Mechanisms of trace-metal incorporation in ferromanganese deposits: implications for reconstructing ocean history

锰铁（FeMn）结核是在130多年前的挑战者号探险期间首次发现的。从那时起，在世界各地的海洋中发现了大量的结核和结壳。结核（在海底）和结壳（在裸露的岩石露头上）主要是由数百万年来海水沉淀的铁（Fe）和锰（Mn）矿物夹层形成的。这些矿物是海水中痕量金属的极其有效的清除剂，因此，FeMn沉积物富含痕量金属，往往达到具有经济价值的水平。除了它们的经济价值外，科学家们目前还利用FeMn沉积物来重建海洋历史的各个方面。然而，科学家们也可以利用这些沉积物，通过测量被清除的微量金属浓度的变化，来提供关于过去海洋状况的更详细的信息。例如，钒(V)是一种微量金属，它以不同的状态存在，这取决于周围海水中的氧气水平。如果沉积物被发现含有还原态的V，科学家可以推断，在沉积物历史的这一点上，产生V的海水是低氧的——缺氧的——或者完全没有氧——缺氧的。这反过来又影响到微量金属在生物（生物）和非生物（地质）隔室之间的海洋循环，例如影响到海洋垂直混合的程度。然而，为了可靠地重建过去的海洋条件，V海水特征（记录了还原状态下的V）必须自最初的V加入以来保持不变。因此，科学家必须确定，当V被清除时，它被永久地锁定在FeMn沉积物中，也就是说，了解V是如何结合的至关重要。先进的光谱技术-使用同步加速器光-可以揭示FeMn矿床中痕量金属的不同状态（如还原V）和配位环境。在这样做的过程中，这些技术揭示了微量金属是如何被纳入FeMn矿床的。某些结合机制是可逆的，而另一些则是不可逆的。通过不可逆机制结合在FeMn结核中的痕量金属将永久锁定在结核中，因此，这些沉积物将忠实地记录该痕量金属的原始海水特征。尽管对FeMn沉积物进行了大量的科学研究，但先进的光谱技术从未被用来揭示一系列微量金属（有助于重建过去的海洋状况）是如何结合在一起的。因此，科学家们对微量金属的结合只有一种经验的理解，即他们知道一种特定的微量金属在一种特定类型的矿床中富集，他们知道这种微量金属是否与富铁或富锰相相关联，但不清楚这种富集是如何发生的，以及为什么某些微量金属更喜欢富铁或富锰的矿物。目前的认识水平还不足以确定海水痕量金属特征是否被永久记录下来。因此，我们可能会错过大量关于过去海洋状况的详细信息，而这些信息是无法通过其他方式确定的。这项工作将使用先进的光谱技术来揭示5种地球化学上重要的微量金属是如何被纳入FeMn矿床的。这将是第一个旨在揭示微量金属是否被永久清除到这些沉积物中的协调研究，因此，将提供关于天然FeMn沉积物中微量金属结合的基本新信息。此外，拟议的工作将开启FeMn沉积物可用于提供有关当地和全球海洋历史的丰富新信息的可能性。

Ferromanganese (FeMn) nodules were first discovered during the Challenger expedition over 130 years ago. Since then, nodules and encrustations have been found in abundance throughout the world's oceans. Nodules (on the seafloor) and crusts (on exposed rock outcrops) are predominantly formed of interlaminated iron (Fe) and manganese (Mn) minerals precipitated from seawater over millions of years. These minerals are extremely efficient scavengers of trace-metals from seawater and, as such, FeMn deposits are enriched in trace-metals, often at economically valuable levels. In addition to their economic value, scientists currently use FeMn deposits to reconstruct aspects of ocean history. Scientists might also, however, be able to use these deposits to provide more detailed information about past ocean conditions by measuring variations in the concentrations of the scavenged trace-metals. For example, vanadium (V) is a trace-metal that exists in different states depending on the level of oxygen in the surrounding seawater. If deposits are found that contain V in a reduced state, scientists can infer that at this point in the deposit's history the seawater that sourced the V was low in oxygen - suboxic - or completely without oxygen - anoxic. This in turn has implications for the oceanic cycling of trace-metals between biotic (biological) and abiotic (geological) compartments and, for example, for the extent of oceanic vertical mixing. However, in order to reliably reconstruct past ocean conditions, the V seawater signature, recording V in a reduced state, must be unaltered since the original V incorporation. Scientists must be sure, therefore, that V is permanently locked-in to FeMn deposits when it is scavenged, i.e., it is essential to know how V is incorporated. Advanced spectroscopic techniques - using synchrotron light - can reveal the different states (like reduced V) and the coordination environment of a trace-metal in a FeMn deposit. In doing so, these techniques reveal how trace-metals are incorporated in FeMn deposits. Certain mechanisms of incorporation are reversible, while others are irreversible. A trace-metal incorporated in FeMn nodules via an irreversible mechanism will be permanently locked into the nodules and, as such, these deposits will have faithfully recorded original seawater signatures of that trace-metal. Despite a vast number of scientific investigations into FeMn deposits, advanced spectroscopic techniques have never been used to reveal how a range of trace-metals (useful for reconstructing past ocean conditions) are incorporated. As such, scientists have only an empirical understanding of trace-metal incorporation i.e., they know that a particular trace-metal is enriched in a particular type of deposit and they know whether the trace-metal is found associated with the Fe-rich or Mn-rich phase, but it is unclear how this enrichment has occurred and why certain trace-metals prefer Fe- or Mn-rich minerals. This current level of understanding is not adequate to establish whether seawater trace-metal signatures are permanently recorded. As such, we are potentially missing out on a wealth of detailed information about past ocean conditions that cannot be ascertained by other means. This work will use advanced spectroscopic techniques to reveal how 5 geochemically important trace-metals are incorporated in the full range of FeMn deposits. This will be the first coordinated study designed to reveal whether trace-metals are permanently scavenged into these deposits and, as such, will provide fundamental new information on trace-metal incorporation in natural FeMn deposits. Moreover, the proposed work will open the possibility that FeMn deposits can be used to provide a wealth of new information about both local and global ocean history.