https://doi.org/10.1130/G48069.1

Manuscript received 26 June 2020 Revised manuscript received 18 September 2020 Manuscript accepted 25 September 2020

© 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 10 December 2020

A novel authigenic magnetite source for sedimentary magnetization Zhiyong Lin1,2,3*, Xiaoming Sun1,3,4,5*, Andrew P. Roberts6, Harald Strauss2, Yang Lu7, Xin Yang1, Junli Gong1, Guanhua Li4, Benjamin Brunner8 and Jörn Peckmann7 1School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China 2Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Münster D-48149, Germany 3Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China 4School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China 5Southern Laboratory of Ocean Science and Engineering (Guangdong, Zhuhai), Zhuhai 519000, China 6Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia 7Institut für Geologie, Zentrum für Erdsystemforschung und Nachhaltigkeit, Universität Hamburg, Hamburg D-20146, Germany 8Department of Geological Sciences, The University of Texas at El Paso, El Paso, Texas 79902, USA ABSTRACT Sulfidic diagenetic environments occur com- We report a novel authigenic nanoscale magnetite source in marine methane seep sedi- monly within continental margin sediments with ments. The magnetite occurs in large concentrations in multiple horizons in a 230 m sediment high organic matter or methane fluxes (Jør- core with gas hydrate–bearing intervals. In contrast to typical biogenic magnetite produced by gensen, 1982; Boetius et al., 2000). Sulfidic magnetotactic bacteria and dissimilatory iron-reducing bacteria, most particles have sizes of conditions cause magnetite dissolution, which 200–800 nm and many are aligned in distinctive structures that resemble microbial precipitates. limits its geological preservation (Riedinger The magnetite is interpreted to be a byproduct of microbial iron reduction within methanic et al., 2005; Roberts, 2015). Typically, surface sediments with rapidly changing redox conditions. Iron sulfides that accumulated at a shallow magnetizations are depleted rapidly at the sul- sulfate-methane transition zone were oxidized after methane seepage intensity decreased. The fate-methane transition zone (SMTZ) where alteration process produced secondary iron (oxyhydr)oxides that then became a reactive iron hydrogen sulfide is released by sulfate-driven source for magnetite authigenesis when methane seepage increased again. This interpretation anaerobic oxidation of methane (Boetius et al., is consistent with 13C depletion in coexisting carbonate nodules. The authigenic magnetite will 2000; Jørgensen et al., 2004), which promotes record younger paleomagnetic signals than surrounding sediments, which is important for pa- magnetite dissolution (Riedinger et al., 2005; leomagnetic interpretations in seep systems. The microbial and possibly abiotic processes that Roberts, 2015). In contrast, we document here caused these magnetic minerals to form at moderate burial depths remain to be determined. abnormally high magnetic susceptibilities at multiple inferred SMTZs from a methane seep INTRODUCTION (MTB) and the other by dissimilatory iron-reduc- site that are due to abundant, well-preserved Magnetic signals preserved in sediments pro- ing bacteria (DIRB) (Moskowitz, 1995; Roberts, magnetite nanoparticles. Magnetic, mineralogi- vide fundamental information for ancient tecton- 2015). Intracellular magnetite produced by MTB cal, and geochemical analyses are presented to ic, geomagnetic field, and environmental recon- has well defined sizes, morphologies, chain ar- reveal the nature of these particles. structions. Sedimentary magnetic signals have rangements, and stoichiometries (Devouard traditionally been thought to be dominated by et al., 1998; Kopp and Kirschvink, 2008). The MAGNETIC SIGNALS RECORDED IN detrital magnetic iron-oxide particles, while bio- magnetic nanoparticulate remains of MTB are METHANIC SEDIMENTS genic magnetite with magnetically ideal stable preserved post-mortem as magnetofossils and The study site GMGS2-16 is situated on the single-domain (SD) properties has proven more are found in diverse sedimentary environments passive continental margin of the northern South recently to be a significant recorder of strong and (Chang and Kirschvink, 1989). In contrast, ex- China Sea (Fig. 1), which contains large basins stable sedimentary remanences over geological tracellular authigenic magnetite produced by with thick sedimentary sequences that have been time scales (Chang and Kirschvink, 1989; Kopp DIRB (Lovley et al., 1987) is thought to have deformed by movement along tectonic linea- and Kirschvink, 2008; Roberts et al., 2012). sizes <20 nm in diameter (Li et al., 2009) with ments (McDonnell et al., 2000). Abundant meth- There are two main pathways for biomineraliza- magnetically unstable superparamagnetic prop- ane-derived carbonates and gas hydrates (Fig. 1) tion of ultrafine biogenic magnetite in sediments, erties (Moskowitz et al., 1993). Although dis- confirm that methane seepage occurs widely in one of which is used by magnetotactic ­bacteria similatory iron reducers occur widely in anoxic the study area (see the Supplemental Material1 subsurface sediments, geological preservation for materials and methods). *E-mails: [email protected]; eessxm@ of extracellular magnetite has been documented Mass magnetic susceptibility (χ) of sedi- mail.sysu.edu.cn only rarely (e.g., Roberts, 2015). ments in core GMGS2-16 has large variations

1Supplemental Material. Geological background, methods, and further morphologies of iron (oxyhydr)oxides and FORC diagrams (Figures S1–S3). Please visit https://doi.org/10.1130/GEOL.S.13150895 to access the supplemental material, and contact [email protected] with any questions.

CITATION: Lin, Z., et al., 2021, A novel authigenic magnetite source for sedimentary magnetization: Geology, v. 49, p. 360–365, https://doi.org/10.1130/G48069.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/360/5252911/360.pdf by guest on 01 October 2021 Figure 1. Location of study site GMGS2-16 in the South China Sea (left), and a seafloor bathymetric map of the locations of five gas hydrate- bearing cores (right) (after Sha et al., 2015). BSR—bottom simulating reflector.

(Fig. 2A). Peak χ values (>2 × 10−4 m3 kg−1) kg−1 in intervals without magnetic aggregates. ments. FORC diagrams for low-χ samples reveal occur in multiple layers with nodular magnetic To characterize the magnetic domain state and a low-coercivity component and a magnetostati- mineral aggregates (Figs. 3A and 3B; Fig. S1 magnetostatic interactions of magnetic particles, cally interacting higher-coercivity SD component in the Supplemental Material). In contrast, χ first-order reversal curve (FORC) measurements (Fig. 2E1; Figs. S2A–S2C); the latter is typical of has low and constant values of ∼1.0 × 10−4 m3 (Pike et al., 1999) were produced for bulk sedi- greigite-bearing sediments that have experienced

A B C D E1 F

E2

E3

E4

Figure 2. Magnetic, geochemical, and mineralogical data for core GMGS2-16 in the South China Sea (mbsf—meters below seafloor; GH—gas hydrate). (A) Magnetic susceptibility (χ) of bulk samples. (B) Magnetic extract as a percentage of bulk sediment. (C) Carbonate nodules and their δ13C values (VPDB—Vienna Peedee belemnite). (D) Sedimentary methane concentrations (Sha et al., 2019). (E) First-order reversal curve (FORC) diagrams for bulk samples. FORC diagrams were processed using the FORCsensei algorithm (Heslop et al., 2020), which was used to search 1350 FORC models using all combinations of VARIFORC smoothing parameters (Egli, 2013) to produce optimal FORC distributions in which noise is smoothed without over-smoothing the underlying signal (https://forcaist.github.io/FORCaist.github.io/). Optimal VARIFORC

parameters for each diagram in this figure are (1) sc,0 = 4, sc,1 = 4, su,0 = 2, su,1 = 3, λ = 0.08, and ψ = 0.25; (2) sc,0 = 3, sc,1 = 3, su,0 = 2, su,1 = 3, λ = 0.08, and ψ = 0.71; (3) sc,0 = 2, sc,1 = 2, su,0 = 2, su,1 = 2, λ = 0.08, and ψ = 0.53; and (4) sc,0 = 2, sc,1 = 2, su,0 = 3, su,1 = 3, λ = 0.12, and ψ = 0.64. (F) X-ray dif- fraction analysis of selected magnetic extracts from different depths. Values shown on the right indicate the depth in core and diagrams at the bottom represent the powder X-ray diffraction peaks of the three minerals.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/360/5252911/360.pdf by guest on 01 October 2021 A B C

D E F

G H I

J K L

Figure 3. Morphologies and structure of magnetite particles in core GMGS2-16 in the South China Sea. (A,B) Polished thin section that reveals a paragenetic sequence of magnetite (M), goethite (G), and hematite (H) (backscattered electron images). Yellow rectangle in A is the area of B. (C,D) Aggregates of magnetite particles (scanning electron microscopy [SEM] images). (E,F) Magnetite particle alignments (SEM images). (G) Spherical particles (SEM images). Yellow arrows indicate smaller crystals. (H,I) Clustered euhedral crystals (SEM images). Yellow arrows indicate twin boundaries. (J) Aggregates of magnetite particles with various sizes (scanning transmission electron microscope [STEM] image). Yellow arrows indicate small single crystals, and inset is selected area electron diffraction pattern. (K,L) Lattice fringes for individual single magnetite crystals in high-resolution TEM images.

diagenetic sulfidization and magnetite dissolution X-ray diffraction (XRD) analysis reveals euhedral crystals (Figs. 3H and 3I). Both par- (Roberts et al., 2018). In contrast, similar FORC that magnetite is the sole magnetic phase in the ticle types also occur as smaller nanocrystals diagrams were produced for samples with peak χ magnetic mineral extracts (Fig. 2F). This was (Figs. 3G–3I) with single-crystal sizes ranging values (Figs. 2E2–2E4; Fig. S2D), which indicate confirmed by observations of clustered ultrafine from 10 to 20 nm (Figs. 3J–3L). the presence of stable SD particles with strong particles within the magnetite aggregates, which magnetostatic interactions and/or vortex-state range mainly from 200 to 800 nm in size (Fig. 3; NATURE OF THE MAGNETITE particles (see Roberts et al. [2017] for signatures Fig. DR3). Rarely, aligned magnetite particles NANOPARTICLES of vortex-state particles). Equidimensional SD were found (Figs. 3E and 3F; Figs. S3F–S3H), Magnetite in marine sediments generally magnetite has sizes in the ∼20–75 nm range, and which differ from magnetofossil chains and are originates from detrital inputs from land or as an vortex-state particles have sizes in the hundreds more similar to microbially formed structures authigenic mineral that forms during diagenesis of nanometers range (Muxworthy and Williams, (e.g., Johannessen et al., 2020). Individual par- (Roberts, 2015). The ultrafine, well-crystallized, 2009). ticles are mainly spherical (Fig. 3G) or clustered and aggregated nature of the studied particles

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/360/5252911/360.pdf by guest on 01 October 2021 Figure 4. Simplified sce- A B C nario for how magnetite precipitates in methanic sediments under the influ- ence of variable seepage intensities. (A) During high seepage activ- ity, the sulfate-methane transition zone (SMTZ) is located near the sedi- ment surface where chemosymbiotic fauna occur. Within the SMTZ, sulfate-driven anaero- bic oxidation of methane results in carbonate and iron-sulfide mineral precipitation. (B) After decline in seepage activ- ity, downward-moving oxidizing fluids promote iron-sulfide mineral oxida- tion at the former SMTZ, which leads to second- ary iron (oxyhydr)oxide formation. (C) When secondary iron (oxyhydr)oxides are exposed to methanic environments (see text for details), microbial iron reduction leads to authigenic magnetite formation. White circles indicate methane seepage flow.

(Fig. 3) excludes a detrital origin and indicates (Riedinger et al., 2005; Roberts, 2015); the un- Magnetite authigenesis has not been identi- that the magnetite nanoparticles formed authi- protected magnetite nanoparticles would be par- fied previously in methanic sediments, although genically within the sediment, through either ticularly unstable (Li et al., 2009) due to their high microbial iron reduction is observed common- biologically mediated or inorganic mechanisms. surface area and reactivity to hydrogen sulfide ly in similar sedimentary settings (Egger et al., Magnetofossils are a common source of ultrafine (Roberts, 2015). Abundant euhedral magnetite 2014; Riedinger et al., 2014; Amiel et al., 2020). magnetite in sediment, which occur as single in such a sulfidic environment is puzzling and Microbial iron reduction in methanic zones can crystals with perfect morphology (Kopp and indicates that magnetite formation postdated most be driven by (1) DIRB outcompeting methano- Kirschvink, 2008). The clustered structures of of the carbonate and pyrite formation at paleo- gens for organic substrates (Thamdrup, 2000), the studied magnetite particles, including crystal SMTZs, after the environment became hydrogen (2) methanogens that switch from methano- twinning (Figs. 3H and 3I), exclude an origin sulfide limited. genesis to iron reduction with an unidentified as intracellular magnetite produced by MTB. In We propose the following scenario for mag- electron donor that does not appear to be meth- contrast, extracellular magnetite produced by netite authigenesis driven by microbial iron re- ane (Sivan et al., 2016), or (3) iron reduction DIRB has irregular shapes without high struc- duction within methanic sediments that undergo coupled to anaerobic oxidation of methane (Beal tural perfection and normally ranges in size from dynamic methane seepage changes (Fig. 4). Verti- et al., 2009; Egger et al., 2014). Extracellular 10 to 50 nm (Li et al., 2009). Most of our identi- cal SMTZ movement and variable redox condi- titanomagnetite has been identified under nearly fied particles are much larger (as large as 800 nm tions occur commonly in this gas hydrate–bearing natural methanic conditions in culture with the in diameter), which makes them distinct from area (Z. Lin et al., 2016). In initial stages with archaeon Methanosarcina barkeri (Shang et al., the reported characteristics of extracellular mag- high methane fluxes, sulfide production would 2020). Based on this observation, microbially netite produced by DIRB. Thus, the nanopar- cause pyrite accumulation at a shallow SMTZ. driven magnetite authigenesis with iron (oxyhy- ticles appear to represent a new type of marine When seepage diminishes, downward-moving, dr)oxides as an electron acceptor (Fig. 3B; Fig. sedimentary magnetite. seawater-derived oxidizing fluids would pro- S1) could be feasible in methanic sediments. Carbonate nodules strongly depleted in 13C mote iron-sulfide mineral oxidation at the former Organic substrates for dissimilatory iron reduc- are distributed throughout core GMGS2-16 SMTZ, leading to secondary iron (oxyhydr)oxide tion are probably scarce when these sediments (Fig. 2C), and pyrite aggregates occur commonly (Fig. 3B; Fig. S1) and gypsum formation (Q. Lin are subjected to methanic conditions, which in the carbonate-bearing layers (Lin et al., 2018). et al., 2016). High porosity and permeability in suggests that coupling of iron reduction to an- These observations indicate the former presence coarse sediments (Chen et al., 2016) and advec- aerobic oxidation of methane is the most likely of locally pronounced sulfate-driven anaero- tive seawater transport due to convective fluxes process. Although the nature of iron reduction in bic oxidation of methane (cf. Jørgensen et al., at seeps (Aloisi et al., 2004) would facilitate sul- methanic sediments is not clear, the presence of 2004), likely coinciding with paleo-SMTZs (cf. fide mineral oxidation. An ensuing change from both microorganisms with iron reducing abilities Lin et al., 2018). Also, molybdenum and uranium oxic to methanic ­environments would then have and reactive Fe3+-bearing minerals is essential enrichments in carbonate nodules (Chen et al., been caused by a resurgence of high methane for the process to occur. Methanogenic and/or 2016), co-occurrence of bivalve shells, and low fluxes (Lin et al., 2018). Rapid sediment burial methanotrophic archaea are present throughout δ34S in pyrite (Lin et al., 2018) indicate that the (e.g., mass wasting; Wang et al., 2016) would the studied core (Cui et al., 2019). The presence carbonate formed close to the seawater-sediment also promote iron (oxyhydr)oxide preservation of iron (oxyhydr)oxides in methanic sediments interface in association with high methane flux during burial into a methanic environment. The at site GMGS2-16 would, thus, allow authi- (cf. Chen et al., 2016; Lin et al., 2018). Most of presence of wüstite (FeO; Fig. 2F) suggests a genic magnetite formation by microbial iron the identified authigenic magnetite accumulated sulfide-free Fe2+-rich environment (cf. Kolo et al., reduction. Irrespective of the lack of laboratory in such sulfide mineral–rich carbonate-bearing 2009) and also indicates rapid burial of reactive culture studies of magnetite formation mecha- layers. This is unusual because magnetite is ex- iron (oxyhydr)oxides without further alteration nisms in methanic sediments, the proposed sce- pected to be reduced in sulfidic environments by sulfidization. nario provides an explanation for why magnetite

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/360/5252911/360.pdf by guest on 01 October 2021 ­accumulates alongside wüstite within methanic Program provided by the China Postdoctoral Coun- rock record: Geobiology, v. 18, p. 31–53, https:// zones at levels coincident with paleo-SMTZs. cil (20180053). We thank Shengxiong Yang and doi.org/10.1111/gbi.12363. Studies of magnetism in methanic sedi- Guangxue Zhang for providing samples. 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ACKNOWLEDGMENTS placha.2013.08.003. Moskowitz, B.M., Frankel, R.B., and Bazylinski, This research was funded by the National Key Heslop, D., Roberts, A.P., Oda, H., Zhao, X., Harrison, D.A., 1993, Rock magnetic criteria for the detec- Research and Development Program of China R.J., Muxworthy, A.R., Hu, P.-X., and Sato, tion of biogenic magnetite: Earth and Planetary (grant 2018YFC0310004 and 2018YFA0702605), T., 2020, An automatic model selection-based Science Letters, v. 120, p. 283–300, https://doi. the National Natural Science Foundation of China machine learning framework to estimate FORC org/10.1016/0012-821X(93)90245-5. (grants 41806049, 41876038, and 91128101), the distributions: Journal of Geophysical Research: Muxworthy, A.R., and Williams, W., 2009, Critical Guangdong Special Fund for Economic Develop- Solid Earth: v. 125, p. e2020JB020418, https:// superparamagnetic/single-domain grain sizes in ment (Marine Economy, grant GDME-2018D001), doi.org/10.1029/2020JB020418. interacting magnetite particles: Implications for the China Geological Survey Project for South Johannessen, K.C., McLoughlin, N., Vullum, P.E., magnetosome crystals: Journal of the Royal So- China Sea Gas Hydrate Resource Exploration (grant and Thorseth, I.H., 2020, On the biogenicity of ciety: Interface, v. 6, p. 1207–1212, https://doi​ DD20160211), and the Australian Research Council Fe-oxyhydroxide filaments in silicified low-tem- .org/10.1098/rsif.2008.0462. (grant DP200100765). Zhiyong Lin acknowledges perature hydrothermal deposits: Implications for Pike, C.R., Roberts, A.P., and Verosub, K.L., 1999, the International Postdoctoral Exchange Fellowship the identification of Fe-oxidizing bacteria in the Characterizing interactions in fine magnetic

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