Greigite and the role of life in its formation - Sarah Slotznick

Presented by Sarah Slotznick (Dartmouth College). Despite being identified over sixty years ago, no consensus exists as to whether the ferrimagnetic mineral greigite (Fe₃S₄) is stable over geologic time or is an intermediate during the formation of pyrite. However, more and more studies highlight the importance of greigite in modern sedimentary systems, where it can cause complex sedimentary remagnetizations, but also be used as a marker for ancient environmental conditions, such as sulfate-to-methane transition zones. Many agree that greigite preferentially forms over pyrite in anoxic regions with low levels of sulfide relative to iron, but the detailed formation pathways of greigite are poorly understood, which makes untangling its environmental implications challenging. Greigite is biomineralized in magnetotactic bacteria, but has also been formed abiotically in laboratory experiments through the ripening of mackinawite, a non-stoichiometric, poorly ordered iron sulfide. Magnetic analyses and modeling suggest that biologically controlled crystals from magnetotactic bacteria have distinct magnetic properties compared to sedimentary and abiogenic greigite, providing a pathway to probe these distinct formation processes in natural settings. However, there are only rare reports of putative greigite magnetofossils with magnetic signatures that overlap those of magnetite magnetofossils, and the effects of diagenesis on them are poorly understood, in contrast to biogenic magnetite. To add further complexity, recent culturing experiments emphasize that, similar to magnetite, greigite can be biologically mediated through an indirect pathway, forming extracellularly from mackinawite precipitates around sulfate- and sulfur-reducing bacteria. These low-temperature studies emphasize that biological mediation forms greigite at significantly faster rates than abiotic experiments under similar conditions, although they have differing results about the importance of dead or inactive bacteria. Further work probing both freshwater and marine bacteria raises questions about the paradigm of sulfide’s importance and instead suggests that organic carbon abundance, iron abundance, and/or iron redox state play an important role in greigite formation. We present new magnetic results analyzing a set of these biologically mediated samples to probe the process of greigite formation by tracking increases in concentration (Ms) and changes in grain size using coercivity, Day plots, and first-order reversal curves (FORCs). Clear growth is noted on the week- to month-long timescale and is distinct from abiotic comparisons; measurements on the year-long timescale show a relative stabilization in grain size and potentially concentration. Notably, this clear growth trajectory occurs before greigite is identified using more traditional tools like powder X-ray diffraction, highlighting the value of magnetic methods for future work. The product of this laboratory greigite formation has grain-size characteristics similar to those observed in natural sedimentary greigite. Could the majority of natural greigite be biological in origin? To further explore this question, more research on greigite diagenesis is needed in laboratory and natural systems.