Influence of open ocean nitrogen supply on the skeletal delta 15N of modern shallow-water scleractinian corals

The isotopic composition of skeleton-bound organic nitrogen in shallow-water scleractinian corals (hereafter, CS- delta 15N) is an emerging tool for studying the marine nitrogen cycle in the past. The CS- delta 15N has been shown to reflect the delta 15N 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 15N of these corals may provide information on past changes in the open ocean regional and global N cycle. Using a recently developed persulfate/denitrifier-based method, we measured CS- delta 15N 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 15N variation on seasonal and annual timescales is most often less than 2ppt. In contrast, a broad range in CS- delta 15N (of 10ppt) is measured across these sites, with a strong correlation between CS- delta 15N and the delta 15N of the deep nitrate supply to the surface waters near the reefs. While CS- delta 15N 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 15N of the subsurface nitrate supply to surface waters drives most of the CS- delta 15N variation across the global ocean. Thus, CS- delta 15N is a promising proxy for reconstructing the open ocean N cycle in the past.