Online ISSN : 2188-5923
Print ISSN : 0386-4073
ISSN-L : 0386-4073
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The Geochemical Society of Japan Award for Young Researchers 2017
  • Yuko Omori
    2019 Volume 53 Issue 2 Pages 47-58
    Published: June 25, 2019
    Released: June 25, 2019

    Organic compounds produced by marine microbial organisms have various roles on the marine carbon cycles and the sea-air interaction. This paper introduces my studies of the dynamics of dissolved organic matter (DOM) and volatile organic compounds (VOCs) in the surface ocean. DOM forms one of the largest organic carbon pools on the surface of the earth. The vertical change in the DOM composition and its controlling factors were examined in the subtropical North Pacific, using fluorescence property of marine humic matter and molecular weight distribution. I revealed that fluorescent DOM is composed of some fractions with different photoreactivity, and that photoirradiation is the main factor controlling the compositional change in DOM with depth. The fate of photo-irradiated DOM is presumably one of the key processes in carbon accumulation in the surface ocean. Dimethyl sulfide (DMS) is one of marine VOCs and plays important roles in climate system. I found that DMS concentrations increased just after depletion of dissolved oxygen in seawater. Incubation experiments combined with an online mass spectrometry and stable isotope techniques were conducted to clarify mechanisms of the DMS increase. The results showed that anoxic stress is one of the important environmental factors controlling the DMS distribution.

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Original paper
  • Atsuyuki Ohta, Masayo Minami
    2019 Volume 53 Issue 2 Pages 59-70
    Published: June 25, 2019
    Released: June 25, 2019

    We have elucidated retention of primary Sr isotopic ratios in source rocks to sediments and sedimentary rocks during repeated sedimentation recycling. Twenty-three fine stream sediments (<180 μm) in Awajishima Island were used for the study. The 87Rb/86Sr and 87Sr/86Sr of stream sediments originated from Cretaceous Ryoke granitic rocks and Cretaceous sedimentary rocks (Izumi Group) were similar to those of the respective source rocks. Incidentally, the 87Rb/86Sr-87Sr/86Sr of stream sediments in Awajishima Island suggests that Cretaceous felsic igneous rocks are the dominant source of clastics in Izumi Group. Stream sediments derived from Neogene and Quaternary sediments, which are originated from granitic rocks and Izumi Group, had comparable isotopic ratios to those of the respective primitive source rocks. Accordingly, 87Rb/86Sr and 87Sr/86Sr can be retained during the repeated recycling process of sediments and sedimentary rocks. Stream sediment is presumed to consist of clastics supplied from respective lithologies according to their exposed areas. However, 87Sr/86Sr of stream sediment is not comparable to the values calculated from exposed areas of lithologies and isotopic data of parent rocks. The result suggests that amount of sedimentation denudation and production differs among different lithologies and is not simply proportional to their exposed areas.

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Abstract of doctoral thesis