Exploring the deep South China Sea: Retrospects and prospects

Rapid developments of deep-sea researches in China over the past 20 years have promoted the South China Sea (SCS) into the international deep-sea frontiers. The “three deep technologies”, namely scientific drilling, long-term seafloor observation and deep submersible vehicles implemented successively in SCS studies helped to achieve a number of scientific breakthroughs. Over the 20 years, five international ocean drilling expeditions to the SCS recovered nearly 10 km of sediment cores from sites at 3–4 km water depths, and drilling into the magmatic basement at 6 sites shed light on the genesis of the SCS basin. Coupled with other deep-sea short core sediments from the SCS, these records demonstrate evidence that water and carbon cycling in the low latitude regions can directly respond to the orbital forcing, and subsequently nurture a new concept of low-latitude forcing of climate changes, which challenges the classical wisdom of the overwhelming role played by the Arctic ice-sheet in climate changes. The exploration in the continent-ocean transition zone also reveals a number of specific features that characterize the SCS as a marginal basin formed at the subduction zone in the Western Pacific. The features include active magmatism and rapid rupture of lithosphere through the basin formation process, and imply that “the SCS is not a mini-Atlantic” as they can be distinguished as “plate-edge rifting” and “inner-plate rifting” respectively, thus challenging the universality of the Atlantic model for passive margins. Many more discoveries can be assembled from long-term mooring observations and deep diving cruises in the deep SCS, such as the cyclonic nature of the deep-water circulation, deep-water sediment transport by contour currents and turbidites, manganese nodules, extinct hydrothermal vents, and cold-water coral forests. In addition, prominent progress achieved in microbiology and biogeochemistry includes the microbial carbon pump and the coupling of carbon and nitrogen cycles. Clearly, most achievements of the deep-sea explorations in the SCS over the last 20 years have always been of international scale and impact. However, the contributions from Chinese scientists are most prominent, particularly with the research activities undertaken from the major program “Deep Sea Processes and Evolution of the South China Sea (2011–2018)” supported by the National Natural Science Foundation of China.