Tracing Combustion Regime Shifts through Char and Soot Black Carbon Records in Beibu Gulf Sediment Cores: Burial Flux Controls and Carbon Storage Assessment.

Black carbon (BC) in marine sediments provides a unique archive of combustion history and a reservoir for long-term carbon storage. However, the distinct roles of char BC (CBC) and soot BC (SBC) in combustion-source tracing and carbon-cycle assessment remain poorly resolved in marginal seas. Here, we analyze four sediment cores from the Beibu Gulf, South China Sea, using benzene poly(carboxylic acid) (BPCA) and chemothermal oxidation at 375 °C (CTO-375) methods to resolve subtype-specific BC records. CBC accounted for 52-57% of the estimated total BC burial across all settings, consistent with sustained biomass-burning contributions. SBC exhibited higher spatiotemporal variability and sensitivity to fossil fuel expansion, particularly near industrial zones. Estimated BC burial fluxes varied spatially, largely reflecting differences in representative sedimentation rates rather than BC content. Notably, nearshore sites exhibited higher burial despite lower BC content. Centennial-scale profiles revealed that SBC effectively traced regional transitions from biomass to fossil fuel combustion, while CBC preserved the long-term fire legacy. These findings highlight the complementary roles of CBC and SBC in reconstructing combustion histories and assessing BC burial in marginal seas. Burial of biomass-derived BC can contribute to net atmospheric CO2 sequestration, whereas fossil-derived BC burial represents the transfer of fossil carbon into long-term sedimentary storage.