Journal article
Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals: A new method and proxy evaluation at Bermuda
Geochimica et Cosmochimica Acta, Vol.148, pp.179-190
01/01/2015
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Source: InCites
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-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 5 mg 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.
Details
- Title
- Isotopic composition of skeleton-bound organic nitrogen in reef-building symbiotic corals: A new method and proxy evaluation at Bermuda
- Creators
- X T Wang - Princeton UniversityD M Sigman - Princeton UniversityA L Cohen - Woods Hole Oceanographic InstitutionD J Sinclair - Rutgers University, New BrunswickR M Sherrell - Rutgers University, New BrunswickM A Weigand - Princeton UniversityDirk V Erler - Southern Cross UniversityH Ren - Columbia University
- Publication Details
- Geochimica et Cosmochimica Acta, Vol.148, pp.179-190
- Grant note
- 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.).
- Identifiers
- 3577; 991012821216602368
- Copyright
- © 2014 Elsevier Ltd. All rights reserved.
- Academic Unit
- Centre for Coastal Biogeochemistry; Faculty of Science and Engineering; School of Environment, Science and Engineering; Science
- Resource Type
- Journal article