Coral record of anthropogenic N perturbation in the remote open ocean

Marine phytoplankton, like all organisms, require nitrogen to live and grow. Although the majority of the air we breathe is N2, the nitrogen in the atmosphere is unavailable for use by most phytoplankton, except certain bacteria and cyanobacteria capable of breaking the strong N–N triple bond. In order for organisms to be able to use nitrogen, N2 gas must first be converted to a more chemically available form such as ammonium, nitrate, or organic nitrogen. The inert nature of N2 means that biologically available nitrogen is often in short supply in the ocean, limiting phytoplankton growth.

Within the last century, humans have become an important source of fixed nitrogen. In the early 20th century, Fritz Haber and Carl Bosch developed an industrial process to produce reactive nitrogen from atmospheric N2 stores. The Haber-Bosch reaction soon became the means to produce nitrogen-rich fertilizers. Meanwhile, the adoption of internal combustion engines and other industrial burning processes led to widespread release of oxidized nitrogen (NOx) to the atmosphere. Through these activities, humans have more than doubled the amount of fixed nitrogen that is pumped into the biosphere every ear. Most of these are deposited on land, but some have escaped and traveled far. Modeling studies suggest that the oceans far away from the continents are not immune from the impacts of humankind's nitrogen fertilization experiment. However, little evidence exists to support this. The signal of anthropogenic nitrogen deposition is diluted by ocean mixing, and its impact may also be counteracted by the organisms capable of fixing atmospheric N2.

Anthropogenic sources of nitrogen are often isotopically lighter than the nitrogen circulating through natural processes in ecosystems. Using 15/14N stable isotope analysis, Ren et al. track the appearance of this isotopically light nitrogen in seasonally resolved coral from Dongsha atoll, a semi-closed circular coral reef atoll located 300km away from the nearest continents in the northern South China Sea. The authors find that the light-nitrogen signal increased just before 2000, coincident with massive increases in fossil fuel combustion in Asia, but decades later than predicted by modeling work. The amplitude of change suggests that, by 2010, anthropogenic atmospheric N deposition represented about one fifth of the annual N input to the surface ocean in this region, which appears to be at the lower end of other estimates.

This study provides two insights into human amplifications of the nitrogen cycle in the South China Sea: 1) Fossil fuel energy use (including coal consumption and vehicle exhaust) may have been the major driver of increasing anthropogenic nitrogen deposition to the open ocean, and their effect may be immediate; 2) The differences between observations and modeling work highlight our unknowns for the sources and pathways for nitrogen deposition to the open ocean.

This discovery on the spreading extent of humankind's unintentional nitrogen experiment should add urgency to recent concerns about the multidimensional planetary boundaries that humanity is pressuring. And it also highlights the urgency to monitor the ocean environment with significant spatial coverage and through time for better understanding the human footprint on the open ocean. This study points to the tremendous potential value of a network of coral-bound N isotope records from ocean islands and offshore reefs to deliver this.

The paper, "21st Century Rise in Anthropogenic Nitrogen Deposition on a Remote Coral Reef," was published in the May 19, 2017, issue of the journal Science.

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Figure A. Most of southeastern China has been covered by a thick greyish shroud of aerosol pollution in January 2002. The smog is so thick it is difficult to see the surface in some regions of this scene, acquired on January 7, 2002. The city of Hong Kong is the large brown cluster of pixels toward the lower lefthand corner of the image (indicated by the faint black box). The island of Taiwan, due east of mainland China, is also blanketed by the smog.This true-color image was captured by the Moderate-resolution Imaging Spectroradiometer (MODIS) sensor, flying aboard NASA’s Terra satellite. This image is from NASA ( Dongsha can be seen as the green circular ring located to the southwest of Taiwan in the South China Sea, marked with a red circle.

Figure B. The increase in the 14N signal observed in Dongsha coral is paced with increase of fossil fuel combustion (including coal consumption and vehicle use) in China.