Rivers transport 200 million tons of carbon to the oceans every year, according to new research that calculates the role of rivers in carbon storage.

The study, conducted by researchers at the Woods Hole Oceanographic Institution, assessed samples from 43 river systems that put out more than 20 percent of the world’s river sediments. Their calculations show that the levels of carbon transported by rivers are equivalent to 0.02 percent of carbon in the atmosphere, but that over thousands of years, this could add up to significant chunks of carbon extracted from the atmosphere.

Most carbon in rivers is picked up from plant and rock debris. As the carbon makes its way downstream, some is released back into the atmosphere when it decays, while the rest makes it out to open ocean. Upon reaching the ocean, a fraction of the sediment sinks to the ocean floor, where it can be stored for millennia in the form of rocks. These rocks eventually make their way back to the surface, but the process is lengthy.

“It’s a bit of a complicated story because it’s a cycle,” said Valier Galy, lead author of the new study, published in Nature. “The process basically takes some CO2 from the atmosphere and puts it in the bottom of the ocean.”

Galy’s study is the first to quantify the amount of carbon transported by rivers. Understanding the role rivers play in the carbon cycle can help researchers model non-natural (or human-caused) carbon emissions and more accurately assess the impact these emissions have on the atmosphere, he said.

Erosion drives process
Until recently, researchers couldn’t differentiate between sources of carbon, but new technology—namely the Accelerator Mass Spectrometer—allowed researchers to identify which carbon comes from plants and which carbon comes from rocks.

Plants absorb carbon dioxide from the atmosphere and convert it to organic carbon during photosynthesis. Most of the carbon returns to the atmosphere through respiration or when plants are eaten or die, but some washes into rivers.

At the same time, particles are worn down from rocks and end up in rivers. As with plants, some carbon is released back into the atmosphere, but some eventually accumulates at the bottom of the ocean.

Though these processes are influenced by factors including climate, vegetation and human activity, erosion is the main factor that affects the amount of carbon that ends up in rivers.

“The more erosion there is, the more organic matter is being transported to the oceans,” Galy said. His study offers ways to calculate how much carbon ends up in rivers based on levels of erosion.

This could have implications for understanding how much carbon storage might occur from increased rainfall in some parts of the world, added Galy, especially at a time when the world’s climate systems are changing rapidly.

“Where it does matter is that it gives a better idea of natural processes,” he said. “We really have to have these natural processes down in such a way that others can calculate anthropogenic carbon contributions.”

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500