Most of the carbon resulting from wildfires and fossil fuel combustion is rapidly released into the atmosphere as carbon dioxide. In a recent study published in Nature geoscience, we have shown that the leftover residue, so-called black carbon, can age for millennia on land and in rivers en route to the ocean, and thus constitutes a major long-term reservoir of organic carbon. The study adds a major missing piece to the puzzle of understanding the global carbon cycle. The whole project was led by Alysha Coppola, a postdoctoral researcher in my group.
Figure 1: Age and flux of black carbon in the particulate organic matter
In this study we address for the first time the flow of black carbon in sediments by rivers on a global scale. We found that a surprisingly large amount of black carbon is exported by rivers. It includes some of the largest rivers worldwide, such as the Amazon, Congo, Brahmaputra, and major Arctic rivers. We found that the more total river sediment is transported by rivers to the coast, the more black carbon travels with it and is ultimately buried in ocean sediments, forming an important long-term sink for atmospheric carbon dioxide. The ages we measured could be up to 17,000 years old, particularly in the Arctic, which explains the mystery as to why black carbon is continuously present in river waters, regardless of wildfire history.
This study is a collaboration between UZH, ETH Zurich and the Woods Hole Research Center.
Reference
Coppola, A.I., Wiedemeier, D.B., Galy, V., Haghipour, N., Hanke, U.M., Nascimento, G.S., Usman, M., Blattmann, T.M., Reisser, M., Freymond, C. V., Zhao, M., Voss, B., Wacker, L., Schefuß, E., Peucker-Ehrenbrink, B., Abiven, S., Schmidt, M.W.I., Eglinton, T.I., 2018. Global-scale evidence for the refractory nature of riverine black carbon. Nature Geoscience 1. doi:10.1038/s41561-018-0159-8