Edited by Donald E. Canfield, Institute of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Odense M., Denmark, and approved August 28, 2018 (received for review January 18, 2018)
Human activities have altered the global nitrogen cycle through nitrogen fertilizer usage and fossil fuel burning. It has been suggested that even the remote open ocean is influenced by anthropogenic nitrogen through atmospheric transport and deposition. However, the magnitude and timing of the anthropogenic N effect on the open ocean remain elusive, mainly because of very limited long-term measurements. Here we provide an ∼130-year coral record of N cycle dynamics from the subtropical North Atlantic. This record shows minimal influence of anthropogenic N deposition, in contrast to published model simulations, suggesting that previous estimates of anthropogenic N deposition on the open ocean were too high.
Human alteration of the global nitrogen cycle intensified over the 1900s. Model simulations suggest that large swaths of the open ocean, including the North Atlantic and the western Pacific, have already been affected by anthropogenic nitrogen through atmospheric transport and deposition. Here we report an ∼130-year-long record of the 15N/14N of skeleton-bound organic matter in a coral from the outer reef of Bermuda, which provides a test of the hypothesis that anthropogenic atmospheric nitrogen has significantly augmented the nitrogen supply to the open North Atlantic surface ocean. The Bermuda 15N/14N record does not show a long-term decline in the Anthropocene of the amplitude predicted by model simulations or observed in a western Pacific coral 15N/14N record. Rather, the decadal variations in the Bermuda 15N/14N record appear to be driven by the North Atlantic Oscillation, most likely through changes in the formation rate of Subtropical Mode Water. Given that anthropogenic nitrogen emissions have been decreasing in North America since the 1990s, this study suggests that in the coming decades, the open North Atlantic will remain minimally affected by anthropogenic nitrogen deposition.
Author contributions: X.T.W., A.L.C., G.H.H., and D.M.S. designed research; X.T.W., A.L.C., and V.L. performed research; X.T.W., V.L., and H.R. contributed new reagents/analytic tools; X.T.W. and Z.S. analyzed data; and X.T.W. and D.M.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The data in this paper are available from the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/dataset/683097).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1801049115/-/DCSupplemental.
Published under the PNAS license.
