Duke Professor's Cruise Data Suggest Antarctic Seas Are Major Carbon Dioxide Burial Site

February 10, 1998

BEAUFORT, N.C. -- A frigid summer cruise to the south polar ice pack's edge has convinced the expedition's chief scientist that the Antarctic region holds the key to how the world's oceans will affect global warming.

"We're talking about a process that occurs in a special, particular area, and it's going to determine the answer for the whole system," predicted Richard Barber, Harvey W. Smith professor at Duke University's Nicholas School of the Environment's Marine Laboratory.

Barber supervised the activities of 37 scientists during the mission south from Christchurch, New Zealand aboard the research vessel R/V Roger Revelle. The mission, which was held Nov. 26, 1997 through Jan. 6, 1998, is funded by the National Science Foundation.

During that fact-finding journey to the iceberg-festooned Southern Ocean, he posted weekly summaries of his group's experiences on a Nicholas School "Antarctic Journal" website that is still available on-line at <http://www.env.duke.edu/antdispatch.html>.

Barber studies how carbon dioxide is taken up and emitted by biological and physical processes in the world's oceans. Excess atmospheric carbon dioxide (CO2), is expected to cause global warming in the next century because human activities have almost doubled the natural amounts of that gas in the atmosphere.

The expedition aimed to learn more about the complicated uptake and release of carbon dioxide in one of the planet's least-studied regions. Learning how carbon dioxide is processed in the Antarctic will help scientists predict the impact of global warming, Barber said in his first post-cruise interview.

The processes by which carbon dioxide is absorbed and release are extraordinarily complex, he said.

Scientists know in general that carbon dioxide is much more soluble in cold seawater than is oxygen, and that a significant amount of atmospheric CO2 thus gets dissolved into cold ocean surface currents.

They also know that cold currents tend to sink below the surface. However, they know that after CO2-rich cold ocean water warms up, it tends to upwell and release much of its carbon dioxide back into the atmosphere.

Microscopic plants -- called phytoplankton -- that live in nutrient-rich ocean waters also take up carbon dioxide, which they convert into sugars with the aid of sunlight and chlorophyll. When those plants die and sink they can carry that trapped carbon with them, possibly burying it for eons in bottom sediments.

But microscopic animals -- called zooplankton -- will eat many of those plants before they have time to die and sink. When they do, these animal "grazers" will instead recycle the carbon back into the atmosphere as CO2.

The Revelle cruise's scientists visited the Antarctic Polar Front Zone, a globe-circling region where hardy phytoplankton and zooplankton live and die in near-freezing CO2-laden water that seems to flow north near the surface before descending along a frontal boundary.

The scientists filled parts of the deck with Plexiglas incubators where they could grow trapped native phytoplankton in natural sunlight. They used radioactive tracers to study the rates that plant specimens took up CO2 and essential nutrients.

The investigators lowered optical sensors to measure the seawater's turbidity and chlorophyl levels -- both signs of biotic activity. They used radioactive thorium, a chemical that sticks to the bodies of phytoplankton, to measure the rates that those plants disappeared from the sunlit surface zone. They also sampled the ocean water's chemistry and kept careful records of its currents.

While data from the 32-day cruise are only beginning to be analyzed, Barber said the trip definitely produced some "surprises." One was the discovery that the northward surface circulation in the region is not caused exclusively by the wind, as the "textbook explanation" suggests.

Visiting at the summer's height, when the sun stayed above the horizon for 24 hours a day, his group found "tongues"of ice-cold water emerging from underneath the edge of the Antarctic ice pack.

"There was a fresher layer on top that was diluted by melting ice, and then there was very cold and salty water going north," Barber said. "The wind cannot blow through the ice, and it was not windy all the time. So this current appeared not to be locally wind-driven while we were there, and yet it was very dramatic."

Another "puzzle," Barber said, was that large concentrations of silicon-containing phytoplankton -- called diatoms -- appeared to be thriving in the study area without reducing levels of the plant nutrient, nitrate. "I was surprised that the diatoms were doing as well as they were without drawing down the nitrate," Barber said.

The scientists couldn't figure out where the diatoms were getting the nitrogen they would need to survive if it wasn't in the form of nitrates. But careful measurements showed that the water's high nitrate levels remained relatively constant, meaning they were not being drawn down by the diatoms. "The growth rates, considering the low temperature, were surprisingly high," he said.

Meanwhile, the scientists found that zooplankton were grazing off only half the diatoms that were generated. Strong thorium depletions indicated that the other half of the diatoms were sinking out of the sunlit zone before they could be eaten.

If further studies confirm these early observations, Barber said they could have implications for global warming. It would mean that, unlike the tropical and temperate oceans, the Antarctic Polar Front Zone may be a burial site for significant amounts of atmospheric carbon dioxide.

Extensive studies by Barber and others already have shown that warm equatorial waters are where the oceans emit most of their dissolved carbon dioxide into the atmosphere. Studies also reveal that grazing zooplankton in tropical and temperate waters keep phytoplankton growth in balance, thus preventing much CO2 burial there.

Barber also suspects that the Southern Ocean, and perhaps the north polar seas as well, may play a strong role in the evolution of ice ages. While an ice age may be triggered by changes in sunlight levels, it could be intensified by increasing phytoplankton-zooplankton imbalances there. That would cause increasingly larger amounts of carbon dioxide to be removed from the atmosphere, he speculated.

Conversely, what his team witnessed last month there may also be an early signal of global warming, he added.

"Remember that the high growth we saw was in a zone that was affected by meltwater," Barber said. "And the plants grew in it dramatically. It is entirely conceivable that a high rate of warming in the spring would result in more of a zooplankton-phytoplankton imbalance. And, of course, the effect of that imbalance would be to pull down CO2.

"We think that for the temperate and tropical oceans, the effect of global warming will be to reduce carbon dioxide uptake from the atmosphere. But for the Antarctic, the effect could easily be increased uptake."

But he was quick to caution that this cruise collected results at only one time of the year. "The process may be strongly one way in the winter and strongly another way in the summer," he said. "All I could think of was that we were there in the best weather -- and it was terrible.

"The sleet and snow was blowing, and the instruments we were using were at their limits all the time. And this was in middle summer. I kept asking myself, ?What's it like all of the rest of the year?' You can't rule out that the important time in the Southern Ocean is when that first roaring, howling storm comes through."

Answering that question will require a new way of doing science, perhaps using remote controlled instruments that are tethered to the sea bottom or affixed to the many uninhabited islands that dot the Antarctic region, he said.
-end-


Duke University

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