How human activities reshape carbon burial in estuarine sediment systems: Evidence from the Yangtze Estuary

Estuaries are critical zones for understanding the dynamics of organic carbon (OC) burial, particularly in the context of human intervention, yet detailed information remains limited. This study presents comprehensive field observations aimed at assessing the spatial and temporal variations in sedimentary OC sources, composition, and distribution within the Yangtze Estuary. By employing a three-endmember estimation, we determined the contributions of fluvial, marine, and salt marsh plants to OC in the mouth bar as 52 ± 6 %, 28 ± 12 %, and 20 ± 9 %, respectively. To address the complexities introduced by human activities and dispersal processes, we utilized a PCA-MC four-endmember model, which revealed that redispersal terrigenous organic matter from the outer estuary, categorized as "marine origin", accounted for 26 ± 10 % re-deposited within the estuary. During the construction of the Deep Channel Navigation Project (DCNP), we observed a shift in terrigenous OC deposition from the North Passage (NP) to the South Passage (SP), along with an increased contribution from salt marsh plants. Post-DCNP completion, sedimentary OC compositions exhibited significant differences between the NP and SP, with the SP enriched in fresh terrigenous OC and the NP showing degraded signals due to dredging activities and source discrimination. Human interventions not only altered the estuarine geomorphology and hydrodynamics but also impacted OC burial and sequestration by modifying sources and compositions. This study highlights the transition of estuarine regions from carbon sinks to potential sources, underscoring the necessity for a comprehensive understanding of carbon cycle dynamics in Anthropocene-influenced estuarine environments.