# The Enrichment Mechanism of REE at Sulfate and Methane Transition Zone (SMTZ) of the Northern Part of the South China Sea

Authors: Xie Rui; Wu Daidai; Wei Jiangong; Wang Liaoliang; Liu Lihua; Li Wenjing; Wu Gang; Guo Xudong
Journal: Geofluids
 (2022)

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## Abstract

Rare earth elements (REE) are important raw materials for electronic products and green-energy technology and deep-sea sediment as an important potential source of REE mineral. It is of great scientific significance to understand the influence of REE in deep-sea sediments. In this paper, thirty sediments samples and eleven sediments samples were collected at the site SH3 and SH1 of Shenhu area, respectively. The trace elements, major elements, and different Fe-Mn minerals were analyzed to investigate the role of anaerobic oxidation of methane (AOM) in the enrichment of rare earth elements in the marine sediments. The result showed that the content of ∑REE/Ti increased at the top of sulfate and methane transition zone (SMTZ) at site SH1 and SH3. Combined with the analysis of different forms of ferromanganese minerals and the REE patterns, it is inferred that there is little influence of terrigenous input to the REE content. In contrary, the REE-rich carbonate phase and REE-rich Fe-Mn oxides (hydroxides) phase have important contribution of REE enrichment at the top of SMTZ. Meanwhile, the calculations of the quantitative relationship between the REE-rich carbonate phase and REE-rich Fe-Mn oxides (hydroxides) phase also confirm this conclusion. And the AOM caused the decrease of the REE-rich Fe-Mn oxides (hydroxides) phase and increase of the REE-rich carbonate phase. This is the potential explanation of ∑REE/Ti enrichment at the top of SMTZ. This study is of great scientific significance for the study of REE enrichment mechanism in the methane seeps and provided a new sight of REE enrichment mechanism in deep-sea sediments.


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## Body

## 1. Introduction

Anaerobic oxidation of methane (AOM) is a very important biogeochemical process in marine sediments. It is considered to be of significant impact on climate change, atmospheric evolution, and related element cycle [1–5]. During AOM, the residual porewater sulfate was consumed at the sulfate-methane transition zone (SMTZ). Here, an upward flux of methane reacts with a descending flux of sulfate [6–10], which is mediated by the metabolisms of methane-oxidizing archaea (MOA) and sulfate-reducing bacteria (SR) [3, 11–16]. During the AOM, the methane-driven authigenic carbonates (MDAC), pyrite, and other minerals are enriched at the sulfate and methane transition zone (SMTZ) [11, 17–20]. Of course, SMTZ was not static and moved with the accumulation of sediments and the continuous supply of methane from below (Xie et al., 2021). Therefore, the sediment profile appeared multistage carbonate, and pyrite increases with the depth. The anomalies of the content of pyrite can help identify the paleo-SMTZ. Previous researches have shown that metal oxides and hydroxides could be served as the electron acceptor during methane oxidation [15, 21–24]. In addition, seafloor observation at seeps revealed that chemical and physical parameters, such as fluid flux, temperature, thermal diffusivity, pH, and alkalinity, are also changed at the SMTZ [25–27]. Numerous studies have also demonstrated the AOM plays an important role in the cycle of P, S, Fe, Ca, Mg, B, and Cl [28–31]. However, up to now, there is little research about the influence of AOM on REEs.Kato et al. [32] suggested that the deep-sea sediments would be an important source of REEs in the future [32]. Therefore, it is particularly important to understand the REE enrichment mechanism and the influencing factors in marine sediments. At present, the REE patterns are often used in indicating the source, reconstructing the redox environment, and investigating the impact of climate change [33–36]. While carbonates formed in the oxygenated bottom water usually show a smooth seawater REE profile characterized by the enrichment of heavy REEs (HREEs) and positive La and negative Ce anomalies [37]. And Pol et al. [38] suggested that four lighter REEs (LREES) supported the growth of acidophilic methanotroph in volcanic mud pots, however, the higher atomic weight REEs (HREEs) were less conducive to growth [38]. These conclusions confirmed that the methane-rich marine sediments influence the REE distribution. Keltjens et al. also showed that light REE (LREEs) are an important constitute of methanotrophs [39, 40]. Shiller et al. [40] found the depletion of LREEs at methane-rich sediments of Gulf of Mexico [40]. Bayon et al. [41] observed Nd in the methane enriched plums above the cold seeps on the Niger Delta margin [41]. Otosaka and Noriki [42] discussed the compositional variations among REEs in settling particles to elucidate the sources of the REEs ([42]; Xiong et al., 2012; [10]). Although the REE enrichment mechanism has been studied in some sea areas by predecessors, the REE enrichment mechanism in the methane seeps has been rarely studied. In general, the Fe-Mn oxyhydroxides are sensitive to the early diagenetic remobilization in marine sediments [15, 16, 36, 43], which can be used in understanding the REEs influenced by the early diagenetic processes. In this paper, we want to answer the question of the mechanism of REE enrichments at the methane seep sediments and reveal the relationship between the AOM and REE cycle.


## 2. Geological Settings

The South China Sea is the largest marginal sea in the Western Pacific Ocean, formed under the convergent plate boundary of the Eurasian plate, the India Australia plate, and the Pacific plate [44–46]. Large and medium-sized Cenozoic sedimentary basins were developed on the northern slope of the South China Sea. Since the late Pliocene, a large amount of terrigenous organic matter has been transported and deposited on the northern slope of the South China Sea, which results in rapid sediment deposition and provides favorable conditions for the formation of gas sources [10, 47]. The studied sites SH1 and SH3 are distributed in the Shenhu area (Figure 1). The Shenhu area is located in the zhu II depression area of the Pearl River Mouth Basin on the northern slope of the South China Sea. Influenced by the formation and evolution of the Cenozoic sedimentary basin in the Pearl River Estuary, a wide range of submarine canyons, central diapir belts, and various types of diapir structures were developed in the Shenhu area [48], which provide good channels for the migration of deep source fluids [49]. The deposition rate in the Shenhu area increased and the content of organic matter was high during the Neocene. Guangzhou Marine Geological Survey carried out five gas hydrate drilling expeditions in the Shenhu area [50]. Geochemical analysis of the sediments and pore water at several stations in the Shenhu area also indicated that methane enriched fluid is active in this area [51, 52].Figure 1
The location of the study site SH1 and site SH3.


## 3. Samples and Methods

### 3.1. Sampling Locations and Sample Collection

The samples used in this study were obtained at the sites SH1 and SH3 by the Guangzhou Marine Geological Survey in the Shenhu area of the South China Sea (SCS) in 2007 (Su et al. 2015). Among them, SH3 is of particular interest because of its high gas hydrate saturation and thick hydrate-bearing sediments. The sediments at sites SH1 and SH3 are mainly silty clay. In this study, 31 samples were obtained from site SH3 at 0-25 mbsf, and 11 samples were collected from site SH1.


### 3.2. Separation of Different Forms of Fe-Mn Minerals

The following Fe-Mn minerals were separated from all collected samples a. Fecarb-carbonate iron, mainly siderite (FeCO3) and iron dolomite [Ca (Fe2+, Mg2+, Mn2+) (CO3)2]; b. Feox1—easy to reduce (amorphous) iron (hydrogen) oxides; c. Feox2 reducible (crystalline) iron oxides, Femag—magnetite; d. Femag—iron contained in magnetite (Fe3O4).The separation of said Fe-Mn minerals was carried out by the following procedure:(1)
Accurately weigh 0.25 g of dry sediment in a 50 mL clean centrifuge tube, drop 25 mL of 1 mol·L-1 NaAc-HAc buffer (pH=4.5) into it, and place it after sealing. Shake on a shaker for 24 hours, adjust the speed to ensure that the sediment is suspended in the solution. After shaking, centrifuge at 4,000 rpm for 15 minutes, pass the supernatant through a 0.4 μm filter membrane, and put the filtrate into a clean and dry centrifuge tube, 4°C cold storage is used to determine Fecarb content(2)
Add 25 mL HONH3Cl solution with a concentration of 50 g·L-1 dropwise to the solid residue in (1), seal it with a parafilm, put it in a shaker and shake for 2 h, then put it in a centrifuge and centrifuge at 4000 rpm 15 min, pass the supernatant through a 0.4 μm filter membrane, put the filtrate into a compiled centrifuge, and store it in a refrigerator at 4°C for determination of Feox1 content in the sediment(3)
Add 25 mL of 50 g·L-1 Na2S2O4 solution dropwise to the residual sediment in (2), seal and shake for 2 hours, centrifuge at 4 000 rpm for 15 minutes, and filter the supernatant through 0.4 μm membrane, and filtrate is stored in a refrigerator at 4°C for determining Feoxl content(4)
Add 25 mL of 0.2 mol·L-1 (NH4)2C2O4 dropwise to the residual sediment in (3), seal and shake for 6 hours, centrifuge at 4 000 rpm for 15 min, and pass the supernatant through 0.4 μm membrane, the filtrate is stored in a refrigerator at 4°C for determination of Femag content


### 3.3. The Calculation Methods ofδCe and δEu

δCe and δEu anomalies were calculated from the relationships.δCe=CeN/LaN×PrN1/2.δEu=EuN/SmN×GdN1/2,whereN represents chondrite normalization (data from Taylor and McClennan, 1985).


### 3.4. Major and Rare Earth Element Analysis of Bulk Sediments

In all samples, the major elements were determined at the Analytical and Testing Center of the Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. A Thermo ARL ADVANTta IntelliPower TM 2000 X-ray diffraction spectrometry (XRF) instrument was used to determine the contents of the major elements (Fe, Mn, and Ti) in sediments. Measured XRF spectral data were converted to elemental and oxide contents by UniQuant semiquantitative analysis software.The contents of rare earth elements in the whole rock were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7700e instrument at the Wuhan Shangpu Analysis Technology Co., Ltd. And rare earth element analysis was carried out by the following procedure:(1)
The 74μm sample was placed in an oven at 105°C for 12 h; (2) 50 mg of the powder sample was weighed in a Teflon bomb; (3) 1 mL of high-purity nitric acid followed by 1 mL high-purity hydrofluoric acid was added to the sample; (4) the Teflon sample bomb was put into the steel sleeve, tightened, and placed in an oven at 190°C for more than 24 h; (5) the sample bomb was cooled and place on a 140°C hotplate after opening the lid, the mixture was evaporated, and 1 mL of HNO3 was added prior to conducting another evaporative step (to a liquid free state); (6) 1 mL of high-purity nitric acid, 1 mL of Milli-Q water, and 1 mL of the internal standard (concentration, 1 ppm) were then added, and the Teflon bomb material was dissolved again, after which the bomb was placed into a steel jacket, tightened, and placed in an oven at 190°C for more than 12 h; (7) lastly, the solution was transferred to a polyethylene bottle and diluted to 100 g with 2% HNO3 for ICP-MS testing


## 3.1. Sampling Locations and Sample Collection

The samples used in this study were obtained at the sites SH1 and SH3 by the Guangzhou Marine Geological Survey in the Shenhu area of the South China Sea (SCS) in 2007 (Su et al. 2015). Among them, SH3 is of particular interest because of its high gas hydrate saturation and thick hydrate-bearing sediments. The sediments at sites SH1 and SH3 are mainly silty clay. In this study, 31 samples were obtained from site SH3 at 0-25 mbsf, and 11 samples were collected from site SH1.


## 3.2. Separation of Different Forms of Fe-Mn Minerals

The following Fe-Mn minerals were separated from all collected samples a. Fecarb-carbonate iron, mainly siderite (FeCO3) and iron dolomite [Ca (Fe2+, Mg2+, Mn2+) (CO3)2]; b. Feox1—easy to reduce (amorphous) iron (hydrogen) oxides; c. Feox2 reducible (crystalline) iron oxides, Femag—magnetite; d. Femag—iron contained in magnetite (Fe3O4).The separation of said Fe-Mn minerals was carried out by the following procedure:(1)
Accurately weigh 0.25 g of dry sediment in a 50 mL clean centrifuge tube, drop 25 mL of 1 mol·L-1 NaAc-HAc buffer (pH=4.5) into it, and place it after sealing. Shake on a shaker for 24 hours, adjust the speed to ensure that the sediment is suspended in the solution. After shaking, centrifuge at 4,000 rpm for 15 minutes, pass the supernatant through a 0.4 μm filter membrane, and put the filtrate into a clean and dry centrifuge tube, 4°C cold storage is used to determine Fecarb content(2)
Add 25 mL HONH3Cl solution with a concentration of 50 g·L-1 dropwise to the solid residue in (1), seal it with a parafilm, put it in a shaker and shake for 2 h, then put it in a centrifuge and centrifuge at 4000 rpm 15 min, pass the supernatant through a 0.4 μm filter membrane, put the filtrate into a compiled centrifuge, and store it in a refrigerator at 4°C for determination of Feox1 content in the sediment(3)
Add 25 mL of 50 g·L-1 Na2S2O4 solution dropwise to the residual sediment in (2), seal and shake for 2 hours, centrifuge at 4 000 rpm for 15 minutes, and filter the supernatant through 0.4 μm membrane, and filtrate is stored in a refrigerator at 4°C for determining Feoxl content(4)
Add 25 mL of 0.2 mol·L-1 (NH4)2C2O4 dropwise to the residual sediment in (3), seal and shake for 6 hours, centrifuge at 4 000 rpm for 15 min, and pass the supernatant through 0.4 μm membrane, the filtrate is stored in a refrigerator at 4°C for determination of Femag content


## 3.3. The Calculation Methods ofδCe and δEu

δCe and δEu anomalies were calculated from the relationships.δCe=CeN/LaN×PrN1/2.δEu=EuN/SmN×GdN1/2,whereN represents chondrite normalization (data from Taylor and McClennan, 1985).


## 3.4. Major and Rare Earth Element Analysis of Bulk Sediments

In all samples, the major elements were determined at the Analytical and Testing Center of the Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. A Thermo ARL ADVANTta IntelliPower TM 2000 X-ray diffraction spectrometry (XRF) instrument was used to determine the contents of the major elements (Fe, Mn, and Ti) in sediments. Measured XRF spectral data were converted to elemental and oxide contents by UniQuant semiquantitative analysis software.The contents of rare earth elements in the whole rock were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7700e instrument at the Wuhan Shangpu Analysis Technology Co., Ltd. And rare earth element analysis was carried out by the following procedure:(1)
The 74μm sample was placed in an oven at 105°C for 12 h; (2) 50 mg of the powder sample was weighed in a Teflon bomb; (3) 1 mL of high-purity nitric acid followed by 1 mL high-purity hydrofluoric acid was added to the sample; (4) the Teflon sample bomb was put into the steel sleeve, tightened, and placed in an oven at 190°C for more than 24 h; (5) the sample bomb was cooled and place on a 140°C hotplate after opening the lid, the mixture was evaporated, and 1 mL of HNO3 was added prior to conducting another evaporative step (to a liquid free state); (6) 1 mL of high-purity nitric acid, 1 mL of Milli-Q water, and 1 mL of the internal standard (concentration, 1 ppm) were then added, and the Teflon bomb material was dissolved again, after which the bomb was placed into a steel jacket, tightened, and placed in an oven at 190°C for more than 12 h; (7) lastly, the solution was transferred to a polyethylene bottle and diluted to 100 g with 2% HNO3 for ICP-MS testing


## 4. Result

### 4.1. The Data of Rare Earth Elements at Site SH1 and Site SH3

All data of rare earth elements at site SH1 and site SH3 are listed in Table1, the ∑REE at site SH1 and site SH3 shows continuous fluctuation with the depth. There is no obvious anomaly, however, the∑REE/Ti (Table 1) displays abrupt increase of at 25 mbsf of site SH1 and the∑REE/Ti represents an abrupt increase of at 16 mbsf of site SH3. The content of Al and Fe shows the same tendency of varied ∑REE/Ti. The UCC-normalized patterns of site SH1 and Site SH3 show LREE and HREE deficits and MREE enrichment.Table 1
The content of REE at site SH3.
Core nameDepthLa (ppm)Ce (ppm)Pr (ppm)Nd (ppm)Sm (ppm)Eu (ppm)Gd (ppm)Tb (ppm)Dy (ppm)Ho (ppm)Er (ppm)Tm (ppm)Yb (ppm)Lu (ppm)∑REE\ (ppm)Ti (%)REE/Ti (10-4)SH1-12.57 m32.062.77.2126.45.121.074.660.724.110.832.420.352.250.36150.170.40376.37SH1-24.27 m34.069.27.6328.65.591.084.930.754.360.882.580.392.370.36162.730.45364.04SH1-38.87 m33.867.17.6528.25.521.164.990.764.440.872.510.382.330.37160.150.45357.48SH1-412.27 m29.657.06.5424.64.701.034.460.693.940.812.250.342.130.34138.340.41337.42SH1-513.97 m30.160.06.8525.84.941.024.440.664.110.792.290.332.140.34143.890.43333.85SH1-618.57 m30.359.16.8625.64.791.034.480.703.990.812.350.352.240.33142.910.43333.90SH1-720.27 m34.367.97.8229.05.461.145.000.774.510.862.560.402.370.35162.350.45364.02SH1-824.87 m31.762.37.1626.65.141.094.830.734.210.852.430.372.340.35150.040.37407.73SH1-926.57 m33.665.87.5627.95.551.115.150.754.410.872.560.392.410.37158.500.46345.32SH1-1036.28 m27.051.46.0723.44.340.964.140.623.850.742.140.342.070.30127.370.38331.70SH1-1137.98 m24.246.05.4220.73.870.863.800.543.290.641.940.291.820.29113.610.36318.22


### 4.2. The Date of Fe-Mn Oxides (Hydroxides) and Fe-Mn Carbonate at Site SH3

All Fe-Mn oxides (hydroxides) and Fe-Mn carbonate are listed in Table2. At site SH3, the concentration of Fe (oxl) and Mn (oxl) decreased at SMTZ (Figure 2). The concentration of Mn (carb) increased and the concentration of Fe (carb) slight increase at the SMTZ (Table 2). What is more, the content of REE has a positive correlation with the change of Fe-Mn oxides (hydroxides) and Fe-Mn carbonate phase.Table 2
The content of different forms of Fe-Mn at site SH3.
Core nameDepth (m)Fe(crab) (ppm)Mn(crab) (ppm)Fe(oxl) (ppm)Mn(oxl) (ppm)SH30.873.6931.547//1.684.131.60510.640.0612.570.5211.24210.340.0574.272.4591.04810.230.0835.934.1710.9683.750.0376.220.6260.7143.4480.1869.680.6630.9333.5980.18210.571.1090.8588.6350.08311.427.7891.0248.9240.07513.121.0890.9819.0240.06314.825.6631.1325.820.01616.851.1241.0012.010.08517.680.8771.01615.130.10918.570.3640.89613.420.11519.420.7261.24111.290.12520.271.5811.46710.760.12921.081.1941.03410.640.06121.930.7111.0895.5810.02822.820.9021.0387.680.096Figure 2
The Fe (crab), Mn (crab), Fe (oxl), and Mn (oxl) change with the depth at site SH3. The gray area is the AOM action area.


## 4.1. The Data of Rare Earth Elements at Site SH1 and Site SH3

All data of rare earth elements at site SH1 and site SH3 are listed in Table1, the ∑REE at site SH1 and site SH3 shows continuous fluctuation with the depth. There is no obvious anomaly, however, the∑REE/Ti (Table 1) displays abrupt increase of at 25 mbsf of site SH1 and the∑REE/Ti represents an abrupt increase of at 16 mbsf of site SH3. The content of Al and Fe shows the same tendency of varied ∑REE/Ti. The UCC-normalized patterns of site SH1 and Site SH3 show LREE and HREE deficits and MREE enrichment.Table 1
The content of REE at site SH3.
Core nameDepthLa (ppm)Ce (ppm)Pr (ppm)Nd (ppm)Sm (ppm)Eu (ppm)Gd (ppm)Tb (ppm)Dy (ppm)Ho (ppm)Er (ppm)Tm (ppm)Yb (ppm)Lu (ppm)∑REE\ (ppm)Ti (%)REE/Ti (10-4)SH1-12.57 m32.062.77.2126.45.121.074.660.724.110.832.420.352.250.36150.170.40376.37SH1-24.27 m34.069.27.6328.65.591.084.930.754.360.882.580.392.370.36162.730.45364.04SH1-38.87 m33.867.17.6528.25.521.164.990.764.440.872.510.382.330.37160.150.45357.48SH1-412.27 m29.657.06.5424.64.701.034.460.693.940.812.250.342.130.34138.340.41337.42SH1-513.97 m30.160.06.8525.84.941.024.440.664.110.792.290.332.140.34143.890.43333.85SH1-618.57 m30.359.16.8625.64.791.034.480.703.990.812.350.352.240.33142.910.43333.90SH1-720.27 m34.367.97.8229.05.461.145.000.774.510.862.560.402.370.35162.350.45364.02SH1-824.87 m31.762.37.1626.65.141.094.830.734.210.852.430.372.340.35150.040.37407.73SH1-926.57 m33.665.87.5627.95.551.115.150.754.410.872.560.392.410.37158.500.46345.32SH1-1036.28 m27.051.46.0723.44.340.964.140.623.850.742.140.342.070.30127.370.38331.70SH1-1137.98 m24.246.05.4220.73.870.863.800.543.290.641.940.291.820.29113.610.36318.22


## 4.2. The Date of Fe-Mn Oxides (Hydroxides) and Fe-Mn Carbonate at Site SH3

All Fe-Mn oxides (hydroxides) and Fe-Mn carbonate are listed in Table2. At site SH3, the concentration of Fe (oxl) and Mn (oxl) decreased at SMTZ (Figure 2). The concentration of Mn (carb) increased and the concentration of Fe (carb) slight increase at the SMTZ (Table 2). What is more, the content of REE has a positive correlation with the change of Fe-Mn oxides (hydroxides) and Fe-Mn carbonate phase.Table 2
The content of different forms of Fe-Mn at site SH3.
Core nameDepth (m)Fe(crab) (ppm)Mn(crab) (ppm)Fe(oxl) (ppm)Mn(oxl) (ppm)SH30.873.6931.547//1.684.131.60510.640.0612.570.5211.24210.340.0574.272.4591.04810.230.0835.934.1710.9683.750.0376.220.6260.7143.4480.1869.680.6630.9333.5980.18210.571.1090.8588.6350.08311.427.7891.0248.9240.07513.121.0890.9819.0240.06314.825.6631.1325.820.01616.851.1241.0012.010.08517.680.8771.01615.130.10918.570.3640.89613.420.11519.420.7261.24111.290.12520.271.5811.46710.760.12921.081.1941.03410.640.06121.930.7111.0895.5810.02822.820.9021.0387.680.096Figure 2
The Fe (crab), Mn (crab), Fe (oxl), and Mn (oxl) change with the depth at site SH3. The gray area is the AOM action area.


## 5. Discussion

### 5.1. The Influence Factors on the Source and Composition of REE

REEs are usually used to identify the sediment sources and help to reconstruct the bottom-water redox conditions [28, 53–55]. At present, the contents of REEs in marine sediments are often influenced by the input of terrigenous sediments, the removal of REEs from the water column by authigenic minerals, and diagenetic alteration [56, 57]. Previous studies have also demonstrated that the REE patterns show the same characteristics as the source characteristics, therefore, the REE patterns can be applied to understand the REE provenance. In the Shenhu area, the sediments are mainly from the Pearl River, SW Taiwan, Luzon Rivers, and some authigenic minerals [58, 59]. Beyond that, Wu et al. (2017) also proved that aeolian transport from land and volcanic ash accounts for less of Shenhu marine sediments [15, 60]. Moreover, most sediments transported into the SCS by the Pearl River and Mekong River are deposited on the inner shelf and along the shore. In contrast, SW Taiwan rivers supply a large amount of sediments (>70 Mt yr−1) to the northern SCS through a deep-water bottom current along the 2000 m isobath.In this paper, the upper continental crust- (UCC-) normalized REE patterns at sites SH1 and SH3 are characterized by slight light REE (LREE (La, Ce, Pr, and Nd)) and heavy REE (HREE (Ho, Er, Tm, Yb, and Lu)), and the enrichment of middle REE (MREE (Sm, Eu, Gd, Tb, and Dy)) (Figure3). This MREE-bulge type pattern is a typical feature of anoxic pore water in the marine sediments [61], and the anoxic environment is interpreted as the consequence of AOM, because, during the AOM process, oxygen will gradually consume [62, 63]. Previous studies have demonstrated that Ce occurs as Ce3+ like other REE elements in anoxic conditions, whereas under oxic conditions, the soluble Ce3+ is oxidized to insoluble Ce4+ [64, 65]. In the studied area, all the samples at sites SH1 and SH3 showed weak negative Ce anomalies, indicating a reduced environment (Figure 3). And this conclusion is consistent with the previous result. Furthermore, sites SH1 and SH3 show lower content of HREE and LREE compared with those of the SW Taiwan, Luzou Rivers, and Parel Rivers. The result suggests that the REE receives less influence of the input of Pearl River, SW Taiwan, and Luzhon Rivers (Figure 4(b)). Ce3+ is oxidized to Ce4+ under oxidizing conditions, resulting in a negative Ce anomaly, which therefore is an effective indicator of oxic conditions, and negative Ce anomalies in marine carbonates have been shown to reflect seawater oxygenation. At site SH1 and SH3, the δCe and δEu were obviously higher than the δCe and δEu of the Parel rivers, SW Taiwan, and Luzhon rivers (Figure 4(a)). Indicating an anoxic and methane seep sediment environment at site SH1 and SH3, such a sediment environment is very favorable for AOM. The conclusion is consistent with the previous study. Therefore, the contents of REE at sites SH1 and SH3 are mainly influenced by the authigenic minerals and later diagenesis induced by the AOM. Combined with the reduced environment, we believe that the REEs are mainly influenced by the authigenic minerals and diagenetic alteration rather than detrital minerals that usually contain much higher REES [61]. However, the northern part of The South China Sea is a high incidence area of detrital input; to eliminate the impact of detrital input on REE, the REE/Ti ratios were used to eliminate the influence of terrigenous sediment input. At site SH1, the ∑REE/Ti (Table 1) contents changed from 318 ug/g to 407 ug/g and show high values at 25 mbsf with decreased content of Fe and Al. At site SH3, the ∑REE/Ti contents varied from 318 ug/g to 375 ug/g, and the highest value is at 17 mbsf with a slightly increase of Fe and Al (Figure 5). This characteristic indicates that the abrupt increase at 25 mbsf of site SH1 and 17 mbsf of site SH3 is not influenced by the input of terrigenous material. It is more likely to be influenced by the later diagenesis or effects of authigenic minerals.Figure 3
The UCC-normalized patterns of site SH1 and site SH3.Figure 4
(a) The plot ofδEu and δCe. (b) The plot of the HREE and LREE. Meanwhile, the distribution of the SW Taiwan referend from the (Li et al. 2013), Pearl river referend from the (Xu and Han, 2009), and the Luzhon river referend from (Marini et al. 2005).
(a)(b)Figure 5
The content of REE, REE/Ti, Al, and Fe change with the depth. The gray area is the current AOM action area. Among them, ∑REE/Ti has a remarkable increase at the top of the SMTZ. The black arrow indicates the increased position of REEs.


### 5.2. The Influence of AOM on the Enrichment of REEs

REE content is usually increased with the sediment depth [66, 67]. However, the content of REEs does not increase with the depth at sites SH1 and SH3. As mentioned above, the∑REE/Ti increases abruptly at sites SH1 and SH3 at 25 mbsf and 17 mbsf, respectively (Figure 5). However, the sulfate and methane transition zone (SMTZ) is at 27-36 msbf of site SH1 and the SMTZ at 18-25 mbsf of site SH3 [15, 16, 24]. The results show that the ∑REE/Ti increased above the SMTZ. As the previous discussion, the REE is mainly influenced by the later diagenetic and authigenic minerals. Theoretically, REE should be enriched in SMTZ instead of above of SMTZ. Hence, the main question is to explain the enrichment of REE at the top of SMTZ. Akagi et al. [53] found that the content of REE contributed from the carbonate with a higher REE concentration at the North Pacific Ocean and Bering Sea [53]. Liao et al. [68] considered that biogenetic apatite is one of the most important REY carriers in REY-rich deep-sea sediment [68]. Kato et al. [32] also suggested that the main host of the REE is an Fe-hydroxide precipitate from hydrothermal plumes that have taken up rare earth elements from ambient seawater. No matter how the REE forms, as a major process of early diagenesis, AOM often results in the enrichment of authigenic minerals in marine sediments. Meanwhile, AOM also influences the metal oxides and the apatite [15], indicating that it is likely to affect the distribution of REE. To further investigate the relationship between AOM and the content of REE, the Fe-Mn oxides (hydroxides) and Fe-Mn carbonate phase at site SH3 were analyzed.At site SH3, there is no correlation between the content of REE and the content of Fe-Mn oxides (hydroxides) at 0-6 mbsf. However, at the paleo-SMTZ (8-15 mbsf) and current-SMTZ, it showed an obviously positive correlation (Figure6). At the same time, the Fe-Mn carbonate phase also showed a positive correlation with the content of REE (Figure 6). This characteristic shows that the REE-carbonate phase and REE-metal oxides (hydroxides) have an important contribution to the content of REE at the top of SMTZ. To better comprehend the role of LREE and HREE in SMTZ, the relationship between HREE and Fe-Mn oxides (hydroxides) is also investigated (Figure 7). The result showed the HREE has a positive correlation with the authigenic mineral (Figure 7). Therefore, it is proved that the REE-rich Fe-Mn carbonate phase and REE-rich Fe-Mn oxides (hydroxides) affect the HREE. During the AOM, HS- and HCO3- are generated, and the HCO3- will react with the Ca2+ to form the carbonate. Therefore, it is often found the vast carbonate at methane seep areas. Meanwhile, HS- is also produced by the AOM, which reductively dissolves iron oxides (FeOx), leading to the precipitation of Fe sulfides (e.g., greigite, mackinawite, and pyrite) (Riedinger et al., 2014). During the reductive of Feox dissolution, the Feox-associated REE is released to the ambient pore water and accumulates in different forms, such as the REE-rich carbonate phase.Figure 6
The correlation of REE and different forms metal oxides or carbonate presented at paleo-SMTZ sediments and modern-SMTZ sediments.Figure 7
The correlation of HREE and different forms metal oxides or carbonate presented at paleo-SMTZ sediments or modern-SMTZ sediments.


### 5.3. Quantitative Calculation of Enrichment of REE

During the AOM, the Fe-Mn oxides (hydroxides) were reduced by the HS- and the content of authigenic minerals increased at SMTZ. The specific reaction is as follow:(1)CH4+SO42−⟶HCO3−+HS−+H2O,(2)2HCO3−+Ca2+Mg2+Sr2+⟶CaMgSrCO3+CO2+H2O,(3)2FeOOH+H2S+4H+⟶2Fe2++S0+4H2O.As can be seen from formulas (1), (2), and (3), when 1 mole of carbonate is produced, 4 moles of metal oxide are reduced.
(4)aCarbonatephaseincreaseaFe−Mnoxideshydroxidesreduces=14.[69] analyzed the different forms of REE in different sediments. And conclude that the content of REE in Fe-Mn oxides phase (hydroxides) is about 12.3 times than the content of REE in the carbonate phase.
(5)REE−richcarbonatepahseRee−richFe−Mnxoideshydroxides=112.3.According to formulas (4) and (5), it can be seen that the content of rare earth elements in the sediment will decrease by 50 ppm when 1 mol carbonate is produced and 4-mole Fe-Mn oxide reduction. The above discussion has confirmed that REE is primarily affected by the carbonate phase and Fe-Mn oxides (hydroxides). Therefore, the content of REE change should follow this quantitative relationship at SMTZ. At site SH1, the content of REE at 20 mbsf is 162.35 ppm, and the content of REE at 37.98 mbsf is about 113 ppm, the difference is about 49 ppm. The same quantitative relationship can also be found in site SH3. Therefore, the conclusion is consistent with the above.


### 5.4. The Enrichment Mechanism of REE Influenced by AOM

Previous studies have demonstrated that sulfate-driven anaerobic oxidation of methane (SD-AOM) can cause the enrichment of carbonate. At the same time, most of the metal oxides will convert to metal sulfide in the presence of excess hydrogen sulfide [2, 3, 70]. With the deepening of research, metal-driven anaerobic oxidation of methane (MD-AOM) is found at the bottom of the SMTZ worldwide [15, 24, 38, 71]. During the metal-driven anaerobic oxidation of methane (MD-AOM), more metal oxides are converted to metal sulfides compared to the SD-AOM. The previous discussion suggested that the REE-rich carbonate phase and REE-rich Fe-Mn oxides (hydroxides) have an important contribution to the REE. The question is that why the REEs are not enriched at the SMTZ while enriched on top of the SMTZ. Based on our results, it is inferred that the metal oxides (hydroxides) are reduced at the SMTZ; meanwhile, the REE-rich Fe-Mn oxides (hydroxides) will release REE in the pore water during the AOM. The released REE will reconvert to the REE-rich carbonate phase. At the SMTZ, the REE-rich Fe-Mn carbonate phase increased and the REE-rich Fe-Mn oxides (hydroxides) decreased. Therefore, the REE is not enriched in the SMTZ while enriched at the top of the SMTZ (Figure 8). And this study revealed the influence of AOM on REE at methane seeps areas. It can help us to better understand the mechanism of REE in methane seep area.Figure 8
At first, it is inferred that the metal oxides (hydroxides) are reduced at the SMTZ, meanwhile, the REE-rich Fe-Mn oxides (hydroxides) will release REE in the pore water during the AOM. The released REE will reconvert to the REE-rich carbonate phase. At the SMTZ, the REE-rich Fe-Mn carbonate phase increased and the REE-rich Fe-Mn oxides (hydroxides) decreased. Therefore, the REE is not enriched in the SMTZ while enriched at the top of the SMTZ.


## 5.1. The Influence Factors on the Source and Composition of REE

REEs are usually used to identify the sediment sources and help to reconstruct the bottom-water redox conditions [28, 53–55]. At present, the contents of REEs in marine sediments are often influenced by the input of terrigenous sediments, the removal of REEs from the water column by authigenic minerals, and diagenetic alteration [56, 57]. Previous studies have also demonstrated that the REE patterns show the same characteristics as the source characteristics, therefore, the REE patterns can be applied to understand the REE provenance. In the Shenhu area, the sediments are mainly from the Pearl River, SW Taiwan, Luzon Rivers, and some authigenic minerals [58, 59]. Beyond that, Wu et al. (2017) also proved that aeolian transport from land and volcanic ash accounts for less of Shenhu marine sediments [15, 60]. Moreover, most sediments transported into the SCS by the Pearl River and Mekong River are deposited on the inner shelf and along the shore. In contrast, SW Taiwan rivers supply a large amount of sediments (>70 Mt yr−1) to the northern SCS through a deep-water bottom current along the 2000 m isobath.In this paper, the upper continental crust- (UCC-) normalized REE patterns at sites SH1 and SH3 are characterized by slight light REE (LREE (La, Ce, Pr, and Nd)) and heavy REE (HREE (Ho, Er, Tm, Yb, and Lu)), and the enrichment of middle REE (MREE (Sm, Eu, Gd, Tb, and Dy)) (Figure3). This MREE-bulge type pattern is a typical feature of anoxic pore water in the marine sediments [61], and the anoxic environment is interpreted as the consequence of AOM, because, during the AOM process, oxygen will gradually consume [62, 63]. Previous studies have demonstrated that Ce occurs as Ce3+ like other REE elements in anoxic conditions, whereas under oxic conditions, the soluble Ce3+ is oxidized to insoluble Ce4+ [64, 65]. In the studied area, all the samples at sites SH1 and SH3 showed weak negative Ce anomalies, indicating a reduced environment (Figure 3). And this conclusion is consistent with the previous result. Furthermore, sites SH1 and SH3 show lower content of HREE and LREE compared with those of the SW Taiwan, Luzou Rivers, and Parel Rivers. The result suggests that the REE receives less influence of the input of Pearl River, SW Taiwan, and Luzhon Rivers (Figure 4(b)). Ce3+ is oxidized to Ce4+ under oxidizing conditions, resulting in a negative Ce anomaly, which therefore is an effective indicator of oxic conditions, and negative Ce anomalies in marine carbonates have been shown to reflect seawater oxygenation. At site SH1 and SH3, the δCe and δEu were obviously higher than the δCe and δEu of the Parel rivers, SW Taiwan, and Luzhon rivers (Figure 4(a)). Indicating an anoxic and methane seep sediment environment at site SH1 and SH3, such a sediment environment is very favorable for AOM. The conclusion is consistent with the previous study. Therefore, the contents of REE at sites SH1 and SH3 are mainly influenced by the authigenic minerals and later diagenesis induced by the AOM. Combined with the reduced environment, we believe that the REEs are mainly influenced by the authigenic minerals and diagenetic alteration rather than detrital minerals that usually contain much higher REES [61]. However, the northern part of The South China Sea is a high incidence area of detrital input; to eliminate the impact of detrital input on REE, the REE/Ti ratios were used to eliminate the influence of terrigenous sediment input. At site SH1, the ∑REE/Ti (Table 1) contents changed from 318 ug/g to 407 ug/g and show high values at 25 mbsf with decreased content of Fe and Al. At site SH3, the ∑REE/Ti contents varied from 318 ug/g to 375 ug/g, and the highest value is at 17 mbsf with a slightly increase of Fe and Al (Figure 5). This characteristic indicates that the abrupt increase at 25 mbsf of site SH1 and 17 mbsf of site SH3 is not influenced by the input of terrigenous material. It is more likely to be influenced by the later diagenesis or effects of authigenic minerals.Figure 3
The UCC-normalized patterns of site SH1 and site SH3.Figure 4
(a) The plot ofδEu and δCe. (b) The plot of the HREE and LREE. Meanwhile, the distribution of the SW Taiwan referend from the (Li et al. 2013), Pearl river referend from the (Xu and Han, 2009), and the Luzhon river referend from (Marini et al. 2005).
(a)(b)Figure 5
The content of REE, REE/Ti, Al, and Fe change with the depth. The gray area is the current AOM action area. Among them, ∑REE/Ti has a remarkable increase at the top of the SMTZ. The black arrow indicates the increased position of REEs.


## 5.2. The Influence of AOM on the Enrichment of REEs

REE content is usually increased with the sediment depth [66, 67]. However, the content of REEs does not increase with the depth at sites SH1 and SH3. As mentioned above, the∑REE/Ti increases abruptly at sites SH1 and SH3 at 25 mbsf and 17 mbsf, respectively (Figure 5). However, the sulfate and methane transition zone (SMTZ) is at 27-36 msbf of site SH1 and the SMTZ at 18-25 mbsf of site SH3 [15, 16, 24]. The results show that the ∑REE/Ti increased above the SMTZ. As the previous discussion, the REE is mainly influenced by the later diagenetic and authigenic minerals. Theoretically, REE should be enriched in SMTZ instead of above of SMTZ. Hence, the main question is to explain the enrichment of REE at the top of SMTZ. Akagi et al. [53] found that the content of REE contributed from the carbonate with a higher REE concentration at the North Pacific Ocean and Bering Sea [53]. Liao et al. [68] considered that biogenetic apatite is one of the most important REY carriers in REY-rich deep-sea sediment [68]. Kato et al. [32] also suggested that the main host of the REE is an Fe-hydroxide precipitate from hydrothermal plumes that have taken up rare earth elements from ambient seawater. No matter how the REE forms, as a major process of early diagenesis, AOM often results in the enrichment of authigenic minerals in marine sediments. Meanwhile, AOM also influences the metal oxides and the apatite [15], indicating that it is likely to affect the distribution of REE. To further investigate the relationship between AOM and the content of REE, the Fe-Mn oxides (hydroxides) and Fe-Mn carbonate phase at site SH3 were analyzed.At site SH3, there is no correlation between the content of REE and the content of Fe-Mn oxides (hydroxides) at 0-6 mbsf. However, at the paleo-SMTZ (8-15 mbsf) and current-SMTZ, it showed an obviously positive correlation (Figure6). At the same time, the Fe-Mn carbonate phase also showed a positive correlation with the content of REE (Figure 6). This characteristic shows that the REE-carbonate phase and REE-metal oxides (hydroxides) have an important contribution to the content of REE at the top of SMTZ. To better comprehend the role of LREE and HREE in SMTZ, the relationship between HREE and Fe-Mn oxides (hydroxides) is also investigated (Figure 7). The result showed the HREE has a positive correlation with the authigenic mineral (Figure 7). Therefore, it is proved that the REE-rich Fe-Mn carbonate phase and REE-rich Fe-Mn oxides (hydroxides) affect the HREE. During the AOM, HS- and HCO3- are generated, and the HCO3- will react with the Ca2+ to form the carbonate. Therefore, it is often found the vast carbonate at methane seep areas. Meanwhile, HS- is also produced by the AOM, which reductively dissolves iron oxides (FeOx), leading to the precipitation of Fe sulfides (e.g., greigite, mackinawite, and pyrite) (Riedinger et al., 2014). During the reductive of Feox dissolution, the Feox-associated REE is released to the ambient pore water and accumulates in different forms, such as the REE-rich carbonate phase.Figure 6
The correlation of REE and different forms metal oxides or carbonate presented at paleo-SMTZ sediments and modern-SMTZ sediments.Figure 7
The correlation of HREE and different forms metal oxides or carbonate presented at paleo-SMTZ sediments or modern-SMTZ sediments.


## 5.3. Quantitative Calculation of Enrichment of REE

During the AOM, the Fe-Mn oxides (hydroxides) were reduced by the HS- and the content of authigenic minerals increased at SMTZ. The specific reaction is as follow:(1)CH4+SO42−⟶HCO3−+HS−+H2O,(2)2HCO3−+Ca2+Mg2+Sr2+⟶CaMgSrCO3+CO2+H2O,(3)2FeOOH+H2S+4H+⟶2Fe2++S0+4H2O.As can be seen from formulas (1), (2), and (3), when 1 mole of carbonate is produced, 4 moles of metal oxide are reduced.
(4)aCarbonatephaseincreaseaFe−Mnoxideshydroxidesreduces=14.[69] analyzed the different forms of REE in different sediments. And conclude that the content of REE in Fe-Mn oxides phase (hydroxides) is about 12.3 times than the content of REE in the carbonate phase.
(5)REE−richcarbonatepahseRee−richFe−Mnxoideshydroxides=112.3.According to formulas (4) and (5), it can be seen that the content of rare earth elements in the sediment will decrease by 50 ppm when 1 mol carbonate is produced and 4-mole Fe-Mn oxide reduction. The above discussion has confirmed that REE is primarily affected by the carbonate phase and Fe-Mn oxides (hydroxides). Therefore, the content of REE change should follow this quantitative relationship at SMTZ. At site SH1, the content of REE at 20 mbsf is 162.35 ppm, and the content of REE at 37.98 mbsf is about 113 ppm, the difference is about 49 ppm. The same quantitative relationship can also be found in site SH3. Therefore, the conclusion is consistent with the above.


## 5.4. The Enrichment Mechanism of REE Influenced by AOM

Previous studies have demonstrated that sulfate-driven anaerobic oxidation of methane (SD-AOM) can cause the enrichment of carbonate. At the same time, most of the metal oxides will convert to metal sulfide in the presence of excess hydrogen sulfide [2, 3, 70]. With the deepening of research, metal-driven anaerobic oxidation of methane (MD-AOM) is found at the bottom of the SMTZ worldwide [15, 24, 38, 71]. During the metal-driven anaerobic oxidation of methane (MD-AOM), more metal oxides are converted to metal sulfides compared to the SD-AOM. The previous discussion suggested that the REE-rich carbonate phase and REE-rich Fe-Mn oxides (hydroxides) have an important contribution to the REE. The question is that why the REEs are not enriched at the SMTZ while enriched on top of the SMTZ. Based on our results, it is inferred that the metal oxides (hydroxides) are reduced at the SMTZ; meanwhile, the REE-rich Fe-Mn oxides (hydroxides) will release REE in the pore water during the AOM. The released REE will reconvert to the REE-rich carbonate phase. At the SMTZ, the REE-rich Fe-Mn carbonate phase increased and the REE-rich Fe-Mn oxides (hydroxides) decreased. Therefore, the REE is not enriched in the SMTZ while enriched at the top of the SMTZ (Figure 8). And this study revealed the influence of AOM on REE at methane seeps areas. It can help us to better understand the mechanism of REE in methane seep area.Figure 8
At first, it is inferred that the metal oxides (hydroxides) are reduced at the SMTZ, meanwhile, the REE-rich Fe-Mn oxides (hydroxides) will release REE in the pore water during the AOM. The released REE will reconvert to the REE-rich carbonate phase. At the SMTZ, the REE-rich Fe-Mn carbonate phase increased and the REE-rich Fe-Mn oxides (hydroxides) decreased. Therefore, the REE is not enriched in the SMTZ while enriched at the top of the SMTZ.


## 6. Conclusion

In this paper, there is an obvious increase of ∑REE/Ti at the top of SMTZ of sites SH1 and SH3. Based on the analysis ofδEu vs. δCe, HREE vs. LREE, and the REE-UCC normalized patterns, the REE characteristics of SH1 and SH3 are different from the Pearl River, SW Taiwan, and the Luzhon river, indicating that the terrigenous sediment input has little contribution to the REE enrichments. Hence, the REE was suggested to be more influenced by the authigenic minerals and later diagenesis. Combined the analysis of the different forms of Fe-Mn oxides (hydroxides) minerals, carbonate phase, and the REE patterns, the Fe-Mn oxides (hydroxides) and Fe-Mn carbonate phase have a positive correlation with the REE content, indicating that the REE-rich carbonate phase and REE-rich Fe-Mn oxides (hydroxides) phase have the important contribution of REE enrichment at the SMTZ. However, there is little increase of REE at the SMTZ, it increased at the top of the SMTZ. It is inferred that the AOM results in the decrease of the REE-rich Fe-Mn oxides (hydroxides) phase and a decrease of the REE-rich carbonate phase. This is the reason why the ∑REE/Ti is enriched at the top of the SMTZ. Our results are of great scientific significance for understanding the REE enrichment mechanism in the methane seeps.



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*Source: 7571792-2022-02-24.xml*