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 (delta N-15 and delta C-13) 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 delta N-15 and delta C-13 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 delta N-15 and delta C-13 values between sites was present. Skeletal delta N-15 values significantly increased and delta C-13 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 parts per thousand. m(-1) and +0.023 parts per thousand. m(-1) are recommended for delta N-15 and delta C-13, respectively. Average depth-corrected delta N-15 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 delta C-13 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 parts per thousand is recommended for black corals when comparing their delta C-13-based proxy records to soft corals. After correcting for both the depth and order effects, variability in delta N-15 values among corals within each genera was low (standard deviation (SD) of the mean <+/- 0.5 parts per thousand), with the exception of Acanthorgorgia. The calculated SD of <+/- 0.5 parts per thousand. provides a first order guideline for the amount of variability that could be expected in a delta N-15 record, and suggests that these corals may be useful for delta N-15-based paleoceanographic reconstructions. Variability in delta C-13 values among corals within genera was also low (standard deviation of the mean <+/- 0.5 parts per thousand) with the exception of Rhipidipathes and Villogorgia. Similar to delta N-15, records from the genera studied here with the exception of Rhipidipathes and Villogorgia may be useful for delta C-13-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. (C) 2010 Elsevier Ltd. All rights reserved.