Sedimentation on the inner shelf of the East China Sea: Magnetic properties, diagenesis and paleoclimate implications

High-resolution magnetic analysis was performed on the 19.22 m core MD06-3040 (27°43.3663´N, 121°46.8822´E; 46 m water depth) from the East China Sea (ECS) inner continental shelf, which was recovered during the IMAGES XIV MD155-Marco Polo 2 cruise of the R/V Marion Dufresne (IPEV). Magnetic grains are mainly transported to the inner shelf from erosion on mainland China, delivered by the Yangtze River. Nine 14C AMS dates indicate that the core spans about the last 10.4 ka, with an average sedimentation rate of nearly 2 mm/yr. The primary magnetic mineral assemblage of the uppermost sediments contains magnetite, with some contributions from hematite and greigite. Two main stepwise changes are observed in the magnetic mineral content, due to post-depositional reductive diagenesis. The first occurs from the top of the core to 3.5 m and is characterized by reduction of both the magnetite and hematite contents. The second occurs at 8.45–9 m and is characterized by a loss of fine-grained ferrimagnetic material. Variations in the magnetic mineral content of this core are related to geochemical and redox environment changes in the ECS, through the oxic–suboxic–sulfidic–methanic zones from top to bottom. Despite significant reduction of a large amount of the ferrimagnetic mineral content due to diagenesis, intervals characterized by lower magnetic mineral concentrations and higher coercivities are still identified at 0.45–0.6, 1.5–1.7, 2.9–3.1, 4.4–4.7, 5.3–5.7, 5.9–6.1 and 8.2–8.4 ka. These intervals are in phase, within dating errors, these intervals are in phase with weak summer monsoon events, which indicate that a relationship between magnetic results and paleoclimatic events is partially preserved.