Differential mobilization and sequestration of sedimentary black carbon in the East China Sea

Black carbon (BC) derived from incomplete combustion of biomass and fossil fuels on land can be mobilized and transported to the ocean. Burial of BC in the ocean sequesters atmospheric CO2 into a long-term carbon sink, likely exerting a positive influence on mitigating global warming. However, the abundances, sources, and burial of sedimentary BC in marine sediments remain poorly constrained, hindering us from accurately understanding the mobilization and sequestration of BC and its roles in the ocean carbon cycle. Here, we investigate concentrations and isotopes (13C and 14C) of BC among grain size-fractionated surface sediments along a across-shelf transect from the Yangtze River prodelta to the Okinawa Trough to decipher the fate of BC in the East China Sea (ECS). Our results show that the bulk BC concentrations decrease firstly from the Yangtze River prodelta to the outer shelf and then increase to the Okinawa Trough. Grain size-fractionated BC concentrations vary along the transect, which we mainly attribute to the differential mobilization of BC driven by hydrodynamic processes. We argue that BC is aged during the mobilization, which results in an older 14C ages of BC found seaward. After considering biomass- and fossil-derived BC apportionments based on 14C balance calculation, we think that BC aging may be verified by more fossil-derived BC burial in the Okinawa Trough. We estimate that BC may account for ∼15% of sedimentary organic carbon (SOC), and up to ∼30% of terrestrial SOC buried in the ECS. BC burial fluxes decrease along the transect, and are heterogeneous in different size fractions, indicating differential sequestration of BC in the shelf and trough. We further estimate that 685 Gg/yr of BC is sequestered in the ECS, and 491 Gg/yr in the prodelta area, with ∼30% being continental biomass-derived BC. We suggest that increasing biomass-derived BC production on land and burying it in the ocean may serve as a powerful means for sequestrating atmospheric CO2, potentially contributing to carbon neutrality.