Greenhouse Gas CO2 Sidetracked in Oceans' Twilight Zone

WOODS HOLE, Massachusetts, April 30, 2007 (ENS) - Climate experts have relied on the oceans to absorb enough of the greenhouse gas carbon dioxide to slow global warming, but new research by an international team of scientists shows that the oceans may have little impact on changes in the atmosphere or climate.

The research indicates that instead of sinking, carbon dioxide is often consumed by animals and bacteria and recycled in the "twilight zone," a dimly lit area 100 to 1,000 meters below the surface.

Because the carbon often never reaches the deep ocean, where it can be stored and prevented from re-entering the atmosphere as a green-house gas, the oceans may not be able to perform the crucial role in greenhouse gas absorption and storage that has been assumed.

The research, funded by the National Science Foundation, is the result of two international expeditions to the Pacific Ocean, and is reported in the April 27 issue of the journal "Science," published by the American Association for the Advancement of Science.

"These results are particularly important to our efforts today to improve the predictive capacity of numerical models that relate ocean carbon to global climate change on different time scales," said Don Rice, director of National Science Foundation's chemical oceanography program.

The study adds a new complication to proposals to mitigate climate change by fertilizing the oceans with iron. The iron was supposed to promote blooms of photosynthetic marine plants and transfer more carbon dioxide from the air to the deep ocean.

"The twilight zone is a critical link between the surface and the deep ocean," said Ken Buesseler, a biogeochemist at Woods Hole Oceanographic Institution and lead author of the new study, which is co-authored by 17 other scientists.
"We're interested in what happens in the twilight zone, what sinks into it and what actually sinks out of it," said Buesseler. "Unless the carbon goes all the way down into the deep ocean and is stored there, the oceans will have little impact on climate change."

Buesseler was the leader of a project called Vertical Transport In the Global Ocean, VERTIGO.

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Scientists deploy a Neutrally Buoyant Sediment Trap in the ocean off Hawaii on a VERTIGO research voyage in 2004. (Photo courtesy UH)
The researchers found that the twilight zone acts as a "gate" that allows more sinking particles through in some regions and fewer in others, complicating scientists' ability to predict the ocean's role in offsetting the impacts of greenhouse gases.

These sinking particles, often called "marine snow," supply food to organisms deeper down, including bacteria that decompose the particles. In the process, carbon is converted back into dissolved organic and inorganic forms that are re-circulated and reused in the twilight zone and that can make their way to the surface and back into the atmosphere.

The problem, say scientists, is that particles sink slowly, perhaps 10 to a few hundred meters per day, but they are swept sideways by ocean currents traveling many thousands of meters per day. To collect sinking particles, oceanographers use cones or tubes that hang beneath buoys or float up from sea floor. That, Buesseler said, "is like putting out a rain gauge in a hurricane."

Using new technology, the researchers found that only 20 percent of the total carbon in the ocean surface made it through the twilight zone off Hawaii, while 50 percent did in the northwest Pacific near Japan.

The sites off Hawaii and Japan were selected because they had been the focus of long-term ocean observations by co-authors David Karl of the University of Hawaii and Makio Honda of the Japan Agency for Marine-Earth Science and Technology.

"As we learn more and more about the sea around us, including the deep ocean, we gain an appreciation for how important and how fragile life in these regions can be, says Karl.

"By gaining a better understanding of how organic matter and energy are processed in the mid-depths of the ocean we can better predict how life in the sea may be affected by human-imposed climate change including greenhouse gas production," he said.

"The twilight zone may be the largest habitat for life on our planet in terms of volume, yet we are just beginning to understand who lives there and how they make their living.

While many studies have investigated the surface of the ocean, little research has been conducted on the carbon cycle below. The VERTIGO team examined a variety of processes to open a new window into the difficult-to-explore twilight zone. They successfully used a wide array of new tools, including an experimental device that overcame a longstanding problem of how to collect marine snow falling into the twilight zone.

James Bishop of the Lawrence Berkeley National Laboratory and University of California, Berkeley was funded by the U.S. Department of Energy to deploy new autonomous optical sediment traps designed to follow the hourly changes in sedimentation as well as ship deployed particle sampling systems to quantify the abundance and composition of particles in the twilight zone.

More than 40 biologists, chemists, physical oceanographers, and engineers from 14 institutions and seven countries participated in the two VERTIGO oceanographic research cruises in 2004 and 2005 to investigate how marine plants die and sink, or are eaten by animals and converted into sinking fecal pellets.