Magnetic properties and mineralogy in non-steady state diagenetic conditions: Study in IODP Expedition 341 Site U1417 marine sediments, Gulf of Alaska

In anoxic marine sediments, in steady state conditions, organoclastic sulfate reduction and anaerobic oxidation of methane release hydrogen sulfide to pore waters. Hydrogen sulfide, in turn, reacts with either solid-phase Fe(III) (oxyhydr)oxides or dissolved Fe²⁺ liberated by dissimilatory Fe(III) reduction to form iron sulfides with greigite as a precursor phase and pyrite as the end member of the reaction sequence. This process is characteristic for sulfate-methane transition zones (SMTZ). However, non-steady state conditions are common in marine sediments and the sequence described above may be disrupted. These conditions may lead to the preservation of early diagenetic greigite and/or to a late formation of greigite during burial enabled by reactivated biotic or abiotic processes. Site U1417 drilled during Integrated Ocean Drilling Program (IODP) Expedition 341 in the Gulf of Alaska has no shallow SMTZ and a deep inverse SMTZ at ~650 meters below sea floor, with a thick sulfate-free and methane-free zone above the deep inverse SMTZ, which makes non-steady state diagenetic conditions at this site unique. The deep inverse SMTZ is likely caused by tectonically-induced fluid circulation related to plate bending fractures. In this study, we aim to investigate how the inverse diagenetic zonation and related tectonically-induced fluid circulation impact the magnetic mineral assemblage and the paleomagnetic record in the sediment. We will pay special attention to the authigenic phases such as greigite which is responsible for secondary magnetizations. We will also assess the role of the diagenetic processes on the original detrital magnetic minerals, and their impact on the iron, sulfur, and carbon elemental cycles.