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Article

Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities

by
Jiangong Wei
1,2,
Tingting Wu
1,*,
Wei Zhang
1,2,
Yinan Deng
1,
Rui Xie
1,*,
Junxi Feng
1,
Jinqiang Liang
1,
Peixin Lai
1,
Jianhou Zhou
1 and
Jun Cao
1
1
MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China
2
Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
*
Authors to whom correspondence should be addressed.
Minerals 2020, 10(12), 1135; https://doi.org/10.3390/min10121135
Submission received: 9 November 2020 / Revised: 27 November 2020 / Accepted: 10 December 2020 / Published: 17 December 2020
(This article belongs to the Special Issue Trace Elements in Carbonates: Isotopic and Geochronological Record)
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Abstract

:
Cold seep carbonates are important archives of pore water chemistry and ancient methane seepage activity. They also provide an important contribution to the global carbon sink. Seep carbonates at three sediment layers (3.0, 52.1, and 53.6 mbsf) were collected at site W08B in the Qiongdongnan Basin of the South China Sea. This study investigated the mineralogy, microstructure, stable carbon and oxygen isotopes, trace elements, and U-Th dates of these carbonates to identify the relationship between methane flux and authigenic carbonate precipitation. The results showed that the δ13C and δ18O values of all carbonates are similar, indicating that the carbon source for shallow carbonates and deep carbonates has remained constant over time and included biogenic and thermogenic methane. Although carbonates were found in three sediment layers, the two main stages of methane seepage events were discernible, which was likely caused by the dissociation of gas hydrates. The first methane seep took place at 131.1–136.3 ka BP. During a dramatic drop in the sea level, the seep carbonate at 52.1 mbsf formed at 136.3 ka BP through the anaerobic oxidation of methane (AOM). The carbonate at 53.6 mbsf resulted from the vertical downward movement of the sulfate-methane transition zone with decreasing methane flux at 131.1 ka BP. This is the reason for the age of carbonates at 52.1 mbsf being older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonate formed at that time via AOM and is now located at 3 mbsf. Moreover, thin-section photomicrographs of deep carbonate mainly consisted of matrix micrite and biological debris and acicular aragonite occurred as vein cement filling the pore spaces between the matrix micrite. The acicular aragonite was mainly influenced by the timing of the carbonate precipitation of minerals. This research identified a long history of methane seep activity reflected by the vertical distribution of carbonates.

1. Introduction

A deep-sea cold seep is a unique environment where deeply sourced fluids (containing methane and other hydrocarbon gases) migrate upward and seep from the seafloor [1]. Cold seep development is mainly driven by overpressure, which is caused by tectonic compression and rapid sediment deposition [2,3]. Therefore, unlike hydrothermal systems, which are mainly distributed in mid-ocean ridges [4,5], cold seeps are widely dispersed throughout the active and passive continental margins and sedimentary basins with thick sediments [3,6,7,8]. Methane reacts with seawater sulfate via the sulfate-driven anaerobic oxidation of methane (SR-AOM), which produces hydrogen sulfide and bicarbonate [9], and provides nutrients for chemosynthetic communities [10,11,12]. Therefore, a special chemosynthetic ecosystem [13], widely distributed carbonate on the seafloor [14], and gas bubble emission from the seafloor [15,16] are important indicators for the identification of marine cold seeps. It is of great significance to study the distribution and activity of these cold seeps because they are not only used as indicators for deep-water petroleum exploration at the early stage [17], but are also closely related to global climate change [18], ocean acidification [19], and the global carbon cycle [20,21].
Seep carbonates represent important records of cold seep activities [22]. Comprehensive studies, investigating mineralogy, stable carbon isotope (δ13C) ratios, oxygen isotope (δ18O) ratios, rare earth elements (REEs), and dating (U-Th and 14C), have been conducted. Seep carbonates have been sampled from various sea areas worldwide, thus providing the possibility to reconstruct the cold seep environment and its activity [23,24,25,26,27,28]. The δ13C provides particularly valuable clues for developing a better understanding of the carbon source [23], while the δ18O plays a significant role in identifying the surrounding temperature and fluid source during carbonate formation [29]. REEs are an important proxy for reconstructing the prevailing redox conditions [24]. Furthermore, dating based on U-Th and 14C is often used to identify the relationship between cold seep activity and global climate change [30].
Over the past two decades, several active cold seeps have been discovered on the northern slope of the South China Sea [31], including the Jiulong seeps [32], Haima cold seeps [10], and station F [33]. In recent years, a large number of seep carbonates have been collected from these cold seeps by multiple research expeditions [30,31,32,34]. More information on cold seep activities has been increasingly reported. These samples were usually obtained by a gravity corer or a remotely operating vehicle (ROV) in shallow sediment. However, most carbonates in shallow sediments have ages younger than 20 ka, which cannot help to study a longer period of cold seep activity.
Controlled by various factors, such as the gas supply, deep strata pressure, changes in the sea level, global climate change, and gas hydrate and carbonate formation, cold seeps are usually intermittently and periodically active [10,14,35]. Therefore, seep carbonates buried at different depths are valuable for reconstructions of the historical activities of ancient cold seeps [26]. However, since they are buried deep in the marine sediment, these samples can only be obtained by drilling, which is very costly. Therefore, very few drilling expeditions are specifically designed for cold seep research. In 2018, the Guangzhou Marine Geological Survey (GMGS) carried out the fifth natural gas hydrate drilling expedition in the Qiongdongnan Basin of the South China Sea (Figure 1) [36]. Seep carbonates were obtained from multiple layers of site W08B (Figure 1) [30]. Based on both field data and literature research, preliminary hypotheses of the cold seep activity were formulated [30]; however, these hypotheses have not been validated by geochemical data. This study systematically investigated the seep carbonates in multiple horizons of site W08B, using mineralogy, δ13C, δ18O, U-Th dating, and REE analysis. The age, carbon and oxygen sources, and redox environment of carbonate formation were discussed. Based on the U-Th ages, the sedimentation rate in this area was further calculated. Finally, the relationship between methane flux and different carbonate textures, as well as the relationship between the historical activity of methane seeps and the distribution of carbonate, were identified.

2. Geological Settings and Site Description

The study area is located in the Qiongdongnan Basin of the South China Sea (Figure 1). The Red River Fault, which is the largest N–S strike-slip belt, marks the western boundary of the Qiongdongnan Basin [37]. The sedimentary sequence of the Qiongdongnan Basin includes Eocene lacustrine deposits, Oligocene lacustrine and shallow water deposits, and Miocene deep-water deposits [38]. Thermogenic gas is mainly derived from mature to over-mature Oligocene source rocks [39], while biogenic gas mainly originates from immature to low-maturity Miocene to Pliocene source rocks [40]. Channels, fans, and gravity deposits are widely developed in the study area [41]. In recent years, a large number of bottom simulating reflectors (BSR) and gas chimneys have been widely discovered [42], indicating the existence of both natural gas hydrates and fluid flow in the Qiongdongnan Basin.
In 2018, site W08B was drilled for natural gas hydrate exploration by the GMGS in the Qiongdongnan Basin [30]. The water depth is around 1740 m and the drilling depth was about 164 m. ROV seafloor observation identified the sporadic distribution of seep bivalve remnants and tube worms on the seafloor, indicating that the ancient cold seeps at this site were historically active; however, gas emission was not observed [30]. Seep carbonates were recovered from multiple subseafloor strata, implying intermittent cold seep activity. The drilling results identified a complex natural gas hydrate system, which had developed in the gas chimney [30]. Logging while drilling and coring results showed that gas hydrates are distributed at 6–174 mbsf. The hydrate-bound gas consists of a large volume of high-order hydrocarbons, such as ethane and propane [30]. δ13C and gas composition analysis indicated that the identified hydrate-bound gases are a mixture of biogenic and thermogenic gas. The paleo-uplift resulted in the differential compaction and formation of fractures and faults, which ultimately controlled the development of gas hydrate [43]. An intermediate gas reservoir lies beneath 190 mbsf, which has been suggested to bridge the deep gas reservoir and shallow gas hydrate system [36]. Ascribed to the gas chimney and fluid flow system, the geothermal gradient at this site reaches up to 102 °C/km, which is almost twice the background value [30].

3. Materials and Methods

The carbonate samples used in this study were recovered from sediment layers at 3, 52.1, and 53.6 mbsf of borehole W08B, for which the carbonate sample numbers 3, 52.1, and 53.6 were used, respectively. Twelve sub-samples (Table 1) were collected from the three samples for X-ray diffraction and REE analysis, five of which were prepared for δ13C and δ18O. Petrographic observations were performed on samples 52.1 and 53.6 (Figure 2). U-Th dating was also performed on the three samples. All analyses were conducted at the laboratory of the GMGS, except for the U-Th dating, which was conducted in the Institute of Geology and Geophysics, Chinese Academy of Sciences.
Petrographic analyses of the carbonate samples were conducted by using polarized light microscopy (Zeiss Axioskop 40). Scans were run from 5° to 65° 2θ (where θ is the scan angle), with a step size of 0.02° and count time of 5 s per step. X-ray diffraction was used to analyze and identify the carbonate mineral composition (Table 1). The samples were first dried for 7 h and then gently ground by a mortar. The ground sample was packed into X-ray sample holders for analysis. Scans were run at room temperature using a Rigaku D/Max 2500 X-ray diffractometer with 18 kW power. The species and content of the minerals were analyzed by using the software Jade2010 [44].
Stable carbon and oxygen isotope ratios were measured using a Thermo MAT-253 isotope ratio mass spectrometer. CO2 gas was extracted by a reaction with supersaturated phosphoric acid on a Thermo Kiel IV Carbonate Device and was introduced into the MAT-253 dual inlet system. The isotope ratios were reported relative to Vienna Peedee Belemnite (VPDB). The precisions of δ13C and δ18O were ±0.1‰ and ±0.25‰, respectively.
REE analysis was performed following the procedure established by Rongemaille et al. [45]. Fifty milligrams of sample was weighed into a pre-cleaned Teflon centrifuge tube. Then, an ultra-pure solution of 5% acetic acid was added and the mixture was left to react for 24 h in a water bath to separate carbonate mineral and residue phases. Subsequently, the mixture was centrifuged to obtain the supernatant, and the process of washing and centrifugation was repeated three times. Finally, the solution was evaporated on a hotplate until complete dryness and was dissolved in 3% HNO3. REE was measured via ICP-MS (Therme Fisher Scientific, Waltham, MA, USA). The analytical precision was better than 5%. In this paper, Ce/Ce * = 3CeN/(2LaN + NdN), Pr/Pr * = 2PrN/(CeN + NdN), Eu/Eu * = 2EuN/(SmN + GdN). N refers to the normalized concentration against the standard Post-Archaean Australian Shale [43].
Furthermore, 10–50 mg of each subsample from three carbonates was used to conduct the U-Th dating analyses. The chemical procedures used to separate uranium and thorium for 230Th dating have been described in [46]. 230Th dating was performed by using a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS, Thermo-Scientific Neptune). Details of instrumental approaches have been described in Wang et al. [47].

4. Results

4.1. Petrology and Mineralogy

The microphotograph shows that shallow carbonates mainly consist of matrix micrite. The thin-section photomicrographs of deep carbonate mainly consist of matrix micrite and biological debris (Figure 3). In general, acicular aragonite occurs as vein cement filling the pore spaces between the matrix micrite (Figure 3a–c). It was common to observe the accretionary overgrowth of rim cement around the foraminifera test (Figure 3d). All carbonates contain seep bivalve shells (Figure 2). XRD analysis showed that, except for sample 53–7 (Table 1), all carbonate samples are dominated by aragonite (61.1–98.0%), with a certain amount of calcite (0.5–18.3%) and a high amount of magnesium calcite (1.9–7.6%). In sample 53-7, the content of high-magnesium calcite (35.9%) is approximately as high as that of aragonite (40.5%). Sample 52–1 was collected from the loose translucent part of the carbonate and contains the highest content of aragonite (98%). In addition, most samples also contain a small amount of other minerals, such as quartz (3.2–14.0%), illite (2.5–3.5%), and albite (0.6–2.4%). Gypsum (0.4%) occurred in sample 3–2.

4.2. Rare Earth Elements

The total content of REEs varies significantly from 2.64 to 64.5 ppm (Table 2). The Ce/Ce * values show a normal to slightly negative anomaly (Ce/Ce * < 1.05), which ranges between 0.79 and 1.04. Compared with heavy REEs (Ho to Lu) and light REEs (La to Nd), medium REEs (Sm to Dy) in the carbonates are relatively enriched.

4.3. δ13C and δ18O Values

The δ13C and δ18O of the five carbonate sub-samples are presented in Figure 4 The result shows that the δ13C and δ18O of samples 53–1 and 52–1 are −38.1‰ and −38.0‰, and 4.6‰ and 5.0‰, respectively. Compared with deeply buried samples (53–1 and 52–1), shallowly buried samples (3–1, 3–2, and 3–3) have higher δ13C values (between −33.8‰ and −25.6‰) and lower δ18O values (between 3.6‰ and 3.8‰).

4.4. U-Th Age of Carbonates

The U-Th ages of the three carbonate samples are presented in Table 3. The result shows that the age of sample 3 (~3 mbsf) is 12.2 ± 2.3 ka. The ages of the samples at 52.1 and 53.6 mbsf are very similar, namely 136.3 ± 3.6 and 131.1 ± 2.5 ka, respectively.

5. Discussion

5.1. Fluid Source and Environment of Seep Carbonate Formation

At cold seeps, the anaerobic oxidation of methane (AOM) typically proceeds in anoxic sub-surface sediments, which leads to increased alkalinity levels in pore waters and, as a consequence, often results in carbonate precipitation [27,31,44,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. Occurrences of authigenic carbonate deposits at the sea floor or in sedimentary records are therefore often related to the paleo-seepage of methane-rich fluids at continental margins. At GMGS5, carbonates were collected at 3, 52, and 53 mbsf layers of the Qiongdongnan Basin of the South China Sea. Gas hydrates have been found at 6 and 136 mbsf and the current sulfate and methane transition zone (SMTZ) is located at about 0.47 mbsf [43]. Therefore, GMGS5 is an ideal experimental site for studying the long history and the relationship between carbonate formation and methane leakage.
REEs have been widely applied as a redox proxy in the marine environment [37,38,39,40,41,42,43,44,45,46]. Diagenetic processes generally alter the REE composition of carbonates and cause a negative correlation between Ce/Ce * and DyN/SmN and a positive correlation between Ce/Ce * and ∑REE [47]. In the studied samples, no such correlation was found (Figure 5d,f). Hence, it can be assumed that all studied samples retained their original REE budget. Feng et al. (2009) considered that small MREE enrichment or flat REE patterns of methane-seep carbonates are indicative of intermittent redox conditions [46,48,49,50,51]. The REE/PAAS patterns in shallow carbonates and deeper carbonates are flat REE patterns with small MREE enrichments in both carbonates. The intermittent redox conditions indicate that the deeper carbonates and shallow carbonates formed in the sulfate methane transition zone. This is because the redox environment of deep sediments remains stable and will not be affected by debris input. On the contrary, the redox environment of the shallow sediments, which is close to the water-sediment surface, is stable under the influence of debris input. Large number of seep bivalve shells that can only survive on the seafloor were cemented in the carbonates [52,53]. This also indicates that the methane flux was high at the time of carbonate formation. Moreover, none of the carbonate samples show Ce anomalies (Figure 5a). There were no Ce anomalies or slightly negative Ce anomalies ranging from 0.79 to 1.05 (Table 2), mainly indicating that seep carbonates were formed in a reducing condition [23,48,49,50,54,55]. Remarkably, the deeper carbonates at 52.1 and 53.6 mbsf show positive Eu anomalies, indicating the mixing of reducing porewater (positive anomaly) with variable proportions of seawater (no Eu anomaly). An additional factor to be considered is the possible contribution of deep-sourced fluids that generally carry an Eu-enriched signal generated by Eu fractionation at elevated temperatures [42]. Based on the widely distributed gas chimneys and deep faults in the area [43], it is reasonable to assume that deeper fluids may also have influenced the REE budget and partly contributed to the development of positive Eu anomalies in the porewater [43]. These data are indicative of precipitation under variable redox conditions as the frequency observed in modern and ancient cold seep carbonates.
In marine sediments, methane is mainly produced via biogenic and thermogenic processes [12,13,27,28,29,30,31,32,33,34,35,48]. Generally, the δ13C values of biogenic methane typically fall within the range of −110‰ to −50‰, whereas the values of thermogenic methane typically range from −50‰ to −30‰ [31,34,56,63]. Moreover, δ13C values of sedimentary organic matter are about −25.0‰ on average. The δ13C values of carbonates always inherit the δ13C characteristic of the carbon source for carbonates. The pore water dissolved inorganic carbon (DIC) includes different carbon sources (methane-derived carbon, seawater DIC, and DIC from organic matter degradation). Therefore, δ13C of the seep carbonates can be used to indicate the carbon source of the seeping fluids [42,43]. The δ13C values of deeper carbonates and shallow carbonates varied from −38.1‰ to −25.6‰, covering the range for biogenetic gas and thermogenic gas, and also covering the sedimentary organic matter. Accordingly, it is reasonable to propose that carbon sources for shallow carbonates and deep carbonates are diverse, including biogenic and thermogenic methane and sedimentary organic matter. Among them, biogenic methane is mainly derived from the Miocene source rock and its overlying strata, while thermogenic methane mainly originates from Oligocene source rocks [48]. However, it is difficult to identify methane sources, based on the δ13C values of carbonates, because the great variability in δ13C values demonstrates the geochemical complexity of these sites.
The δ18O values of shallow carbonates range from 3.6‰ to 3.8‰ and the δ18O values of deep carbonates range from 4.6‰ to 5.0‰. Both samples show obviously higher δ18O values than seawater. The fact that anomalously positive δ18O values of carbonate are obviously different from those of modern seawater points to the precipitation of carbonate from 18O-enriched fluids. At a constant bottom water temperature during carbonate formation, the anomalous δ18O of carbonate can be explained by the presence of exotic 18O-enriched fluids. In general, the source of 18O-enriched fluids is the dehydration of clay minerals at great depths and or dissolution of gas hydrate [57,63,64,65]. For the first case, the reaction of clay mineral dehydration takes place at least 2 km below the seafloor and the gas source rock depth in our study area is not compatible with that process. Therefore, the possible influence of the dehydration of clay minerals on δ18O values could be excluded in the first place. For the second case, the enrichment in δ18O of interstitial fluids in the study area is best explained by gas hydrate dissolution. The δ18O values of carbonates 52–1 and 53–1 (5.0 ‰ and 4.6 ‰, respectively) are rather heavy compared to most recent samples (3.6–3.8‰) and might reflect paleo-gas hydrate dissociation. It can be speculated that in the process of the upward migration of methane-rich fluids, released by the deeper hydrate (136 mbsf), a large amount of released methane will reform at the shallow hydrate layer (6 mbsf) under an appropriate temperature and pressure environment.

5.2. Influence of Methane Flux on the Formation of Authigenic Carbonate

This section addresses three questions: (1) As mentioned above, the deeper carbonates and shallow carbonates are formed by AOM, and the organic characteristic of different layers is consistent. What mechanism causes methane seeps? (2) The U-Th date of carbonate at 3 mbsf is the 12.2 ± 2.2 ka period, while for carbonate at 52.1 mbsf, it is the 136.3 ± 3.6 ka period, and for carbonate at 53.6 mbsf, it is the 131.1 ± 2.5 ka period. What are the causes of the older age of carbonate at 52 mbsf than the age of carbonate at 53 mbsf? (3) There are two major types of aragonite: The shallow carbonates cement by microcrystalline aragonite in the matrix, while the surface of the cavities in the deep carbonates cement by acicular aragonite (Figure 3). What are the causes of the different morphologies of aragonite?
With regard to the first question, in general, methane flux has been assumed to be influenced by changes in sea level and tectonic activities at continental margins [55,56,57,58,59,60,63,64,65]. Among these changes, the change of sea level has been regarded as the prominent factor, leading to the decomposition of gas hydrate in geological history [61]. The U-Th dates of all samples fall within periods in which the sea level was 110 m lower than the present sea level (Figure 6) [61,62,66,67,68]. The coincidence with a low stand is very strong evidence for relationships between seep activity and the sea level. This suggests that sea level change was the main factor that triggered the leakage of methane-rich fluid [68]. Combined with the gas hydrate distribution at 6 and 136 mbsf, this study proposes that the methane-rich fluids were mainly produced by gas hydrate dissociation. This result is consistent with other research conclusions, indicating that the seep carbonates formed in a period of carbonate formation with lower sea levels [52,53,54,55,56].
With regard to the second question (why carbonates at 52 mbsf are older than those at 53 mbsf), in this study area, the sulfate and methane transition zone was not stable during the geological history. At about 136 ka BP, the global sea level dropped rapidly, and the methane-rich fluids diffused from the bottom and reacted with sulfate when it was close to the sediment–water interface. With the AOM, the carbonate gradually precipitated in the sulfate and methane transition zone. Then, between 131 and 136 ka BP, with an increasing temperature, the sea level gradually rose. With this change in sea level, the methane flux gradually decreased. Previous studies have suggested that the sulfate and methane transition zone (SMTZ) was influenced by the methane flux and sedimentation rate [46,47]. The sediments in the Qiongdongnan Basin are mainly from rivers and gravity deposits with a sedimentation rate of up to 1 m/ka [30,47]. However, the Qiongdongnan Basin is very broad, with large spatial differences in sedimentation rates. Based on the ages of the three samples obtained by U-Th dating, the deposition rate of this site was calculated to be approximately 0.41 m/ka. However, due to the scarcity of dating data, the calculated deposition rate is unreliable. It can be found from the seismic profile of site W08 that there is no development of turbidity sediments. Therefore, the influence of turbidity can be eliminated (Figure 7); therefore, the age reversal was not caused by debris input or the turbidity of the sediment. Hence, the main reason for the age reversal was a change in methane flux. In general, the more intense the methane flux, the shallower the SMTZ, and the weaker the methane flux, the deeper the SMTZ. Therefore, as the methane flux decreased with the changing sea level, the SMTZ moved downward. The younger carbonate at 53.6 mbsf formed at a deeper SMTZ.
With regard to the third question (the causes of the difference between shallow carbonates and deep carbonates), the petrology and mineral composition of seep carbonates are useful for understanding the environmental changes underlying carbonate formation [25,51,52]. XRD showed that both aragonites are pure minerals, containing no other components. Furthermore, allogenic terrigenous minerals, quartz, and feldspar comprise a minor portion of authigenic carbonate rock (Figure 8a). Moreover, XRD analyses showed that the aragonite content is high, except for the carbonate at 53.6 mbsf, which has high contents of high-Mg calcite. It is indicated that shallow carbonates and deeper carbonates were formed under high SO42− environments. As a typical authigenic carbonate mineral, aragonite is more likely to be affected by the fluid environment [56,57,58,59,60,61,63,64,65]. Of the allogenic terrigenous minerals, quartz is the most thermodynamically stable as most natural fluids achieve supersaturation concerning quartz [53]. In this study, the plot of aragonite content vs. quartz content shows an obvious negative correlation (Figure 8b). This relationship was also observed in the seep carbonates of the Gulf of Mexico [53,54,55,56,63,64]. The quartz and aragonite content can be used as indicators of the evolution of a seepage system, to a certain extent. In fact, the formation and dissolution of carbonate minerals in authigenic carbonates follow a time sequence. At the early stage, high-Mg calcite and aragonite are the primary cement, while the quartz content is low. However, at the dissolution stage, aragonite starts to dissolve. At the last stage, the quartz increases because the HMC and aragonite are replaced by silica and quartz [57,65,67,68]. At the studied site, the deeper carbonate and shallow carbonate show a high content of aragonite and low content of quartz. This indicates that all carbonates are not significantly affected by diagenetic alteration. In general, aragonite seems to be favored in high SO42− environments with high total alkalinity concentrations [69,70]. The crystallization of magnesium calcite preferentially occurs under conditions with lower SO42− and total alkalinity concentrations [71]. Furthermore, it is suggested that the occurrence of aragonite reflects high methane flux and efficient methane oxidation [71,72]. In deep and shallow carbonates, minerals are mainly composed of aragonite. It is indicated that all the carbonates formed under high SO42−. However, the microphotographs show that shallow carbonates mainly consist of matrix micrite. The thin-section photomicrographs of deep carbonate mainly consist of matrix micrite and biological debris (Figure 3). In general, acicular aragonite occurs as vein cement filling the pore spaces between the matrix micrite (Figure 3a–c). It was common to observe the accretionary overgrowth of rim cement around the foraminifera test. Harry et al. considered that seep carbonates precipitated in a high sulfate concentration environment are characterized by sparitic and acicular aragonite cements [73,74]. Higher sulfate concentrations tend to promote aragonite precipitation [69,70,71,72,73]. Such conditions represent deeper carbonates formed in an environment where HCO3−-rich fluids mix with sulfate-rich bottom water. This conclusion is supported by the large fragments of chemosynthetic bivalves observed. Xi et al. found that the in situ Raman spectra of the authigenic carbonates exhibited the highest crystallinity in the fauna-rich area. Scanning electron microscopy and X-ray diffraction analyses of the samples supported the conclusion that the aragonite in the fauna-rich area is more euhedral than that in the fauna-poor area [69]. Therefore, this suggests that the differences can also be caused by different methane fluxes [69]. The difference between the morphologies of shallow and deep aragonite in this article are also more likely caused by the growth time of the minerals. A longer growth time leads to euhedral minerals [54,55,56,63,67,68].

5.3. Mechanism of Seep Carbonate Formation and Methane Seep Evolution

In the studied area, the REE patterns and the lack of Ce anomalies indicate that all carbonates formed in a reduced environment. The low δ13C values indicate that carbonate formation has a close relationship with methane seep. In the studied area, three stages of methane seepage could be identified.
The first stage: About 136 ka BP years ago, the sea level decreased dramatically. This caused a decrease in hydrostatic pressure. Under such a condition, gas hydrate became unstable and released methane into shallower sediments. When it reached the sediment–water interface, the released methane reacted with sulfate and carbonate precipitated as a by-product of AOM. At first, the carbonates at 51.2 mbsf were mainly cement by microcrystalline aragonite under a high concentration of SO42− (Figure 9a).
The second stage: From 131 to 136 ka BP, the methane flux gradually decreased with the gradually increasing sea level. Hence, the SMTZ moved down and was fixed at new balanced layers. At the same time, the carbonate at 53.6 mbsf formed at the bottom of carbonate at 51.2 mbsf. As time went by, the formed carbonate mineral at 51.2 mbsf was gradually cemented by sparitic and acicular aragonitein cavities (Figure 9b).
The third stage: At about 12 ka BP, the newly formed hydrate was influenced by the drop of the sea level and released methane into shallow sediments. With the help of AOM, carbonate at 3 mbsf formed. Since the growth of aragonite is short, the carbonate matrix mainly cemented by microcrystalline aragonite (Figure 9c).

6. Conclusions

Systematic analyses, including those on the mineralogy, petrography, stable carbon and oxygen isotopes, REE, and U-Th dating, were conducted on three seep carbonate samples (3, 52.1, and 53.6 mbsf) recovered from site W08B in the Qiongdongnan Basin, China. Based on comprehensive analyses, the following major conclusions were drawn: Two distinct stages of methane seepage events are discernible, which were likely caused by the dissociation of gas hydrates. The first methane seep took place at 131–136 ka BP. The methane seepage caused carbonates at 52.1 and 53.6 mbsf, formed during 131–136 ka BP. Meanwhile, the methane flux changes caused the vertical downward movement of the sulfate-methane transition zone. Furthermore, the carbonate at 53.6 mbsf formed at the bottom of carbonates at 52.1 mbsf. This is the reason why the age of carbonate at 52.1 mbsf is older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonates formed at that time via AOM and are now located at 3 mbsf. The shallow carbonate and deep carbonate minerals are mainly composed of aragonite, indicating that all carbonates formed at a high SO42− concentration. The higher SO42− concentration close to the seafloor also indicates a higher methane flux. However, the thin-section photomicrographs of deep carbonate mainly consist of matrix micrite and biological debris and acicular aragonite occurs as vein cement filling the pore spaces between the matrix micrite. It was common to observe the accretionary overgrowth of rim cement around the foraminifera test. Th thin-section photomicrographs of shallow carbonate mainly consist of matrix micrite and biological debris. The differences between shallow carbonates and deeper carbonates were mainly caused by the duration of the carbonate precipitation of minerals. Moreover, the δ18O values and δ13C values of the deeper carbonates and shallow carbonates indicate that the carbon source for carbonates remained constant through time and included biogenic and thermogenic methane. This research reveals the history of methane seep and the relationship between the methane seep activity and the vertical distribution of carbonates.

Author Contributions

Conceptualization, R.X. and T.W.; methodology, R.X.; software, Y.D.; validation, J.C., J.F. and J.L.; formal analysis, W.Z.; investigation, R.X.; resources, W.Z.; data curation, W.Z.; writing—original draft preparation, J.W.; writing—review and editing, R.X. and T.W.; visualization, P.L.; supervision, J.Z.; project administration, J.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (No. 2018YFC0310000), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0201), Key Laboratory of Marine Mineral Resources, Ministry of Land and Resources (No. KLMMR-2015-A-07, KLMMR-2017-A-12), and China Geological Survey Project (No. DD20160227).

Acknowledgments

We thank all the participants in the GMGS5 expedition for their great support. We also thank the reviewers for their comments during the review process.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the drill site GMGS5-W08B. The topographic map and inset map were obtained from the website https://maps.ngdc.noaa.gov/viewers/fishmaps/.
Figure 1. Location of the drill site GMGS5-W08B. The topographic map and inset map were obtained from the website https://maps.ngdc.noaa.gov/viewers/fishmaps/.
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Figure 2. Carbonate samples recovered from core W08B. Color differences resulted from different dryness. Above: Photographs of carbonate after recovery [30]. Below: Photographs of carbonate in the onshore laboratory prepared for subsampling.
Figure 2. Carbonate samples recovered from core W08B. Color differences resulted from different dryness. Above: Photographs of carbonate after recovery [30]. Below: Photographs of carbonate in the onshore laboratory prepared for subsampling.
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Figure 3. Thin-section photomicrographs of carbonate samples. (a,b) Sample at 52.1 mbsf. (c,d) Sample at 53.6 mbsf. m: micrite; ar: aragonite; for: foraminifer; and pore: pore space.
Figure 3. Thin-section photomicrographs of carbonate samples. (a,b) Sample at 52.1 mbsf. (c,d) Sample at 53.6 mbsf. m: micrite; ar: aragonite; for: foraminifer; and pore: pore space.
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Figure 4. Stable carbon isotope (δ13C) values and oxygen isotope (δ18O) values of authigenic carbonates at deferent sediment layers. Shallowly buried samples (3–1, 3–2, and 3–3) have higher δ13C values (between −33.8‰ and −25.6‰) and lower δ18O values (between 3.6‰ and 3.8‰). The δ13C values of 53–1 and 52–1 are −38.1‰ and −38.0‰. Modified from reference [31].
Figure 4. Stable carbon isotope (δ13C) values and oxygen isotope (δ18O) values of authigenic carbonates at deferent sediment layers. Shallowly buried samples (3–1, 3–2, and 3–3) have higher δ13C values (between −33.8‰ and −25.6‰) and lower δ18O values (between 3.6‰ and 3.8‰). The δ13C values of 53–1 and 52–1 are −38.1‰ and −38.0‰. Modified from reference [31].
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Figure 5. (a) Shale-normalized REE pattern of the seep carbonates. (b) Plot of Ce/Ce * with the LaN/SmN; (c) plot of Ce/Ce * with the DyN/SmN; (d) plot of Ce/Ce * with the Pr/Pr *; and (e) plot of Ce/Ce * with the REE.
Figure 5. (a) Shale-normalized REE pattern of the seep carbonates. (b) Plot of Ce/Ce * with the LaN/SmN; (c) plot of Ce/Ce * with the DyN/SmN; (d) plot of Ce/Ce * with the Pr/Pr *; and (e) plot of Ce/Ce * with the REE.
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Figure 6. (a) The sea level change [59] and the distribution of carbonate. The sun symbols represent the time of carbonate formation. (b) The sedimentation rate estimated based on the U-Th carbonate ages.
Figure 6. (a) The sea level change [59] and the distribution of carbonate. The sun symbols represent the time of carbonate formation. (b) The sedimentation rate estimated based on the U-Th carbonate ages.
Minerals 10 01135 g006
Minerals 10 01135 g007 550
Figure 7. Pseudo-3D seismic profiles showing the characteristics of the bottom simulating reflector (yellow line: BSR) and mass transport depositions (MTDs) at site GMGS5-W08-2018 [43].
Figure 7. Pseudo-3D seismic profiles showing the characteristics of the bottom simulating reflector (yellow line: BSR) and mass transport depositions (MTDs) at site GMGS5-W08-2018 [43].
Minerals 10 01135 g007
Minerals 10 01135 g008 550
Figure 8. (a) Mineral distributions of all carbonate samples. (b) Plot of the aragonite content and the quartz content.
Figure 8. (a) Mineral distributions of all carbonate samples. (b) Plot of the aragonite content and the quartz content.
Minerals 10 01135 g008
Minerals 10 01135 g009 550
Figure 9. (a) Formation mechanism of carbonate at 52 mbsf: the sea level decreased dramatically. This caused a decrease in hydrostatic pressure. Under such a condition, gas hydrate became unstable and released methane into shallower sediments. When it reached the sediment–water interface, the released methane reacted with sulfate and carbonate precipitated as a by-product of AOM. (b) formation mechanism of carbonate at 53 mbsf: The methane flux gradually decreased with the gradually increasing sea level. Hence, the SMTZ oved down and was fixed at new balanced layers. At the same time, the carbonate at 53.6 mbsf formed at the bottom of carbonate at 51.2 mbsf and (c) formation mechanism of carbonate at 3 mbsf: At about 12 ka BP, the newly formed hydrate was influenced by the drop of the sea level and released methane into shallow sediments. With the help of AOM, carbonate at 3 mbsf formed.
Figure 9. (a) Formation mechanism of carbonate at 52 mbsf: the sea level decreased dramatically. This caused a decrease in hydrostatic pressure. Under such a condition, gas hydrate became unstable and released methane into shallower sediments. When it reached the sediment–water interface, the released methane reacted with sulfate and carbonate precipitated as a by-product of AOM. (b) formation mechanism of carbonate at 53 mbsf: The methane flux gradually decreased with the gradually increasing sea level. Hence, the SMTZ oved down and was fixed at new balanced layers. At the same time, the carbonate at 53.6 mbsf formed at the bottom of carbonate at 51.2 mbsf and (c) formation mechanism of carbonate at 3 mbsf: At about 12 ka BP, the newly formed hydrate was influenced by the drop of the sea level and released methane into shallow sediments. With the help of AOM, carbonate at 3 mbsf formed.
Minerals 10 01135 g009
Table
Table 1. Mineral composition of carbonate subsamples from core W08B.
Table 1. Mineral composition of carbonate subsamples from core W08B.
Sample No.Approximate Relative Percentages (%)
AragoniteCalciteHigh-Mg CalciteQuartzIlliteGypsumAlbite
3–165.318.35.411.0---
3–383.513.0-5.2---
3–261.112.57.614.02.70.42.4
52–198.00.5-----
52–280.54.2-11.22.5-0.6
53–184.03.9-8.9--1.2
53–283.82.42.29.6--1.2
53–383.13.31.98.7--0.6
53–483.18.71.93.3--0.6
53–588.03.82.05.1---
53–694.81.9-3.2---
53–740.55.835.99.83.5-1.4
Table
Table 2. Rare earth element (REE) contents of the authigenic carbonates.
Table 2. Rare earth element (REE) contents of the authigenic carbonates.
ElementApproximate Content (ppm)
3–13–23–352–152–253–153–253–353–453–553–653–7
La11.413.65.60.69.04.99.210.19.12.86.19.2
Ce23.428.310.90.917.19.317.718.717.55.19.417.6
Pr2.42.91.10.11.91.01.92.11.90.61.21.9
Nd9.511.94.90.47.34.07.48.27.32.25.07.3
Sm1.92.10.90.11.30.81.41.61.40.40.81.5
Eu0.40.40.20.10.30.20.30.30.30.10.20.3
Gd1.71.90.80.11.20.71.21.41.20.40.71.2
Tb0.20.30.10.00.20.10.20.20.20.10.10.2
Dy1.11.40.70.10.90.50.90.90.90.30.50.8
Ho0.20.20.10.00.20.10.20.20.20.10.10.2
Er0.60.70.40.10.50.30.50.50.50.20.30.5
Tm0.10.10.00.00.10.00.10.10.10.00.00.1
Yb0.50.60.30.10.40.30.50.50.40.10.20.4
Lu0.10.10.00.00.10.00.10.10.10.00.00.1
∑.EE53.36 64.50 26.17 2.64 40.37 22.21 41.54 44.75 40.92 12.27 24.70 41.07
Ce/Ce *1.03 1.04 0.99 0.82 0.96 0.97 0.98 0.94 0.98 0.93 0.79 0.97
Eu/Eu *0.95 1.04 0.99 3.64 1.06 1.03 1.08 0.94 1.05 1.48 1.50 1.07
Pr/Pr *0.96 0.94 0.92 1.02 0.99 0.94 0.97 0.99 0.98 0.98 1.05 0.98
Ce/Ce *—3CeN/(2LaN + NdN). Pr/Pr *—2PrN/(CeN + NdN). Eu/Eu *—2EuN/(SmN + GdN). N refers to the normalized concentration against the standard Post-Archaean Australian Shale [43].
Table
Table 3. Activity ratios used for age calculation and isochron U-Th ages.
Table 3. Activity ratios used for age calculation and isochron U-Th ages.
Sample No.230Th/232Th234U230Th/238U230Th age (yr)230Th age (yr)
(AT × 10−6) (not calibrated)(calibrated)
320.0±0.4144.0±3.70.1512±0.001315,408±15512,210±2269
5292.9±2.0112.9±3.90.8190±0.0069139,969±2585136,281±3605
53113.9±2.3101.1±2.40.7913±0.0037134,088±1351131,143±2461
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Wei, J.; Wu, T.; Zhang, W.; Deng, Y.; Xie, R.; Feng, J.; Liang, J.; Lai, P.; Zhou, J.; Cao, J. Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities. Minerals 2020, 10, 1135. https://doi.org/10.3390/min10121135

AMA Style

Wei J, Wu T, Zhang W, Deng Y, Xie R, Feng J, Liang J, Lai P, Zhou J, Cao J. Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities. Minerals. 2020; 10(12):1135. https://doi.org/10.3390/min10121135

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Wei, Jiangong, Tingting Wu, Wei Zhang, Yinan Deng, Rui Xie, Junxi Feng, Jinqiang Liang, Peixin Lai, Jianhou Zhou, and Jun Cao. 2020. "Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities" Minerals 10, no. 12: 1135. https://doi.org/10.3390/min10121135

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