Magnetic Properties Of Core Sediments Off The Yangtze River Estuary And East China Sea Inner Shelf And Their Influencing Factors

Sediments from the Yangtze River Estuary (YZE) and inner shelf of the East China Sea (ECS), are important recorders of the changes in the watershed and marine environment. Currently, few studies has carried out sedimentary record studies using environmental magnetic technology in this area. Based on magnetic measurements, particle size and geochemical analyses on eight cores from the Yangtze River Estuary and East China Sea shelf, this paper discusses the factors influencing magnetic properties of the sediments and its implications for environmental changes.It is shown that the cores consist mostly of clayey silt, with mean size mostly ranging between 5 and 30μm. Magnetite dominates magnetic properties of the samples. The pattern of variation of magnetic susceptibility (χ) with depth divides the cores into three groups, i.e., slight increasing susceptibility, slight decreasing susceptibility and a sharp decreasing susceptibility with depth, respectively. On a whole, sediments with a higher percentage of the<16um fraction are magnetically stronger, which is best manifested in core YZE. It suggests that particle size is a main factor influencing magnetic properties of sediments.χvalues of core surface sediments display a decreasing trend from west to east as well as from south to north, which is similar to the variation in the<16μm particle size fraction.Iron (Fe) species analysis using BCR method indicates that residual iron in sediments accounts for more than 80%, which is followed by reducible iron (up to 10%). Manganese (Mn) is dominated by acid soluble carbonate phase, which can be as high as 50%, and followed by residual phase. Contents of Fe, Mn and total sulfur (TS) have no significant vertical variations in YZE, CX38, CX32 and CX2 cores, indicating that influence of early diagenesis is minor. Oxidizable iron and TS increase in the lower part of core F17 and CX21, and the presence of pyrite as revealed by themomagnetic analysis, suggest a relatively reducing condition in the lower part of these two cores. As a result, magnetite dissolves, resulting in decrease of susceptibility and S ratios. Magnetic properties of core F17 are heavily impacted by diagenesis in comparison to core CX21.Based on 137Cs dating, sedimentation rate of core CX32 is averaged to be 3.11 cm/a. A mean sedimentation rate of 1.85 cm/a in core CX21 is derived from core correlations. Magnetic and geochemical results suggest that sediment provenance also play a role in the variations of magnetic properties of CX21 and CX32. The increase of Ti, x and SIRM toward the surface is most likely caused by increased sediment contribution of Jinsha River to the Yangtze River in recent years.