Petrographical and Geochemical Signatures Linked to Fe / Mn Reduction in Subsurface Marine Sediments from the Hydrate-Bearing Area, Dongsha, the South China Sea

: Fe and Mn oxides and (oxy)-hydroxides are the most abundant solid-phase electron acceptors in marine sediments, and dissimilatory Fe / Mn reduction usually links with the anaerobic oxidation of methane (AOM) and organic matter oxidation (OMO) in sediments. In this study, we report the results from subsurface marine sediments in the Dongsha hydrate-bearing area in the South China Sea. The petrological and geochemical signatures show that the Fe / Mn reduction mediated by AOM and OMO might occur in sediments above the sulfate-methane transition zone. X-ray di ﬀ raction and scanning electron microscopy analyses of sediments indicate that Fe(III) / Mn(IV)-oxides and authigenic carbonate minerals coexisted in the Fe / Mn reduction zone. The lower δ 13 C values of dissolved inorganic carbon, coupled with an evident increase in total inorganic carbon contents and a decrease in Ca 2 + and Mg 2 + concentrations indicate the onset of AOM in this zone, and the greater variation of PO 43 − and NH 4 + concentrations in pore water suggests the higher OMO rates in subsurface sediments. Geochemical and mineralogical analyses suggest that the previously buried Fe(III) / Mn(IV) oxides might be activated and lead to the onset of Fe / Mn reduction induced by AOM and OMO. These ﬁndings may extend our understanding of the biogeochemical processes involved in Fe / Mn reduction in continental shelves with abundant methane, organic matter, and terrigenous metal oxides.


Introduction
Iron (Fe) and manganese (Mn) oxides and (oxy)-hydroxides are the most abundant solid-phase electron acceptors in marine sediments [1]. Following the discovery and confirmation of microorganisms with the capacity to conserve energy from Fe(III) and Mn(IV) reduction in marine sediments, it has been found that dissimilatory microbial metal reduction plays a very important role in early diagenesis [2]. Based on the 'redox cascade' in the diagenetic process, organic matter oxidation (OMO) occurs through  [36,40]); (b) Bathymetric map showing the study area and location of the sediment core DH-CL14 (modified from references [32,35]). BSR, bottom-simulating reflectors.  [36,40]); (b) Bathymetric map showing the study area and location of the sediment core DH-CL14 (modified from references [32,35]). BSR, bottom-simulating reflectors.
After retrieval, the core was immediately cut into segments on board at the interval of 70 cm from the top down. A total of 22 sediment samples (20 cm thick for each sample) were collected and packed in the zip-lock plastic bags. The air in the bags was pumped out, and the samples were stored at −20 • C. The sampling depths in detail are presented in Table S1.

Analysis of Porewater Geochemistry
Core DH-CL14 was obtained by a gravity piston sampler, and the contents of methane were measured onboard using the headspace equilibration technique by the Agilent 6890N gas chromatograph (Agilent, Palo Alto, CA, USA) equipped with a flame-ionization detector [41]. The pore water was extracted on board at room temperature by a vacuum apparatus [42] from the top 20 cm of a 70 cm thick core section before core freezing. These porewater samples were sealed in numbered plastic bottles and stored at 4 • C for further chemical analyses. Total alkalinity (TA) was analyzed on board by titration with an HCl standard solution [43]. The relative standard deviation (RSD) was less than 2% (n = 12). The concentrations of phosphate (PO 4 3− ) and ammonium (NH 4 + ) were photometrically measured on board using a Shimadzu UVmini-1240 spectrophotometer (Shimadzu, Kyoto, Japan) with an analytical error of ±4.0% [44]. About 1.5 mL of each subsample of porewater were added to a 2.5% zinc acetate solution onboard to fix all sulfides as the form of ZnS and stored at 4 • C. The concentrations of sulfides were determined in the laboratory using a spectrophotometer [45] by the complexion of the ZnS precipitate in an acidified solution of phenylenediamine and ferric chloride with a detection limit of less than 1 nmol/L. The sulfide standard was validated by titration with thiosulfate to achieve improved accuracy. The contents of sulfate (SO 4 2− ), calcium (Ca 2+ ), and magnesium (Mg 2+ ) in pore water were determined by using a Metrohm 790 ion chromatograph (IC) (Metrohm, Herisau, Switzerland), with an analytical error of ±1% [46]. The carbon isotope ratios of dissolved inorganic carbon (δ 13 C DIC ) in pore water were analyzed by a Delta Plus XP multiflow-isotope ratio mass spectrometer (Finnigan, Bremen, German), with an analytical error of ±0.1% [42].

Grain Size Analysis
The grain sizes of sediments were analyzed by a Mastersizer 2000 laser particle analyzer (Malvern, Worcestershire, UK) with the measurement range of 0.02-2000 µm, size resolution of 0.01 µm, and measurement error of <2%. Before measuring the grain sizes, these samples were homogenized by ultrasonic vibration, oxidized using 10% H 2 O 2 to remove organic matters, and dispersed in a 0.05% solution of sodium hexametaphosphate to separate discrete particles. Grain sizes were classified into three fractions, i.e., sand (>63 µm), silt (4-63 µm), and mud (<4 µm).

Analysis of Total Organic Carbon and Total Inorganic Carbon in Sediments
The contents of total carbon (TC) and total organic carbon (TOC) in sediments were quantitatively measured with a CS230 carbon-sulfur analyzer (Leco, MI, USA) according to an improved method with the optimal pretreatment procedure [47]. The limit of quantification (LOQ) (defined as the mean blank value plus 10 times the standard deviation) was 2.3 ± 0.3 µg/g TC or TOC per crucible in this study. The total inorganic carbon (TIC) was determined by the difference between TC and TOC.

Major Element Analysis of Sediments
The major element composition of sediments was determined by the PW2424 X-ray fluorescence spectrometry (PANalytical, Almelo, Netherlands) after fusion in lithium nitrate. Lower limits of detection (LLD) for major elements were 0.01%. Spearman's linear correlation among the contents of major elements (wt%) and grain parameters was analyzed by the software IBM SPSS Statistics (Ver. 19, Microsoft, Chicago, IL, USA).

Analysis of Minerals and Sediment Textures
The minerals and sediment textures in core DH-CL14 were analyzed by the system of Quanta 400F FEG field emission environmental scanning electron microscopy (SEM) (FEI, Hillsboro, OR, USA), with the Oxford INCA energy-disperse X-ray spectroscopy (EDS) (Oxford, Bucks, UK), and HKL electron backscattered diffraction (EBSD) facility (HKL, Hobro, Denmark). The analytical conditions are described as below: resolutions for SEM, 2.0 nm (30 kV) and 3.5 nm (3 kV); range of element analysis for EDS: 5B−92U; resolution for EBSD: 0.1 µm (Al, 20 kV); accelerating voltage: 20 kV; and magnification factor: 7~1,000,000. Elemental identification via mapping was performed on the EVO MA 10 SEM (CARL ZEISS, Oberkochen, Germany) with an XFlash 6/30 detector (Bruker, Karlsruhe, Germany). The mineralogical analysis was conducted on powdered sediment samples using a PANalytical Empyrean powder X-ray diffractometer (PANalytical, Almelo, Netherlands) by the monochromatic Cu-Kα radiation at 40 kV and 40 mA for identifying mineral phases. The measurement patterns for X-ray powder diffraction (XRD) were recorded between 30 • and 80 • (2θ) at the scanning speed of 1 • /min for mainly focusing on iron-manganese minerals. The identification of minerals via XRD was analyzed by the software X'Pert High Score Plus (Ver. 3.0, PANalytical, Almelo, Netherlands), with the International Center for Diffraction Data (ICDD) Database.

Grain Composition of Sediments
The grain composition and its related parameters of sediments, including mean grain size (Mz) and median grain size (Md), are presented in Table S1. It can be seen from Table S1 that the core is mainly composed of fine-grained silt (56.46-69.82%) and mud (26.16-38.09%) (Table S1; Figure S1). For convenience, the core can be roughly divided into three units (A, B, and C) on the basis of its multiple parameters. It is shown from Table S1 and Figure S1 that the maximum (7.57%) and minimum (6.69%) of its Mz values appear at the depths of 360 cm (the boundary between units A and B) and 500 cm (the border between units B and C), respectively. The Md values have the same variation trend as that of the Mz values.

Porewater Geochemistry
The chemical composition of pore water in core DH-CL14 is listed in Table S2  Similarly, from units A to B, the sulfate concentrations decrease smoothly from 29 mmol/L (near to the average value in seawater) in the upper sediments to 24 mmol/L at the bottom of unit B. However, the sulfate concentrations decline abruptly down to 11 mmol/L in unit C, and then decrease slightly below the depth of 640 cm (Figure 2a). At the end of this core, the sulfate concentration still maintains at a relatively high level of 10 mmol/L.
The concentrations of dissolved sulfides in units A and B are nearly close to the detection limit, and then rapidly increase to 62 mmol/L in unit C ( Figure 2b). Overall, the concentrations of dissolved sulfides exhibit a similar variation trend to those of methane.
The stable carbon isotope composition of dissolved inorganic carbon (δ 13 C DIC ) in this core exhibits a relatively monotonic decreasing trend, i.e., from units A to C, the δ 13 C DIC values decrease gradually from −4.42% to −37.31% , except the lowest value (−46.20% ) at a depth of 640 cm in unit C (Table S2, Figure 2b (Table S2), with the second-highest PO 4 3− concentration at a depth of 430 cm.

Contents of TOC and TIC in Sediments
The TOC contents in sediments range from 0.40% to 0.67% with a mean of 0.56 ± 0.08% (Table  S2). It can be seen from Figure 2c that the contents of TOC progressively increase from units A to B from 0.40% to 0.67%, but with a slightly decreasing trend to 0.61% in unit C. The TIC contents suddenly increase from 0.55% to 1.43% in unit B, and then decrease with depth in unit C (Table S2, Figure 2c). and stable carbon isotope ratios (δ 13 C) of dissolved inorganic carbon (DIC) in pore water; (c) Contents of total organic carbon (TOC) and total inorganic carbon (TIC) in sediments; and (d) Contents of CaO and Fe2O3 T in sediments; Contents of phosphate (PO4 3− ) (e), ammonium (NH4 + ) (f), and total alkalinity (TA) (g) in pore water; (h) Copies of apsA as the functional gene of SRBs (the data referred from [32]). Units A, B, and C are roughly divided on the basis of diverse geochemical parameters.  (Table S2; Figure 2g). The concentrations of Ca 2+ and Mg 2+ in pore water start to evidently decrease at a depth of 360 cm (9.5 mM and 52 mM, respectively; Table S2), with values much lower than those (Ca 2+ , 10.3 mM; Mg 2+ , 53.2 mM) for the typical seawater [48].

Contents of TOC and TIC in Sediments
The TOC contents in sediments range from 0.40% to 0.67% with a mean of 0.56 ± 0.08% (Table S2). It can be seen from Figure 2c that the contents of TOC progressively increase from units A to B from 0.40% to 0.67%, but with a slightly decreasing trend to 0.61% in unit C. The TIC contents suddenly increase from 0.55% to 1.43% in unit B, and then decrease with depth in unit C (Table S2, Figure 2c

Composition of Major Elements in Sediments
The contents of major elements in sediments are presented in Table S3. The core is characterized by the high contents of SiO 2 (53.46-61.80%), Al 2 O 3 (13.06-16.80%), and Fe 2 O 3 T (4.80-6.42%), and the relatively low contents of CaO (2.51-6.75%). It can be seen from Figure 2d that the contents of Fe 2 O 3 T gradually increase from 5.47% to 6.42% at a depth of 220-360 cm, with the highest value at the depth of 360 cm; followed by an abrupt decrease to 4.80% in unit B. Spearman's relation analysis shows that there is a significant positive correlation (significance level, p < 0.01) between Mz and Al 2 O 3 (correlation coefficient, ρ = 0.8744), Fe 2 O 3 T (ρ = 0.8304), or mud (ρ = 0.7811) (Table S4). However, there is a significant negative correlation (p < 0.01) between the contents of CaO and Fe 2 O 3 T (ρ = −0.7525) ( Table S4). The contents of CaO continuously decrease from 4.23% to 2.51% in unit A, while starting to abruptly increase from 360 cm to 590 cm, with a peak of 6.75% at a depth of 590 cm (Figure 2d).

Sediment Textures
Scanning electron microscopy-Energy Dispersive Spectrometer (SEM-EDS) analysis was performed to observe pyrite aggregates and Fe/Mn oxyhydroxides/ carbonates textures. Various pyrite aggregates occurred in sediments, such as general framboids, specific framboids with radial overgrowths, just like sunflowers in shape, and long pyrite bands ( Figure 3). Pyrite framboids in units A, B, and C exhibit secondary overgrowths, and framboidal cores could be clearly distinguished from their overgrowths (Figure 3a,e,g,h). The initial pore space among the framboids was filled by their overgrowth (Figure 3h). Some pyrite bands were converted into aggregates of euhedral crystals (Figure 3c). Subhedral to euhedral pyrite crystals homogeneously distributed in the sediments showing the octahedral shape with a size of 5-45 µm (Figure 3b,d,f). In fact, the sectional shapes of both the pyrite band and sunflower-shaped framboid may configure a three-dimensional pyrite tube just as revealed by the previous report [49]. Several pyrite tubes are found in units A and C. Most of the tubes vary from 200 µm to 600 µm in length and from 85 µm to 140 µm in diameter; they consist of clustered pyrite framboids (Figure 3c  SEM-EDS analysis, combined with elemental identification via mapping, was employed to observe the textures and element distribution of sediments. The analytical results reveal the occurrence of one Fe/Mn microscopic concretion with a size of 285 μm × 245 μm at a depth of 380 cm in this core (Figure 4a). The elemental spectra of minerals indicate that the concretion consists of columnar or rhombohedral Mn-oxides and fibrous or botryoidal Fe-oxides (Figures 4 and S3).   (Figure 4a,c,f). Scaly iron minerals with the obviously high peak in element O are siderite (FeCO 3 ) (Figure 4e and Figure S3d). SEM elemental mapping images exhibit that this concretion is mainly composed of metal elements of Fe (red) and Mn (green) (Figure 4g) Fe-oxides (red) account for about two-thirds of this concretion (Figure 4h,i). Furthermore, Fe-and Mn-oxides with different shapes represent a typical intergrowth (Figure 4g,i). Minerals 2019, 9, x FOR PEER REVIEW 9 of 18

Sulfate Reduction Coupled with AOM in Subsurface Sediments in the Dongsha Area, the SCS
Dongsha is located in the northeastern continental slope of the SCS (Figure 1), where the extensive occurrence of natural gas hydrates and authigenic carbonates indicates high-flux methane seeps [39,[50][51][52][53]. Numerous studies have also confirmed the onset of AOM in submarine sediments in this region [32,54,55].
In this study, the geochemical profile of the core DH-CL14 exhibits the following signatures of AOM. It could be seen from Figure 2a

Sulfate Reduction Coupled with AOM in Subsurface Sediments in the Dongsha Area, the SCS
Dongsha is located in the northeastern continental slope of the SCS (Figure 1), where the extensive occurrence of natural gas hydrates and authigenic carbonates indicates high-flux methane seeps [39,[50][51][52][53]. Numerous studies have also confirmed the onset of AOM in submarine sediments in this region [32,54,55].
In this study, the geochemical profile of the core DH-CL14 exhibits the following signatures of AOM. It could be seen from Figure 2a that the current SMTZ occurs in unit C, where both methane and sulfate are depleted (Figure 2a). Due to the coupling between AOM and sulfate reduction, the concentrations of dissolved sulfide (ΣH 2 S) and CaO display their maximal concentrations while the concentrations of Ca 2+ and Mg 2+ decrease rapidly in unit C (Tables S2 and S3, Figure 2b,e).
Meanwhile, abundant authigenic mineral aggregates (Figure 3g,h), especially pyrite tubes (Figure 3i), are accordingly formed in unit C. The δ 13 C DIC values in pore water continuously decrease until reaching the minimum of −46.2% at the depth of 640 cm (Figure 2b), which is consistent with the reported 13 C-depleted DIC in the AOM zone [56]. Specific SRBs can utilize methane as their electron donor and co-exist with ANMEs for participating in AOM [57]. Therefore, the quantification of the functional gene apsA for SRBs is a rapid detection method for enumerating SRBs in natural environments [26]. As shown in Figure 2h, the highest apsA copies (1.0-3.1 × 10 4 copies/g dry sediments) occurs in unit C (Table S2) [32]. The above evidence may demonstrate the onset of sulfate-driven AOM in unit C [48].

Petrological and Geochemical Signatures for Fe/Mn Reduction in Unit B
Although the microbially mediated sulfate reduction dominates the pathway with AOM in unit C, it may have not always coupled with AOM in unit B. As revealed in Table S2 and Figure 2h, the relative abundances of SRBs drop dramatically in unit B only with 301−459 copies/g dry sediments, which are one to two orders of magnitude lower than those at the depths of 220−360 cm (2.9−5.1 × 10 3 copies/g dry sediments) and 500−730 cm (1.0-3.1 × 10 4 copies/g) [32]. The above results suggest that sulfate reduction may be weakened, whatever is coupled with AOM/OMO, in spite of the maintenance of higher sulfate concentrations. On the other hand, the contents of TIC and CaO abruptly increase in unit B (Figure 2c,d), indicating that the formation of authigenic carbonates may result from the coupling between AOM/OMO and other electron acceptors (i.e., Fe/Mn-oxides) [18,32].
Therefore, sulfate reduction may be limited due to the occurrence of higher Fe contents (4.86-6.42 wt%); and the coupling between Fe(III)/Mn(IV) reduction and AOM/OMO may accordingly be induced in unit B. The above inference is supported by the following petrological and geochemical signatures.
Petrologically, Hein, and Koski pointed out that the 13 C-depleted authigenic carbonates associated with rhodochrosite in sediments of the Franciscan Complex (a paleo-analog of the Eel River Basin) recorded the onset of Mn-driven AOM [58]. González [26][27][28]. In core DH-CL14, the authigenic minerals in unit B mainly include calcite, aragonite, dolomite, siderite, and rhodochrosite (Figures 3, 4 and S2). Particularly, the well-preserved concretion, consisting of rhodochrosite, manganite, siderite, and goethite (Figures 4 and S3) with the specific intergrowth texture (Figure 4g-i) was found in the same environment at a depth of 380 cm. Goethite is usually considered to act as an ideal substrate for Fe-reducing bacteria in natural ecosystems due to its low crystallinity and high specific surface area [26,[60][61][62]. Moreover, siderite has frequently been found in association with calcite in seep environments [63][64][65]. In this study, the presence of siderite and rhodochrosite indicates that they were formed in an alkaline and non-sulfidic environment with sufficient supplies of Fe-oxides [66,67]. Actually, pyrite tubes are not observed in unit B (Figure 3), and the contents of dissolved sulfides in pore water are as low as 2.0 × 10 −6 mmol/L in unit B (Figure 2b), which are propitious to the formation of siderite and rhodochrosite (Figure 4d,e). Obviously, the occurrence of authigenic Fe/Mn microscopic concretions, Fe(III)/Mn(IV)-oxides or -hydroxides (e.g., ferrihydrite, goethite, lepidocrocite, and birnessite), and authigenic minerals (e.g., calcite, Fe-rich dolomite, siderite, and rhodochrosite) indicates that Fe(III)/Mn(IV) reduction may become the dominant process of carbon turnover in unit B [27,59].
Geochemically, core DH-CL14 from the Dongsha area, the SCS has the higher contents of Fe 2 O 3 T (4.80-6.42%, with a mean of 5.53%) (Table S3, Figure 2d) in comparison with methane-rich sediments in the Nile deep-sea fan, the eastern Mediterranean Sea (mean = 2.05%) [68], iron-enriched carbonates in the Secchia River, the Northern Apennines (mean = 2.48%) [69], and Fe-rich carbonate pipes in the northern Okinawa Trough (4.09-4.63%) [27]. The evident correlation between grain parameters (particularly mud percentage and Mz) and Fe 2 O 3 T contents (Table S4) suggests that iron may result from the terrestrial input of Fe-oxides previously deposited in the continental shelf. The sufficient previously buried Fe(III)-minerals, together with minor Mn(IV)-minerals provided the material sources and created the thermodynamically favorable conditions for metal reduction rather than sulfate reduction for consuming methane or organic matters, and thereby Fe/Mn-rich carbonates precipitated in unit B. Also, it has been proved that the addition of Fe 3+ can inhibit the reduction of sulfate in sediments [70,71]. Besides, the evident increase in TIC and CaO in unit B (Figure 2c,d) implies the high production of authigenic carbonates and sufficient supply of methane or organic matters as electron donors. Therefore, we infer that the occurrence of excessive high-valence Fe/Mn-oxides (clearly observed in the micro-concretion) in unit B may inhibit microbially-mediated sulfate reduction, and further induce the vertical transition from sulfate reduction in unit C to a high-valence metal reduction in unit B.

AOM vs. OMO Coupled with Fe/Mn Reduction in Unit B
In anoxic marine sediments, Fe/Mn-oxides are mainly consumed via dissimilatory Fe/Mn reduction, associated with microbially-mediated OMO [72] or AOM. Therefore, how to identify the mechanisms for Fe/Mn reduction is a key issue for evaluating the quantity of methane consumed by metal-driven AOM.
The δ 13 C DIC value in pore water is a key parameter for distinguishing AOM from OMO. There are four main sources of DIC in pore water: (1) derived from seawater (δ 13 C DIC ≈ 0% ); (2) produced during OMO; (3) originated from AOM; (4) from residual DIC after methanogenesis [73]. The sources of inorganic carbon and types of redox reactions could be discriminated against by the δ 13 C DIC of pore water. In this work, the δ 13 C DIC values range from −22.67% (360 cm depth) to −26.12% (500 cm depth) in unit B (Table S2; Figure 2b), which are lower than the δ 13 C DIC values from the typical marine organic matters in the SCS (−20% ) [74,75]. Therefore, the lower δ 13 C DIC values indicate the occurrence of AOM [76], and in turn, lead to the precipitation of carbonates and decrease in the Ca 2+ and Mg 2+ concentrations (Table S2, Figure 2c,d).
In this study, the DIC concentrations in this core failed to be detected, but the data of total alkalinity were obtained. In general, the increase in total alkalinity (TA) and concentration of hydrogen sulfide (ΣH 2 S) in pore water imply the onset of active sulfate-driven AOM [77]. At the bottom of the core, both TA and ΣH 2 S show an evident increase, implying that AOM contributed to sulfate reduction in the SMTZ (Figure 2b,g) [33]. However, not similar to ΣH 2 S, TA displays the second peak value at a depth of 430 cm (7.79 mmol/L) (Table S2; Figure 2g), suggesting that AOM was not associated with sulfate, and had a weak effect on TA above the SMTZ.
The profiles of PO 4 Figure 2f) [43]. Nevertheless, the NH 4 + maximum in core DH-CL 14 is still far lower than that in the Black Sea (<4.5 mM) [80] and the SCS (<3 mM) [81]. The quasi-linear relation of NH 4 + and logarithmic trend of PO 4 3− in pore water with depth display the consistent OMO rates, except that at a depth of 430 cm in unit B (Figure 2e,f). The abrupt rise for PO 4 3− in unit B indicates that the reactions between organic matters and Fe/Mn-oxides may be more intense in this sediment layer.
Since OMO is usually homogeneous in a local region, its rate generally depends on the abundances of organic matter and electron acceptors. In the Dongsha area of the SCS, the maximal sedimentation rate after late Pleistocene (41.7 cm/ka) [82] may be comparable to the average in the continental slope [79], but the TOC contents in this core are less than 0.67%. Besides, the evident increase in TIC in unit B (Figure 2c) and the progressive decrease in its δ 13 C DIC values with depth (Figure 2b) indicate the contribution of methane. Furthermore, previous studies have proved that the dissimilatory Fe/Mn reduction associated with OMO generally occurs at a depth of several centimeters in the upper surface sediments [11,12,17]. Therefore, the rapid consumption of excessive high-valence Fe/Mn-oxides in the micro-concretion and evident increase in carbonates imply the onset of the reduction from Fe(III)/Mn(IV) to Fe(II)/Mn(II) induced by AOM and the precipitation of authigenic siderite or rhodochrosite in sediments (Figure 4d,e).

Probable Biogeochemical Processes for Fe/Mn Reduction in the High-Flux Methane Emission Region
The new petrological and geochemical data, as well as the apsA abundances, suggest that Fe/Mn reduction occurred at a depth of several centimeters above the SMTZ and might couple with AOM and/or OMO in the hydrate-bearing region with abundant Fe/Mn-oxides, as described as follows.
Previous studies have shown that metal-driven AOM is widely distributed in the marine sediments with the high input of metal oxides and high-flux methane seeps [29,30,83]. In this case, the study area is located in the northeastern unstable continental slope of the SCS [84] (Figure 1), and the sediments with abundant Fe/Mn-oxides from the shelf of the SCS might be integrally transported into the Dongsha area due to the structural or gravity instability. The previously deposited Fe/Mn-oxides, in particular, goethite, ferrihydrite, and birnessite with large specific surface areas [28,60,61,85], would spontaneously trigger the onset of AOM in the sediments with high-flux methane seeps. Obviously, the Fe/Mn source, in this case, evidently differs from those previously reported in the northern Okinawa Trough (i.e., Fe 3+ derived from deep-source fluids at methane seeps) [26][27][28].
Abundant high-valence metal-oxides are widely distributed in various environments, in which AOM decouples with sulfate reduction, but couples with these metal-oxides [83,86]. In a recent co-culture experiment of ferrihydrite reduction and methane oxidation, ferrihydrite was reduced, and ferrous minerals were formed [87]. In submarine sediments, Fe/Mn-oxides such as ferrihydrite and birnessite are important and ubiquitous geosorbents, which have large specific surface areas of more than 200 m 2 /g [85,88,89] and 63−300 m 2 /g, respectively [90,91]. In addition, the similar biogeochemical processes of Fe and Mn reduction are found in the hydrocarbon-derived Fe-rich nodules from the Dongsha area and carbonate chimneys from the Gulf of Cadiz [34,92,93]. The input of high-valence Fe-oxides derived from terrigenous sediments in the continental shelf of the Gulf of Cadiz was proved to support the formation of siderite-rhodochrosite nodules from goethite-Mn oxides nodules [59]. Therefore, the sufficient Fe/Mn-oxides may react with methane derived from the deep hydrate-bearing layers.
Meanwhile, the above-mentioned scenario on AOM in core DH-CL14 reveals a specific coexistence between metal-and sulfate-driven AOM in unit B, despite the maintenance of the higher concentrations of sulfate in this unit. Fe(III) in solid Fe-oxides can be activated via microbial mediation [33]. Iron-reducing bacteria, which obtain Fe(III) through the siderophore uptake mechanism, can cooperate with ANMEs to consume Fe(III) and methane, and accordingly, Fe(II) and bicarbonates are formed in Fe-driven AOM in association with the migration of Fe and its accompanying elements [24]. Spontaneously, some sulfides, as the products of sulfate-driven AOM/OMO, react with the byproducts [Fe(II)] of Fe-driven AOM to form sulfide minerals (e.g., pyrite) in this unit (Figure 3d,f). The excessive Fe(II) results in the absence of dissolved sulfide in pore water (Figure 2b) and the occurrence of siderite (Figure 4e). The reducing capacity of hydrocarbons and the role of hydrocarbon-rich fluids as a metal-transporting agent were proved in hydrocarbon escape structures, such as mud volcanoes, related expelled waters, sediments, and authigenic mineralization [59,94,95]. This hydrocarbon flow promoted the migration of fluids with methane, metallic ions, and their complexes in sediments, commonly together with microbial mediation like SRBs [34,59].
In summary, this study provides a good case on the potential methane sink linked with metal-driven AOM in the hydrate-bearing area with abundant methane and terrigenous metal oxides in the northeastern continental slope of the SCS. Although metal-driven AOM probably occurs in the nitrite-, nitrate-, and sulfate-depleted and metal-rich natural environments, its significance still remains unclear at the global scale [18]. Therefore, it is necessary to further clarify the contribution of metal-driven AOM to the methane sink. If the metal reduction is ignored, its importance will be undervalued, especially in continental shelves with abundant metal oxides [82,96].

Conclusions
The petrographical and geochemical signatures indicate the coexistence between metal-driven AOM and OMO at a depth of 380-500 cm above the SMTZ in the sediment core from the Dongsha hydrate-bearing area, the SCS. The profiles of the methane and sulfate concentrations in pore water suggest that the current SMTZ depth in this core is around 600 cm. Above the SMTZ, high-valence Fe/Mn-oxides (e.g., ferrihydrite and birnessite), together with the reaction products (e.g., siderite and rhodochrosite) are firstly discovered in the Fe/Mn microscopic concretions with the intergrowth texture. The lower δ 13 C DIC values (−22.67% to −26.12% ), the increase in TIC, and decrease in Ca 2+ and Mg 2+ indicate the onset of AOM in unit B; and the increase in PO 4 3− and NH 4 + in pore water suggests the occurrence of OMO. Based on these findings, Fe/Mn reduction may occur in subsurface sediments at a depth of several centimeters above the SMTZ, which may be mutually mediated by AOM and OMO. Accordingly, a novel probable mechanism has been proposed to expound the complicated biogeochemical processes involved in Fe/Mn reduction in the hydrate-bearing area with abundant methane, organic matter, and terrigenous metal oxides.