Influence of hydroxamate siderophores on redox-sensitive trace elements and their isotopes and their relevance for the application of redox proxies in paleo-climate reconstruction studies

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In the natural environment, plants, microbes and fungi excrete a range of iron-specific organic chelators called siderophores. These molecules occur in almost all natural settings and are known to not only bind to Fe(III), but also have high complex stability constants with a range of other environmentally important trace metals, esp. to those that have a high charge and low ionic radius. These elements, i.e., Zr, Hf, the rare earth elements, Th, U and others, are referred to as high-field strength elements (HFSE) in geochemistry and are traditionally considered immobile during water-rock interaction unless a specific change in the oxidation state occurs, which may cause a different behavior of the redox-sensitive HFSE (i.e., U, Ce) and their isotopes in comparison to redox-insensitive HFSE. This observation is commonly used in paleoclimate studies to reconstruct past atmospheric oxygen levels. My pilot research indicates that the siderophore desferrioxamine B is able to bind to some of these redox-sensitive HFSE and enhances their mobility relative to the redox-insensitive HFSE during leaching from certain rock matrices. In the present proposal, I want to systematically investigate the potential impact of the DFOB siderophore on a range of redox-sensitive trace elements and their isotopes. In order to achieve this, I will conduct batch leaching experiments on natural rocks and I will also conduct scavenging experiments, both under oxic and anoxic conditions, in presence of siderophores as well as other organic ligands. The trace element and isotope studies will be complemented by XANES/EXAFS synchrotron radiation measurements to verify whether siderophores are actually able to oxidize redox-sensitive trace metals even under low oxygen conditions or whether other mechanisms cause the pronounced mobilization of redox-sensitive HFSE in presence of the DFOB siderophore. This study is filling an important research gap as the effects of extracellular biological compounds on these elements and isotopes in the natural environment are fairly unknown and it is also not clear whether the potential presence of such biogenic compounds may set limitations on the application of certain paleoredox proxies and whether intense biological activity during or after sediment deposition could lead to false-positives in paleoclimate reconstruction studies. The results of this project may also provide an idea on whether certain redox proxies, under certain circumstances, might indicate the presence of ‘life’ rather than the presence of oxygen, e.g., in the Meso- or Neoarchean. To achieve this goal, I will be collaborating with internationally renowned experts in the fields of trace element and isotope geochemistry and x-ray absorption spectroscopy.