Tracing Sediment Erosion in the Yangtze River Subaqueous Delta Using Magnetic Methods

We assessed the usefulness of magnetic properties for tracing sediment erosion in a deltaic environment. Surface and core sediments from the Yangtze River subaqueous delta were subjected to magnetic, granulometric, geochemical, and radionuclide analyses. Based on magnetic properties and particle size, the surface sediments can be divided into three groups. Groups I and II have a similar particle size distribution and geochemical composition, but the former has higher values of magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM). We interpret Group I as consisting of modern sediments, while Group II represents previously buried sediments that have undergone significant reductive diagenesis and that have been subsequently exposed by erosion. Group III has coarser particle size, which reflects the mixing of delta sediments with Pleistocene relict sands. Two cores from the areas occupied by Groups I and II have significantly different magnetic profiles. Core A3-4 (Group II) records a partial loss of magnetic mineral concentration due to diagenesis and has much lower χ, SIRM, and S₋₃₀₀ values than core A6-2 (Group I). Radionuclide dating reveals that core A3-4 sediments are older. We conclude that core A3-4 location has undergone erosion, which is consistent with recent bathymetric survey results. We propose that the ratio of SIRM/Fe can be potentially used to trace mobilized old sediment in this environment. Our study demonstrates that magnetic properties provide a robust approach for studying sediment dynamics in this type of environment. In addition, our findings provide insights into the nature of biogeochemical processes associated with deltaic erosion.