Biogenic magnetite as an index for Fe availability

Magnetotactic bacteria produce biogenic magnetite in aquatic environments with reduced oxygen (O₂) and high Fe concentrations. Increased biogenic magnetite contents in geological archives have been associated with marine deoxygenation, Fe fertilization and productivity. However, these conditions, which depend on amplified nutrient supply to marine settings, enhance organic matter production and subsequent magnetic mineral dissolution due to reductive diagenesis. This suggests that the paleoenvironmental significance of biogenic magnetite content variability remains elusive, and more records are needed to clarify the mechanisms that control the abundance of magnetotactic bacteria biomineralization products in sedimentary records. Accelerated nutrient input from the continents to the oceans and reduced seawater O₂ concentrations are recurrent during global warming events due to temperature controls on both O₂ solubility and hydroclimate. This has motivated the generation of multiple early Eocene biogenic magnetite records, which based on rock magnetic and electron microscopy experiments, have related biogenic magnetite contents with productivity and seawater O₂ variability. Here, we present new geochemical and rock magnetic data from the Contessa Road section (Gubbio, Italy), which records a series of early Eocene global warming events. Our new data reveal that biogenic magnetite content variability cannot be directly used as an index for increased productivity and/or marine deoxygenation; alternatively, it can be seen an index for Fe availability in marine settings. Our observations indicate that biogenic magnetite does not exclusively reveal a specific process, which suggests that its content variability may depend on different local paleoenvironmental conditions.