Metal enrichment and magnetic properties of core sediments from the eastern Yellow Sea, East Asia: Implications for paleo-depositional change during the late Pleistocene/Holocene transition

To investigate the paleo-depositional environments, sediment cores YJ1 and YJ2 from a tidal flat of the eastern Yellow Sea were analyzed for the vertical variations of metal concentrations, particle size, clay mineralogy, magnetic properties, and total organic carbon content (TOC). Fe (and Mn) concentrations determined by both partial and total acid extraction increased at 15 m depth in YJ1 and 10 m in YJ2. The layer directly below these Fe-rich brownish horizons showed dramatic decreases in clay mineral content and trace metals (Zn, Pb, Cu, Cr, Co, Ni, and As), but increases in sand fraction, magnetic susceptibility, and TOC. All results were inverse in the upper metal-rich layer. These dramatic changes in multi-parameters are likely due to an environmental change during the late Pleistocene/Holocene transition. The rapid environmental change (sand-rich and oxic → clay-rich and anoxic) resulted in aqueous dissolution of redox-sensitive metals (e.g., Fe and Mn) from the lower sandy layer materials. The dissolved metals diffused upward and concomitantly were immobilized within the upper clay-rich layer. The reprecipitated Fe (or Mn) (hydro)oxides then scavenged trace metals by coprecipitation and/or adsorption. This study suggests that metal distribution patterns can provide new insights into the paleo-depositional processes during the Pleistocene–Holocene period.