Influence of open ocean nitrogen supply on the skeletal δ15N of modern shallow-water scleractinian corals

The isotopic composition of skeleton-bound organic nitrogen in shallow-water scleractinian corals (hereafter, CS-delta N-15) is an emerging tool for studying the marine nitrogen cycle in the past. The CS-delta N-15 has been shown to reflect the delta N-15 of nitrogen (N) sources to corals, with most applications to date focusing on the anthropogenic/terrestrial N inputs to reef environments. However, many coral reefs receive their primary N sources from the open ocean, and the CS-delta N-15 of these corals may provide information on past changes in the open ocean regional and global N cycle. Using a recently developed persulfate/clenitrifler-based method, we measured CS-delta N-15 in modern shallow-water scleractinian corals from 8 sites proximal to the open ocean. At sites with low open ocean surface nitrate concentrations typical of the subtropics and tropics, measured CS-delta N-15 variation on seasonal and annual timescales is most often less than 2 parts per thousand. In contrast, a broad range in CS-delta N-15 (of similar to 10 parts per thousand) is measured across these sites, with a strong correlation between CS-delta N-15 and the delta N-15 of the deep nitrate supply to the surface waters near the reefs. While CS-delta N-15 can be affected by other N sources as well and can vary in response to local reef conditions as well as coral/symbiont physiological changes, this survey indicates that, when considering corals proximal to the open ocean, the delta N-15 of the subsurface nitrate supply to surface waters drives most of the CS-delta N-15 variation across the global ocean. Thus, CS-delta N-15 is a promising proxy for reconstructing the open ocean N cycle in the past. (C) 2016 Elsevier B.V. All rights reserved.