South China Sea Sediments Research Articles

Impact of Terrestrial Input on Deep-Sea Benthic Archaeal Community Structure in South China Sea Sediments.

Archaea are widespread in marine sediments and play important roles in the cycling of sedimentary organic carbon. However, factors controlling the distribution of archaea in marine sediments are not well understood. Here we investigated benthic archaeal communities over glacial-interglacial cycles in the northern South China Sea and evaluated their responses to sediment organic matter sources and inter-species interactions. Archaea in sediments deposited during the interglacial period Marine Isotope Stage (MIS) 1 (Holocene) were significantly different from those in sediments deposited in MIS 2 and MIS 3 of the Last Glacial Period when terrestrial input to the South China Sea was enhanced based on analysis of the long-chain n-alkane C31. The absolute archaeal 16S rRNA gene abundance in subsurface sediments was highest in MIS 2, coincident with high sedimentation rates and high concentrations of total organic carbon. Soil Crenarchaeotic Group (SCG; Nitrososphaerales) species, the most abundant ammonia-oxidizing archaea in soils, increased dramatically during MIS 2, likely reflecting transport of terrestrial archaea during glacial periods with high sedimentation rates. Co-occurrence network analyses indicated significant association of SCG archaea with benthic deep-sea microbes such as Bathyarchaeota and Thermoprofundales in MIS 2 and MIS 3, suggesting potential interactions among these archaeal groups. Meanwhile, Thermoprofundales abundance was positively correlated with total organic carbon (TOC), along with n-alkane C31 and sedimentation rate, indicating that Thermoprofundales may be particularly important in processing of organic carbon in deep-sea sediments. Collectively, these results demonstrate that the composition of heterotrophic benthic archaea in the South China Sea may be influenced by terrestrial organic input in tune with glacial-interglacial cycles, suggesting a plausible link between global climate change and microbial population dynamics in deep-sea marine sediments.

Complete genome sequence of a high cesium ion- tolerating bacterium Bacillus sp. Cs-700 isolated from the South China Sea sediment.

Generally, cesium ion (Cs+) which concentration of above the millimolar order is toxic to microorganisms. In this study we report the isolation and genomic characteristics of Bacillus sp. Cs-700, which can grow in media supplemented with 700mM Cs+ and was one of the most Cs+ tolerating bacteria been reported. Bacillus sp. Cs-700 was isolated from the South China Sea, and comprised one circular chromosome of 4,297,839bp (40.32mol%G+C content), containing 4366 predicted protein-coding sequences. We predict that numerous genes related to high cesium tolerating, and the genome of Bacillus sp. Cs-700 will be helpful for further insights into the mechanism of bacterial resistance to excess Cs+ conditions and the bioremediation of radiocesium-polluting environments.

Source identification and co-occurrence patterns of major elements in South China Sea sediments

South China Sea (SCS) sediments have multiple sources. The quantitative assessment of the potential sources is crucial to understanding the source-to-sink transport processes in the SCS. In this study, 1987 surface sediments were collected from the entire SCS (excluding the Gulf of Thailand). Major elements were measured, and the co-occurrence relationships among the major elements and grain sizes were investigated by utilizing network analysis. Positive matrix factorization (PMF) was used for the identification of potential sources and the quantitative assessment of their relative contributions. Only nine elements (SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, TiO2, and loss on ignition (LOI)) were chosen for the PMF analysis. The calculated signal-to-noise ratios of MnO and P2O5 were extremely low, making these two elements unsuitable for analysis. Four potential sources were identified from the combined PMF factor profiles and factor contributions, representing a northern and western terrigenous source, a southeastern terrigenous source, a biogenic carbonate source, and a source of sandy sediment on the shelf. The PMF factor profiles correspond to the four communities detected based on the co-occurrence relationships. These results support the validity of the four-factor PMF model. In addition, the spatial trends of the relative contributions of terrigenous elements (TiO2, Fe2O3, Al2O3, K2O, and MgO) are comparable to the pattern of the oceanic currents in the SCS. Therefore, the transport of the terrigenous elements in the SCS is controlled by hydrodynamic conditions.

Effect of methane hydrate dissociation and reformation on the permeability of clayey sediments

Natural gas hydrates in the South China Sea generally exists in clayey sediments, whose permeability is significantly affected by the swelling-shrinkage characteristics of clay that are caused by hydrate dissociation and reformation, and these characteristics influence the efficiency of gas production. In this study, montmorillonite with the most significant swelling characteristics and kaolinite with weak swelling characteristics in South China Sea sediments were investigated experimentally, respectively. The gas permeability was experimentally investigated for varying hydrate saturations during hydrate dissociation and reformation. The results showed the following: The clayey samples after hydrate dissociation had lower permeability than those without dissociation due to the clay particle swelling and the existence of bound water, but the swelling and hydrophilicity of kaolin are lower than for montmorillonite, which is confirmed by observation of SEM photography. Furtherly, the hydrate reformation improved the gas permeability of clayey samples to a certain extent except for those kaolin samples with initial ice saturations of 5% and 10% because of the increased pore space due to dehydration shrinkage of clay particle and the formation of typical aggregate structures, which is in contrast to the results for sand. Furthermore, the damage value of permeability after hydrate dissociation and the ratio of the permeability after reformation of hydrate to the initial permeability in both the montmorillonite and kaolin were obtained in this study.

Family-level diversity of extracellular proteases of sedimentary bacteria from the South China Sea

Protease-producing bacteria and their extracellular proteases are key players in degrading organic nitrogen to drive marine nitrogen cycling and yet knowledge on both of them is still very limited. This study screened protease-producing bacteria from the South China Sea sediments and analyzed the diversity of their extracellular proteases at the family level through N-terminal amino acid sequencing. Results of the 16S rRNA gene sequence analysis showed that all screened protease-producing bacteria belonged to the class Gammaproteobacteria and most of them were affiliated with different genera within the orders Alteromonadales and Vibrionales. The N-terminal amino acid sequence analysis for fourteen extracellular proteases from fourteen screened bacterial strains revealed that all these proteases belonged to the M4 family of metalloproteases or the S8 family of serine proteases. This study presents new details on taxa of marine sedimentary protease-producing bacteria and types of their extracellular proteases, which will help to comprehensively understand the process and mechanism of the microbial enzymatic degradation of marine sedimentary organic nitrogen.

Mercury methylation-related microbes and genes in the sediments of the Pearl River Estuary and the South China Sea

Methylmercury (MeHg) is a toxicant that mainly originates from in situ microbial methylation of inorganic mercury (Hg) in the environment and poses a severe health risk to the public. However, the characteristics of the Hg-methylating microbial community and its relationship with MeHg production in various environments remain to be understood. In the present study, Hg-methylating microbial communities and genes (hgcAB cluster) in the sediments of the Pearl River (PR), Pearl River Estuary (PRE) and South China Sea (SCS) were investigated at a large spatial scale using high-throughput sequencing-based approaches. The results showed that sulfur-reducing bacteria (SRB) and iron-reducing bacteria (IRB) were consistently the dominant microbial strains responsible for the methylation of inorganic Hg in all three regions investigated. The abundance and diversity of Hg-methylating communities and genes were both found to be higher in the PR sediments compared to that in the PRE and SCS sediments, and in good agreement with the spatial distribution of MeHg. Furthermore, a significant correlation was observed between the MeHg concentration and the abundance of both hgcA and hgcB genes in the sediments of the PR, PRE and SCS regions. Overall, the present study suggested that there was the presence of a close link between MeHg and Hg-methylating communities or genes in the ambient aquatic environment, which could be used to reflect the potential of in situ MeHg production.

Experimental investigation on the decomposition characteristics of natural gas hydrates in South China Sea sediments by a micro-differential scanning calorimeter

Marine natural gas hydrate production has become a hot topic in recent years. There is still a long way to achieve the commercial production of marine hydrate reservoirs. The purpose of this work is to investigate the natural gas hydrate decomposition characteristics in the real marine sediments obtained from the South China Sea. A high-pressure micro-differential scanning calorimeter was employed to study the thermodynamic properties of natural gas hydrates. The results indicated that the increase in gas pressure or sediment salinity decreased the ice melting temperature by changing the vapor pressure of water. In addition, the decrease in sediment moisture from 40% to 30% increased the decomposition pressure of hydrates by 4.4 MPa at 278 K of sea bottom temperature, due to the water absorption capacity of the sediments increases the actual salinity furtherly. The influence of mixed gases on the decomposition pressure of hydrates was also investigated. The findings of this work on the effects of salinity and gas components on the decomposition pressure are significant to the spot safety production of marine natural gas hydrate reservoirs.

Spatiotemporal variation of organic geochemical properties since the mid-Miocene in the deep South China Sea (IODP Expedition 349)

We performed an extensive survey of total organic carbon (OC) and stable OC composition of in the deep South China Sea sediments from the International Ocean Discovery Program (IODP) Expedition 349 Sites U1431, U1433 and Ocean Drilling Program (ODP) Leg 184 Site 1148. Our results reveal dramatic spatial and temporal variations in organic geochemical characteristics among the three Sites. Before ~8 Ma, OC deposited at Site U1431 may be predominantly terrigenous, whereas OC deposited at Sites U1433 and 1148 is mainly of marine origin. After ~8 Ma, OC accumulation at the three Sites progressively increases, mainly due to enhancement of marine primary productivity and input of terrestrial organic matter; the three sites receive mainly marine OC, whereas Sites U1433 and 1148 receive relatively more terrestrial OC compared with Site 1431. Additionally, methane concentrations vary from ~20 to ~100,000 ppmv at Site U1433 and from ~10 to ~600 ppmv at Site 1148 with higher geothermal gradients (78 and 84 °C/km, respectively). In contrast, methane concentrations are <5 ppmv at Site U1431 with lower geothermal gradient (14.8 °C/km). Such high methane concentrations and ratios of methane to ethane at Site U1433 suggest that the gaseous hydrocarbons in the southwestern sub-basin (deep water depth, >4000 km) are primarily sourced from microbial production. Our findings suggest that the sources, trajectories, and fates of OC buried in the deep South China Sea since the mid-Miocene are spatially and temporally variable, controlled by sedimentological processes in the sub-basins, and regional tectonic evolution and long-term paleoenvironmental changes.

Rare earth element fractionations of the northwestern South China Sea sediments, and their implications for East Asian monsoon reconstruction during the last 36 kyr

Rare earth elements (REEs) fractionations in siliciclastic marine sediments are related to the parent rock properties, weathering degree, grain size, and heavy minerals. The relations between marine sediment REE compositions and regional paleoclimate are not fully understood. Here, the REEs for detrital components of a core retrieved from the northwestern South China Sea (SCS) were measured. Results show the REE (LREE/HREE, (La/Yb)N) and Light REE ((La/Sm)N) fractionation degrees have good responses to the regional paleoclimate over the past 36 kyr. Sea level played a key role in REE fractionations during ~36-24 kyr BP, manifesting as lower REE and LREE fractionation degrees correspond to the decreased sea level. The entire trends for LREE/HREE, (La/Yb)N and (La/Sm)N over the past 20.5 kyr are almost in phase with the regional East Asian summer monsoon (EASM) records. Because REEs are mainly concentrated in heavy minerals, the density difference of heavy minerals is probably the main controlling factor on REE fractionations. Clay minerals and feldspars released during the weathering process have a high content of Light REEs (LREEs). In the warm, wet periods (Holocene Optimum, and Bølling-Allerød), more precipitation promoted weathering and strengthened the estuarine hydrodynamic force, which respectively resulted in high content of clay minerals and feldspars, and high-density heavy minerals (e.g., monazite). The process led to LREE enrichment because the enriched minerals contain higher contents of the LREEs. In contrast, the dry, cold climate conditions during the periods Heinrich 1, Younger Dryas, and Last Glacial Maximum resulted in the reversed REE fractionations, as the weak hydrodynamic force can only carry the low-density heavy minerals (e.g., zircon) that are enriched in HREEs. This model does not seem to apply to 36- 24 kyr BP, probably because of the re-transport of ancient shelf sediments. The study facilitates the understanding of REE application in paleoclimate reconstruction within slope sediments.

Hydrocarboniclastica marina gen. nov., sp. nov., a marine hydrocarbonoclastic bacterium isolated from an in situ enriched hydrocarbon-degrading consortium in sea sediment.

A Gram-stain-negative, motile, non-spore-forming, aerobic and rod-shaped bacterial strain, Soil36-7T, was isolated from an in situ enriched hydrocarbon-degrading consortium in South China Sea sediment. Strain Soil36-7T grew at 4-40 °C (optimum 28-32 °C), at pH 5-10 (pH 7-8) and in the presence of 1-12 % (w/v) NaCl (3-6 %). Phylogenetic analyses based on 16S rRNA gene sequences and a genome-based approach using UBCGs (up-to-date bacterial core genes) showed Soil36-7T formed a distinct branching lineage within the family Alteromonadaceae. 16S rRNA gene sequence similarity was 92.9, 92.1 and >88.3 % between strain Soil36-7T and the type species of the genera Marinobacter, Tamilnaduibacter and the other genera of the family Alteromonadaceae, respectively. The major fatty acids in Soil36-7T were C16 : 0, C16 : 1ω6/7c, C16 : 0 10-methyl, C18 : 1ω7c, C12 : 0 and C18 : 0. The predominant respiratory quinone was Q-9, with a minor amount of Q-10 (3.5 %). The major polar lipids were phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol, and various unidentified glycolipids, phospholipids, aminophospholipids and other polar lipids. The DNA G+C content was 57.9 mol%. On the basis of phylogenetic, genomic, phenotypic and chemotaxanomic characteristics, strain Soil36-7T could be classified as representing a novel species of a new genus within the family Alteromonadaceae, for which the name Hydrocarboniclastica marina gen. nov., sp. nov. is proposed. The type strain of the type species is Soil36-7T (=MCCC 1A12105T=KCTC 62334T).

Dissociation characteristics of methane hydrates in South China Sea sediments by depressurization

Marine methane hydrate is a considerable energy source for use in the near future. With great obstacles to spot production, researchers are focusing on the production characteristics of hydrates in various experimental systems, such as glass beads, clay, silica sand and so on. This study investigated the production behaviors of methane hydrate in the real South China Sea sediments using depressurization method. The hydrate saturations of the remolded hydrate-bearing sediments ranged from 10.10% to 23.76%. The results indicate that an 8% increase of hydrate saturation can prolong the dissociation duration under the same backpressure of 2 MPa by 120 min. In addition, the excess temperature drop caused by the depressurization may induce the unpredictable occurrence of hydrate reformation or icing; therefore, a double depressurization method that depressurizes to 2 MPa (second stage) after 20 min maintenance at 4 MPa (first stage) is employed in order to shorten the temperature drop in the first stage and increase the dissociation rate in the second stage. The results of this study are significant for the spot production of marine hydrates in order to achieve high efficient gas production.

DSC Measurement on Formation and Dissociation Process of Methane Hydrate in Shallow Sediments from South China Sea

Marine methane hydrates exploitation has become one of the hottest research topics in recent years. In this study, we investigated thermodynamic characteristics on methane hydrates in shallow sediments of South China Sea (SCS). The formation and dissociation process of methane hydrate in SCS sediments were conducted by use of a high pressure micro-differential scanning calorimeter (μDSC). The ice melting peak and hydrate dissociation peak were well distinguished by DSC and the repeated cycles proved the accuracy of DSC measurement. According to the DSC thermograms and pressure profiles, we found the hydrate formation from ice and meltwater respectively in two heating stages. The stage of transformation from ice to hydrate had obvious pressure decrease without little heat effect. Hydrates may come unstable when close to the dissociation temperature and affected the heat flow and pressure irregularly and frequently by a small degree. Comparing the heat flows of different pressures, we also noticed the decrease of ice melting point and the increased of hydrate dissociation point along with the pressure got higher. This study aimed to provide fundamental data for spot exploitation of marine methane hydrate reservoirs.

Antimicrobial tunicamycin derivatives from the deep sea-derived Streptomyces xinghaiensis SCSIO S15077

Tunicamycin E (1), featuring a methyl substitution at C-10′, was isolated from marine-derived Streptomyces xinghaiensis SCSIO S15077 originated from the South China Sea sediment together with six known compounds, tunicamycin B (2), tunicamycin X (3), tunicamycin A (4), streptovirudin D2 (5), tunicamycin C (6), and tunicamycin C3 (7). The structure of compound 1 was elucidated by detailed spectroscopic data analyses. All the compounds exhibited strong to moderate antibacterial activity against Gram-positive bacteria Bacillus thuringiensis BT01 and B. thuringiensis W102 with MIC values ranging from 0.008 to 2 μg/mL. Moreover, compounds 1–7 exhibited moderate antifungal activity against Candida albicans ATCC 96901 and C. albicans CMCC (F) 98001 with MIC values ranging from 2 to 32 μg/mL. This is the first report that tunicamycins exhibit antimicrobial activities against B. thuringiensis, C. albicans CMCC (F) 98001 and a fluconazole resistant strain C. albicans ATCC 96901.

Influence of Sea Level Change and Centennial East Asian Monsoon Variations on Northern South China Sea Sediments Over the Past 36 kyr

AbstractSouth China Sea (SCS) sediments can provide important paleoclimate records because of their generally high deposition rates and strong regional influence of East Asian summer monsoon (EASM) variations. However, interpretation of sedimentary records from the northwestern SCS slope is complicated because sedimentation is sensitive to sea level and provenance variations. We seek to develop a paleoclimatic record of SCS sediments by analyzing sediment magnetic properties over the past ∼36 kyr from core PC338. The dominant magnetic minerals are magnetite and hematite in most parts of the core. The hematite concentration increased profoundly over the ∼15–32 kyr B.P. period when sea level was lower than −90 m below the present level. This suggests that Red River inputs were enhanced significantly in this period. Moreover, increased coarse magnetite contents were recorded in association with an enhanced East Asian winter monsoon (EAWM) during the Last Glacial Maximum and Heinrich event 1. Low‐field and frequency‐dependent magnetic susceptibility variations were coeval with EASM‐controlled climate change recorded in South China and the SCS over the past 15 kyr, probably because regional terrestrial weathering and erosion of iron‐bearing minerals, and their transportation are dominated by climate. Rhythmic coarsening of Holocene magnetic mineral concentrations coincided with global cooling events, which indicates that core PC338 also contains signals of enhanced EAWM variations. The study demonstrates that magnetic proxies are indicative of sediment supply and deposition variations on the SCS continental slope that are useful for understanding the effects of sea level and climate change on SCS sediments.

Bioprospecting of Novel and Bioactive Compounds from Marine Actinomycetes Isolated from South China Sea Sediments.

Marine actinomycetes are less investigated compared to terrestrial strains as potential sources of natural products. To date, few investigations have been performed on culturable actinomycetes associated with South China Sea sediments. In the present study, twenty-eight actinomycetes were recovered from South China Sea sediments after dereplication by traditional culture-dependent method. The 16S rRNA gene sequences analyses revealed that these strains related to five families and seven genera. Twelve representative strains possessed at least one of the biosynthetic genes coding for polyketide synthase I, II, and nonribosomal peptide synthetase. Four strains had anti-Mycobacterium phlei activities and five strains had activities against methicillin-resistant Staphylococcus aureus. 10 L-scale fermentation of strains Salinispora sp. NHF45, Nocardiopsis sp. NHF48, and Streptomyces sp. NHF86 were carried out for novel and bioactive compounds discovery. Finally, we obtained a novel α-pyrone compound from marine Nocardiopsis sp. NHF48, an analogue of paulomenol from marine Streptomyces sp. NHF86 and a new source of rifamycin B, produced by Salinispora sp. NHF45. The present study concluded that marine actinomycetes, which we isolated from South China Sea sediments, will be a suitable source for the development of novel and bioactive compounds.

Alcohol compounds in Azolla imbricata and potential source implication for marine sediments

This study investigated the composition of long-chain alkyl diols, triols, sec-alcohols, hydroxyl acids, and other hydroxylated compounds in Azolla imbricata and compared the organic alcohol components of Azolla filiculoides, Azolla microphylla, and South China Sea (SCS) sediments in order to investigate the possible indication of Azolla being the biological source of diols and triols in SCS sediment. Large amounts of diols, monohydroxy acids, and sec-alcohols with internal hydroxy groups at omega 20 were detected in the three types of Azolla. Among these, 1,omega 20-diol and omega 20-hydroxy acid exhibited strong even-odd predominance distribution, whereas omega 20-sec-alcohol exhibited strong odd-even predominance distribution. In addition, small amounts of diols, triols, and dihydroxy acids with internal hydroxy groups at 9, 10 or omega 9, omega 10 were detected, among which the chain length of C-29 was predominate. Compounds having similar structures as those in Azolla reflected a similar biosynthetic pathway: omega 20-hydroxy acid exhibiting even-odd predominance distribution is decarboxylated to omega 20-sec-alcohol exhibiting odd-even predominance distribution and converted to 1, omega 20-diol with even-odd predominance distribution by acyl reduction; omega 9, omega 10-hydroxy acid is converted to 1,20,21(1, omega 9, omega 10)-triol by acyl reduction, and then converted to 9,10-diol by hydrogenation and dehydration. The alcohol components in A. imbricata were clearly not the biological source of 1,13/1,14/1,15-C-28,C- 30,C- 32 diols and 1,3,4-C27-29 triols in the SCS sediment. Certain marine diatoms might be the source of 1,14-C-28,C- 30 diol in inshore sediment, but the biological source of diols and triols in the SCS sediment requires further investigation.

Influence of foraminifera on formation and occurrence characteristics of natural gas hydrates in fine-grained sediments from Shenhu area, South China Sea

Marine gas hydrates accumulate primarily in coarse-grained, high-permeability layers; however, highly saturated natural gas hydrates have been discovered in the fine-grained sediments of Shenhu area, South China Sea (SCS). This may be explained by key factors, such as the great abundance of foraminifera shells. In this paper, by analyzing the SCS foraminifera structure and performing hydrate formation experiments in the foraminifera shells, the contribution of foraminifera to hydrate accumulation in the SCS was investigated from a microscopic point of view. Simulations of hydrate formation were carried out in both pure SCS foraminifera shells and the host sediments. Pore structures in typical foraminifera were studied by use of micro-focus X-ray computed tomography (CT) and scanning electron microscopy (SEM). Hydrate growth and occurrence characteristics in the foraminifera shells were observed in-situ . The results showed that the presence of foraminifera significantly enhanced the effective porosity of the SCS sediments. Moreover, while the hydrates grew preferentially in the chambers of the coarse-grained foraminifera by adhering to the inner walls of the foraminifera shells, no apparent hydrate accumulation was observed in the fine-grained or argillaceous matrix. These findings provide a basis for further studies on the accumulation mechanism of hydrates and physical properties of hydrate reservoir in the South China Sea.

Particle sizes effect of the elements in the South China Sea sediment off Pahang coastal during the Northeast monsoon (pre-) and the Southwest monsoon (post-) periods

The Pahang water determined an effect of the sizes of sediment particles on 11 elements distribution during the monsoon season. The elements were of very high concentration in a sediment particle size of 40, 63, and 90 µm. The lower concentration of the elements was associated with a sediment size of 125, 250, and 500 µm. It has been observed in this research study that a concentration of elements increases with a decrease in a size of the sediment particles. The sediment from the Northeast (pre-) monsoon consisted of a finer particle size than the Southwest (post-) monsoon sediments. Overall, the concentration of the elements in the Northeast (pre-) monsoon sediment was higher than the Southwest (post-) monsoon sediment. The Northeast monsoon is highly influenced due to an input of fresh sediment from the rivers into the South China Sea as well as, due to a redistribution in the surface sediment. There were many differences in sediment element content between the two monsoon seasons, but an anthropogenic impact was only found in lead (Pb).

Neodymium isotopic composition in foraminifera and authigenic phases of the South China Sea sediments: Implications for the hydrology of the North Pacific Ocean over the past 25 kyr

AbstractεNd and normalized Rare Earth Elements (REE) patterns of benthic and planktonic foraminifera and Fe‐Mn coatings precipitated on sediments have been investigated for the South China Sea (SCS) to (1) assess the reliability of the extraction of past seawater εNd in the SCS and to (2) reconstruct past hydrological changes during the last 25 kyr. Reductively cleaned mono‐specific planktonic foraminifera (Globigerinoides ruber) and mixed benthic foraminifera in core‐top sediments from 1500 to 2400 m display similar εNd values to those of the modern Pacific Deep Water (PDW) (εNd of −3.9 to −4.4). Furthermore, the εNd of the reductive cleaning solutions shows similar εNd values to ones obtained on cleaned foraminifera. Combined with PAAS‐normalized REE patterns, these results confirm that the oxidative and reductive cleaning procedure applied to foraminifera does not totally remove all of the Fe‐Mn coatings and that εNd values yielded by cleaned planktonic foraminifera retain the εNd imprint of the bottom and/or pore water. εNd values obtained from a leaching procedure carried out on the bulk non‐decarbonated sediments are comparable to the εNd values of the modern PDW, whereas a similar leaching procedure applied to decarbonated sediments reveals a bias due to contamination with Nd deriving from lithogenic particles. In core MD05‐2904, seawater εNd, reconstructed from planktonic foraminifera, indicates that the last glacial period is characterized by lower εNd (−5.2 ± 0.2 to −6.4 ± 0.3) than the late Holocene (−4.1 ± 0.2). Assuming that Nd input from river does not change strongly the εNd of the PDW of the northern SCS, these εNd variations suggest a higher relative proportions of southern‐sourced water in the deep water of the western subtropical Pacific Ocean during the last glacial period.

Elemental sulfur in northern South China Sea sediments and its significance

Elemental sulfur (ES) is one of the intermediates in the inorganic sulfur cycle and thus plays a key role in the fractionation of stable sulfur isotopes in different reservoirs and the marine environment. In this study, solid ES is discovered in sediments near the Jiulong Methane Reef in the northern South China Sea by scanning electron microscopy and Raman spectroscopy. Combining the morphology and distribution of ES, pyrite concentrations, and sulfur isotopes, we conclude that: (1) solid ES coexists with pyrite microcrystals and sulfide (oxyhydr)oxides as well as clay minerals, and they are mainly distributed on the surface of mineral aggregates; (2) ES mainly occurs within and near the sulfate-methane transition zone (SMTZ) despite little morphological diversity; (3) ES formation might be related to hydrogen sulfide oxidation and is therefore linked with fluctuations in the SMTZ. Within the SMTZ, hydrogen sulfide is produced and pyrite precipitates because of enhanced anaerobic oxidation of methane coupled with dissimilatory sulfate reduction. This enhances the efficiency of the inorganic sulfur cycle and provides favorable conditions for ES formation. The discovery of solid ES in sediments near the Jiulong Methane Reef suggests an important relationship with SMTZ fluctuations that could have implications for the evolution of methane hydrate in the South China Sea.