Where the river turns old: Urbanized deltas imprint a fossil signature on black carbon exported to the ocean

Black carbon (BC) from biomass burning is a key component of the global carbon cycle, and radiocarbon (14C) age of riverine BC is used to constrain its terrestrial residence time. However, whether 14C-free BC from fossil fuel combustion significantly contributes to riverine BC and biases these ages remains arguable. Here, we track dissolved and particulate BC (DBC, PBC) along a river continuum from forested headwaters to the urbanized estuary. Such a gradient allowed us to distinguish fossil BC from pre-aged biomass-derived BC. We estimate that fossil sources contribute 18 ± 3% of DBC and 24 ± 6% of PBC in the urban zone, causing the apparent 14C ages to be older by 1,475–1,972 years for DBC and 1,619–2,877 years for PBC. The aging effect on DBC was primarily linked to less-condensed aromatics, while that of PBC—especially pronounced during the dry season—was driven by highly condensed aromatics. Our findings reveal that urbanized deltas are critical conduits of fossil BC to the ocean, necessitating land-use contextualized assessments when applying riverine 14C ages to estimate terrestrial residence times.