Mineral magnetic properties of Holocene sediments in the subaqueous Yangtze delta and the implications for human activity and early diagenesis

This study examines the temporal and spatial distribution of magnetic properties in Holocene sediment cores taken from the subaqueous Yangtze delta, China, to evaluate depositional environmental changes including the sediment with an anthropogenic provenance and changes in the bottom-water chemistry. Our approach is to compare the magnetic properties in Holocene cores with those from a suite of surficial sediment samples taken at various locations from the Yangtze delta and the adjacent continental shelf. The results indicate that the magnetic properties in sediment cores change generally with sedimentary facies, mainly due to their in-phase changes with sediment grain size and redox conditions, but that they have also been significantly altered by effects of human activity and early diagenesis. Magnetic parameters that exhibit soil erosion associations show remarkable increases over the past ∼800 years, reflecting an increase in the terrestrial supply of fine-grained magnetic minerals induced by the intensification of human activity in the Yangtze River catchment. Early diagenesis was strong in core HZK8, located at the depocenter of the subaqueous Yangtze delta, including dissolution of fine-grained ferrimagnetic minerals and the precipitation of authigenic greigite and pyrite, as evidenced by both room-temperature and thermo-magnetic analyses. The dissolution of ferrimagnetic minerals coincides with changes in sedimentation rate at different sites and time periods, suggesting that this factor is important for controlling the rate of early diagenesis. Authigenic iron sulfides suggest the function of sulfate-reducing bacteria, which implies a hypoxic environment at this site from ∼6000 cal. yr BP.