Sediment Mineralogy and Geochemistry and Their Implications for the Accumulation of Organic Matter in Gashydrate Bearing Zone of Shenhu, South China Sea

Abstract

The Total Organic Carbon (TOC) content plays a crucial role in gas hydrate exploration because a higher TOC content signifies a greater potential for buried gas hydrates. The regulatory mechanisms governing organic matter in sediment are intricate and influenced by various predominant factors unique to different regions. Notably, the Shenhu area in the South China Sea stands as a pioneering region for methane hydrate research. Despite its significance, limited research has focused on the burial patterns of TOC, resulting in an insufficient dataset to draw definitive conclusions. Consequently, a comprehensive understanding of the burial patterns and controlling factors of TOC within this area remains elusive. This study examines the pore-water characteristics, mineral composition, geochemistry, and sedimentary factors of four distinct sites within the Shenhu region of the South China Sea. The current depths of the Sulfate-Methane Interface (SMI) for sites CL54, CL56, CL57, and CL60 are identified as 28.6, 8.5, 31.9, and 8.1 m below the seafloor (mbsf), respectively. It’s noteworthy that these SMI depths align with locations known to harbor underlying gas hydrates. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses reveal that the primary sediment sources within this region encompass microbial shells (such as foraminifera and diatoms), clay, and terrestrial detritus. In addition, marine productivity exhibits a reverse correlation with TOC content, and both TOC content and Ce/Ce* ratios exhibit synchronous fluctuations with sedimentation rate. Drawing from the sedimentation rate, TOC content, as well as indicators of redox conditions (Mo_EF, Ce/Ce*, Mo/U) and productivity proxies (Ba/Al, P/Al) within the sampled sites, it becomes apparent that high sedimentation rate coupled with ‘anaerobic’ conditions foster favorable conditions for TOC accumulation. This comprehensive investigation not only provides valuable datasets but also offers insights into the intricate processes governing TOC accumulation.

1. Introduction

Natural gas hydrate boasts high energy density and yields cleaner products after combustion, positioning it as a promising clean future energy source [1,2,3,4,5,6,7,8]. As a future energy source, many countries worldwide, including Japan, Germany, Russia, and others, have increased their investments in natural gas hydrate exploration, with commercial development gradually progressing [9,10]. Continental margins are the primary reservoirs of natural gas hydrates, with carbon storage in continental margin hydrates estimated at approximately 500–2000 Gt [11]. Therefore, investigating the controlling factors behind natural gas hydrate accumulation in continental margin areas holds significant importance for the development and utilization of this ‘new energy’ [12,13,14].

The Shenhu area in the South China Sea serves as the forefront of hydrate research [6,15,16,17,18]. In 2017 and 2020, the Guangzhou Marine Geological Survey Bureau successfully conducted two natural gas hydrate trial mining operations in the Shenhu area, achieving two world records for the highest gas production and daily gas production during the second trial mining [16,17,18,19]. Therefore, the study of controlling factors influencing natural gas hydrate accumulation in the Shenhu area holds immense significance for expediting the commercialization of natural gas hydrate mining in this region.

Organic matter serves as the substrate for natural gas hydrate formation, with the burial of organic matter exerting significant control over hydrate formation [4,20,21,22,23,24]. Currently, only a handful of scholars are investigating organic matter burial in natural gas hydrate areas [25,26,27,28,29]. Consequently, understanding of the organic matter burial mechanism in the Shenhu area remains incomplete, necessitating further data and research to confirm or unveil the burial patterns of organic matter in this region.

So far, we have gained some understanding of the sedimentary conditions that control the accumulation of natural gas hydrates. Natural gas hydrates form in high-pressure, low-temperature, and methane-oversaturated environments [2,14,30,31]. Such environments easily develop beneath 300 m of seawater in continental margins, making the availability of methane the most crucial factor for natural gas hydrate accumulation in these regions [2,31,32]. Current research suggests that the organic matter content in continental margins determines the concentration of methane [33]. Since methane is generated through acetate fermentation or CO₂ reduction of organic matter after burial [34], the organic matter content serves as the fundamental condition for natural gas hydrate accumulation. In most hydrate-bearing areas, the Total Organic Carbon (TOC) content in sediments exceeds 1 wt% [35,36,37]. It is typically challenging for natural gas hydrates to form in environments where the TOC content in sediments is less than 0.5 wt% [38].

The TOC content in sediments is influenced and regulated by a multitude of sedimentary factors, including sedimentation rate, marine productivity levels, redox conditions, and more [39]. In different regions, the sedimentary factors that control the TOC content may also vary greatly [5,40,41]. For example, some studies suggest that the higher the productivity level in marine sediments, the higher the TOC content [42], but some studies have found that the TOC content sometimes shows an opposite trend to marine productivity indicators [16,43]. The sedimentary condition is not unidirectional for regulating the TOC content. For example, as the sedimentation rate increases within a certain range, the organic matter content will increase, but too high a sedimentation rate will dilute the organic matter content [44]. Therefore, controlling the TOC content in a region is the result of multiple sedimentary factors acting together, and it has a certain regional specificity. The main controlling factors affecting the TOC in sediments may be completely different in different regions.

There are few studies on the controlling factors of TOC content in the South China Sea area, especially in the Shenhu area. Previous studies based on biostratigraphy suggested that the accumulation of natural gas hydrates in the Dongsha area was controlled by the sedimentation rate [45]. Dongsha and Shenhu are adjacent and may have similar factors controlling the distribution of TOC content, but some studies have found that there is a weak correlation between methane flux and TOC content [46]. Therefore, from a sedimentary perspective, the sedimentary factors that dominate the TOC content in the Shenhu area still need further research.

Based on measurements of sulfate concentrations, XRD analysis, SEM imagery, geochemical trace element analysis, AMS-¹⁴C dating of foraminifera, and TOC content measurements, we provide a detailed analysis of the current Sulfate-Methane Interface (SMI) depth, methane flux, sediment provenance, sedimentation rate, redox conditions, productivity, weathering intensity, and TOC content at four sites within the Shenhu region in the South China Sea. The objective of this study is to gain a comprehensive understanding of the mechanism governing organic matter accumulation in gas-hydrate-buried regions.

2. Geological Setting

The South China Sea, one of the largest marginal seas in the Pacific Ocean, is situated at the convergence point of the Pacific, Eurasian, and Indian-Australian tectonic plates (Figure 1). It represents a typical transitional continental margin characterized by the accumulation of organic-rich sediment layers up to 4000 m thick, owing to its relatively short spreading history [16,47,48]. The Shenhu area, which is part of the Pearl River estuary basin, lies on the northern slope of the South China Sea. Since the Cenozoic era, three primary tectonic stages have shaped the Shenhu area: the Paleocene rifting stage, the Eocene thermal subsidence stage, and the Neotectonic movement stage [49]. These tectonic events have generated numerous faults and fractures, providing favorable conditions for the migration of hydrocarbon gases and the formation of gas hydrates [50]. Moreover, due to the high sedimentation rate of the Shenhu area since the Mesozoic and the low geothermal gradient (45.0 to 67.7 °C/km) there [48,51], gas hydrate reservoirs are likely to exist there. The presence of Bottom Simulating Reflectors (BSRs) is widespread in the Shenhu area, and numerous methane seeps have been previously reported [48]. Moreover, methane anomalies in pore water have been identified in the Shenhu region [46]. The extensive distribution of authigenic carbonate further indicates the occurrence of gas hydrates [29,32,52]. In June 2007, gas hydrate samples were successfully recovered from the Shenhu area [53]. Subsequently, successful trial productions of gas hydrates in the Shenhu area were conducted in 2017 and 2020. As of now, the Shenhu area remains at the forefront of gas hydrate exploration and exploitation, solidifying its status as a pioneer region in this field.

3. Materials and Methods

Four sediment cores (CL54, CL56, CL57, and CL60) were collected from the Shenhu area during an August 2018 cruise aboard the vessel ‘Haiyang Sihao’ using a gravity plunger. The lengths of these cores are 6.0 m for CL54, 4.0 m for CL56, 5.0 m for CL57, and 5.0 m for CL60. They were retrieved from water depths of 960 m, 1534 m, 990 m, and 1491 m, respectively (

Table 1. The information of cores from Shenhu.

Cores ID	Type	Depth (m)	Length (m)
CL54	Gravity plunger	960	6
CL56	Gravity plunger	1534	4
CL57	Gravity plunger	990	5
CL60	Gravity plunger	1491	5

). Following retrieval, the four cores were stored at a temperature of 4 °C. The sediments in these cores are characterized as organic-rich and primarily consist of dark gray, unconsolidated silty clay. Sediment samples were systematically collected at 10 cm intervals from the top to the bottom of each core, individually packed in zip-lock plastic bags, and subsequently stored at a temperature of −20 °C. A total of 200 sediment samples were collected. These samples were then subjected to drying in a freeze-drier for 48 h at the Marine Resources and Coastal Engineering Key Laboratory (MRCEL) at Sun Yat-sen University (SYSU). Following drying, the samples (200 in total) were pulverized using an agate mortar until the grain size was reduced to less than 63 μm.

3.1. Measurement Method

3.1.1. Scanning Electron Microscopy (SEM)

SEM images were obtained to elucidate sediment petrography using a Zeiss IGMA instrument at the School of Earth Sciences and Engineering, SYSU. These images were captured in secondary electron mode with a 12 kV acceleration voltage. SEM analysis was conducted on a total of 12 sediment samples taken from various depths within the four cores, specifically at depths of 10–20 cmbsf, 250–260 cmbsf, and 490–500 cmbsf, respectively.

3.1.2. X-ray Diffraction (XRD)

Approximately 1 mg of powdered sample was loaded into a fiberglass tube for analysis. XRD analysis was conducted at the MRCEL facility using a D/Max RAPID II instrument (Japan, Rigaku). The radioactive source employed was Mo, and the diffraction angle ranged from 2° to 40°. The step length used was 0.02°, and each sample was measured for a duration of 8 min. Subsequently, the data obtained were subjected to conversion using the 2DP software (Ver 2.1.6), and data analysis was carried out using the PDXL software (Ver 2.8.1.1). Mineral contents were calculated based on the identified phases to fit the intensity results, following the principles of Rietveld analysis [54]. All 200 samples underwent XRD measurements and content calculations. In the XRD signals of whole-rock sediment samples, distinct diffraction peaks of quartz and carbonate minerals can be readily detected, with minor peaks of other minerals present in a limited number of samples. Through full-pattern fitting using PDXL2, the percentages of carbonate, quartz, feldspar, and other minerals can be determined; however, the sum of these mineral percentages typically falls below 100%. Through observations using scanning electron microscopy (SEM) and visual inspection, it is evident that these sediments predominantly consist of a high clay content, serving as the primary constituent or matrix of the sediment. Therefore, in this study, it is assumed that the missing portion of the mineral content in the full-pattern analysis is largely attributed to clay minerals or non-crystalline components or organic matter. Consequently, in this research, we estimate this missing portion of the mineral content as the clay content. Clay content was estimated by reducing the percentage content of carbonate, quartz, and other minerals by one hundred percent.

$$w_{cla}=100\%-w_{carb}-w_q{-w}_o{-w}_f$$

The$w_{cla}$ represents clay content. The$w_{carb}$ represents carbonate content. The$w_q$ represents quartz content. The$w_o$ represents other minerals content (less than 5%). The$w_f$ represents feldspar content.

3.1.3. Chemistry Elements Analysis

Dried powdered samples (n = 200) were submitted to ALS Minerals and ALS Chemex-Co., Ltd. (Guangzhou, China) for trace element analysis. The concentration of trace elements in all powdered samples was determined using the ALS standard marine sample method M61-MS81. These samples underwent digestion using a mixture of perchloric acid, nitric acid, and hydrofluoric acid. Subsequently, the resulting solutions were dissolved in dilute hydrochloric acid and analyzed using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to determine their respective concentrations.

3.1.4. TOC Content and Isotope Measurement

TOC content analysis was conducted in MRCEL. The sediment samples were acidified with 10% HCl for 24 h and washed three times with deionized water. Subsequently, they were subjected to freeze-drying for TOC content measurement [55]. TOC contents were quantified using a CNS-HO rapid elements analyzer, with a precision and accuracy level better than 3%. TOC carbon isotopes were determined in MRCEL by MAT 253 plus. The sample was converted to CO₂ in the Elements Analyzer (EA) at 1020 °C and subsequently measured by the MAT 253 Plus mass spectrometer. Isotope result precision was better than ±0.2‰ and calibrated with international reference materials NBS-18 (δ¹³C = −5.014‰, δ¹⁸O = −23.2‰).

3.1.5. AMS-¹⁴C Dating of Foraminifera

The AMS-¹⁴C analyses were carried out on specimens of planktonic foraminifera shells weighing around 12 mg, which were carefully extracted from 6 samples, respectively. The BETA laboratory (Miami, FL, USA) conducted the AMS-¹⁴C age determinations. The obtained conventional AMS radiocarbon ages were subsequently calibrated utilizing the MARINE 13 calibration curve [56]. Accounting for the regional reservoir effect, the corrected age was determined as 11 ± 40 years [56].

3.2. Analysis Method

3.2.1. Estimate Porosity of Sediment Samples

Because sediment samples underwent freeze-drying pretreatment, sodium salts present in seawater were preserved within the sediment after water sublimation during the freeze-drying process. Consequently, sodium (Na) in the sediments originates primarily from two sources: one being residual Na-salts from the sublimation of seawater and the other being terrestrial minerals (such as feldspar and clay minerals). Correlation analysis demonstrates that Na exhibits weak associations with Al, Ti, and K (

Table 2. The elements correlation in cores (n = 200).

	Na	K	Al	Ti	Zr	Ca	Ba	Cd	Mn	Mo	P	S
Na	1.00	−0.02	0.23	0.16	−0.24	−0.32	0.80	0.47	0.62	0.09	0.33	−0.05
K	−0.02	1.00	0.94	0.90	0.15	−0.66	0.20	−0.13	0.02	0.36	0.19	0.47
Al	0.23	0.94	1.00	0.95	0.18	−0.77	0.39	0.01	0.15	0.39	0.24	0.48
Ti	0.16	0.90	0.95	1.00	0.40	−0.87	0.23	−0.09	0.01	0.48	0.12	0.59
Zr	−0.24	0.15	0.18	0.40	1.00	−0.51	−0.38	−0.23	−0.40	0.29	−0.44	0.29
Ca	−0.32	−0.66	−0.77	−0.87	−0.51	1.00	−0.21	0.05	−0.02	−0.58	0.13	−0.62
Ba	0.80	0.20	0.39	0.23	−0.38	−0.21	1.00	0.53	0.71	−0.05	0.56	−0.14
Cd	0.47	−0.13	0.01	−0.09	−0.23	0.05	0.53	1.00	0.24	−0.15	0.12	−0.40
Mn	0.62	0.02	0.15	0.01	−0.40	−0.02	0.71	0.24	1.00	−0.01	0.50	−0.13
Mo	0.09	0.36	0.39	0.48	0.29	−0.58	−0.05	−0.15	−0.01	1.00	−0.15	0.66
P	0.33	0.19	0.24	0.12	−0.44	0.13	0.56	0.12	0.50	−0.15	1.00	0.06
S	−0.05	0.47	0.48	0.59	0.29	−0.62	−0.14	−0.40	−0.13	0.66	0.06	1.00

), suggesting that the sodium in the sediments primarily derives from Na-salts formed during the seawater sublimation process. This conclusion is supported by the fact that Al, K, and Ti are typical terrestrial elements, and Na displays a weak association with them. Consequently, we devised a formula to estimate the porosity of freeze-dried sediments based on the Na content in the dried sediment samples.

Porosity ($\varnothing$), as defined, represents the ratio of pore volume to the total volume. In the context of marine sediments, the volume of seawater ($v_{seawater}$) within the pores is considered equivalent to the pore volume, as the pores are filled with seawater. The total volume of sediment comprises both the volume of dried sediment ($v_{dry sediment}$) and the volume of seawater ($v_{seawater}$). Consequently, the formula for calculating porosity is defined as:

$$\varnothing =v_{seawater}/(v_{dry sediment}+v_{seawater})$$

(1)

The concentration of Na⁺ in both pore water and seawater is considered conservative. The average concentration of Na⁺ in global seawater (${\rho }_{Na}$) is 11.06128 g/L. Consequently, the volume of seawater can be calculated once the mass of Na in dried sediment ($m_{Na}$) is known. This calculation is based on the Na⁺ concentration in seawater:

$$V_{seawater}=m_{Na}/{\rho }_{Na}$$

(2)

The mass of Na in the dried sample can be determined by multiplying the mass of the dry sediment ($m_{dry sediment}$) by the Na concentration (${\omega }_{Na}$) in the dry sediment. This calculation is based on the Na concentration in the dry sediment:

$$m_{Na}={\omega }_{Na}\ast m_{dry sediment}$$

(3)

Because the sodium (Na) in sediments consists of both halite and some terrigenous minerals, it is essential to subtract the sodium contribution from terrigenous minerals when calculating the Na derived from seawater. This calculation involves determining the Na content originating from halite:

$${Na}_{Halite}={Na}_{total}-{Na}_{terrigenous}$$

(4)

$${Na}_{terrigenous}={Al}_{total}\times \left(\frac{{Na}_{PAAS}}{{Al}_{PAAS}}\right)$$

(5)

Combine Equations (1)–(5), the calculation formula of porosity is extrapolated:

$$\varnothing =\frac{\left[{\omega }_{Na}-{\omega }_{Al}\times \left(\frac{{Na}_{PAAS}}{{Al}_{PAAS}}\right)\right]/{\rho }_{Na}}{\frac{\left[{\omega }_{Na}-{\omega }_{Al}\times \left(\frac{{Na}_{PAAS}}{{Al}_{PAAS}}\right)\right]}{{\rho }_{Na}}+1/{\rho }_{dry sediment}}\%$$

(6)

ω represents the concentration of elements in dry sediment, ${\rho }_{Na}$ represents the concentration of Na⁺ in seawater, and ${\rho }_{dry sediment}$ represents the density of dried sediment. With the known value of the normal ocean Na⁺ concentration being 11.06128 g/L and the density of dried sediment established at 1790 g/L [57], this study can proceed to estimate the porosity for all samples (n = 200), then the porosity of all samples (n = 200) was estimated.

3.2.2. Calculation of SMI Depth and Methane Flux

Linear regression was used to calculate SMI depth. Fick’s First Law was used to calculate the flux of sulfate [46,57]:

$$F=-\varnothing D·\frac{\partial C}{\partial x}$$

In sediment:

$$D_{se}=\frac{D_{sw}}{1-\mathrm{l}\mathrm{n}\left({\varnothing }^2\right)}$$

where F is the flux, $\varnothing$ is porosity; ∂C/∂x is the concentration gradient; D_se is the free-solution diffusion coefficient in marine sediments and D_sw is the free-solution diffusion coefficient in seawater (Figure 2). The calculated porosity result is in agreement with the majority of previous research findings [46,57].

3.2.3. Calculation of Element Enrichment Factor

The element enrichment factor is calculated by the following equation [58]:

$$X_{\mathrm{EF}}=\frac{X_{sample/{Al}_{sample}}}{X_{standard}/{Al}_{standard}}$$

X_sample and X_standard represent elements in the sample and the standard, respectively.

3.2.4. Constraining the Chronological Framework

The establishment of a chronostratigraphic framework was facilitated by utilizing AMS ¹⁴C dating ages obtained from sediment cores CL57 and CL60 in our study. These dating results were further corroborated by chronological constraints derived from the dating outcomes of core SH2. Additionally, we incorporated a calibration curve for calcium carbonate to accurately identify key chronological milestones within the framework. In instances where specific chronological constraints were unavailable, this study employed linear interpolation to estimate ages. The ages are reported in thousands of years before the present era (Figure 3 and

Table 3. The AMS ¹⁴C dating of the study region.

Samples NO	Method	Material	Conventional Age (BP)	δ13C (‰)	δ18O (‰)	Reference
CL57-80-90	AMS-Standard	foraminifera	9470 ± 30	1.4	−0.37	this study
CL57-160-170	AMS-Standard	foraminifera	15,220 ± 50	1.9	−0.27
CL57-330-340	AMS-Standard	foraminifera	35,150 ± 270	2	−0.71
CL60-60-70	AMS-Standard	foraminifera	3970 ± 30	1.7	−0.86
CL60-160-170	AMS-Standard	foraminifera	9080 ± 30	1.5	−0.46
CL60-490-500	AMS-Standard	foraminifera	15,790 ± 40	1.4	−0.62
SH-CL52-20-21	AMS-Standard	foraminifera	4800 ± 30	1.4	−2.21	[8]
SH-CL52-199-200	AMS-Standard	foraminifera	32,110 ± 190	2.6	−0.62
SH-CL52-179-180	AMS-Standard	foraminifera	23,260 ± 80	2	−0.14
SH-CL52-249-250	AMS-Standard	foraminifera	>43,500	2.4	−1.42

).

4. Results

4.1. SMI Depths and Methane Fluxes

Figure 2 presents the sulfate concentrations for cores CL54, CL56, CL57, and CL60 (Table S1). Sulfate concentrations exhibit a broad range within these cores: CL54 (ranging from 25 to 30.52 mmol/L, with an average of 27.83 mmol/L, n = 30), CL56 (ranging from 15 to 27.86 mmol/L, with an average of 22.96 mmol/L, n = 20), CL57 (ranging from 24.27 to 28.85 mmol/L, with an average of 26.72 mmol/L, n = 25), and CL60 (ranging from 11.46 to 27.81 mmol/L, with an average of 20.47 mmol/L, n = 25). Sulfate concentrations in these cores exhibit a linear decrease with depth. Notably, cores CL56 and CL60 display steeper sulfate gradients compared to cores CL54 and CL57 (Figure 2).

Porosity values for the four cores have been calculated. Overall, sediment porosity decreases with depth due to the compression caused by sediment accumulation. The average porosities for sites CL54, CL56, CL57, and CL60 are 61.0%, 70.0%, 63.6%, and 71.2%, respectively. Notably, the porosity at sites CL56 and CL60 is significantly higher than that at sites CL54 and CL57. Based on these average porosity values, the methane flux at sites CL54, CL56, CL57, and CL60 is estimated to be 3.46, 26.2, 4.94, and 27.6 mmol·m⁻²·yr⁻¹, respectively.

4.2. AMS ¹⁴C Dating

Sediment samples were extracted from specific depths within both the CL57 core (at 80–90 cmbsf, 160–170 cmbsf, and 330–340 cmbsf) and the CL60 core (at 60–70 cmbsf, 160–170 cmbsf, and 490–500 cmbsf). It’s worth noting that no instances of age inversions were detected within the CL57 and CL60 cores. Specifically, for the CL57 core, the ages obtained for the three sampled locations are as follows: 9470 ka BP, 15,220 ka BP, and 35,150 ka BP. Similarly, for the CL60 core, the ages for the corresponding three sample locations are 3970 ka BP, 9080 ka BP, and 15,790 ka BP (Figure 3 and Table 3 and Table S2).

4.3. Petrography

The minerals present in the four cores consist primarily of quartz, clay, and carbonates (Figure 4). Carbonates, in particular, predominantly originate from foraminifera shells (Figure 4a). Notably, foraminifera shells exhibit relatively intact surface features (Figure 4c). Clay minerals appear fine-grained, forming aggregates, dispersed structures, and occasionally short columnar or granular shapes (Figure 4b,f). Quartz grains are mostly tabular or granular in nature (Figure 4d). It’s worth noting that the four sites share very similar mineral species and morphological characteristics (Tables S3 and S4).

XRD patterns of sediments from different depths (at 10–20 cmbsf, 250–260 cmbsf, and 490–500 cmbsf, respectively) of the four cores are presented (Figure 5). The identified minerals include quartz, low-Mg-calcite, halite, illite, albite, orthoclase, kaolinite, chlorite, and montmorillonite (Figure 5a). XRD patterns of sediment from the four sites exhibit similarity. Peaks (104) of carbonates are magnified in Figure 5b, with low-Mg-calcite dominating the carbonate species (Figure 5b).

The mineral contents of the four cores are depicted in Figure 5 and Figure 6. The clay content exhibits a wide range, spanning from 20.3 to 78.1 wt%, with an average of 45.3 wt% (n = 200). Carbonate content varies between 5.49 and 47.4 wt%, with an average of 16.6 wt% (n = 200). Quartz content falls within the range of 8.96 to 44.9 wt%, with an average of 28.9 wt% (n = 200). Vertically, clay content remains relatively stable, while quartz and carbonate exhibit opposite trends. Frequency distribution graphs of mineral contents are presented in Figure 6. The clay content frequency distribution across the four cores displays a normal distribution (n = 200). Cores CL56 and CL60 exhibit higher clay content than cores CL54 and CL57 (Figure 6a). The frequency distribution of carbonate content in the four cores shows a left-leaning trend (n = 200) (Figure 6b), whereas the quartz content frequency distribution displays a right-leaning trend (n = 200) (Figure 6c). To distinguish the provenance of the sampled sediment, this study utilized the La-Sc-Th triangular diagram (Figure 7). The La-Sc-Th triangular diagrams of sediment from all four cores (n = 200) exhibit a high degree of similarity.

4.4. Sedimentary Environment and TOC Accumulation

Sedimentation rates: The sedimentation rates for the four cores were calculated based on the chronological frameworks presented in Figure 8. These sedimentation rates exhibit a noticeable increase during the Younger Dryas (YD) period (Figure 8). The average sedimentation rates for sites CL54, CL56, CL57, and CL60 are 14.3, 18.2, 11.9, and 20.8 cm/ka, respectively. Throughout the historical period, the sedimentation rates of CL56 and CL60 consistently exceeded those of CL54 and CL57.

TOC content: TOC contents of the cores are depicted in Figure 8 and Figure 9. The TOC content ranges from 0.53 to 1.35 wt%, with an average of 0.91 wt% (n = 43) (Figure 9f and

Table 4. TOC content and isotope in different sites.

Depth (cmbsf)	CL54-TOC Content (wt%)	Depth (cmbsf)	CL56-TOC Content (wt%)	Depth (cmbsf)	CL57-TOC Content (wt%)	Depth (cmbsf)	CL60-TOC Content (wt%)
5	0.80	5	0.82	10	0.69	10	1.03
60	0.75	50	0.68	60	0.76	50	0.87
110	0.77	90	0.73	110	0.81	90	0.78
160	0.67	130	0.92	160	1.08	130	0.82
210	0.79	170	0.94	210	1.07	170	0.92
310	0.58	210	1.29	260	0.96	210	0.94
360	0.53	250	1.10	310	0.83	250	1.11
410	0.68	290	1.29	360	0.78	290	1.23
460	0.52	330	1.29	410	0.68	330	1.25
510	0.67	370	1.19	460	0.61	370	1.26
560	0.66	400	1.17	490	0.62	410	1.18
						450	1.35

). The average TOC content for cores CL54, CL56, CL57, and CL60 is 0.69, 1.04, 0.83, and 1.06, respectively (Figure 9f). The average TOC contents of cores CL56 and CL60 are higher than those of cores CL54 and CL57 (Figure 9f). Additionally, the TOC contents of the four cores exhibit a noticeable increase during the YD period (Figure 8). The carbon isotopes of TOC range from −19.2 to −21.7‰, exhibiting similar trends (

Table 5. TOC carbon isotope in the cores.

Depth (cmbsf)	CL54 (V-PDB‰)	Depth (cmbsf)	CL56 (V-PDB‰)	Depth (cmbsf)	CL57 (V-PDB‰)	Depth (cmbsf)	CL60 (V-PDB‰)
5	−21.00	5	−20.93	10	−20.67	10	−20.93
60	−20.76	50	−20.71	60	−20.69	50	−20.71
110	−20.41	90	−20.87	110	−20.70	90	−20.87
160	−20.77	130	−21.76	160	−20.97	130	−21.76
210	−20.88	170	−20.44	210	−20.85	170	−20.44
310	−21.29	210	−20.18	260	−19.70	210	−20.18
360	−21.14	250	−20.40	310	−19.36	250	−20.40
410	−21.24	290	−20.26	360	−19.85	290	−20.26
460	−20.92	330	−20.27	410	−21.13	330	−20.27
510	−20.67	370	−19.86	460	−21.28	370	−19.86
560	−20.94	400	−19.66	490	−20.65	410	−19.66
						450	−20.40

).

Redox condition: Molybdenum Element Enrichment Factor (Mo_EF), Ni/Co, Mo/U, and Ce/Ce* are employed to reconstruct the redox condition of the paleo-environment (Table S5). The Ce/Ce* ratio ranges from 0.91 to 1.04, with an average of 0.97 (n = 200) (Figure 9a). The average Ce/Ce* ratios for cores CL54, CL56, CL57, and CL60 are 0.97, 0.98, 0.99, and 0.95, respectively (Figure 9a). Moreover, the average Ce/Ce* ratios for cores CL56 and CL60 are higher than those for CL54 and CL57 (Figure 9a). The scatter plot of Ce/Ce* vs. Pr/Pr* shows a higher value in cores CL56 and CL60 (Figure 9b). Ce/Ce* ratios in all four cores exhibit distinct increases during the Younger Dryas periods (Figure 8). Mo_EF varies from 0.35 to 6.30, with an average of 1.09 (n = 200) (Figure 9c). The average Mo_EF values for cores CL54, CL56, CL57, and CL60 are 1.22, 1.13, 0.78, and 1.22, respectively (Figure 9c). Mo/U varies between 0.05 and 1.24, with an average of 0.21 (n = 200) (Figure 9d). The average Mo/U ratios for cores CL54, CL56, CL57, and CL60 are 0.26, 0.22, 0.11, and 0.24, respectively (Figure 9d). Ni/Co ratios range from 2.67 to 3.63, with an average of 3.15 (n = 200) (Figure 9e). The average Ni/Co ratios for cores CL54, CL56, CL57, and CL60 are 2.89, 3.34, 2.97, and 3.31, respectively (Figure 9e).

Paleo-productivity: Ba/Al and P/Al ratios are utilized to assess paleo-productivity and are presented in Figure 9 (Figure 9). The Ba/Al ratio ranges from 0.0055 to 0.011, with an average of 0.0073 (n = 200) (Figure 9i). Specifically, the Ba/Al ratios of cores CL56 and CL60 are higher than those of cores CL54 and CL57 (Figure 9i). The average Ba/Al ratios for cores CL54, CL56, CL57, and CL60 are 0.0063, 0.0079, 0.0069, and 0.0082, respectively (Figure 9i). Additionally, Ba/Al ratios in all four cores exhibit distinct decreases during the Younger Dryas (Figure 8). The P/Al ratio ranges from 0.0053 to 0.014, with an average of 0.0072 (n = 200) (Figure 9g). The average P/Al ratios for cores CL54, CL56, CL57, and CL60 are 0.0072, 0.0071, 0.0073, and 0.0073, respectively (Figure 9g and Table S6).

5. Discussion

5.1. Pore-Water Characteristic Is Consistent with Underlying Gas Hydrate Region

The sulfate concentration in pore water is typically conservative, averaging 28 mmol/L [4]. When the rate of sulfate consumption exceeds the sulfate supply rate from seawater, the sulfate concentration in pore water drops below 28 mmol/L [19,59,60,61,62]. Within pore water, sulfate depletion can be attributed to both methane anaerobic oxidation (AOM) and organic matter sulfate reduction (OSR). The AOM reaction equation is as follows [24]: CH₄ + SO₄²⁻ → HCO₃⁻ + HS⁻ + H₂O. The OSR reaction equation is as follows [63]: 2(CHO) + SO₄²⁻ → 2HCO⁻ +H₂S. Both OSR and AOM processes can result in the reduction of dissolved sulfate concentrations in pore water [22,64,65]. The sulfate profile generated by OSR typically exhibits a curved pattern, whereas that of AOM follows a linear trend [4,5,66].

In general, organic matter sulfate reduction (OSR) is commonly recognized as the primary pathway for sulfate depletion, with organic matter content playing a pivotal role in controlling sulfate concentration and the depth of the SMI in marine sediment [5,67]. However, within gas hydrate burial regions, methane anaerobic oxidation (AOM) emerges as the predominant reaction, leading to a more rapid consumption of sulfate compared to OSR [4,5,33]. In these gas hydrate burial regions, the rate of AOM and sulfate flux is primarily controlled by methane flux [4]. Methane flux also governs the depth of the SMI in gas hydrate-buried regions [4].

The pore water data were compiled from a previous study [68]. The relationship between depth and sulfate concentration in pore water from the four cores was analyzed using linear regression. The linear regression results indicate a high level of agreement (R² = 0.68 for core CL54, R² = 0.97 for core CL56, R² = 0.91 for core CL57, and R² =0.99 for core CL60), suggesting that the sulfate profiles in these four cores exhibit typical linear sulfate depletion patterns, influenced by AOM (Figure 2).

Extrapolating sulfate concentrations reveals values approaching zero, with SMI depths of 28.6 mbsf for core CL54, 8.5 mbsf for core CL56, 31.9 mbsf for core CL57, and 8.1 mbsf for core CL60 (Figure 2). In continental margins characterized by gas hydrates or methane seepage, the typical SMI depth is less than 50 mbsf [46]. The SMI depths at our study sites are shallower compared to those in typical gas hydrate-bearing locations [57,69], suggesting that AOM has led to sulfate depletion in the study sites. This indicates the presence of underlying gas hydrates in the study sites. Notably, near the study sites, gas hydrate samples (site SH2) were successfully obtained [53], providing further consistency with the potential of underlying gas hydrate.

5.2. Minerals and Geochemistry Reveal Sedimentary Provenance

Provenance plays a crucial role in determining the organic matter content of sediment. Marine sediments originating from different sources exhibit varying organic matter levels.

The four study sites are closely situated, as depicted in Figure 1. Sites CL56 and CL60 are located in valleys with wide contours, while sites CL54 and CL57 are situated on hillsides with dense contours (Figure 1). Given their proximity, it is likely that these four sites share similar provenance. Bhatia and Crook (1986) introduced the La-Th-Sc ternary diagram as a tool to distinguish sediment provenance [70,71]. The La-Th-Sc triangle diagrams for the four cores exhibit striking similarities, indicating a common provenance (Figure 7). The points corresponding to the four cores in the La-Th-Sc triangle diagram fall within a zone typically associated with clastic sediment components primarily derived from Asian aeolian dust, with limited input from river clasts [72]. Consequently, the four study sites appear to be minimally influenced by terrigenous sources and are predominantly shaped by marine deposition and aeolian dust input. Supporting evidence from scanning electron microscopy and X-ray diffraction (XRD) further reinforces the dominance of marine deposition and aeolian dust input. (1). The mineral composition and morphology of sediments in all four cores closely resemble one another, with a notable presence of foraminiferous shells (Figure 4a,c). (2). XRD analysis of whole-rock mineralogy indicates similar mineral species across the four cores (Figure 5a). The sediment in all four cores contains a significant amount of carbonate (most of microbial origin), quartz (fine grain from aeolian dust), and limited feldspar (of terrigenous origin), reinforcing the mixture provenance. In addition, the carbon isotope composition of TOC varies from −21.7 to −19.2‰, indicating that it is a typical marine organic matter (with values ranging from −22 to −20‰) and is mainly influenced by marine organic matter.

In summary, considering the proximity of the four study sites, sediment morphology, mineral species, TOC carbon isotope, and the La-Th-Sc diagram, it is evident that the provenance of all four cores is notably similar and primarily characterized by mixture source. The organic matter in the study area primarily originates from marine organic matter, while the contribution of organic matter transported by aeolian dust is limited.

5.3. Sedimentary Environment and TOC Accumulation

Methane flux in sediment is primarily governed by the source of methane and the TOC content. The source of methane is the key factor determining methane flux and the depth of the SMI. There are two primary methane sources: biogenic methane and thermogenic methane. Biogenic methane in marine sediment is generated by interstitial microbes [63]. The production of biogenic methane is closely linked to the organic matter content in marine sediment. There are two microbial processes involved in methane generation within marine sediment. One is CO₂ reduction to generate methane (CO₂ + 4H₂ → CH₄ + 2H₂O), and another is acetate fermentation to generate methane (CH₃COOH → CH₄ + CO₂) [63]. Both processes rely on the presence of organic matter in the sediment since they utilize organic matter as a substrate to produce either CO₂ or CH₃COOH [63]. Regions with a high influx of organic matter are more likely to exhibit significant biogenic methane production. However, thermogenic methane primarily originates from fracture transport and may have a weaker correlation with local organic matter content. In reality, most gas hydrates are generated from biogenic methane sources [1,2,72,73]. The TOC content in sediment exerts control over methane flux and the depth of the SMI. On a global scale, the scatter plot of SMI depth versus TOC content reveals a linear negative correlation (R² = 0.7) [33], indicating that TOC accumulation significantly influences SMI depth.

The average TOC contents for cores CL54, CL56, CL57, and CL60 are 0.69, 1.04, 0.83, and 1.06, respectively (Figure 10f). Cores CL56 and CL60, which exhibit shallower SMI depths, display higher TOC content compared to cores CL54 and CL57, which have deeper SMI depths. This observation is consistent with the idea that a higher organic matter content is conducive to shallower SMI depths.

5.3.1. Marine Productivity

Productivity is intricately linked to the organic matter content of sediment. Marine primary productivity encompasses the capacity of phytoplankton, benthic plants (including fixed algae, mangroves, and seaweed), and autotrophic bacteria to generate organic matter through photosynthesis [74]. Elevated productivity signifies a higher abundance of organisms and a greater organic matter content in the environment. Barite serves as the primary carrier of barium in marine sediments. Biogenetic barite, also known as authigenic barite, is directly associated with marine productivity [74]. Organisms either accumulate barium in their skeletal structures or facilitate its precipitation into barite microcrystals [74]. Consequently, the Ba/Al ratio is considered a reliable proxy for assessing primary marine productivity [42]. Furthermore, phosphorus is a fundamental element essential for the growth of organisms and can serve as an indicator of marine productivity within sediment [74]. Therefore, the P/Al ratio can also be employed as a representative measure of marine productivity in sedimentary environments.

Using the Ba/Al ratios, the productivity of the four cores was reconstructed (Figure 8). The average productivity of cores CL56 and CL60, which exhibit shallower SMI depths, demonstrates higher values compared to cores CL54 and CL57, which have deeper SMI depths (Figure 9g,i). However, it’s worth noting that the Ba/Al ratio and P/Al ratio versus TOC content show little correlation (Figure 10d), and the variation in Ba/Al is not synchronized with the variation in TOC content (Figure 8). These observations imply that a high level of productivity may not confer benefits in terms of TOC accumulation in the study area.

5.3.2. Redox Conditions

Redox conditions play a significant role in influencing the organic matter content of sediment. Aerobic respiration by microbial cells can lead to the consumption of substantial amounts of organic matter, thereby reducing the organic matter content within the sediment [42]. Consequently, reducing conditions are more conducive to the preservation of organic matter, resulting in an increased organic matter content in sediment. To assess the redox conditions at the four sites, paleo-redox conditions were reconstructed using three proxies: Mo_EF, Mo/U, and Ce/Ce* (Figure 9a,e).

Trace elements serve as valuable proxies for reconstructing paleoenvironmental conditions due to their sensitivity to redox changes. The trace element enrichment factor is a reliable method for reflecting the redox conditions in sediment [75,76]. Molybdenum (Mo) is particularly sensitive to redox conditions because its content is high relative to hydrogen sulfide concentrations in pore water. Moreover, Mo readily co-precipitates with hydrogen sulfide, leading to its enrichment in sediment [77,78,79]. Consequently, Mo_EF tends to have high values in anaerobic environments. Mo/U is another valuable proxy for indicating redox conditions. It serves as a robust indicator of redox conditions and is particularly useful for distinguishing redox processes in seep environments [80]. Mo is influenced by the scavenging effect of dissolved sulfur species, while U is correlated with iron reduction [80]. Consequently, Mo tends to accumulate in sulfate reduction zones, whereas U tends to be enriched in iron reduction zones [80,81]. As a result, Mo/U ratios are higher in anaerobic conditions. Additionally, the Ce/Ce* ratio could also serve as a proxy for assessing redox conditions. The rare earth elements (REEs) system exhibits similar geochemical characteristics, and the parent fluid REE pattern can be preserved when REEs are incorporated into minerals. In sediment, a negative Ce anomaly REE pattern emerges when minerals precipitate from oxygenated seawater [72,82,83]. In an oxidizing environment, Ce³⁺ can be oxidized to Ce⁴⁺ when incorporated into iron-manganese phases, resulting in the depletion of Ce from the REE system and pore water, thereby presenting a negative Ce anomaly pattern [84].

In the sampled sites, there is a notable positive correlation between the Mo_EF and TOC content. Furthermore, Mo/U ratios also exhibit a positive correlation with TOC content. Additionally, a positive correlation is observed between Ce/Ce* and TOC content. These pieces of evidence collectively suggest that anaerobic conditions are more conducive to the accumulation of organic matter. (Figure 10e).

5.3.3. Sedimentation Rates

The rate of sedimentation plays a pivotal role in determining the organic matter content within sediment. Previous research has investigated the impact of sedimentation rate on organic matter [85]. Generally, organic matter content tends to increase with a higher sedimentation rate up to a certain threshold. However, once the sedimentation rate surpasses a specific point, it can lead to a dilution of the organic matter content in the sediment [86]. Additionally, the sedimentation rate can influence sediment porosity, with higher rates of sedimentation resulting in reduced compaction [85].

Based on a chronological framework, this study reconstructed the variations in sedimentation rate for the cores over time (Figure 8). From 0 to 10.7 thousand years before the present (ka BP), there is a consistent increase in sedimentation rate with age. During the period from 10.7 to 13 ka BP, a significant drop in K/Al (weathering index) observed in study cores indicates an increase in weathering, potentially leading to an increase in terrigenous input and a higher sedimentation rate. The study sites may have been influenced by paleoenvironmental changes, as the shift change period coincides with the Younger Dryas (YD) event [87,88,89,90].

Site CL60, with shallower SMI depths, consistently exhibits a higher average sedimentation rate than site CL57, which has deeper SMI depths. These findings suggest that a higher sedimentation rate may be advantageous for achieving shallower SMI depths. The sedimentation rates and SMI depth data compiled by Borowski et al. [4] revealed a slight negative correlation between sedimentation rate and SMI depth (Figure 10f). This supports the notion that sedimentation rate can influence the depth of the SMI, with higher sedimentation rates favoring shallower SMI depths. On a global scale, a scatter plot comparing SMI depth and sedimentation rate shows a linear negative correlation (R² = 0.7) [33], further confirming this hypothesis.

In study sites of CL57 and CL60, Ce/Ce*, TOC content, and sedimentation rates exhibited pronounced synchronous variations, implying that an increase in sedimentation rates could result in a more “anaerobic” sedimentary environment conducive to greater organic matter accumulation (Figure 8). Interestingly, there was an inverse correlation between Ba/Al and sedimentation rates, suggesting that marine productivity is hampered by an increase in sedimentation rates (Figure 8). Porosity and sedimentation rates did not show significant trends, likely due to compaction leading to a shrink in porosity in deeper sediment layers (Figure 8). These findings underscore the influence of sedimentation rate on the accumulation of organic matter.

Based on the above discussion, a model emerges that illustrates how the sedimentary environment influences organic matter accumulation in the Shenhu region (Figure 11). In an aerobic environment, organic matter is susceptible to consumption through aerobic respiration. However, when the sedimentation rate increases, organic matter is buried rapidly, reducing its exposure to oxygen. This burial process enhances the preservation of organic matter within the sediments. Furthermore, sediments with higher organic content are more likely to generate significant methane flux and result in the formation of shallow SMI.

6. Conclusions

This study investigates pore water characteristics, mineral composition, geochemistry, and sedimentary factors at four sites in the Shenhu region. The depths of the SMI in the four cores correspond to known gas hydrate locations. Analyses reveal that sediment sources predominantly consist of marine microbial shells, clay, and terrestrial detritus, indicating a mixture provenance. Marine productivity exhibits an inverse correlation with TOC content, but both sedimentation rate and Ce/Ce* ratios exhibit synchronous fluctuations with TOC content. These fluctuations suggest that high sedimentation rates and anaerobic conditions favor the accumulation of TOC. This comprehensive study provides valuable data and insights into the intricate processes governing TOC accumulation in the Shenhu region, the pioneering region for gas hydrate study.