexcess concentration (TIMS). The slope of the best fit yields a growth rate of 110 mm/Ma up to 24.5 mm depth. The section from 25 to 30.3 mm depth shows constant230Thexcess concentrations probably due to growth rates even faster than those in the top section of the nodule. From 33 to 50 mm depth, the growth rate is approximately 60 mm/Ma. Two layers in the nodule with distinct laminations (11–15 and 28–33 mm depth) probably formed during the transition from isotopic stage 8 to 7 and in stage 5e, respectively. The Mn/Fe ratio shows higher values during interglacials 5 and 7, and lower ones during glacials 4 and 6. A comparison of our data with data from adjacent sediment cores suggests (a) a variable supply of hydrothermal Mn to sediments and Mn nodules of the Peru basin or (b) suboxic conditions at the water sediment interface during periods with lower Mn/Fe ratios.
    Type of Medium: Electronic Resource
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  • 4
    Publication Date: 2011-08-19
    Description: Results are presented from a systematic search for CO emission from regions around 34 young open clusters in the outer Galaxy. The clusters have well-determined distances ranging from about 1 to 5 kpc and ages not greater than about 100 Myr. It was found that some moderately young clusters have no associated CO emission. All the surveyed clusters younger than about 5 Myr have associated with them at least one molecular cloud more massive than 10,000 solar mass, while the molecular clouds associated with clusters older than about 10 Myr are not more massive than a few thousands solar masses. It was also found that molecular clouds are receding from young clusters at a rate of about 10 km/sec, and that they seem to be destroyed by their interaction with the stars. Sites of ongoing star formation were identified in a number of clouds associated with young clusters.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 70; 731-812
    Format: text
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  • 5
    Publication Date: 2019-07-12
    Description: H-alpha and VLA radio continuum observations at wavelengths of 6 and 20 cm are used to study the distribution of extinction and the distribution of giant radio H II regions along the spiral arms in M81. The radial distribution of visual extinction Av in the plane of M81 shows no trend, with an upper limit 0.1 mag/kpc for the radial extinction gradient and a mean Av of 1.1 + or - 0.4 mag. Nearly all the giant radio H II regions lie along the spiral arms or the inner H I ring. The radial distribution of the set of giant radio H II regions exhibits a strong maximum at a galactocentric distance R roughly 300 arcsec. Unless molecular hydrogen in M81 is also concentrated near R roughly 300 arcsec, the data disagree with Visser's (1980) model for star formation associated with a density wave. Some suggestions are made about how to change the ballistic particle model of Leisawitz and Bash to agree with the observed radial distribution of giant radio H II regions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 319; 61-75
    Format: text
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