High sensitivity measurements of nitrogen isotopic ratios in coral skeletons from Palau, western Pacific: Temporal resolution and seasonal variation of nitrogen sources

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  • UCHIDA ASAMI
    Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo
  • NISHIZAWA MANABU
    Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo
  • SHIRAI KOTARO
    Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo
  • IIJIMA HIROKO
    Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo
  • KAYANNE HAJIME
    Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo
  • TAKAHATA NAOTO
    Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo
  • SANO YUJI
    Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo

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Abstract

By using a step-wise heating method in conjunction with static vacuum mass spectrometry, we have measured at high temporal resolution (one month) the nitrogen isotopic ratio of a coral skeletal sample collected from Palau in the northern part of the Western Pacific. The δ15N value from the 900°C combustion step varies significantly, from +1.1‰ to +10.9‰ for the estimated time period from July 1995 to January 1998. The higher values occurred during winter and the lower values in summer. The δ18O and δ13C values of the skeleton were also analyzed for the same period, using conventional methods. The ranges of δ18O and δ13C variations are from -6.1‰ to -5.4‰ and from -1.6‰ to -0.9‰, respectively, which are significantly smaller than the δ15N variation. The δ13C change is possibly attributable to kinetic isotope effects, as deduced from a negative correlation between the δ13C value and mean sea surface temperature (SST) in the region. On the other hand, the δ15N variation could be derived from seasonal change of the relative ratio of two components utilized by symbiotic algae: a 15N-depleted component from N2 fixation and 15N-enriched nitrate from the tropical oligotrophic open ocean.

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