Abstract
Magnetotactic bacteria produce chains of magnetite1,2 and/or greigite3,4,5 crystals within their cell bodies called magnetosomes that are permanently magnetized6. They use these magnets to navigate along geomagnetic field lines to reach their preferred habitat7. Greigite magnetosomes have been well documented in modern sedimentary environments, but their identification in the fossil record remains controversial. Here we use transmission electron microscopy, electron diffraction patterns and rockmagnetic analyses to assess the origins of nanometre-scale greigite crystals found in Pliocene claystones from the Carpathian foredeep of Romania. We find that, like modern magnetosomal greigite grains, the crystals are single domain8, with few crystallographic defects and an overall shape consistent with an intracellular origin. We suggest these crystals are magnetosomal in origin, which would place them among the oldest greigite magnetofossils identified so far. The crystals also carry a primary magnetic signal, which has remained stable since its acquisition 5.3–2.6 million years ago. We suggest that greigite magnetofossils could therefore provide reliable records of ancient geomagnetic field variations, and that they could also be used as a proxy to assess palaeoenvironmental conditions in low-oxygen sedimentary environments.
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Acknowledgements
This work was financially supported by the Netherlands Research Centre for Integrated Solid Earth Sciences (ISES) and the Netherlands Geosciences Foundation (ALW) with support from the Netherlands Organization for Scientific Research (NWO). J. Kirschvink is thanked for providing the new code for the single-domain stability field of greigite.
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I.V. initiated the project, undertook the analyses and provided the interpretation. C.F. and J.D.M. assisted and advised on TEM microscopy. C.G.L. assisted with the NRM analyses and the TK03.GAD correction. M.J.D. and W.K. advised and assisted throughout.
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Vasiliev, I., Franke, C., Meeldijk, J. et al. Putative greigite magnetofossils from the Pliocene epoch. Nature Geosci 1, 782–786 (2008). https://doi.org/10.1038/ngeo335
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DOI: https://doi.org/10.1038/ngeo335
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