第四紀後期の海洋環境と生物生産　　堆積有機物の炭素・窒素安定同位体比による古海洋解析

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タイトル別名
  • Environments of Ocean and Primary Productivity during the Late Quaternary. Paleoceanographic Analyses Using Stable Carbon and Nitrogen Isotopic Ratios of Sedimentary Organic Matter.
  • タイセキ ユウキブツ ノ タンソ チッソ アンテイ ドウイタイヒ ニヨル コ

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Recently stable carbon and nitrogen isotopic ratios (δ13C and δ15N) of the sedimentary organic matter have become to be applied to reconstruct past changes of the biogeochemical processes in surface waters, although they were used only as the mixing tracers of terrestrial organic matter into coastal sediments during 1970's and 80's. While both of the δ13C and δ15N values of the organic matter produced in the surface water change due to the isotopic fractionation processes during the uptake of inorganic substance (CO2 and NO3- etc.) by phytoplankton, the reconstructed paleoceanograhic factors are completely different between the δ13C and δ15N, such as the [CO2] aq and the phytoplankton growth rate for the δ13C and the nitrate availability for δ15N, reflecting the difference in Σ CO2 and nitrate budgets in the sea surface water.<BR>Because the magnitude of carbon isotopic fractionation during the photosynthesis of phytoplankton are influenced by [CO2] aq in the surface water, many recent studies of the organic δ13C in the sediment cores discuss about past changes in pCO2 in the surface water. However, the phytoplankton growth rate also governs the δ13C values of phytoplankton, andtherefore, it is actually very difficult to extract purely the [CO2] aq data from the organic δ13C values. On the other hand, δ15N of sediment is now believed to reflect the past changes in nitrate utilization rates in the surface water (i.e. biological pumping efficiency). Because the isotope fractionation during the nitrate uptake by phytoplankton makes theδ15N values of the remaining nitrate much higher in the surface water body, the relativelylow δ15N values of organic matter can be used to trace a upwelling center of the surface water.<BR>The δ13C and δ15N values in the particulate organic matter change largely during their sinking and sedimentation processes. Both of the two isotopic values in the sinking particledecrease downward irrespective of their sample sites, probably due to the selective decomposition of amino acids which have relatively higher δ13C and δ15N values than the bulk organic matter. Contrary to this, the δ13C and δ15N values increase again on the sediment surface. Because the changes in δ13C and δ15N values of organic matter in the sinking and sedimentation processes are universal and somewhat constant processes, it seems reasonable to apply the downcore δ13C and δ15N variations to reconstruct the past changes in the surface water processes.

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  • 地学雑誌

    地学雑誌 107 (2), 203-217, 1998

    公益社団法人 東京地学協会

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