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

摘要

By using a step-wise heating method in conjunction with static vacuum mass spectrometry, we have measuied 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 <515N value from the 900°C combustion step varies significantly, from +l..l%o to +10.9%o for the estimated time period from July 1995 to January 1998, The higher values occurred during winter and the lower values in summer. The <518O and 513C values of the skeleton were also analyzed for the same period, using conventional methods. The ranges of <518O and <513C variations are from -6. Woo to -5.4%o and from -1.6%o to -0.9%o, respectively, which are significantly smaller than the <515N variation The <513C change is possibly attributable to kinetic isotope effects, as deduced from a negative correlation between the <513C value and mean sea surface temperature (SST) in the region. On the other hand, the <515N 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.