Atmospheric deposition, aquatic transformation and marine carbon sequestration potential of black carbon: A critical review.

Black carbon (BC) is a highly condensed carbon-rich substance mainly derived from incomplete combustion of fossil fuels, biomass, and other carbonaceous materials. Atmospheric BC is ubiquitous with strong light absorption capacity and long-range transport potential. With increasing anthropogenic emissions and climate-driven perturbations, its deposition into marine systems exerts profound impacts on regional biogeochemical cycles, yet large uncertainties remain regarding its aquatic ingress pathways, fraction-dependent transformation dynamics and long-term carbon sequestration potential. This review systematically synthesizes recent advances in the composition continuum, global distribution, and cross-sphere transport of atmospheric BC, as well as its subsequent transformation and fate following deposition into marine environments. Evidence indicates that BC is not a chemically inert pool, but a heterogeneous assemblage of fractions with distinct sources, structures, solubilities, and reactivities, whose fraction-dependent properties govern divergent environmental behaviors in marine systems. We clarify key drivers behind the uncertainties in BC flux estimates, elaborate divergent geochemical behaviors of heterogeneous BC fractions governed by physicochemical and biological reactions in the water column, and illustrate how refractory BC moieties are selectively preserved to sustain marine carbon sequestration. This work fills major knowledge gaps concerning the aquatic geochemical evolution of deposited atmospheric BC, advances the understanding of BC cycling across global and regional scales, and provides a theoretical foundation for evaluating the oceanic carbon sink potential of atmospherically derived BC. Future research should prioritize standardized BC characterization protocols, refined flux and biogeochemical process constraints, and integration of dynamic dissolved and particulate BC cycling into mainstream marine carbon budget models to ultimately quantify the carbon sequestration potential of atmospherically deposited BC.