Grain size composition and transport of sedimentary organic carbon in the Changjiang River (Yangtze River) Estuary and Hangzhou Bay and their adjacent waters

Surface sediments from the Changjiang River (Yangtze River) Estuary, Hangzhou Bay, and their adjacent waters were analyzed for their grain size distribution, organic carbon (OC) concentration, and stable carbon isotope composition (δ13C). Based on this analysis, about 36 surface sediment samples were selected from various environments and separated into sand (>0.250 mm, 0.125–0.250 mm, 0.063–0.125 mm) and silt (0.025–0.063 mm) fractions by wet-sieving fractionation methods, and further into silt- (0.004–0.025 mm) and clay-sized (<0.004 mm) fractions by centrifugal fractionation. Sediments of six grain size categories were analyzed for their OC and δ13C contents to explore the grain size composition and transport paths of sedimentary OC in the study area. From fine to coarse fractions, the OC content was 1.18%, 0.51%, 0.46%, 0.42%, 0.99%, and 0.48%, respectively, while the δ13C was–21.64‰,–22.03‰,–22.52‰,–22.46‰,–22.36‰, and–22.28‰, respectively. In each size category, the OC contribution was 42.96%, 26.06%, 9.82%, 5.75%, 7.09%, and 8.33%, respectively. The OC content in clay and fine silt fractions (<0.025 mm) was about 69.02%. High OC concentrations were mainly found in offshore modern sediments in the northeast of the Changjiang River Estuary, in modern sediments in the lower estuary of the Changjiang River and Hangzhou Bay, and in Cyclonic Eddy modern sediments to the southwest of the Cheju Island. Integrating the distribution of terrestrial OC content of each grain size category with the δ13C of the bulk sediment indicated that the terrestrial organic material in the Changjiang River Estuary was transported seaward and dispersed to the Cyclonic Eddy modern sediments to the southwest of the Cheju Island via two pathways: one was a result of the Changjiang River Diluted Water (CDW) northeastward extending branch driven by the North Jiangsu Coastal Current and the Yellow Sea Coastal Current, while the other one was the result of the CDW southward extending branch driven by the Taiwan Warm Current.