| The coastal region in Nantong, Jiangsu Province, China is located in the northern part of the Yangtze river delta. Its Holocene environmental evolution has received much attentions. This paper aims to apply environmental magnetism, a important tool in paleoenvironmental study, to the study of environmental change of Nantong, and get a better understanding of the evolution history of Nantong in Holocene.In this study,4drill cores from Caoyuan (CY,29.90m in length), Sanyu(SY,41.60m in length)、LvSi (LS,30.25m in length)、Xinan (XA,41.90m in length) were obtained from the present coast of Nantong, which covers the period of mid-late Holocene according to AMS14C dating. Rock magnetism were carried out on samples from the4cores. In combination with analyses of particle size, geochemistry, X-ray diffraction(XRD), diffuse reflectance spectroscopic(DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), this paper discusses the magnetic properties of the cores, its influencing facotors and environmental implications. The conclusion is as follows:1. According to AMS14C dating, the recovered cores was deposied since the mid-late Holocene. The deposition rates of core XA and CY are relatively slower, with mean sedimentation rate of0.43cm/a and0.51cm/a, in respectively. The rate of core SY is faster, with an average value of1.75cm/a.2. Magnetite dominates magnetic properties of the cores, and the grain size is mainly pseudo single domain (PSD) and multi-domain (MD). Greigite occurs at certain depth in the cores, which can be recognized by higher SIRM/χ, S-100, S-300and lower L-ratio values. In addition, goethite, hematite, maghemitite are more abundant in the top～2m of each cores.3. The similar geochemical composition suggests a relatively stable provenance. However, the correlation relationships between geochemical elements and mean size in cores CY and SY are different from those in cores LS and XA. It may reflect that a different sediment source in the study area, with Yellow River materials dominatin in the north and Yangtze River materials in the south.4. Particle size plays a dominant role in the varations of magnetic properties. Mean size is positively correlated with magnetic susceptibility (x). It is due to the fact that coarser PSD-MD grains, which are enriched in the sand fractions, dominate magnetic properties of sediments.χARM shows positive correlations with the fraction less than32μm in core SY and LS. But this correlation is not evident in core CY and XA, which is probably caused by the dissolution of fine magnetic minerals under reductive diagenesis.5. Early diagenesis drived by organic matter decay has a remarkable influence on magnetic properties in layer3of core XA. It is characterized by the dissolution of fine magnetite and the relative increase of antiferromagnetic minerals proportions. The presence of greigite in the organic layer of core CY suggests the change of sedimentary environment during its formation and preservation.6. Enrichment of goethite, hematite and maghematite in the top-2m layer of tidal flat facies is caused by pedogenesis. It results in lower S-ratios and higher HIRM, which provides a new means for buried pelaosol identification.7. The variations of magnetic properties respond sensitively to hydrodynamics and geochemical characteristics of sedimentary environments. Magnetic properies of the cores can reflect well the environmental evolution process in the study area since the mid-late Holocene. |
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