Density of Nitrous oxide
Researchers in the eastern tropical North Pacific used the sampling device shown here to gather water samples from various depths, at three different sampling locations. Credit: Clara Fuchsman
Nitrous oxide (N2O) is a potent greenhouse gas that can contribute to climate change and damage the ozone layer. But its cycling in and out of ocean waters has remained poorly understood, making it difficult to predict how the gas might impact the climate.
Now new research by MIT postdoc Andrew Babbin and three others has provided a way to quantify this cycle, in which N2O—commonly known as laughing gas—is rapidly formed and destroyed in oxygen-poor layers of seawater, and some of the gas is released into the air. The findings, based on computer analysis and sampling of ocean waters from different depths, are presented this week in the journal Science, and show that this source of atmospheric nitrous oxide has been drastically underestimated.
Babbin, a postdoc in MIT's Department of Civil and Environmental Engineering and the study's lead author, says that while nitrogen and its compounds are crucial to life, it has been difficult to accurately measure the processes by which these compounds cycle through the land, air, and water.
"There have been a lot of estimates on what the sources and sinks are, both on land and in the ocean, " Babbin says. But the new measurements, he adds, show that in parts of the ocean, those estimates were off by at least a factor of 10.
It turns out that a particular zone of the ocean—a boundary between oxygen-rich surface waters and oxygen-free, or "anoxic, " deep waters—plays a key role in nitrogen cycling. This "suboxic" zone experiences an imbalance between bacterial processes that create N2O and those that break it down—and the excess of N2O created by this imbalance is given off to the atmosphere.
Ocean nitrification begins with nitrogen entering the sea as runoff from agricultural fertilizers and other sources. Marine microbes take in nitrogen compounds, such as ammonia, and chemically modify them, releasing N2O as a byproduct. Other bacteria carry out denitrification, a process that breaks down nitrogen compounds through steps that ultimately lead to nitrogen gas—but which can also release some N2O.
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