Biogenic and hydrogenous magnetic mineral formation in upward oxygenated sediments of the East Pacific Rise

The Clarion-Clipperton Zone is a deep-sea area in the eastern Pacific Ocean. Due to low sedimentation rates, the sediment cover in this region is thin and discontinuous. This allows low-temperature hydrothermal activity in the underlying crustal rocks, bringing oxygen and other solutes into the sediments from below. Therefore, the geochemical zonation in the deeper sediments is reversed. The sediment composition, mineral diagenesis and (bio)geochemical processes in this environment have not yet been studied in detail. This work, based on a combination of sediment-magnetic methods and geochemical analyses, investigates the mineralogy and the processes in these hydrothermally influenced sediments. The influx of oxygen from below supports the preservation of magnetic minerals in the sediments and enables reliable magnetic dating over several million years. Hydrothermal oxygen and chemoremanent overprinting have no significant effects. However, it was discovered that microaerophilic magnetotactic bacteria can live deep inside the hydrothermally oxygenated sediments, where no such bacteria have been expected in the past. The secondary biogenic magnetite produced by these bacteria accumulates locally and results in magnetic anomalies and overprinting. This discovery has far-reaching consequences for hydrothermally active deep-sea regions and, in particular, for ridge flank systems.