【作者】 董艳；

【导师】 张卫国；

【作者基本信息】 华东师范大学 ， 自然地理学， 2014， 博士

【摘要】 南通滨海地区地处长江三角洲北翼,其全新世环境演变过程备受关注。本文将环境磁学这一古环境研究的重要手段应用于该地区,探讨环境磁学在滨海古环境重建的应用机制及影响因素,以期对南通地区的环境演变过程有更为深刻的认识。本文对采自南通滨海地区曹园(CY,长29.90m)、三余(SY,长41.60m)、吕四(LS,长30.25m)、新安(XA,长41.90m)四个中晚全新世钻孔进行了系统的环境磁学研究,结合AMS14C测年、粒度、地球化学、X射线衍射(XRD)、漫反射光谱(DRS)、电镜分析等手段,分析了沉积物的磁性特征及其影响因素,探讨了磁学参数的古环境意义。主要结论如下：1、AMS14C测年数据显示,本文研究的四个钻孔为中晚全新世以来的堆积。其中XA、CY孔沉积速率相对较慢,平均速率分别为0.43cm/yr和0.51crn/yr；SY孔沉积速率较快,平均为1.75cm/yr。2、南通滨海地区中晚全新世沉积物的磁性特征为亚铁磁性矿物所主导,磁铁矿为主要的亚铁磁性矿物,颗粒大小以假单畴(PSD)-多畴(MD)为主；部分层位存在胶黄铁矿,具有较高的SIRM/χ, S-100、S-300和较低的L-ratio。此外,潮滩相上部(约表层2m)沉积物中针铁矿、赤铁矿、磁赤铁矿等磁性矿物的贡献较为显著。3、南通滨海地区中晚全新世沉积物具有相似的地球化学组成,说明物质来源较为稳定,不是引起沉积物磁性特征变化的主要因素。但CY、SY孔和LS、XA孔元素含量与粒度相关关系存在的差异,可能反映了研究地区南北物源存在差异,即北部受更多的黄河物质影响,南部受更多的长江物质影响。4、粒度是导致磁性特征变化的重要因素,其中平均粒径与磁化率(X)呈显著正相关,即粒度越粗,磁化率越高。这与较粗的PSD-MD磁性矿物主导了磁性特征,而这些磁性颗粒主要赋存在较粗的砂粒级组分中有关。SY和LS孔XARM与<32μm各粒级含量具有显著正相关,但是CY和XA两孔中非磁滞剩磁(XAMR)与细颗粒组分不具相关性,这可能是由于细颗粒磁性矿物在早期成岩作用过程中的优先溶解所致。5、有机质驱动的早期成岩作用对磁性特征的影响,在XA孔的层3浅海相沉积中最为显著,表现为细颗粒磁铁矿的溶解以及不完整反铁磁性矿物比例的上升。而CY孔的有机质层则含有大量的胶黄铁矿,指示了沉积环境的转变。6、受成土作用影响,钻孔顶部潮滩沉积物中针铁矿、赤铁矿和磁赤铁矿富集,使得沉积物具有较低的S-ratios和较高的HIRM,这为识别埋藏古土壤提供了新的手段。7、利用磁性特征对沉积动力和地球化学特征的指示,可以很好地反映研究区中晚全新世以来的环境演变过程。更多还原

【Abstract】 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.更多还原