Particle-size dependent magnetic property variations in the Yangtze delta sediments of late Holocene: Effects of pedogenesis and diagenesis

Abstract

Late Holocene is a key period for the formation of modern deltas, and environmental magnetism is commonly used to infer paleoenvironmental information. Magnetic properties of bulk sediments are influenced by both hydrodynamic sorting and post-depositional pedogenesis and diagenesis, and the relationship between them is unclear. Detailed magnetic study on particle size fractions is a tool for understanding this issue and has been used in surface sediments, while the applications in delta core sediments are rare. Here we investigated the bulk and particle size dependent magnetic properties of the Yangtze River Delta core sediments over the past 3 ka, to determine the main mechanism for bulk magnetic property changes under the effects of pedogenesis and diagenesis. Our results showed that, in all particle size fractions, pedogenesis occurred in the top ∼2 m (within 0.3 ka) and the dissolution of fine magnetic minerals driven by early diagenesis appeared below this depth and completed at ∼20 m in depth (∼1.5 ka). These effects, including the dissolution of fine-grained ferrimagnetic minerals and the precipitation of authigenic greigite, were most obvious in the <16 μm fraction and decreased with coarsening particle sizes. The grain sizes of magnetic minerals were the finest in the <16 μm sediments and the coarsest in the 32–63 μm sediments, with intermediate similar features observed in the 16–32 and 63–125 μm sediments. Bulk magnetic properties were dominated by the <16 μm sediments. Our findings distinguished the differences between the effects of pedogenesis and early diagenesis among size fractions, which may provide a better approach to recognize redox states and provenance in delta sediments since the late Holocene.