Award: OCE-1924191

Black carbon is a byproduct of the incomplete combustion of fossil fuels and biomass burning. It is a subfraction of the organic carbon present in soils and sediment. Commonly reported total organic carbon contains both organic and black carbon. Unlike organic carbon however, black carbon cannot be used by biological organisms, effectively transferring carbon from the biological carbon cycle, commonly referred to as just the carbon cycle, to the geological carbon cycle, which can be considered a sink in the tractional carbon cycle. Overall, the transport and fate of black carbon in the ocean are not well constrained. Most of the black carbon resides (pre-ages) on land before being transported to the ocean by rivers during major rainstorms that mobilize soil and sediment particles. However, recent studies have found unaged black carbon in the equatorial Atlantic Ocean, derived from annual crop burning in sub-Saharan Africa. Fine soot particles from these fires are transported westward by trade winds to the equatorial Atlantic. In this study, we examined black carbon aerosols and sediments to determine the origin, extent, and depositional rate of black carbon particles in equatorial Atlantic sediments and a Congo River outflow sediment core to assess the sedimentary and atmospheric transport processes of black carbon to better understand the global carbon cycle.

We collected gaseous compounds, aerosols, and surface sediments from the equatorial Atlantic Ocean in early March 2020. To measure the black carbon, we used a method called chemothermal oxidation at 375 ˚C, where we essentially burned off the organic matter in an oven at 375 ˚C with air pumped in for 24 hours, then acidified the samples to remove any inorganic carbon (foraminifera), leaving only the black carbon and other inorganic minerals, allowing the black carbon to be detected the same way as traditional organic carbon.

We found higher concentrations of black carbon aerosols near the African coast, with levels decreasing westward, ranging from 0.39 to 17.86 ng m^-3 (Figure 1). Using the National Oceanic and Atmospheric Administration’s Hybrid Single-Particle Lagrangian Integrated Trajectory model, we calculated the back trajectory of the air masses that originated over the Canary Islands, the northwestern African coast, southern Spain, and Portugal. Using polycyclic aromatic hydrocarbons, compounds formed during combustion, as a references for the combustion origin, most were from fossil fuel emissions and only a few closest to Africa had a biomass signature. As our field sampling occurred after the larger northern sub-Saharan African biomass burning season (December to February), these results are in line with expectations.

Surface sediment samples revealed that black carbon deposition in this region ranged from 0.10 to 0.35 mg cm^-2 kyr^-1 (Figure 2). Near the African coast, carbon dating values of black carbon are comparable to those for the total organic carbon, indicating that the black carbon was deposited relatively quickly after it was produced. Further, the stable carbon isotope values mimic the black carbon aerosol values found in central Africa. Moving westward across the Atlantic, the carbon dating values of black carbon increase, suggesting that the African biomass black carbon is mixing with the radiocarbon-dead black carbon from fossil fuels. These results confirm that modern black carbon is transported to deep ocean sediments across the entire Atlantic and that aerosol deposition regionally can significantly contribute to oceanic black carbon concentrations.

We also collaborated with other researchers, and used multiple methods to analyze a sediment core from the Congo River outflow spanning the last 15,000 years. These methods included the breakdown of condensed aromatic compounds into benzene polycarboxylic acids (BPCAs), chemothermal oxidation (CTO), and ¹³C nuclear magnetic resonance (NMR), each targeting different fractions of black carbon. Despite differences in these approaches, all showed consistent trends, with higher black carbon concentrations during dry periods and lower concentrations during humid periods, reflecting shifts in climate and vegetation. Stable carbon isotope analysis revealed changes in terrestrial black carbon inputs, with both BPCA and CTO results indicating rainforest expansion during wetter periods.  The isotopic signatures showed a higher proportion of carbon derived from trees (C3 plants), which take up carbon-13 at a lower rate compared to grasses (C4 plants), indicating a shift toward increased tree cover and a corresponding decrease in grasslands. Although black carbon fluxes to the sediment were similar across methods, significant changes were noted in the last 1,000 years, with increased BC deposition during drier periods when fires were more frequent. These findings enhance our understanding of black carbon’s role in the global carbon cycle and help us reconstruct past climates and landscapes.

Lastly, we worked with The SMILE (Science and Math Investigative Learning Experience) Program and their middle and high school clubs in Rhode Island. Their students visited the URI Graduate School of Oceanography and introduced them to different scientists and science topics, including fish evolution, robotics, paleo/geology, ocean exploration, and autonomous research vessels.

Last Modified: 10/17/2024
Modified by: Rainer Lohmann