上白垩统黑色页岩和大洋红层的Fe同位素特征及其古海洋学和古气候学意义

发布时间：2018-02-26 17:14

  本文关键词： 晚白垩世 大洋缺氧事件 大洋红层 Fe同位素 古海洋　出处：《中国地质大学(北京)》2016年博士论文　论文类型：学位论文

【摘要】：白垩纪是地质历史记录中极端温室气候的典型的代表,研究白垩纪的古海洋和古气候系统变化对于预测正在升温的地球未来温室效应变化方面具有重要的意义。上白垩统的海相地层存在富有机质黑色页岩向贫有机质的大洋红层的转变,可能记录了发生在Cenomanian-Turonian界线附近的白垩纪极端大洋缺氧事件OAE 2之后全球大洋逐渐转为富氧状态。这个转变涉及温室气候条件下全球海洋和气候系统波动,以及碳-氧循环等重要科学问题,因此已成为国际白垩纪研究领域新的重要方向。本文选取了东特提斯西藏床得剖面、西特提斯意大利Contessa Quarry剖面和北大西洋ODP 103航次Site 641A站点这三个剖面上白垩统黑-红转变层位的样品,开展了系统的沉积学、元素地球化学、有机碳、Fe组分和Fe同位素组成分析研究,试图重建晚白垩世古海洋-古气候变化。结果显示,三个剖面的大洋红层与下伏黑色页岩的Fe同位素组成及其他地球化学指标均显示有系统变化。本文是国际上首次报道上白垩统大洋红层的Fe同位素组成,其Fe同位素组成相对均一,但某些样品存在一定的变化,δ56Fe的变化范围为-0.400~0.505‰,说明在大气和表层水氧化条件下,岩性和区域Fe循环对Fe同位素组成的影响。代表大洋缺氧事件的黑色页岩的Fe同位素组成有相对较大的变化范围,随三个剖面不同的沉积位置和沉积环境的不同而不同。结合Fe同位素、Fe组分和元素地球化学特征,在OAE 2黑色页岩沉积期间东特提斯床得剖面处于弱氧化沉积环境,西特提斯Contessa Quarry剖面处于缺氧/硫化环境,而北大西洋的Site641A为硫化环境。三个剖面黑-红过渡层段的Fe同位素组成变化范围大,可能反映了转变过程中水体环境的波动。三个剖面沉积时的氧化还原环境和Fe循环模式的不同,指示了在全球气候变化的背景下,区域古地理、构造等作用对古海洋环境和地球化学循环的作用。北大西洋Site641A站点的大洋红层的Fe指标和古生物化石证据可能暗示了在Turonian(晚)期赤道大西洋初期打开,初期打开之后经历了约0.6 Ma才有稳定的海水连通,使大西洋演化为通风良好的开放大洋环境。相比较而言,在西特提斯洋(意大利)从OAE 2的硫化环境过渡到富氧、通风大洋环境用了大约1.1 Myr,在东特提斯洋(西藏南部)需要更长时间(9Myr)。根据本文的数据分析,并结合全球古气候古海洋学和古气候学指标表明,从上白垩统黑色页岩到大洋红层的沉积,可能是这可能是由气候变冷和大洋海道打开共同驱动的。
[Abstract]:The Cretaceous is a typical representative of the extreme greenhouse climate in the geological history records. The study of the Cretaceous paleooceanic and paleoclimate systems is of great significance in predicting the future Greenhouse Effect changes of the warming earth. The marine strata of the Upper Cretaceous are rich in organic matter, black shale and poor in organic matter. The transformation of the red layer of the ocean, It may have been recorded that after the Cretaceous extreme oceanic anoxic event OAE _ 2, near the Cenomanian-Turonian boundary, the global ocean gradually became oxygen-rich. This transition involves fluctuations in the global ocean and climate system under greenhouse climate conditions. As well as important scientific issues such as the carbon-oxygen cycle and so on, it has become a new important direction in the international Cretaceous research field. In this paper, the East Tethys Tibetan bed profile has been selected. The samples of the Cretaceous black-red transition horizon in the three sections of the Contessa Quarry section of West Tethys Italy and the North Atlantic ODP 103 voyage Site 641A station have developed systematic sedimentology and elemental geochemistry. The study on the composition of organic carbon and Fe isotopes in order to reconstruct the paleo-ocean-paleoclimate change of late Cretaceous shows that, The Fe isotopic compositions and other geochemical indices of the oceanic red beds and underlying black shale in the three sections show systematic variation. This is the first time that the iron isotopic compositions of the upper Cretaceous oceanic red beds have been reported in the world. Its Fe isotopic composition is relatively uniform, but some samples have some changes. The range of 未 56Fe varies from -0.400 to 0.505 鈥，

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