Observations of Aeolian Transported Black Carbon in the Subtropical Atlantic Ocean

Black carbon (BC) particles are ubiquitous in the marine environment and could be elevated in the Subtropical Atlantic environment due to the transport of biomass burning emissions from the African continent. In the summer of 2010, we collected atmospheric boundary layer particulate matter samples and sediment cores in distinct regions throughout the Subtropical Atlantic ocean basin with the goals of evaluating the concentration and source of BC in this remote environment. For the atmospheric samples, four different BC methodologies were applied which measure different portions of the BC combustion continuum to provide unique insights into the BC form: chemothermal oxidation at 375°C, thermal optical transmittance, pyrene fluorescence loss, and optical transmission attenuation. The greatest BC concentrations were measured in the Caribbean Sea and within the African aerosol plume, with regional averages of 0.6 µgm−3 for both. The lowest average concentrations were measured off the coast of South America at 0.2 to 0.3 µgm−3. Soot-like aerosols composed up to 45% of the carbonaceous aerosols in the Caribbean Sea to as little as 4% within the African aerosol plume. In pelagic and deltaic sediments, BC comprised between 17±6% of the sedimentary organic carbon in the Northwest Argentina Basin up to 65±18% in the Amazon Delta. Black carbon sediment accumulation rates were six times greater in the Sierra Leone Rise (8.4±4.1 mg cm-2 kyr-1) compared to the remote Northwest Argentina Basin (1.3±0.4 mg cm-2 kyr-1), possibly due to enhanced regional atmospheric deposition from Savanna grassland fires. This study demonstrated that Subtropical Atlantic Ocean sediments serve as an important sink for aeolian transported BC.