Nitrogen Isotopes In Ice Cores Record Human Activity

A study of ice cores from Greenland reveals that the nitrogen isotope ratio in nitrates has undergone a marked change during the past 300 years—ever since humans began generating nitrogen oxides (NO_x) by burning large quantities of fossil fuels and fixing nitrogen from the atmosphere to make synthetic fertilizers (Science 2009, 324, 1288). It's known that NO_x readily converts to nitrates in the atmosphere. And scientists have suggested in the past that the ¹⁵N/¹⁴N makeup of nitrates should hold information about where the nitrogen came from, says geological scientist Meredith G. Hastings of Brown University, who led the research team. The new ice core study provided "a unique opportunity to look at whether isotopes of nitrate have changed with the sources," Hastings says. And they have: The researchers found that the percentage of ¹⁵N in the nitrates began a dramatic decline in the mid-1800s. With this temporal association with the onset of anthropogenic NO_x emissions, scientists can now begin to "quantitatively reconstruct the influence of different NO_x sources on preserved nitrate" in ice cores and other nitrate-containing samples, Hastings and coworkers write.