Biomass burning is a source of modern black carbon to equatorial Atlantic Ocean sediments

Black carbon is a refractory form of organic carbon formed from the incomplete combustion of fossil fuels and biomass. Riverine transport is considered the dominant pathway of black carbon to the coastal oceans. However, the provenance and pathways of black carbon to the open ocean remain unknown. Here we use both stable and radiogenic isotopes of carbon to show that sedimentary black carbon across the equatorial Atlantic Ocean is aeolian and primarily derived from biomass burning of C4-plants. Fluxes of surface sedimentary black carbon measured along an equatorial Atlantic Ocean transect using chemothermal oxidation at 375 °C were relatively consistent across the Atlantic, ranging from 0.10–0.35 mg cm−2 kyr−1. Carbon isotope values near Africa suggest the black carbon was mostly young and derived from C4 plants, whereas offshore South America, the black carbon was older and dominated by C3 plants. The black carbon radiocarbon values were similar to the reservoir corrected total organic carbon near Africa, implying little pre-aging on land and increased westwards. These findings highlight the influence of C4-biomass burning in the tropical Atlantic and the importance of aeolian deposition as a black carbon source within the global carbon cycle. Black carbon in equatorial Atlantic Ocean sediments is largely derived from biomass burning, with young material from burning grass nearer Africa, and older black carbon from tree burning near South America, according to stable and radiogenic isotope measurements along a transect.