Influence of Early Low‐Temperature and Later High‐Temperature Diagenesis on Magnetic Mineral Assemblages in Marine Sediments From the Nankai Trough

Abstract

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...

Greigite Formation Modulated by Turbidites and Bioturbation in Deep‐Sea Sediments Offshore Sumatra

“…Greigite is considered to be metastable but has been reported to be generated and preserved in diverse environmental settings with ages ranging from the Ediacaran to the Holocene, although the greigite could be younger than its host sediment. Environments include organic‐rich lacustrine and marine sediments, restricted anoxic basins, gas hydrate systems, and hydrothermal/cold seep environments (e.g., Badesab et al., 2020; Dewangan et al., 2013; Dong et al., 2013; Greve et al., 2021; Horng, 2018; Housen & Musgrave, 1996; Kars, Greve, & Zerbst, 2021; Kars, Köster, Henkel, et al., 2021; Kelder et al., 2018; Krs et al., 1990; Larrasoaña et al., 2007; Reynolds et al., 1994; Roberts et al., 1996; Sagnotti & Winkler, 1999; Snowball, 1991; Vasiliev et al., 2007, 2008).…”

“…Thermal maturation due to burial typically drives progressive diagenesis: the continual loss of fine-grained magnetite and gradual pyritization of greigite. These processes are argued to occur from the subsurface to depths corresponding to a burial temperature of ∼60°C (e.g., Aubourg et al, 2012;Kars, Greve, & Zerbst, 2021;Kars, Köster, Henkel, et al, 2021;Kars et al, 2012Kars et al, , 2014Musgrave et al, 2019).…”

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

“…High‐pressure flows result in mechanically weak areas (∼800–1050 mbsf) beneath the décollement zone (Heuer et al., 2020; Hirose et al., 2021), where Fe‐ and Mn‐rich hydrothermal fluids intrude via lateral advection (LaRowe et al., 2020; Torres et al., 2015). Such environments favor Fe‐Mn (oxyhydr)oxide formation, which explains the elevated contents of Mn and Fe oxides between 800 and 900 mbsf (Figure S3 in Supporting Information ) (Kars et al., 2021; Torres et al., 2015). Meanwhile, hydrothermal fluids carry large amounts of trace metals (Wheat et al., 2002), which are, in turn, adsorbed onto the Fe‐Mn (oxyhydr)oxides.…”

“…The porewater sulfate and methane profiles display a reverse SMTZ sulfides (Figure 2a, Figure S4 in Supporting Information S1; Heuer et al, 2020;Kars et al, 2021;Köster et al, 2021), where iron oxides are typically transformed into Fe sulfides (e.g., Riedinger et al, 2005Riedinger et al, , 2017. During this alteration, previously adsorbed Mo is released into porewater (Figure 4).…”

“…To understand postdepositional Mo behavior and the use of Mo‐based proxies for paleoredox reconstruction, we investigate deep sediments from the International Ocean Discovery Program (IODP) Expedition 370 Site C0023, which have undergone extensive early diagenesis (Kars et al., 2021; Köster et al., 2021) and hydrothermal alteration (Tsang et al., 2020). By examining RSE contents and Mo‐isotope ratios of bulk sediments, along with porewater trace element concentrations, we aim to reveal factors that exert the first‐order control on the Mo redistribution and δ 98 Mo composition during postdepositional processes.…”