Low-temperature magnetic properties of deeply buried gas hydrate bearing sediments: a case study from IODP expedition 375 site U1519C in the Hikurangi subduction zone, New Zealand

The diagenetic pathways, mineral types, and products of magnetic minerals in gas hydrate-bearing sediments are closely linked to burial depth. During IODP Expedition 375, drilling at the northern Hikurangi margin recovered 83.09 m of core from the gas hydrate stability zone (518.4–640.0 m depth) at Site U1519C. This provides an exceptional opportunity to investigate progressive diagenesis and fluid-driven late-stage diagenesis in deeply buried gas hydrate-bearing sediments. We conducted low-temperature magnetic measurements on 13 samples from this interval, including: (i) Low-temperature cycling (LTC) cycles, (ii) Zero-field-cooled (ZFC) and field-cooled (FC) curves, (iii) Low-temperature hysteresis loops, and (iv) Low-temperature alternating current (AC) magnetic susceptibility. Using features such as low-temperature transitions and curve trajectory patterns, we determined the types, concentrations, and assemblages of magnetic minerals, analyzed the origins of magnetic particles. Key results reveal: (1) Deeply buried sediments exhibit notably low SIRM intensity, indicating scarce magnetic minerals dominated by superparamagnetic (SP) and single-domain (SD) particles. This indicates that the deeply buried sediments experienced extensive pyritization under sustained reducing diagenetic conditions; (2)Despite the dominance of SP signals in the low-temperature FC/ZFC curves, the observation of the Verwey transition at ∼118 K—a characteristic low-temperature phase transition stemming from magnetite’s structural transformation—provides definitive evidence for the presence of trace magnetite even at such depths (>580 mbsf); (3) A double Verwey transition (∼106 K and ∼118 K) was observed in some samples, which indicates the coexistence of biogenic magnetite and nearly stoichiometric magnetite; (4) Authigenic Greigite (Fe₃S₄), an intermediate product of pyritization (FeS₂), is detected. Some greigite likely exists as SP particles, while a low index of hysteresis parameters (D_JH) indicates limited contributions from stable SD greigite among ferrimagnetic minerals.This study provides low-temperature magnetic evidence for diagenetic processes affecting magnetic minerals in deeply buried gas hydrate-bearing sediments. It reveals partial magnetite preservation, greigite formation and transformation, and ultimate pyritization, offering new insights into magnetic mineral evolution in such environments.

1 Introduction

Gas hydrates are solid crystalline compounds formed when gas (primarily methane) and water combine under high-pressure and low-temperature conditions. Authigenic magnetic iron sulfides in marine sediments are strongly associated with gas hydrate occurrence. Moreover, their presence may represent a key indicator for identifying hydrate-bearing zones. Numerous studies have investigated magnetic mineral variations in shallow sediments from gas hydrate-bearing zones. Housen and Musgrave (1996) conducted detailed rock magnetic analyses on cores from ODP Leg 146 Sites 892 and 889/890 along the Cascadia margin. This work showed that hydrate-bearing sediments primarily contain fine-grained magnetic iron sulfides, whereas sediments adjacent to the upper and lower boundaries of the gas hydrate-bearing intervals contain mixtures of magnetite and iron sulfides. Larrasoaña et al. (2007) combined detailed mineral magnetic analyses and electron microscopy observations on gas hydrate-bearing sediments from ODP Leg 204 Sites 1244 and 1252. This work revealed micrometer-scale authigenic greigite and monoclinic pyrrhotite as the primary magnetic carriers in hydrate-bearing intervals. Within sulfide zones, anaerobic oxidation of methane (AOM) zones, and methane hydrate stability zones, these magnetic iron sulfides form as byproducts of microbially mediated reductive diagenesis. Kars and Kodama (2015) conducted a high-resolution rock magnetic study on 218 Pleistocene sediment samples from 70 to 110 m below seafloor (mbsf) in Hole C0008C, IODP Expedition 316, Nankai Trough. This work revealed three magnetic mineral assemblages: magnetite, greigite-pyrrhotite, and their mixtures. Sediments carrying the authigenic greigite-pyrrhotite assemblage were associated with gas hydrate occurrences. This study demonstrated the feasibility of using rock magnetic parameters to rapidly detect and quantify hydrate-related authigenic iron sulfides. In addition, the rock magnetic properties of sediments in gas hydrate-dominated systems have been extensively investigated using magnetic methods to understand the diagenesis of magnetic minerals, as reported in areas such as the Sea of Marmara (Yang et al., 2023) and the Bay of Bengal (Gaikwad et al., 2021; Gaikwad et al., 2022; Badesab et al., 2020). Collectively, these pioneering studies demonstrate that biogeochemical processes associated with gas hydrate formation generate authigenic magnetic iron sulfides with distinct, detectable magnetic signatures. Yet magnetic mineral evolution in deeply buried systems remains enigmatic. Increasing burial depth elevates geothermal gradients and enhances fluid flux, transitioning early reductive diagenesis in shallow sediments to progressive and fluid-driven late-stage diagenesis in intermediate-to-deep strata. This raises a critical question: How do diagenetic authigenic magnetic sulfides associated with gas hydrate occurrence evolve in shallow sediments? Two potential evolutionary pathways emerge: (1) During late diagenesis, hydrate formation at the base of sulfate reduction zones restricts H₂S concentration and pore-water exchange. This inhibits complete pyritization, favoring preservation and continued growth of authigenic greigite-monoclinic pyrrhotite from earlier diagenetic iron sulfides. (2) Alternatively, sustained microbial activity fueled by hydrates could supply continuous H₂S, dissolving earlier iron sulfides and driving complete conversion to paramagnetic pyrite with minor residual ferrimagnetic oxides. Determining the dominant pathway requires further investigation.

Low-temperature magnetism offers distinct advantages for detecting magnetic carriers in weakly magnetic sediments. With high sensitivity (10⁻⁹ emu), minimal thermal noise, and reduced electromagnetic interference, it excels at identifying biogenic single-domain magnetite (Moskowitz et al., 1993). Biogenic magnetite forms through synthesis by magnetotactic bacteria, microorganisms commonly found in reducing sediments and porewaters with high concentration of hydrogen sulfide (H₂S) (Bazylinski et al., 1991). Remanence measurements during temperature cycling reveal key magnetic mineral characteristics—including mineral species, grain size distribution, and oxidation state—providing insights into their origin and diagenetic history (Smirnov and Tarduno, 2000; Chang et al., 2013). Roberts et al. (2011) applied low-temperature magnetometry to tectonically uplifted sediment samples from the Neogene marine sequence of the Hikurangi Margin. The authors identified a monotonic and rapid decrease in saturation remanence between 0 and 50 K in the zero-field-cooled (ZFC) and field-cooled (FC) curves and demonstrated that it was caused by superparamagnetic (SP) greigite. Applying low-temperature magnetic methods to hydrate-bearing sediments (0–492 mbsf) at IODP Site U1518, Kars et al. (2021) precisely identified magnetite and found indirect evidence for greigite. They further demonstrated how rock magnetic parameters can rapidly quantify key properties of authigenic iron sulfides—including concentration and grain size—in hydrate-associated sediments.

During IODP Expedition 375 at the northern Hikurangi margin, drilling at Site U1519C recovered Miocene turbiditic silts from 518.4 to 640 mbsf. We characterized the magnetic mineralogy and grain-size distribution of the deeply buried gas hydrate stability zone by conducting the first systematic low-temperature magnetic measurements on 13 samples. The methodology comprised (i) low-temperature cycling (LTC) cycles, (ii) ZFC and FC curves, (iii) low-temperature hysteresis loops, and (iv) low-temperature alternating current (AC) susceptibility. All measurements were conducted following standard low-temperature rock magnetic protocols. These analyses were performed using a Quantum Design MPMS XL-7 magnetometer at the Paleomagnetic and Rock Magnetic Laboratory of the University of Bremen, Germany. By integrating diagnostic features such as low-temperature transitions and curve trajectories, this study aims to identify magnetic mineral assemblages and their unique “magnetic fingerprints”. This study establishes the physical foundation for magnetic identification of hydrate-bearing strata and core magnetic logging techniques. Furthermore, these fingerprints, when preserved from paleo-gas hydrate activity, can serve as a scientific basis for recognizing fossil gas hydrate zones—sedimentary layers formed by the dissociation of ancient gas hydrate.

1.1 Study area

The Hikurangi subduction zone, situated in the South Pacific Ocean east of New Zealand (Figure 1A), features westward subduction of the Pacific Plate beneath the Australian Plate along the Hikurangi Trench at 4.5–5.5 cm/yr (Barnes and Wu, 2019). The subducting Pacific Plate comprises the Hikurangi Plateau—a Cretaceous oceanic plateau dominated by mafic-ultramafic rocks—overlain by ∼1 km of Mesozoic-Cenozoic sedimentary cover. The northern margin hosts multiple seamounts on the subducting plate, creating an irregular subduction interface with localized frontal tectonic erosion features associated with seamount subduction.

Figure 1

Figure 1. (A) Bathymetry map showing seismic line 05 CM-04 (black) and IODP Expedition 375 drilling locations (red circles) off the East Coast of New Zealand’s North Island. Drilling targeted tectonic elements: the Papaku thrust fault at the accretionary wedge front (Site U1518), the forearc Tuaheni Basin (Site U1519), the leading edge of the subducting Turangahui Knoll (Site U1520), and its summit (Site U1526). Site U1519 is situated on the upper continental slope of the Hikurangi margin, ∼38 km offshore at ∼1000 m water depth within the landward portion of the mid-slope Tuaheni Basin. Modified from Saffer et al. (2019). (B) Interpreted seismic profile along line 05 CM-04 at Site U1519. VE = vertical exaggeration. Modified from Barnes et al. (2019).

IODP Expedition 375 (8 March - 5 May 2018) investigated slow-slip earthquakes through drilling operations at the northern Hikurangi margin. The study employed core sampling, and long-term borehole monitoring to characterize hydrogeological, geochemical, and physical processes. Drilling to 650 mbsf at Site U1519 revealed three major unconformity-bounded lithostratigraphic units (Table 1). Integrated shipboard analyses—including infrared thermal imaging of cores, porewater chloride measurements, bottom-simulating reflector (BSR) identification (Figure 1B)—confirmed the gas hydrate stability zone within the Miocene turbiditic silts (518.4–640.0 mbsf). Centimeter-scale hydrate layers were encountered at 587.25–587.32, 605.74–605.93, and 617.15–617.20 mbsf. Below the sulfate-methane transition zone (SMTZ), hydrates exhibit low saturation (<10%) and heterogeneous distribution patterns.

Table 1

Table 1. Summary of lithostratigraphic units encountered at Site U1519.

At Site U1519, the Late to Middle Pleistocene section (from 4.40 to 536.32 mbsf) is characterized by a high sedimentation rate of 0.84 m/ky, indicating rapid deposition (Barnes and Wu, 2019). Recent research in the Bay of Bengal has also reported that high sediment accumulation rates facilitate the rapid burial and preservation of detrital magnetic grains shortly after their deposition (Badesab et al., 2019; Badesab et al., 2022). In contrast, the age of the lower section (below 536.32 mbsf) is poorly constrained, and its sedimentation rate remains uncertain. Therefore, it remains unclear whether the magnetic minerals in the study area’s sediments are favorably influenced by rapid sedimentation for their preservation.

2 Materials and methods

2.1 Experimental overview

In this study, a suite of low-temperature magnetic measurements was employed to characterize the magnetic mineralogy of the sediment samples. These measurements include LTC cycles, FC/ZFC remanence, AC susceptibility, and low-temperature hysteresis experiments. This section briefly outlines the experimental procedures underpinning each technique. The interpretation of these measurements allows for the identification of magnetic phases (e.g., magnetite, greigite) and their domain states, which is crucial for understanding the magnetic signal recorded in the geological record.

2.2 Site description and drilling generalities

Drilling at Site U1519 employed both the Rotary Core Barrel (RCB) system for discrete intervals (108–163.6, 250–288.4, and 520–640 mbsf in Hole U1519C) and the Advanced Piston Corer (APC) system which continuously cored from seafloor to 85.8 mbsf in Holes U1519D and U1519E. Thirteen sediment core samples were collected from the 587–618 mbsf interval in Hole U1519C (Table 2).

Table 2

Table 2. Rock magnetic parameters of samples from different depths.

2.3 Low-temperature magnetic measurements

Low-temperature rock magnetic experiments, including FC, ZFC, and LTC measurements, were conducted on all thirteen sediment core samples from the 587–618 mbsf interval in Hole U1519C. All measurements employed a Quantum Design MPMS XL-7 magnetometer at the University of Bremen’s Paleomagnetism Laboratory.

For the FC measurements, samples were cooled from 300 K to 5 K in a 5 T field to impart a saturation isothermal remanent magnetization (SIRM), after which the remanence was recorded during zero-field warming to 300 K. For the ZFC protocol, samples were first cooled to 5 K in zero field, then subjected to a 5 T pulse to impart a SIRM, with remanence subsequently measured during warming in zero field. The LTC procedure involved magnetizing samples at 300 K in a 5 T field, cooling them to 5 K in zero field, and then measuring remanence during stepwise warming to 300 K in 2 K increments.

A subset of nine samples was selected for further analysis. AC susceptibility was measured on this subset at frequencies of 1, 10, and 111 Hz during warming after zero-field cooling to 5 K. Hysteresis loops were also acquired for the same nine samples at temperatures of 5, 70, 130, and 300 K by sweeping the magnetic field between ±1 T.

2.4 Shipboard data

Onboard measurements provided magnetic susceptibility (MS) and porewater geochemical data (Figure 2). Within the sampled interval (570–620 mbsf), MS values are consistently low (0–30 × 10⁻⁵ SI; Figure 2). No systematic depth-dependent trends were observed in porewater chemistry or MS. Discrete anomalies occur locally, most notably at 605–606 mbsf showing concurrent increases in MS, chloride (Cl⁻), and methane (CH₄), alongside decreased sulfate (SO₄^2-) concentrations.

Figure 2

Figure 2. Downhole data at Site U1519 showing: (A) magnetic susceptibility (MS), (B) chloride (Cl⁻) concentration, (C) iron (Fe) concentration, (D) sulfate (SO₄^2-) concentration, and (E) methane (CH₄) concentration. The blue shaded area indicates the sampling interval (587–618 mbsf). Core recovery and lithologic description are from Wallace et al. (2019).

3 Result

3.1 Low-temperature SIRM warming

Figure 3 presents low-temperature SIRM warming results for samples from site U1519C captured in this Expedition following FC and ZFC treatments. The FC curve consistently lies slightly above the ZFC curve (Figures 3A–C), indicating the absence of multi-domain (MD) magnetite (Housen and Moskowitz, 2006). Below 50 K, all curves exhibit rapid decay, characteristic of magnetic responses from SP particles (Tarduno, 1995). First-derivative analysis (10–40 K) reveals no significant peaks during warming. This absence of phase transitions precludes pyrrhotite (∼34 K) and siderite (∼37 K) within the 30–40 K interval (Frederichs et al., 2003). Convergence of ZFC and FC curves at ∼50 K (Figures 3A–C) may reflect Ti impurities in magnetite (Wang et al., 2021). Remanence losses exceeding 90% during warming confirm SP particles dominate the SIRM signal. No distinct transitions appear in raw FC/ZFC curves, including features characteristic of greigite. First-derivative magnification reveals subtle Verwey transitions at ∼106 K and ∼118 K (Figure 3D), indicating coexisting biogenic magnetite (∼106 K) and near-stoichiometric magnetite (∼118 K) (Chang et al., 2016a). Furthermore, the FC/ZFC curves coincide beyond the Verwey transition at 118 K, suggesting the absence of goethite; no evidence of the Morin transition (characteristic of hematite) was observed near 260 K (Liu et al., 2006).

Figure 3

Figure 3. Representative rock magnetic measurements: FC/ZFC curves for samples from Expedition U1519. (A–C) FC/ZFC curves and the first derivative of the FC curve. (D–F) Close-up view of the first derivative of the FC curve within the 95–130 K range. The blue shaded region highlights the peak corresponding to the Verwey transition, diagnostic of both biogenic and inorganic magnetite.

3.2 Low-temperature cycling of RTSIRM

LTC results reveal distinct magnetic behaviors. At 587.13 mbsf (Figure 4A), the LTC warming and cooling branches separate at ∼110 K, with no distinct Verwey transition. The relatively low remanence loss of ∼9% over the entire cycle indicates the presence of fine-grained, nanoscale magnetite particles (Özdemir et al., 2002). Upon cooling from room temperature (300 K) to 5 K, the remanence increased by a factor of 2.4. Subsequently, during warming from 5 K, the remanence decreased rapidly by ∼50% between 5 K and 50 K. The LTC behavior differed within the discrete anomaly zone (∼605 mbsf). Sample CUBE9503381 (Figure 4A) displayed more than double the SIRM intensity of the other two samples and exhibited reduced remanence decay between 5 K and 40 K. The warming and cooling curves separated above 100 K. A distinct hump appeared in the cooling curve between 150 K and 200 K, which was absent in the warming curve. This pattern is characteristic of nearly stoichiometric, SD magnetite grains (Özdemir et al., 2002). In contrast, according to Özdemir and Dunlop (2010), partially oxidized magnetite exhibits an irreversible hump in both cooling and warming cycles within the 150–200 K range. The overall remanence loss over the full LTC cycle was higher here, reaching ∼13%, consistent with the presence of coarser magnetite grains. Collectively, the LTC data indicate that samples from the main study area contain minor amounts of fine-grained SD magnetite. A significantly higher content of sub-micron single-domain (SD) magnetite were found exclusively within the discrete anomaly zone (∼605 mbsf).

Figure 4

Figure 4. (A–C) RTSIRM curves under a 5 T field. Blue and red circles represent cooling and warming data points, respectively. (D–F) Corresponding alternating current (AC) magnetic susceptibility for LTC samples at frequencies of 1, 10, and 111 Hz (colored curves). (G–I) Hysteresis loops measured at 300 K with a maximum applied field of 1 T.

3.3 Low-temperature AC susceptibility

AC magnetic susceptibility decreased rapidly and monotonically during warming for all samples, showing no discernible peaks (Figures 4D–F), confirming paramagnetic dominance through its characteristic temperature dependence and negligible ferromagnetic contribution. The in-phase susceptibility (χ′) exhibited pronounced temperature dependence across all measured frequencies (1, 10, and 111 Hz), decreasing by >90%. Crucially, χ′ displayed no frequency dependence, consistent with paramagnetic mineralogy (e.g., pyrite). As temperature increases, susceptibility gradually diverges at different frequencies. As proposed by Chang et al. (2013), this behavior occurs as some SD particles become gradually unblocked and exhibit SP behavior with increasing temperature. This effect was most pronounced in the 605.8 mbsf sample (Figure 4E). The frequency dependence of magnetic susceptibility, as determined by the frequency-dependent parameter (${\mathit{\chi }}_{\mathit{d}\mathit{f}}\left(\%\right)$), was calculated using Equation 1:

${\mathit{\chi }}_{\mathit{d}\mathit{f}}\left(\%\right)\left(=\mathbf{100}\times \left({\mathit{\chi }}_{\mathit{l}\mathit{f}}-{\mathit{\chi }}_{\mathit{h}\mathit{f}}\right)/{\mathit{\chi }}_{\mathit{l}\mathit{f}}\right)(1)$

For sample CUBE-9503381, measurements show ${\mathit{\chi }}_{\mathit{d}\mathit{f}}\left(\%\right)$ values of approximately 10% at 1–10 Hz frequencies, reaching ∼13% when comparing 1 Hz–111 Hz. These elevated values indicate significant concentrations of SP magnetic minerals (Kars et al., 2018).

3.4 Hysteresis loops

Prior to high-field slope correction, hysteresis loops exhibited negligible opening at low fields (Figures 4G–I), except for those within the discrete anomaly zone. Central loop width increased systematically with temperature (Figures 5A–D), confirming paramagnetic dominance consistent with AC susceptibility results (Wang et al., 2020). The enhanced paramagnetic signatures within the reducing gas hydrate stability zone are ascribed to pyrite formation through reductive dissolution and sulfidation of magnetite. The temperature-dependent variation of magnetization follows paramagnetic behavior, further confirming the predominance of paramagnetic minerals. The paramagnetic magnetization (M_p), as described by Curie’s law, was calculated using Equation 2:

${\mathit{M}}_{\mathit{p}}\left(\mathit{B},\mathit{T}\right)=\frac{\mathit{C}\mathit{B}}{{\mathit{u}}_0\mathit{T}}(2)$

where C is the Curie constant and μ₀ is the vacuum permeability (Paterson et al., 2018). When cooling from 130 K to 70 K, the uncorrected maximum magnetization increased by a factor of 1.92 (Figures 5B,C). The excess enhancement beyond theoretical predictions likely stems from magnetite’s Verwey transition near 118 K amplifying magnetization effects.

Figure 5

Figure 5. Hysteresis loops for sample CUBE9503381 at 605.8 mbsf under a 1 T applied field, showing raw and slope-corrected data at (A) 5 K, (B) 70 K, (C) 130 K, and (D) 300 K.

Most samples plot within the pseudo-single-domain (PSD) region on the Day-plot (Figure 6). This apparent distribution may reflect a known limitation of Day-plots: mixtures of SD and SP grains can artifactually shift data into the pseudo-single-domain (PSD) region (Tauxe et al., 2002; Pick and Tauxe, 1994). The data trend parallels theoretical SD + SP mixing curves, indicating that magnetic minerals exist predominantly as mixtures of SD and SP particles. The proportion of SP particles shows an increasing trend with depth.

Figure 6

Figure 6. Day-plot showing hysteresis parameter ratios for all samples (figure modified from Dunlop (2002)). The curves in the Day-plot represent SD-SP and SD-MD particle mixing trends, with volume fractions explicitly annotated for both the SP and MD components.

4 Discussion

4.1 Magnetite preservation mechanisms in deep gas hydrate systems

Site U1519, situated on the upper continental slope of the Hikurangi Margin, represents a sulfate-reducing setting where microbially produced hydrogen sulfide (H₂S) reacts with iron phases to 7s that contributes to extensive magnetite depletion in sulfidic marine sediments (Chang et al., 2016a). However, our low-temperature measurements reveal that deeply buried marine sediments below 580 mbsf show that while hysteresis loops and AC susceptibility curves both indicate dominant paramagnetic pyrite signatures, detectable Verwey transitions in first-derivative FC curves confirm persistent minor magnetite preservation. The surviving magnetite consists of nearly stoichiometric sub-micron particles. Day-plot analysis confirms these SD grains coexist with SP minerals (Figure 6). This suggests that preserved magnetite grains in deeply buried samples likely exist as a broad distribution of nanoscale particles. Although fine-grained magnetite typically undergoes preferential dissolution in deep-sea reducing environments, analyses of samples from Cretaceous seafloor basalts, mid-ocean ridge basalts, and the Oman margin (NW Arabian Sea) reveal that silicate-hosted magnetite inclusions provide effective physical protection against dissolution (Chang et al., 2016a; Pick and Tauxe, 1994; Smith, 1979). Chang et al. (2016b) identified signals indicative of magnetite inclusions within silicate particles in first-order reversal curve (FORC) diagrams of these samples. This confirms that silicate grains remain structurally stable in sulfide-rich environments, thereby protecting encapsulated magnetite from dissolution. Transmission electron microscopy (TEM) analysis further verifies biogenic magnetite within these samples (Figure 7). This inclusion mechanism accounts for the limited magnetite preservation observed below 580 mbsf.

Figure 7

Figure 7. High-resolution TEM and SAED analyses of titanomagnetite inclusions within a plagioclase host mineral. (a–l) Results from two distinct areas in sample “MD01-2421-7-110”. Images in (b–d) and (h) correspond to magnetic mineral (titanomagnetite) inclusions, while (e,f) and (i–l) correspond to the silicate host mineral (plagioclase). Clear lattice fringes for both phases are observed. Arrows in (g) indicate crystal twinning within a magnetic nanoparticle inclusion. Modified from Chang et al. (2016b).

4.2 Authigenic greigite preservation in deep-buried sediments

During the sulfate reduction stage, the transformation of iron oxides to pyrite proceeds through intermediate iron sulfide phases (Liu et al., 2014; Roberts et al., 2011). As documented by Larrasoaña et al. (2007) at two ODP Leg 204 sites on the Cascadia margin, gas hydrate-bearing sediments contain authigenic greigite and monoclinic pyrrhotite as the dominant magnetic carriers. Greigite preservation potential in deep sediments is well-documented and controlled by environmental parameters including methane concentration, dissolved Fe²⁺, and sulfate availability (Kars et al., 2021; Roberts et al., 2011).

Low-temperature measurements on deep-seated samples (>580 mbsf) from IODP Expedition 375 Site U1519 reveal uniformly weak SIRM across all specimens (Figures 3A–C). All samples contained significant proportions of SP minerals, as evidenced by pronounced monotonic changes in low-temperature FC/ZFC and LTC measurements that substantially suppressed the Verwey transition (Figures 3, 4). The frequency-dependent susceptibility (F_SP) parameter, an estimator of SP magnetic mineral content, was calculated using Equation 3:

${\mathit{F}}_{\mathit{S}\mathit{P}}=\frac{\left({\mathit{M}}_{\mathit{r}\mathit{s}}\left(5\right)-{\mathit{M}}_{\mathit{r}\mathit{s}}\left(300\right)\right)}{{\mathit{M}}_{\mathit{r}\mathit{s}}\left(5\right)}(3)$

where Mrs(T) denotes the saturation remanent magnetization measured at temperature T during FC measurement (Kars et al., 2017). While residual fields in MPMS magnetometers may overestimate F_SP in weakly magnetic samples (Roberts, 1995), our measurements consistently show high SP fractions (F_SP >90%) in deep sediments (Table 2), confirming SP dominance. The absence of a Verwey transition in SP magnetite at low temperatures, combined with greigite’s lack of characteristic magnetic transitions, complicates the discrimination of SP particle sources. However, studies from the Hikurangi subduction zone and vicinity report authigenic greigite occurring as broadly distributed SP particles (Moskowitz et al., 1993; Qian et al., 2021). Rowan and Roberts (2006) identified authigenic greigite—formed during diagenesis through magnetite dissolution—in tectonically uplifted Neogene marine sediments from the Hikurangi margin (Qian et al., 2021), confirming abundant SP greigite within these deposits. Subsequently, our FC curves closely match those reported by Roberts et al. (2011) for the same region, demonstrating a monotonic rapid decrease in saturation remanence (SIRM) between 5 and 50 K. This behavior was conclusively attributed to SP greigite (Liu et al., 2014). Low sulfate concentrations further inhibit pyritization, promoting greigite preservation (Kars et al., 2021). Dominant paramagnetic pyrite signatures in low-temperature measurements indicate pervasive pyritization throughout the reducing environment of deeply buried sampling zones. Below 580 mbsf at Site U1519C, low sulfate concentrations and reactive iron concentrations (Figure 2) restrict microbial H₂S production. This likely arrested greigite-to-pyrite conversion near cessation, explaining authigenic greigite preservation. These conditions support greigite as partially source of the observed SP signatures in FC/ZFC curves.

The contribution of single-domain (SD) greigite to ferrimagnetic assemblages, as determined by the D_JH parameter (Kars et al., 2021), was calculated using Equation 4:

$D_{JH}=\left(\frac{{\mathit{M}}_{rs}}{{\mathit{M}}_{\mathrm{s}}}\right)/\left(\frac{{\mathit{B}}_{\text{cr}}}{{\mathit{M}}_{\mathrm{c}}}\right)(4)$

Elevated D_JH values typically indicate substantial SD greigite fractions. Kars et al. (2021) reported that D_JH values typically fall below 0.2 in low-coercivity systems and exceed 0.3 in high-coercivity assemblages. Our low-coercivity samples exhibit depressed D_JH values ranging from 0.01 to 0.09. These low values indicate a diminished contribution of SD greigite within the ferrimagnetic mineral assemblage, consistent with dominant SP particle signatures. Isolated anomalously high values occur exclusively within the 605 mbsf interval, indicating an elevated proportion of authigenic greigite within the magnetic mineral assemblage. Greigite adopts a cubic spinel structure isostructural with magnetite, yet lacks distinctive low-temperature transitions analogous to magnetite’s Verwey transition (Kars et al., 2017). Furthermore, the dominance of SP particles in samples suppresses magnetic contributions from other minerals, thereby constraining precise determination of their distribution characteristics solely through low-temperature magnetic methods.

4.3 Identification of biogenic magnetite

Rock samples from deep-burial zones exhibit two distinct Verwey transition peaks (∼106 K and ∼118 K) in first-derivative FC curves (Figure 4B). Following Chang et al. (2016a), these dual transitions indicate both biogenic magnetite and near-stoichiometric inorganic magnetite (Bazylinski et al., 1991). The ∼118 K transition reflects near-stoichiometric composition, with deviations from the ideal ∼120 K transition likely caused by impurities (e.g., Ti). Conversely, chain fragmentation or oxidation of magnetosomes can suppress the Verwey transition below 110 K (Chang et al., 2016a; Roberts et al., 2012), consistent with the ∼106 K peak. As LTC tests revealed no oxidation features, we attribute the ∼106 K peak to biogenic magnetite. The ${\mathit{\delta }}_{\mathit{F}\mathit{C}}/{\mathit{\delta }}_{\mathit{Z}\mathit{F}\mathit{C}}$ ratio further discriminates biogenic magnetite and was calculated using Equation 5:

$\mathit{\delta }=\left(\mathit{J}\left(80\mathit{K}\right)-\mathit{J}\left(150\mathit{K}\right)\right)/\mathit{J}\left(80\mathit{K}\right)(5)$

where J(T) represents remanence measured at temperature T after FC or ZFC treatment (Moskowitz et al., 1993). Fresh magnetotactic bacteria typically exhibit a ${\mathit{\delta }}_{\mathit{F}\mathit{C}}/{\mathit{\delta }}_{\mathit{Z}\mathit{F}\mathit{C}}$ ratio >2; in contrast, our samples average ∼1.03 (Table 2). This suppression primarily results from dominant SP particle signals overwhelming the Verwey transition - a documented interference mechanism that impedes magnetofossil identification via ${\mathit{\delta }}_{\mathit{F}\mathit{C}}/{\mathit{\delta }}_{\mathit{Z}\mathit{F}\mathit{C}}$ ratios (Moskowitz et al., 1993; Housen and Moskowitz, 2006). The preservation mechanism for biogenic magnetite remains unresolved; however, its co-occurrence with magnetite inclusions has been documented in several studies (Chang et al., 2016b).

4.4 Diagenesis in gas hydrate-bearing sediments: Shallow vs. deep comparison

The LTC data comparisons reveal significantly higher saturation remanence (SIRM > 1E^-3Am²/kg) in shallow samples from the Hikurangi Margin (68–460 mbsf) and the Nankai Trough (70–110 mbsf) compared to our deeper samples. FC/ZFC curves exhibit similarly elevated characteristics, with SIRM values substantially exceeding those in most deep samples (Kars et al., 2021; Kars and Kodama, 2015). FC/ZFC curves exhibit similarly elevated characteristics, with SIRM values substantially exceeding those in most deep samples. Furthermore, these curves display comparatively enhanced Verwey transitions near 120 K relative to deeper sediments. A notable drop in remanence occurs at ∼490 mbsf in the Hikurangi Margin. Furthermore, the δ parameter (δ_FC and δ_ZFC) quantifies the relative proportions of magnetite versus ferrimagnetic iron sulfides. Plotting δ values (Figure 8) shows most samples fall within the magnetite-dominant field. Only a discrete anomalous sample at ∼605.8 mbsf plots within the mixed magnetite/iron sulfide field, resembling the magnetic composition of shallow Hikurangi Margin samples. This indicates that magnetite—despite its depleted concentrations—remains the predominant magnetic phase throughout most deep sediment sections. Greigite content variations align with D_JH data, showing low overall abundance of single-domain greigite grains, except at the discrete anomaly where it is higher. Collectively, comparison with low-temperature experimental data from shallow gas hydrate-bearing sediments reveals that deep burial sediments (>580 mbsf) exhibit: significantly depleted magnetic mineral concentrations, advanced reductive dissolution of magnetite, and pyritization progressing toward completion.

Figure 8

Figure 8. δ values from FC/ZFC experiments across samples. Values of 0–0.18 indicate iron sulfide dominance, 0.18–0.4 reflect mixed magnetite and iron sulfides, and values > 0.4 signify magnetite dominance. (Adapted from Kars and Kodama (2015)).

5 Conclusion

Compared to gas hydrate-bearing shallow sediments in other marine settings, magnetite in the deep anoxic sediments at Site U1519 has been nearly completely depleted by intense pyritization. Consequently, the magnetic assemblage is now dominated by SP grains (F_SP >90%), as evidenced by a prominent monotonic decay in the FC/ZFC curves. Within the reducing environment, the preservation of authigenic greigite is controlled by low sulfate concentrations and reactive iron concentrations. These conditions suppress microbial H₂S production and pyritization, allowing greigite to persist as an intermediate reaction product. Furthermore, dual Verwey transition peaks (∼106 K and ∼118 K) in select samples indicate the coexistence of biogenic magnetite and inorganic magnetite. Suppression by SP particle signals resulted in low ${\mathit{\delta }}_{\mathit{F}\mathit{C}}/{\mathit{\delta }}_{\mathit{Z}\mathit{F}\mathit{C}}$ ratios, complicating the identification of biogenic magnetite compared to fresh magnetosomes. Nevertheless, the presence of biogenic magnetite demonstrates its potential for preservation even within deep reducing settings, challenging the assumption of its inevitable dissolution. Collectively, this study demonstrates that low-temperature magnetism can be used to decipher the preservation and alteration of magnetic minerals, thereby providing a new magnetic perspective for understanding diagenetic evolution within gas hydrate systems of the Hikurangi margin.

Data availability statement

The data collected for this study can be found at https://doi.org/10.5281/zenodo.15639564. All Expedition 375 shipboard data are available at https://web.iodp.tamu.edu/OVERVIEW.

Author contributions

SL: Writing – original draft. XL: Writing – review and editing. KX: Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was funded by the National Natural Science Foundation of China (grant number 42164003).

Acknowledgments

Acknowledgements

Samples and shipboard data used in this study were provided by the International Ocean Discovery Program (IODP). We thank shipboard scientists and staff on IODP Expedition 375. We thank Tilo von Dobeneck and Thomas Frederichs for kindly providing access to the paleo- and rock magnetic laboratory facilities at the University of Bremen’s Department of Geosciences and for conducting experiments on the samples.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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