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-delta N-15) 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 delta N-15. We explored the variability of CB-delta N-15 within specimens, among corals collected at different depths in a given ocean region, and among different ocean regions. Modern D. dianthus CB-delta N-15 is strongly correlated with the delta N-15 of N export as estimated from sediment traps, shallow subsurface nitrate, and surface sediments, suggesting that CB-delta N-15 is a reliable proxy for delta N-15 of N export. D. dianthus CB-delta N-15 is consistently 8-9 parts per thousand higher than delta N-15 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. (C) 2014 Elsevier B.V. All rights reserved.