The Ubiquity of Dissolved Black Carbon in Freshwater Environments

Black carbon (BC) is abundant in soils and generally ubiquitous in the environment including in the deep ocean. As such, it is of importance in the global carbon cycle. However, little is known about the processes controlling its transport and fate in freshwater environments, particularly in its dissolved form. It has been proposed that dissolved BC enters the ocean environment via fluvial transport to become a significant component of marine dissolved organic carbon (DOC). However, the flux of dissolved BC (DBC) from freshwater to the ocean has not been constrained, and no information regarding the BC content in terrestrial DOC on a globally relevant scale has been reported. Here we present a database of DBC from over 200 freshwater samples, including samples from 6 different LTER sites, ranging from headwater streams to the world’s major rivers, from tropical wetlands to arctic rivers, covering a wide range of climatic regions worldwide. Our data show that DBC is ubiquitous in freshwater environments representing 10.6 ± 0.7% of the terrestrial DOC pool, and its environmental dynamics seem closely coupled to those of bulk DOC. Consequently, the estimated DBC flux to the ocean is 26.14 ± 1.73 Mt/yr, making a significant contribution to the global carbon transfer between terrestrial and marine systems.