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-δ¹⁵N) is an emerging tool for studying the marine nitrogen cycle in the past. The CS-δ¹⁵N has been shown to reflect the δ¹⁵N 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-δ¹⁵N 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-δ¹⁵N 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-δ¹⁵N variation on seasonal and annual timescales is most often less than 2‰. In contrast, a broad range in CS-δ¹⁵N (of ~10‰) is measured across these sites, with a strong correlation between CS-δ¹⁵N and the δ¹⁵N of the deep nitrate supply to the surface waters near the reefs. While CS-δ¹⁵N 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 δ¹⁵N of the subsurface nitrate supply to surface waters drives most of the CS-δ¹⁵N variation across the global ocean. Thus, CS-δ¹⁵N is a promising proxy for reconstructing the open ocean N cycle in the past.