Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments

Black carbon (BC), a group of environmentally concentrated organic pollutants, is widely distributed in marine sediments via riverine run-off and atmospheric deposition. The fate of BC transformation and cycling in marine sediments, however, has not been well studied. Here, we present radiocarbon measurements for sedimentary solid-phase BC (SBC) and porewater-dissolved BC (DBC) in surface sediments collected from the Yangtze and Yellow River estuaries and their adjacent coastal regions. Radiocarbon results revealed that two distinct BC pools in the sediments of the SBC had ancient radiocarbon ages (7110-15,850 years BP) that were 5370-14,935 years older than the ¹⁴C ages of porewater DBC. Using a radiocarbon mass balance model, we calculated that modern biomass-derived BC contributed 77-97% of the DBC pool and that fossil material-produced BC accounted for 61-87% of the SBC pools. This discrepancy between modern and dead BC contributions was associated with the BC budget after particulate BC (PBC) deposition; 38 ± 13% of the PBC was transferred to porewater as DBC and 62 ± 13% was sequestrated as SBC in sediments, serving as an important CO₂ sink in marine sediments. We also provide evidence suggesting that DBC likely comprises some very fine particulate forms that are not completely dissolved as molecules. The nature and transformation mechanisms of DBC in natural aquatic systems need to be further studied.