Abstract
Understanding sediment dynamics and anthropogenic impacts in the land-sea interaction zone of the Abandoned Yellow River Delta is challenged by complex material sources. This study aims to reconstruct sedimentary environment evolution and evaluate human influences on silty-mud tidal flats. Sediment cores were collected from a bare flat (GT) and a Spartina alterniflora marsh (HM) in the Yancheng coastal wetland, Jiangsu, China. A multi-proxy approach was applied, including grain size, organic carbon and nitrogen content, 210 Pb - 137 Cs dating, analysis of major elements and iron speciation analysis. Results indicate: (1) The GT core exhibits sandy silt characteristics, indicative of an intertidal environment. The HM core is dominated by clayey silt and silt, displaying a silt-sand binary structure, reflecting a typical silty tidal flat. Grain size characteristics suggest tidal flat accretion is collectively driven by reclamation, storm surges, tidal action, and sediment trapping by S. alterniflora . (2) Organic matter provenance analysis reveals marine sources dominate the GT core, while terrigenous sources dominate the HM core. Increasing trends of TOC, TN, and C/N with depth suggest S. alterniflora introduction likely promoted organic matter accumulation. (3) 210 Pb - 137 Cs dating yields deposition rates of 1.25 cm/a (GT) and 1.39 cm/a (HM). Combined with previous studies, regional average deposition rates show a declining trend with enhanced accretion, potentially linked to increased hydrodynamics from reclamation and the Jiangsu Coastal Current. (4) Major elements follow the Na > Al > K > Fe > Ca > Ti > Mg > Ba > Mn > Sr and the distribution of major elements is synergistically controlled by provenance, grain size effects, and biological processes; the variations in element-grain size correlations reveal the modulating effect of sedimentary composition on geochemical behavior. (5) Iron speciation content follows the order Fe ox > Fe mag > Fe carb > Fe py and increases with depth, primarily sourced from terrigenous inputs influenced by chemical weathering and marine dynamics. The sedimentary environment is oxic/suboxic, promoting TOC mineralization and hindering preservation. The Fe HR /Fe T ratio characteristics resemble those of shelf-margin marine sediments and Yellow River particulates. However, influenced by weathering, runoff, and sea-level changes, this ratio is unreliable for accurately indicating redox conditions.