Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals: A new method and proxy evaluation at Bermuda. (1st January 2015)

• Record Type: Journal Article
• Authors: Wang, X.T.; Sigman, D.M.; Cohen, A.L.; Sinclair, D.J.; Sherrell, R.M.; Weigand, M.A.; Erler, D.V.; Ren, H.
• Is Part Of: Geochimica et cosmochimica acta. Volume 148(2015:Jan. 01)
• Page Start: 179
• Page End: 190
• Publication Date: 2015-01-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2014.09.017
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The skeleton-bound organic nitrogen in reef-building symbiotic corals may be a high-resolution archive of ocean nitrogen cycle dynamics and a tool for understanding coral biogeochemistry and physiological processes. However, the existing methods for measuring the isotopic composition of coral skeleton-bound organic nitrogen (hereafter, CS-δ 15 N) either require too much skeleton material or have low precision, limiting the applications of this relatively new proxy. In addition, the controlling factors on CS-δ 15 N remain poorly understood: the δ 15 N of source nitrogen and the internal nitrogen cycle of the coral/zooxanthellae symbiosis may both be important. Here, we describe a new ("persulfate/denitrifier"-based) method for measuring CS-δ 15 N, requiring only 5 mg of skeleton material and yielding a long-term precision better than 0.2‰ (1σ). Using this new method, we investigate CS-δ 15 N at Bermuda. Ten modern Diploria labyrinthiformis coral cores/colonies from 4 sampling sites were measured for CS-δ 15 N. Nitrogen concentrations (nitrate + nitrite, ammonium, and dissolved organic nitrogen) and δ 15 N of plankton were also measured at these coral sites. Among the 4 sampling sites, CS-δ 15 N shows an increase with proximity to the island, from ∼3.8‰ to ∼6.8‰ vs . atmospheric N2, with the northern offshore site having a CS-δ 15 N 1–2‰ higher than the δ 15 N of thermocline nitrate in the surrounding Sargasso Sea. Two annually resolved CS-δ 15 N time series suggest that the offshore-inshore CS-δ 15 N gradient has persisted since at least the 1970s. Plankton δ 15 N among these 4 sites also has an inshore increase, but of only ∼1‰. Coral physiological change must explain the remaining (∼2‰) inshore increase in CS-δ 15 N, and previous work points to the coral/zooxanthellae N cycle as a control on host tissue (and thus carbonate skeletal) δ 15 N. The CS-δ 15 N gradient is hypothesized to result mainly from varying efficiency in the internal nitrogen recycling of the coral/zooxanthellae symbiosis. It is proposed that, in more productive inshore waters, greater food uptake by the coral causes a greater fraction of its low-δ 15 N regenerated ammonium to be excreted rather than assimilated by zooxanthellae, raising the δ 15 N of the inshore corals. If so, coral tissue- and CS-δ 15 N may prove of use to reconstruct and monitor the state of the coral/zooxanthellae symbiosis over space and time.