Calcium isotope fractionation during transport and recrystallisation processes in marine deep sea sediments

生物成因的海洋碳酸钙壳是地球历史上系统地球变化的重要档案，其元素和同位素组成可用于许多古环境代用指标。这些代理的可靠性不仅取决于主要因素，而且取决于保存，而保存又取决于部分溶解和再结晶等过程。在IODP 320/321考察期间获得的沉积物为整个新生代提供了保存完好的高分辨率档案；因此，它们是重要的古气候重建的基础。然而，尽管整体保存良好，孔隙水分析显示碳酸盐再结晶的迹象。钙（Ca）的同位素组成是研究海洋沉积物成岩过程（包括再结晶）的工具。本研究的主要目的之一是利用Ca同位素测定IODP Exp. 320/321期间钻探的碳酸盐沉积物的再结晶速率，以评估代理档案的保存情况。在多个钻孔位置，孔隙水和相应沉积物的钙同位素组成呈现出特征剖面。该项目获得的初步结果表明，除了再结晶外，孔隙水中Ca的扩散也起着重要作用，在模拟再结晶速率时必须考虑到这一点。扩散输运过程中的同位素分馏是一个尚未完全描述的过程。在正在进行的项目中，我们将进行实验来量化在沉积物扩散过程中依赖质量的同位素分馏。结果将纳入再结晶速率的确定，并将有助于避免建模过程中的系统误差。该项目的最终目标是有助于更好地了解海洋沉积物孔隙水的过程。

Biogenic marine calcium carbonate shells are important archives for changes in the System Earth through Earth history, as their element and isotopic composition is used for many paleo-environmental proxies. The reliability of those proxies depends not only on primary factors but also on the preservation, which in turn depends on processes like partial dissolution and recrystallization.Sediments retrieved during IODP Expeditions 320/321 provide well preserved high resolution archives throughout the Cenozoic; they are hence the basis for important paleoclimate reconstructions. Nevertheless, despite the overall good preservation, porewater analyses revealed indications for carbonate recrystallization. A tool for the investigation of diagenetic processes in marine sediments, including recrystallization, is the isotopic composition of calcium (Ca). One of the main aims of this study is to use Ca isotopes to determine recrystallization rates of the carbonate sediments drilled during IODP Exp. 320/321 to evaluate the preservation of the proxy archives. At several drill locations, the Ca isotope composition of the porewater and corresponding sediment shows characteristic profiles. First results obtained in the project indicate that besides recrystallisation, diffusion of Ca in the porewater plays an important role and has to be considered when modelling recrystallization rates. Isotope fractionation during diffusive Ca transport is a process that has not been fully characterised yet. During the ongoing project we will conduct experiments to quantify mass dependent isotope fractionation during diffusion through sediments. The results will be incorporated in the determination of recrystallization rates and will help to avoid systematic errors during modelling. The final goal of the project is to contribute to a better understanding of processes in porewaters of marine sediments.