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URN etd-0719101-103520
Author Kevin Chu
Author's Email Address m8753606@student.nsysu.edu.tw
Statistics This thesis had been viewed 5347 times. Download 1586 times.
Department Marine Geology and chemistry
Year 2000
Semester 2
Degree Master
Type of Document
Language zh-TW.Big5 Chinese
Title Pb-210 and Po-210 in the Western South Okinawa Trough:Distribution Pattern and Radioactive Disequilibrium
Date of Defense 2000-07-31
Page Count 82
Keyword
  • Po-210
  • particulate scavenging process
  • mass fluxes
  • lateral transport
  • Wwstern South Okinawa
  • Pb-210
  • Abstract  In this study, the settling particulates
    collected by time-series sediment traps, sediment
    cores, and seawater samples taken in the western
    South Okinawa Trough were analyzed for 210Pb and
    210Po in order to understand the characteristics
    of the particulates and sediments based on the
    distribution of these two nuclides and the extent
    of their radioactive disequilibrium.
     Two sediment traps were deployed at T18 (24°45'N 122°18'E, about 300m and 100m above bottom)
    from February to August 1999. The results show
    that higher mass fluxes were observed from
    February to April, ranging between 16 and 56
    g/m2/d. The 210Pb activity increases
    systematically with time from about 80 to 100
    dpm/g, and the upper trap has slightly higher
    values. Similar to 210Pb, the 210Po activity
    also increases, but from near zero to only 27
    dpm/g for both traps, much lower than the
    corresponding 210Pb activity.
     The 210Pb activities are quite different
    between the settling particulates collected by
    traps at T18 and the surface sediment taken
    nearby (the former have a mean activity of about
    85 to 90 dpm/g; the latter has a value of only 11
    dpm/g). The size distribution and elemental
    composition are also different between the
    trapped particulates and the sediment. Thus the
    underlying sediments were not directly derived
    from the overlying trapped particulates. Except
    for T4 core below 15cm and T19 entire core, the
    210Po/210Pb activity ratio in the sediments
    varies greatly, from near zero to 0.7, indicating
    deficiency of 210Po. Both 210Po and 210Pb
    activities are much lower in the sediments than
    in the settling particulates. A T19 both 210Po
    and 210Pb activities in the core decrease steeply
    from 120 dpm/g at surface to 60 dpm/g at 4 cm,
    indicating radioactive equilibrium. This is
    entirely different from the cores taken at other
    stations.
     The total 210Po/210Pb activity ratio in the
    water column at T17 is nearly constant at about
    0.6, but drops to 0.3 below 700m, i.e. 210Po
    activity is deficient in the entire water
    column. At T18, the activity ratio is about 1.3
    between 100m and 300m, indicating a 210Po excess
    in this layer. Below 400m, the ratio is about
    0.6 to 0.7, showing 210Po deficit again. These
    observations indicate that the water columns at
    T17 and T18 are stratified, probably due to
    lateral transport. Based on the 210Po/210Pb
    activity ratio the mean residence time of 210Po
    with respect to particulate scavenging ranges
    from 3 to 16 months.
    Advisory Committee
  • S. Lin - chair
  • C. C. Chen - co-chair
  • Y. C, Chung - advisor
  • Files
  • etd-0719101-103520.pdf
  • indicate access worldwide
    Date of Submission 2001-07-19

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