Carbon and nitrogen isotopic tracing of organic matter sources and sinks in anthropogenic estuaries

River estuaries are critical zones for global organic matter (OM) burial; however, the impacts of human perturbations on OM sources and sinks remain poorly understood. Here, we integrate δ¹³C and δ¹⁵N values with environmental parameters and grain-size data to fingerprint OM source variations and their controlling mechanisms across the water-sediment continuum in the Pearl River Estuary (PRE) and adjacent coasts. Our results suggest that OM primarily originates from riverbank soil, river phytoplankton, C₃ plant detritus, marine primary production, and anthropogenic sewage. The sewage influence, indicated by elevated δ¹⁵N signatures (17.6‰ at river mouths), decreases markedly from the water column to sediments. Marine sources dominate surface water OM, whereas terrestrial sources (particularly riverbank soil) prevail in bottom water and sediments. The stable non-sewage OM contribution across the water-sediment continuum suggests effective sequestration during settling. This efficient transfer, characterized by minimal isotopic alteration, could be conceptually explained by a microbial conversion mechanism analogous to the microbial carbon pump, wherein labile particulate OM is transformed into refractory biomass. Grain size analysis further reveals associations that fine particles are correlated with enhanced marine OM preservation, whereas riverbank soil persists due to its inherent recalcitrance. Our results highlight that investigating the water-sediment continuum is essential for understanding OM dynamics. This approach provides a robust framework for elucidating OM sources and sinks in anthropogenic estuaries, offering new insights into global carbon cycling under human perturbations.