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

生物成因的海洋碳酸钙贝壳是地球历史中地球系统变化的重要档案，因为它们的元素和同位素组成被用于许多古环境代理。这些代用指标的可靠性不仅取决于原生因素，还取决于保存情况，而保存情况又取决于部分溶解和重结晶等过程。在IODP 320/321考察期间获得的沉积物提供了整个新生代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重结晶的迹象。钙的同位素组成是研究海洋沉积物成岩过程（包括重结晶）的一个工具。本研究的主要目的之一是利用Ca同位素来确定在IODP Exp期间钻探的碳酸盐沉积物的重结晶速率。320/321评估代理档案的保存情况。在几个钻孔位置，孔隙水和相应的沉积物的钙同位素组成显示出特征剖面。在该项目中获得的第一个结果表明，除了重结晶，钙在孔隙水中的扩散起着重要的作用，必须考虑建模时重结晶率。同位素分馏扩散钙运输过程中是一个过程，尚未得到充分的特点。在正在进行的项目中，我们将进行实验，以量化质量依赖的同位素分馏过程中通过沉积物扩散。结果将纳入再结晶速率的测定，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促进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.