Integrated Approach to Investigate Magnetite Cycle in Marine Methanic Sediments

Magnetite (Fe3O4), a ubiquitous sedimentary iron mineral, is crucial for paleomagnetic records preservation. However, reactive ferric iron minerals, including magnetite, can undergo reduction in aquatic sediments above and within the sulfidic zone and at the Sulfate-Methane Transition Zone (SMTZ), resulting in the production of dissolved ferrous iron. Partial reoxidation of the reduced iron at the oxic-anoxic interface can lead to authigenic magnetite precipitation. Yet, magnetite persistence and behavior in deeper methanic sediments have remained poorly understood. Here we explore magnetite dynamics in different sub-methanic zones (deeper, middle and upper) of Mediterranean continental shelf sediments, including the potential for its authigenic precipitation. Sequential extractions revealed increasing magnetite concentrations accompanied by low-temperature magnetization (Verwey transition). First-order reversal curve (FORC) analyses supported nanoscale authigenic magnetite presence. The results highlight a net increase in single-domain magnetite in the middle methanic zone with declines in the upper and deeper zones. Microbial analyses pointed to iron reduction throughout the methanic zone, alongside potential precipitation of magnetite and a decline in methanogenesis functional genes. Sediment incubations with spiked 57Fe-ferrihydrite showed gross precipitation of isotopically enriched 57Fe-magnetite in the upper methanic zone. Our combined findings distinguish between gross and net magnetite precipitation, suggesting authigenic magnetite formation within the methanic zone, with reshaping and smoothing of the original magnetic signal. They also emphasize the limitations of relying on a single-method approach to unravel such complex processes. We propose that the methanic zone plays a critical role in the early diagenesis of magnetic minerals, driven by dynamic cycles of magnetite dissolution and authigenic precipitation.