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Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea
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Junli GONG1, Xiaoming SUN1, 2, 3, *, Zhiyong LIN1, Hongfeng LU4, Yongjun LU5
Acta Oceanologica Sinica | 2017, 36(9) : 52 - 64
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Acta Oceanologica Sinica | 2017, 36(9): 52-64
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Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea
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Junli GONG1, Xiaoming SUN1, 2, 3, *, Zhiyong LIN1, Hongfeng LU4, Yongjun LU5
Affiliations
  • 1 School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
  • 2 School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China
  • 3 Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China
  • 4 Guangzhou Marine Geology Survey, Guangzhou 510075, China
  • 5 School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China
Published: 2017-09-01 doi: 10.1007/s13131-017-1111-2
Outline
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The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11 (gas hydrate free) and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane (AOM) which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ18O bulk-sediment values indicate that methane involved in AOM might be released from the “episodic dissociation” of gas hydrate. Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria (Gammaproteobacteria and Epsilonproteobacteria), while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B (MBGB), Halobacteria, Thermoplasmata, Methanobacteria, Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members (Firmicutes and MBGB) are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore, a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.

Geochemistry  /  Microbial community  /  16S rRNA  /  Gas hydrate  /  Taixinan Basin  /  South China Sea
Junli GONG, Xiaoming SUN, Zhiyong LIN, Hongfeng LU, Yongjun LU. Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea[J]. Acta Oceanologica Sinica, 2017 , 36 (9) : 52 -64 . DOI: 10.1007/s13131-017-1111-2
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1 Introduction
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Gas hydrate is an ice-like crystalline substance, composed of cages that are predominantly consisted of water and nature gas (Kvenvolden, 1993). It occurs both in polar region and offshore deep-water marine sediments under appropriate conditions including low temperature, high-pressure as well as adequate methane concentrations (Lu et al., 2007). Owing to its high potential for being an alternative energy resource (Kvenvolden, 1988), increasing attentions have been attracted into studying gas hydrate recently. Among these, much more efforts have been put into finding evidences that can indicate the occurrence of gas hydrate (Lu et al., 2015; Zhang et al., 2014a).
Geochemical analysis is a commonly used method in gas hydrate investigation. Changes in pore water chlorinity concentration as well as δ18O and δD value with depth are conclusive evidences for the presence of gas hydrate (Hesse, 2003; Kvenvolden and Kastner, 1990). In addition, obviously decreasing variation in pore water sulfate concentration (Borowski et al., 1996, 1999; Dickens, 2001; Tréhu et al., 2004; Yang et al., 2008, 2010), strong depleted δ13C value of pore water dissolved inorganic carbon (DIC) or authigenic carbonates in sediments (Campbell, 2006; Han et al., 2008; Lu et al., 2005, 2006; Yang et al., 2008), as well as the changing contents of redox-sensitive trace elements (Helz et al., 1996, 2011; Hu et al., 2014, 2015; McManus et al., 2006; Sato et al., 2012; Zheng et al., 2000, 2002), are also suggested to be linked to the presence of deep buried gas hydrate (e.g., Bhatnagar et al., 2008; Borowski et al., 1996; Niewöhner et al., 1998). This is owing to an upward flux of methane that fuels anaerobic oxidation of methane (AOM), during which sulfate is reduced by methane with 1:1 stoichiometry and the isotope features as well as the redox state of the ambient sediment or pore water are also changed correspondingly (Lin et al., 2016a, b, 2017). The overall reaction of AOM is represented as following:
${\rm{C}}{{\rm{H}}_4} + {\rm{S}}{{\rm{O}}_4}^{2 - } \to {\rm{HC}}{{\rm{O}}_3}^ - + {\rm{H}}{{\rm{S}}^ - } + {{\rm{H}}_{\rm{2}}}{\rm{O }}.$
Alternatively, studies have also suggested that there is potential specialization of microbial communities in gas hydrate as well as related sedimentary niches (Inagaki et al., 2006; Jiao et al., 2015; Mills et al., 2003; Yanagawa et al., 2014). Through comparison analyses, studies found that the dominant microbial taxa in hydrate-containing areas differ with that in the hydrate-free sediments (Inagaki et al., 2006; Jiao et al., 2015; Yanagawa et al., 2014). It is also suggested that the dominance of JS1 and Marine_ Benthic_Group_B (MBGB) are significantly related to the occurrence of gas hydrates (Parkes et al., 2014). These demonstrate that special microbial communities, though often not unequivocal, might be used as complementary indicators for identifying gas hydrate occurrence. So far, consistent constraints from both geochemical and microbial analyses regarding to the occurrence of gas hydrate in the South China Sea (SCS) have been rarely documented.
In this study, geochemistry and 16S rRNA gene phylogenetic analyses are integrated used to study the sediments from a shallow piston Core DH_CL_11 and a hydrate-bearing Core GMGS2-16. Our scientific interests are:
(1) characterizing the geochemical and microbiological features of the two cores;
(2) exploring the main biogeochemical process occurs in the two cores;
(3) exploring the possible relationships between the aforementioned features and process with gas hydrate occurrence.
2 Method
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2.1 Site description and sampling
The SCS is one of the largest marginal seas of the western Pacific, located at the conjunction region of the Eurasian Plate, the Pacific Plate and the Indian Plate (Lu et al., 2011). The two study sites are located in the Taixinan Basin (Fig. 1), which is in the direction of northeast (NE), with a length of about 400 km, average width of 150 km, a water depth of 200–3 500 m, and an area of more than 60 000 km2 (He et al., 2006). Two depressions respectively distribute in the north and the south of this basin, separated by an uplift in the NE direction. The thickness of the Cenozoic sediments in the depressions can reach 8 000 m, and the thickness of the sediments in the eastern part of the basin can reach 7 000 m offshore Kaohsiung, which provide suitable pressure-temperature regime to form and host gas hydrate (McDonnell et al., 2000). Tectonically, the evolution of the Taixinan Basin can be divided into three periods: rifting (eventually caused the formation of the basin), compression (developed a lot of reverse faults and related folds) and thermal subsidence (generated horizontal sediment layers which are basically not controlled by the effects of faulting) (Ding et al., 2004).
Various types of seafloor morphologies, such as the abyssal trough, undersea cliffs, ocean plateau, steep slope, valley, sea slide and submarine fan, are common observed in this basin (Lu et al., 2011; Yang et al., 2010). Fault-fold system, diapirs and gravity flow deposits are also widely distributed (Chen et al., 2006). These provide favorable geological settings and structures for gas hydrate formation. Heat flow data in the Taixinan Basin sub-seabed are highly variable. A much higher heat flows had been found just above the mud diapirs, showing that deep fluid migration occurred there (Shyu et al., 1998). Strong bottom-simulating reflectors (BSRs), generally interpreted to be related to the occurrence of gas hydrate, have been reported existed in this basin, demonstrating its potential for the occurrence of gas hydrate (e.g., Li et al., 2013; McDonnell et al., 2000; Song et al., 2001; Shyu et al., 2006; Wu et al., 2005). Meanwhile, Structure I gas hydrates characterized by shallow burying, thick layers, multiple types and high saturation have been discovered in this basin in 2013 (Zhang et al., 2014a).
The two studied sediment cores (DH_CL_11 and GMGS2-16) were collected during HY4-2012-06 and GMGS2 cruises. Core DH_CL_11 (21°56′N, 118°53′E) was collected by gravity piston, is about 767 centimeters long and showed a strong smell of hydrogen sulfide gas during onboard sampling. This core is located in one of the three gas hydrate prospect areas in the south and west of Taiwan (McDonnell et al., 2000), and vast area of seep carbonates called “Jiulong Methane Reef” has ever been reported in its immediate vicinity (Li et al., 2013). Estimated thickness of the gas hydrate stability zone under Core DH_CL_11 is around 280 m (Bi, 2010). GMGS2-16 is one of the gas hydrate-bearing cores recovered from voyage GMGS2, which is about 235 meters long and located on the boundary between the Zhujiang River Mouth Basin and the Taixinan Basin (Zhang et al., 2014a). Double hydrate horizons were identified in this core, with nodular hydrates presenting at 15–30 mbsf (meters below seafloor) and fine vein-like hydrates presenting at 189–226 mbsf.
Sediment subsamples were collected from each sediment core. For molecular analyses, three samples (CL_11_2, CL_11_11 and CL_11_35) from Core DH_CL_11 and four samples (16_1, 16_13, 16_15 and 16_39) from Core GMGS2-16 were selected. The corresponding sampling depth of each sample is marked in Fig. 2 by dash lines. Upon retrieving, samples were carefully peeled and the inner part of each sample were frozen at –20°C until analyses.
2.2 Geochemical analysis
Total organic carbon (TOC) and total sulfur (TS) in sediments were determined with an Elementar Vario EL Cube CHNOS analyzer in the Instrumental Analysis and Research Center of Sun Yat-sen University (Guangzhou) according to the procedures described in Freire et al. (2012). Briefly, sediment samples were firstly powdered and treated using 10% HCl solution to remove carbonate, rinsed with distilled water, and then dried in the oven until analysis. Carbon- and O-isotope ratios were measured on a Finnigan MAT 252 mass spectrometer. The CO2 used for δ13C and δ18O measurements was extracted from the carbonate component of the bulk sediment with pure H3PO4 at 75°C. All data are reported relative to the PDB standard. Headspace methane content was analyzed on board, following the operation procedures and methods described by Yin et al. (2011). Pore water sulfate concentration was measured using ion chromatography (Metrohm790IC), and the standard deviation from repeated measurement of sulfate content in standard sea water was less than 2%.
The SMI (sulfate-methane interface) depth was calculated by Linear Regression. Fick’s First Law was used to calculate the flux of sulfate:
$F = - \emptyset {D_{{\rm{sed}}}}\cdot\frac{{\partial C}}{{\partial x}},$
in marine sediment:
${D_{{\rm{sed}}}} = \frac{{{D_{{\rm{sw}}}}}}{{1 + n\left({1 - \emptyset } \right)}},$
where F is the flux, $\emptyset $ is porosity, ∂C/∂x is the concentration gradient, Dsed and Dsw are the free-solution diffusion coefficient in marine sediments and sea water, respectively. Here, the assessed Dsw, porosity, n value are inherited from Wu et al. (2013), which are 0.55×10–9 m2/s, 68.39% and 3, respectively. The flux of sulfate can be calculated out by Eqs (2) and (3).
2.3 Nucleic acid extraction and purification
DNA was extracted from 0.5 g sediments using the E.Z.N.A Soil DNA kit (OMEGA, USA) according to the instruction of the manufacturer. Duplicated DNA extractions were combined before purification through gel extraction (AXYGEN, USA). The concentration was quantified in duplicate using BioTek Epoch (USA). The total volume of the elution buffer was 80 μl.
2.4 Pyrosequencing
Bacterial and archaeal 16S rRNA genes covering the V1-V3 and V3-V5 regions were selected to construct community libraries through tag-encoded 454 pyrosequencing. Amplicon pyrosequencing was performed using a 454/Roche A sequencing primer kit on a Roche Genome Sequencer GS-FLX Titanium platform at Majorbio Bio-Pharm Technology Co., Ltd., Shanghai, China. After pre-experimenting, the minimum number of thermal cycles was set to 25 and 33 for the bacterial and archaeal community analyses, respectively. The thermal cycling scheme used for the amplification of the partial bacterial 16S rRNA genes was as follows: initial denaturation at 95°C for 2 min, 25 cycles of denaturation at 95°C for 30 s, annealing at 55°C for 30 s, extension at 72°C for 30 s, and final extension period at 72°C for 5 min. The thermal cycling scheme used for the amplification of the partial archaeal 16S rRNA genes was as follows: initial denaturation at 95°C for 2 min, 33 cycles of denaturation at 95°C for 30 s, annealing at 53°C for 30 s, extension at 72°C for 30 s, and final extension period at 72°C for 5 min. Negative controls contained the entire reaction mixture without the template DNA.
2.5 Phylogenetic analysis
The filtered sequences were trimmed using the trimseq script from the EMBOSS package (Rice et al., 2000). The average length of the edited reads for bacterial and archaeal community are 252 and 256 bp, respectively. These reads were aligned with the bacterial and archaea SILVA (SSU111) database (http://www.arb-silva.de) using “align.seqs” commands (http://www.mothur. org/wiki/Align.seqs). UCHIME (http://drive5.com/uchime) was used to determine chimeric sequences. Furthest neighbor commands were used and unique sequences were clustered into operational taxonomic units (OTUs) defined by 97% similarity. For taxonomic classification, sequences with identity scores greater than 97% identity (<3% divergence) to known or well characterized 16S rRNA sequences were resolved at the species level, between 95% and 97% at the genus level, between 90% and 95% at the family, between 85% and 90% at the order, 80%–85% at the class, 70%–80% at phyla (Stackebrandt and Goebel, 1994). The data preprocessing and OTU-based analysis were performed on Mothur Win.
2.6 Nucleotide sequences accession number
All 454-GS Junior sequence data in this study were submitted to the NCBI Sequence Read Archive (SRA) under accession number SRP072268.
3 Results
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3.1 Geochemical features
The TOC of sediments from the DH_CL_11 and GMGS2-16 ranges from 0.53% to 1.26% and 0.51% to 1.15%, respectively. The TS of sediments from both sites are 0.04% to 0.40% and 0.02% to 0.75%, respectively. On the depth profile of Core DH_CL_11, the TS show an increasing variation trend. It increases steadily from 0.04% at 20 cmbsf (centimeters below seafloor) to 0.15% at 707 cmbsf, and then sharply increases to 0.40% at 747 cmbsf. The TS in core GMGS2-16 illustrates a fluctuating trend, with multiple layers having the maximum TS values (Figs 2b and f).
The stable C, O isotopes of bulk sediment carbonates was analyzed. The δ13C and δ18O values of sediments from Core DH_CL_11 range from –1.9‰ to –0.2‰ and –4.2‰ to –1.7‰, respectively, and those of core GMGS2-16 range from –12.1‰ to 0.4‰ and –8.2‰ to –0.7‰, respectively (Figs 2c and g). On the depth profile of Core DH_CL_11, the variation trends of the δ13C and δ18O values opposite to each other. The δ13C values generally decreases with depth. While the δ18O values increases from –4.2‰ in the near surface layer to –1.7‰ at around 500 cmbsf, and then slight decreases to –2.3‰ or –2.4‰. The δ13C and δ18O values of Core GMGS2-16 illustrate a fluctuating trend, but the contrasting changing trends between the δ13C and δ18O values still persists.
Pore-water sulfate concentration and the methane content in headspace gases from Core DH_CL_11 were also detected. Generally, the sulfate concentration displays a decrease pattern along the depth profile. Methane contents above 650 cmbsf are only at a much lower level, but a dramatic increase is documented at 700 cmbsf (Fig. 2d).
3.2 Microbial features
3.2.1 Overview of pyrosequencing
Bacterial and archaeal 16S rRNA gene (SSU) amplicons prepared from seven sediment samples were analyzed. Fourteen pyrosequencing dataset containing 7,077–19,524 sequences were generated with an average length range between 350 and 600 bases (Table 1). The number of OTUs (at 97% similarity level) detected across the two sampling cores ranges from 130 to 1 541, and 36 to 1 114 in the bacterial and archaeal communities, respectively. The richness and diversity index of microbial community structure were summarized in Table 1. The coverage rates in the bacterial and archaeal community range from 0.904 2 to 0.999 9, demonstrating that the majority of the phylotypes had been covered by surveying effort. Comparative analysis indicated that, as a whole, the microbial richness of the shallow piston core (DH_CL_11) was much higher than that of the gas hydrate-bearing core (GMGS2-16). Similar, the richness of archaea was also higher in Core DH_CL_11.
3.2.2 Taxonomic composition
Firmicutes and Proteobacteria are the most predominant members in the bacterial community (Fig. 3a). The former accounts for a large proportion (80.65%–92.48%) in samples from core DH_CL_11 as well as the near seafloor sample (16_1) from core GMGS2-16. The latter account for a significant part (81.29%–93.26%) of the bacterial communities of the other three layers from core GMGS2-16. Other phylum or groups, like Actinobacteria, Bacteroidetes, Candidate_division_OP8, Chloroflexi, Planctomycetes and Spirochaetae are also frequently detected in the bacterial communities, and each of them account for at least 0.5% of the bacterial community.
A detailed phylogenetic analysis of the top 20 OTUs of each community were conducted (Table 2). From it we can see, 16/20 of the OTUs in Core DH_CL_11 are assigned to Lactococcus, whichalso account for a large portion of the top 20 OTUs from sample 16_1. Ectothiorhodospiraceae, a family belonging to Gammaproteobacteria, has the largest sequence numbers in the bacterial community of core GMGS2-16. These sequences are mainly assigned to sample 16_13 and 16_15. Sulfurimonas, a genus belonging to Helicobacteraceae in Epsilonproteobacteria, has the third largest number, sequences of which are originally from sample 16_39.
For the archaeal community, the dominant members in each sample are different (Fig. 3B). Halobacteria and Thermoplasmata are highly abundant in CL_11_2 and CL_11_11. Sequences belonging to Halobacteria are mainly assigned to Deep Sea Hydrothermal Vent Group-6 (DSHVGp-6) (Table 3). Lineages within Thermoplasmata are mainly assigned to Marine_Benthic_ Group_D (MBGD), South Africa Gold Mine Archaea-R (SAGMA-R) and South Africa Gold Mine Archaea-I (SAGMA-I) (Table 3). MBGB are predominant in all detected archaeal communities but that of 16_39. MCG were generally frequently detected (3.36%–11.09%) in all chosen samples, while MGI were only detected abundantly in CL_11_2 (13.03%) and CL_11_35 (15.48%). Group C3 were abundantly detected (17.70%) in sample 16_15. Methanobacteria and Methanomicrobia are the dominant archaeal class in 16_39, accounting for 20.57% and 40.53% of the detected community, respectively. Besides, Methanomicrobia also represented 39.98% of the detected archaeal members in 16_13. Lineages assigned to class Methanomicrobia and Methanobacteria at the genus or family level mainly consist of Methermicoccus, Methanosarcina, Methanosaeta, Methanobacterium, Methanomicrobiaceae and ANME-1b, with most of them being methanogens except for ANME-1b (Table 3).
4 Discussion
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4.1 Indications from the geochemical parameters
TOC in marine sediments are residual carbon survived seafloor and shallow subseafloor diagenesis (Johnson et al., 2014). Those preserved below the sulfate reduction zone in marine sediments could be used as substrates for methanogenesis (Johnson et al., 2014). The TOC contents in the studied sediment cores (0.51%–1.26%) are in the range of that in the surface sediments from the Taixinan Basin (0.04%–1.37%), and they have reached the threshold (0.4%–0.5%) for in situ biogenic gas hydrate formation (Klauda and Sandler, 2005; Waseda, 1988). However, the inconsistency variation trends between the headspace methane and TOC in Core DH_CL_11 suggest that the sharply increasing methane at the bottom layer is not generated by methanogenesis in situ but originally come from deep.
The total sulfur in sediments comprises organic sulfur and inorganic sulfur (e.g., sulfate, sulfide). On the depth profile of Core DH_CL_11, the pore water sulfate concentration decreases gradually, TS show an increasing trend, and only a weakly correlation exists between the TS and TOC. These demonstrate that inorganic sulfur, specially the sulfide, is the main component of the total sulfur in sediments. TOC/TS keep decreasing with depth, with TOC/TS from the bottom part being lower than that of the normal marine sediments (2.8±0.8) (Zhang et al., 2015) (Fig. 4a). This indicates that extra sulfur, which is not generated from sulfate reduction by TOC, contributes to the sulfur pool at the bottom part. The significant correlation between the TS and the headspace methane suggests that upward methane from deep is the main cause of the increasing TS in sediments. Therefore, we proposed that the sharply elevated TS are affected by AOM.
In Core GMGS2-16, many layers have the maximum TS values, which roughly correspond to layers with lower TOC/TS values than the normal marine sediments (the shadow area) (Fig. 4b). Generally, there is a positive correlation between TS and TOC contents, with an average TS/TOC ratio of 0.36 in “oxic-suboxic marine sediments” (Berner, 1982; Sato et al., 2012). A nearly constant TS/TOC ratio always turns out at non-seep sediments with oxic seawater, while sediments from the “cold seep” sites do not follow this correlation owing to the hydrogen sulfide generated during AOM (Goldhaber, 2003). In this study, high TS/TOC ratios, ranging from 0.03 to 1.35 and falling out of non-seep marine sediment facies, are observed in core GMGS2-16. Three anomalous TS/TOC areas (in dashed circle), which are consistent well with layers having TOC/TS lower than 2.8±0.8, are delineated (Fig. 4c). These suggest that ancient AOM processes had repeatedly occurred during its sedimentary history.
AOM has been described to play an important role in influencing the pore-water sulfate concentration and gradient in methane-rich sediments, and it has also suggested to be linked to the presence of deep buried gas hydrate (e.g., Bhatnagar et al., 2008; Borowski et al., 1996; Niewöhner et al., 1998). It occurs within the seafloor sediments, where pore water sulfate, meet methane from methanogenesis and/or thermogenesis zones (Freire et al., 2012). That interface is named sulfate-methane interface (SMI), where SO42– and CH4 are consumed to zero concentrations at significantly high rate (Boetius et al., 2000). The calculated SMI depth in Core DH_CL_11 is 8.6 mbsf, which is comparable to other adjacent cores characterized by high methane flux from the same basin (Lu et al., 2011). Such depth is even lower than that in the gas hydrate bearing area from Blake Ridge (Borowski et al., 1999), falling into a range of the SMIs from the gas hydrate occurrence areas worldwide. The SO42– flux in Core DH_CL_11 is 23.19 mmol/m2a, close to the upper limit (26.9 mmol/m2a) of that from the Shenhu area of SCS and slightly higher than in the gas hydrate area at the Blake Ridge (8.2–18 mmol/m2a) (Borowski et al., 1996). Such shallow SMI depth, high sulfate flux as well as the sharply elevated headspace methane concentration suggests high methane flux in this core. As for the source of methane, it could be migrated from a gas hydrate decomposing layer or other types of deep gas source (Ye et al., 2016)
For core GMGS2-16, a trend of increasing TS observed at 0–3 mbsf is similar to that in the Core DH_CL_11 (Fig. 5b). Considering that Core DH_CL_11 is affected by the high methane flux and that there is a shallow buried gas hydrate-bearing layer (15–30 mbsf) in core GMGS2-16, we suppose that the increase of TS, resulting from AOM, might be related to the underlying gas hydrate dissociation. In addition, bicarbonate released during AOM, which could result in a noticeable increase in alkalinity at the SMI depth (Nauhaus et al., 2005). High alkalinity is beneficial to precipitation of authigenic carbonates (Boetius et al., 2000; Jørgensen et al., 2004; Snyder et al., 2007), if there is enough Ca2+, Mg2+ in the ambient pore water. In the current study, the δ13Cbulk-sediment value in Core DH_CL_11 ranges from –1.9‰ to –0.2‰, close to that of the marine carbonates, suggesting that normal marine carbonates are the major components in carbonate rocks. This is consistent with our microscope observation that biological carbonates appeared in large numbers in sediments (data not shown). A decreasing trend in δ13Cbulk-sediment values was observed on the depth profile, similar to the trend of δ13CDIC in this core (Ye et al., 2016). Relative depleted δ13Cbulk-sediment values occurred near the inferred SMI, comparable with that in the adjacent piston cores GC10 and HD319 reported by Lu et al. (2011). This suggests that the gradually depleted δ13Cbulk-sediment values are related to AOM, and that AOM is ubiquitous in sediment cores characterized by high methane flux in the Taixinan Basin. Contrast to δ13Cbulk-sediment, the δ18Obulk-sediment shows an increasing trend with depth, with relative δ18O-rich values at the 13C-depleted zone (Fig. 2c). Compared to the seep carbonates samples acquired from the vicinity area (Han et al., 2008, 2013, 2014), these δ18Obulk-sediment values are much closer to that of the normal marine carbonates. However, it seems unlikely that such a regular trend is due to an internal random fluctuation. Thus, an ascending fluids enriched in 18O from deep sea are suggested to influence this sediment core. Generally, ancient seawater, water from gas hydrate decomposition and clay mineral dehydration are the main sources of δ18O-rich fluids (Dählmann and de Lange, 2003; Pierre et al., 2000; Takeuchi et al., 2002). Until now, the dehydration of clay mineral in large-scale in this basin has not been reported yet (Lu et al., 2011). Besides, it is unlikely that the ancient seawater is a main factor because of the opposite changing trend between δ18O and δ13C. Therefore, the best explanation of the δ18O values in Core DH_CL_11 should be that it has been influenced by fluids from gas hydrate dissociation. Moreover, the pore water salinity at the lower part of Core DH_CL_11 decreased with depth, which furtherly supported that gas hydrate is dissociating underlying (Gong et al., 2014). This interpretation is only tentative, and more persuasive evidences are needed to indicate the occurrence of underlying gas hydrate.
In Core GMGS2-16, the 13C-depleted and 18O-enriched values of bulk sediment turn up intermittently along the depth profile, which can be attributed to the occurrence of the authigenic seep carbonates in these depths (unpublished data). These indicate that intense AOM appeared intermittently, which is generally interpreted as a response to the “episodic dissociation” of gas hydrate. Such an interpretation has been drawn from other neighboring gas hydrate bearing cores in this area. For example, Chen et al. (2016) suggested that at least 6 episodes of gas hydrate release existed in the geologic record of Core GMGS2-08, and that the varying sizes of authigenic carbonate among the layers are indicators of different intensity of methane flux in each episode. Zhuang et al. (2016) suggested that the integrated 13C-depleted and 18O-enriched values of foraminifera in sediment core are important evidences for the occurrence of gas hydrate decomposition during sedimentary history. Thus, we conclude that the gas hydrate reservoirs in this area have been running a circulation of accumulation, release and re-accumulation. Aerobic oxidation, anaerobic oxidation, released into the seawater/atmosphere, or staying in the sediment core are the possible fates of methane from gas hydrate decomposition. While, from the point of view of a long geological history, it seems that the gas hydrate in one sediment core is undergoing a cycling of decomposition, migration and re-storage.
The crossplot of δ13C versus δ18O is useful to differentiate seep (δ13C-depleted and δ18O-rich) from normal marine (non-seep) sediments. In this study, sediments from DH_CL_11 and GMGS2-16 plot closely with another two hydrate potential cores in the Taixinan Basin (Lu et al., 2011). Some of these data are shifting to sediments from the Hydrate Ridge (Wang and Suess, 2002), but most of the data are still separated from them (Fig. 6). The possible reason for this may be owing to the existence of large amount of biological carbonates in the sediment cores and few carbonates from AOM. However, the opposite changing trends of the δ13C and δ18O values on the depth profiles has demonstrated that AOM has happened or/and is happening in our studied cores.
4.2 Indications from the microbial community analyses
The predominance (80.65%–92.48%) of the Firmicutes group in the shallow sediments (including all three layers from the shallow piston core and the near surface layer from Core GMGS2-16) is one of the prominent features observed in the bacterial diversity. Compared to the microbial-investigating studies that have been conducted in sediments from the SCS (Table 4), the dominant microbial groups in this study are quite different from that in the surface sediments of northern/southern slope (Liao et al., 2009; Li et al., 2008a), Xisha Trough (Li et al., 2008b) and the Qiongdongnan Basin (Jiang et al., 2007). This indicates that the distribution of microbial community varies under different geographic location. Nevertheless, the dominant bacterial phyla in Core DH_CL_11 and a cold seep site from the same basin also differ with each other (Zhang et al., 2012). This indicates that other factors aside from geographic location affected the acquired microbial community. Besides, shared dominant groups cannot be observed in the gas hydrate-bearing Core GMGS2-16 and the gas hydrate-bearing cores from the Shenhu area, indicating that the presence of hydrate presence or not might not be the controlling factor for the microbial distribution in hydrate-containing sediments (Jiao et al., 2015). Consistent conclusion can also be drawn from the microbial diversity analyses conducted in worldwide gas hydrate-existing or related areas (e.g., Boetius and Suess, 2004; Briggs et al., 2012, 2013; Jiao et al., 2015; Lanoil et al., 2005; Nunoura et al., 2012; Yan et al., 2006; Zhang et al., 2012), in which the dominant members are always quite variable among different areas.
Parkes et al. (2014) proposed that the choice of different “universal” bacterial or archaeal-specific PCR primer/probe for 16S rRNA genes can add biases into the microbial community structure analyses (Parkes et al., 2014). It reminds us of the need of caution in comparing microbial communities between different studies. Adding that the distribution of microbial community varies under different geographic location, we proposed that valid comparison is limited to geographical proximity as well as parallel experimental operations. For example, results from the comparative analyses of the microbial communities in hydrate-containing and hydrate-free cores from geographically close sites revealed dominant members in the two kinds of isolation places different with each other (Jiao et al., 2015; Inagaki et al., 2006; Yanagawa et al., 2014). These actually verify that the geographic location and especially the parallel experimental operations may play an important role in distinguishing different niches. In this study, under parallel experimental operations, shared predominant taxa——Firmicutes and MBGB, are observed in samples from the piston core (DH_CL_11) and the near surface layer of the gas hydrate-bearing drilling core (GMGS2-16). Therefore, a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses.
Firmicutes have been abundantly detected in a crude oil-impacted gas hydrate-bearing site (Station 156) from the Gulf of Mexico (Orcutt et al., 2010), a deep layer of Site UBGH2-10 from the Ulleung Basin (Lee et al., 2013), as well as a long hydrate-bearing drilling core (Site 17A) from the Andaman Sea (Briggs et al., 2012). Besides, Phylotypes within this phylum are commonly abundantly detected in deep oil or coal deposits, such as the crude oil deposits in Japan, oil field in Malaysian and subsurface coal beds from Canada (Li et al., 2012; Penner et al., 2010; Yamane et al., 2011). Firmicutes have been reported as major decomposers of organic matter in biogas reactors (Tang et al., 2005). These members are mainly involved in hydrolyzing complex organic compounds and converting them to oligomers and monomers which can be utilized directly by methanogenic Archaea or further degraded by the so-called secondary fermenting bacteria (Kampmann et al., 2012). Thus, it is reasonable that Firmicutes can be dominant in gas hydrate-containing environment, since intense methanogenesis usually carried out at depth (Marchesi et al., 2001). This also helps to explain why the dominance of Firmicutes only occurs at a deep layer of Site UBGH2-10 from the Ulleung Basin. However, previous research pointed out that Firmicutes could be transported by wind over short or long distances (Polymenakou and Mandalakis, 2013). Therefore, they can be abundantly detected in any layers of a sediment core.
As for MBGB, the predominant of MBGB has been reported in many gas hydrate-related areas, such as the hydrate zones in the Nankai Trough (Reed et al., 2002), the Sea of Okhotsk (Inagaki et al., 2003a), the Ulleung Basin (Briggs et al., 2013; Lee et al., 2013) and the “cold seep” area from the Taixinan Basin. Besides, by statistical analysis, the dominant MBGB has been found significantly correlated with the occurrence of gas hydrates (Parkes et al., 2014). Thus, the predominant of MBGB in most of the detected subsections in this study might also be related with a gas hydrate system, but its specific relationship to gas hydrates is still not clear.
In summary, conditional comparison results of the dominant members between DH_CL_11 and GMGS2-16, and the abundant distribution of Firmicutes and MBGB in other gas hydrate-bearing cores both suggest that there might be gas hydrate buried at depth of Core DH_CL_11. However, it is noteworthy that the dominant bacterial/archaeal phylogroups detected here are mainly involved in organic matter decomposition (Lactococcus in Firmicutes) (Schleifer et al., 1985), sulfur-oxidation (Ectothiorhodospiraceae in Gammproteobacteria and Sulfurimonas in Epsilonproteobacteria) (Inagaki et al., 2003b); Tourova et al., 2007), and methane generation (lineages within Methanomicrobia and Methanobacteria). While, very few sequences are related to known sulfate reduction bacteria (SRB) and ANME, although a coupled biogeochemical process including sulfate reduction and AOM is expected to exist in both Cores DH_CL_11 and GMGS2-16. Since AOM mainly occurs in the sulfate methane transition zone (SMTZ) (Harrison et al., 2009; Knittel and Boetius, 2009; Reeburgh, 2007), a possible explanation for this phenomenon might be that we fail to choose layers characterized by typical SMI features. Meanwhile, the chosen of different PCR primers for 16S rRNA genes may further aggravate this bias.
5 Conclusion
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In this study, we analyzed the geochemical and microbial characters of sediments from a shallow piston core in a potential gas hydrate-existing area and a gas hydrate bearing core. Geochemical analysis suggests that there is an ongoing gas hydrate decomposing and AOM process in Core DH_CL_11. In Core GMGS2-16, ancient AOM processes are suggested to dominate its diagenetic process and the gas hydrate there is supposed to be still active. The predominant groups of the microbial communities in the DH_CL_11 are consistent with those in the near surface layer of GMGS2-16 as well as other known gas hydrate-bearing sites, suggesting a high possibility for the existing of gas hydrate underlying Core DH_CL_11. Overall, integrated geochemical and microbiological analyses are informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.
Acknowledgements
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The authors are grateful to Yang Shengxiong, Zhang Guangxue, and Liang Jinqiang from the Guangzhou Marine Geological Survey for providing samples and valuable suggestions. Special appreciation is extended to staffs from the School of Life Sciences in the Sun Yat-sen University. They also acknowledge the editor and reviewers.
Funding
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  • The Natural Science Foundation of China under contract Nos 91128101, 41273054 and 41373007; the China Geological Survey Project for South China Sea Gas Hydrate Resource Exploration under contract No. DD20160211; the Fundamental Research Funds for the Central Universities under contract No. 16lgjc11; the Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme under contract No. 2011.
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doi: 10.1007/s13131-017-1111-2
  • Receive Date：2016-09-02
  • Online Date：2026-04-16
  • Published：2017-09-01
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  • Received：2016-09-02
  • Accepted：2016-12-06
Funding
The Natural Science Foundation of China under contract Nos 91128101, 41273054 and 41373007; the China Geological Survey Project for South China Sea Gas Hydrate Resource Exploration under contract No. DD20160211; the Fundamental Research Funds for the Central Universities under contract No. 16lgjc11; the Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme under contract No. 2011.
Affiliations
    1 School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
    2 School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China
    3 Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China
    4 Guangzhou Marine Geology Survey, Guangzhou 510075, China
    5 School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China

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genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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