【作者】 李琳；

【导师】 王张华；

【作者基本信息】 华东师范大学 ， 自然地理学， 2013， 硕士

【摘要】 本文利用在太湖平原松江地区小昆山获得的一个全新世钻孔(SL67),进行AMS14C测年,并综合运用地层学、沉积学手段进行沉积地貌环境演变恢复。在此基础上,再进行元素地球化学和环境磁学分析,旨在揭示有机地球化学元素以及自生硫复铁矿在全新世钻孔沉积微相中的分布规律,以探讨硫元素和铁硫化物指标在全新世沉积地貌环境演变研究中的应用价值和局限性。SL67孔AMS14C年龄、岩性以及粒度特征表明钻孔所在地全新世以来沉积环境演变如下：末次盛冰期干冷气候条件下,本区发育硬土层。全新世早期,海平面快速上升,本区受到海水影响并在其控制下发育潮上带-盐沼-高、中潮滩-高潮滩-中潮滩沉积层序。自全新世中期,随着海平面上升速率的减慢和长江大量泥沙的供应,发育沉积海退序列：低潮滩-中潮滩-高潮滩-潮上带-湖沼平原。研究发现,有机地球化学元素在全新世钻孔沉积微相中的分布具有一定的规律。在元素含量方面,海退沉积体系元素含量显著低于海进体系。海进体系中,盐沼-高潮滩-中潮滩这一沉积序列,各元素含量依次降低。而海退序列中,中潮滩各元素含量最高。在有机质组成上,海进体系潮上带、盐沼TOC/TN均大于10,其有机质以陆生植被贡献为主。从高潮滩到中潮滩,TOC/TN值小于10,并呈现出下降的趋势,反映出有机质中海洋藻类贡献逐渐加强。海退层序中,除湖沼平原外,TOC/TN均较低,表明有机质以藻类贡献为主。此外,太湖平原全新世钻孔潮滩沉积物TS与TOC的比值及其相关关系与沉积微相具有一定的关系。海进层序盐沼和高潮滩TS异常高,且高潮滩TS/TOC也呈现异常高值。潮上带TS/TOC最低,盐沼次之,高潮滩达最高值后向中潮滩降低。TS-TOC一元线性回归斜率变化基本与TS/TOC一致。海退层序中低潮滩都波动出现TS及TS/TOC的较高值,高潮滩和潮上带较低。且海退层序TS与TOC相关性较差。研究还发现,本孔早期成岩作用显著,黄铁矿普遍存在于本孔潮滩沉积环境中,而自生硫复铁矿仅显著出现在海进层序盐沼相和海退层序中潮滩中上部,盐沼相还包含一定的磁黄铁矿。海进层序高中潮滩及海退层序低潮滩也出现较为微弱的自生硫复铁矿信号。这种特征与我们在长江口现代高潮滩普遍发现硫复铁矿不同,说明在沉积物埋藏后,黄铁矿化作用继续发生。海进层序中盐沼沉积的TS/TOC远低于高潮滩沉积,反映了硫酸盐的供应相对受限制,这正解释了本孔盐沼沉积中硫复铁矿和磁黄铁矿的信号远强于高潮滩沉积。海退层序中低潮滩TS/TOC的高值及硫复铁矿的出现,推测与海退层序潮上带-高潮滩沉积物发生淋滤作用以及中低潮滩下伏地层的早期成岩作用密切相关,即下伏地层早期成岩作用产生的甲烷、硫化氢气体向上运移,储存在颗粒较粗的海退中低潮滩沉积物并继续发生早期成岩作用。本研究还显示,TS/TOC指标的应用,能更好地协助沉积微相判断。更多还原

【Abstract】 We obtained a Holocene sediment core SL67at Xiaokunshan from the Taihu Lake plain. The sedimentary environmental evolution was reconstructed by AMS14C dating, stratigraphic and sedimentological analyses. On this basis, we employed element geochemistry and environmental magnetism in order to reveal the distribution pattern of the organic geochemical elements and authigenic greigite in the sedimentary micro-facies of tidal flat in the Holocene sediment core. Furthermore, we evaluated the application and limitation of sulfur and iron-sulfides to distinguish sedimentary micro-facies of the tidal flat in Holocene sediment core.After measuring the AMS14C ages, we decided the Holocene sedimentary environmental evolution according to the lithology and sediment grain size at the core site. There is layer of a stiff mud at the bottom of core, formed during the Last Glacial Maximum (LGM). A transgressive succession, i.e. supratidal-saltmarsh-upper tidal flat-mid-tidal flat-upper tidal flat-mid-tidal flat was formed during the early to mid-Holocene when the study area was influenced by rapid sea-level rise. As the sea-level rise decelerated from the mid-Holocene, a large amount of sediment input was derived from the Yangtze River. This subsequently formed a regressive succession of lower tidal flat-mid-tidal flat-upper tidal flat-supratidal-lacustrine plain.Results demonstrate some certain rules in the distribution of geochemical element in different sedimentary micro-facies of the tidal flat. Firstly, the content of TOC, TN and TS in the regressive succession was much lower than that in the transgressive one. In the transgressive succession, content of TOC, TN and TS is the highest in the saltmarsh sediments, followed by that in the upper tidal flat and mid-tidal flat. In the regressive succession, content of of TOC, TN and TS is the highest in the mid-tidal flat sediments.The composition of organic matters varies in different sedimentary facies. In the transgressive succession, the ratios of TOC/TN exceed10in the supratidal and saltmarsh, reflecting their major origin of terrestrial vegetation. TOC/TN declines from supratidal to mid-tidal flat, and its value is consistently lower than10, indicating the increased contribution from marine algae. In the regressive succession, the ratio of TOC/TN is low in each sedimentary facies except the lacustrine plain, implying major organic contribution from algae.Besides, ratios of TS/TOC in sediments of SL67are also related to the sedimentary micro-facies of the tidal flat. In the transgressive succession, TS content is extremely high in the saltmarsh and upper tidal flat sediments. The ratio of TS/TOC is the minimum in the supratidal flat and saltmarsh sediments while it is the highest in the upper tidal flat sediments. The regression line slope of TS-TOC plot has the similar change trend as TS/TOC. In the regressive succession, TS content and TS/TOC are generally in high values but with large fluctuations in the mid-and lower tidal flat sediments. By contrast, the values in the supratidal and upper tidal flat are much lower. TS and TOC shows a poor correlation in the sediments of regressive succession.Early diagenesis prevails in core SL67. Pyrite was found all over the tidal flat sediments in core SL67while greigite just markedly showed up in the basal saltmarsh and the mid-tidal flat sediments of regressive succession. In addition, pyrrhotite also occurred in the basal saltmarsh sediments. In the upper and mid-tidal flat of transgressive succession and the lower tidal flat of the regressive succession, there also appeared weak signals of greigite.TS/TOC is much lower in the basal saltmarsh than that in the upper tidal flat of transgressive succession, suggesting the sulfate limitation in the saltmarsh. This explains the much stronger signals of greigite and pyrrhotite in the basal saltmarsh than in upper tidal flat sediments. We also suggest that the eluviation of the supratidal-upper tidal flat sediments and the early diagenesis of the underlying sediments result in the high values of TS/TOC and occurrence of greigite in mid-and lower tidal flat of regressive succession. Seepage of methane and H2S produced in the underlying sediments made storage and further early diagenesis in the coarser-grained sediments of lower and mid-tidal flat sediments of regressive succession.Present study also shows that the application of TS/TOC can improve the differentiation of sedimentary facies.更多还原