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.