Sedimentary Accumulation of Black Carbon on the East Coast of The United States

The occurrence, trends and sources of soot black carbon (BC) in coastal sediments are poorly understood, particularly during the Anthropocene. Two sediment cores, covering the last ∼100 years from the US East Coast, off North Carolina and in the Florida Straits, were analyzed for organic carbon (OC), BC fluxes and BC sources. BC fluxes were 0.1 g cm−2 year−1 at both sites and accounted for 8%–22% of total OC. Carbon stable isotope values indicated OC to be of marine origin, while the BC was mostly terrestrially derived, C3‐plant material. Radiocarbon values revealed BC originating mostly from fossil fuels or pre‐aged carbon (fraction modern of 14%–31%) at North Carolina, while in the Florida Strait the BC was mostly derived from biomass burning (fraction modern of 70%–74%), in‐line with continental (NC) or marine (FS) air mass origins. Ratios of polycyclic aromatic hydrocarbons broadly supported different BC sources at the two sites. Plain Language Summary Black carbon particles are the byproduct of incomplete combustion, either from wildfires or the deliberate burning of biomass and fossil fuels. Given its chemical and physical properties, black carbon particles are stable and accumulate in sediment. In coastal sediments, the black carbon composition will reflect changes in the burning of different fuels over time. This study provides information of how sources and intensity of black carbon deposition changed over a ∼100 year period using sediment cores from two sites along the US East Coast. The result suggests black carbon deposition increased over time and was dominated by fossil fuel combustion in the North Carolina shelf sediment, while in the Florida Straits the black carbon production did not change through the period investigated (1930s–2020s) and was influenced by vegetation burning. Key Points Black carbon fluxes were 0.1 g cm−2 year−1 at both Northeast shelf sites, accounting for 8%–22% of total carbon North Carolina had mostly old black carbon (14C fraction modern 14%–31%), likely from fossil fuel combustion Black carbon in the Florida Strait sediment was mostly biomass‐derived (14C fraction modern ∼70%), likely reflecting biomass burning