Biogeochemical evolution and iron speciation in the abandoned Yellow River Delta: deciphering the impact of anthropogenic engineering and introduced Spartina alterniflora

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, 210Pb - 137Cs 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) 210Pb - 137Cs 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 Feox > Femag > Fecarb > Fepy 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 FeHR/FeT 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.