The Arctic Ocean may be reaching its limit as a carbon sink, suggests a new study.

Some scientists had projected that, as sea ice cover receded and the ocean warmed, its waters could sop up increasing amounts of carbon dioxide. Removing the barrier between ocean and air, the reasoning went, would help provide a steady stream of CO2 to feed microbes in the surface waters.

But the study, published yesterday in Science, says that water sampled in the Arctic Ocean's Canada Basin in 2008 showed higher levels of CO2 than scientists expected to find, except in areas still covered by sea ice.

That implies that the microbe population hasn't expanded to gobble up the increased amount of CO2 entering surface waters from the air above.

"When we got to the Arctic Basin, the CO2 [in the water] was not that low," said Wei-Jun Cai of the University of Georgia, who led an international science team that sampled waters in the Canada Basin in the summer of 2008.

Cai said it appears there aren't enough nutrients in the surface waters to support a microbe population boom, despite the influx of CO2.

"What I am really emphasizing is in the basin area, it's a much smaller [carbon] sink than we expected," he said.

But it's not clear whether the Canada Basin is representative of the entire Arctic Ocean, other experts said.

Arctic ocean carbon cycle remains a mystery
Jeremy Mathis, a chemical oceanographer at the University of Alaska, Fairbanks, said the new paper "highlights the uncertainties in an environment that is changing very rapidly."

More research is needed to understand the Arctic's carbon cycle, Mathis said, and how it will change as the level of CO2 in the atmosphere rises and summer sea ice cover declines.

That's something Cai agreed with wholeheartedly. "Because of this rapid change, we need more research, sensors out there, multiple years of research," he said.

Another scientist who has studied the Arctic's potential as a carbon sink, Kevin Arrigo of Stanford University, said that "the jury is still out on how strong a sink for atmospheric CO2 the Arctic Ocean really is."

Just a handful of studies have examined the topic, and they arrived at different conclusions, Arrigo said in an e-mail as he finished up a five-week NASA cruise to the Arctic, where he investigated, among other things, the role the northern waters play in the carbon cycle (ClimateWire, June 9).

Arrigo also said he's not surprised that the new study found low levels of biological activity in the Canada Basin.

China joins research with 'Snow Dragon'
"I agree that this region isn't, and probably never has been, a big carbon sink," he said. That could change in the future if, as sea ice recedes, winds mix the water more thoroughly and bring more nutrients into water near the ocean's surface.

As it now stands, nutrient-rich water from the deep Pacific Ocean upwells into the Bering Sea and travels through the Bering Strait into the Arctic Ocean. But by the time it reaches the Canada Basin, Cai said, marine life has gobbled up much of the nutrient content.

Cai and his colleagues -- who included researchers from Canada, China, Japan, Korea and the United States -- based their findings on data they gathered during a research cruise in the summer of 2008. They sampled water in the Canada Basin and compared what they found to conditions documented in 1994 and 1999.

Cai said the research began in part because the Chinese government was looking for a project to help train a new generation of Chinese scientists who will be able to take advantage of the country's increasing investment in accessing the Arctic. China now owns an icebreaking ship outfitted for scientific expeditions -- the Xuelong, or "Snow Dragon" -- and is building another.

"My involvement in the Arctic research was somewhat accidental," Cai said. The Chinese and American governments were looking to collaborate on Arctic research, "and they were looking for someone who knew the science and could speak Chinese."

The Chinese were eager to test out a new instrument to analyze CO2 in seawater, he said. For the Americans, the arrangement allowed access to an icebreaker at a time when just one of three U.S. icebreakers is in good shape.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500