A major international project involving two research expeditions to the Pacific Ocean has revealed unexpected details about the fate of carbon dioxide taken up by photosynthesizing marine plants in the sunlit surface layer of the ocean. The study, published April 27 in the journal Science, identifies a critical link in the processes that determine the ocean's ability to absorb and store the carbon dioxide that is accumulating in our atmosphere.
Some of the carbon incorporated into the cells of marine plants ultimately sinks into the depths of the ocean, where it is stored and prevented from reentering the atmosphere as a greenhouse gas. But a surprisingly large amount of the carbon carried below the surface layer by sinking particles of organic matter (known as "marine snow") gets consumed by animals and bacteria and recycled within the "twilight zone," a dim region from 100 to 1,000 meters below the surface.
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 study builds on pioneering investigations of marine snow begun in the 1970s by Mary Silver, professor of ocean sciences at the University of California, Santa Cruz. Silver, a coauthor of the new paper, was the first scientist to take a close look at the small flocs and flakes of nonliving particles that drift down through the water. Her findings showed that marine snow is a major source of sinking organic matter in the world's oceans and is the site of intense microbial activity.
Now, researchers have found that the twilight zone acts as a "gate," allowing more sinking particles through in some regions and fewer in others. These findings complicate the efforts of climate modelers to predict the ocean's role in offsetting the impacts of greenhouse gases.
"The twilight zone is a critical link between the surface and the deep ocean. Unless the carbon that gets into the ocean goes all the way down into the deep ocean and is stored there, the carbon can still make its way back into the atmosphere," said Ken Buesseler, a biogeochemist at Woods Hole Oceanographic Institution (WHOI) and lead author of the new study in Science, coauthored by 17 other scientists. "Without long-term carbon storage at depth, the ocean can do little to stem the increase in atmospheric levels of carbon dioxide," he said.
Buesseler was a leader of the ambitious project, funded primarily by the U.S. National Science Foundation, called VERTIGO (VERtical Transport In the Global Ocean). More than 40 biologists, chemists, physical oceanographers, and engineers from 14 institutions and seven countries participated in the two VERTIGO cruises in 2004 and 2005 to investigate the fate of the carbon in marine plants that die and sink or are eaten by animals and converted into sinking fecal pellets.
Silver was one of two biologists who conducted shipboard studies of seawater samples collected during the cruises--identifying plankton, characterizing the communities of organisms in each location, and trying to understand how different communities influence the marine carbon cycle. Working alongside Silver was her former graduate student Deborah Steinberg, who did her doctoral research on marine snow at UCSC and is now at the Virginia Institute of Marine Sciences.
"It's the biology of the system--the plants that fix the carbon and the animals that graze on them--that makes the system variable," Silver said. "More or less carbon gets carried into the deep waters depending on which organisms are present."
Sinking particles of 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 recirculated and reused in the twilight zone and can make their way to the surface and back into the atmosphere.
In the North Pacific, an ecosystem in which diatoms (a type of microscopic algae) dominate in the surface layer proved to be more efficient in transporting organic carbon through the twilight zone than the ecosystem at the site near Hawaii. Understanding exactly which factors are responsible for this difference will require further study.
"We know very little about this layer, the twilight zone, where organic material either passes through or gets burned up and returns as carbon dioxide to the atmosphere," Silver said.
But the new findings show that the ocean is not as big a sink for atmospheric carbon as was once thought, she said. Widely used estimates of carbon sequestration in the oceans have been largely derived from an earlier project, called VERTEX, in which Silver also participated in the 1980s.
"Our new work is showing that the earlier model of carbon sequestration was very simplistic. We now have to make a more sophisticated model that takes into account the variations in the amount of carbon sequestered in the deep sea," she said.
Silver noted that the VERTIGO study benefited from a diverse group of scientists contributing perspectives from a wide range of disciplines, including chemistry, physics, biology, and engineering. The sites off Hawaii and Japan were selected because they had been the focus of long-term ocean observations by coauthors David Karl (University of Hawaii) and Makio Honda (Japan Agency for Marine-Earth Science and Technology). Thomas Trull (University of Tasmania, Australia), Philip Boyd (University of Otago in Dunedin, New Zealand), and Frank Dehairs (Free University of Brussels, Belgium) all study the Southern Ocean and could provide important perspectives contrasting ocean carbon cycles off Antarctica and in VERTIGO. David Siegel (UC Santa Barbara) helped track the ocean currents and pathways of the sinking particles. Along with WHOI engineers, James Bishop (UC Berkeley) brought new tools to study the abundance and composition of particles in the twilight zone.
"This combination of expertise could not be found in any single lab or country," Buesseler said. "We were fortunate to attract such a diverse group of talented scientists willing to unravel the secrets of the twilight zone and its role in the global carbon cycle."
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