Most of the carbon being exhaled ?or outgassed ?as carbon dioxide from Amazonian rivers and wetlands has spent a mere 5 years sequestered in the trees, other plants and soils of the surrounding landscape, U.S. and Brazilian researchers report in the July 28 issue of Nature.
It had been hoped that regions such as the nearly 2.4 million-square-mile Amazon River basin ?where tropical forests rapidly gulp carbon dioxide during photosynthesis ?were holding onto that carbon for decades, even centuries, says Emilio Mayorga, University of Washington oceanographer and lead author of the Nature piece with Anthony Aufdenkampe of the Stroud Water Research Center in Pennsylvania.
As policy makers turn increasingly to carbon-credit trading as a means of grappling with the impacts of human-induced climate change, knowing how much carbon can be stored ?and where and for how long ?is critical, the authors say.
"Our results were surprising because those who've previously made measurements found carbon in the rivers that came from the surrounding forests to be 40 to more than 1,000 years old," Aufdenkampe says. "They assumed that the return of this forest carbon to the atmosphere must be a slow process that offered at least temporary respite from greenhouse effects.
"As part of the largest radiocarbon age survey ever for a single watershed, we show that the enormous amount of carbon dioxide silently being returned to the atmosphere is far younger than carbon being carried downstream," he said. "Previous studies failed to detect the rapid recycling of forest carbon because they never dated the invisible greenhouse gas as it is literally exhaled by the river organisms."
"River breath is much deeper and faster than anyone realized," says Jeff Richey, UW oceanographer and another co-author.
Carbon is carried by rains and groundwater into waterways from soils, decomposing woody debris, leaf litter and other organic matter. Once in waterways it is chewed up by microorganisms, insects and fish. The carbon dioxide they generate quickly returns to the atmosphere, some 500 million tons a year, an amount equal to what is absorbed each year by the Amazonian rainforest.
"Having established that the amount of carbon outgassing is much greater than anyone imagined, the issue then becomes, where does it come from," Mayorga says. "If it's young, that indicates the carbon pool is dynamic, which could make the system much more reactive to deforestation and climate change."
For example, data from a region of active deforestation in the southern Amazon already shows that the carbon leaving rivers has an identifiable isotopic signature of pasture grasses.
"You're changing the land use, changing vegetation and other conditions. In terms of what's being respired, the system is responding fairly quickly," Mayorga says. "Human and natural systems, in turn, will be impacted."
No previous tropical study has used both radioactive carbon-14 and stable carbon-13 isotopes to address these questions. Funding from the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory made the analysis by Mayorga and Aufdenkampe possible. The samples were collected by Richey's research group and Brazilian scientists on expeditions going back as far as 1991 that were funded by the National Science Foundation, National Aeronautics and Space Administration and the Research Support Foundation for the State of San Paulo (FAPESP), Brazil.
Other co-authors are Paul Quay and the late John Hedges, both UW oceanographers; Caroline Masiello of Rice University; Alex Krusche of the University of São Paulo, Brazil; and Thomas Brown of the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.