Nitrous oxide record sheds light on glacial carbon dioxide

University Park, Pa. -- A 106,000-year-long record of nitrous oxide concentrations and a shorter record of nitrogen and oxygen isotopes show that both marine and terrestrial nitrous oxide production increased in unison and effectively by the same proportional amount during the end of the last glacial period, according to Penn State researchers.

Equal terrestrial and marine production of nitrous oxide also suggest that increased storage of carbon in the oceans was not the cause of low atmospheric carbon dioxide during ancient glacial periods, the researchers report in today’s (Aug. 15) issue of Science.

“Nitrous oxide is a greenhouse gas, but there is so little of it in the atmosphere, that it hardly contributes to climate change through changes in the radiation budget,” says Todd Sowers, research associate in geosciences. “Changes in nitrous oxide loading can, however, provide clues about systems that control carbon dioxide in the atmosphere.”

Sowers, working with Richard B. Alley, the Evan Pugh professor of geosciences, and Jennifer Jubenville, former graduate student, looked at nitrous oxide from the Greenland Ice Core Project II ice core to catalog atmospheric nitrous oxide concentrations through time.

“This is a new record of concentration variations back this far, only a small portion had been done before” says Sowers. “We found a 40 percent increase in the concentration of nitrous oxide in the atmosphere as the Earth warmed at the end of the last glacial period.”

The concentration data alone shows how much nitrous oxide was in the atmosphere at any particular time. It cannot, however, suggest how much of that gas came from the oceans or land. The researchers also looked at an ice core from the Taylor Dome, Antarctica, to create a 30,000-year history of the isotopic composition of the nitrogen and oxygen in the nitrous oxide.

Bacteria on land and in the oceans produce nitrous oxide in one of two ways. Ocean bacteria tend to create nitrous oxide that has more of the heavier isotopes of nitrogen and oxygen, while terrestrial bacteria tend to produce nitrous oxide with the lighter atoms. By looking at proportions of isotopes in the trapped gases, the researchers could determine how much was made on land and how much in the oceans.

“Before we had the isotope records, common wisdom suggested changes in terrestrial emission were probably the major player responsible for the observed concentration changes,” says Sowers. “Our isotope data, however, show that both oceanic and terrestrial emissions changed in roughly the same proportion throughout the last 30,000 years.”

Carbon dioxide in the atmosphere hits lows during glacial periods and some researchers have suggested that increased productivity in the glacial oceans could have removed carbon dioxide from the atmosphere. If the oceans behaved as they do today, then increased oceanic productivity during the glacial period would have produced elevated oceanic nitrous oxide production. However, if the relationship between terrestrial and marine nitrous oxide did not change, then this cannot be an explanation for the low levels of carbon dioxide in the atmosphere during glacial periods.

“When we thought terrestrial emissions were the dominant control on atmospheric nitrous oxide concentrations, then this hypothesis could have been true,” says Sowers. “Now that we know that the land and oceans contributed equally, we have to look for another explanation for the low carbon dioxide levels.”