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Volume 41 ， Issue 2 ， 2023
- Abstract：The South China Sea (SCS), situated in southern China, at the junction of the Pacific Plate, the Eurasian Plate, and the Indian Ocean Plate, is a northeast-southwest trending semi-enclosed sea. It spans an area of approximately 3.5 million square kilometers and has an average water depth of about 1 200 m, its deepest point reaching 5 559 m. In 2021, a scientific expedition (called as U1 voyage) in the South China Sea was organized by the Innovation Research Team of Guangdong Special Key Program from March to April, this marks the first comprehensive scientific research voyage to the southern U-boundary corridor. Consisting of a total of 30 papers, this special issue is to share a portion of the research findings from this scientific expedition U1 voyage, covering six aspects: 1) characteristics of the marine ecosystem in the SCS and its response to marine dynamic processes; 2) multi-scale marine dynamic processes, sea-air interactions, and forecasting techniques in the SCS; 3) geomorphology and geological structure; 4) sedimentary processes and resource potential in the SCS; 5) geostrategy, rights and interests maintenance and strategic countermeasures in the SCS; 6) marine scientific instruments. By integrating the scientific research with the study of history, jurisprudence and international strategies, this issue presents new insights into the formation history and scope evolution of the SCS, and it also seeks to establish a new scientific framework based on the marine governance and development of the SCS.Keywords：South China Sea;U-boundary;geological science;U1 voyage;ecological environment;ocean remote sensing13|4|0Updated：2024-10-12
- Abstract：The role of sea surface temperature (SST) variability in the pre-monsoonal (April to July) intraseasonal oscillation (ISO) over the South China Sea (SCS) is investigated using the Community Earth System Model Version 2 (CESM2). An Atmospheric Model Intercomparison Project (AMIP) simulation forced by daily sea surface temperatures (SSTs) derived from a parallel coupled general circulation model (CGCM) run was compared with observations and the mother coupled simulation. In the coupled model, the SST warming leads the peak convection about 1/4 period as in observations. The paralell uncoupled model fails to simulate this phase relationship, implying the importance of air-sea coupling in reproducing realistic ISO. Due to the near-quadrature phase relationship between SST and precipitation ISOs during the ISO events, it is difficult to distinguish the active/passive role of SST from observations alone. Significant correlation in intraseasonal precipitation between the daily SST-forced AMIP and mother CGCM runs indicates that SST plays a role in driving the atmospheric ISO.Keywords：intraseasonal variability;South China Sea;Community Earth System Model Version 2 (CESM2);Sea surface temperature13|2|0Updated：2024-10-12
- Abstract：Internal waves (IWs) are small-scale physical processes that occur frequently in stratified marginal seas. IWs are ubiquitous and well documented in the northern South China Sea (nSCS), but few studies have explored the ecosystem responses to the IWs. MODISA chlorophyll-a (Chl-a) data from 2002 to 2014 were used to examine the distribution of Chl a near the Dongsha Atoll (DSA). Composite Chl a from about 40 IWs during spring and summer showed stronger response on the northern side than on the southern side of the DSA. One day after the passage of IWs, composite surface Chl a on the northern side increased from 0.11 mg/m3 to a maximum mean value of 0.18 mg/m3. It decreased to 0.13 mg/m3 after two days and maintained that level for several days after the passage of IWs. The enhanced surface Chl a likely caused subsurface Chl-a maximum and nutrients in the surface layer. Approximately 64% of the increase in surface Chl a was due to the uplift of the subsurface Chl-a maximum one day after the passage of IWs, while nutrient-induced new phytoplankton growth contributed about 18% of the increase a few days later. When the IWs occurred frequently in spring and summer, Chl-a level on the northern side was about 30% higher than that on the southern side. IW dissipation and its impact on nutrients and chlorophyll were stronger on the northern side of the DSA than on the south, which caused a north-south asymmetric distribution of Chl a in the region.Keywords：internal waves (IWs);phytoplankton;nutrients;Dongsha Atoll;northern South China Sea6|0|2Updated：2024-10-12
- Abstract：The calculation results of marine environmental design parameters obtained from different data sampling methods, model distributions, and parameter estimation methods often vary greatly. To better analyze the uncertainties in the calculation of marine environmental design parameters, a general model uncertainty assessment method is necessary. We proposed a new multivariate model uncertainty assessment method for the calculation of marine environmental design parameters. The method divides the overall model uncertainty into two categories: aleatory uncertainty and epistemic uncertainty. The aleatory uncertainty of the model is obtained by analyzing the influence of the number and the dispersion degree of samples on the information entropy of the model. The epistemic uncertainty of the model is calculated using the information entropy of the model itself and the prediction error. The advantages of this method are that it does not require many-year-observation data for the marine environmental elements, and the method can be used to analyze any specific factors that cause model uncertainty. Results show that by applying the method to the South China Sea, the aleatory uncertainty of the model increases with the number of samples and then stabilizes. A positive correlation was revealed between the dispersion of the samples and the aleatory uncertainty of the model. Both the distribution of the model and the parameter estimation results of the model have significant effects on the epistemic uncertainty of the model. When the goodness-of-fit of the model is relatively close, the best model can be selected according to the criterion of the lowest overall uncertainty of the models, which can both ensure a better model fit and avoid too much uncertainty in the model calculation results. The presented multivariate model uncertainty assessment method provides a criterion to measure the advantages and disadvantages of the marine environmental design parameter calculation model from the aspect of uncertainty, which is of great significance to analyze the uncertainties in the calculation of marine environmental design parameters and improve the accuracy of the calculation results.Keywords：South China Sea;marine environmental design parameters;model uncertainty;information entropy;Monte Carlo method8|0|0Updated：2024-10-12
- Abstract：Sediment incipient motion is the first step in sediment resuspension. Previous studies ignored the effect of seepage flow on the mobility of sediment particles and simplified the seabed surface as a rigid boundary. A flume experiment was designed to innovatively divide the seabed into two parts to control the dynamic response of the seabed and control the seepage conditions. In the experiment, the seabed sediments and the amplitude of internal solitary waves (ISWs) were changed to compare and analyze the impact of seepage flow on the sediment resuspension by shoaling ISWs. Moreover, parametric research and verification were carried out. Results indicate that seepage flow can greatly influence fine sand, promote sediment resuspension, and increase the amount of suspension by two times on average. However, seepage flow had a little effect on the suspension of clayey silt and sandy silt. Besides, seepage force was added to the traditional gravity, drag force, and uplift force, and the parameterization of threshold starting shear stress of coarse-grained sediments was developed. The results of this parameterization were verified, and seepage force was critical to parameterization. The threshold starting shear stress was reduced by 54.6% after increasing the seepage force. The physical mechanism of this process corresponded to the vertical reciprocating transient seepage in and out the seabed interface caused by the wave-induced transient excess pore water pressure. This quantitative study on seepage flow for shear stress of coarse-grained sediments induced by ISWs is critical to geohazard assessment.Keywords：internal waves;seepage flow;sediment resuspension;parameterization;mechanism6|0|0Updated：2024-10-12
- Abstract：Spatial and temporal distribution of abundance, community structure and succession, and mortality of zooplankton were examined in a subtropical bay—the Qinzhou Bay, Guangxi, South China, to evaluate the impact of oyster culture on zooplankton. Data on zooplankton abundance, phytoplankton biomass, and environmental parameters were collected in 12 routine samplings at four sites in the Qinzhou Bay in 2018. Results show that the hydrological environment in the estuary affected the temporal variation of zooplankton of the Qinzhou Bay. However, the oyster culture affected the spatial distributions of zooplankton. The abundance of zooplankton outside the oyster farm was higher than that inside, whereas the mortality outside the oyster farm, on the contrary, was lower than that inside. Oyster culture in the Qinzhou Bay had built a significant impact on the abundance and community composition of zooplankton, and the food shortage caused by the ingestion of oyster might exacerbate the mortality of zooplankton in the Qinzhou Bay. In addition, the large-scale oyster culture increased the spatial heterogeneity of the community structure of zooplankton inside and outside the oyster farms. Findings of this study provide a reference for future policy-making in aquaculture management.Keywords：zooplankton;community structure;mortality;oyster culture;Qinzhou Bay6|0|1Updated：2024-10-12
- Abstract：Luzon Strait is the main channel connecting the South China Sea (SCS) and the western Pacific, with complex atmospheric and oceanic dynamic processes. Based on 44 days of glider measurements and satellite observations, we investigated the temporal and vertical variations of chlorophyll-a (Chl-a) concentration in the Luzon Strait from July 25 to September 6, 2019. The Chl a was mainly distributed above 200 m and concentrated in the subsurface chlorophyll maximum (SCM) layer. The depth of SCM ranged between 50 m and 110 m, and the magnitude of SCM varied from 0.42 mg/m3 to 1.12 mg/m3. The variation of Chl a was identified with three stages responding to different dynamic processes. Under the influence of Kuroshio intrusion, the SCM depth sharply deepened, and its magnitude decreased in Stage 1. Afterward, a prominent Chl-a bloom was observed in the SCM layer from August 6 to August 16. The Chl-a bloom in Stage 2 was related to the influence of a cyclonic eddy, which uplifted of the thermocline and thus the deep nutrients. During Stage 3, prolonged heavy rainfall in the northeastern SCS resulted in a significant salinity decrease in the upper ocean. The convergence of upper water deepened the thermocline and the mixed layer. Thus, the Chl a decreased in the SCM layer but increased in the surface layer. In particular, a typhoon passed through the Luzon Strait on August 24, which induced the Chl a increase in the upper 50 m. However, there was little change in the depth-integrated Chl a (0–200 m), indicating that the Chl a increase in the surface layer was likely associated with physical entrainment of SCM caused by strong mixing, rather than the phytoplankton bloom in the upper water column. Underwater gliders provide frequent autonomous observations that help us understand the regional ocean’s complex dynamic processes and biological responses.Keywords：Luzon Strait;glider observations;chlorophyll a;Kuroshio;cyclonic eddy;typhoon7|0|2Updated：2024-10-12
- Abstract：Mirs Bay is a semi-enclosed bay neighboring the Zhujiang (Pearl) River estuary, one of the largest estuarine systems in the world. The long-term historical observational data (1994–2017) of temperature, salinity, dissolved oxygen (DO), and biochemical parameters were used to examine the spatiotemporal distribution of hypoxia in Mirs Bay and adjacent coastal waters. Results show that bottom hypoxia varied seasonally and interannually. Hypoxia mainly occurred from June to September in Mirs Bay and the transition zone in the southern waters of Hong Kong, and the recorded hypoxia events have increased from 2007. The density difference between the bottom and surface layers was positively related to the bottom apparent oxygen utilization (AOU) (R=0.620, P<0.001) and negatively related to the bottom DO (R=0.616, P<0.001), indicating that water column stratification was an essential prerequisite for the formation of bottom hypoxia in summer. The bottom oxygen consumption and hypoxia had higher positive correlation with the seasonal thermocline (R=0.683, P<0.001) than the halocline (R=0.540, P<0.001), including in the area was affected by freshwater plume. The insignificant relationship between AOU and nutrients indicated that local eutrophication was not the only important factor in the formation of the hypoxic zone during summer. The decrease in phosphorous owing to the pollutant reduction policy and the increase in nitrate may have led to an increase in hypoxia events in the bay where waters therein are characterized by nitrogen-limitation. The increase in chemical oxygen demand in wastewater also promoted oxygen consumption. Compared to the adjacent coastal waters influenced by Zhujiang River plume water, the Mirs Bay experienced more hypoxia events. The high concentrations of ammonium and total Kjeldahl nitrogen in the sediment of Mirs Bay increased the oxygen depletion in the bottom water. The long residence time of the near-bottom water in Mirs Bay increased the risk of bottom hypoxia events, although the nutrient concentrations were lower than those in the transition zone. These factors lead to differences in hypoxia occurrence in Mirs Bay and adjacent coastal waters.Keywords：hypoxia;stratification;organic matter decomposition;water residual time;Mirs Bay10|0|0Updated：2024-10-12
Marine ecological environment and its response to marine dynamic processesin the South China Sea
- Abstract：Wind and wave data are essential in climatological and engineering design applications. In this study, data from 15 buoys located throughout the South China Sea (SCS) were used to evaluate the ERA5 wind and wave data. Applicability assessment are beneficial for gaining insight into the reliability of the ERA5 data in the SCS. The bias range between the ERA5 and observed wind-speed data was -0.78–0.99 m/s. The result indicates that, while the ERA5 wind-speed data underestimation was dominate, the overestimation of such data existed as well. Additionally, the ERA5 data underestimated annual maximum wind-speed by up to 38%, with a correlation coefficient >0.87. The bias between the ERA5 and observed significant wave height (SWH) data varied from -0.24 to 0.28 m. And the ERA5 data showed positive SWH bias, which implied a general underestimation at all locations, except those in the Beibu Gulf and central-western SCS, where overestimation was observed. Under extreme conditions, annual maximum SWH in the ERA5 data was underestimated by up to 30%. The correlation coefficients between the ERA5 and observed SWH data at all locations were greater than 0.92, except in the central-western SCS (0.84). The bias between the ERA5 and observed mean wave period (MWP) data varied from -0.74 to 0.57 s. The ERA5 data showed negative MWP biases implying a general overestimation at all locations, except for B1 (the Beibu Gulf) and B7 (the northeastern SCS), where underestimation was observed. The correlation coefficient between the ERA5 and observed MWP data in the Beibu Gulf was the smallest (0.56), and those of other locations fluctuated within a narrow range from 0.82 to 0.90. The intercomparison indicates that during the analyzed time-span, the ERA5 data generally underestimated wind-speed and SWH, but overestimated MWP. Under non-extreme conditions, the ERA5 wind-speed and SWH data can be used with confidence in most regions of the SCS, except in the central-western SCS.Keywords：ERA5;reanalysis data;wind speed;significant wave height;mean wave period;South China Sea (SCS)10|0|3Updated：2024-10-12
- Abstract：The South China Sea suffers strongly from the typhoon storm surge disasters in China, and its northern coastal areas are facing severe risks. Therefore, it is necessary and urgent to establish an assessment system for rating typhoon storm surge disaster. We constructed an effective and reliable rating assessment system for typhoon storm surge disaster based on the theories of over-threshold, distribution function family, and composite extreme value. The over-threshold sample was used as the basis of data analysis, the composite extreme value expansion model was used to derive the design water increment, and then the disaster level was delineated based on the return period level. The results of the extreme value model comparison show that the Weibull-Pareto distribution is more suitable than the classical extreme value distribution for fitting the over-threshold samples. The results of the return period projection are relatively stable based on different analysis samples. Taking the 10 typhoon storm surges as examples, they caused landfall in the Guangdong area in the past 10 years. The results of the assessment ranking indicate that the risk levels based on the return period levels obtained from different distributions are generally consistent. When classifying low-risk areas, the classification criteria of the State Oceanic Administration, China (SOA, 2012) are more conservative. In the high-risk areas, the results of the assessment ranking based on return period are more consistent with those of the SOA.Keywords：risk classification;South China Sea;typhoon storm surge;extreme value expansion;over threshold sampling6|0|1Updated：2024-10-12
Multi-scale marine dynamics, sea-air interactions and forecasting technology in the South China Sea
- Abstract：Whether the South China continental margin had shifted from active subduction to passive extension in the Late Cretaceous remains controversial. Located in the northernmost of the South China Sea continental margin, Sanshui Basin developed continuous stratigraphy from Lower Cretaceous to Eocene and provides precious outcrops to study the regional tectonic evolution during the Cretaceous. Therefore, we conducted field observations, petrology, clay mineralogy, geochemistry, and detrital zircon chronology analyses of sedimentary rocks from the Upper Cretaceous Sanshui Formation in Sanshui Basin. Results suggest that the Sanshui Basin is characterized as an intermoutane basin with multiple provenances, strong hydrodynamic environment, and proximal accumulation in the Late Cretaceous. An angular unconformity at the boundary between the Lower and Upper Cretaceous was observed in the basin. The sedimentary facies of the northern basin changed from lacustrine sedimentary environment in the Early Cretaceous to alluvial facies in the Late Cretaceous. The zircon U-Pb ages of granitic gravelly sandstone from Sanshui Formation prominently range from 100 Ma to 300 Ma, which is close to the deposition age of Sanshui Formation. The major and trace elements of the Late Cretaceous sedimentary samples show characteristics of active continental margin, and are different from the Paleogene rifting sequences. Hence, we propose that the northern South China Sea margin underwent an intense tectonic uplift at the turn of the Early and Late Cretaceous (around 100 Ma). Afterward, the northern South China Sea margin entered a wide extension stage in the Late Cretaceous (~100 to ~80 Ma). This extensional phase is related to the back-arc extension in the active continental margin environment, which is different from the later passive rifting in the Cenozoic. The transition from active subduction to passive extension in the northern South China Sea may occur between the late Late Cretaceous and the Paleogene.Keywords：continental margin;South China Sea;Sanshui Basin;late Cretaceous;tectonic transition6|0|1Updated：2024-10-12
- Abstract：Magnetic data has been widely applied in the tectonic division. High-resolution magnetic data were used to analyze the geotectonic zoning of the South China Sea. Based on the newly compilated magnetic data, the processing results and the distribution of known faults, we consider that the U-shaped line approximately along the South China Sea national boundary of China shown in the magnetic map is a significant geological and geophysical boundary. We first described the linear characteristics of the magnetic data and then applied pseudo-gravity, Euler deconvolution, tilt derivatives, and the texture segmentation method to process the data. Results show that the dividing line between the South China Sea and the surrounding blocks is approximately along this U-shaped line. The dividing line between the South China domain and the South China Sea domain is along with the Dongsha Islands to Xisha Trough, which is different from the previous geophysical zoning results. Our results are almost consistent with those of the gravity data indicating roughly the tectonic zonation along the U-shaped line.Keywords：South China Sea;U-shaped line;tectonic zoning;aeromagnetic map;texture segmentation6|0|0Updated：2024-10-12
- Abstract：Principal component analysis (PCA) was employed to determine the implications of geochemical and isotopic data from Cenozoic volcanic activities in the Southeast Asian region, including China (South China Sea (SCS), Hainan Island, Fujian-Zhejiang coast, Taiwan Island), and parts of Vietnam and Thailand. We analyzed 15 trace element indicators and 5 isotopic indicators for 623 volcanic rock samples collected from the study region. Two principal components (PCs) were extracted by PCA based on the trace elements and Sr-Nd-Pb isotopic ratios, which probably indicate an enriched oceanic island basalt-type mantle plume and a depleted mid-ocean ridge basalt-type spreading ridge. The results show that the influence of the Hainan mantle plume on younger volcanic activities (<13 Ma) is stronger than that on older ones (>13 Ma) at the same location in the Southeast Asian region. PCA was employed to verify the mantle-plume-ridge interaction model of volcanic activities beneath the expansion center of SCS and refute the hypothesis that the tension of SCS is triggered by the Hainan plume. This study reveals the efficiency and applicability of PCA in discussing mantle sources of volcanic activities; thus, PCA is a suitable research method for analyzing geochemical data.Keywords：volcanic rocks;geochemical indicators;mantle source;principal component analysis;South China Sea6|0|0Updated：2024-10-12
- Abstract：As one of the four largest bay areas with strong economic activities in the world, the Guangdong-Hong Kong-Macao Greater Bay Area (GHMGBA) is located in the zone of interaction between the South China Block (SCB) and the South China Sea (SCS). Under the influence of complex geologic evolution, basin-range structures, fault systems and hot springs are well developed here. However, the characteristics of geological structures and the genetic mechanism of these geological phenomena are still unclear. Therefore, we performed ambient noise tomography to obtain 3-D upper crust (0–7.5 km) S-wave velocity structures of the GHMGBA by using 40-day continuous waveform data from 130 seismic stations in the GHMGBA. Our results show that sedimentary basins in the GHMGBA are mainly characterized by low-velocity anomalies. S-wave velocities of sediment formation in basins are about 2.8–3.1 km/s. Rapid changes in velocity appear at the edges of the basins, which correspond to the NE-, NEE-, and NW-trending faults, indicating prominent basin-controlling effects of the faults. The Sanshui Basin (SSB), the largest in the GHMGBA, has a developmental depth of about 4 km, and there is a significant difference in velocity gradient between the east and west sides of the basin, indicating that SSB has experienced east-west asymmetric expansion. Moreover, there are prominent low-velocity anomalies at a depth of about 4.5 km beneath the hot springs at the west of the Zhujiang (Pearl) River estuary (ZRE). We infer that the low-velocity anomalies are fluid reservoirs of the hot springs, which lead to the development of the hot springs on the surface. In addition, the distribution of main cities in the GHMGBA shows a spatial correlation with low-velocity areas at shallow depths (<3 km). The population development trend in the GHMGBA in the past 20 years is also mainly concentrated in the structural province of relatively low-velocity. In combination with the GHMGBA basin structures and drainage distribution characteristics, we suggest that the basic geological environment to some extent affects the habitability of the human settlement and thus determines the distribution and development trend of the main urban context. We believe that the 3-D S-wave velocity structure of the upper crust of the GHMGBA obtained in this study, as well as the deep structural characteristics of the basins and hot springs, will provide support to urban construction planning and geological hazards research of the GHMGBA.Keywords：Guangdong-Hong Kong-Macao Greater Bay Area (GHMGBA);ambient noise tomography;crustal structure;basin;hot spring6|0|0Updated：2024-10-12
- Abstract：The Zengmu and Beikang basins, separated by the West Baram Line (WBL) in the southwestern South China Sea margin, display distinct geological and geophysical features. However, the nature of the basins and the WBL are debated. Here we explore this issue by conducting the stratigraphic and structural interpretation, faults and subsidence analysis, and lithospheric finite extension modelling using seismic data. Results show that the WBL is a trans-extensional fault zone comprising normal faults and flower structures mainly active in the Late Eocene to Early Miocene. The Zengmu Basin, to the southwest of the WBL, shows an overall synformal geometry, thick folded strata in the Late Eocene to Late Miocene (40.4–5.2 Ma), and pretty small normal faults at the basin edge, which imply that the Zengmu Basin is a foreland basin under the Luconia and Borneo collision in the Sarawak since the Eocene. Furthermore, the basin exhibits two stages of subsidence (fast in 40.4–30 Ma and slow in 30–0 Ma); but the amount of observed subsidence and heat flow are both greater than that predicted by crustal thinning. The Beikang Basin, to the NE of the WBL, consists of the syn-rift faulted sub-basins (45–16.4 Ma) and the post-rift less deformed sequences (16.4–0 Ma). The heat flow (~60 mW/m2) is also consistent with that predicted based on crustal thinning, inferring that it is a rifted basin. However, the basin shows three stages of subsidence (fast in 45–30 Ma, uplift in 30–16.4 Ma, and fast in 16.4–0 Ma). In the uplift stage, the strata were partly folded in the Late Oligocene and partly eroded in the Early Miocene, which is probably caused by the flexural bulging in response to the paleo-South China Sea subduction and the subsequent Dangerous Grounds and Borneo collision in the Sabah to the east of the WBL.Keywords：tectonic subsidence;foreland basin;West Baram Line;Zengmu Basin;Beikang Basin;South China Sea6|0|1Updated：2024-10-12
- Abstract：Submarine volcanism is widely developed in the South China Sea (SCS). However, the characteristics, distribution, and genesis of submarine volcanoes in the southern margin of the SCS remain obscure. In this study, we analyzed the characteristics of submarine volcanoes and identified a total of 43 submarine volcanoes in the southern margin of the SCS, based on a newly acquired 310-km seismic reflection profile, along with previous 45 multi-channel seismic (MCS) profiles, petrological results from volcanic rocks sampled by dredging and drilling, nearby ocean bottom seismometer (OBS) wide-angle seismic profiles, and gravity and magnetic data. The study ascertains that most of these volcanoes are located in fault-block belts and graben-horst zones with strong crustal stretching and thinning. These volcanoes exhibit positive high-amplitude external seismic reflections, weak and chaotic internal seismic reflections, and are accompanied by local deformation of the surrounding sedimentary strata. Meanwhile, they have higher positive gravity anomalies and higher magnetic anomalies than the background strata. The petrological dating results show that volcanic ages are primarily in the Pliocene-Pleistocene, with geochemical characteristics indicating dominance of oceanic island basalt (OIB)-type alkali-basalts. Extensional faults have obviously spatial correspondence with post-spreading volcanism, suggesting these faults may provide conduits for submarine volcanism. The high-velocity bodies (HVBs) in the lower crust and magma underplating exist in the southern SCS, which could provide a clue of genesis for submarine volcanism. The inference is that the intensity of post-spreading volcanism in the southern margin might be affected by stretching faults, crustal thinning and magma underplating.Keywords：submarine volcanoes;magmatism;extensional fault;Nansha Block;South China Sea (SCS)6|0|0Updated：2024-10-12
- Abstract：The stretched structure and heterogeneity of the crust of the Nansha Block, the southern continental margin of the South China Sea (SCS), are not well understood. We used published ocean bottom seismic (OBS)/multichannel reflection seismic (MCS) profiles across the Nansha Block to establish five two-dimensional crustal structure models. Using gravity modelling with magnetic anomaly inversion, we obtained the distribution of density and local magnetic susceptibility of the crust. The models show that the distribution of density and thickness of the upper crust in the Nansha Block is uneven, and the thick upper crust is prevalent in the regions close to the continent-ocean transition (COT) showing different characteristics. The interpreted Mesozoic granite blocks and Precambrian rigid basement reflects the heterogeneity in the material composition of the SCS continental margin. Based on the thinning styles of different crustal layers, we suggest that the Nansha Block has a three-layer thinning pattern. The uppermost pre-rift layer was deformed via brittle fractures, the upper crust was sheared by discrete shear zones, and the lower crust experienced ductile deformation. The inherited pre-rift thermal regime, mechanical state, and material composition of the SCS continental margin affected the extensional structure of the crust.Keywords：Nansha Block;crustal architecture;gravity inversion;crustal boudinage6|0|0Updated：2024-10-12
- Abstract：The Kuiyang-ST2000 deep-towed high-resolution multichannel seismic system was designed by the First Institute of Oceanography, Ministry of Natural Resources (FIO, MNR). The system is mainly composed of a plasma spark source (source level: 216 dB, main frequency: 750 Hz, frequency bandwidth: 150–1 200 Hz) and a towed hydrophone streamer with 48 channels. Because the source and the towed hydrophone streamer are constantly moving according to the towing configuration, the accurate positioning of the towing hydrophone array and the moveout correction of deep-towed multichannel seismic data processing before imaging are challenging. Initially, according to the characteristics of the system and the towing streamer shape in deep water, travel-time positioning method was used to construct the hydrophone streamer shape, and the results were corrected by using the polynomial curve fitting method. Then, a new data-processing workflow for Kuiyang-ST2000 system data was introduced, mainly including float datum setting, residual static correction, phase-based moveout correction, which allows the imaging algorithms of conventional marine seismic data processing to extend to deep-towed seismic data. We successfully applied the Kuiyang-ST2000 system and methodology of data processing to a gas hydrate survey of the Qiongdongnan and Shenhu areas in the South China Sea, and the results show that the profile has very high vertical and lateral resolutions (0.5 m and 8 m, respectively), which can provide full and accurate details of gas hydrate-related and geohazard sedimentary and structural features in the South China Sea.Keywords：Kuiyang-ST2000 system;deep-towed system;seismic data process;plasma spark source;high resolution;gas hydrate;South China Sea6|0|0Updated：2024-10-12
- Abstract：Bivalve shell fossils, cemented by authigenic carbonates, are widely spread in the Haima cold seep, Qiongdongnan Basin of the South China Sea (SCS). In this study, we examined an element profile of Gigantidas platifrons in the Haima cold seeps at a water depth of 1 381 m. Based on the scanning electron microscope (SEM) analyses, the prismatic layer and nacreous layer were identified, which are characterized by prismatic structure and stratified structure, respectively. In addition, the profile can be subdivided into two parts: altered and unaltered zones. Laser inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping shows that the element concentrations of the altered zones were influenced by the authigenic carbonate rocks, whereas the element concentrations of unaltered zones remain stable. In-situ X-ray diffraction (XRD) analyses show that the mineral constituent of the prismatic layer is mainly composed of aragonite. Along with the growth profile, Mg/Ca ratios of unaltered zones have minor variations, ranging 0.72–0.97 mmol/mol (mean=0.87 mmol/mol), with estimated temperatures of 3.8–4.1 °C, indicating that the temperature of the surrounding seawater remains constant and agree with the measured data of 3.9 °C which was conducted by a conductivity-temperature-depth system (CTD). The minor variations of Ba/Ca ratios (0.01–0.06 mmol/mol; mean=0.04 mmol/mol) indicate a relatively stabilized salinity of the surrounding seawater. S/Ca ratios show large variations of 0.04–4.15 mmol/mol (mean=1.37 mmol/mol). S/Ca ratios have regular variations which generally correspond to the variations of the Mg/Ca ratios, highlighting that the S/Ca ratios of bivalve shells show the potential to reflect the growth rate of the Gigantides. However, further studies should be carried out on the understanding of the links between the S/Ca ratios and seepage intensity of cold-seep fluids.Keywords：Mg/Ca, Ba/Ca, and S/Ca ratios;laser inductively coupled plasma mass spectrometry (LA-ICP-MS);Gigantidas platifrons;Haima cold seep;South China Sea10|0|1Updated：2024-10-12
- Abstract：Submarine seep plumes are a natural phenomenon in which different types of gases migrate through deep or shallow subsurface sediments and leak into seawater in pressure gradient. When detected using acoustic data, the leaked gases frequently exhibit a flame-like structure. We numerically modelled the relationship between the seismic response characteristic and bubble volume fraction to establish the bubble volume fraction in the submarine seep plume. Results show that our models are able to invert and predict the bubble volume fraction from field seismic oceanography data, by which synthetic seismic sections in different dominant frequencies could be numerically simulated, seismic attribute sections (e.g., instantaneous amplitude, instantaneous frequency, and instantaneous phase) extracted, and the correlation between the seismic attributes and bubble volume fraction be quantitatively determined with functional equations. The instantaneous amplitude is positively correlated with bubble volume fraction, while the instantaneous frequency and bubble volume fraction are negatively correlated. In addition, information entropy is introduced as a proxy to quantify the relationship between the instantaneous phase and bubble volume fraction. As the bubble volume fraction increases, the information entropy of the instantaneous phase increases rapidly at the beginning, followed by a slight upward trend, and finally stabilizes. Therefore, under optimal noise conditions, the bubble volume fraction of submarine seep plumes can be inverted and predicted based on seismic response characteristics in terms of seismic attributes.Keywords：seismic oceanography;submarine seep plumes;bubble volume fraction;seismic response characteristics;seismic attribute analysis;quantitative analysis6|0|0Updated：2024-10-12
Geomorphology and geological structure of South China Sea
- Abstract：The acoustic properties of seafloor sediment are essential parameters in the exploration of marine resources, ocean scientific research and ocean engineering. Seafloor sediment samples were collected at the southern U-boundary of the South China Sea (SCS), and the acoustic and physical properties were measured in the laboratory. The correlation between physical and sound speed ratio (SSR) was discussed, and SSR-physical property empirical regressions in the Sunda Shelf were established for the first time. Compared with the northern continental shelf of SCS, the Sunda Shelf are mainly silty and sand sediment, and the SSR ranges from 0.994 9 to 1.094 4, which has higher SSR than the northern continental shelf, implies that the Sunda Shelf is a high SSR area. Since the same kind of sediment has different physical properties, the single physical parameter of sediment cannot fully represent the acoustic properties of sediment, therefore, the multiple parameter prediction model should develop in the future to improve the prediction precision.Keywords：acoustic properties;seafloor sediment;South China Sea (SCS);U-boundary7|1|2Updated：2024-10-12
- Abstract：High concentrated and heterogeneous distribution of gas hydrates have been identified in the gas hydrate production test region in the Shenhu area, South China Sea. The gas hydrate-bearing sediments with high saturation locate at two ridges of submarine canyon with different thickness and saturations just above the bottom simulating reflection. The crossplots of gamma ray, acoustic impedance (P-impedance) and porosity at four sites show that the sediments can be divided into the upper and lower layers at different depths, indicating different geotechnical reservoir properties. Therefore, the depositional environments and physical properties at two ridges are analyzed and compared to show the different characteristics of hydrate reservoir. High porosity, high P-wave velocity, and coarse grain size indicate better reservoir quality and higher energy depositional environment for gas hydrate at Sites W18 and W19 than those at Sites W11 and W17. Our interpretation is that the base of canyon deposits at Sites W18 and W19 characterized by upward-coarsening units may be turbidity sand layers, thus significantly improving the reservoir quality with increasing gas hydrate saturation. The shelf and slope sliding deposits compose of the fine-grained sediments at Sites W11 and W17. The gas hydrate production test sites were conducted at the ridge of W11 and W17, mainly because of the thicker and larger area of gas hydrate-bearing reservoirs than those at Sites W18 and W19. All the results provide useful insights for assessing reservoir quality in the Shenhu area.Keywords：reservoir properties;gas hydrate production test region;depositional environment;South China Sea6|0|2Updated：2024-10-12
- Abstract：The southwestern depression of the Qiongdongnan Basin (QDNB) hosts thick Cenozoic sediments and awaits major hydrocarbon discovery. Multichannel seismic (MCS) profile CFT2011 across the southwestern QDNB reveals a ~60-km-wide fuzzy reflection zone (FRZ) within the sediments, but its origin and distribution remain unclear. Here ocean bottom seismometer (OBS) data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles. The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth, likely resulting from enhanced fluid infilling. The MCS profile exhibits that the giant FRZ is about 3–9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments. Within the shallow overlying sediments, multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset, consistent with the features of gas-charged sediments. They are probably shallow gas reservoirs with gases sourced from the deep FRZ. Therefore, the FRZ is proposed to be a giant gas-charged zone, which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure. This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km, respectively. It is roughly estimated to cover an area of ~1 900 km2 and host a volume of ~11 400 km3 assuming an average thickness of 6 km, implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea.Keywords：Southwestern Qiongdongnan Basin;fuzzy reflection zone;low velocity;gas charging;natural gas potential6|0|1Updated：2024-10-12
- Abstract：Natural gas hydrate is a potential clean energy source and is related to submarine geohazard, climate change, and global carbon cycle. Multidisciplinary investigations have revealed the occurrence of hydrate in the Qiongdongnan Basin, northern South China Sea. However, the spatial distribution, controlling factors, and favorable areas are not well defined. Here we use the available high-resolution seismic lines, well logging, and heat flow data to explore the issues by calculating the thickness of gas hydrate stability zone (GHSZ) and estimating the inventory. Results show that the GHSZ thickness ranges between mostly ~200 and 400 m at water depths >500 m. The gas hydrate inventory is ~6.5×109-t carbon over an area of ~6×104 km2. Three areas including the lower uplift to the south of the Lingshui sub-basin, the Songnan and Baodao sub-basins, and the Changchang sub-basin have a thick GHSZ of ~250–310 m, 250–330 m, and 350–400 m, respectively, where water depths are ~1 000–1 600 m, 1 000–2 000 m, and 2 400–3 000 m, respectively. In these deep waters, bottom water temperatures vary slightly from ~4 to 2 °C. However, heat flow increases significantly with water depth and reaches the highest value of ~80–100 mW/m2 in the deepest water area of Changchang sub-basin. High heat flow tends to reduce GHSZ thickness, but the thickest GHSZ still occurs in the Changchang sub-basin, highlighting the role of water depth in controlling GHSZ. The lower uplift to the south of the Lingshui sub-basin has high deposition rate (~270–830 m/Ma in 1.8–0 Ma); the thick Cenozoic sediment, rich biogenic and thermogenic gas supplies, and excellent transport systems (faults, diapirs, and gas chimneys) enables it a promising area of hydrate accumulation, from which hydrate-related bottom simulating reflectors, gas chimneys, and active cold seeps were widely revealed.Keywords：gas hydrate stability zone;gas hydrate inventory;Qiongdongnan Basin;South China Sea8|0|2Updated：2024-10-12
- Abstract：The Shenhu Submarine Canyon Group on the northern slope of the South China Sea consists of 17 slope-confined canyons, providing a good example for investigating their hosting sediments. Three drilling sites, including W07, W18, and W19, have proven the occurrence of gas hydrate reservoirs in the inter-canyon area between canyons C11 and C12. Whereas, variations of the geomorphology and seismic facies analyzed by high-resolution 3D seismic data indicate that the gas hydrate-bearing sediments may form in different sedimentary processes. In the upper segment, a set of small-scale channels with obvious topographic lows can be identified, revealing fine-grained turbidites supplied from the shelf region during a very short-term sea-level lowstand. In the middle part, gas hydrate units at Site W07 showing mounded or undulation external configuration are interpreted as sliding sedimentary features, and those features caused by gravity destabilization were the main formative mechanism of gas hydrate-bearing sediments that were sourced from the upper segments. In contrast, for the canyon transition zone of lower segments between C11–C12 inter-canyon and C12 intra-canyon areas, where W18 and W19 sites are located, the gas hydrate-bearing sediments are deposited in the channelized feature in the middle to lower segment and slide erosive surface. Gas hydrate-bearing sediments of the lower segment were migrated through channelized features interconnecting with the middle to lower slope by gravity-driven flows. The majority of deposits tended to be furtherly moved by lateral migration via erosive surface created by sediment failed to intra-canyon area. The conclusion of this study may help better understand the interaction between the formation mechanism of gas hydrate-bearing sediments and the geomorphologic effects of inter-canyon areas.Keywords：submarine canyons;gas hydrate-bearing sediments;seismic geomorphology;inter-canyon transport process6|0|1Updated：2024-10-12
- Abstract：The Zengmu Basin located in the shallow water area of the southern South China Sea, is rich in oil and gas resources, within which faults and mud-diapir are developed, but it is unknown whether oil and gas migrate to the seafloor surface. The newly collected multibeam data across the Zengmu Basin reveal a large number of depressions, with depths of 2–4 m, widths of several tens of meters, large distribution range of 1.8–8 km along survey line, up to ~50 km, and their backscatter intensity (-26 dB) is much greater than that of the surrounding area (-38 dB). Combined with the developed mud-diapir and fracture structures, and abundant oil and gas resources within this basin, these depressions are presumed to be pockmarks. Furthermore, more than 110 mono-sized small circular pockmarks, with a depth of less than 1 m and a width of 5 m, are observed in an area of less than 0.03 km2, which are not obliterated by sediment infilling with high sedimentation rate, implying an existence of unit-pockmarks that are or recently were active. In addition, seismic profiles across the Zengmu Basin show characterization of upward migration of hydrocarbons, expressed as mud-diapir structures, bright spots in the shallow formation with characteristics of “low frequency increase and high frequency attenuation”. The sub-bottom profiles show the mud-diapir structures, as well as the gas-bearing blank zones beneath the seafloor. These features suggest large gas leaking and occurrence of large amounts of carbonate nodules on the seafloor. This indicates the complex and variable substrate type in the Zengmu Basin, while the area was once thought to be mainly silty sand and find sand. This is the first report on the discovery of pockmarks in the Zengmu Basin; it will provide basic information for submarine stability and marine engineering in China’s maritime boundaries.Keywords：South China Sea;Zengmu Basin;dense pockmarks;gas leaking;substrate type6|0|1Updated：2024-10-12
- Abstract：Knowledge of nepheloid layers is important to improve the understanding of physical, geological, and sedimentary processes from continental shelf to abyssal environments. We had not tried to study the nepheloid layers in a hydrate-associated tank until unexpected results occurred. Tank experimental results show that gas eruptions triggered intermediate nepheloid layers. Thus, we proposed a new mechanism of intermediate nepheloid layer generation by eruptions. The intermediate nepheloid layers were generated in uniform-density fluid, which indicated that stratified fluid is not a necessary condition for intermediate nepheloid layers. Sufficient space for advection and an oblique slope for detachment are the key ingredients for intermediate nepheloid layer generation by eruptions. Our experiments also offer a new experimental evidence for bottom nepheloid layer generation by earthquakes. Given the scale effects of laboratory experiment, it is important to determine whether submarine volcanic eruption or hydrate-associated venting causes intermediate nepheloid layer in the nature.Keywords：intermediate nepheloid layer;gas eruption;bottom nepheloid layer;gas hydrate;tank experiment;seafloor instability6|0|0Updated：2024-10-12
Sedimentary processes and resource potential in the South China Sea
- Abstract：The ecological suitability of marine resources and environmental conditions for marine ranching in Guangdong, South China was evaluated. Niche theory was used to establish an ecological suitability evaluation model for marine ranching site selection, and suitability evaluation of marine ranching was conducted in. Results show that the ecological suitability index of marine ranching site selection was greater than 64.0 in 20 sites, including offshore Shantou Bay, offshore Zhanjiang Port, and Longdou Bay etc., which are the priority areas for marine ranching. In other 13 sites, the ecological suitability index ranged between 8.0 and 32.0, including Houjiang Bay, Haimen Bay, and Jieshi Bay etc., indicating the suitability for marine ranching. However, the ecological suitability index was 0 in Shantou Bay, Zhujiang (Pearl) River estuary, Huangmao Sea, Guanghai Bay, Zhanjiang Port, Qiongzhou Strait, and Anpu Port. These sites are thus unsuitable for marine ranching. This study provides a theoretical basis for site selection and planning of marine ranching in Guangdong Province.Keywords：marine ranching;site suitability;evaluation;ecological niche;Guangdong Province6|0|1Updated：2024-10-12
Geostrategy, rights and interests maintenance and strategic countermeasures for the South China Sea
- Abstract：A new gravity sensor based on electromagnetic damping for the JMGrav marine gravimeter is presented. The new gravity sensor considered the advanced construction methods of the electromagnetic damping system of the gravimeter. The design features of the new system are discussed and the research survey data in the South China Sea are shown. Numerical simulations are applied to model the magnetic and mechanical characteristics of the system using finite element analysis and to evaluate the force distribution and the resulting damping effects. The performance characteristics of the system were tested on a motion simulator in laboratory, and the gravimeter was subjected to vertical accelerations of up to 100 Gal in 1–1 000 s. It was found that the amplitude reduction of vertical accelerations in 3–15 s is 30–45 dB, with a time lag of 2–5 s, while the effect on gravity in period greater than 600 s is less than 0.5 dB, with a time lag of less than 100 s. The accelerations cause discrepancies of approximately only 1 mGal between the static value and the mean dynamic value. The sea tests were conducted in September 2020. Gravity measurements were taken with a JMGrav marine gravimeter onboard the R/V Dongfanghong 3, and the effective survey line exceeded 2 000 km. Completely irregular accelerations with peaks up to 100 Gal yielded a reduction of approximately 40 dB in amplitude. The survey data were evaluated using ocean gravity field models and grid line tests. The results show that the accuracy of the gravity measurements is better than 2 mGal.Keywords：electromagnetic damping;JMGrav marine gravimeter;marine gravity survey6|0|2Updated：2024-10-12
- Abstract：The seafloor vector magnetometer is an effective tool for marine geomagnetic surveys and seafloor magnetotelluric (MT) detection. However, the noise, power consumption, cost, and volume characteristics of existing seafloor vector magnetometers are insufficient for practical use. Therefore, a low-noise, low-power-consumption seafloor vector magnetometer that can be used for data acquisition of deep-ocean geomagnetic vector components is developed and presented. A seafloor vector magnetometer mainly consists of a fluxgate sensor, data acquisition module, acoustic release module, glass sphere, frame, burn-wire release, and anchor. A new low-noise data acquisition module and a fluxgate sensor greatly reduce power consumption. Furthermore, compact size is achieved by integrating an acoustic telemetry module and replacing the acoustic release with an external burn-wire release. The new design and magnetometer characteristics reduce the volume of the instrument and the cost of hardware considerably, thereby improving the integrity and deployment efficiency of the equipment. Theoretically, it can operate for 90 days underwater at a maximum depth of 6 000 m. The seafloor vector magnetometer was tested in the South China Sea and the Philippine Sea and obtained high-quality geomagnetic data. The deep-water environment facilitates magnetic field data measurements, and the magnetometer has an approximate noise level of 10 pT/rt (Hz)@1 Hz, a peak-to-peak value error of 0.2 nT, and approximate power consumption of 200 mW. The fluxgate sensor can measure the magnetic field in the lower frequency band and realize geomagnetic field measurements over prolonged periods.Keywords：seafloor vector magnetometer;low noise;low power consumption6|0|1Updated：2024-10-12
- Abstract：To dampen periodic off-levelling motions within an inertial platform while undergoing horizontal accelerations of the same period and to achieve a levelling accuracy of a few tens of arcseconds with that system, an internally damped inertial platform for a marine scalar gravity system was the developed. Methods for attenuating horizontal acceleration and reducing off-levelling error by a satisfactory gyro-levelling loop, which are fundamental to the internally damped inertial platform, were designed and implemented. In addition, phase delays are introduced by the levelling loop. The resulting off-levelling gravity errors were analyzed and modeled. A series of tests on a motion simulator were performed in laboratory for a variety of simulated sea conditions. We found that the motion of the platform is a function of the amplitude and period of the simulated ship motions and ranges between 10 and 40 arcseconds. In addition, the phase lag between platform motion and ship motion is not constant but ranges 180°–270°, depending on the period and amplitude of the motion. Then, the platform, on which a gravimeter was mounted, was installed on the R/V Shiyan 2 to conduct a gravity survey in the South China Sea. Despite rough sea conditions, it was shown that in short periods of 2–30 s, the off-levelling angle was less than 30 arcseconds, and the phase lagged the horizontal acceleration by 230°–260°. From a repeated survey line and intersecting survey points, the estimated errors of gravity measurements were between 1.3 and 1.7 mGal. The marine measurements results were compared with those of satellite altimetry data and show a mean value of 0.5 mGal in a standard deviation of 1.5 mGal.Keywords：internally damped platform;gyro-levelling loop;marine gravimetry6|0|1Updated：2024-10-12
Marine Scientific Instruments
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