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

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 δ 15 N 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 δ 18 O and δ 13 C values of the skeleton were also analyzed for the same period, using conventional methods. The ranges of δ 18 O and δ 13 C variations are from -6.1‰ to -5.4‰ and from -1.6‰ to -0.9‰, respectively, which are significantly smaller than the δ 15 N variation. The δ 13 C change is possibly attributable to kinetic isotope effects, as deduced from a negative correlation between the δ 13 C value and mean sea surface temperature (SST) in the region. On the other hand, the δ 15 N variation could be derived from seasonal change of the relative ratio of two components utilized by symbiotic algae: a 15 N-depleted component from N 2 fixation and 15 N-enriched nitrate from the tropical oligotrophic open ocean.

Seasonal variations in the nitrogen isotope composition of Okinotori coral in the tropical western Pacific: A new proxy for marine nitrate dynamics

“…The total nitrogen (TN) content in coral ranged from 0.007 to 0.048%, with a mean value of 0.015% (Figure 3b). The mean value of TN agrees with that of the previous studies (0.001 ∼ 0.1‰ [ Marion et al , 2005; Uchida et al , 2008]). The TN profile generally trended high with high δ 15 N coral values, with the exception of August 2003.…”

“…The average values of the δ 15 N coral (+3.7‰) agree with the symbiotic coral skeletal value (+4.09 ± 1.51‰) reported by Muscatine et al [2005]. The δ 15 N in coral tissues and skeletons has been used as an indicator of the δ 15 N DIN [ Sammarco et al , 1999; Heikoop et al , 2000; Risk et al , 1994; Marion et al , 2005; Uchida et al , 2008]. The DIN assimilated by zooxanthellae is assumed to be the main source of nitrogen for coral, according to D'Elia and Cook [1988].…”

“…However, Yamamuro et al [1995] reported the δ 15 N of coral tissue in Ishigaki and Palau (+4 to +6‰) and suggested that zooplankton were not the main source of nitrogen for corals. Uchida et al [2008] suggested that feeding PON may not affect the δ 15 N coral in more insolated environments because photoautotrophy was suggested for symbiotic coral living at 0 to 30 m water depth, while heterotrophy was suggested for coral at 50 m for a reef in Jamaica [ Muscatine and Kaplan , 1994]. As the insolation for Okinotori coral (2 m depth) may have been enough for photoautotrophy, heterotrophy might not exert a controlling effect on the δ 15 N coral .…”

“…In the results of Marion et al [2005], the temporal resolution of the δ 15 N coral was 3 to 6 months because the nitrogen abundance in the coral skeleton was too low (10 to 500 ppm) to be analyzed at a higher resolution; therefore, they targeted baseline changes in the δ 15 N coral at an annual time step. Uchida et al [2008] analyzed 2 year‐changes in the δ 15 N coral in Palau coral with a higher resolution (8 points/year) and suggested that the δ 15 N coral could be controlled by the seasonal mixing of a 15 N‐depleted component from N 2 fixation and 15 N‐enriched nitrate from the tropical open ocean. However, they did not demonstrate the occurrence of seasonal mixing with any observation data and other coral proxies.…”

Nitrogen isotopes of organic nitrogen in reef coral skeletons as a proxy of tropical nutrient dynamics

“…Geochemical proxies in coral skeletons may enable us to detect the origins of nutrients and their historical changes. The nitrogen isotope of organic matter in coral skeletons ( δ 15 N coral ) can vary with that of nitrogenous sources [ Marion et al , 2005; Uchida et al , 2008]. The 30‐year record of δ 15 N coral in Bali, for instance, captured the historical increase in fertilizer use through agricultural runoff into the ocean as a decline in δ 15 N coral [ Marion et al , 2005].…”