Changes in Pyrogenic Tracers Over the Last 15,000 Years in the Congo River Catchment Through Multi-Method Analysis

Black carbon (BC), the most recalcitrant part of the pyrogenic carbon continuum, is formed by the incomplete combustion of biomass and fossil fuels. Methods for detecting BC include the chemical degradation of condensed aromatic compounds into benzenepolycarboxylic acids (BPCA), chemothermal oxidation of organic carbon at 375°C (CTO), ¹³C nuclear magnetic resonance combined with a molecular mixing model, thermogravimetry-differential scanning calorimetry, and the use of polycyclic aromatic hydrocarbons as tracers. However, there is limited knowledge about the comparability of these methods in marine sediments and their suitability as wildfire proxies. Here, we examined a sediment core from the Congo River outflow using a multi-methodological approach with environmental data and proxies to assess pyrogenic tracers from the Congo River basin over the last 15,000 years and determine commonalities between the methods. Despite differing analytical windows, both dry-weight and total organic carbon concentrations, and δ¹³C values for most methods showed a congruous trend. Higher BC concentrations and higher δ¹³C values were present during arid periods and lower during humid periods, reflecting changes in vegetation and terrestrial organic matter inputs. For all methods, the sedimentation flux identified significant variations in BC deposition only in the last 1,000 years BP due to anthropogenic land use changes. These findings deepen our understanding of BC in the global carbon cycle and show that BC proxies can reveal distinct transport pathways, with CTO-BC representing atmospheric deposition and BPCA-BC and NMR-BC indicating fluvial inputs to coastal margins, aiding in the reconstruction of past climates and landscapes.