Magnetic fingerprinting of hydrodynamic variations and channel erosion across the turbidity maximum zone of the Yangtze Estuary, China

摘要

Magnetic measurements were conducted on surface sediments and suspended particles collected across the turbidity maximum zone of the Yangtze Estuary, in order to examine the spatial distribution of magnetic mineral assemblages and the factors responsible for this distribution. The results indicate that magnetic properties are dominated by ferrimagnetic grains. Bulk magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) values show positive correlations with the proportion of the ＞63 μm fraction in the North Channel, while anhysteretic remanent magnetization (χ_(ARM)) is significantly correlated with the proportion of the ＜ 16 μm fraction in both surface sediments and suspended particles. Such a bimodal distribution of ferrimagnetic minerals in the sand and finer fractions is confirmed by particle size-specific measurements. Sediments in the North Channel have the highest χ and SIRM values but lowest χ_(ARM)/SIRM ratios, which is consistent with the coarsest particle size due to strong hydrodynamics, i.e. currents. Within each channel, χ and SIRM values are higher in sediments from shallower water depth due to energetic conditions resulting from waves. Compared with surface sediments, suspended particles have lower χ and SIRM values but higher χ_(ARM)/SIRM ratios due to lower sand fractions. The increasing trend of χ_(ARM)/SIRM of suspended particles along the pathway of sediment transport indicates weakening hydrodynamics from the inner estuary to the outer estuary. Diagenesis is another factor influencing magnetic properties in addition to particle size. Channel erosion leads to local exposure of buried sediments on the channel bed. As a result of a stronger diagenetic imprint, they show magnetic properties different from the recently deposited sediments. Our results indicate that magnetic properties cannot only indicate spatial variations in hydrodynamics, but also provide insight into sediment erosion/deposition processes. Combined granulometric and magnetic methods can therefore be used to interpret hydrodynamics and track changes in estuary morphology.