Stable nitrogen and carbon isotope (δ15N and δ13C) variability in shallow tropical Pacific soft coral and black coral taxa and implications for paleoceanographic reconstructions

Soft corals and black corals are useful proxy tools for paleoceanographic reconstructions. However, most work has focused on deep-water taxa and few studies have used these corals as proxy organisms in shallow water (<200 m). To facilitate the use of stable nitrogen and carbon isotope (δ15N and δ13C) records from shallow-water soft coral and black coral taxa for paleoceanographic reconstructions, quantification of the inherent variability in skeletal isotope values between sites, across depth, and among taxa is needed. Here, skeletal δ15N and δ13C values were measured in multiple colonies from eleven genera of soft corals and two genera of black corals from across a depth transect (5–105 m) at two sites in Palau located in the tropical western Pacific Ocean. Overall, no difference in skeletal δ15N and δ13C values between sites was present. Skeletal δ15N values significantly increased and δ13C values decreased with depth. This is consistent with changes in isotope values of suspended particulate organic matter (POM) across the photic zone, suggesting that the primary food source to these corals is suspended POM and that the stable isotopic composition of POM controls the skeletal isotopic composition of these corals. Thus, to compare the isotope records of corals collected across a depth range in the photic zone, first order depth corrections of −0.013‰ m−1 and +0.023‰ m−1 are recommended for δ15N and δ13C, respectively. Average depth-corrected δ15N values were similar between black corals and soft corals, indicating that corals in these orders feed at a similar trophic level. In contrast, average depth-corrected δ13C values of black corals were significantly lower than that of soft corals, potentially resulting from metabolic processes associated with differing skeletal compositions among the orders (i.e., gorgonin vs. chitin based). Thus, a correction of +1.0‰ is recommended for black corals when comparing their δ13C-based proxy records to soft corals. After correcting for both the depth and order effects, variability in δ15N values among corals within each genera was low (standard deviation (SD) of the mean <±0.5‰), with the exception of Acanthorgorgia. The calculated SD of <±0.5‰ provides a first order guideline for the amount of variability that could be expected in a δ15N record, and suggests that these corals may be useful for δ15N-based paleoceanographic reconstructions. Variability in δ13C values among corals within genera was also low (standard deviation of the mean <±0.5‰) with the exception of Rhipidipathes and Villogorgia. Similar to δ15N, records from the genera studied here with the exception of Rhipidipathes and Villogorgia may be useful for δ13C-based paleoceanographic reconstructions. Overall, using the recommendations developed here, stable isotope records from multiple sites, depths and taxa of these corals can be more rigorously compared.