TAKING A DIP. Water sampling suggests that most carbon in the carbon dioxide dissolved in the rivers of the Amazon Basin was locked in plants for only a few years.Stroud Water Research Center

The Amazon and its tributaries release about 500 million tons of carbon dioxide into the atmosphere each year. Some of that gas comes from the oxidation of organic material in soil or from the decomposition of woody debris, which may have sequestered the carbon for decades or centuries. To figure out how old the carbon in this gas actually is, Emilio Mayorga, a biogeochemist at the University of Washington in Seattle, used carbon-dating techniques.

Analyses of water samples collected at 60 river sites in the Amazon Basin between 1991 and 2003 suggest that the carbon dioxide dissolved in those samples contained mostly carbon that had originated from transient reservoirs such as leaves, wetland grasses, or the animals that consumed them.

The carbon had been locked away in such sources only about 5 years on average, Mayorga and his colleagues estimate in the July 28 Nature.

That surprisingly rapid turnover in carbon suggests that Amazonian ecosystems may respond surprisingly quickly to influences such as climate change or deforestation.

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