Decoupled transport pathways and source materials of black carbon in the Pearl River Estuary and the North Yellow Sea

Black carbon (BC) buried in marginal sea sediments represents a significant global carbon sink, yet the relative contributions of different transport pathways and source materials under varying climatic and emission regimes remain poorly constrained. In this study, we analyzed BC content, BC/TOC ratio, stable carbon isotopes (δ 13 C BC ), and sediment grain size in surface sediments from two contrasting Chinese marginal seas: the subtropical Pearl River Estuary (PRE) and the temperate North Yellow Sea (NYS). Using a Bayesian mixing model (MixSIAR), we apportioned BC sources along two dimensions: geographic transport pathway and specific source material. BC concentration ranges overlapped widely, yet PRE had a higher mean than NYS, with distinct controls. In the energetic PRE, grain size sorting drove a clear east-west zonation. In the muddy NYS, this sorting was regionally masked by atmospheric deposition of coal combustion BC at nearshore stations. Pathway apportionment revealed two distinct receiving modes: the open PRE was dominated by marine-proximal 13 C-enriched pathway (52.8%) against a uniform atmospheric background (27.2%), whereas the NYS acted as a steady-state sink dominated by terrestrial inputs (riverine: 34.5%; atmospheric: 38.4%). In contrast, source-material apportionment was highly consistent across both regions, following the order: fossil fuel combustion (∼45%) > biomass burning (∼35%) > rock weathering (∼20%). This decoupling, regionally divergent pathways but nationally convergent source materials, arises because BC transport is governed by local physical environments, while its composition reflects larger-scale emission patterns. Our findings clarify BC source–sink processes in Chinese marginal seas and provide scientific support for coordinated atmosphere–river–ocean pollution control.