Physical separation of combustion and rock sources of graphitic black carbon in sediments

In order to discriminate between graphitic black carbon (GBC) from different sources, we separated surface sediments from two contrasting environments according to size and density and analyzed the fractions for GBC. In Lake Washington, an urban lake, over 70% of the GBC was in the two lightest fractions (<2.0 g/cm3), and over half the GBC was smaller than 3 μm, suggesting that most of the GBC in this lake is light, small combustion-derived soot-black carbon (BC). In contrast, over 70% of the GBC from the Washington Coast sediment fell into the two heaviest fractions (>2.0 g/cm3), and 70% was silt-sized (3–63 μm). These data suggest that Washington Coast Slope GBC is dominated not by combustion-derived soot but by mineral-associated carbon, probably weathered out of continental rocks, and support previous research suggesting that petrogenic graphite is the source of this carbon (Dickens, A.F., Gélinas, Y., Masiello, C.A., Wakeham, S., Hedges, J.I., 2004. Reburial of fossil organic carbon in marine sediments. Nature 427 336–339). Based on the physical separation of the two types of GBC, we estimate that the Lake Washington GBC contains 73% soot-BC, whereas the Washington Coast GBC is 70% rock-derived GBC. The evidence for two different sources of sedimentary GBC indicates that this pool of carbon is not a homogenous subfraction of BC, and caution must be used when interpreting the results of this and other BC analyses. The different physical properties of the two GBC subfractions suggest that they will follow different distribution pathways in the environment.