Isotopic composition of carbonate-bound organic nitrogen in deep-sea scleractinian corals: A new window into past biogeochemical change

Over the last two decades, the skeletal remains of deep-sea corals have arisen as a geochemical archive of Pleistocene oceanographic change. Here we report the exploration of the isotopic composition of the carbonate-bound organic nitrogen (hereafter, CB-δ¹⁵N) in the deep-sea scleractinian coral Desmophyllum dianthus as a possible tool for reconstructing past changes in the ocean nitrogen cycle. The measurement protocol is adapted from a high-sensitivity method for foraminifera shell-bound δ¹⁵N. We explored the variability of CB-δ¹⁵N within specimens, among corals collected at different depths in a given ocean region, and among different ocean regions. Modern D. dianthus CB-δ¹⁵N is strongly correlated with the δ¹⁵N of N export as estimated from sediment traps, shallow subsurface nitrate, and surface sediments, suggesting that CB-δ¹⁵N is a reliable proxy for δ¹⁵N of N export. D. dianthus CB-δ¹⁵N is consistently 8-9‰ higher than δ¹⁵N of N export, indicating that D. dianthus acquires its nutrition primarily from suspended particulate organic matter (POM) that derives from sinking POM, not directly from sinking POM.