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 The tectonic migration of ocean floor led to a transition from an organic carbon-starved to an elevated organic carbon burial environment  Diagenetic transformation of iron oxides into pyrite within the carbon-lean sediments occurred several millions of years after deposition Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

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Dissimilatory iron reduction plays an important role in iron cycling in reducing environments (e.g., Lovley, 1991). Detrital and authigenic iron oxides are used by microbes as electron acceptors to oxidize organic matter, releasing dissolved iron (Fe 2+ ) into porewater. In marine environments, early formation of authigenic iron oxides results from Fe 2+ diffusion into the overlying oxic and nitrogenous zones. In underlying sulfidic environments, dissolved iron and iron-bearing solid phases react with dissolved sulfide (HS − /H 2 S) to form iron sulfides. HS − /H 2 S results from organoclastic sulfate reduction associated with organic matter decomposition (e.g., R. A. Berner, 1981;Jørgensen et al., 2019) and microbial anaerobic oxidation of methane (AOM) above and at the sulfate-methane transition (SMT) zone (e.g., Knittel & Boetius, 2009;Treude et al., 2005). These processes strongly affect iron-bearing magnetic minerals, which affects the magnetic properties of sediments and sedimentary rocks (Roberts, 2015). Iron reduction and the subsequent incorporation of released Fe 2+ into iron sulfides, is an important mechanism responsible for magnetic mineral alteration. Destruction of detrital and biogenic ferrimagnetic minerals during diagenesis can destroy primary magnetic records. Limited exposure of sediments to dissolved sulfide can prevent pyritization (e.g., R. A. Berner, 1984;Canfield & Berner, 1987) and favor preservation of metastable iron sulfide precursors such as greigite (Kao et al., 2004). Greigite is ferrimagnetic and can carry a secondary magnetization that can poten-Abstract Diagenesis can have a major impact on sedimentary mineralogy. Primary magnetic mineral assemblages can be modified significantly by dissolution or by formation of new magnetic minerals during early or late diagenesis. At International Ocean Discovery Program Site C0023, which was drilled in the protothrust zone of the Nankai Trough during Expedition 370, offshore of Shikoku Island, Japan, non-steady state conditions have produced a complex sequence of magnetic overprints. Detailed rock magnetic measurements, which characterize magnetic mineral assemblages in terms of abundance, grain size, and composition, were conducted to assess magnetic mineral alteration and diagenetic overprinting. Four magnetic zones (MZs) are identified down-core from ∼200 to 1,100 m below sea floor based on rock magnetic variations. MZ 1 is a high magnetic intensity zone that contains ferrimagnetic greigite, which formed at shallow depths and is preserved because of rapid sedimentation. MZs 2 and 4 are low magnetic intensity zones with fewer magnetic minerals, mainly coarse-grained (titano-)magnetite and hematite. This magnetic mineral assemblage is a remnant of a more complex assemblage that was altered diagenetically a few million years after deposition when the site entered the Nankai Trough. MZ 3 is a high magnetic intensity zone between MZs 2 and 4. It contains authigenic single-domain magnetic particles that probably formed from fluids that circulat...

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