River water carbon studied

Virginia scientists seek clues to changes in climate

March 11, 2001|By ASCRIBE NEWS SERVICE

GLOUCESTER POINT, Va. - The ability of scientists to predict long-term climate change requires an understanding of Earth's carbon cycle.

Research by Virginia Institute of Marine Science scientists suggests that the carbon in river water may be much older - and vary more in age - than previously thought. Their findings may help resolve a fundamental paradox facing those trying to assess Earth's carbon budget.

The research, conducted by scientists Peter Raymond and James Bauer at the Virginia Institute of Marine Science, was published in a recent issue of the journal Nature.

By studying water from the Amazon and several North American rivers, including the York, Raymond and Bauer determined that much of the carbon in these waters was up to thousands of years old. Previous thinking held that river carbon was at most a few decades old, and was due to inputs from recent land plant production.

Researchers have long assumed that rivers play an important role in Earth's carbon cycle by carrying to the sea the carbon that land plants remove from the atmosphere via photosynthesis.

When land plants die and decay, the carbon in their tissues enters the soil. As rivers erode the soil, they carry this carbon to the ocean. Once in the ocean, this carbon may sink to deep waters. There, it can become trapped for thousands of years, effectively removing it from the atmosphere and thus helping to lessen the greenhouse effect.

The missing carbon

Based on this assumption, oceanographers expect to find large amounts of river-derived carbon in the ocean. "We know that continents supply a lot of carbon to the oceans," says Bauer, "but current techniques tell us it's not there."

The paradox of this missing carbon has vexed oceanographers. But Raymond and Bauer's results suggest that the land-derived carbon may still be there in the ocean - scientists have simply been unable to detect it because it gets altered by bacterial degradation and no longer "looks" like it did when it started out on land.

Traditional thinking holds that the carbon that rivers carry to the ocean was only recently extracted from the atmosphere by land plants. This "young" carbon can be detected in a water sample by measuring the proportion of stable and radioactive carbon atoms it contains. Oceanographers have thus looked for large amounts of young carbon and other land-plant substances in their seawater samples as a fingerprint of land-derived carbon - but in vain.

670 years old

Raymond and Bauer took a different approach, using the carbon-dating technique on water samples from rivers rather than the ocean. Their results show that the carbon in the rivers they studied is on average 670 years old, much older than previously suspected. They also found that the age of the various types of carbon in the samples varied much more than once thought.

"Our research," says Bauer "shows that young carbon in rivers is probably the exception rather than the rule. We'll know better once we do similar measurements in a larger number of rivers."

River-borne carbon may thus indeed be abundant in the ocean, but scientists will need to look at something other than its age in order to identify it.

About 40 percent of the carbon in river water occurs as tiny suspended particles. The rest of river carbon is dissolved in the water.

Raymond and Bauer found that particulate river carbon is especially old - the average age of such samples from the rivers studied ranges from around 700 to more than 4,700 years old. This suggests that much of the particulate carbon in these rivers derives not from land plants but from ancient carbon stored in rocks and deep soil layers around the rivers' drainage basins. The dissolved carbon is younger - but still much older than expected.

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