Coral reefs over the Nansha Islands have characteristic geomorphological patterns; they exist in the form of atolls. The Niu’e (also known as Mckennan) and Ximen (Whitsum) reefs of the Jiuzhang Giant Atoll are two secondary atolls with newly-formed debris deposits, which may represent an important stage of landform evolution from a submerged atoll towards a reef island. Grain-size analysis of the surficial sediment samples collected from the two reefs shows that gravel-sized coral debris is the main component of the deposit. Medium grain sizes of the samples (measured by the long axis) range from 14 to 45 mm. Sorting coefficients range from 4.5 (being poorly sorted) to 31 with a sphericity value of 0.52 to 0.68 (far from a sphere shape). Medium grain sizes show a trend of increasing away from the shoreline. Medium grain sizes of the sediment samples are positively correlated with sorting coefficients, whereas they are more or less negatively correlated with sphericity values. Based on an analysis of historical records and satellite images, these beach ridge- or sand bar-like deposits represent modern accumulation landforms; their formations are related to storm waves and to some extent to the winter monsoon waves. The sand bar on the Niu’e reef had been extending towards the southeast and at the same time moving towards the north. The sand bar on the Ximen reef had an overall migration towards the southeast, with recurved sand spits on both sides. The mechanisms for the formulation of these features, in terms of changes in coral material supply and storm wave pattern, need further investigations.

Marine planktonic ostracods are small crustaceans with a wide distribution, playing an important role in ocean carbon cycle. The ostracods in smaller size are usually underestimated when marine zooplankton samples were collected with 505 μm mesh. Based on the zooplankton samples collected with 505 and 160 μm meshes in the northern South China Sea from July to August 2006 (summer) and from December 2006 to January 2007 (winter), the capture efficiency for the ostracods using two planktonic mesh sizes was compared, and the community structure and the influence of environmental factors on their distributions were analyzed. We found that the abundance of ostracods obtained by the 160 μm mesh net was six times that by the 505 μm net in summer. The community structure of planktonic ostracods collected by the 160 μm mesh was analyzed with the following findings. 1) The species richness of planktonic ostracods of a total of 32 species was identified to increase from nearshore to offshore, and the difference between the areas with water depth shallower and deeper than 100 m was extremely significant (p<0.01). 2) The abundance of planktonic ostracods was higher in summer, when the high abundance area appeared mainly in the coastal waters of the Leizhou Peninsula and northeast Hainan Island higher than that in offshore significantly (p<0.05), and lower in winter, when the distribution of ostracods abundance was homogeneous. 3) Euconchoecia aculeata, dominant species, contributed the most to high abundance in the nearshore. 4) The species richness of ostracods was positively correlated with temperature and water depth; and the abundance was negatively correlated with temperature and salinity, and positively correlated with chlorophyll α concentration. Qiongdong coastal upwelling and Yuexi Coastal Current boosted the abundance of planktonic ostracods up to 1252 ind·m ^-3 in the coastal waters of the Leizhou Peninsula and northeast Hainan Island. Results suggest that smaller mesh net should be used for the overall assessment of community characteristics of planktonic ostracods.

Marine ecosystem dynamics model is an important means to study marine ecological environment. As the model complexity increases, the number and uncertainty of biological parameters increase, which has a great impact on model results; therefore, optimization of model parameters is particularly important. In this paper, a one-dimensional physical-biological model is applied to the northern South China Sea, and the key biological parameters obtained through sensitivity analysis are optimized by using genetic algorithm. The results show that the sensitive parameters in the model are related to phytoplankton growth, zooplankton growth, feeding and death, and detritus sinking. Based on the genetic algorithm optimization of the above-mentioned parameters, we find that the surface and vertical simulation errors of the model are reduced by 27.80% and 21.40%, respectively, by using only surface satellite data; the surface and vertical simulation errors are reduced by 14.90% and 32.70%, respectively, by adding observed profile data. The success of applying genetic algorithm in the one-dimensional model provides the basis for its further application in three-dimensional marine ecosystem models.

In situ hydrographic data of sea surface salinity (SSS) revealed a freshening trend in the northern South China Sea (SCS) during 2004—2012, which was the freshest in 2012. Such freshening was also found in the UK Met Office EN4 objective analysis product and the ocean general circulation model for the Earth Simulator (OFES) output during 1993-2014, particularly west of Luzon Island and in the southeastern SCS. The salinity budget analysis in the upper layer indicated that the fresh water forcing and horizontal salinity advection contributed to the trends of SSS west of Luzon Island and in the western SCS, respectively. The excessive precipitation in summer accounted for the decrease of SSS to a great extent west of Luzon Island, which was associated with the strengthening trend of the Walker Circulation.

Driven by the southwest monsoon, an offshore jet is usually formed in the western South China Sea (SCS) and sandwiched by a cyclonic eddy in the north and an anticyclonic eddy in the south, which affects the ecosystem of the region. Using in-situ and satellite data in September 2014, we analyzed the impact of this jet with two eddies on phytoplankton size structure in this region. The data showed that picophytoplankton (0.2~2 µm) dominated the surface, averaged at 76.7% of the total chlorophyll. The contributions of nanophytoplankton (2~20 µm) and microphytoplankton(20~200 µm)in the jet area were respectively higher, and had a positive relationship with the total chlorophyll. Comparatively higher percentage of microphytoplankton appeared in the anticyclonic eddy at surface (average 10.3%) than in the cyclonic eddy (average 3.6%). The results suggested that physical processes significantly influenced summertime surface phytoplankton size structure in the western South China Sea. Both the jet and eddies can affect phytoplankton size structure by increasing the contribution of microphytoplankton. Surface horizontal advection of phytoplankton by the northeastward jet formed the coastal upwelling area and was the main source of microphytoplankton in the open sea. The interactions of convergence and divergence in the eddies with the jet formed a chlorophyll front and increased the microphytoplankton component. Upwelling in the center of the cyclonic eddy brought up nutrients, which increased the percentage of the microphytoplankton component.

For deep understanding of the evolution of a fault in the passive continental margin slope, we analyzed the characteristics of fault activities and sedimentary response on the northern slope of the Baiyun Sag in this paper. Based on geological structural interpretation of 3D seismic profiles, fault activities were studied by using growth index. The results showed that the fault strike mainly is NWW-trending, while the rest are NEE- to EW-trending of small faults in the eastern and northern slope zones of the Baiyun Sag. In the vertical sections, these faults exhibited ladder and conjugated shaped combinations, while a single fault may be of planar shape. There were three episodes of post-rift fault activities, namely, 23.03-21 Ma, 19.1-15.97 Ma and 10-2.59 Ma. There has been a significant increasing activity since 10 Ma. The study of faults’ orientation indicates that Cenozoic stress field rotated clockwise from NS to NNE and the deflection angle is about 8 degrees. The main controlling factor of fault activities on the northern slope zone of the Baiyun Sag was the continuing strong abnormal subsidence and rapid sedimentation, while the rifting of the South China Sea was also an important factor that formed weak compressive stress on the northern margin; and the convergence between South China Sea plate and Philippine Sea plate played a key role in faults’ reactivation. Sedimentary units responded to tectonic movements directly in a series of deformation. The Dongsha Event was most obvious by canyon incisions, strata truncation and fault reactivation. We suggest the driving force came from the east.

To better understand deep reflectors in the multi-channel seismic profile D80 in the northern South China Sea, a coincident ocean bottom seismic survey line (OBS2015-1) with 15 OBSs was inversed. This OBS line extends ESE 300 km, from 800 m in the upper slope to 3760 m in the abyssal plain. OBS data processing includes position and time corrections, phase identification using Obstool software, and velocity inversion with FAST tomography software. The resultant velocity shows that Cenozoic strata has a velocity of 1.6-3.5 km·s^-1 and a depth of 2 km. Mesozoic velocity is 3.5-5.5 km·s^-1 with an average depth of 3 km. In the oceanic and continental transition zone (OCT), the sediment basement is greatly affected by Late Cenozoic magmatic activity. Obvious high velocity anomaly and isoline uplift (5 km) were detected in the upper crust of the upper slope, corresponding to chaotic reflection anticline structure in D80; and the overlying Late Cenozoic strata also presents as synchronous deformation, which may result from the Late Cenozoic magmatic intrusion. The crustal thickness decreases gradually from 23-20 km beneath the continental slope to 8 km in the oceanic basin. A high velocity (7.0-7.6 km·s^-1) layer was seen in the lower crust, which changes from 5 km in the upper slope to 2 km in the ocean basin. Because of the Late Cenozoic magmatic activity in the continental slope and OCT, we interpret that the high crustal velocity layer originated from magmatic intrusion after the cession of seafloor spreading.

Recent investigations of the ultraslow-spreading (full spreading rate: 12~18mm#cod#x000b7;yr^-1) Southwest Indian Ridge revealed two kinds of crustal structure: Magmatic and amagmatic accretionary crust. Magmatic accretionary segments appear as the axial rise, relatively low mantle Bouguer gravity anomaly, strong magnetization and thick crust. Amagmatic accretionary segments feature detachments and abundant high-angle normal faults, lack of transform faults, deep water, relatively high mantle Bouguer gravity anomaly and weak magnetization. There are also significant amount of serpentinized peridotites exposed on the seafloor, and the igneous crust is thin, even absent. The Southwest Sub-basin of the South China Sea (SWSB) has relatively slow-spreading rates (full spreading rate: 50~35mm#cod#x000b7;yr^-1). The central part of SWSB also presents thin crust and there might be some serpentinized peridotites in the basin area, which are similar to the characteristics of the amagmatic accretionary crust in the ultraslow-spreading Southwest Indian Ridge.

To improve the accuracy of Chaetognatha identification, we combined morphological identification and DNA barcoding based on the mitochondrial CO#cod#x02160;(cytochrome oxidase subunit #cod#x02160;) gene to identify some arrow worms from the northern South China Sea. The results showed that the morphologically and molecularly identified results were consistent, for the species of Ferosagitta robusta, Aidanosagitta neglecta and Aidanosagitta regularis. Furthermore, the neighbor-joining phylogenetic tree was constructed with the amplified CO#cod#x02160; sequences and those homologous sequences from GenBank based on Kimura 2-parameter, and the phylogenetic positions of Sagiitta (Sagittidae, Sagittoidea) were discussed. The results indicated that these CO#cod#x02160; sequences formed monophylotic clades compared with GenBank (91%~100%). The intraspecific genetic distance ranged from 0.000 to 0.015 with an average value of 0.004, while the interspecific genetic distance ranged from 0.081 to 0.173 with an average value of 0.127. The barcoding gap was detected between intra and interspecific variations, suggesting that the CO#cod#x02160; gene could be a standard DNA barcode for the Chaetognatha identification. This study was the first to apply DNA barcoding to phylogenetic analysis of Sagittidae, and to discuss the phylogenetic position at molecular level.

The four-season environment ecological comprehensive surveys were carried out at 34 stations in the southern South China Sea by the research vessel “NanFeng” from March to December in 2013. Results show that the average chlorophyll a (Chl a) concentration of shallow water at 150 m in early spring (0.14 mg•m^-3) and early summer (0.12 mg•m^-3) were lower than that in early autumn (0.18 mg•m^-3) and early winter (0.16 mg•m^-3). This was mainly due to the smallest spring wind speed in early spring and the highest water temperature in early summer, which were not conducive to vertical mixing, limiting deep nutrients being brought up. The subsurface Chl a maximum in the four seasons all appeared at both 50 m (0.24~0.26 mg•m^-3) or 75 m (0.22~0.26 mg•m^-3), and the difference of Chl a concentration at these two depths was small, with little season change. The 50 m and 75 m layers were located in the middle part and near the bottom of the euphotic seawater, and the light received by the two layers can meet the growth need of phytoplankton. With the combined impact of mixed layer depth and the upper bound of the thermocline, the Chl a concentration in the 50 m layer was mainly controlled by the supply of nutrients, and that in the 75 m layer, by ocean temperature, both being important factors affecting phytoplankton growth.

Cold-water corals, also known as deep-water corals, are newly found ecological resources with high biological diversity and attract scientific research on environmental changes. In this study, we investigated cold-water corals retrieved from the cold seep area in the northern South China Sea. We identified two species (Crispatotrochus sp.1, Crispatotrochus sp.2) and four genera [Balanophyllia (Balanophyllia), Balanophyllia (Eupsammia), Lochmaeotrochus, and Enallopsammia]. Stable carbon and oxygen isotopic analyses revealed that the δ¹³C and δ¹⁸O of cold-water corals were -7.36‰ ~ -1.15‰ and -1.38‰ ~ 3.67‰, respectively. The δ¹³C and δ¹⁸O results were close to the carbon and oxygen isotopic values of cold-water corals in other parts of the world, but were quite different from those of warm-water corals, cold-seep carbonate rocks and low-temperature hydrothermal carbonate rocks. This research discovered cold-water corals in a cold seep region of the northern South China Sea. This discovery is important for the studies on environmental effects of deep-water currents and of cold springs in the region.

The third scientific drilling in the South China Sea (SCS) will be carried out during Feb. to Jun. 2017 under the system of the International Ocean Discovery Program (IODP), namely, IODP Legs 367 and 368. The drilling project will improve the research on geology and geophysics of the SCS and bring it to an unprecedented stage. The combination of Ocean Bottom Seismometers (OBS) deep seismic survey with IODP drilling data will improve drilling achievement, and contribute greatly to our understanding of the specific mechanism of rifting and breakup processes in the northern SCS. We first built three original velocity models based on the three geological presumptions of IODP Legs 367 and 368 as follows. 1) the exhumed lower crust, 2) the oldest oceanic crust and 3) the exhumed upper mantle. Second, the ray-tracing and travel-time modelling were performed for different OBS intervals with employment of Rayinvr and Tomo2d software, as well as check-board tests for the models. The testing results suggest that the ray-tracing paths and ray density with the 7 km interval of OBS stations are better than those with the 10 km interval. However, the seismic survey line is 100 km, enough in length to make sure to detect the crustal structure at 30 km depth. And the results of check-board tests show that the 7 km interval of OBS stations is necessary to difference the 20-km-in-size velocity anomaly, which is the uncertainty zone in the seismic profile. The design analysis on the optimal deployment scheme of OBS stations will not only provide a good suggestion for the design of future seismic survey, but also contribute to our understanding of the mechanism of rifting and breakup processes in the SCS.

The northern part of the South China Sea (SCS) is one of the important oil and gas bearing areas in China. However, the evolution history of the basins located in the northern SCS and their relationship with the surrounding tectonic events are still unclear. Based on the drilling and seismic data, the tectonic stress evolution characteristics and the initial crustal thickness of the Qiongdongnan Basin and Pearl River Mouth Basin are calculated by using mathematical simulation. The results show that the basins of the northern SCS have thinner initial crust thickness and lithospheric thickness. There exist two stress relaxation periods in the Pearl River Mouth Basin, and two stress relaxation periods in the Qiongdongnan Basin, one in shallow water and the other in deep water. The first period of stress relaxation in the northern SCS was continuous in space, which is mainly distributed in the deep water area, and was earlier in the east and later in the west. The second period was different in the east and west. The analysis shows that the stress relaxation periods in the deep water of the northern SCS were related with shear rifting from east to west of the northwest sub-basin of the SCS. The second period of stress relaxation in the Pearl River Mouth Basin was related to local magmatic intrusion, and the stress relaxation period in the shallow water of the Qiongdongnan Basin corresponded to the quiet period of the Red River fault.

Well LF14, drilled in the Lufeng Sag of the northern South China Sea, discloses marine sediment archives ranging from the mid-to-late period of the early Miocene to the early Pliocene. An abrupt rise in paleo-water depth up to 275 m during the early Miocene is recorded at the lowermost part of the well, followed by several fluctuations thereafter. The early Miocene to early Pliocene deposits are interpreted as formed under middle-to-outer shelf environment, and five complete transgressive-regressive sea-level cycles are identified. The results of backstripping calculations indicate a rapid subsidence in the study area during 17.5-10 Ma. The high sedimentation rate in this period may be caused by rapid tectonic subsidence and high terrigenous input. The subsidence rate was low from 10 to 4.53 Ma, characterized by a low sedimentation rate mainly due to less supply of terrigenous materials and less accommodation space caused by the Dongsha Movement. Finally, several short-lived uplift events were identified in the well based on quantitative reconstruction of paleo-water depth.

Marine geological survey is the strategic demand of our country to build a powerful marine nation. The survey data have important application values. With the rapidly developing technologies of computer, database and network, combining the ways of digital management, application and sharing of marine geological survey data has become an important indicator to measure the level of marine science and technology in China. In view of disadvantages of poor interactivity and low scalability in traditional WebGIS application, the sharing service platform of marine geological survey data in the South China Sea on the web was developed by Flex API and ArcGIS Flex Viewer, to develop widget for functional expansion based on the application framework. The users can browse and inquire these data easily. In this paper, the centralized management, display and service of the marine geological survey data in the South China Sea are illustrated. The results show that the platform can meet information service demands of marine users.

Mesoscale energy can be effectively extracted from geostrophic flows via sub-mesoscale processes and forward cascade to smaller dissipation scales. These ubiquitous sub-mesoscale processes in the upper ocean play an important role in the transport of mass and energy, mesoscale variability and re-stratification of mixed layer. Using the high-resolution (500-m) ROMS results, we preliminarily analyze the sub-mesoscale dynamic processes of typical anticyclonic eddy in the northern South China Sea in winter. Our results show that the strong lateral buoyancy gradient at eddy periphery can efficiently reduce the Ertel potential vorticity of the front, which exacerbates frontal instabilities and is favorable for the development of symmetric instability (SI). In this case, one of the most important mechanisms is the frontogenesis for the generation of frontal SI. Furthermore, sub-mesoscale processes and associated instabilities can trigger a strong vertical secondary circulation across the front. The vertical velocity is up to 95 m·d^-1, suggesting significant vertical exchanges of mass and energy in the mixed layer.

We selected two rapidly intensifying, locally-originated typhoons Wutip (1321) and Mirinae (1603), and analyzed upper- and low-level circulations, vertical wind shear, and ocean heat content in and around the South China Sea during typhoons’ passages over the sea. We also used WRF (Weather Research and Forecasting Model) model to discuss the environmental factors for their different intensities. Although the underlying conditions were more favorable for Mirinae, it did not develop into a strong typhoon due to weak upper- and low-level circulations and strong vertical wind shear. On the contrary, Wutip developed into a super typhoon rapidly due to the cold southward airflow and the outburst of warm humid air from the southwest. The results from WRF model’s sensitivity experiments about the effect of sea surface temperature (SST) on the intensity of locally-originated typhoon indicated that locally-originated typhoon intensity and SST have nonlinear positive correlation. However, as SST increases, the intensification trend of locally-originated typhoon slows down. From July to September, the SST in the South China Sea is higher than 28℃, meeting the condition for typhoon intensification. Therefore, attention should be paid to different environmental factors other than SST in locally-originated typhoons’ forecast in the South China Sea.

Using remote sensing to accurately estimate phytoplankton absorption coefficient a_ph(l) can provide basic data and useful method to distinguish different functions of phytoplankton species for long time and large spatial scale. In this paper, the characteristics of a_ph(l) spectral are compared and analyzed in four typical areas of the South China Sea (SCS), east area of Qiongdong (QD), Guangdong Coastal area (GD), and the Pearl River Estuary (PE) by using field data collected during2003-2012.Then, the phytoplankton population structure differences are preliminarily identified. Furthermore, the performances of MODIS-Aqua a_ph(l) products derived from the semi-analytical algorithm QAA and empirical algorithm PL by using MODIS-Aqua remote sensing reflectance R_rs(l) products are compared in the SCS and QD waters based on the relaxed match-ups between MODIS-Aqua products and field data. The results show the differences of a_ph(l) spectral features are obvious among the clear water represented by the SCS and QD and turbid waters represented by GD and PE. In the clear waters, the a_ph(l) value is small but in a dominant position of particle absorption, while in the GD and PE areas, the a_ph(l) value is relatively large but not in a dominant position. The a_ph(l) coefficient have obvious spatial differences, and the possible causes are pigment packaging effect and the variation of pigment composition and concentration. MODIS-Aqua a_ph(l) products derived from the empirical algorithm PL perform better than those from the semi-analytical algorithm QAA. The algorithm QAA-derived a_ph(l) products underestimate the results compared to the field data, while the algorithm PL overestimate the results, with the average relative error (APD)less than 22% for both algorithms. There is a great improvement in the accuracy of the PL algorithm by using the Chl-a products derived from the optimized algorithm of OCI (named algorithm NOCI), with the APD less than 14%. In summary, there are strong application prospects to discuss different functions of ocean phytoplankton species by using remote sensing products.

The shallow gas reservoir in the northern deep water area of the South China Sea (SCS) is a new type of oil-gas reservoir associated with gas hydrate, which has the characteristics of shallow burial and large scale, and its burial depth is generally less than 300 m. The shallow gas reservoir was formed by the gas hydrate cap in the deep cracking gas along the fault, and the identification of bottom simulating reflector is an effective method to find the shallow gas reservoir. The gas sources mainly include: pyrolysis gas, biogas and mixture. Slope tensile rupture is a major channel for gas migration, a sand layer at the bottom of the hydrate is the main reservoir of shallow gas accumulation, and the hydrate layer is blocked. Looking from gas hydrate distribution characteristics found in the SCS, shallow gas reservoir, widely distributed in slope deep-water area, with big reservoir thickness, and potentially significant resources, is a new type of reservoir while mining costs are relatively low.

Our Argo trajectory simulation system for the South China Sea (SCS) contains the high-resolution ambient velocity field, a Lagrangian particle tracking model and the parameterization that represents the vertical motions of profiling Argo floats. This system is applied to simulate both conventional Argo floats (typically parked at 1000 m depth and profiling to 2000 m depth) and Deep Argo floats (parked at 500 m above the seafloor) within the SCS. By conducting the simulation with the counterparts of six core Argo floats serviced in the SCS, we find the displacements of synthetic floats from the simulation system resemble the real float displacements over 100-day time intervals. We therefore judge the simulations for core Argo are robust and further apply the system to simulate a potential Deep Argo array (with the resolution of 2°×2°×30 day). The results explore both the representativeness and the predictability of float displacements, which may provide a basis to understand float displacements in the deep layer as well as to contribute to planning deep Argo array program.

Dongsha area is a critical gas hydrate area of the northern South China Sea, and a submarine slide is extensively developed on its slope. In this paper, we analyzed the sedimentological particle size, species characteristic and stable isotope of benthic foraminifera in the cores 973-4 and 973-5, respectively, recovered from the middle of the slope and the flat area at the base. The results of the distinctly negative δ¹³C values and the heavier δ¹⁸O values in both cores in the Last Glacial period suggested that there were persistent gas hydrate dissociation events in the Dongsha area during this period. The δ¹³C negative gradually disappeared and the δ¹⁸O values decreased since the Last Deglacial period, indicating that gas hydrate dissociation was prevented because of global sea level rise. In the core 974, obvious submarine slide deposits only occurred in 440~600 cm according to the Last Glacial Maximum and the number of Uvigerina spp. and Bulimina spp. sharply increased in this location, which implied the submarine slide was probably caused by an intense methane release event induced by the descending sea level in the Last Glacial Maximum period. A smaller scale submarine slide was also recorded in core 973-5, but the deposition time was later than that of core 973-4.

The seabed pockmark is widely found in the Qiongdongnan Basin in the northern slope of the South China Sea. The formation of pockmark is mainly regarded as related to the seabed fluid seepage in recent studies. However, it is still unclear what the geochemical characteristics of the seabed sediment are and what corresponding activities of the pockmark are. Two sediment cores of C14 and C19 from the Southern Qiongdongnan Basin were recovered and investigated in this study. Here, we examine the contents of total sulfur (TS), total carbon (TC) and total organic carbon (TOC) contents as well as chromium reducible sulfur (CRS) content and its stable isotopes (δ³⁴S_CRS) values. Combined with the ratio of total nitrogen to total organic carbon (TN/TOC) and reduced sulfate content in pore water, we analyze the geochemical characteristics. The results suggest that there are two types of sulfate reduction reaction (SR) at the C14 site - the anoxygenous oxidation of organic matter controlled SR (OSR) at the depth of 0~3.91 meters below seafloor (m bsf) and the anaerobic oxidation of methane (AOM) derived SR at the depth of 3.91~7.55 m bsf, and the depth of 3.91 m bsf is the boundary of the two types of reaction of SR. The contents of TS and TC below 3.91 m bsf are higher than those above 3.91 m bsf, and the concentration of sulfate in pore water below 3.91 m bsf decreases linearly with depth, Therefore, the depth of 3.91 m bsf is a significant indicator for geochemical characteristics in sediment core. Moreover, the SR below 3.91 m bsf is controlled by the seepage of methane-bearing fluid. The contents of TS and TC at the C19 site increase with depth. However, the variation of TN/TOC ratios is almost contrary to the patterns of TS and TC contents, that is, because OSR is the main SR type of the whole sediment core at the C19 site and the organic matter is revealed to be deposited in early diagenetic stage. The δ³⁴S_CRS values from cores C14 and C19 are -50.2‰~-46.9‰ and -50.1‰~-42.0‰, respectively. The negative δ³⁴S_CRS values suggest that the major biogeochemical process is the result of sulfate reduction in a relatively open system. Therefore, the methane-bearing fluid for the pockmark in the Qiongdongnan Basin had leaked and the pockmark is now seemingly in a weak seepage stage and/or even to be inactive.

Physical parameters such as water content, density and porosity of marine sediments are important for studying acoustic characteristics of sediments. Due to the complex and variety of the marine sediments in the South China Sea and precious deep-sea sediment samples, it is necessary to accurately determine the physical parameters of these sediment samples and make sure no damage to their chemical properties. However, the current measuring methods cannot meet the demands. In this study, we determined and analyzed the changing trends and characteristics of the water content, density, porosity of the clay, silt, and sand in three typical sediments with the drying time under different temperature (60, 80, 100, and 120℃) by using circular knives method and dryer method. The results are as follows. 1) The higher the temperature, the shorter the time of the total loss of pore water, and the periodic characteristics of the water loss process for the same type of sediment. 2) The times of typical sediments completely lose the pore water are in the order of t_sand<t_silt<t_clay; and at different time periods, the rate of pore water loss is quite different, which is mainly related to the particle size, the gap between particles and the loss of moisture contained in sediments in the later stage of drying. 3) The drying temperature of 80℃ is advisable, if the sediments is also needed for further comprehensive utilization and analysis, and the reference time and critical time are calculated for the complete drying of the three sediments. 4) Physics parameter of the critical time generation into the experience equation for velocity prediction is feasible at 80℃.

Chlorophyll a (Chla) concentration is an important indicator of phytoplankton biomass for estimating primary production, which exhibits inhomogeneous vertical distribution. In this study, optical measurements with high vertical resolution were used for studying the spatio-temporal variability of Chla profiles and its influencing mechanism, providing us much insights for understanding marine environmental dynamics in the South China Sea. In this study, an optical inversion method was proposed to retrieve Chla profiles from in situ measurements with high-spectral absorption/attenuation spectrophotometer in the northeastern South China Sea. Based on the in situ data of the Kuroshio Cruise in summer 2015, an empirical relationship between the absorption line height at 676 nm and Chla was developed for inverting Chla with a high accuracy (Chla=49.96×(a_LH(676) (676))^0.9339, the coefficient of determination R²=0.87, the root mean square error RMSE=0.16 mg·m^-3). Moreover, the response characteristics of Chla profiles to different hydrodynamic processes were studied. Results showed that large spatial differences existed in the vertical distribution of Chla. In the nearshore area, surface Chla varied from 0.42 to 1.57 mg·m^-3, which generally followed a decreasing trend with depth. Uniform vertical distribution of Chla in upper ocean was observed in the coastal upwelling region. The Subsurface Chlorophyll Maximum (SCM) layer is nearly ubiquitous in stratified waters of the open ocean, with its vertical structure being influenced by mesoscale processes. The depth of SCM ranged between 34 and 100 m, which showed consistent variability with the isopycnal depth of 1023 kg·m^-3. The SCM layer was uplifted to about 34 m due to the mixing effect in the upper ocean caused by Kuroshio intrusion in the western area of Luzon Island. The uplifting and widening of the SCM layer were also observed in the area affected by a cold eddy, with obviously different features in the eddy center from that on the eddy edge.

Particle size distribution (PSD) describes the relationship between particle concentration and particle size, influencing marine ecosystem environment, the optical properties of sea water, and so on. Based on the in-situ profiles of biological and optical properties during summer 2016 in the South China Sea basin, characterization of PSD was studied. The power-law model was fit to describe the PSD, and the results indicated that the mean coefficient of determination between in-situ PSD and simulated PSD could reach 0.95 in the logarithmic space. The PSD slope (ξ) ranged in [1.27, 7.65] with a mean of 3.93±0.56. The mean of ξ in the surface water of the South China Sea basin was similar to the mean of ξ in global ocean surface water, but higher than that in the surface water of other areas such as the bay. There was a strong negative relationship between ξ and the mean diameter (D_A). Taking section T1 as an example, we analyzed the mean PSD profile of these stations. The features of PSD profiles are as follows. 1) At the surface, there were high values of ξ with low values of D_A because the dominate particle was pico-phytoplankton. 2) The minimum value of ξ appeared in the subsurface chlorophyll maximum layer (SCML) with higher D_A, which may result from the higher proportion of big phytoplankton. 3) In the twilight layer, the values of ξ as well as the values of D_A were between those at the surface and in the SCML. This phenomenon may be related to the process of flocculation, decomposition, and settlement of phytoplankton. The traits of PSD would influence the inherent optical properties (IOP) of seawater. We found that both particulate beam scattering coefficient at 532 nm (b_p(532)) and particulate beam backscattering coefficient at 532 nm (b_bp(532)) would be higher in the SCML because of the increasing chlorophyll-a concentration. However, the lowest mean b_p(532) and the lowest mean b_bp(532) were found in the twilight layer. Furthermore, although the relationship between ξ and the particulate beam attenuation sepctral slope was weak, the model of Boss et al. (2001b) is suitable for estimating the regional range and mean value of ξ roughly.

Ocean dynamic processes in the South China Sea driven by the monsoon are characterized by seasonal and multi-scale variations, which have significant impacts on the evolution of marine ecosystems. Marine bacteria, as an important component of material cycle and energy flow in marine ecosystems, have active responses to the environmental changes associated with multi-scale dynamic processes. The interdisciplinary research on microbial community structures, their dynamic variation and associated physical processes is one of the hot issues in marine sciences, which combines marine microbiology, microbial ecology and physical oceanography. Based on the primary results of the Key Research Program, "Study on bacteria diversity coupled with the monsoon circulation in the South China Sea and significance in microbial oceanography" supported by the National Natural Science Foundation of China, we discuss in this paper current research progress, including coastal upwelling, seasonal thermal fronts, mesoscale eddy, sub-mesoscale processes, and their potential impacts on the marine bacterial diversity and local ecosystem in the South China Sea.

In this study, the vertical characteristics of near-inertial oscillations (NIOs) induced by Typhoon Kalmaegi are analyzed by using data from two moorings about 20 km apart in the northeastern South China Sea in Sept. 2014. The results show that the energy of typhoon-induced NIOs, which propagates downward into the ocean interior, increases with depth in the upper layer and becomes maximum in the mid layer before dissipating with depth. However, we find that there is a large difference in vertical characteristics of typhoon-induced NIOs between the two moorings. The NIOs at mooring A had three different vertical phase velocities at different depths while the NIOs at mooring B showed invariable phase velocity. The value and depth of the maximum near-inertial kinetic energy (NIKE) are also different at the two moorings. The maximum NIKE was 15% greater at mooring B than at mooring A. After the passage of Kalmaegi, fD₂ via nonlinear wave interaction between f and D₂ occurred at the two moorings, except that the intensities of fD₂ at the two moorings were different. At mooring A, fD₂ changed with depth and its interaction between f and D₂ was weak. The energy of fD₂ at mooring B enhanced at the full depth, and the kinetic energy and velocities of fD₂ and f had a good correlation for their time-space distributions and variations. The difference of vertical phase velocity of NIOs and fD₂ at the two moorings may be caused by the effects of eddy and different stratifications.

Sea surface wind is not merely a major driving force to the upper-ocean currents; the wind energy input is the main source of mechanical energy to keep these currents moving. To analyze the secular trend of wind energy input into the South China Sea (SCS), we calculate the wind energy input into the surface current, surface geostrophic current and surface ageostrophic current from 1901 and 2010 by using SODA (v 2.2.4) data. The results show that during the past 110 years, the trend of the wind energy input into surface current, surface geostrophic current and surface ageostrophic current decreased on the whole. The reducing amplitude is 56%, 65% and 49%, respectively. The dominant factor is the decline of wind stress (about 35%). Due to the monsoon systems, seasonal variation of wind energy input into the SCS is significant. In winter, the wind energy input is the strongest, mainly in the north and west parts of the basin, and the shape of energy input distribution is like a “boomerang.” Our results have some implication for further understanding SCS circulation in terms of energy.

The basaltic basement in the South China Sea was cored for the first time during International Ocean Discovery Program (IODP) Exp 349. The basalt was collected from Sites U1431 and U1433 close to the fossil spreading ridge in East and Southwest Sub-basins, respectively. Carbonate veins (n=16) within the basalt were investigated under microscope and by the laser Raman spectra. At Site U1431, the carbonate veins consisted of calcite veins or aragonite veins, while at Site U1433 the carbonate veins are composed of either calcite veins, or aragonite veins, and calcite-aragonite mixed veins. Meanwhile, the calcite veins and aragonites alternated, and at ~42.1 m below basement, two calcite veins and one aragonite vein occurred parallel, likely indicating that multiple hydrothermal fluids of different sources have circulated there. At both sites, the textures of aragonite are generally the same-blocky, fibrous, and radiating fibers. However, it is not the case for calcite. At Site U1431, the calcite is clotted blocky, blocky, granule, and fibrous, while at Site U1433 the calcite only occur as blocks. The abundance of carbonate veins at Site U1431 is much higher than that at Site U1433. These suggest that the low-temperature hydrothermal circulation at Site U1431 is stronger than that at Site U1433. Different local geology and environment mostly lead to the differences in hydrothermal activity at these two sites: Site U1431 is close to a giant seamount that serves as a recharge site, while Site U1433 is far away from any recharge or discharge site.

A coupled physical-biological 3D model was established, which is forced by realistic forcing, to quantify the effects and underlying mechanisms of upwelling and river plume on the spatial distribution of phytoplankton biomass during summer in the northern South China sea (NSCS). The model was validated by using cruise data of 2006 and 2008 and remote-sensing observations from 2006 to 2008. The results suggest that the model satisfactorily captures the processes of coastal upwelling and river plume dynamics that determine phytoplankton distribution. Model results indicate that summer phytoplankton in the NSCS were mainly distributed within 50m isobath. In Qiongdong and Shantou, phytoplankton distributed evenly. Upwelling processes accounted for up to 90% of the biomass, and horizontal advection in the upper layer was the main sink, while biological processes were the main sources for the phytoplankton biomass. In the Pearl River and Shanwei, phytoplankton shows surface and subsurface maxima: the river plume contributed 35%~40% (mainly to the upper layer) of the phytoplankton biomass, and upwelling contributed 60%~65% (mainly to the middle and bottom layers). In Yuexi, the phytoplankton biomass was extremely low in the upper layer, and mainly distributed in the middle and bottom layers; in total, upwelling contributed 92% of phytoplankton biomass. In the NSCS, both upwelling and river plume processes stimulated phytoplankton through nutrient supply. In the upwelling process, nutrient supply reflected the combination of along- and across-shore nutrient transports. The thermocline, which alters vertical nutrient transfer, is the key factor affecting the variable contributions of upwelling and river plume processes for phytoplankton in different layers. Overall, the spatial variation of summer phytoplankton is mainly driven by upwelling and river plume processes, and by the circulation-nutrient-biological coupling effect.