A survey of Fe-isotope fractionation in the marine biogeochemical cycle

尽管铁对浮游生物生产和全球碳循环具有重要意义，但对海洋中可溶铁的来源的量化很少，对铁的海洋生物地球化学循环的了解也很少。我们建议使用新的铁同位素系统来表征这个循环。这现在是可能的，因为在许多主要的生物地球化学过程中，铁同位素以一种特有的方式分馏。我们对沉积反应铁的研究表明，在早期海相成岩作用过程中，沉积反应铁具有系统的同位素趋势，并认为逸回海中的溶解铁具有独特的、非常轻的同位素组成。在陆相铁和成岩变质铁两种通量的简单情况下，海水的同位素组成应反映各自的混合比例。此外，过程反应物和产物的铁同位素组成取决于给定反应的完整性，因此可能表明特定生物地球化学过程发生的程度。我们试图在整个海洋生物地球化学循环中研究海洋铁同位素分馏。大部分的研究将在波罗的海进行，它包括海洋铁循环的所有主要方面，并且仅由两个溶解铁来源主导。波罗的海的高铁负荷将保持样本规模可控，并支持方法开发。铁同位素分馏将在河口混合、海洋颗粒形成和海洋成岩作用以及水柱中进行研究，目的是开发一个适用于公海的工具箱。

Sources of soluble Fe to the ocean are poorly quantified and the marine biogeochemical cycle of Fe is poorly understood despite the importance of Fe for plankton production and the global carbon cycle. We propose to use the novel Fe isotope system to characterise this cycle. This is now possible because Fe isotopes fractionate in a characteristic way during many of the principal biogeochemical processes. Our previous study of sedimentary reactive Fe shows a systematic isotopic trend during early marine diagenesis and suggests that the dissolved Fe escaping back into the sea has a unique and very light isotopic composition. In the simple case of two fluxes, continental Fe and diagenetically modified Fe, the isotopic composition of seawater should reflect the respective mixing proportions. In addition, the Fe isotopic composition of process reactant and product depends on the completeness of a given reaction and may therefore indicate to what extent a particular biogeochemical process takes place. We seek to investigate marine Fe isotope fractionation in the full marine biogeochemical cycle of Fe. Much of the study will be done in the Baltic Sea, which comprises all principal aspects of the marine Fe cycle and is dominated by only two sources of dissolved Fe. The high Fe load of the Baltic Sea will keep sample sizes manageable and support method development. Fe isotope fractionation will be investigated during estuarine mixing, marine particle formation and marine diagenesis and in the water column with the aim to develop a toolbox applicable to the open ocean.