Sediments in water bodies and seas help to bind carbon in the form of organic particles. How effectively this happens in a river, however, depends on how it flows, as researchers have now discovered. If the river flows slowly and in many meanders, more carbon is bound to the sediment. If it flows straight and quickly, a larger part of the organic matter is oxidized to CO2 and is released into the atmosphere.
It takes around 8,500 years for a grain of sand from the Andes to be washed from the Rio Bermejo over the Argentine lowlands into the Río Parana. The 1200-kilometer journey is interrupted by many breaks in meanders and floodplains, where the grain is deposited over thousands of years and then transported on again. The sand is accompanied by organic carbon washed in from the soil and plants.
CO2 sink or source?
This is where the climate comes into play: the more organic carbon in the form of plant remains or animal relics is bound in the sediment of the river or deposited by it in the sea sediment at the mouth, the better it is for the climate. This carbon is then stored for a long time and is not immediately oxidized again by bacteria and other decomposers to form the greenhouse gas carbon dioxide (CO2). “The amount of particulate organic carbon that is buried in the long term is determined by the balance between oxidation and conservation during transport in the river,” explain Marisa Repasch from the GFZ German Research Center for Geosciences in Potsdam and her colleagues.
How the shape and flow of the river influences this equilibrium, Repasch and her team have now investigated in more detail using the example of the Rio Bermejo. “The Rio Bermejo was an ideal natural laboratory for us because it has no noteworthy tributaries,” explains her colleague Dirk Sachse. The river flows relatively isolated from the Andes to Rio Parana and forms countless meanders, especially in the lowlands. Where and how much organic carbon is transported in the water, deposited in the sediment or converted into CO2 by oxidation, the research team used isotope analyzes to investigate more closely at six points along the 1200-kilometer-long river.
It all depends on the meander
It turned out that there are undisturbed meandering sections of a river that deposit sediment together with the carbon and thus stabilize it over the long term. In the slow-flowing loops, the organic particles can attach themselves to sediment grains from the floodplains and are thus bound. As a result, only a small amount of carbon is prematurely oxidized in these river sections and thus released back into the atmosphere. According to the measurements, the oxidation rate in the slow, meandering river areas was around 30.4 tons of carbon per year, while in the fast flowing river sections it was 6050 tons of carbon per year.
“This means that natural rivers, which have space to remove the floodplains, can extract more carbon from the atmosphere than straight river sections,” explains Sachse. Such slow, natural rivers therefore play an important role as CO2 sinks in the climate system. That can change, however, if rivers are straightened and prevented from slowly meandering through other human interventions: “Artificially stabilized rivers are far less effective carbon sinks,” says Repasch. This knowledge can help to better limit the contribution of rivers to the CO2 budget, at the same time it shows possibilities of how the fluvial CO2 sinks could be reactivated: “Now it is exciting to answer the question of whether we can help the climate, if we give the rivers more space again and don’t hinder the natural river loops, ”says Sachse.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences; Technical article: Nature Geoscience, doi: 10.1038 / s41561-021-00845-7