TY - JOUR
T1 - Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals
T2 - A new method and proxy evaluation at Bermuda
AU - Wang, X. T.
AU - Sigman, Daniel Mikhail
AU - Cohen, A. L.
AU - Sinclair, D. J.
AU - Sherrell, R. M.
AU - Weigand, M. A.
AU - Erler, D. V.
AU - Ren, H.
N1 - Funding Information:
We thank Dr. Ross Jones for assistance with coral collection at Bermuda. This work is supported by the NSF Grants OCE-1060947 and OCE-1234664 to D.M.S. and OCE-82698600 to A.L.C, the MacArthur Foundation (D.M.S.), the Grand Challenges Program at Princeton University (D.M.S.), and the Tuttle Fund of the Department of Geosciences at Princeton University (X.T.W.). We acknowledge Ms. M. Shailer (Department of Conservation Services, Government of Bermuda) and Dr. Andreas Andersson (Scripps Institute of Oceanography) for providing the bathymetry map of Bermuda. We also thank Dr. David Baker, two anonymous reviewers, and the editor for thoughtful reviews.
Publisher Copyright:
© 2014 Elsevier Ltd.
PY - 2015/1/1
Y1 - 2015/1/1
N2 - 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.
AB - 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.
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U2 - 10.1016/j.gca.2014.09.017
DO - 10.1016/j.gca.2014.09.017
M3 - Article
AN - SCOPUS:84918589651
SN - 0016-7037
VL - 148
SP - 179
EP - 190
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -