Magnetotactic bacteria produce biogenic magnetite in Fe-rich, low-O₂ aquatic environments in which these bacteria carry out ecological functions and move along geomagnetic field lines. The biomineralization products of magnetotactic bacteria have been found in sedimentary archives of different geological periods and have been called magnetofossils. The environmental conditions in which magnetofossils are produced have led to use these crystals as proxies for marine productivity shifts, O₂ concentration reductions and/or Fe availability changes in geological timescales; however, the occurrence of these processes is not necessarily synchronic, and some of them can affect magnetite preservation. For instance, enhanced export production in the ocean may produce magnetofossil dissolution due to microbial oxidant consumption in organic-rich environments. The specific controls of each environmental shift in magnetofossils formation and preservation remain elusive, and their understanding is critical to elucidate the paleoenvironmental significance of the biomineralization products of magnetotactic bacteria. Past global warming events offer an excellent alternative to analyze the driving mechanisms of magnetofossil formation and preservation, because of warming-induced accelerated nutrient/hydrological cycle conditions and widespread deoxygenation. Here, we present geochemical, rock magnetic and electron microscopy data from the Contessa Road section (Gubbio, Italy), which is an early Eocene marine sedimentary section with records of contrasting global warming events. Using well-developed geochemical proxies for export production, O₂ availability and nutrient supply, we identified that Fe variability drove magnetofossil formation at the Contessa Road setting, and indicate that export production and O₂ cannot be assessed using magnetofossil contents, at least at this location. These observations provide important information to improve the development of magnetofossil-based proxies, and suggest that the use of these crystals as paleoenvironmental indices should be accompanied by other datasets with a more reliable theoretical basis. 

magnetofossils,Fe availability,magnetotactic bacteria,paleoenvironments