Southern South China Sea Research Articles

Contrasting Patterns of Population Genomic Structure Between Broadcast‐Spawning and Brooding Corals in Southeast Asia

ABSTRACTAimAs climate change increasingly threatens the world's coral reefs, enhancing their resilience by improving population connectivity for key reef species is crucial for ensuring their persistence. Here, we evaluate the population genomic structure of two common coral species, Pocillopora acuta and Porites sp., chosen due to their divergent life histories. Thousands of single‐nucleotide polymorphisms (SNPs) were sequenced and analysed to infer regional connectivity patterns in Southeast Asia, a region that harbours a tremendous diversity of marine life.LocationCoasts of the Malay Peninsula and northern Borneo, covering ~1 million km2.MethodNextRAD genotyping‐by‐sequencing of 185 Porites sp. and 221 Pocillopora acuta colonies. Libraries were prepared and sequenced on Illumina NovaSeq 6000. Genotyping involved initial quality controls, allele frequency filtering and checks for contamination. Genetic structure was assessed with Bayesian clustering, and relationships between genetic variation and environmental factors were studied through redundancy analysis. Contemporary gene flow was estimated using BayesAss.ResultsWe observed panmixia among the broadcasting Porites sp. populations, while for the brooding Pocillopora acuta, the Malay Peninsula acts a strong barrier to dispersal between the Malacca Strait and the southern South China Sea. Moreover, its genomic structure seems to follow current marine ecoregion delineation. By analysing contemporary migrant movement, we can prioritise reef localities for conservation. In particular, localities at the Andaman Coral Coast are contemporarily isolated from the other localities, and Tioman is identified as a major larval source for both species.Main ConclusionOur analyses highlight contrasting population differentiation patterns between the two species that can be explained by the disparity in their reproductive strategies. These findings are important for biodiversity managers in Southeast Asia; incorporation of regional connectivity considerations into conservation planning can help safeguard ecosystem resilience and persistence.

Riverine sediment geochemistry and its dispersal pattern on the western Sunda Shelf

Understanding sediment provenance in continental shelf basins is essential for reconstructing paleoenvironmental changes, enhancing insights into sedimentary dynamics, and elucidating their contributions to the global carbon cycle. To decipher sediment provenances and enhance comprehension of the sediment dispersal patterns and the factors governing geochemical compositions on the Sunda Shelf, we conducted an exhaustive analysis of trace elemental concentrations and the isotopic ratios of strontium (87Sr/86Sr) and neodymium (εNd) in the silicate fractions of 35 surface sediment samples. These samples were collected from the western Sunda Shelf and its proximate major river end-members, namely, the Mekong, Rajang, Pahang, and Kelantan Rivers. Through the application of various statistical methodologies, including classical cluster analysis, Principal Component Analysis (PCA), the La-Sc-Th discrimination diagram, and an array of elemental ratios, we identified three distinct geochemical provinces on the Sunda Shelf. Each province is defined by unique geochemical signatures indicative of varied sediment sources or provenances. This distinction was primarily attributed to pronounced sediment heterogeneity, reflecting lithologic variances from the diverse river end-members. In pursuit of a holistic understanding of sediment provenance in the region, SrNd isotopic data was also integrated from prior studies encompassing the eastern Sunda Shelf and the southern South China Sea. By utilizing the SrNd mixing model, complemented with Monte-Carlo simulations, we estimated the sediment contributions from surrounding river end-members to the southern South China Sea basin. According to the model, the Mekong River emerges as the principal sedimentary source of the Sunda continental shelf and the southern South China Sea, attributed to its substantial sediment outputs. Additionally, the model has identified significant contributions from the Rajang, Pahang, and Kelantan rivers, particularly in offshore regions near their estuaries. Further, this study revealed the previously underappreciated influence of South China's rivers, namely the Red and Pearl Rivers, on the eastern Sunda Shelf and deeper southern South China Sea region beyond the continental shelf. This study not only delineates the dominant sediment sources influencing the Sunda Shelf and the South China Sea but also underscores the importance of considering a broad spectrum of river end-members to understand sedimentary dynamics in an active marine environment.

Multi-proxy reconstructions of paleotemperature in the southern South China Sea since the last deglaciation

The accuracy of paleothermometers is a prerequisite for understanding the past sea surface temperature (SST) changes in the tropical seas. Here, we analyzed the SST estimates reconstructed by four lipid proxies with common linear and newly advanced models in parallel in a sediment core collected from the southern South China Sea (SCS). After excluding the impact of terrestrial input, all of the four proxies-inferred SSTs displayed a gradually warming pattern since 18.3 ka. Our long-chain alkenones-derived annual SST at seawater depth of 0–30 m (SST 0–30 m) record closely matched the regional synthetic SST record from the entire southern SCS, corresponding to high-latitude climate events during the deglaciation. The temperatures reconstructed by long-chain diols (LCDs) showed an upper limit of 27 °C, and we thus proposed that they reflected the optimal survival temperature for organisms producing LCDs when SST was higher than 27 °C. Isoprenoid and hydroxy glycerol dialkyl glycerol tetraethers (iGDGTs and OH-GDGTs)-derived temperatures likely reflected the subsurface temperature (subT) at seawater depth of 30–125 m and SST towards the warm season in the tropical sea, respectively.

Southeast Asian rainforest lost biodiversity during the range expansion to ice-age Sunda Shelf

The capability of tropical forest to recover from extensive land-use remains a matter of debate, despite its importance for guiding conservation and restoration policies. This is especially the case for Southeast Asia. Fortunately, the Sunda Shelf was extensively exposed during the Last Glacial Maximum sea-level lowstand, providing a unique window to study the long-term and landscape-scale development of tropical forest in new accommodation spaces that may be analogous to those emerge after the cessation of human activities. Here we conduct pollen analyses on three sediment cores from the southern South China Sea – two located in front of the Sunda Shelf paleo-river mouths and one close to the northern Borneo – in order to evaluate the similarity/difference between the “primary” and “secondary” tropical forest of Southeast Asia. The assemblages of major pollen taxa and the life-form composition of identified plant types are found to be quite similar among the sites, indicating that well-structured rainforest should have expanded to the Sunda Shelf despite the sandy and potentially saline, nutrient-poor soils. The plant biodiversity, however, was obviously lower on the Sunda Shelf than on Borneo as inferred from the pollen richness. This indicates a loss of biodiversity during large-scale range expansions. Our findings suggest the potential of reforestation in the lowlands of Southeast Asia, but unfortunately the incapability of restoring biodiversity to pre-disturbance levels through natural regeneration alone. Moreover, the old forests in the mountainous northern Borneo appears to be irreplaceable and thus a priority of conservation efforts.

Seismic Reflection Imaging Reveals Relict Subduction Zone of the Paleo-Pacific Plate in the Northeastern South China Sea

The South China Sea (SCS) is widely accepted as an active margin that is associated with the subduction of the Paleo-Pacific plate during the Mesozoic. However, the exact location of the subduction or suture zone remains unclear. Understanding the location of the subduction zone is crucial for comprehending the Mesozoic tectonic evolution of the South China Block and the Cenozoic rifting process of the SCS. To clarify the position of the subduction zone and the influences of preexisting structures, we used a multichannel seismic reflection profile to investigate the crustal architecture. The seismic profile reveals a crustal “crocodile” structure that is interpreted as relict subduction in the Chaoshan Depression and a set of south-dipping crust-mantle reflectors related to the initial rifting in the continent–ocean transition (COT) zone. The results indicate that a Mesozoic subduction zone is located at the northeastern margin of the SCS and that preexisting structures (subduction-related structures) facilitated the rifting process. Combined with previous studies on the oceanic plateau collision-accretionary zone of the northern SCS and the Mesozoic accretionary zone in Palawan of the southern SCS, we infer that a section of the suture zone of the Paleo-Pacific plate subduction is preserved at the northeastern SCS margin and that the rifting of the SCS may have initiated at the suture zone.

3D Structure of Low‐Angle Normal Faults and Tectono‐Sedimentary Processes of Nascent Continental Core‐Complexes in the SE South China Sea

AbstractThis contribution explores the formation and evolution of hyper‐extended basins, associated with the early stage of core complex formation, controlled by low‐angle normal faults active at <30°. Based on a high‐resolution industrial 3D seismic reflection survey along the southern margin of the South China Sea (SCS) (Dangerous Grounds), we mapped and analyzed the 3D geometry of low‐angle normal fault systems and the related stratigraphy. Two main hyper‐extended basins were documented, filled by up to 6 km of sediments including pre‐ to post‐rift sequences. The observed normal faults on depth migrated seismic sections show an average dip angle of <30° and appear planar, characterized by continuous reflections with no clear steepening at depth and sole‐out on distinct decollement levels. Detailed fault surface mapping reveals the occurrence of km‐scale corrugations together with large wavelength undulation. The formation of these hyper‐extended basins is associated with polyphased syn‐rift infill during the development of the low‐angle normal faults. The first syn‐rift sequence appears as chaotic and discontinuous packages that has been dismembered and fragmented during the activity of low‐angle normal faults. The second syn‐rift package shows unexpected sedimentary wedges developing successively toward the footwall and the hangingwall. This geometry results from the interplay between the main low‐angle normal fault and antithetic faults defining a so‐called extensional fishtail. The deep structure of these basins shows nascent domes with limited evidence of magmatism. Eventually, these basins likely capture the earliest stage of core complex development in the proximal margin of the southern SCS.

Expansion of grasslands across glacial Sundaland caused by enhanced precipitation seasonality

The distribution of vegetation across glacial Sundaland and the underlying mechanisms have long been debated. We address this issue by comparing a compilation of vegetation reconstructions with climate simulations from the Community Earth System Model 2 (CESM2). A new n-alkane δ13C record derived from higher plants, covering the period from 21 to 5 cal ka BP in the southern South China Sea, is presented. This record, combined with previous pollen-based reconstructions, indicates that C3 forests dominated the outer-to-mid shelf regions of the northern Sundaland between the Last Glacial Maximum (LGM) and Heinrich Stadial 1, while open grasslands occupied the inner shelf region. A compilation of vegetation reconstructions shows a glacial contraction of lowland rainforests in longitudinal extent compared to the Holocene, especially in southern Sundaland. Furthermore, the previously proposed “savannah corridor” or “dry tropical forests” existed in the interior of Sundaland during the LGM, extending from the Malay Peninsula to the Lesser Sunda Islands. Modelling studies suggest that changes in the seasonality of precipitation, rather than mean annual rainfall, are more relevant to vegetation changes between the LGM and the Holocene. The glacial emergence of Sundaland caused an eastward shift of the ascending branch of the Walker Circulation and a substantial reduction in atmospheric convection activity over Southeast Asia, leading to a decrease in water availability during the dry season in both hemispheres. The enhanced water stress thus favors the expansion of open environments.

A comparative analysis of real-time quantitative PCR and metabarcoding methods for eDNA-based detection of the toxic dinophyte Alexandrium tamiyavanichii (Dinophyceae)

The marine dinophyte Alexandrium tamiyavanichii is a toxigenic species that produces a group of neurotoxins that is responsible for paralytic shellfish poisoning in humans. Early detection of the species is essential for efficient monitoring. Harmful microalgal monitoring systems have evolved over the years with the advent of environmental DNA (eDNA)-based species detection techniques. In this study, eDNA samples were collected from a large-scale sampling covering the southern South China Sea. The sensitivity and specificity of metabarcoding of the V4 and V9 18S ribosomal DNA barcodes by high-throughput sequencing (HTS) were compared to the species-specific real-time qPCR targeting the A. tamiyavanichii ITS2 region. Environmental samples were screened for A. tamiyavanichii by qPCR (n = 43) and analyzed with metabarcoding (n = 30). Our results revealed a high occupancy profile across samples for both methods; 88% by qPCR, and 80–83% by HTS. When comparing the consistency between the two approaches, only two samples out of 30 were discordant. The V4 and V9 molecular units detected in each sample were positively correlated with the qPCR ITS2 gene copies (V4, rs = 0.67, p < 0.0001; V9, rs = 0.65, p < 0.0001), indicating that metabarcoding could be used as a useful tool for early detection of the species. Our results also revealed that the estimation of A. tamiyavanichii cell abundances based on the HTS read abundances was comparable to that of the qPCR quantification. For long-term monitoring, metabarcoding could serve as a cost-effective screening of detecting not only single HAB species but also simultaneously detecting a multitude of potentially harmful species, which is valuable in informing the subsequent implementation of species-specific monitoring strategies.

A Pilot Study on the Diel Vertical Migration Pattern of Mesopelagic Fishes in the Southern and Central South China Sea

The diel vertical migration of mesopelagic fishes in the southern (S-May station and S-Nov station) and central (C-Jun station and C-Dec station) South China Sea was investigated through a series of continuous field surveys conducted in May, June, November, and December 2017. These surveys employed a combination of mid-water trawl and acoustics techniques. The diel migration process, vertical distribution, acoustic migration proportion, and migration pattern of mesopelagic fishes were analyzed. The results revealed that mesopelagic fishes initiated an upward migration before sunset, with the process concluding within 30–120 min after dark. Subsequently, they commenced a downward migration before dawn, which terminated within 10–50 min after sunrise. The mesopelagic deep-sea layers of mesopelagic fishes at the S-May, S-Nov, C-Jun, and C-Dec stations ranged from 360 to 700 m, 350 to 680 m, 350 to 520 m, and 300 to 700 m, respectively. The acoustic migration proportions of mesopelagic fishes at the corresponding stations were found to be approximately 44.5%, 25.7%, 29.8%, and 58.0%, respectively. There were seasonal and regional differences in the vertical migration and distribution patterns of mesopelagic fishes in the South China Sea. A total of 228 species were identified, including 203 fish species, 23 cephalopod species, and 2 shark species. Among these, a subset of 43 fish and cephalopod species exhibited extensive diel vertical migrating behavior. Specifically, this subset comprised 23 lanternfish species, 8 cephalopod species, and 12 other fish species. Lanternfishes were the predominant diel vertical migratory species, while cephalopods also played a significant role in diel vertical migration. The diel migration behavior was found to be slight for Diaphus lucidus, Melamphaes microps, Argyropelecus affinis, and six other fish species. Non-migratory behavior was observed in Sternoptyx obscura, Argyropelecus sladeni, Sternoptyx diaphana, and 13 other fish species. The diel migration habits of 178 additional species of fish, cephalopods, and sharks could not be definitively determined.

Myxobolus acanthogobii Hoshina, 1952 and Myxobolus selari n. sp. (Myxosporea: Myxobolidae) infecting brain of commercial fishes in Terengganu, Malaysia.

Myxosporean infection in marine water fishes has drawn less attention than in freshwater fishes, which resulted in a higher taxonomic variety in freshwater in Malaysia. This study aimed to address the gap by conducting a myxosporean survey on two commercially significant marine fish species, Nemipterus furcosus (Valenciennes) (Eupercaria incertae sedis: Nemipteridae) and Selar crumenophthalmus (Bloch) (Carangiformes: Carangidae), collected from the northeastern part of peninsular Malaysia. During the examination of the organs, two distinct Myxobolus Bütschli, 1882 species were discovered in the brain tissue of these fishes, despite the absence of any observable pathological signs. The two Myxobolus species were characterized through morphometry, morphology, and analysis of partial small subunit ribosomal RNA (18S rDNA) gene. As a result, Myxobolus acanthogobii Hoshina, 1952, which infects 2.3% of N. furcosus, is synonymous with a myxobolid species commonly found in Japanese waters, based on its morphological traits, tissue tropism, and molecular diagnostics. Furthermore, a novel species, Myxobolus selari n. sp., was described, infecting the brain of one (11%) individual S. crumenophthalmus. This unique species displayed distinctive features, placing it within a well-supported subclade primarily comprising brain-infecting myxobolids. Maximum likelihood analysis further revealed the close relationships among these brain-infecting myxobolids, underscoring the significance of tissue tropism and host taxonomy for myxobolids. This study represents the initial documentation of Myxobolus species within the southern South China Sea, shedding light on the potential diversity of marine myxosporean in this region. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:7C400E35-7CB8-4DEE-92B7-F75FF3926441.

Modulations of the South China Sea Ocean Circulation by the Summer Monsoon Intraseasonal Oscillation Inferred from Satellite Observations

The South China Sea (SCS) displays remarkable responses and feedback to the summer monsoon intraseasonal oscillation (ISO). This study investigates how the SCS summer ocean circulation responds to the monsoon ISO based on weekly satellite data. In summer, the largest amplitudes for intraseasonal (30–90 days) sea surface height variations in the SCS occur around the northeastward offshore current off southeast Vietnam between a north–south eddy dipole. Our results show that such strong intraseasonal sea surface height variations are mainly caused by the alternate enhancement of the two eddies of the eddy dipole. Specifically, in response to the intraseasonal intensification of southwesterly winds, the northern cyclonic eddy of the eddy dipole strengthens within 1–2 weeks, and its southern boundary tends to be more southerly. Afterwards, as the wind-driven southern anticyclonic gyre spins up, the southern anticyclonic eddy gradually intensifies and expands its northern boundary northward, while the northern cyclonic eddy weakens and retreats northward. Besides the local wind forcing, westward propagations of the eastern boundary-originated sea surface height anomalies, which exhibit latitude-dependent features that are consistent with the linear Rossby wave theory, play an important role in ocean dynamical adjustments to the monsoon ISO, especially in the southern SCS. Case studies further confirm our findings and indicate that understanding this wind-driven process makes the ocean more predictable on short-term timescales.

Origin of a high-velocity layer: Insights from seismic reflection imaging (South China Sea)

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin.

The variability of Intermediate water intrusion into the South China Sea during the last deglaciation

Millennial-scale variations of the intermediate water in the South China Sea (SCS) have not been well understood due to limited data. Here we investigate the temperature and δ18O of seawater (δ18Osw, a proxy of salinity) evolution history of the intermediate water during the last deglaciation by analyzing Mg/Ca and oxygen isotopes of planktonic foraminifera Globorotalia inflata in the southern SCS. The results indicate that intermediate water was warm and salty during the Heinrich 1 (H1) and Younger Dryas (YD) cold events, while it was cold and fresh during the Bølling-Alleød (B-A) warm event in the southern SCS. By comparison with other paleorecords, we find that there was a weak intrusion of the low-salinity intermediate water from the western Pacific into the SCS during H1 and YD, whereas the opposite occurred during the B-A. Our study reveals that the variability in the intrusion intensity of intermediate water into the SCS from the western Pacific was primarily influenced by the changes in winter wind intensity during the last deglaciation.

Rare earth element geochemistry of reef carbonates in the South China Sea since the Miocene: Insights into paleoclimatic significance

Seawater chemistry critically affects the functioning of ocean ecosystems. However, the evolution of the seawater chemistry in the South China Sea (SCS) since the Miocene is not fully resolved, and its links to tectonic and climate changes remain unclear. In-situ, shallow marine carbonates can provide insights into long-term changes in seawater chemistry and associated environmental processes. In this study, we conducted the rare earth element (REE) analysis of the NK-1 core in the southern SCS since the Miocene. The results reveal poor relationships between siliciclastic/diagenetic indicators (mineralogical, isotopic, and elemental data) and REE proxies, which demonstrate that the REE compositions are insensitive to non-carbonate contaminations and diagenesis. The results exhibit seawater-like REE patterns with LREE depletion, positive La anomaly, superchondritic Y/Ho ratio, and negative Ce anomaly, which demonstrate that this core has preserved a signal of the primary seawater REE composition. The Y/Ho ratios range from 45 to 80 and La anomalies range from 1.0 to 2.3 during ∼23–13 Ma, and they then decrease after ∼13 Ma, with the Y/Ho ratios ranging from 30 to 52 and La anomalies ranging from 0.6 to 1.5. Considering that seawater REE components are mainly sourced from terrigenous sources which exhibit lower La anomalies and Y/Ho ratios, our results indicate an enhanced terrigenous input since ∼13 Ma. We consider that the climate cooling and aridity since ∼13 Ma might be responsible for the phenomenon.

Neogene carbonate platform development in the southern South China Sea: Evidence from calcareous microfossils

Biogenic reefs are unique geologic bodies that are the products of biological activity in tropical and subtropical oceans and critical to global carbon cycling. The South China Sea (SCS) has developed largest carbonate platforms in the Western Pacific. The fully-cored Well NK-1, which is drilled through the reef complex on the Meiji Atoll, Nansha Islands, offers good opportunity for assessing the long-term evolutions of biogenic reefs in the SCS. The stratigraphic ages based on palaeomagnetic and Sr isotopic data are controversial in some intervals of core NK-1. To reveal the evolution history of Nansha Islands and their controlling factors, more credible age constraints on core NK-1 are needed. This research presents new records of calcareous microfossils (calcareous nannofossil and foraminifer) abundances and diversities in the biogenic reef carbonates of core NK-1. Based on the assemblages of the calcareous microfossils, seven biostratigraphic datums are recognized from the sequence of core NK-1, revealing a general continuous succession of the earliest Miocene to Pleistocene age. By combining with the magnetostratigraphic data, the development and evolution of carbonate platform in Nansha Islands are reconstructed. The bottom age of the biogenic reef sequence of NK-1 is estimated for 23.03 ± 1.06 Ma, close to the Oligocene/Miocene boundary. The new chronology highlights three short hiatuses of 3.6–2.4 Ma, 11.1–10.0 Ma, and 13.5–11.9 Ma, which are well correlated with the prominent exposed surfaces at 210.3 m, 464.6 m and 561.6 m, respectively. It reveals remarkably similar evolution history of carbonate platforms in the SCS since Neogene by comparing with records of stratigraphy and growth rate from deep drilling cores on isolated coral reefs in Nansha and Xisha Islands. The results showed that the SCS carbonate platform developments, which go through Late Oligocene / Early Miocene initiation phase, Early-Middle Miocene rapid growth phase, Middle-Late Miocene erosion phase, Miocene-Pliocene stable development phase and Quaternary growth phase, are mainly influenced by global sea level changes, environmental factors and regional tectonism such as the SCS basin spreading, Sabah Orogeny Movement and Dongsha Movement.

Lipid biomarker evidences of natural and anthropogenic organic matter inputs in sediments from the eastern Sunda Shelf in the southern South China Sea

Understanding the exact origin of sedimentary organic matter (OM) was crucial for assessing carbon cycle processes and environmental contamination in continental margin. However, such studies are less performed on shelves far away from the mainland. Here, the abundance and composition of lipid biomarkers were determined in surface sediments from 58 sites at the eastern Sunda Shelf, South China Sea, to identify the sedimentary OM sources and characterize the spatial variation of OM input from specific sources. The results indicated that the sedimentary OM were derived mainly from algae, bacteria, seagrass, terrestrial plants, mangroves, petroleum, and rock erosion products, and the relative significance of these sources varied between different sites. Algal OM input, as indicated by the abundances of short-chain n-alkanes, cholest-2-ene, dinosterol, brassicasterol, and C30 1,15 alkyl diol, was relatively high at five sites (ZW 47, 51, 55, 82, and 86). Conversely, long-chain n-alkane and long-chain n-alkanol proxies suggested terrestrial plant input was most predominant at three other sites (ZW68, 103, and 106), and high abundances of taraxerol and β-amyrin in these sites imply the dominance of mangrove vegetation in the terrestrial OM. UCM (unresolved complex mixture) and αβ-hopanes indicated a relatively high input of petroleum-derived OM at another five sites (ZW3, 8, 25, 51, and 97). The analysis of abundance variations in the biomarkers suggested a substantial spatial heterogeneity in terrestrial plant OM input and a much lower heterogeneity in algal OM input, probably resulting from great variation in riverine transport intensity and relatively homogeneous water environment across the studied region. Correlation analysis indicated that both algal and terrestrial plant OM significantly contributed to the OM buried in the sediments. These results can form a basis for understanding the carbon cycle processes in Sunda Shelf. The finding of petroleum contamination in several sites can benefit for environmental protection and management of the shelf.

Characteristics of Paleontological Communities in Surface Sediments of the Southern South China Sea and Their Paleoclimatic and Paleoenvironmental Significance

Characteristics of Paleontological Communities in Surface Sediments of the Southern South China Sea and Their Paleoclimatic and Paleoenvironmental Significance

Anthropogenic multipollutant input to the offshore South China Sea

The remote region of the South China Sea (SCS), situated far from urban mainland areas, is commonly perceived to experience minimal pollution. However, this may evolve into a considerably polluted region owing to increasing anthropogenic pollutants. In this study, we employ a multidisciplinary approach to analyze the surface sediments collected from the offshore area of the southern SCS. Our aim is to explore potential anthropogenic pollutants, their interactions, and the related controlling factors. This research endeavors to enhance our understanding of the current pollution status in the SCS and help making relevant policy management decisions. Comparison with previous reports reveals that now, the area is more extensively and increasingly contaminated by petroleum hydrocarbons and heavy metals (Cd and As) than before. For the first time, we report the recognition of coprostanol and long-chain alkyl mid-chain ketones, unveiling the noticeable incorporation of sewage fecal matter and biomass burning into offshore sediments. Moreover, sedimentary multipollutants (except ketones) exhibit strong correlations with terrestrial elements and fine-sized particles, displaying a roughly high-west/low-east spatial variability in pollutant accumulation or enrichment. These signatures evidently demonstrate the major impact of river discharges (e.g., the Mekong River to the west and the Pearl and Red Rivers to the north) on the SCS. They have hydrodynamic effects on the subsequent basin-wide dispersal of pollutants, driven by monsoon-induced large- and regional-scale currents. The different behavior of burning-related ketones may be partly due to their aerosol form, leading to atmospheric transportation. Because anthropogenic multipollutants pose compounded threats, exacerbating oceanic warming and acidification to marine ecosystems such as the widespread coral reefs in the southern SCS, scientific management of urban emissions is required to mitigate ecosystem degradation in the Anthropocene era.

The Northeast Mindoro block is the conjugate margin of the southern East China Sea Basin: Insight from detrital zircon data

There is a hot debate over whether the initial opening of the South China Sea (SCS) took place in continental crust or oceanic crust. In this study, we present new detrital zircon U–Pb ages from late Eocene sedimentary rocks in the East China Sea Basin (ECSB). Our results reveal a broad U–Pb age spectrum with a substantial proportion of grains dating >1500 Ma. Notably, the provenance characteristics of the late Eocene ECSB are markedly distinct from other regions along the South China margin, as evidenced by kernel density estimation, nonmetric multidimensional scaling, and cumulative age distribution plots. The ECSB shared nearly identical provenance features with the northeast Mindoro block on the southern SCS margin during the late Eocene. Considering the previously proposed widespread breakup unconformity across the southern ECSB and the slab-based plate reconstruction from tomography of the SCS, we propose that northern Mindoro was the conjugate margin of the southern ECSB and underwent southward drifting during the SCS opening. This study provides a new plate reconstruction model of the SCS and evidence supporting the contention that the initial breakup of the SCS occurred within the continental crust.

Diversity of Culturable Bacteria from the Coral Reef Areas in the South China Sea and Their Agar-Degrading Abilities.

The South China Sea (SCS) is abundant in marine microbial resources with high primary productivity, which is crucial for sustaining the coral reef ecosystem and the carbon cycle. Currently, research on the diversity of culturable bacteria in the SCS is relatively extensive, yet the culturable bacteria in coral reefs has been poorly understood. In this study, we analyzed the bacterial community structure of seawater samples among Daya Bay (Fujian Province), Qionghai (Hainan Province), Xisha Islands, and the southern South China Sea based on culturable methods and detected their abilities for agar degradation. There were 441 bacterial strains, belonging to three phyla, five classes, 43 genera, and 101 species, which were isolated by marine agar 2216E (MA; Becton Dickinson). Strains within Gammaproteobacteria were the dominant group, accounting for 89.6% of the total bacterial isolates. To investigate vibrios, which usually correlated with coral health, 348 isolates were obtained from TCBS agar, and all isolates were identified into three phylum, three classes, 14 orders, 25 families, and 48 genera. Strains belonging to the genus Vibrio had the greatest number (294 strains), indicating the high selectivity of TCBS agar for vibrios. Furthermore, nineteen strains were identified as potentially novel species according to the low 16S rRNA gene similarity (<98.65%), and 28 strains (15 species) had agar-degrading ability. These results indicate a high diversity of culturable bacteria in the SCS and a huge possibility to find novel and agar-degrading species. Our study provides valuable microbial resources to maintain the stability of coral ecosystems and investigate their roles in the marine carbon cycle.