A River Flipped: Humans Trump Nature on Texas RiverAug. 24, 2010
Human activity eclipses Brazos River's native carbon cycle
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A new study by geochemists at Baylor University and Rice University in Houston has found that human activity, like dams, have completely obscured the natural carbon dioxide cycle in Texas' longest river, the Brazos.
The study, which is available online in the journal Biogeochemistry, is the first to document such an overwhelming influence of human activity on carbon dioxide in a major river.
"Rivers are a wildcard in our understanding of climate and atmospheric carbon dioxide," said Dr. Bill Hockaday, a co-author of the study and an assistant professor of geology, College of Arts and Sciences, at Baylor. "We were surprised by the behavior of carbon in the Brazos River water. Near Waco, the main source of dissolved CO2 in the Brazos is not the limestone bedrock, as we originally thought it would be. It actually comes from more modern sources. We think this is caused by the construction of dams, which increase the water surface area, allowing for more CO2 exchange with the air, and also impound nutrients that promote the growth of aquatic plants, like algae."
With humans adding some 8.5 gigatons of carbon dioxide to the atmosphere each year through the burning of fossil fuels, scientists are increasingly interested in studying how the atmosphere and biosphere exchange carbon dioxide. Plants take up carbon dioxide from the air via photosynthesis and store it in their leaves and stems. Some of that stored carbon gets buried in the soil and locked away for hundreds or thousands of years. But much is also washed into rivers, where rapid decomposition can quickly return it to the atmosphere. Understanding when and where that plant carbon dioxide is returned to the atmosphere is essential if policymakers are to plan effective carbon-sequestration strategies.
One method scientists use to gauge how effectively ecosystems store carbon is radiocarbon dating. The technique involves precisely measuring the amount of radioactive carbon-14 in samples from an ecosystem. Because about half of the carbon-14 atoms in a material will decay and become nitrogen-14 every 5,730 years, scientists can determine the age of a material based on how much carbon-14 it still contains.
In a 2005 study, researchers used radiocarbon dating and found that the Amazon River, the world's largest, cycles carbon into the atmosphere in less than five years. Previous radiocarbon dating studies in cooler climates, like the U.S. Northeast, had found that rivers could store carbon from plants for thousands of years. The Amazon study indicated that rapid carbon cycling could be the norm for tropical and subtropical rivers where warmer temperatures aid decomposition.
To test the idea, researchers began collecting and analyzing water samples at seven sites along the Brazos in 2007. The Brazos is the U.S.'s 11th longest river. Researchers gathered samples from as far south as Freeport, Texas, near the river's mouth, to Granbury, Texas, about 300 miles inland. Hockaday used radiocarbon dating to determine the age of the carbon dioxide in the samples.
The lab results showed the exact opposite. At the head of the river, the carbon is from rapidly cycling organic matter, and at the base of the river, the radiocarbon tests revealed ancient carbon that had been locked away from the atmosphere for several thousand years.
"Downstream of Waco, near Houston, there is no limestone bedrock. So we were shocked when we discovered that the carbon in the water was, in fact, coming from limestone," Hockaday said. "We attribute this to the extensive use of limestone and oyster shells in the construction of roadways around Houston. Our hypotheses about how the Brazos River moves carbon between the land, atmosphere and the ocean turned out to be completely wrong. We think the natural carbon transport processes in the Brazos have been overridden by human activities."
In a 2009 study, researchers had found that human activity -- namely the use of seashells as road base material in the late 19th and early 20th centuries -- skewed the radiocarbon results in Buffalo Bayou, a heavily urbanized watershed that runs through downtown Houston. Based on those results, the team believes the "old" carbon from the southern Brazos is also from dissolved seashells in old roads.
Scientists currently believe Earth's rivers take up about 1 gigaton of carbon each year and give off about the same amount. But the exact dynamics of the process are largely unknown. For example, the residence time of carbon dioxide -- how long it stays in the river -- has been studied in fewer than a half a dozen rivers worldwide. If a significant number of those rivers are like the Brazos, scientists may need to adjust the way they think about rivers inhaling and exhaling carbon dioxide.
The research was funded by the Texas Water Resources Institute, the U.S. Geological Survey, the National Institutes for Water Research, and Hans O. and Suse Jahns.