The soils and sediments at the bottom of rivers are rich in organic material. They can store carbon for thousands of years according to a study from the Woods Hole Oceanographic Institution (WHOI). Despite often high rates of erosion and sediment transport, the riverbed can hold organic carbon for 500 to 17,000 years. The researchers focused their studies on the Ganges-Brahmaputra basin in India, which feeds off waters from the Himalaya Mountains. The fact that riverbeds store much carbon is a cause for concern. In a changing climate, the soils could be destabilized, releasing the carbon back to the atmosphere.

!ADVERTISEMENT!

The WHOI researchers used radiocarbon dating to determine the age of carbon stored in the riverbed. Upstream in the Ganges watershed, carbon was stored up to 17,000 years. However, downstream in the floodplain, the longest residence times were only 3,500 years.

The researchers were very surprised by the carbon residence times because the region generally has very high rates of physical erosion. "We thought it was likely that the organic matter there was young," Valier Galy said, one of the Woods Hole researchers. "But what gets exported there sits in the soil for quite some timeâ€”3,000 years on average. That's pretty old."

Galy said that this has great implications for the global carbon cycle because when carbon is stored in the ground, it cannot be in the atmosphere, causing global warming. Therefore, it is a good thing that this river basin is not significantly contributing to CO2 in the atmosphere.

However, the researchers warn that the Ganges may be susceptible to global warming, as well as other great river basins around the world.

"If carbon has a short residence time in soils, global warming canâ€™t speed up the rates too much of exporting carbon to the atmosphere," Galy explained. On the other hand, if carbon is stored for thousands of years, as it has been shown to do, then global warming can have a much larger impact, hastening the speed of transfer from the soils to the atmosphere.

"Our study shows that ancient soil carbon exists in a globally significant tropical system," he said. "We therefore hypothesize that similar stocks of ancient carbon may exist elsewhere at low latitude. Global warming would likely destabilize this ancient carbon, generating an extra flux of CO2 to the atmosphere, hence further warming. This may not be too important over the short-termâ€”decades, for example. But over a longer time scaleâ€”tens of thousands of yearsâ€”it can be important."

This study has been published in the journal, Nature Geoscience.

Link to published article: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1293.html

Image shows Ganges River. Image credit: Nadina/Shutterstock