Three-dimensional patterns and contrasting driving mechanisms of sediment organic carbon in the highly urbanized subtropical Pearl River Estuary, China.

Although estuarine sediments are critical reservoirs for organic carbon, the three-dimensional variability and source composition of sediment organic carbon (SOC) in highly urbanized subtropical estuaries remain poorly constrained. This study investigated the three-dimensional variation and source composition of SOC in the Pearl River Estuary, southern China. Using data from 24 surficial sediment sites and 5 sediment cores, we identified the primary environmental controls on horizontal versus vertical SOC distributions. Surface SOC content ranged from 0.58% to 2.35%, with a clear decreasing trend from the upper to lower estuary. The extrapolated SOC stock to 1 m depth averaged 75.88 ± 23.02 Mg C ha-1. The SOC content of sediment cores ranged from 0.52% to 2.26%, and declined with increasing depth in core S24. A four-end-member Bayesian mixing model revealed that terrestrial and mangrove-derived sources dominated the upper estuary, while marine sources prevailed seaward, with sewage-derived organic matter playing a minor role. Multivariate analyses revealed contrasting control mechanisms. Horizontally, SOC variation was negatively correlated with silt content and δ13C, but positively correlated with total nitrogen, reflecting hydrodynamic sorting and land-sea source gradients. Vertically, SOC variation was positively correlated with total nitrogen and silt content, with moisture content mediating the effects of burial depth and compaction, while SOC was negatively correlated with pH and δ13C, suggesting alkaline conditions enhance mineralisation. Our findings demonstrate that horizontal SOC patterns are governed by contemporary hydrodynamic sorting and land-sea source gradients. In contrast, vertical SOC dynamics are shaped by early diagenesis, compaction-driven density changes, and depth-related chemical conditions-a dual framework that should be explicitly considered in blue carbon assessments of rapidly urbanising coastal zones.