The major research programDeep Sea Processes and Evolution of the South China Sea (2011-2018)" supported by the National Natural Science Foundation of China, was completed with some major achievements. Together with the implementation of three and a half ocean drilling expeditions (IODP 349, 367, 368, 368X), rapid developments of deep-sea researches have promoted the South China Sea into the international deep-sea frontier. Substantial progress was made in three fields:the oceancontinent interactions in deep sea, the plate-edge rifting of marginal basin, and the low-latitude forcing of climate changes, all with innovative discoveries challenging the conventional wisdom originated from the West Europe and North Atlantic. The research progress in recent years have shown the leading role of the Chinese community in the South China Sea. With the further enhancement of research activities, the South China Sea is expected to become a natural laboratory for the global ocean science.

The South China Sea (SCS) is a semi-enclosed and multi-connected marginal deep sea with a large number of islands (seamounts). The deep SCS circulation is very robust with active deep water renewed. This paper reviews the current understanding of the deep water in the SCS, focusing on its origin and development, as well as the unique features of the SCS deep circulation.

The deep-sea storm involves the rapid movement of the water with high suspended particle concentration near the bottom of the deep sea. It can damage or even destroy all obstacles, change the submarine topography, and affect the benthic ecological environment. However, in-situ observational studies of its occurrence and the sedimentary record were few and far between. This paper studies the sediment dispersion process of a deep-sea storm in September 2016 caused by the turbidity current activity and observed in situ by deploying a mooring system on the levee of the Gaoping Submarine Canyon in the northeastern South China Sea. It is shown that the suspended particle concentration in places near the bottom increases rapidly with the increase of the temperature and the decrease of the salinity. It is believed that the sediment dispersion of the deep-sea storm is the main dynamic process for the formation of sedimentary laminae in the submarine canyon.

This paper reviews the spatially and temporally varied air-sea CO₂ fluxes in the South China Sea (SCS), to show that its basin area is a weak source to the atmospheric CO₂, while its northern shelf is a CO₂ sink. On an annual average basis, the SCS emits carbon of (1.33±1.88)×10¹⁰ g. The northern shelf includes a River-dominated Ocean Margin (RiOMar) during the peak discharges, and an SCS basin as an Ocean-dominated Margin (OceMar). The OceMar is characterized by dynamic exchange with the open ocean via a two-dimensional or even three-dimensional process, i.e., the horizontal intrusion of the open ocean water and the subsequent vertical mixing and upwelling. Depending on the different ratios of the dissolved inorganic carbon (DIC) and nutrients from the source waters into the margins, the relative consumption or removal between the DIC and the nutrients, while being transported into the euphotic zones taken over by biogeochemical processes, determines the CO₂ fluxes. Thus, the excess DIC relative to the nutrients in the upper layer will lead to the CO₂ degassing. Similar diagnosis can also be made to the RiOMar systems with typical features of significant excess nutrients relative to the DIC. It is suggested that the framework of the carbon cycle revealed from the SCS has important implications in better understanding world's other coastal systems.

The South China Sea (SCS) is the largest marginal sea in the western Pacific Ocean. Significant breakthroughs have been made in the SCS researches, especially through the South China Sea Deep Initiative and International Ocean Discovery Program (IODP). One of the surprising discoveries is that the expected mantle serpentinites at the IODP drill sites are not found at the northern SCS continental margin; instead, the magma is found to erupt rapidly, indicating the significant magmatism at the SCS soon after the continental rifting and probably due to the strong influence of surrounding subduction zones. Thus, the SCS might be regarded as a new type of rift basin of "plate-edge rifting", different from the classic Atlantic type of "intra-plate rifting". It is also suggested that the subduction-induced mantle upwelling is likely to play an important role in the magmatism of the SCS.

The South China Sea is roughly divided by the Zhongnan Fault into the eastern and western parts, with big differences in the Mesozoic sedimentation and the residual thickness, the nature and the characteristics of the continent-oceanic boundary, the age of the continental breakup, the width of the thinned continental lithosphere, the age and the magnetic layer structure of the oceanic crust, the magnetic anomaly, the lithochemistry, and the magmatic activity. They are mainly controlled by the east-west difference in the pre-rift tectonic background, the lithospheric extension rates, and the tectonic settings related with seafloor spreading. These factors profoundly affect the subsequent regional sedimentary zonation and thermal subsidence.

There are two types of volcanic rocks in the South China Sea (SCS):the mid-ocean ridge basalts (MORB) and the ocean island basalts (OIB). The International Ocean Discovery Program (IODP) Expeditions 349, 367, 368 and 368x have successfully drilled out the basement of the SCS basin, for the first time, with samples of the oceanic crust during the initial opening (~34 Ma) and the final spreading (~15-16 Ma) of the SCS. The East Subbasin, the Southwest Subbasin and the continent-ocean transition (COT) zone in the northern margin of the SCS represent different evolution stages of the basin. Due to the differences of the mantle evolution, the mantle potential temperature and the recycled materials in the mantle sources, the MORBs generated at the mid-ocean ridges with different spreading rates show distinct compositions. The large-scale mantle upwelling beneath the SCS during the post-spreading, probably induced by the continuous subduction in the surrounding area, has produced the seamounts at the fossil ridges in the SCS, unlike the volcano chain generated by the mantle plumes. Although the SCS is a small marginal sea, it has recorded incrediblly abundant information and thus provides a rare window for probing the deep earth.

The present-day South China Sea (SCS) was evolved from land to sea in the Eocene 34 million years ago, and its paleo-water was very deep. At the turn of the Miocene/Oligocene, 24 million years ago, due to the T60 tectonic movement, the entire SCS basin became a deep-sea environment. Since the Miocene, 10.0, 6.5 and 1.2 million years ago, along with the collision of the Luzon island arc with the Eurasian plate, the semi-closed degree of the SCS basin increased, so that the SCS deep-water could only come from the Pacific above the sill depth (~2600 m) of the Bashi Strait. After that, due to the global sea level change, the deep-water exchange between the SCS and the Pacific on both sides of the Bashi Strait displayed the glacial/interglacial mode.

The fluvial systems in the South China Sea have experienced prominent variations since the SCS was originally formed. During the early Oligocene, the drainage area of the Pearl River was constrained within the coastal South China. It gradually extended westward into the plateau margin of Yunnan-Guizhou during the late Oligocene. It is until Miocene that the modern Pearl River fluvial network has been well established. A source-to-sink analysis also indicates that a paleo-river "Kontum-Ying-Qiong" was originated from the western South China Sea paleo-ranges and played a significant role in the sedimentary infilling processes, but finally buried under the sediments with the following South China Sea seafloor spreading. The evolution reconstruction of the northern South China Sea fluvial systems and the sedimentary environment has provided considerable insights into the paleogeographic reconstruction of the South China Sea as well as the Eurasian southeastern margin since the early Cenozoic, as well as the petroleum exploration within the South China Sea sedimentary basins.

Based on the data obtained from ocean drilling wells and the seismic profile data, differences and changes in sediment sources of the deep-sea basin in the South China Sea (SCS) during syn- and post-seafloor spreading are determined quantitatively. It is shown that the oceanic basin of the SCS started to be formed as far back as the Oligocene (32Ma), and it extends with two spreading events of the SCS. The basin is mainly filled with volcanic conglomerate and ash, carbonate rock, ultra-microfossil mud, muddy clay, silty clay, mudstone and silt-fine grained sandstone. The main sediments in the deep-sea basins are terrigenous deposits since the Late Miocene (11.6Ma). The supply of plentiful terrigenous deposits is closely related to the regional tectonic events (such as the rapid uplift of the Qinghai-Tibet Plateau and the subduction of the Philippine plate) during the closing process of the SCS, as well as the strengthening of the East Asian monsoon since the Late Miocene and the strong weathering and erosion of the source area.

The evolution of the Cenozoic reefs-carbonate platform is considered a part of the South China Sea opening, and plays an important role in the deepwater hydrocarbon reservoirs, the global carbon circulation, and the South China Sea tectonic history. The platforms were formed during the early Miocene, and flourished during the early Middle Miocene. However, they became drowned since the Middle Miocene, and many platforms disappeared during the Quaternary. It is found that the carbonate platform of the South China Sea sees a developmental pattern from south to north and from east to west. The highest accumulation rate of the platform was reached during the Middle Miocene. The control of the platform life history is very complex. But the most important four factors are the tectonic activities, the changes in the relative sea level, the changes in the input of the terrestrial detrital materials, and the changes in the paleo-ocean environment, and they are considered to control the development and the death of the reefs-carbonate platform.

Submarine landforms associated with turbidity currents are well developed in the South China Sea (SCS). There are numerous submarine canyons in the continental slopes. Sediment waves and cyclic step bedforms related to the supercritical turbidity currents are distributed along the thalwegs, on the overbank areas or off the outlets of some submarine canyons. However, large-scale submarine fans at the foot of the continental slopes are rarely observed; on the other hand, a high proportion of turbidites are found being preserved in the pelagic to the hemipelagic succession of the abyssal plain. We suggest that the turbidity currents traversing the submarine canyons could be of high energy, enough to sustain a longer distance transport along the abyssal plain even after the deceleration at the foot of the continental slopes. The turbidity-current associated landforms in the SCS were mostly initiated in the late Miocene, and might be closely related to the active plate tectonics in the SCS and surrounding areas.

In the South China Sea, about one tenth of the deep basin area with water depth more than 2500 m is occupied by various seamounts. Supported by the NSFC program "Deep Water in South China Sea", more than ten seamounts were explored for the first time by four dive cruises with the HOV and the ROV from 2013 to 2019, respectively. Among the achievements in these cruises, three exciting discoveries are made, including two large ferromanganese nodule fields, the fossil hydrothermal field and the extensive deep-water coral forests. The two large ferromanganese nodule fields are on two seamounts, the Jiaolong seamount of about 3500 m deep and the Taimao seamount of 1700 m in water depth, respectively. The size of a ferromanganese nodule field on the top of the Jiaolong seamount is about 500000 m² with a nodule coverage from 30% to 50%. Nearly all nodules are hydrogenetic in their origin. Besides the exceptionally high Th,Pb,Ce contents, the contents of potential economic valued elements Mn,Cu,Co,Ni etc. are found between open oceans and other marginal seas. The fossil hydrothermal field is named the "Nanming", with at least 16 extinct hydrothermal mounds along a 700m long diving survey track on the top of the Longxi seamount (about 3019 m in water depth) close to the residue spreading ridge of the South China Sea. Preliminary studies show that the low-temperature hydrothermal precipitates (mainly Fe-Si-Mn-P assemblages) are of a young age from about 5200 years BP to 14300 years BP, similar to the age of the active magmatism in the area. The fascinated deep-water corals or coral forests are discovered to live extensively on the seamounts over the South China Sea, mainly at the water depth from 800m to 1800m. There are also some corals growing on seamounts as deep as in water depth from 3000 m to 3700 m. All these discoveries shed a new light for us to understand the mechanism of the South China Sea system from earth interior to exosphere in multiple scales.

Several basin groups with wide and deep grabens were formed through intensive stretching and thinning of the continental-margin crust in northern shelf margin of the South China Sea, with various types of hydrocarbon source rocks and massive reservoir sand bodies being developed. It is the coupling control of large-scale basins, widely distributed multiple source rocks and high-variable geothermal fields that established the generic foundation for deep-water hydrocarbon resources. With the interaction of large drainage systems, tectonic relief and relative sea-level fluctuation, three types of depositional systems were formed to control the development and the distribution of large-scale reservoirs whilst the high heat-flow accelerated the compaction and the diagenesis of deep-water sediments. The deep-water hydrocarbon exploration is very promising at the northern shelf margin of the South China Sea although with some challenges due to the complexity and the variability of three factors:source rocks, effective reservoir rocks and heat flow.

Large deep-sea debris dumps that were never recorded before on the seafloor worldwide have been discovered in the submarine canyons to the north of Xisha trough in the South China Sea (SCS). The plastics primarily came from fishery and navigation activities, as implied by the categories of plastics in the dumps. The pollution history of microplastics reconstructed by 210Pb dating shows that the microplastics pollution started in the 1980's and it is characterized by the terrestrial input. The continental shelf in the SCS is heavily polluted by microplastics. It is further shown that the deep-sea canyons on the continental slope provide important conduits for carrying the microplastics to the deep-sea basin. The turbidity currents play a critical role in the transportation of plastics in the canyon system. This paper reviews recent progresses of studies of plastics and microplastics using manned submersible in the SCS. For the first time, the concept of the deep-sea plastic dump ecosystem is proposed.