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 commonly used to constrain its terrestrial residence time. However, whether 14C-free BC from fossil sources significantly contributes to riverine BC and biases these ages remains unclear. Here, we track dissolved and particulate BC (DBC, PBC) along a river continuum from forested headwaters to an urbanized estuary. This urbanization gradient allows us to distinguish fossil BC from pre-aged biomass-derived BC. We estimate fossil sources contribute 18 ± 3% of DBC and 24 ± 6% of PBC in the urban zone, increasing their apparent 14C ages by 1,475-1,972 and 1,619-2,877 years, respectively. DBC aging is primarily linked to less-condensed aromatics, while PBC aging, especially during the dry season, is driven by highly condensed aromatics. Our findings identify urbanized deltas as critical conduits of fossil BC to the ocean and underscore the importance of incorporating land-use context when interpreting riverine BC 14C ages as estimates of terrestrial residence time.