白垩纪中期OAE1a-ORB1沉积转变的矿物学研究：以意大利Gorgo a Cebara和土耳其Yenicesihlar剖面为例

The Cretaceous oceanic black shales and red beds were correlated with oceanic anoxic and oxic events, respectively. The high frequency transition between black shales and red beds in the mid-Cretaceous oceans indicates the instability of paleoceanographic and paleoclimatic condition, and provides an important way to investigate the Cretaceous paleoceanographic and paleoclimatic systems. The present application is focused on the stratigraphic transition from the mid-Cretaceous OAE1a to ORB1 in the Gorgo a Cebara section, Italy, and the Yenicesihlar section, Turkey, and will ferret out the variations of bulk mineral composition, the clay mineral assemblage and the occurrence state of iron and iron-bearing mineral during the transition from OAE1a to ORB1. Through systematic mineralogical studies, the potential mechanism of mineral composition variation, and the low-temperature geochemical cycle model of iron for response to the oceanic anoxic-oxic transition will be established. Based on the studies of species, concentrations and coloration mechanism of ferric oxides in ORB1, the origin of oceanic red beds will be discussed. At last, by comparing the analysis results of both two studied sections acquired in this study and combining the existing relevant geological data, the formation mechanism of ORB1 and its relationship with OAE1a, as well as the oceanic anoxic-oxic transition mechanism will be further discussed. This study will deepen our understanding of Cretaceous paleoceanographic and paleoclimatic systems.

黑色页岩和大洋红层沉积分别与大洋的缺氧和富氧作用有关。白垩纪中期黑色页岩与大洋红层高频交替出现，反映了当时的古海洋和古气候状态的不稳定性，因此成为研究白垩纪海洋-气候系统的重要窗口。本申请拟以意大利Gorgo a Cerbara剖面和土耳其Yenicesihlar剖面从缺氧事件OAE1a到大洋红层ORB1转变的地层为研究对象，通过系统的矿物学研究，查明黑红转变过程中矿物组成变化、粘土矿物组合特征、铁的赋存状态及含铁矿物相的变化，建立黑红转变的矿物相响应机制、揭示大洋缺氧-富氧转变过程中铁的低温地球化学循环；查明大洋红层ORB1的致色铁氧化物类型、含量和致色机制，探讨大洋红层的成因。通过两个剖面的对比，结合已有的基础地质资料，探讨ORB1形成与OAE1a之间的关系以及古海洋缺氧－富氧作用转化机制。该研究的开展有助于深化对白垩纪古海洋和古气候系统的认识。

白垩纪中期黑色页岩与大洋红层高频交替出现，是研究白垩纪海洋-气候系统的重要窗口。本项目主要以意大利Gorgo a Cerbara 剖面上大洋缺氧事件OAE1a向大洋红层ORB1转换带为研究对象，通过系统的沉积学、矿物学以及地球化学的研究，获得了意大利Gorgo a Cerbara 剖面大洋缺氧事件（OAE1a）向大洋红层（ORB1）转变的岩性特征、矿物组成变化和地层格架，建立了意大利Gorgo a Cerbara剖面OAE1a向ORB1转变的碳同位素地层学；赤铁矿的出现使过渡层之上的泥灰岩呈现红色。相对于伊利石含量，黑色页岩中的蒙脱石含量较OAE1a向ORB1转换层以及ORB1中均高，可能与较多的火山物质输入有关。采用反射光谱与多元线性回归相结合的方法对ORB1中的赤铁矿含量进行了测定，获得赤铁矿含量变化范围为0.03%至0.86%；透射电镜显微电子图像及电子衍射花样显示赤铁矿可能以纳米级的微晶形式存在于红层样品中；赤铁矿的形成主要受铁的来源、沉积时的氧化还原条件以及成岩作用的影响，这些因素也成为制约红层形成的关键因素。本课题组在多年研究的基础上，结合前人研究成果从矿物学的角度提出了大洋红层的成因模式，深化了对白垩纪时期古海洋和古气候系统的认识。