Ice cores show fossil fuels disrupt nitrogen cycle

BROWN (US)—An ice core from Greenland indicates the greatest change in the global nitrogen cycle occurred between 1950 and 1980, following a period of rapid increases in fossil fuel emissions.

Fossil fuel combustion releases nitric oxides into the air that combine with other elements to form smog and acid rain.

In a paper published in Science, Meredith Hastings, assistant professor of geological sciences at Brown University and the paper’s lead author, and Julia Jarvis and Eric Steig from the University of Washington, trace the source of nitrates to nitric oxides released through fossil fuel burning that parallels the beginning of the Industrial Revolution.

“What we find is there has been this significant change to the nitrogen cycle over the past 300 years,” says Hastings. “So we’ve added this new source—and not just a little bit of it, but a lot of it.”

To make the link, two isotopes of nitrogen found in nitrates in a 100-meter long Greenland ice core were examined at high resolution. The core contains nitrates from 1718 to 2006. Tests showed the ratio of the nitrogen-15 isotope to the more common nitrogen-14 isotope had changed from pre-industrial times to the present.

“The only way I can explain the trend over time,” Hastings says,  “are the nitric oxide sources, because we’ve introduced this whole new source—and that’s fossil fuels burning.”

Eric Steig, from the Department of Earth and Space Sciences at the University of Washington, says the work also addresses a long-standing question about changes in lake chemistry in remote regions.

“Sediment cores in Arctic lakes show that there have been significant 20th-century declines in the nitrogen isotopic composition of organic nitrogen,” Steig says. “It’s been unclear whether these are due to changes in the lake biogeochemistry or to the direct effect of changes in the isotopic composition of the incoming nitrate from the atmosphere. Our study makes it clear that it is primarily the latter.”

The group now wants to determine the ratio of nitrogen-14 and nitrogen-15 isotopes for individual sources of nitric oxides, including lightning, biomass burning, bacterial “fixing” of nitrogen, and fossil fuel burning in order to pinpoint sources of nitrogen overloading, whether natural or human-caused.

“For example in Narragansett Bay, we could distinguish between nitrogen caused by sewage overflows or vehicular pollution, power plants, fertilizers, or other sources—and know how to attack the problem,” Hastings says.

Even more, the researchers want to quantify changes in the natural sources of nitric oxides and see whether climate change is influencing those processes.

The task is complicated, however, because nitrogen, when cycling through the atmosphere or deposited on land or in water, is subject to influences that can alter the isotopic ratios, thus masking the source. So, the scientists will need to tease out the extent of those alterations to trace the isotopic signatures of nitric oxide sources accurately.

The research was funded by the National Science Foundation’s Office of Polar Programs and the Joint Institute for the Study of the Atmosphere and the Ocean.

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