BLACK CARBON SOURCES, CONCENTRATIONS, AND FLUXES TO SUBTROPICAL ATLANTIC SEDIMENTS

Black carbon (BC) is the highly graphitized byproduct of incomplete combustion. While it is considered a solar absorber in the atmosphere, it is a potential sink for fixed carbon when deposited to sediments. Little data is currently available for BC fluxes to deep sediments, especially in remote marine regions. A multicorer was used to collect sediment at eight sites across the subtropical Atlantic in the summer of 2010; additional sediments from the Niger Delta and South Atlantic were obtained from the University of Bremen. BC concentrations were determined using a thermal oxidation method in which sediment was heated at 375°C for 24 hours with excess oxygen to oxidize the organic carbon. The remaining carbon is operationally defined as soot BC and analyzed on an isotope ratio mass spectrometer. The study area was divided into four regions: Amazon, Niger Delta, Sierra Leone Rise (SLR), and South Atlantic. The SLR and South Atlantic sites were removed from fluvial influence, thus atmospheric deposition was the primary source of BC to these sediments. BC concentrations were greatest at the Niger Delta and SLR sites at 0.7% and 0.6%, with lower concentrations at the Amazon (0.4%) and South Atlantic (0.1%). The ratio of BC to total organic carbon decreased from 60% at the Amazon site to 40% at the SLR and Niger Delta sites, and 15% at the South Atlantic. The BC flux to the sediments was greatest in the Niger Delta region at 20.8 μg cm^-2 yr^-1, followed by the Amazon at 7.8 μg cm^-2 yr^-1. The SLR had a flux of 3.1 μg cm^-2 yr^-1 compared to the remote South Atlantic with a flux of 1.2 μg cm^-2 yr^-1. Based on location, BC at the South Atlantic and SLR was derived mostly by atmospheric deposition. This could imply that 6-15% of the BC in the Niger Delta fluvial sediments could be from atmospheric deposition. We hypothesized that the SLR had elevated BC fluxes and concentrations due to grass burning transported by the prevailing easterly winds to the study area. The δ¹³C of the BC at the SLR sediments was enriched (-22‰) relative to the other regions (average -26‰), suggesting a large input of C₄ plant material, such as Savanna grass. Radiocarbon ages of the top sediments were modern and indicated that <1.2% of the SLR BC is from fossil fuel inputs. Results suggest that atmospheric deposition of BC to remote sediments may be significant in areas with elevated biomass burning.