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

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

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.

Publications

10 25 50
 
Description Fate and mobility of bioessential and redox sensitive trace-metals associated with marine ferromanganese precipitates were investigated with a view to using measurements of their concentration and stable isotope composition in ferromanganese deposits as new records of palaeocean chemistry and associated environmental processes. Work determined the molecular-level mechanisms controlling the sequestration of Ni, Cu and Tl to iron and manganese (hydr)oxides prevalent in marine ferromanganese precipitates, which determines the long-term stability of these metal signatures recorded in ferromanganese precipitates like crusts. Work spawned 6 peer-reviewed high impact publications in Geochimica et Cosmochimica Acta.
Exploitation Route Understanding the fate of Cu, Ni and Tl in industrial waste water streams released into the terrestrial near-surface environment, for example during accidental spill and subsequent remediation of Red Mud Leachates.
Sectors Environment

 
Description Fate and mobility of bioessential and redox sensitive trace-metals associated with marine ferromanganese precipitates were investigated with a view to using measurements of their concentration and stable isotope composition in ferromanganese deposits as new records of palaeocean chemistry and associated environmental processes. Work determined the molecular-level mechanisms controlling the sequestration of Ni, Cu and Tl to iron and manganese (hydr)oxides prevalent in marine ferromanganese precipitates, which determines the long-term stability of these metal signatures recorded in ferromanganese precipitates like crusts. Work has been used to predict the concentration of heavy metals in rivers and streams following the Ajkai Hungarian Red Mud environmental disaster in 2010. Work has also been used to understand the partitioning of bioessential metals between ancient seawater and iron (hhydr)oxides found in Banded Iron Formations, to help interpret the chemistry of ancient seawater and the associated Earth system, for example relating to the Great Oxidation Event some 2.3 billion years ago.
First Year Of Impact 2010
Sector Environment
Impact Types Societal,Economic

 
Description Partitioning of Ni into Banded Iron Formations 
Organisation University of Alberta
Country Canada 
Sector Academic/University 
PI Contribution Determining the fate and mobility of Ni sequestered to ancient Banded Iron Formations, based on our knowledge of Ni behaviour from this NERC grant, which might be used to interpret the chemistry of contemporaneous ancient seawater, related to the Great Oxidation Event soem 2.3 billion years ago.
Collaborator Contribution Synchrotron x-ray absorption spectroscopy to determine the crystal-chemistry of Ni in Banded Iron Formations and associated experimentally produced iron (hydr)oxide minerals.
Impact Paper submitted to Chemical Geology: Robbins L.J., Swanner E.D., Lalonde S.V., Eickhoff M., Paranich M.L., Reinhard C.T., Peacock C.L., Kappler A., Konhauser K.O. (2014) Limited Zn and Ni mobility during simulated Iron Formation diagenesis. Submitted to Chemical Geology.
Start Year 2013