Scientists had not known the age of the carbon that reaches the ocean: was it recently derived from contemporary plant material, or had it been locked in soils for hundreds or thousands of years and therefore not part of Earth's recent carbon cycle? Now, using carbon-14 data, scientists from the United States and Germany have been able to determine the approximate age of dissolved organic carbon in the Arctic for the first time.

Most of the carbon that reaches the ocean is relatively young at present, but that this could change. Warming of the Arctic, which has been documented in recent years, could affect northern peats, collectively one of the largest reservoirs of organic carbon on Earth. As the carbon-rich soils warm, the carbon is more susceptible to being transported to the ocean by rivers small and large, they say.

The researchers, headed by Ronald Benner of the University of South Carolina, studied four rivers in northern Russia and in Alaska, along with the Arctic Ocean itself. The carbon-14 dating method is not precise, because, for example, old and new dissolved organic carbon is typically mixed in a given sample, resulting in an average reading, and content of rivers varies by season as well. The scientists concentrated their study in periods of peak river discharge.

"Our results are not applicable to the sedimentary fraction of river discharge," Benner notes. "However, most of the organic carbon exported from land to the ocean is in dissolved form, and it is the dissolved components that track river water in the ocean."

River water tends to remain near the surface of the Arctic Ocean for five to 15 years, and the land-derived dissolved organic carbon from all sources and years is therefore mixed. Various samples gave radiocarbon average ages varying from 680 to 3,770 years, including both carbon from land-derived and marine sources. The researchers analyzed dissolved lignin phenols to determine the portion of a particular sample that had originated on land, as the compound is related only to terrestrial plant material.

The East Greenland Current is the major source of both Arctic Ocean water and its dissolved organic carbon component reaching the North Atlantic Ocean. The study concludes that the land-derived dissolved organic carbon reaching the Atlantic via this current is much younger than the marine component.

In fact, up to half of it reaches the Atlantic, some three to 12 teragrams [three million to 13 million tons]. The fate of the young land-derived dissolved organic carbon in the Atlantic Ocean is uncertain, but there is no evidence of this material at lower latitudes in the Atlantic, the researchers say.

"This suggests most of the land-derived organic carbon ends up being oxidized to carbon dioxide and thus eventually cycles back into the atmosphere," says Benner. "If current warming trends in the Arctic continue, we can expect to see more of the old carbon now sequestered in northern soils enter the carbon cycle as carbon dioxide. This will act as a positive feedback, tending to enhance the greenhouse effect and accelerate global warming."

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