South China Sea Shelf Research Articles

Baroclinic responses of Qiongzhou Strait throughflow to different forcings

The Qiongzhou Strait (QS) throughflow is affected by the seasonal circulation in both the northern South China Sea (SCS) shelf and the Beibu Gulf. The water masses with distinct properties between these two regions signify the baroclinic processes in the QS. In this study, a high-resolution unstructured-grid Finite Volume Community Ocean Model (FVCOM) was used to investigate the features and dynamic mechanisms of baroclinic-induced currents (BCC) in the QS. Different forcings were considered such as tides, river discharges, winds, and remote forcings (low-frequency sea surface level and current variations). The model results indicate that the volume transport of the BCC contributes to 31.26% of the total in the QS, determining the transport direction in summer. The baroclinic response to low-frequency remote forcings is most pronounced during the summer-autumn season, but less than winds during the winter-spring season. This finding underscores the importance of low-frequency remote forcings on the QS throughflow, which has often been overlooked in previous studies. Momentum balance suggests that the BCCs are predominantly driven by both the barotropic and baroclinic pressure gradient forces. Decrease (increase) in either of these two forces will lead to an intensified eastward (westward) flow. The contribution of the horizontal advection is relatively weak but cannot be ignored. This study highlights the baroclinic responses of QS throughflow to different forcings, which is important when investigating the circulation in the northern SCS, especially in the Beibu Gulf.

Nonnegligible contribution of terrigenous sediment inputs from local small watersheds to west regions of the Pearl River Estuary, northern South China Sea shelf

As a crucial “sink” for terrigenous material entering the ocean, provenance of sediments within the coastal zone and continental shelf sea regions holds significant scientific importance in the investigation of global material cycling. Environmental magnetism was proved as an effective and rapid approach for sediment source identification. Detail and systematic magnetic measurements were performed for surface sediment samples collected from the mega Pearl River, local small rivers (Tan, Moyang and Jian rivers) of west Guangdong, coastal zone (CZ) and continental shelf (CS) to analyze the contribution of local small watersheds to marine sediments. The results indicate that detrital river input magnetite and hematite are coexisted in both CZ and CS sediments. Compared to CZ sediments, relatively higher concentration of hematite and finer grain size of magnetite are appeared at CS sediments. By comparing of magnetic parameters, it was inferred that magnetic minerals from the Pearl and Tan rivers are transported and deposited on the coastal zone by Guangdong longshore current. Meanwhile, sediment magnetic parameters in the Moyang and Jian Rivers are similar to the CS sediments, implying the two rivers-sourced materials were directly transported and deposited at the CS due to their narrow and restricted estuaries, and/or the CS preserves the residual deposits from the two rivers formed during the Last Glacial. Except for the Pearl River, sources of local small rivers significantly contribute to marine sediments at the coastal zone and continental shelf.

Seasonal variations of coastal trapped waves (CTWs)’ propagation in the south China sea

Coastal trapped waves (CTWs) are sub-inertial waves with energy confined to the continental shelf and play a crucial role in the coastal dynamics. In this study, we focus on analyzing the seasonal characteristics of CTWs in the South China Sea shelf region by using observations from tide gauges and reanalysis data. Our findings reveal significant seasonal variations in both the phase speed and amplitude of CTWs in the South China Sea. It is shown that phase speed tends to be higher during the winter, followed by spring and autumn, and with less distinct propagation characteristics during the summer. In terms of amplitude, apart from seasonal variations, the CTW amplitudes on the narrower Vietnam shelf are relatively smaller. Drawing on classical theories, we analyze the Kelvin wave mode, the first shelf wave mode, and the second shelf wave mode in CTWs, and investigate the effects of winds, background currents, and stratification. Results indicate that the winds have a significant impact on the generation of CTWs, and the greatest influence of winds occurs upstream relative to the direction of CTW propagation. Background currents can have a significant impact on the phase speed of CTWs, and the seasonal variations of CTWs are closely linked to the background circulations. Additionally, we discuss the influence of seawater stratification on the phase speed of CTWs (measured by Burger number, Bu). Under conditions of strong stratification in the summer, the Bu is larger and leads to higher phase speed of CTWs, particularly when the Bu reaches its maximum on the continental slope. This study provides a better understanding of the CTWs dynamics in the South China Sea shelf region. Our findings can be useful for predicting and managing coastal hazards in the region.

Dynamics of the surface carbonate system in oil fields with a high concentration of wells on the northwestern South China Sea shelf

Oil exploitation may pose adverse effects on marine ecosystems, but its impacts on surface carbonate dynamics remain unknown. In a carbonate system with low air-sea ∆pCO2, such as the South China Sea (SCS), human activities may affect the pCO2 distribution patterns and potentially alter CO2 sink or source at the surface. This study investigates the surface carbonate system in two oil fields, namely the Wenchang Oil Feld and Enping Oil Feld, located on the northwestern SCS (NWSCS) shelf. In Enping Oil Field, although there is a slight increase in surface pCO2 due to probable total alkalinity (TA) consumption from CaCO3 precipitation, strong biological production makes the plume water a strong CO2 sink. Similarly, the biological processes dominated the pCO2 variability in Wenchang Oil Feld, exhibiting high values in its central area. In NWSCS, the influence of shelf water was observed during both cruises. And the pCO2 drawdown caused by the decreased sea surface temperature (SST) and CO2 outgassing outweighed their increases via enhanced vertical mixing, leading to a pCO2 drawdown from September to October within this water mass. More importantly, there were no significant disparities observed in carbonate parameters at stations along transects with and without wells, and the observed parameter values in this study fell within the range reported previously on the nSCS shelf with similar controlling processes. Thus the impact of oil exploitation on carbonate dynamics is negligible, and the characteristics of the carbonate system in oil field are primarily governed by natural processes such as the mixing of plume water and basin water, CaCO3 precipitation and the changes in SST. The provided data establish a crucial baseline for detecting future alterations in carbonate chemistry within oil fields, and the rapid fluctuations in sea surface pCO2 highlight the need for higher spatiotemporal resolution observation.

Dissolved carbon dynamics and exchange in a high permeability beach aquifer

The biogeochemical transformation of dissolved carbon in tidal beaches affects the coastal carbon cycle and budget. Yet, the influence of groundwater flow on the behavior of dissolved carbon and CO2 flux across the different interfaces in these dynamic beach systems remains poorly understood, especially in deep beach aquifers. In this study, we investigated the biogeochemical reactions and origins of dissolved carbon and quantitatively evaluated CO2 outgassing driven by submarine groundwater discharge (SGD) in a high permeability beach aquifer (9–13 m depth) located in the South China Sea shelf. Results revealed that both dissolved inorganic carbon (DIC) and total alkalinity (TA) exhibited non-conservative additions across the salinity gradient. The excess DIC and TA most likely resulted from organic carbon decomposition in the brackish and saline zones. Aerobic respiration (upper saline plume zone), anaerobic reactions, and tidal mixing were identified as the dominant processes controlling DIC and TA dynamics. The mean CO2 outgassing flux across the beach sediment-air interface was estimated to be 1.05 g m−2 d−1, significantly higher than the CO2 flux across the sea-air interface. The outgassing pattern of groundwater CO2 from beach aquifer appeared to be driven by tidal pumping. Considering the combined effects of SGD-derived dissolved carbon and nutrients, we suggest that SGD yielded a net CO2 flux, contributing to 45 % of the CO2 flux across the sea-air interface. SGD directly and indirectly delivered substantial CO2 from the beach aquifer to the atmosphere, approximately (4.89–9.21) × 103 tons of CO2 per year along the 76.2 km long sandy coast. These findings demonstrate that intertidal beach aquifers, as active biogeochemical reactors have the strong potential to intensify CO2 emissions to the atmosphere through both beach sediment-air and sea-air interfaces. This study provides a better understanding of the links between carbon biogeochemical cycles and hydrologic processes in the coastal zones.

A tri-point correlation method for ocean current estimation and its applications in near-bottom turbulent mixing on the shelf of the Northern South China Sea

We propose a tri-point correlation method to estimate ocean currents using moored temperature observation data. The transit time for two time series of temperature is determined with the cross correlation technique, enabling the formulations of an equation system to determine the current speed and angle. To validate our method, we utilize a high-resolution temperature dataset collected at the shelf bottom of the northern South China Sea and compare the inferred currents with direct measurement results with a current meter. The results demonstrate a high level of agreement between the inferred currents and the directly measured ones. Using this temperature dataset and the synchronously inferred currents, we further estimate near-bottom turbulent mixing, including diapycnal mixing and vertical heat flux. During the observation period, the mean values of the turbulent kinetic energy dissipation rate, turbulent diffusivity, and vertical heat flux are 3.8×10−8 m2/s3, 2.6×10−4 m2/s, and 10.0 W/m2, respectively. These inferred turbulent mixing quantities exhibit a positive correlation with the dynamics of internal waves.

Neogene morphotectonic evolution of the East Asian Continental Shelf

The offshore basins in the East Asian Continental Shelf have undergone distinct tectono-geomorphic-sedimentary evolution since the Neogene, driven by complex interactions between tectonic and surface Earth processes. However, the contribution of the latter to the evolution of the offshore basins has been less studied. The landscape simulation software Badlands serves as an ideal tool to investigate the impacts of tectonic and surface processes on sedimentation as it simulates the sediment routing in a source-to-sink context. Focusing on the southern East China Sea Basin (ECSB) and the northern South China Sea Shelf Basin (SCSSB), two key components of the East Asian Continental Shelf, we used Badlands to reconstruct the tectono-geomorphic-sedimentary evolution since the Neogene. It revealed two-phase evolutionary processes: (1) the southern ECSB and the northern SCSSB were in the unified shelf environment and represented as an integral during the Early Miocene, though the northern SCSSB experienced anomalous subsidence due to the southward jump of the SCS mid-ocean ridge. The integrated southern ECSB and northern SCSSB experienced uplift and denudation since the Middle Miocene, under the influence of the Philippine Sea Plate extrusion. (2) Subsequently, the southern ECSB and the northern SCSSB were separated into two independent basins in the Late Miocene, when Taiwan Island was significantly uplifted due to the Philippine Sea Plate extrusion and the arc-continental collision. Then Taiwan Island experienced a multi-stage uplift characterized by southward and westward migration. It concluded that tectonics (i.e. Taiwan Orogeny and the subduction of the Philippine Sea Plate) primarily dominated the differential evolution of the southern ECSB and the northern SCSSB. Surface Earth processes (i.e. the intensification of the East Asian Monsoon and the sea-level fluctuation) are the secondary contributors.

Responses of Phytoplankton Communities to the Effect of Both River Plume and Coastal Upwelling

AbstractRiver plumes and coastal upwelling systems are both nutrient‐abundant habitats compared to the open ocean, and the mechanisms that structure the phytoplankton community when these occur together are unclear. In this study we investigated the dynamics and drivers of phytoplankton biomass and community composition in the Pearl River plume‐coastal upwelling system on the northern South China Sea shelf during summer. We found that phytoplankton biomass was lower in the plume than in upwelled water. Diatoms were the only dominant group in the upwelled water. In contrast, diatoms and Synechococcus were co‐dominant in the plume, and the negative correlation between the proportions of these two groups indicated that they occupied distinct niches. The lower phytoplankton biomass in plume waters was due to nutrient limitation, and there was a transition from limitation by phosphorus to limitation by nitrogen in the plume along its path. This bottom‐up control limited only diatoms, whereas Synechococcus was limited mainly by top‐down control via microzooplankton grazing. This difference led to niche differentiation of the dominant phytoplankton groups in the plume‐upwelling system. This discovery of niche differentiation enhances understanding of food web structure and will facilitate modeling of marine biogeochemical cycles.

Contrasting sources and fates of sedimentary organic carbon in subtropical estuary-marginal sea systems

Sediments deposited in subtropical deltas and continental shelves are the “recorders” of sources, transport processes, and accumulation of organic carbon (OC) at the land-ocean interface, but their role in the global carbon cycle remains poorly constrained. In this study, we determined the abundance and carbon isotopic composition (δ13C and Δ14C) of bulk OC, as well as characteristics of specific biomarker compounds (plant wax n-alkanes, n-fatty acids, and phytoplankton sterols) in surface sediments from the Pearl River estuary (PRE) and adjacent continental shelf of the northern South China Sea (SCS) in order to examine the sources and fates of OC delivered to a large subtropical river-marginal sea system. We observed marked spatial variability in the sources, hydrodynamic processes and accumulation of sedimentary OC in PRE and northern SCS shelf, with the emergence of two distinct regimes reflected in the bulk OC isotopic characteristics of surface sediments. In Pearl River impacted regions, relatively fresh terrestrial OC (OCterr) was subject to substantial degradation and aging during transport within the estuary and from the estuary to the shelf. In contrast, sedimentary OC in the Taiwanese river impacted region comprised a mixture of fluvially-derived refractory OCterr from the Choshui River and modern marine OC. Burial efficiencies of OCterr are also spatially heterogeneous as a result of these different OC sources and hydrodynamic processes. The overall OCterr burial efficiencies are relatively low in the Pearl River influenced region, with an average value of 27 ± 10% in the PRE and an average value of 17 ± 6% in the western shelf, in contrast to the higher values in the region impacted by Taiwanese rivers (55% ± 39%).These findings suggest both sources and hydrodynamic processes during transport exert strong influence on the fate of OC, with important implications for the role of river-dominated marginal seas in the global carbon cycle.

Relict and Modern Sediments on the Continental Shelf of the Northern South China Sea: A Reconsideration

AbstractUnderstanding the temporal‐spatial patterns of modern and relict sediments is of importance for assessing changes in the Quaternary environment and sea‐level. Sedimentological and geochemical data is presented, along with in situ shell‐based accelerator mass spectrometry 14C ages of 30 samples from the surface sediments on the northern shelf of the South China Sea (NSSCS). The authors’ data show that the NSSCS surface sediments exhibit considerable diversity in composition. Modern sediments are primarily constrained to the NW inner shelf, which is fed by fluvial sands sourced from Coastal South China river systems and dominated by the Pearl River Estuary delivery. The transport and discharge of the terrestrial sediments to the NSSCS is highly influenced by the Guangdong Longshore Current and its secondary cyclonic eddies. Relict sediments dominated by well‐sorted, medium‐ and coarse‐grained sands were identified in the NW Shenhu and NW Dongsha areas of the outer NSSCS. The sedimentology and geochemistry of the relict sediments imply that they were deposited in a dry and cold environment either during the low sea levels of the late Pleistocene (∼40 ka) or the early Holocene (∼10 ka). To the east, the Taiwan Shoal and vicinity are dominated by a sand mixture, at which the relict sediments were reworked by terrigenous supply and modern hydrodynamic environment due to the compound action of the Guangdong Longshore Current, seasonal cyclones, and Kuroshio Intrusion. The present isobaths of ∼−90 m in the NSSCS might be the reflective of the early Holocene coastal delta or the last glacial maximum shoreline.

Different vertical heat transport induced by submesoscale motions in the shelf and open sea of the northwestern South China Sea

Submesoscale currents are known to be associated with strong vertical velocities (O (10) m/day), regulating the redistributions of energy and matter balances. The northern South China Sea (SCS) is fulfilled with submesoscale motions, which might induce strong vertical heat transport (VHT). We set up a 1-km horizontal resolution Massachusetts Institute of Technology General Circulation Model (MITgcm) to study the seasonal variations in submesoscale vertical heat transport in shelf regions and open seas. Spectrum analysis shows that the spatial scale separating submesoscale and mesoscale motions are 14 and 30 km for the shelf and open regions, respectively. The submesoscale VHT in the shelf region is one order of magnitude larger than that in the open ocean. The former has the largest value in summer and winter, which might be induced by summer upwelling and winter downwelling, while the latter is strongest in winter and weakest in summer in open regions. The submesoscale VHT also appears to have intra-seasonal variations and might be attributed to the disturbances of tropical cyclones and life stages of submesoscale eddies. The submesoscale VHT is strongest in the pregeneration phase of the eddies, and the maximum VHT belt has an entrainment type at the developing and mature stages. The chlorophyll-a concentration also has the same temporal variation as the different life-stage of eddies. This study provides local VHT induced by submesoscale motions, which is expected to improve our understanding of submesoscale air–sea interactions and their biological effects.

The contrast in suspended particle dynamics at surface and near bottom on the river-dominated northern South China Sea shelf in summer: implication on physics and biogeochemistry coupling

To understand the process-response relations among physical forcing and biogeochemical properties of suspended particles (SPs) in the river-dominated northern South China Sea shelf, a 5-day shipboard observation was conducted at a fixed location on the dispersal pathway of the Zhujiang (Pearl) River plume (ZRP) in the summer of 2016. Instrumented moorings were deployed near the sampling site to record the flow and wave fields every 10 minutes. Hydrographic properties were measured hourly to identify different water masses. Water and SPs samples at the surface (3 m) and near the bottom (3 m above the bed) were taken every 3 h for the analyses of nutrients, chlorophyll-a (Chl-a), and particulate organic matter (POM including POC, PN, and δ13CPOC). Meanwhile, the grain-size composition of SPs and seafloor sediment were also analyzed. Results showed that monsoon winds drove cold upwelling and ZRP waters at the surface. Both the upwelling and ZRP regimes contained newly produced marine phytoplankton based on low POC/Chl-a ratio (PC ratio) and enriched δ13CPOC. However, SPs in the ZRP regime were smaller (<153 µm), having denser particle bulk density, and less enriched δ13CPOC, indicating different bio-communities from the upwelling regime. EOF analysis of the surface data suggested that mixing processes and the dispersal of the ZRP regime were mainly controlled by far-field storm winds, tidal modulation, and strength of mixing. On the other hand, a bottom nepheloid layer (BNL) was observed, mainly consisting of SPs<63 μm with higher bulk density than SPs at the surface. POM in the BNL was degraded and δ13CPOC-depleted according to the PC ratio and δ13CPOC. EOF analysis of the near-bottom data indicated that the dominant physical processes influencing the biogeochemical properties of SPs in the BNL were jointly the upwelling-associated lateral transport (first order) and tide-related resuspension (second order). Our study identified the contrast between the surface and near-bottom regimes with the coupling patterns among physical forcing and physiochemical properties of SPs using good constraints on particle dynamics and particle sources.

Magnetovariational Sounding in South Vietnam according to the Dalat Observatory Data

Abstract —This paper describes a three-dimensional inversion of magnetovariational tippers calculated for the Dalat observatory in South Vietnam on the basis of the geomagnetic variation records obtained from the INTERMAGNET network. The frequency dependences of the tippers are inverted using the ModEM software, which makes it possible to construct a geoelectric model of the Dalat zone in a 300 × 300 × 150-km spatial region (x, y, and z axes, respectively) with its center located at the observatory. The resulting model of the geoelectric section contains both surface and deep conductive blocks located at the top and bottom of the Earth’s crust and in the upper mantle. The central part of the upper local conducting block with a resistivity of 0.5–1.0 Ohm⋅m is located near the observatory at a depth of 12–14 km. A more massive conductive block is located to the east of the observatory in the coastal region. Its central part with a resistivity of 0.3 Ohm⋅m is located at a depth of 24–28 km and extends under the continent along the coast for about 80 km. The roots of this block are visible to depths of ~100 km in the South China Sea shelf region. It is assumed that the high electrical conductivity of these blocks is provided by melting in the presence of aqueous fluids and granite series rocks, which are widely distributed in the Dalat zone. The crystallization depths of the samples of these granites estimated from the geothermobarometry data are close to the depth intervals of conductive blocks in the model section, which was also observed in the interpretation of magnetotelluric soundings in Southern Tibet, where granitoid rocks are widespread.

A Discussion on the relationship between prominent unconformities on the SCS shelf margins and the end of seafloor spreading in the South China Sea

In an effort to reconcile different data sources (seafloor spreading, seismic and gravity images, well calibrations, outcrop studies) in the South China Sea (SCS), we reviewed the unconformity records, in particular in the context with the shut-off of seafloor spreading in the SCS. With respect to the start of spreading, there is a consensus: ca. 34-41 Ma. Recent data infer an end of spreading, near to the magnetic Anomaly 5 = ca. 15.5 Ma (Langhian age). In Northwest Borneo, it is suggested that this event is coeval with the Deep Regional Unconformity (DRU) in Sabah and Brunei, and the Mid-Miocene Unconformity (MMU) in Sarawak. The MMU is also recognized in offshore Vietnam and Palawan, on the Western and Southeastern margins of the SCS, respectively. The MMU/DRU may constitute the border between active margin and passive margin deposits within the marine SCS sub-basins. The progradation of clastic shelves post-MMU/DRU may have hampered growth of bioherms whilst creating prolific sandstone reservoir sequences offshore Northwest Borneo and Vietnam.

Intermittent Generation of Internal Solitary‐Like Waves on the Northern Shelf of the South China Sea

AbstractPredicting the occurrence of internal solitary‐like waves (ISWs) is important for parameterizing turbulent dissipation. ISWs in the northern South China Sea (SCS) are well understood to mainly originate from the Luzon Strait (LS). They are generally predictable due to their phase‐locked relation with tidal forcing in the LS. However, irregular occurrence of ISWs is noted from our 9‐day moored measurements on the SCS shelf. More energetic ISWs unexpectedly appeared during the neap tide than the spring tide. Their occurrence was related to subtidal shelf flows. The tidal and shelf flows had comparable magnitudes. We thus hypothesize that when the total flow meets critical conditions ISWs are generated locally over irregular seafloor. This hypothesis is supported by numerical experiments and validated by observations. Consequently, local generation of ISWs enhances both the intensity and complexity of internal wave field on the shelf, and thus the unpredictability of ISW occurrence.

Provenance and dispersal patterns of sediments on the continental shelf of northern South China Sea: Evidence from detrital zircon geochronology

The northern shelf of South China Sea is an important passageway for the transportation of sediments from the terrigenous source to the oceanic sink, but the provenance and dispersal patterns of sediments remain largely unclear. Here we report the results of zircon grain-size analysis and U-Pb geochronology to investigate the sediment provenance and illustrate the transport and dispersal patterns of sediments on the continental shelf of the northern South China Sea. The results show that detrital zircon U-Pb ages within sediments are characterized by a major peak at ca. 150 Ma, as well as three minor age populations at ca. 240 Ma, 400–500 Ma, and 900–1000 Ma. A detailed provenance study suggests that these sediments were primarily derived from both the Pearl River catchment and the drainage systems in Taiwan Island. Our study confirms that the transportation and dispersion of sediments were largely controlled by both topography and hydrodynamics of sea. The sediments in the inner continental shelf were transported from the Pearl River drainage system and subject to the Guangdong Coastal Current, whereas the sediments in the middle and outer continental shelf were markedly influenced by the South China Sea warm current. This study elucidates the transport and dispersal patterns of detrital sediments in the marginal sea by comprehensive analyses of detrital zircon grains.

Microstructure observations of three-layered mixing on the shelf of the northwestern South China Sea

This study presents microstructure observations on the northwestern South China Sea (nwSCS) shelf, where energetic turbulence drove a high dissipation rate on the order of 10−5 W kg−1 in May 2021. Together with the hydrography and current observations, the results show a typical three-layered structure of dissipation on the shelf and reveal different mixing regimes for different layers. Below the mixed layer, mixing over the shallow shelf displays periodically varying characteristics, indicating low-mode internal tides. Strong mixing occurred in the stably stratified layer, reaching an approximate magnitude of 1 × 10−5 W kg−1. Our analysis suggests that the strong mixing in this layer is attributed to the shear behavior of the low-mode internal waves because the observed dissipation rate increases with increasing vertical shear and stratification as described by the MacKinnon and Gregg parameterization. In the bottom boundary layer, mixing by tidal shear is dependent on the Richardson number and trends with velocity and vertical shear. In addition, our analysis also reveals that mixing continuously contributed to downward heat transport to the deep layers. This study can improve our understanding of the vertical structure of fine-scale mixing and its regimes, and their effects on the shelf of the nwSCS.

Spatio-temporal distribution and mechanism of Cenozoic magmatism in the South China Sea and adjacent areas: Insight from seismic, geochemical and geochronological data

ABSTRACT Cenozoic magmatism was highly active in the South China Sea (SCS), and mostly dominated by basalts. The magmatic rocks are mainly distributed in the lower slope and subbasins of the SCS, in addition to sporadic and small-scale Cenozoic volcanism on the SCS shelf. This paper presents a complete map of the Cenozoic magmatic rocks of the SCS with magmatic rock boundaries. This paper also presents an analysis of geological survey and published data in terms of seismic profiles, ages, geochemistry and isotopic systematics of the Cenozoic magmatic complexes of the SCS and adjacent areas. Many of wells or dredges are located in magmatic rocks identified by seismic profiles. Three periods of magmatic activity are distinguished: (1) a pre-spreading period (>33 Ma); (2) a syn-spreading period (33–15 Ma) and (3) a post-spreading period (<15 Ma). The thickest volcanic rocks encountered more than 1000 m (possibly Oligocene to Miocene) in the Boayan 1/1A well in the Palawan Trough of the southern SCS during Cenozoic. The oldest magmatic rocks are andesites (62.5 ± 2.2 Ma) in the northern SCS (offshore) and andesites (65 ± 3 Ma) in Penghu (on land). The latest modern magmatic activity was detected in 1923 offshore of a submarine volcanic eruption (10°09´29″N, 109°00′50″E, 20 m below sea level) and along the shore of a volcanic eruption in the Philippines. Cenozoic magmatic rocks of the SCS include at least 1008 rock bodies: the smallest is 0.00012 km2, the largest is 1528 km2 and the total area covers 46,309 km2. The Yangjiang-Yitong fault and Zhongnan-Reedbank fault divide the magmatic rocks into two different sections in the eastern and western SCS. The ages of the magmatic rocks in the eastern SCS are mostly older than those in the western SCS. The magmatic rocks in the northern, southern, eastern, western SCS and adjacent areas, and SCS basin are related to extension, subduction+collision, subduction, extrusion, extension+cooling+subsidence, respectively.

Effects of Mixing and Stratification on the Vertical Distribution and Size Spectrum of Zooplankton on the Shelf and Slope of the Northern South China Sea

The northern shelf and off-shelf regions of the South China Sea (SCS) present a stark contrast between being eutrophic from terrestrial runoffs of nutrients and biota and being oligotrophic with multiple nutrient limitations due to the distance to land sources and stable permanent stratification. The abundance, size, and trophic structures of plankton in the shelf and off-shelf regions were studied in a joint Sino-German cruise conducted between September 1 and 24, 2018. A laser optical plankton counter was mounted on a water sampler-CTD (conductivity–temperature–depth) system for studying the horizontal and vertical distributions of plankton structures in a size range between 0.1 and 35 mm and their relationships with hydrographic and biological features in the northern SCS and its shelf region. Results revealed the subsurface chlorophyll maximum layer (SCM) below the pycnocline and plankton aggregation near the SCM about a depth of 50–60 m. The distributions of small plankton between 0.1 and 0.5 mm were strongly correlated with stratification and SCM compared to those of large plankton. Analyzing the intercept and slope of a normalized biovolume spectrum (NBVS) as an indicator for abundance and size structure of a plankton community, results revealed that in the shelf region, the slopes exhibited no significant vertical variations in the water column regardless of stratification and SCM. In contrast, in the off-shelf stratified water column, the intercepts and slopes were lower and flatter in the surface layer, higher and steeper in the SCM layer, and the lowest and flattest in the deep layer. Stirring by the typhoon also altered both the abundances and size structures of the plankton communities, with significant regional differences. This study elucidates the variances of plankton abundances, distributions, NBVS slopes, and intercepts among different water column structures in both shelf and off-shelf regions of the northern SCS.

Geographic Scale Influences the Interactivities Between Determinism and Stochasticity in the Assembly of Sedimentary Microbial Communities on the South China Sea Shelf.

Determinism and stochasticity in microbial community composition decisions have attracted wide attention. However, there is no consensus on their interrelationships and relative importance, and the mechanism controlling the interaction between the two ecological processes remains to be revealed. The interaction of the two ecological processes on the continental shelf of the South China Sea was studied by performing 16S rRNA gene amplicon sequencing on 90 sediments at multiple depths in five sites. Three nearshore sites have higher microbial diversity than those two close to the shelf margin. Different microbial composition was observed between sites and microbial composition of nearshore sites was positively correlated with total nitrogen, total sulfur, total organic carbon, and dissolved oxygen, while that of offshore was positively correlated with total carbon, salinity, and photosynthetically active radiation. The null model test showed that the community composition among layers of the same site and between nearby sites was mainly dominated by the homogeneous selection, while that between distant sites was mainly affected by dispersal limitation, which indicates that geographic scale influences the interactivities of determinism and stochasticity. Our research indicates that the balance of these two ecological processes along the geographic scale is mainly determined by the dispersal ability of microbes and environmental heterogeneity between areas. The study provides new insights into how deterministic and stochastic processes shape microbial community composition on the continental shelf.