Rio Bermejo. Credit: Kristen Cook
A grain of sand from The Andes takes 8500 years to cross the Argentine Lowlands and into the Ro Paran. The 1200-kilometer journey along the river Ro Bermejo is interrupted at many points by river floodplains where the grain is sometimes deposited over thousands of years and then washed away again. Organic carbon is also present in the sand, which has been derived from soil and plants. Transport in water is therefore important for climate change: Rivers transport carbon from the atmosphere as sediment to the sea where it can be stored for thousands of year without affecting the climate.
The GFZ German Research Centre for Geosciences has now quantified each step of the journey for its first time. They report their findings in Nature Geoscience. One important result is that carbon is deposited, reabsorbed and transported further into the ocean in the unaffected sections of rivers. On the other side, the river sections with straight banks allow only suspended particles to pass through. The carbon in river floodplains is slow decomposed into CO 2 by microorganisms. Dirk Sachse, GFZ working group leader, says that the Ro Bermejo is an ideal natural laboratory because it doesn't have any significant tributaries. Sachse is also the director of the Helmholtz topic "Landscapes of the Future" in the Helmholtz program, "Changing EarthSustaining our Future." Sachse says this means that natural river courses with floodplains that can be eroded by flooding can remove more carbon from our atmosphere than straight sections. This could be because humans can also increase atmospheric CO2 concentration by straightening rivers. The exciting question now is whether we can help the environment by giving rivers more space and not limiting natural river meandering.
Marisa Repasch, first author of GFZ, led an international team that studied the river and floodplains using a variety of instruments. For example, analyses of cosmogenic beryllium-10 content indicated the length of sediment transport. The age of the organic origin particles can be determined using dating based on unstable carbon isotopes 14C. Samples were collected from the river at several stations along the source to sink pathway during fieldwork in Argentina. Marisa Repasch summarizes the results by saying that "naturally meandering rivers erode material out of floodplains and transport them to the sea, where they remain for a long period."
Further exploration of Minimal evidence for permafrost carbon at Siberia's Kolyma river
Marisa Repasch and colleagues, Fluvial Organic Carbon Cycling Regulated by Sediment Transit Time and Mineral Protection, Nature Geoscience (2021). Information from Nature Geoscience Marisa Repasch and colleagues, Fluvial organic Carbon Cycling Regulated by Sediment Transit Time and Mineral Protection, (2021). DOI: 10.1038/s41561-021-00845-7
