the incorporation of observational findings that
a substantial fraction of gross open-ocean N deposition reflects an ocean-atmosphere N cycle
rather than a net continental input (27). Our results thus appear to provide additional support
for the importance of ocean-atmosphere cycling
in the reactive N chemistry of the marine atmosphere (28), which complicates the quantification of AAN input from direct deposition rate
Another significant finding is that the CS-d15N
decline was minor until ~2000, decades later than
anthropogenic N emissions increased in China
(5). The timing of CS-d15N change resembles the
increase in the nitrogen oxide emissions associated with fossil fuel burning more than it does
the ammonia emissions from food production
(N fertilizer use and animal husbandry) (Fig. 4
and fig. S8). In particular, coal consumption and
the number of vehicles have increased exponentially over the past decades, coinciding with
the more rapid CS-d15N decline since the late
1990s. Thus, the data suggest that fossil fuel
energy use has been the major driver of increasing AAN deposition in the South China Sea. This
is consistent with the dominant role of energy
use in the N deposition on land from the 1990s
to the 2000s (29).
Superimposed on the decreasing trend, CS-d15N
shows seasonal and decadal variations. The sea-
sonal changes coincide with (but slightly lag)
changes in d18O, indicating that low d15N values
occurred in the winters and high values in the
summers (Fig. 2 and figs. S1 and S4). The sea-
sonal variation in CS-d15N is between 0.5 and 1‰,
appearing stronger since the late 20th century
(fig. S4). One possible driver of the seasonal
variation is a seasonal cycle in atmospheric N
sources. During winter, the strong northeasterly
monsoon blows polluted air from northeast Asia
to Dongsha, delivering more low-d15N AAN relative
to the summer, when southwesterly monsoons im-
port cleaner maritime air (30). Consistent with
this explanation, nitrate in wet deposition in
Dongsha is observed to be ~5‰ lower in winter
than summer (11). An increase in AAN deposi-
tion since the late 20th century may thus also
explain the recent increase in d15N seasonality.
Further work will be required to test this inter-
pretation. The cause of the decadal variability in
CS-d15N is difficult to assess with current infor-
mation; El Niño–Southern Oscillation variation
may be involved (fig. S9).
Anthropogenic emissions have fundamentally
changed the biogeochemical cycling of nitrogen
on land and in coastal regions [e.g., (31, 32)] and
have been suggested to impact the open ocean,
especially its nutrient-limited low-latitude regions
(1, 33). Our Dongsha CS-d15N record provides
a multidecadal accounting of AAN deposition
on the open ocean, showing that AAN has indeed
increased, but the reconstructed increase falls in
the lower range of prior estimates. Our findings
point to energy use and vehicles as the major
concern for impacts on this region of the open
ocean and thus raise the prospect that reduc-
tions in nitrogen oxide emissions in East Asia
achieved by upgrading combustion technology
or switching to renewable energy sources could
largely spare the open western North Pacific from
anthropogenic N fertilization. A spatially extensive
network of N isotope records from corals and
similar archives from ocean islands and offshore
reefs, when combined with direct measurements
and numerical models of the reactive N cycle,
has great potential to clarify the evolving anthro-
pogenic impact on the open-ocean N cycle.
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Funding was provided by Ministry of Science and Technology
of Taiwan (MOST) grant 105-2628-M-002-007-MY3,
National Taiwan University grants NTU-SINICA-105R104513,
NTU-CESRP-105R7625, and NTU-CDP-105R7719 to H.R., the
Sustainability Science Research Program of the Academia
Sinica to G. T.F. W. and A.L.C., Taiwan MOST grant NSC 101-2611-
M-001-003-MY3 to G. T.F. W., U.S. NSF grants 1536368 and
1537338 to A.L.C. and D.M.S., the Grand Challenges Program of
Princeton University to D.M.S., and an Academia Sinica
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Materials and Methods
Figs. S1 to S10
11 November 2016; accepted 6 April 2017
752 19 MAY 2017 • VOL 356 ISSUE 6339 sciencemag.org SCIENCE
Fig. 4. Comparison of the CS-d15N decline with two contributors to anthropogenic N emissions in China. Higher values of CS-d15N (red) are plotted downward. Data on N fertilizer use (black)
and coal consumption (blue) are from the Earth Policy Institute ( www.earth-policy.org/
data_center/). Coal consumption is shown here as a proxy for nitrogen oxide emissions, although
vehicle use and associated petroleum consumption are also important sources (fig. S8).