Understanding the role of microorganisms in the formation of ironsulfides in the sedimentary record (NERC EAO DTP)

The iron sulfide mineral greigite, Fe3S4, is found widely in nature, especially in sulfate-reducing diagenetic sedimentary environments. Its ferrimagnetic properties mean it is often the main carrier of palaeomagnetic and palaeo-environmental information used in polar wandering and climate change models covering the last 4M years of Earth history. It is therefore essential we understand how this critically important mineral forms in sediments but this is currently poorly constrained. Distinguishing primary, syn-diagenetic or secondary origin of Fe sulfides in sediments is essential to allow interpretation of this paleomagnetic record, but little is known about their formation and, in particular, the late diagnetic growth of greigite. Organic geochemical analyses of iron sulfide nodules containing greigite and pyrite (FeS2) collected from the upper Pliocene Valle Rica section in a clay mine North of Rome (Image 1.), indicate the presence of a microbial community during its formation similar to those found in cold seeps and sediments where anaerobic oxidation of methane (AOM) coupled to sulfate reduction plays a dominant role. The sulfide formed would have been available to react with iron oxides and dissolved Fe2+ which could explain the formation of the nodules. Considering that the migration of methane in marine sediments and the subsequent AOM process is widespread, these results suggest that the above process could be a dominant factor in the formation of secondary iron sulfides in all sediments and could have a profound impact on the associated paleomagnetic records.

In this project, the student will study the critical factors controlling the formation of iron sulfides in the sedimentary record, including concretions, nodules and nano-spheres, with particular focus on the role of micro-organisms in these processes. This will be achieved primarily by organic geochemical analyses combined with compound specific 13C isotope analyses of intact iron sulfide-bearing nodules/ concretions/ sediments from different environments to study the biomarker compositions present and the role of methane during the formation processes. The magnetic properties of the greigite will be studied using state-of-the-art SQUID magnetometry and synchrotron magnetic X-ray spectroscopy (XMCD, PEEM) to identify the nature of their magnetic signals and their relationship to mineral growth. The study of natural material will be complemented by lab experiments to demonstrate the bioprecipitation of the iron reducing bacterial strains and specific natural communities, and correlated with changes in microbial community structure and inorganic geochemistry. This will identify the critical links between methane metabolism and the biogeochemical cycling of iron and sulfate and sedimentary iron sulfide formation.

The student working on this cross-disciplinary project will gain a wide breadth of training in organic and inorganic geochemistry, microbiology and molecular biology and will have access to world-class facilities in the Williamson Research Centre for Molecular Environmental Science at the University of Manchester and the UK synchrotron facility; the Diamond Light Source. The techniques will provide a basis for a future career in environmental science, in the industrial, government or academic sectors, in a rapidly expanding research area of international importance.