The exploration of unconventional and/or new energy resources has become the focus of energy research worldwide, given the shortage of fossil fuels. As a potential energy resource, gas hydrate exists only in the environment of high pressure and low temperature, mainly distributing in the sediments of the seafloor in the continental margins and the permafrost zones in land. The accurate determination of the thickness of gas hydrate stability zone is essential yet challenging in the assessment of the exploitation potential. The majority of previous studies obtain this thickness by detecting the bottom simulating reflectors (BSRs) layer on the seismic profiles. The phase equilibrium between gas hydrate stable state with its temperature and pressure provides an opportunity to derive the thickness with the geothermal method. Based on the latest geothermal dataset, we calculated the thickness of the gas hydrate stability zone (GHSZ) in the north continental margin of the South China Sea. Our results indicate that the thicknesses of gas hydrate stability zone vary greatly in different areas of the northern margin of the South China Sea. The thickness mainly concentrates on 200–300 m and distributes in the southwestern and eastern areas with belt-like shape. We further confirmed a certain relationship between the GHSZ thickness and factors such as heat flow and water depth. The thickness of gas hydrate stability zone is found to be large where the heat flow is relatively low. The GHSZ thickness increases with the increase of the water depth, but it tends to stay steady when the water depth deeper than 3 000 m. The findings would improve the assessment of gas hydrate resource potential in the South China Sea.

In this study, about 220 satellite images between 2000 and 2012 were obtained from FY-series, MODIS, CBERS, HJ-1A and HJ-1B to estimate the impact of duststorms on the South Yellow Sea (SYS), which serve as an important source of particles there. The analyzing results from the images support a total occurrence of 88 duststorms (including the locally-generated dusty weather) that affected the SYS during 2000–2012. The annual occurrence was about 4–10 times (10 times in 2000 and 2004; four times in 2009 and 2012), predominantly in March (29%), April (33%) and May (22%). By mapping the distribution of their frequency, the duststorms influencing the SYS were found primarily moving from the northwest (39 times, 44.3%) and west (37 times, 42%) to the study region with only 11 duststorms (12.5%) coming from the north and 1 duststorm (1%) from the southwest. We estimated that an annual amount of 0.5–3.5 million tons of sediment particles was brought to the SYS by the duststorms during 2000–2012.

Water circulation and sediment transport in the Beibu Gulf are important for its environmental protection and resource exploitation. By employing the Regional Ocean Modeling System (ROMS), we studied the seasonal variation of circulation, sediment transport and long-term morphological evolution in the Beibu Gulf. The simulation results show that the circulation induced by tide and wind is cyclonic both in winter and summer in the gulf and that the wind-driven circulation is stronger in winter than that in summer. The sediment concentration is higher in the Qiongzhou Strait, west of the Hainan Island and the coast of Vietnam and the Leizhou Peninsula. The sediment is transported westwards in winter and eastwards in summer in the Qiongzhou Strait. The west entrance of the Qiongzhou Strait is dominated by westward transport all the year round. The sediment discharged by rivers is deposited near the river mouths. The simulated result demonstrates that the sediment transport is mainly controlled by tidal induced bottom resuspension in the Beibu Gulf. Four characteristics are summarized for the distribution patterns of erosion and deposition. (1) The erosion and deposition are insignificant in most area of the gulf. (2) Sediment deposition is more significant in the mouths of Qiongzhou Strait. (3) The erosion is observed in the seabed of Qiongzhou Strait. (4) Erosion and deposition occur alternatively in the west of Hainan Island.

To evaluate the controlling factors for coastline change of the Changjiang (Yangtze River) Estuary since 1974, we extracted the mean high tide line from multi-temporal remote sensing images that span from 1974 to 2014 at 2-year intervals. We chose 42 scenes to constrain the changing pattern of the Changjiang Estuary coastline, and implemented GIS technology to analyze the area change of the Changjiang (Yangtze) Subaerial Delta. Runoff, sediment discharge and coastal engineering were withal considered in the analysis of the coastline changes. The coastline has transgressed seaward since 1974, and a part of it presents inter-annual variations. The area of the Changjiang Subaerial Delta increased by 871 km², with a net accretion rate of 21.8 km²/a. Based on the change of sediment discharge due to the major projects in the Changjiang River Basin, we divided the changing pattern of the coastline into three stages: the slow accretion stage (1974–1986), the moderate accretion stage (1987–2002), and the rapid accretion stage (2003–2014). Liner regression analysis illustrated that there is a significantly positive correlation between the area changes and sediment discharge in the Chongming Eastern Shoal and Jiuduansha. This suggested that sediment load has a fundamental effect on the evolution of the Changjiang Estuary. Construction of Deep Waterway in the North Passage of the Changjiang River (1998–2010) led to a rapid accretion in the Hengsha Eastern Shoal and Jiuduansha by influencing the hydrodynamics in North Passage. Coastal engineering such as reclamation and harbor construction can also change the morphology of the Changjiang Estuary. We defined a contribution rate of area change to assess the impact of reclamation on the evolution of Changjiang Estuary. It turned out that more than 45.3% of area increment of the Changjiang Estuary was attributed to reclamation.

An understanding of the sedimentary environment in relation to its controlling factors is of great importance in coastal geomorphology, ecology, tourism and aquaculture studies. We attempt to deal with this issue, using a case study from the Xincun Lagoon, Hainan Island in southern China. For the study, surficial sediment samples were collected, together with hydrodynamic and bathymetric surveys, during August 2013. Numerical simulation was carried out to obtain high-spatial resolution tidal current data. The sediment samples were analyzed to derive mean grain size, sorting coefficient, skewness and kurtosis, together with the sand, silt and clay contents. The modern sedimentary environments were classified using system cluster and principal component analyses. Grain size analysis reveals that the sediments are characterized by extremely slightly sandy silty mud (ESSSM) and slightly silty sand (SSS), which are distributed in the central lagoon and near-shore shallow water areas, respectively. Mean grain size varies from 0 to 8.0Ф, with an average of 4.6Ф. The silt content is the highest, i.e., 52% on average, with the average contents of sand and clay being 43% and 5%, respectively. There exists a significant correlation between mean size and water depth, suggesting that the surficial sediments become finer with increasing water depth. Cluster analyses reveals two groups of samples. The first group is characterized by mean grain size of more than 5.5Ф, whilst the second group has mean grain size of below 3.5Ф. Further, these groups also have different correlations between mean grain size and the other grain size parameters. In terms of the tidal current, the average values of the root mean square velocity (RMSV) are 7.5 cm/s and 6.9 cm/s on springs and neaps, respectively. For the RMSVs that are higher than 4 cm/s, a significant positive correlation is found between the content of the 63–125 μm fraction and the RMSV, suggesting that the RMSV determines the variability of the very fine sand fraction. Based on system cluster and principal component analyses (PCA), the modern sedimentary environments are classified into three types according to the grain size parameters, RMSVs and water depth data. The results suggest the importance of grain size parameters and high-spatial resolution hydrodynamic data in differentiating the coastal sedimentary environments.

Large-scaled reclamation modifies the coastal environment dramatically while accelerating the disappearance of salt marshes, which causes the degradation of the coastal ecosystem and the biodiversity function. In this study, we explored the changes of tidal flat and salt marsh coverage in a small-scale tidal flat with an area of ~160 000 m² in the plain coast of Jiangsu Province, China. Human activities (e.g., the construction of dikes) are a crucial contributor that benefits for the tidal flat accretions and the following changes of salt marsh coverage. Located in the front of the man-made “concave coastline”, the study area is suitable for sediment accretion after the dike construction in the end of 2006. On the basis of the annual tidal surface elevation survey from 2007 to 2012, the sedimentation rates in the human influenced tidal flat varied from a few centimeters per year to 23 cm/a. The study area experienced a rapid accretion in the tidal flat and the expansion of the salt marsh, with the formation of a longshore bar, and a subsequent decline of the salt marsh. Breaking waves during the flooding tide brought much sediment from the adjacent tidal flat to the study area, which caused burial and degeneration of the salt marsh. The vertical grain size changes within a 66 cm long core in the study area also demonstrated the above changes in the tidal environment. This study indicates that the responses of small-scale tidal flat changes to reclamation are significant, and the rational reclamation would benefit for the new salt marsh formation in front of the dikes. Further research about the evolution of small scale tidal flat as well as the spatial planning of the polder dike should be strengthened for the purpose to maintain a healthier coastal environment.

In the east of China’s seas, there is a wide range of the continental shelf. The nutrient cycle and the carbon cycle in the east of China’s seas exhibit a strong variability on seasonal to decadal time scales. On the basis of a regional ocean modeling system (ROMS), a three dimensional physical-biogeochemical model including the carbon cycle with the resolution (1/12)°×(1/12)° is established to investigate the physical variations, ecosystem responses and carbon cycle consequences in the east of China’s seas. The ROMS-Nutrient Phytoplankton Zooplankton Detritus (NPZD) model is driven by daily air-sea fluxes (wind stress, long wave radiation, short wave radiation, sensible heat and latent heat, freshwater fluxes) that derived from the National Centers for Environmental Prediction (NCEP) reanalysis2 from 1982 to 2005. The coupled model is capable of reproducing the observed seasonal variation characteristics over the same period in the East China Sea. The integrated air-sea CO₂ flux over the entire east of China’s seas reveals a strong seasonal cycle, functioning as a source of CO₂ to the atmosphere from June to October, while serving as a sink of CO₂ to the atmosphere in the other months. The 24 a mean value of air-sea CO₂ flux over the entire east of China’s seas is about 1.06 mol/(m²·a), which is equivalent to a regional total of 3.22 Mt/a, indicating that in the east of China’s seas there is a sink of CO₂ to the atmosphere. The partial pressure of carbon dioxide in sea water in the east of China’s seas has an increasing rate of 1.15 μatm/a (1μtm/a=0.101 325 Pa), but pH in sea water has an opposite tendency, which decreases with a rate of 0.001 3 a^–1 from 1982 to 2005. Biological activity is a dominant factor that controls the $ {p_{{\rm{C}}{{\rm{O}}_2}{\rm{air}}}} $ in the east of China’s seas, and followed by a temperature. The inverse relationship between the interannual variability of air-sea CO₂ flux averaged from the domain area and Niño3 SST Index indicates that the carbon cycle in the east of China’s seas has a high correlation with El Niño-Southern Oscillation (ENSO).

The comparison of the underwater topographic data in recent four decades shows that main waterways of the radial sand ridges area in the southern Yellow Sea tend to gradually migrate southward (scour depth and southward extension of the main channels in Xiyang, southward approach of Lanshayang Waterway and Xiaomiaohong Waterway on South Flank). Although there are various hypotheses about the cause and mechanism of the overall southward migration of the radial sand ridges, no universal and reliable understanding has been obtained so far. The mechanism of this process becomes a challenging problem which serves a key issue in the morphodynamics of the radial sand ridges and the harbor construction in this area. On the basis of the shoreline positions and underwater terrains at different development stages of the Huanghe Delta coast in northern Jiangsu Province, China since the northward return of the Huanghe River and flowed into the Bohai Sea, combined with the tidal wave numerical simulation study, the characteristics and hydrodynamic changes of the tidal wave system in the southern Yellow Sea at different evolution stages are investigated. It is shown that due to the shoreline retreat and the erosion of underwater delta, tidal current velocity is enhanced, and the enhanced area gradually migrates southward. It is revealed that this southward migration of a large-scale regional hydrodynamic axis is possibly a dominant mechanism leading to the overall southward migration of the radial sand ridges.

It is exceedingly important to estimate the stability of coral reefs. In recent years, growing construction projects have been carried out on the reef flat in the South China Sea. As a special marine geotechnical medium, it is made of the reef debris underwent overwhelmingly long geological age. Reefs grow thickly on the carbonate platform after the Late Oligocene and have five to six main sedimentary facies. It can be used as a recorder to measure the occurrence time of recent earthquake. A model of reef body is presented to study the influence of earthquakes according to the geological structure characteristic of reefs in the Nansha Islands. Furthermore, GeoStudio is used to simulate stress and deformation situations within it under various earthquake intensities. A safety factor is calculated by the limit equilibrium method, and the possible scenarios of earthquake-induced landslides and sliding scale are defined with a Newmark sliding block method, as well as stress distribution and deformation behaviors. Therefore, the numerical results suggest that the connections between the coral reef and the earthquake are as follows: (1) the reef body has a good stability under self-gravity state; (2) after the earthquake, it may cause slope’s instability and bring out slumping when the safety factor is smaller than 1 (F_S<1); (3) the safety factor decreases with the increasing earthquake intensity, and fluctuates around a particular value after a while; and (4) as a new developed part of the reef, the smaller shallow landslide will be easily subject to collapse caused by the earthquake. It is concluded that it is feasible to provide a reference for evaluating the stability of coral reef using a geotechnical engineering simulation method. This can help the engineering constructions in the South China Sea.