No laughing matter: Ocean nitrous oxide emissions greater than thought

by Sara E. Pratt
Thursday, November 12, 2015

Nitrous oxide is a potent greenhouse gas and, since the banning of chlorofluorocarbons in 1987, it has become the main driver of ozone loss from the stratosphere. Most atmospheric nitrous oxide is emitted from agricultural land and soils, but roughly a third is thought to come from the ocean. However, marine sources and sinks of the gas are not well understood.

One of the keys to quantifying these sources and sinks is found in coastal upwelling zones, where deep cold waters carrying abundant nutrients like nitrate and nitrite rise to the surface, fueling high productivity among marine algae. When the algae die and sink toward the seafloor, the decay process consumes and depletes oxygen in the water column. The resulting suboxic waters facilitate both the microbial production and reduction of nitrous oxide.

Two new field studies have measured how much nitrous oxide is being emitted from a pair of oxygen minimum zones (OMZs) in the Pacific Ocean, elucidating the particular biogeochemical processes at work as well as rates of nitrogen cycling. The findings from both studies suggest that previous estimates of marine nitrous oxide emissions have been vastly underestimated.

In a recent study in Nature Geoscience, Damián Arévalo-Martínez, a chemical oceanographer at the Helmholtz Center for Ocean Research in Kiel, Germany, and colleagues examined the upwelling zone off the coast of Peru in the tropical South Pacific. On cruises in late 2012 and early 2013, the researchers sampled the water column in different parts of the region to determine where and at what depth nitrous oxide was being produced. They also towed a laser-based instrument between fixed sampling sites to continuously sample nitrous oxide.

From these measurements, they calculated an overall flux of nitrous oxide from this upwelling zone to the atmosphere ranging from 0.3 to 1.4 teragrams of nitrous oxide per year. These levels of emissions, from a single upwelling zone, are equivalent to 5 to 22 percent of the current estimated total nitrous oxide emissions from the entire global ocean (thought to be about 4 teragrams total). All of the world’s coastal upwelling regions together were thought to contribute only 5 percent of the global total of nitrous oxide.

“Arévalo-Martínez and colleagues find evidence for extremely high emissions of nitrous oxide from the Peruvian coastal region, suggesting that oceans are a larger source of [atmospheric] nitrous oxide than previously thought,” wrote Imke Grefe, an oceanographer at Dalhousie University in Halifax, Nova Scotia, Canada, in an accompanying commentary in Nature Geoscience. “The results … highlight the large uncertainty in current global estimates of marine nitrous oxide emissions.”

The researchers found that most of the nitrous oxide was being produced near the boundary of the OMZ, where oxygen concentrations are slightly higher than in the zone’s interior. Even small amounts of oxygen inhibit the completion of denitrification, leaving higher levels of intermediate products like nitrous oxide and nitric oxide in the water.

The study focused solely on the Peruvian upwelling zone, which the researchers call a hot spot of nitrous oxide production that needs to be taken into account in climate models. Further studies will be needed to assess the contributions of other coastal upwelling zones around the world, they suggested.

In a second study, published in Science a few weeks earlier, scientists sampled OMZs in the eastern tropical North Pacific and also found that these spots are playing an overlooked role in the emission of nitrous oxide.

Andrew Babbin, a postdoctoral researcher in marine biogeochemistry at MIT, and colleagues used isotope tracers to measure the reduction of nitrous oxide directly. They found that in suboxic zones, the processes that produce nitrous oxide and consume it are not as tightly coupled as previously thought.

“It turns out that with a little bit of decoupling — it doesn’t have to be much — there can be large amounts of nitrous oxide production,” said Babbin, who conducted the research while a doctoral student at Princeton, in a statement.

The researchers concluded that current models predicting the rate at which nitrous oxide cycles through the marine system are off by an order of magnitude, as are the climate models that depend on them.

“Current estimates of the marine nitrous oxide sources to the atmosphere … including the Intergovernmental Panel on Climate Change estimate of about 4 teragrams of nitrogen per year, are low,” the authors wrote. “The nitrous oxide production rates predicted [in this study] would account for 4 teragrams of nitrogen per year from the OMZs alone.”

Along with Peru, there are three other major eastern boundary upwelling systems in the world, off the coasts of California, South Africa and the Canary Islands. In addition, recent studies that have shown OMZs in the ocean are growing and are expected to continue to grow in the future now also have implications for the production of more greenhouse gas.

“These findings are highly significant,” said James Galloway, a biogeochemist at the University of Virginia who was not involved in the research, in a statement. “They indicate that now the oceans can be expected to increase their nitrous oxide emissions, just as continents are expected to, due to agriculture.”