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

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.