Shallow streams that wind through the mountains of New Zealand and Taiwan carry more sediment into the ocean than giant rivers like the Amazon or the Nile, according to Ohio State University geologists.
The finding means that even small, mountainous islands may have a big impact on the environmental condition of the world's oceans, said Anne Carey, assistant professor of geological sciences.
Carey and Carmen Nezat, a former research scientist at Ohio State, presented these results November 7 at the annual meeting of the Geological Society of America in Boston.
The geologists found that rivers in Taiwan yield up to 75 times more sediment than the average river worldwide, and rivers in New Zealand yield up to 175 times more.
Scientists study the chemical content of water entering the oceans to gauge the overall health of the environment, Carey explained. Carbon is one important element that washes away from island soils and enters the planet's oceans, where it could play a role in global climate change, she said.
"People who study sediments originally overlooked these small islands, because the volume of water they release into the oceans is correspondingly small," Carey said. "But the role of these islands in the ocean's geochemistry is important."
Their collaborators included W. Berry Lyons, professor of geological sciences and director of the Byrd Polar Research Center at Ohio State; D. Murray Hicks, a hydrologist at New Zealand's National Institute of Water and Atmospheric Research; Shuh-Ji Kao, assistant research scientist at National Taiwan University's Institute of Oceanography; and Jeffrey Owen, post-doctoral researcher at Academia Sinica in Taiwan. Nezat is now a doctoral student in geological sciences at the University of Michigan.
Throughout 1998 and 1999, the researchers gathered water samples from the Cropp, Hokitika, and Haast Rivers in New Zealand. In 2000, they collected samples from the Nan-she-chi, Hualien, and Lanyang Rivers in Taiwan.
In New Zealand, the Cropp River yielded the most sediment, with more than 32,000 tons per square kilometer of its watershed per year. Likewise, the Lanyang River in Taiwan carried almost 14,000 tons of sediment per square kilometer per year.
Both rivers were well above the world average. The Amazon and Nile Rivers, for example, produce 190 and 40 tons per square kilometer per year, respectively.
Taiwan and New Zealand produce so much sediment because they both receive a great deal of rainfall, which washes away exposed mountain rock, Carey said. Some parts of Taiwan receive more than eight feet of rain per year, and parts of New Zealand receive as much as 32 feet. Ohio, by contrast, receives only about three feet of rain per year.
The sediments contain chemical elements such as carbon, copper, and phosphorus, which affect the chemistry of the oceans, Carey explained.
"We're trying to understand how the Earth naturally balances such conditions," she said. "It looks like these small rivers could play a much bigger role in that balance than we thought."
Geological Sciences at Ohio State
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