Diagram showing the formation and preservation of the iron sulfide greigite in bioturbidated anoxic sediments.
Model for the formation and preservation of the iron sulfide greigite in bioturbidated anoxic sediments. (a) Reductively dissolved Fe2+ from iron(oxy)(hydr)oxides (DIR) react with upwelling hydrogen sulfide (HS-) produced by organic sulfate (SO42-) reduction (OSR) to form iron sulfides (iron monosulfide: FeS, pyrite: FeS2, greigite: Fe3S4). (b) Intense bioturbidation (BI) transports organic matter downward. This mechanism fuels the production of all ingredients needed for further redox reactions. (c) The next mud-rich turbidite pulse quickly bury the previous turbidite bed, cuts down the downward oxygen flux and creates a closed system. As soon as Fe2+ exceeds HS-, the pyritization process is arrested and the intermediate iron sulfide greigite is preserved. (d) Accumulation of large amounts of coarse-grained greigite causes magnetic susceptibility peaks at the top of the turbidite bed. Credit: Yang et al. [2022], Figure 11
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

The strongly ferrimagnetic thiospinel of iron, greigite (Fe3S4), forms as a precursor of pyrite (FeS2) in reducing sediments and provides a crucial mineral for paleomagnetic and environmental magnetic studies. The most important factors for greigite formation and preservation is biologically degradable organic matter, reactive iron, and dissolved sulfate that can be biologically reduced to sulfide by sulfate-reducing microorganisms.

Mineral magnetic measurements in combination with electron microscopic observations taken by Yang et al. [2022] revealed the presence of very large greigite grains up to several tens of micrometers in submarine fan sediments from the Sumatra Subduction Margin cored by IODP Expedition 362. Such “giant” crystal sizes of greigite are exceptional compared to grain sizes below a few hundred nanometers, observed normally for this iron sulfide. In the cored sequence, the greigite is confined to multiple organic-rich mudstone horizons with high magnetic susceptibility. The authors propose that formation of greigite is fostered by successive organic-rich turbidite cycles that have been intensively reworked by benthic fauna.

This formation pathway may be an overlooked important iron-sulfur sink in carbon-rich turbidite beds and should be considered when assessing the marine iron-sulfur-carbon cycle.

Citation: Yang, T., Dekkers, M. J., Zhao, X., Petronotis, K. E., & Chou, Y.-M. (2022). Greigite formation modulated by turbidites and bioturbation in deep-sea sediments offshore Sumatra. Journal of Geophysical Research: Solid Earth, 127, e2022JB024734. https://doi.org/10.1029/2022JB024734

—Agnes Kontny, Associate Editor, Journal of Geophysical Research: Solid Earth

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