24The isotopic composition of skeleton-bound organic nitrogen in shallow-water 25 scleractinian corals (hereafter, CS- 15 N) is an emerging tool for studying the marine 26 nitrogen cycle in the past. The CS- 15 N has been shown to reflect the 15 N of nitrogen (N) 27 sources to corals, with most applications to date focusing on the anthropogenic/terrestrial 28 N inputs to reef environments. However, many coral reefs receive their primary N 29 sources from the open ocean, and the CS- 15 N of these corals may provide information 30 on past changes in the open ocean regional and global N cycle. Using a recently 31 developed persulfate/denitrifier-based method, we measured CS- 15 N in modern shallow-32 water scleractinian corals from 8 sites proximal to the open ocean. At sites with low open 33 ocean surface nitrate concentrations typical of the subtropics and tropics, measured CS-34 15 N variation on seasonal and annual timescales is most often less than 2‰. In contrast, 35 a broad range in CS- 15 N (of ~10‰) is measured across these sites, with a strong 36 correlation between CS- 15 N and the 15 N of the deep nitrate supply to the surface waters 37 near the reefs. While CS- 15 N can be affected by other N sources as well and can vary in 38 response to local reef conditions as well as coral/symbiont physiological changes, this 39 survey indicates that, when considering corals proximal to the open ocean, the 15 N of the 40 subsurface nitrate supply to surface waters drives most of the CS- 15 N variation across 41 3 the global ocean. Thus, CS- 15 N is a promising proxy for reconstructing the open ocean 42 N cycle in the past. 43 1. Introduction 44 Coral skeleton-bound organic matter (CSOM) constitutes 0.01-0.1% of the skeleton 45 material by weight and research is ongoing to understand the synthesis, composition and 46 role of organic matter during the calcification process (Drake et al., 2013; Tambutte et al., 47 2011). From a paleoceanographic and biogeochemical perspective, the CSOM is directly 48 synthesized by coral at the time of calcification and may provide important information 49 about coral reef environments in the past. For shallow-water scleractinian corals, CSOM 50 has several key virtues as an archive of past conditions. First, CSOM is protected by the 51 carbonate skeleton and may be preserved for tens or hundreds of millions of years 52 (Muscatine et al., 2005). Second, shallow-water scleractinian corals are widely 53 distributed in the low latitude ocean, and fossil coral samples are found throughout the 54 Mesozoic and Cenozoic Eras (i.e. back to ~240 Ma). Third, shallow-water scleractinian 55 corals have high linear extension rates (e.g., 2 cm/year) and produce annual growth bands.56 Appropriate techniques would allow for the generation of high-resolution records on 57 individual coral cores.58 4 59 Due to the difficulty associated with analyzing this dilute form of organic matter, only a 60 handful of measurements have been made on CSOM: total organic carbon and amino 61 acid...
