Scientists studying the carbon cycle of the Brazos River have found it overwritten by human activities, with the effect of dams, sewage wastewater and paved roads overwhelming the natural riverine ecosystem of the longest river in Texas.
The Brazos, which is 840 miles long, flows south through the center of the state and empties into the Gulf of Mexico. The river basin is home to more than two million people.
Results of the study, which were published last week in the journal Biogeochemistry, were the opposite of what the authors had expected to find if natural factors were still determining the river's carbon cycle.
"We are mucking with the biosphere,” Caroline Masiello, a professor of earth science at Rice University and co-author of the study, said in an interview with SolveClimate News. “What we thought of as the carbon cycle is already dominated by human activities on the Brazos."
Watershed geology and ecology usually determine a river's carbon content. Unlike oceans, rivers don't act as carbon sinks, but the behavior of carbon in riverine ecosystems is part of the global carbon cycle, and understanding the fate of carbon in the biosphere—where it goes, how it moves, what humans have done to change it—could have consequences for climate change and carbon sequestration.
Disrupting the Natural Carbon Cycle
The natural global carbon cycle revolves largely around photosynthesis. Growing plants absorb 120 gigatons of CO2 annually from the atmosphere, said Masiello, but when they die and decompose, they release the same amount of CO2 back into the air. Human activities – largely the burning of fossil fuels, adds an additional 8.5 gigatons of CO2 into the atmosphere every year.
About 1.5 gigatons of the human-emitted CO2 is absorbed annually by the oceans, where it's stored for up to thousands of years.
"The ocean is only a sink for CO2 because we're forcing it to be," said Dr. Jonathan Cole, a limnologist at the Cary Institute of Ecosystem Studies who was not involved in the Brazos River study.
Historically, the oceans have been both a source and a sink for CO2; 10,000 years ago, the oceans were a net producer of CO2 into the atmosphere. Today, the burning of fossil fuels has created a "diffusion pump" that forces CO2 into the ocean, said Cole. And as CO2 dissolves in water, it makes the oceans more acidic.
Not only does the acidity decrease the oceans' ability to take up more CO2 in the future, it also disrupts the lifecycles of small organisms at the base of the ocean food chain.
Rivers receive almost all of their carbon from terrestrial ecosystem; contributors include soil runoff, decomposing plants and eroding bedrock. Most of the CO2 is then degassed back into the atmosphere or washed into the oceans. A small amount remains stored in river sediment, but over time the sediment will end up in the ocean as well.
Radiocarbon Reveals the Story
Masiello and her colleagues began their study by analyzing the carbon-14 found in different sections of the Brazos. Also called radiocarbon, C14 is created in the atmosphere and has a half-life of 5,730 years. Anything that's been in recent contact with the atmosphere—like plant matter—has higher levels of C14. Fossil fuels, which have formed from ancient buried organic material, contain almost no radiocarbon.
Since the middle of the Brazos runs over limestone (calcium carbonate) bedrock that's millions of years old, the scientists expected to find no detectable C14 in the water. However, a dam in the river has created a section of slow-moving, warmer water covered in algae. Treated urban wastewater also flows into this area, promoting organic growth and decay. As a result, contrary to expectations, the C14 levels were quite high.
Further downstream near the Gulf, scientists expected to find more C14 because of the subtropical climate. Again, they found the opposite; roads along that part of the river are paved with crushed limestone containing low C14. Erosion and runoff had carried the limestone powder from the roads into the river.
Masiello's team also tried to measure the rate of carbon exchange near the river's mouth. When trees photosynthesize, they take in CO2 from the atmosphere. Later, when the trees die, they decay in the soil, the soil washes into the river and there releases the carbon that was once in the tree.
By measuring the C14 scientists would be able to tell the turnover time of carbon—how long it takes for carbon to cycle from the atmosphere into a plant and back into the air again.
"We were trying to find the turnover time for the lower Brazos," said Masiello, "but we couldn't get a number because of the limestone (used for road paving)." C14 levels from the limestone overwhelmed natural factors from the surrounding ecosystem.
"We can't really say that the (human-caused changes) have created more or less carbon than what would have been there naturally," cautioned Masiello, because there aren't any pre-dam carbon cycle studies of the river as a point of comparison. Still, the findings point to the need for further research to understand how humans influence other riverine carbon cycles.
"One idea is to look at other river systems so that we can extrapolate this to a larger scale," said Fanwei Zeng, a graduate student in Masiello's lab and lead author of the Biogeochemistry paper.
A Check for Leaking Carbon
The carbon cycle research has another important potential application. By establishing the baseline radiocarbon level of a river, scientists can measure if carbon is unexpectedly leaking into the system; it can provide a check on carbon sequestration projects that attempt to bury CO2 emissions permanently underground in geologic formations.
One way to detect the presence of a leak, said Masiello, is to monitor the radiocarbon levels of nearby rivers. All rivers have their own radiocarbon signatures —the expected C14 levels resulting from a combination of human and natural causes.
If sequestered carbon begins leaking into a river basin, C14 levels would drop quickly because emissions sequestered from fossil fuels sources have no radiocarbon signature.
The scientists also said comparing the carbon cycle of a river in an urban setting with another river with similar ecology and geology in a rural setting could reveal the nature and extent of human impacts.
All of this is important, said Masiello, because humans are already changing the carbon background flux –the 120 gigatons of carbon emitted and absorbed by plants every year.
Modern industrial agricultural practices introduce massive loads of fertilizers into the soil, which cause plants to thrive and absorb more CO2. At the same time, nitrogen—a component of fertilizers—accelerates decomposition and the release of carbon back into the atmosphere.
"Understanding the balance of those two factors is important," said Masiello. At this point, however, the net effect is unclear, and studies like the one Masiello and her colleagues just published will help calrify the increasing human influence over the natural carbon cycle.