Organic carbon cycling in sediments of the Changjiang Estuary and adjacent shelf: Implication for the influence of Three Gorges Dam

Surface sediments collected from the Changjiang Estuary and adjacent shelf were analyzed for elemental and stable carbon isotopic composition, and lignin-phenols to investigate spatial variability of the sources, transport and decay of sedimentary organic carbon (OC). Bulk and molecular proxy data indicated a mixed marine/terrestrial OC sources in the study area. A three end-member mixing model using Monte-Carlo simulation showed that marine OC was the predominant OC source, accounting for an increasing fraction along the coast and seaward, while soil-derived OC and C3 vascular plant detrital OC decreased seaward and southward. Large fragments of lignin-rich C3 vascular plant OC were deposited mainly near the river mouth, whereas fine-grained lignin-poor soil-derived OC was delivered further south alongshore. Higher values of lignin decay indices, seaward and southward, were attributed to selective transport of terrestrial OC on fine-grained particles and efficient remineralization in mobile muds. Λ8 of OC in Changjiang Estuary sediments has slightly decreased in recent years, which could in part be due to the trapping of terrestrial coarse particles by the Three Gorges Dam (TGD). Also, we propose that there has been an increasing input of phytodetritus derived from freshwater phytoplankton to coastal sediments after the construction of the TGD.