Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals: A new method and proxy evaluation at Bermuda

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-δ¹⁵N) 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-δ¹⁵N remain poorly understood: the δ¹⁵N 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-δ¹⁵N, requiring only 5mg of skeleton material and yielding a long-term precision better than 0.2‰ (1σ). Using this new method, we investigate CS-δ¹⁵N at Bermuda. Ten modern Diploria labyrinthiformis coral cores/colonies from 4 sampling sites were measured for CS-δ¹⁵N. Nitrogen concentrations (nitrate+nitrite, ammonium, and dissolved organic nitrogen) and δ¹⁵N of plankton were also measured at these coral sites. Among the 4 sampling sites, CS-δ¹⁵N 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-δ¹⁵N 1-2‰ higher than the δ¹⁵N of thermocline nitrate in the surrounding Sargasso Sea. Two annually resolved CS-δ¹⁵N time series suggest that the offshore-inshore CS-δ¹⁵N gradient has persisted since at least the 1970s. Plankton δ¹⁵N 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-δ¹⁵N, and previous work points to the coral/zooxanthellae N cycle as a control on host tissue (and thus carbonate skeletal) δ¹⁵N. The CS-δ¹⁵N 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-δ¹⁵N regenerated ammonium to be excreted rather than assimilated by zooxanthellae, raising the δ¹⁵N of the inshore corals. If so, coral tissue- and CS-δ¹⁵N may prove of use to reconstruct and monitor the state of the coral/zooxanthellae symbiosis over space and time.